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Charge transfer transitions in glasses - Attempt of a systematic review
Möncke, Doris
Möncke, D. & Ehrt, D. (2021). Charge transfer transitions in glasses - Attempt of a systematic review. Optical Materials: X, 12, 10092. https://doi.org/10.1016/j.omx.2021.100092
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Invited Article
Charge transfer transitions in glasses - Attempt of a systematic review
Doris M ¨ oncke
a,*, Doris Ehrt
baInamori School of Engineering at the New York State College of Ceramics, Alfred University, Alfred, NY, USA
bOtto-Schott-Institute, Friedrich-Schiller-University of Jena, Jena, Germany
A R T I C L E I N F O Keywords:
Charge transfer transition Transition metal ions UV cutoff Absorption edge Photoluminescence
A B S T R A C T
In glasses, even low levels of dopants or impurities can give rise to very intense and broad charge transfer transitions from ligands (e.g. oxygen or fluorine ions) to the metal ion (L→M), absorbing strongly in the short wavelength ultraviolet. In an attempt of a systematic review of charge transfer transitions, we compile data of various glass systems with high intrinsic transmission that allow the observation of charge transfer (CT) tran- sitions involving cations of different electronic configurations. Various glasses of different composition were selected as matrices, including fluoroaluminate glasses with low P2O5 content (FP10 = 35AlF3–10MgF2–30CaF2–15SrF2–10Sr(PO3)2), phosphate [SrP =Sr(PO3)2, NSP =Na2O-40SrO-50P2O5], silicate (NS =15Na2O–85SiO2, DS =33Na2O–67SiO2), aluminosilicate (BCAS =10BaO–10CaO–15Al2O3–65SiO2) and borosilicate (NBS1 = 16Na2O–10B2O3–74SiO2, NBS2 = 4Na2O–1Al2O3–21B2O3–74SiO2, Duran = 5Na2O/
K2O–1Al2O3–12B2O3–82SiO2) compositions. All glasses were prepared from very high purity materials and doped with various metal ions. Charge transfer transitions of electrons to or from these cations induce absorption and photoluminescence phenomena in the ultraviolet and visible spectral region, which were recorded by optical spectroscopy.
Charge transfer transitions were considered for empty valence orbitals, that is, for the high valent 3d, 4d and 5d ions, and for Zn2+, Ag+, Cu+with full d10 orbitals. 3d, 4d and 5d ions with partially filled valence orbitals that could be stabilized in the named glasses are studied as well. Doping concentrations for these allowed transitions typically ranged from 5 to 5000 wt-ppm of metal ions, with some samples also displaying higher dopant levels.
Inter valence charge transfer (IVCT) transitions directly from one metal ion to a neighboring metal ion (M→M) of the same element or metal to metal charge transfer (MM-CT) between ions of different elements can also induce strong visible absorption and deep coloring for which some examples will be discussed.
1. Introduction
Most charge transfer (CT) transitions of an electron occur between an anion and a cation (ligand to metal = LMCT), though intervalence charge transfer transitions of an electron between two cations of the same element with different valences or between different elements (IV- MMCT) may occur if two cations are situated in very close proximity to each other [1–5].
The transfer of an electron between oxygen ligands to metal cations occurs often at relatively high energies in the UV wavelength region.
This results in no apparent coloring, and only if CT bands extend into the visible, they may impart a yellow to brown color. This contrasts with d- d transitions, which give rise to colorful glasses when doped with a range of transition metal ions. S2−→ Fe3+is a well-known example for a visible CT transition, giving beer bottles their brown color; here, the energy of
the electronic transfer from the more polarizable sulfide ion instead of an oxygen ion, shifts the band into to visible range. Since CT transitions are fully allowed, the absorption bands can be very intense and very broad, even for low concentrations of the dopant metal ion.
This paper is the first attempt for glasses to systematically investigate CT transitions of various transition metal ions by comparing a selection of high purity glasses with high transmission in the UV. So far, only very few papers focus in a general way on CT transitions in glasses [6–13], while countless papers mention CT transitions of individual dopants in individual glass compositions, often without any more details. In order to have comparable cations and solvents, concentrations and ligand strengths, we dedicate a central part of this paper to the comparison of spectral data obtained between 1980 and 2011 on a selection of five glass types doped with a series of transition metal ions. The glasses were prepared for a range of different projects and while some spectra have
* Corresponding author.
E-mail address: [email protected] (D. M¨oncke).
Contents lists available at ScienceDirect
Optical Materials: X
journal homepage: www.journals.elsevier.com/optical-materials-x
https://doi.org/10.1016/j.omx.2021.100092
Received 15 August 2021; Received in revised form 13 September 2021; Accepted 15 September 2021
been published before in a different context, many have not yet been made publicly available, or have not been discussed regarding a sys- tematic correlation of CT bands with the ions’ electronic configuration.
We will also look into the impact of the glass matrix, where we will apply the concept of optical basicity, that is, the electron donor power of the glass matrix as a parameter of ligand field strength.
Our own data will be supplemented with examples from the litera- ture on typical CT transitions and applications that range from deeply colored minerals to the Raman Resonance effect. In the following we will give a brief overview of (1) different types of charge transfer tran- sitions and (2) optical basicity as a measure of the electron donor power of the glass matrix which will help to quantify the effect of the ligand strength of different glasses on metal cations. In (3) we will discuss optical absorption and photoluminescence spectra of different glass systems doped with a series of transition metal ions. Many data points are derived from five glass systems: phosphate doped fluoroaluminate glass (FP = 35AlF3–10MgF2–30CaF2–15SrF2–10Sr(PO3)2), two meta- phosphate glasses [––SrP = Sr(PO3)2 and ––NSP = 10Na2O–40S- rO–50P2O5], two silicate glasses with varying sodium content (NS = 15Na2O–85SiO2, DS = 33Na2O–67SiO2), aluminosilicate (BCAS = 10BaO–10CaO–15Al2O3–65SiO2) and borosilicate glasses with varying borate and sodium content (NBS1 =16Na2O–10B2O3–74SiO2, NBS2 = 4Na2O–21B2O3–1Al2O3–74SiO2, Duran =5Na2O/K2O–12B2O3–1Al2O3– 82SiO2). Table 1 shows the composition and some typical properties of these glasses. Fluoroaluminate glasses containing phosphates for better glass formation, are characterized by a very high transparency in the UV which even allow the observation of CT transitions of relatively high energies.
To find potential underlying regularities between the glass matrix
and the electronic configuration, we focused especially on d0 (Ti4+, Zr4+, V5+, Nb5+, Ta5+, Cr6+, Mo6+, W6+) and d10 (Zn2+, Ag+, Cu+) ions.
Transitions metal ions with half and partially filled orbitals are included as well, such as d1 (Ti3+, V4+, Mo5+, W5+); d2 (V3+, Mo4+, Mn5+); d3 (Cr3+, Mo3+); d4 (Mn3+); d5 (Mn2+, Fe3+); d6 (Fe2+, Co3+); d7 (Co2+); d8 (Ni2+); and d9 (Cu2+).
IV-MMCT transitions, which involve two cations, can also induce strong and broad visible absorption bands and deep coloring. We consider some examples in section 7, such as black to brown coloring Fe2+→ Fe3+and Mn2+→ Fe3+(as known from magnetite), or Fe2+→ Ti4+ which is known from ilmenite, as well as Cu+→ Cu2+, or Co2+→ Co3+IV- CT transitions.
2. Types of charge transfer transitions
Contrary to Laporte (symmetry) forbidden intra-atomic d-d transi- tions, inter-atomic charge transfer transitions of an electron are fully allowed and give intensive and broad absorption bands even when ions are present in only the smallest quantities. The relation between the concentration of coloring centers, c in mol per liter or in wt-ppm, the optical pathlength (sample thickness, d) and the transition probability (molar or specific extinction coefficient, ελ) to the measured absorbance is described by the Lambert-Beer Law:
A=c⋅d⋅ελ (1)
The molar or specific extinction coefficient, ελ, varies for d-d transi- tions between 0.5 L mol−1 cm−1 (double forbidden, that is spin and symmetry forbidden Mn2+) to 200 L mol−1 cm−1 for tetrahedral Co2+, where the lack of inversion center relaxes the Laporte or symmetry rule
Table 1
Composition and selected properties of the glasses used as matrix for the study of CT transitions [7]. Tmelt, melting temperature; Tg transition temperature, ne refractive index and ABBE-number νe at λ =546.07 nm; VUV intrinsic absorption edge in the vacuum UV; ρ, density; αO2-, oxygen polarizability; optical basicity Λ from the composition Λtheo, from the oxygen polarizability ΛαO2- and when using the probe ion Pb2+ΛPb.
Mol% Tmelt (◦C)
Tg(±3) (◦C) ne Δn ±2 ×10−4 νe VUV
edge (g/cm3) ±0.01 Å3 theoa ±0.001 O2-b
Pb ±0.001 FP10 10Sr(PO3)2
35 AlF3
10 MgF2
30 CaF2
15 SrF2
1050
440 1.4567 90 160 nm
7.8 eV 3.43 1.031 0.377 n. a.c 0.394
SrP 50 SrO
50 P2O5
1300
485 1.5589 66 170 nm
7.3 eV 3.20 1.496 0.480 0.501 0.465
NSP 10 Na2O 40 SrO 50 P2O5
1300
400 1.5468 165 185 nm
6.7 eV 3.05 1.495 0.481 0.476 0.46
DS 33 Na2O 66 SiO2
1450
460 1.5083 55 2.49 1.635 0.604 0.597 0.56
NS 15 Na2O 85 SiO2
1500
485 1.4833 61 195 nm
6.3 eV 2.34 1.528 0.531 0.523 0.54
BCAS 10 BaO 10 CaO 15 Al2O3
65 SiO2
1650
830 1.5560 60 185 nm
6.7 eV 2.85 1.599 0.580 0.572 0.58
NBS1 16 Na2O 10 B2O3
74 SiO2
1500
550 1.5109 163 195 nm
6.3eV 2.45 1.490 0.520 0.549 0.56
NBS2 4 Na2O 21 B2O3- 1 Al2O3
74 SiO2
1600
440 1.4706 65 175 nm
7.0 eV 2.17 1.442 0.470 0.462 0.50
Duran 5Na2O/K2O 12 B2O3
1 Al2O3
82 SiO2
1650
530 1.4725 66 175 nm
7.0 eV 2.22 1.450 0.484 0.468 0.58
aUsing: Λ(Al2O3) =0.61 for AlO4; Λ(AlF3) =0.40 for AlF6; Λ(B2O3) =0.4; Λ(BaO) =1.33, Λ(CaO) =1.00, Λ(K2O) =1.32; Λ(MgF2) =0.4; Λ(Na2O) =1.08; Λ(P2O4)
=0.36; Λ(SiO2) =0.48; Λ (SrO) =1.08 [31,33].
b Using α(Al3+) =052; α(B3+) =0.003; α(Ba2+) =1.55; α(Ca2+) =0.47; α(K+) =0.83; α(Mg2+) =0.094; α(Na+) =0.179; α(P5+) =0.052; α(Si4+) =0.033;
α(Sr2+) =0.86.
cThe empirical formula of equation (7) applies only to oxide glasses, not for mixed oxyfluoride systems.
[12,14]. However, in fully allowed CT transitions, ελ can reach values between 2500 and 25 000 L mol−1 cm−1 or between 0.01 and 0.5 wt-ppm−1cm−1 [6,15]. This means that even low traces of dopants or impurities can give rise to very intense charge transfer transitions and the only reason that such strong transitions are often overlooked is that they are usually of such a high energy that their maxima are found in the ultraviolet (UV) or vacuum ultraviolet (VUV) wavelength region [16].
Additionally, many oxide glasses have an intrinsic absorption edge that is found at lower energies, so that even if optical spectroscopy is employed to observe potential CT transitions, the bands are hidden under the absorption edge. For example in common window glass SLS, the apparent absorption edge is found around 300 nm due to the pres- ence of trace impurities [17] while the intrinsic UV-edge λ0 has a value of approximately 200 nm when a 0.2 mm thick samples of highest purity was analyzed by transmission and absorption measurements using vacuum-UV and UV spectroscopy [7]. The intersection of the linear extrapolation of the absorption edge with the x-axis gives λ0 in nm, which can then be converted to energy (cm−1 or eV).
In this review, we want to discuss only electronic transitions between differently charged atoms (inter-atomic) not transitions within one atom (intra-atomic). The latter is typical for d-d transitions, electronic tran- sitions between the d-orbitals of one cation. There, the partially filled valence orbitals are lowered or raised in energy due to the coordinating anions that establish a ligand field. Deeper details on dissolution colors in glasses can be found in many references, starting with the funda- mental early compilation of Weyl [18] to a myriad of works discussing ligand field effects [12–14,19,20]. Contrary to d-d transitions, system- atic studies of CT effects are rare. To our the knowledge, only few studies have been attempted for crystals [2], semiconductors [1], or solutions [5], and only recently have models been postulated that might predict CT energies in oxide crystals [21–23].
2.1. LMCT (ligand to metal charge transfer)
Upon excitation by a photon, an electron of a ligand, L (O2− in oxide glasses, F− in fluoride glasses), can be transferred to the metal ion, M.
This L→M transition requires a certain amount of energy that is usually high enough to fall in the short ultraviolet wavelength region. For highly polarizable anions, such as the larger sulfide ion, the transition can shift into the visible, as known for the amber brown glass of beer bottles, the S2−→ Fe3+CT transition peaking in the visible at 410 nm (see Fig. 1).
Thus, oxygen-metal (O→M) transitions can only be observed in high
purity glasses with overall low polarizability for a wide transmittance window in the UV. The anion polarizability of a glass can be related directly to the optical basicity of a glass, which will be discussed briefly in section 3. Glasses with high amounts of glass former and with mod- ifier cations of lower field strength or with fluorine instead of oxygen anions have generally a low optical basicity and high transparency in the VUV [7].
LMCT transitions are of great significance for a wide range of ap- plications, such as transparency and color [7,12,14], attenuation of optical fibers, photo-darkening after irradiation of glasses [7], and even for Raman measurements as will be discussed in section 6. LMCT tran- sitions often arise from impurities introduced by the raw materials of the glass batch or container/refractory materials and practically define the extrinsic absorption edge.
With the exception of amber brown (S2−→ Fe3+), few L→M transi- tions are shifted far enough to the visible to impart a color, e.g. O2−→ Cr6+ in the [Cr6+O4]2- complex, is shifted to extend into the blue wavelength region and as a consequence colors the glasses yellow to brown. Fig. 1 shows a schematic drawing of the S2−→ Fe3+CT, the corresponding absorption spectrum, and the ensuring color on the example of a brown beer bottle. An electron is transferred from the sulfide ion S2− to the transition metal cation Fe3+. The absorption maximum of 410 nm is well separated from the absorption edge at shorter wavelengths.
LMCT transitions are generally broad, as can be understood when visualizing the bonding and electronic transitions between an electron of a nonbonding 2p orbital of oxygen to an unoccupied 3d orbital of the transition metal cation, as depicted in Fig. 2.
As shown on the example of Fig. 1, the much larger sulfide ion compared to oxygen ion, results in a much more polarizable and more floppy electron cloud. Therefore, the more shielded nucleus makes it easier for sulfide to donate an electron or a partial charge to a neigh- boring cation. As a consequence, the CT transition is shifted into the
Fig. 1.(a) Absorption spectrum of the S–Fe CT transition, (b) schematic of LMCT on this example of the transition from S2− to Fe3+as typical for (c) brown beer bottles. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2.Molecular Orbital diagram for the M-O bonding of an MO6 octahedron.
The OMCT transitions from the oxygen 2p-to metal 3d-orbitals are indicated by arrows. Strongest dipole allowed σ–σ and π–π; weaker dipole-allowed π–σ and σ–π; and dashed, weak dipole-forbidden low-energy transitions (after Mos- kin [24]).
visible wavelength region. The coloring of glasses by this sulfur amber complex requires tight control of the redox equilibrium of the melt and production, which has been researched in much detail [19,25,26].
While intra-atomic transitions are well understood and can be pre- dicted by application of the Ligand Field Theory [4,5,14,20,27], no such theory exists yet for the prediction of CT transitions in glasses or crystals.
The authors are aware of several initial attempts for the prediction of CT transitions, such as recently by Takemura and Ogasawara [23], who were able to predict the LMCT energies for a series of trivalent transition metal ions in α-Al2O3 nonempirically by first-principles calculations.
A more extensive data set is provided by Dorenbos et al. who pre- sented several in depth studies of CT energies of rare earth elements and to a lesser degree of transitions metal ions in crystals, relying on a far- reaching literature review of available data [21,22].
2.2. Inter valence charge transfer transitions from cation to cation (IVCT) IVCT transitions involve transitions between two cations of different charge. Typical examples are high iron and manganese containing nat- ural glasses such as obsidian, or glassy materials made from silicate minerals. In historic times, black glass buttons were melted without batching form natural basalt (Ochsenkopf, Germany) [28], while today natural minerals are utilized in the production of insulating foam glass.
Like their historic counterparts, these products are often black, since the used minerals contain polyvalent ion pairs between which an electronic transition might occur, such as Fe2+→ Fe3+, or Mn2+→ Mn3+, or a mix of both, as depicted in the schematic drawing of Fig. 3
As can be seen from the schematic, a requirement for IVCT transi- tions is the close proximity of two polyvalent cations to enable the transfer of an electron [4,12,29]. In glasses, this also means that the overall concentration of the cations has to be in the range of one or more wt%, otherwise the cations are spaced too far apart for such transitions to occur. Thus, CT transitions can be observed for ppm level of cations, but IVCT transitions only for higher weight percentages of the dopants.
The energies of IVCT transitions are often much lower than LMCT transitions and extend far into the visible wavelength range. Since IVCT are often accompanied by CT transitions, in glasses they are not seen as separate bands with a clear maxima and defined separation from the intrinsic absorption edge, but rather as extending the absorption edge with shoulders and broad overlapping bands from CT and IVCT transi- tions reaching far into or beyond the visible wavelength range. In addition to obsidian, we will discuss several examples of IVCT
transitions that are observed in the spectra of multi-valent dopant spe- cies throughout the text and in more detail in the final section 7. In addition to the IVCT couples Fe2+→ Fe3+, Cu+→ Cu2+or Co2+→ Co3+, many other ions show distinct shifts in their UV–Vis cutoff, such as the Mo4+ containing NSP glass. Instances for strong MMCT transitions involving different elements are observed for Mn2+→ Fe3 or Fe2+→ Ti4+. Because of the high extinction coefficient of allowed transitions, even small amounts can shift the apparent absorption edge of glasses that contain multiple oxidation states or polyvalent ions. The less homoge- nous the glass structure and ion distribution, the higher the likelihood to observe IVCT transitions at even low impurity/dopant concentrations.
3. Optical basicity
The concept of optical basicity as first introduced by Duffy and Ingram [30] allows to estimate the electron donor power of a glass matrix from their composition and to subsequently relate the glasses’ anion polarizability to the preferred oxidation state or coordination number of dopants within the respective glass matrix. Even the melts’
solubility of carbonates, or the aggressiveness toward crucible materials can often be connected to the optical basicity.
The optical basicity was initially based on measurements of the nephelauxetic effect in glasses by comparing the shift of s-p transitions of probe ions such as Pb2+in the glass to that of CaO, which has been chosen as unity.
Λ=νfree− νglass
νfree− νCaO
= 60700− νglass
60700− 29700=60700− νglass
31000 (2)
here, νfree is the absorption energy of the free lead ion with 60 700 cm−1, νCaO is the energy of Pb2+in CaO with 29 700 cm−1, and νglass is the measured energy of the maximum of the s-p transition of divalent lead in the respective glass.
However, probe ions often seek out high basicity sites and while they might be good at predicting potential dopant sites, this method does not always display the average basicity of a glass. Here, measurements based on the refractive index give better results. From measurements of many glasses and crystals, review papers with oxide basicities for most oxides based on elements of the periodic table are now available [31,32], which allow the calculation of the average or theoretical glass basicity from the composition alone, using the increment system proposed by Duffy in which the oxide basicities are weighted over the oxygen equivalent fraction.
Fig. 3. (a) Schematic of an IVCT transition of an electron from Mn2+to Fe3+, which like the Fe2+to Fe3+IVCT gives the black appearance of obsidian (b). The orange arrows denote the path of the transferred electron; (c) shows the absorption spectra taken at the two marked spots on sample (b). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Λth=ХA
γA+ХB
γB+…= ya ya+qb+…Λ(
AxOy
)+ qb ya+qb+…A
Λ( BqOp
)+…
(3) here, 1/γА=Λ(AxOy) … is the optical basicity of oxide AxOy, etc. while Xa, Xb, … are the oxygen equivalent fractions of the glass composition rather than the mole fraction. It has been shown that the optical basicity of fluorides is one half of the optical basicity of oxides [33] and the optical basicity of nitrides is three halves the optical basicity of oxides [34], resulting in a wide applicability of the concept of optical basicity to a wide range of glass systems.
The experimental determination of the optical basicity via the refractive index is based on an empirical correlation between the oxygen polarizability and the optical basicity. Using the Lorenz-Lorentz equa- tion, and assuming that the cation polarizabilities remain constant, the oxygen polarizability can be calculated from refractive index and den- sity data.
αm=3Vm
4πN. (n2− 1
n2+2 )
(4) here, αm is the average molar polarizability, N is the Avogadro number, Vm the molar volume, and n is the refractive index at infinity wave- lengths (nλ), though for oxide glasses most often nd is used as an approximation. The electronic oxygen polarizability αO2− can be derived from αm by subtracting the sum of the cation polarizabilities∑
iNCαC, and dividing by the number of oxygen ions:
αA=αm− ∑
iNCαC
NA (5)
where NA and NC are the numbers of anions A and cations C. A list of cation polarizabilities used in the current study is given in Table 1.
Duffy suggested an empirical relation between the electronic oxygen polarizability (αO2−) and the optical basicity [35], which is valid for a wide range of oxide glasses:
Λ=1.67 (
1− 1 αO2−
)
(6) This equation was later updated with equation (7) that has a higher precision, but focuses on a narrower compositional range and is best used for Λ<0.65:
Λ=0.7αO2− − 0.547 (7)
Likewise, the intrinsic absorption edge of a glass can be used if refractive index data is lacking.
However, as will be evident by the manyfold examples shown in the current study, the presence of small concentrations of dopants and im- purities can have a significant impact on the position of the absorption edge. When measuring CT transitions instead of the intrinsic absorption edge, the experimental optical basicity based on Egap will deviate significantly from the intrinsic band gap and overall glass polarizability and optical basicity as calculated from the composition or refractive index data. Thus, the current study can also be a warning of relying too much on Tauc plots for the determination of the intrinsic band gap of glasses, which are often defined by a very broad Urbach tail due to impurities [6]. Viezbicke et al. discuss the variations of the band gap as found in the literature for the simple compound ZnO and the wide spread of data, from 3.1 eV to 3.5 eV, is as much of a warning of using Tauc plots in glasses, especially when the purity level of the glass is not highlighted as well [36].
This section on optical basicity is included for two reasons, for one it helps to understand the selection of the different glass systems used in the experimental part of our study of CT transitions in glasses as all selected systems have a relative low polarizability and high trans- parency in the UV that allows us to observe CT transitions better.
Secondly, when comparing the CT transitions of the same dopant in the different systems, we assume that the electron donor power of the glass matrix relates directly to the intensity and position of the CT transitions.
4. Experimental section
Data of CT and IVCT transitions from optical absorption and/or photoluminescence excitation and emission spectroscopies were collected over the last 30 years, as part of different projects [6–9,15,16, 29,37–48].
4.1. Glass types
Nine different glasses from five glass systems were selected for this study: Fluoroaluminate glasses with low P2O5 content (FP), phosphate (SrP and NSP), silicate (NS and DS), aluminosilicate (BCAS) and boro- silicate (NBS1, NBS2 and Duran) glasses of different optical basicity.
Table 1 shows the exact composition and some properties of the selected glasses, including band gap, refractive index, oxygen polarizability and optical basicity.
Details on glass preparation, structure and properties can also be found in Table 1 and in the references given here and throughout the text. All selected glasses are characterized by a relatively good trans- mission in the UV wavelength range, especially when prepared from high purity raw materials in Pt crucibles (or for phosphate glasses in silica). Carbon crucibles and argon atmosphere were used for reducing conditions, shifting the redox state of the polyvalent ions [6,9,15].
The ionic fluoroaluminate glasses with low P2O5 content (FP), have the lowest optical basicity [6,15,49]. Two meta-phosphate glasses SrP = Sr(PO3)2 or 50SrO–50P2O5 and the more basic NSP = 10Na2O–40S- rO–50P2O5 glass have a higher optical basicity and due to the polyvalent nature of phosphorous have shown exceptional aptness in stabilizing lower oxidation states of dopants when melted under reducing condi- tions [16,46].
The second group comprises silicate containing systems like two sodium silicate glasses (NS, DS) [47], the aluminosilicate glass (BCAS) [39] and three borosilicate glasses. Of the three borosilicates, the optical glass NBS1 is comparable to BK7 and has a significant number of non-bridging oxygen atoms, while NBS2 has the same silica content of 74 mol% but a lower Na2O:B2O3 ratio such that it has a fully polymer- ized network comparable to the Duran-like glass, though the latter has a higher SiO2 content of 82 mol% [7,42,50].
All glasses were of very high purity and prepared and doped with 5–5000 wt-ppm of various metal ions. Ions with empty or full d-orbitals:
d0: Ti4+, Zr4+, V5+, Nb5+, Ta5+, Cr6+, Mo6+, W6+
d10: Zn2+, Ag+, Cu+
Ions with partially filled d-orbitals such as:
d1: Ti3+, V4+, Mo5+, W5+; d2: V3+, Mn5+; d3: Cr3+, Mo3+; d4: Mn3+; d5: Fe3+, Mn2+; d6: Fe2+Co3+; d7: Co2+; d8: Ni2+and d9: Cu2+
4.2. Spectroscopy
Optically polished plane-parallel glass plates with a thickness be- tween 1 and 10 mm were used for transmission measurements on a Shimadzu UV-3101 PC spectrometer. Spectra were obtained from 190 to 3000 nm with an error <1% and a special set up was used for 0.2 mm thick samples to access the vacuum UV region, 120–200 nm, with an error <15%. Absorption spectra were obtained from the transmission spectra as A =-log (I0/IT).
Photoluminescence was studied with a Shimadzu RF-5001 with a 150 W Xe-lamp for excitation and emission in the range of 200–900 nm
in reflection modus with an error <5%. The slit width varied between 1.5 and 5 nm.
5. Results and discussion
The direct measurement of CT bands is not easy, since they are often fully or partially hidden by the intrinsic absorption edge of the glass matrix, or so intense, that the band cannot be separated from the new apparent absorption cutoff in the UV–Visible range. CT bands might be observed as distinct bands with a well separated maximum when measured on very thin samples of glasses with a high intrinsic band gap, made form ultrapure materials and only doped with a very low con- centration of the respective transition metal.
Fig. 4a shows a well-defined CT band and its evolution into the ab- sorption edge, cutoff in the UV–visible region, using the example of Cu doped FP glasses with increasing Cu-concentration from 100 to 500 wt- ppm. The dotted green line in the same plot also shows how the apparent absorption edges used throughout the text were obtained by extrapo- lation of the flank of the absorption edge and intersection with the x- axis. The UV cutoff is the easiest obtainable parameter, though also the least exact, lowest energy limit of CT transitions. Fig. 4b shows the spectra of Cu doped Duran glasses, over a wider range of Cu levels ranging from 40 wt-ppm to 4 wt%.
Here, the LMCT and the IVCT can be well distinguished in Duran. The CT band of O2–→Cu2+(235 nm) is found at a very similar position as the band of O2–→Cu+(240 nm), while the IVCT band covers the full visible wavelength range, enclosing the d-d transition at 800 nm which gives CuO doped glasses their characteristic cyan blue color. For low Cu-levels up to 400 wt-ppm, the LMCT band increases in intensity, but not in its width. For higher Cu-levels, the emergent IVCT band causes a dramatic shift in the UV–Vis cutoff. This is apparent from the photographs in Fig. 5, which show the as prepared glasses as well as 10 mm and 1 mm
thick polished sample plates. The bottom row actually shows remnants of the silica crucible attached to the glass, as the high viscosity of the melt at the melting temperature of 1680 ◦C does not allow to cast the melt. While the d10 ion Cu+is colorless, d-d transitions of the d9 ion Cu2+ result in a cyan coloring, turning with higher Cu-levels first green and finally to a dark green in thin samples while thicker specimen of the glass with 4 wt% Cu appear almost black, due to the broad, intense IVCT transitions.
Photoluminescence allows the assessment of the position of the band maxima of excitation bands which might be hidden by the absorption edge. Often, the excitation bands are found to be positioned in the low energy tail of the respective CT absorptions bands. The reason why, is not yet fully understood. Dorenbos mentions this phenomenon as typical for 3d, 4d, 5d and 5f elements, but not for 4f ions [21,51]. Another explanation for the discrepancy in the excitation and charge transfer absorption bands might be the effective interaction with the excitation photons and the dopants. Photons of too high energies that exceed the transmission window can only be absorbed at the sample’s surface, while lower energy photons might enter deeper into the glass sample and excite the glass volume thus resulting in a higher number of emitted photons despite the fact that only the tail of the CT band is excited. This idea is illustrated by the overlay of absorption and excitation photo- luminescence spectra of Fig. 6, on the example of the Cu-doped Duran samples from Fig. 4b.
Increasing the doping level from 40 to 400 wt-ppm, shifts the exci- tation band from 240 nm to 270 nm, while the photoluminescence in- tensity increases significantly. For higher concentration levels of 4000 wt-ppm, the photoluminescence intensity decreases drastically due to concentration quenching. Similar to the figure of merit for Faraday ro- tator materials, the PL excitation maximum might therefore be a com- bination of two factors, best excitation, and best transparency. Since CT transitions are spin and symmetry allowed, their extinction coefficient is
Fig. 4. Normalized absorption spectra of (a) FP glass doped with increasing amounts of Cu (100–500 wt-ppm) The positions of the LCuCT band maxima and of the d- d transitions are indicated. The green dotted lined, the determination of the absorption edge, cutoff used for the data listed in the supplementary information. (b) shows Duran glasses doped with approximately 40 wt-ppm to 4 wt% Cu (40 000 wt-ppm), broken line denotes reduced melted samples [7]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
very high. In glasses we often see more than one CT band, the higher energy band usually has the higher extinction coefficient, the lower energy bands might therefore add a slope to the absorption edge, or a pre peak type shoulder. Deconvoluted absorption curves for iron doped Duran are shown later in Fig. 11.
Examples for the shifts of CT-transition maxima can be seen in Fig. 7a
and b for Nb and W for three different glass types, FP, NSP and Duran.
The corresponding excitation and emission spectra are shown in the same plot (Fig. 7c and d). Depending on the glass system and concen- tration and dopant species, the cutoff in the UV–Vis appears as a steep edge or to be more sloped. If more than one redox state of the dopant is present, especially for higher concentration levels or inhomogeneous clustering, IVCT bands can also affect the absorption edge. Such an IVCT contribution can be seen in Fig. 4b or later in Fig. 9.
Fig. 8 shows the correlation between the CT band positions (listed in Table 2), photoluminescence excitation bands (listed in Table 3) and the absorption edge (listed in the Supplementary Information).
The apparent cutoff was measured by the intersection of the extrapolation of the absorption slope with the x axis of spectra from our collection, spectra that had been expanded to the highest available ab- sorption units (y-axis) This interpolation is included in Fig. 4a for clarification.
As mentioned, the excitation bands are observed in the low energy tails of the absorption bands, the same low energy site that determines the absorption edge. Fig. 8 shows more scattering when correlating the measured CT band maxima with the absorption edge, partially, due to the presence of more than one observable CT band. When a second, high energy CT absorption band or shoulder was observed, these data points are added in Fig. 8 as hollow symbols. The rare instances that the CT bands are observed at a higher position as the UV cutoff can be explained by concentration effects. Higher dopant levels might overemphasize the second CT absorption band with higher extinction coefficient, which only appears as low energy shoulder or tail in the samples of high cation concentrations but are well resolved for samples with very low dopant levels.
5.1. LMCT shift with charge
Fig. 8 gives an indication over the energy range of LMCT transitions in inorganic oxide and oxy-fluoride glasses. Upon closer analysis, several trends can be recognized. For example, for the same ion, higher oxida- tion states result in lower energies of the CT transitions. This is shown exemplarily for molybdenum in Fig. 9a. The redox state of Mo could be Fig. 6. Photoluminescence excitation and emission spectra (left axis) for the
Cu-doped Duran glasses samples of Fig. 4b (photographs are displayed in Fig. 9 of [7]). The dotted lines show the positions of the respective absorption spectra (right axis). Black 400 wt-ppm, red 40 wt-ppm, cyan 4000 wt-ppm, blue 4 wt%
(40 000 wt-ppm). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Photographs of the Cu-doped Duran samples melted under air for which the optical spectra are depicted in Fig. 4b. The doping levels increase from left to right from 40 wt-ppm to 4 wt% Cu’. See text for more details.
shifted in the NSP glass by changing melting conditions from bubbling with oxygen (Mo6+), to melting under normal atmosphere (Mo5+), to adding sugar (Mo4+), and melting in graphite crucibles under argon atmosphere (Mo3+). Despite the fact, that the intermediate oxidation states are always present in a mix with higher and/or lower oxidation states of molybdenum, a significant IVCT contribution is only apparent for Mo4+, where the absorption edge is shifted far into the visible wavelength region. All other samples show a decrease in the energy of the absorption edge for Mo3+>Mo5+>Mo6+[40,52]. The polyvalent nature of phosphorous supports a high flexibility in oxidation states of polyvalent dopants melted under reducing or oxidizing conditions in NSP glasses [16,46]. In glasses, only few elements are stable in so many different oxidation states, and often different oxidation states can only be realized by changing the glass matrix. Other polyvalent ions with three or more known oxidation states in glasses are V, Cr, Nb and Mn, though some species, such as Cr4+and Mn4+seem to be unstable in glasses.
The spectral shift of the CT band of Mn2+>Mn3+>Mn5+is depicted in Fig. 9b for a standard NCS glass (74SiO2–10CaO–16Na2O) [14] and more basic meta-silicate glasses (50SiO2–10BaO–40Cs2O and 55SiO2–10BaO–40Cs2O) [53].
For better analysis, the ions can be further separated according to equivalent electronic configuration, that is, ions of the same number of valence electrons or ions of the same charge are compared. The latter
concept is used in Fig. 10 where (a) includes d10 ions and figures (e) and (f) include mostly d0 ions as only few fully oxidized cations can reach such a high charge. For most series, the CT transitions of heavier ions of the same charge have higher energies 5d >4d >3d, though the data is not sufficient for M6+ions. For the 3d ions of M2+or M3+distinct zigzag patterns emerge, which have been described before by Takemura [23]
and Dorenbos [21,22]. The strong effect of the glass matrix is apparent for several ions like titanium where many experimental data points were available.
5.2. LMCT shift with the glass matrix
On first glance, it appears from Fig. 10 that for many ions, the most acidic fluoroaluminate glasses (FP), together with the fully polymerized low alkali borosilicate glasses Duran, exhibit higher CT energies than silicate glasses, which have higher modifier oxide levels and therefore higher optical basicities. For fluorides, such a behavior would be ex- pected from the anion polarizability and higher energies of FM-CT over OM-FT transitions (see also [11,21,54]). However, careful analysis shows a few exceptions, for example the Ti-glasses. If Ti4+and Ti3+ions are both present – and the likelihood of this is greater for the more acidic glasses and for NSP - the emergence of IVCT transitions might shift the absorption edge into the visible. Except for the Ti-glass series, CT bands in FP glasses are most often found at the high energy end of the CT Fig. 7. The optical absorption spectra, top, (a) and (b), and photoluminescence excitation and emission spectra (bottom, (c) and (d), are shown for 1000 wt-ppm W (left) and 100 wt-ppm Nb (right) doped glasses, Duran, FP and NSP [15].
transitions variations found for each cation.
Iron is another case for which detailed CT absorption data is avail- able. Two examples of band deconvolution are shown in Fig. 11.
Different CT transitions for Fe ions can be distinguished in Duran, FP and SrP samples doped with 20 and 25 wt-ppm Fe’ (as Fe2+ or Fe3+)
respectively. The difference in energies might reflect on a change in the ligand type. Fig. 11 also allows to determine the half width at half maximum (HWHM) of the CT bands, which vary from roughly 5000 cm−1 to over 11 000 cm−1 for the high energy band of Fe3+and from 4000 to 6000 for Fe2+.
Returning to the discussion of the glass matrix, it is worthwhile to look again at Fig. 7, which showed the dependence of the shifting ab- sorption edge for Nb and W in three different glasses. Undoped FP, NSP and Duran glass display their intrinsic absorption edge at 160 nm (FP), 175 nm (Duran) and 185 nm (NSP) [7,9]. For the Nb and W doped glasses, the absorption edge of Duran is always at higher energies than for NSP, but the absorption edge of the oxy-fluoride glass is shifted to much lower energies than expected from a glass with a high fraction of low polarizable fluoride ions.
Meta-phosphate and FP glasses allow for a very homogenous distri- bution of ions, while Duran glasses are prone to clustering of dopants near borate units [37,55,56]. Electron spin resonance spectroscopy gives evidence of an early loss of hyperfine structure in FP glasses compared to phosphate glasses, but a higher packing density could account for the observed effect. FP glasses will provide a mixed F− and O2− ligand environment to the MZ+dopants, and contributions of OMCT transitions of pyro-phosphate and ortho-phosphate ligands might be better observable than the higher energy FMCT transitions of the fluoride li- gands [21,52]. At the current time, we can only speculate why CT transitions are often found at higher energies in Duran glasses than in FP glasses.
Overall, these variations within one specific dopant series are sig- nificant, but become negligible when comparing other dopant species as in Fig. 10, where greater changes are observed with the ion’s charge or mass. The variations in the slope of the absorption edge speaks for the presence of more than one CT band with different extinction coefficients for the same ion in the different glass systems, and as shown by Dorenbos [21,22], such intermediate states and differences in conduction and valence bands are crucial for the CT energies.
The effect of changing coordination numbers will be discussed later in section 5.7.
Fig. 8.Correlation between the CT band position (blue diamonds, hollow symbols for lower energy second bands or shoulders) and the photo- luminescence excitation bands (red circles) with the UV cutoff of the glasses and dopants for which two or all three values were available from our data listed in Tables 2 and 3 and SI Table. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9.Shift of the absorption edge with increasing charge of the dopant for (a) 5000 wt-ppm Mo in NSP melted under oxidizing conditions by bubbling with oxygen (blue), under air (black, d-d band of Mo5+), melting slightly reducing with added sugar (green, dashed, IVCT with d-d bands of Mo5+, Mo4+and Mo3+) and strongly reducing conditions in a C-crucible under argon (red, d-d band of Mo3+) [40,52]; (b) shows the redox series of Mn-containing silicate glasses in order of decreasing absorption edge energy: NCS (≈5000 wt-ppm, Λ =0.57) (1) melted under reducing condition (Mn2+), and (2) under air (Mn3+); and the more basic CsBaSiX glasses with decreasing SiO2 content X in mol% (3) X =61 (Λ =0.7, Mn3+), (4) X =56 (Λ =0.75, Mn3+/5+), and (5) X =50 (Λ =0.81, Mn5+) [53]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
5.3. CT and ion mass
The energies of CT transition seem to decrease with lower size and lower mass of the cations, 5d >4d >3d. The data scatters for d0 M6+ ions, where for FP glasses the LMCT energies of Mo6+>W6+, and for silicate glasses Cr6+≈Mo6+≈W6+. However, for d10 ions, the CT energy of Ag+>Cu+, as well as for M5+and M4+, where 4dn >3dn for com- parable electron configurations n. Cr3+data scatters but FP glasses seem to reach the same CT energies as does the single measured Mo3+NSP sample.
For the series M2+and M3+, for which the most data points are available, spanning most of the transition metal ions 3d series, certain zigzag patterns are again observed concerning the variation of the ab- sorption edge. These will be discussed in more detail in sections 5.7 and 5.8.
First, we want to discuss ions with full or empty electron configu- ration, and afterwards ions with partially filled orbitals where we will focus on series of similar charge of the transition metal ion.
5.4. d0 ions
Fig. 10 includes data for the d0 ions (3d) Ti4+, V5+, Cr6+; (4d) Zr4+, Nb5+, Mo6+, (5d) Ta5+, and W6+. Spectra of well separated CT transi- tions bands were only available for Ti4+, Nb5+, Ta5+, and Cr6+, while photoluminescence excitation and emission spectra are available for all these ions, except for Cr6+, V5+and Mo6+.
For d0 ions, the highest energies for the CT bands are found for lower charges (M4+>M5+>M6+) for cations with the same number of orbitals, and for the same charge with higher mass (5d0 >4d0 >3d0). Interest- ingly, the trends agree qualitatively well with the average vacuum referred binding energy (VRBE), as introduced by Rogers and Dorenbos as the energy of the single electron in the lowest energy d level of d1 ions in various compounds [21,22]. In Fig. 12a, the average VRBE is used,
based on values extracted by Rogers & Dorenbos from a broad data set of crystalline materials using the chemical shift model. In Fig. 12b, the VRBE of the free ion is used as reference and shows qualitatively a very good agreement with the larger data set derived from photo- luminescence data from our glass samples.
The photoluminescence data of d0 ions covers more glass types than the scarce absorption data set of charge transfer transitions. For W, Nb and Ti, where FP, NSP and borosilicate and silicate glasses were avail- able, the measured excitation bands spread over a relative wide energy range, as discussed earlier.
5.5. d10 ions
Only three d10 ions were analyzed: Zn2+, Ag+, and Cu+. As apparent from Fig. 10a and b, the absorption edge is found at higher energies for the 4d >3d ion, the same is observed for the photoluminescence exci- tation band position.
The excitation bands are also higher for the higher charged d10 ion, that is for Zn2+>Cu+. Fig. 13 shows the CT energies as function of the glass type. As seen before, the highest energies are seen for the low basicity glasses Duran and FP and CT energies fall for the phosphate glass and even more for high dopant concentrations as for 50% ZnO in zinc meta-phosphate (ZnP) and zinc meta-borate (ZnB). Duran data shows some scattering, perhaps due to the fact that samples with higher dopant levels of 5000 wt-ppm of the cations are prone to clustering.
5.6. Ions with partially filled d-orbitals
We already discussed the increase in energy with a decrease of charge for ions of the same element in section 5.1 and in Fig. 9 on the examples of the Mo and Mn redox series.
For most redox pairs studied, we find the same trend, without any other exception, as can be seen from the comparison of the absorption Table 2
Measured absorption maxima of CT transitions in glasses and the extinction coefficient ε.
Ion Dopant Glass Absorption maximum ελ
dn Z+ wt-ppm eV cm−1 nm cm−1 ppm−1
3dn
Ti d0 4+ 5000 50 FP 5.5 44 400 225 0.50
56 100, 200 Duran 6.2 49 750 201 0.30
V d0 5+ 5000 NS, DS, BCAS 3.1 38 000 265 0.1
d1 4+ 25, 50 FP >44 000 <230
5000 FP
d2 3+ 25, 50 FP 5.8 46 500 215
Cr d0 6+ 200 NS, DS, BCAS 3.4 27 400 365 0.2
d3 3+ 25, 50 FP 7.3 57 140 175 0.5
Mn d5 2+ 5000 FP 5.4 43 500 cm−1 230
Fe d5 3+ 5000, 25 FP 6.7 & 4.8 54 000 & 38 500 185 & 260 0.25 & 0.18
SrP 5.3 41 700 240 0.20
20 Duran 5.6 & 4.9 45 050 & 39 200 222 & 255 0.30 & 0.06
50, 100 NS, DS, BCAS 5.8 & 5.0 46 500 & 40 000 215 & 250 0.3 & 0.1
Fe d6 2+ 6, 25, 50 FP 7.3 & 5.6 58 800 & 45 500 170 & 220 0.20 & 0.006
SrP ~5.6 ~45 500 ~220 <0.01
20, 50, 100 Duran 5.8 46 500 215 0.03
50, 100 NS, DS, BCAS 5.6 45 400 220 ~0.01
Ni d8 2+ 50 FP 7.3 58 800 170 0.2
Cu d9 2+ 25, 100 FP 6.9 & 5.4 55 600 & 43 500 180 & 230 0.15 & 0.12
Cu d10 1+ 25, 100 FP 7.1 & 5.2 57 100 & 41 700 175 & 240 0.40 & 0.04
d10 1+ 40, 400, 4000 Duran 6.2 & 5.3 49 800 & 42 500 201 & 235 ~0.1 & 0.02
Ag d10 1+ 50 FP 5.8 46 900 213 0.01
5000 NSP 5.3 42 500 235
100 Duran* 6.0 48 800 205 0.05
4d0
Zr d0 4+ 5000 FP >6.5 >52 600 <190
Nb d0 5+ 50, 100 FP NSP, Duran 5.9 ~47 620 ~210 ~0.2
Mo d0 6+ 5000 FP NSP, Duran >6.2 >50 000 <200
5d0
Ta d0 5+ 50 1000 5000 FP FP, NSP, Duran FP, NSP ~6.5 ~52 600 ≈190 ~0.2
W d0 6+ 1000 5000 FP, NSP, Duran NSP UV D <FP <NSP
