Engineering the A- and B-sites for upconversion luminescence in Ho- and Yb-codoped filled tetragonal tungsten bronze oxides
T. Wei•L. Ye •C. Z. Zhao•W. B. Wang• Q. Z. Ma• Q. Lv• J. M. Liu
Received: 11 October 2014 / Accepted: 18 December 2014 / Published online: 24 December 2014 Springer Science+Business Media New York 2014
Abstract Filled tetragonal tungsten bronze (FTTB) oxi- des represent a huge family of materials which exhibit rich electric and magnetic functionalities deserving for com- prehensive investigations. Recently, some members of this family have established their significance in efficient up- conversion (UC) luminescence upon proper rare-earth species doping, while the signal intensity is yet weak.
In this work, the UC luminescence of BapLn6-pTi8-p Nb2?pO30 codoped by Ho and Yb (BLTNp:Ho–Yb) with differentpare studied. The bright UC green emission, red emission, and near-infrared (NIR) emissions, originating from the two-photon energy-transfer process associated with the5S2?5I8,5F5?5I8, and5S2?5I7transitions of Ho3? ions, were observed. The luminescence intensity demonstrates remarkable dependence on the A- and B-sites occupation, and the UC green, red, and NIR luminescence can be enhanced for 10, 25, and 10 times, respectively, upon a change of p from 3.5 to 5.0. This dependence is found to be tightly correlated with the crystal field envi- ronment, as characterized by the local lattice distortion.
This work sheds light onto an alternative strategy to enhance the UC luminescence in this FTTB family.
Introduction
The upconversion (UC) luminescence is a typically non- linear photonic process in which one higher-energy photon can be generated via the absorption of two or more lower- energy photons [1]. Recent years, UC luminescence in rare-earth lanthanide (Ln)-doped solid-state materials has drawn considerable attention due to the UC luminescence particulars and application potentials in biomedical imag- ing, 3D-displays, phosphors, solar cells, sensors, and solid- state lasers, etc. [1–5]. However, it should be mentioned that these potentials are still far from constrained due to the low UC emission intensity whose enhancement seems to be the core issue nowadays in order to meet any practical requirements [6]. Along this line, substantial efforts have been made to enhance this intensity by various approaches.
One crucial scheme is to engineer the local crystal field environment for those luminescent Ln ions embedded in the host matrix of solid-state materials [7–11].
Indeed, a number of solid-state materials as host for embedding these luminescent centers have been attempted [1–12]. So far, evidences are available for the high sensi- tivity of the UC luminescence to the local crystal field environment [7–11]. For example, Chen et al. reported two orders of magnitude of enhancement of the UC green radiation in Li?-doped Y2O3:Er3?nanocrystals by tailoring the local environment of Er3?ions with Li?ions [7]. Ra- masamy et al. doped Fe3?ions in NaGdF4:Yb3?–Er3?and observed the 30- and 34-times increase in the UC red and green luminescences, respectively [8]. Zhao et al. obtained the 8- and 5-times enhancement of the 452 and 479 nm UC luminescences, respectively, in 7 mol% Li?-doped NaYF4:Yb3?–Tm3? nanocrystals [9]. In addition, the enhanced UC luminescence was also confirmed in Ca2?
ions-doped NaGdF4:Yb3?–Er3? [10]. At present, it is T. Wei (&)L. YeW. B. WangQ. Z. MaQ. Lv
College of Science, Civil Aviation University of China, Tianjin 300300, China
e-mail: [email protected] T. WeiJ. M. Liu
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
e-mail: [email protected] C. Z. Zhao
School of Electronics and Information Engineering, Tianjin Polytechnics University, Tianjin 300160, China
DOI 10.1007/s10853-014-8805-z
believed that the local crystal field environment does act as a critical factor to tune the intensity of UC luminescence [7–12].
The above-mentioned experiments suggest that host matrix of UC luminescence should possess high degrees of freedom for tuning in terms of unit cell volume, vacant size, and lattice distortion. A hypersensitive transition can be generated in these structures by tailoring the local lattice symmetry or distortion around Ln3?ions, so that the local crystal field can be modified [1, 7–12]. A lower local symmetry or larger local distortion is believed to favor the UC luminescence since the forbidden 4f–4f transitions of Ln3? ions can be released in this case, allowing more probabilities for the UC sequence [1, 7–12]. Earlier experiments demonstrated that the substitution of small cations, such as Li?, Fe3?, and Ca2?, into the host structure substantial does benefit to the structural distortion, enhancing the UC luminescence [7–10]. However, the underlying mechanism on one-to-one correspondences of the UC luminescence and local crystal fields has been rarely touched up to now [7–11]. For most cases, authors chose small dopants to maximize the lattice distortion of the host materials, and indeed significant consequence has been obtained [7–13]. However, as mentioned right now, no quantitative relationship between the lattice distortion parameters (bond angles and lengths, ionic occupations, and ionic spatial distribution) and the UC luminescence intensity has been given. In addition, a comprehensive knowledge on engineering the local lattice distortion and chemical occupation, so that the UC luminescence per- formance can be optimized, is highly appreciated for fun- damental researches.
To proceed, a proper choice of the host materials for embedding the Ln species is of the top priority. Recently, the (A2)4(A1)2(C)4(B1)2(B2)8O30-type filled tetragonal tungsten bronze (FTTB) oxides, where Ln3? activators fill the A-sites, have been investigated owing to their relatively low phonon energy and potential integration of electric and optic functionalities [14–20]. The local crystal field of these FTTB-type oxides can be easily modified by substituting the A- or B-site cations. Zhu et al. reported that the (B1/B2)O6 octahedral tilting around Ln3?-sites can be tailored by modulating the radius difference between the A1- and A2- site ions [20]. Moreover, this octahedral tilting may be realized by tailoring merely the A1-site ions, as reported by Levin et al. [14]. Li et al. reported the re-entrant relaxor behavior of FTTB-type Ba5LaTi3Nb7O30, attributed to the structural fluctuations [19]. Because of the abundant crys- tallographic sites and susceptive crystal field variance of the FTTB-type oxides, it is reasonably believed that effective tailoring of the local lattice distortion around Ln3?ions in these materials can be very promising, while substantial effort along this line is definitely needed [14–20].
In the present work, we choose BapLn6-pTi8-pNb2?pO30 (3 BpB5), a quaternary FTTB-type oxide, as the UC host matrix for Ho3? and Yb3?ions as the UC luminescence activators and sensitizers, respectively [1, 4]. For this oxide, both the A- and B- sites have two sub-sites, A1- and A2-sites, B1- and B2-sites, respectively. The A1-site has a 12-fold coordination, while the A2-site has a 15-fold coordination. The B1- and B2-sites both have 9-fold coor- dination, but the local bonding details are different. Earlier reports indicated that variations of all these sub-sites by chemical substitutions have important impact on the dielectric performances of the materials [14–21], reflecting the evident variations of the crystal field. For this reason, it is of interest to explore the dependence of the structural distortion and then the UC luminescence on the ionic occupation and distribution at these sites. For simplifica- tion, we define parameterpto measure these variations in a series of BapLn6-pTi8-pNb2?pO30 samples by fixing the Ho3?and Yb3?concentrations. The influences of the Ba2?, Ln3?, Ti4?, and Nb5? occupation and distribution at the A1-, A2-, B1-, and B2-sites on the UC luminescence behaviors will be carefully investigated. For convenience, we denote this series of samples by BLTNp:Ho–Yb.
Experimental details
The polycrystalline Bap(La0.85Ho0.025Yb0.125)6-pTi8-p
Nb2?pO30 ceramics, abbreviated as BLTNp:Ho–Yb, with different p were prepared by the solid-state processing using highly purified element oxides and carbonates [16, 17, 22]. Ho2O3 (99.99 %), Yb2O3 (99.99 %), and TiO2 (99 %) were supplied by Aladdin Industrial Corporation.
BaCO3 (99 %), La2O3 (99.99 %), and Nb2O5 (99.5 %) were supplied by Sinopharm Chemical Reagent Beijing Co., Ltd. All chemicals were mixed in stoichiometric ratios. After mixing by milling in alcohol for 24 h using agate pots and agate balls in a planetary mill, the as-pre- pared powders were dried and then calcined at 1300C for 5 h. The resultant powders were reground and pelletized under 10 MPa pressure into disks of 13 mm diameter and 1–2 mm thickness. Then, all disks were sintered at 1300C for 3 h. The obtained BLTNp:Ho–Yb ceramics were polished to 0.6 mm in thickness for the following UC luminescence and electrical measurements. Furthermore, Y1.88Ho0.02Yb0.1O3(Y2O3:Ho–Yb) ceramics were synthe- sized by co-precipitation followed by thermal treatment method. Y(NO3)3•6H2O (99.9 %), Ho(NO3)3•6H2O (99.99 %), and Yb(NO3)3•6H2O (99.99 %) were selected as starting materials, and ammonia solution (AR, 25–28 %) as precipitant. Appropriate amounts of Y(NO3)3•6H2O (99.9 %), Ho(NO3)3•6H2O (99.99 %), and Yb(NO3)3•6- H2O (99.99 %) were dissolved in deionized water under
condition of stirring. Then, ammonia solution was drop- wise added to the above mixture. Then, the precipitates were washed and dried at 100C for 12 h. The resultant powders were reground and pelletized under 10 MPa pressure into disks of 13 mm diameter and 1 mm thick- ness. Then, all disks were calcined at 600C for 2 h and sintered at 850C for 2 h. The obtained Y2O3:Ho–Yb ceramics were polished to 0.6 mm in thickness for the following UC luminescence measurements.
The crystal structure of BLTNp:Ho–Yb was identified by an powder X-ray diffractometer (XRD) (X’pert-MPD, Philips) using Cu Ka radiation, with working current and voltage of 40 mA and 40 kV, respectively. The XRD profiles were collected in the range 8B2h B120, with a step of 0.02. The general structure analysis system (GSAS) program was used for the Rietveld structural refinement of the XRD data [23,24]. Surface morphologies of the samples were characterized by scanning electron microscopy (Evo 18, Germany). For UC luminescence
measurement, a 980 nm power-controllable near-infrared (NIR) diode laser (Hi-Tech Optoelectronics Co., Ltd) with a maximum power of 2 W was used to pump the surface of the BLTNp:Ho–Yb ceramics. The UC luminescence spectra were collected under the same experimental con- ditions for all BLTNp:Ho–Yb ceramics by a Zolix SBP300 spectrofluorometer (SBP300, Zolix Instruments Co. Ltd) with a photo-multiplier tube (PMT) as the detector. The signals were recorded using the data acquisition system connected to a computer. To perform dielectric measure- ments, silver electrodes were deposited on the surfaces of BLTNp:Ho–Yb ceramics to construct parallel-plate capacitors. The temperature dependence of dielectric per- mittivity was measured using a dielectric spectrometer (TH2828S, Tonghui electronic Co. Ltd) connected to a tubular furnace (GSL-1100X) with a heating rate of 2 K/min.
Results and discussion
First, we present in Fig.1 the measured UC emission spectra for BLTNp:Ho–Yb with different A- and B-site occupation and distribution (p) at room temperature under the 980 nm near-infrared (NIR) laser excitation. For each case, three distinct UC emission peaks over the range of 400–800 nm can be detected. Referring to literature, it is easy to identify these peaks and their origins. The intense green UC emission peak centered at*550 nm arises from the intra-4f electronic transition 5S2?5I8 of Ho3? ions [16, 25,26]. The red emission peak located at *650 nm and the weak NIR peak located at *760 nm are respec- tively attributed to the5F5?5I8and5S2?5I7transitions of Ho3?ions [16,25,26]. It is noted that ultraviolet-blue UC emissions observed in hexagonal NaYF4:Ho–Yb powders do not appear here [27]. For present BLTNp:Ho–
Yb, the intense green UC luminescence can be easily observed by the naked eyes at room temperature. The inset Fig. 1 Measured UC emission spectra for BLTNp:Ho–Yb (p=3
(a), 3.5 (b), 4 (c), 4.15 (d), 4.35 (e), 4.5 (f), and 5(g)) at room temperature pumped by the 980 nm NIR laser. The inset of this figure presents the photograph of the photoluminescence for samplep=3 (Color figure online)
Fig. 3 UC luminescence spectra for Y2O3:Ho–Yb and BLTNp:Ho–
Yb (p=5) under 980 nm excitation (Color figure online)
(a) (b)
Fig. 2 Variation of the UC luminescence intensities, I (5S2?5I8 and5F5?5I8) (a), and I (5S2?5I7) (b), as a function ofp(Color figure online)
of Fig.1 presents the typical photograph of the photolu- minescence for sample p=3. Furthermore, as shown in Fig.1, it is observed that the positions of the UC emission peaks do not shift for all the samples, suggesting no
substantial change in the macro lattice structure with the variation of p, while the local lattice distortion is signifi- cant, as shown by the X-ray diffraction (XRD) data pre- sented below. In fact, one can see that the UC emission intensity displays remarkable dependence on parameter p. For clearly illustrating this dependence, Fig.2gives the UC intensities of the5S2?5I8,5F5?5I8, and5S2?5I7 emissions as a function ofp. The strongest UC emission is observed for samplep=5, while the weakest one appears in samplep=3.5. For details, the UC emission intensities of the green, red, and NIR peaks of samplep =5 are*10,
*25, and *10 times stronger than those of sample p =3.5, respectively. In addition, to evaluate the UC emission, the comparison of UC luminescence between Y2O3:Ho–Yb and BLTNp:Ho–Yb (p=5) is shown in Fig.3 which indicates the strong UC emissions of BLTNp:Ho–Yb. It should be mentioned that a careful calibration of the intensity data in our measurement is performed by repeated measurements at different surface regions on the BLTNp:Ho–Yb ceramics.
We also measure the UC luminescence intensities of the three emissions as a function of the pump laser power. It is known that for the unsaturated UC process, the number of photons required to populate the upper emitting state can be described by the following relation [7–13]:
I/ Pn; ð1Þ
(a)
(b)
(c)
Fig. 4 Dependence of the 5S2?5I8 (a), 5F5?5I8 (b), and
5S2?5I7 (c) UC emission intensities on pumping power (Color figure online)
Table 1 Slopes (nvalues) obtained from the linear fitting results of UC emission intensity and pump laser power for BLTNp:Ho–Yb
p 3 3.5 4 4.15 4.35 4.5 5
Slope (n)
5S2–5I8 2.003 (12) 2.344 (26) 1.930 (25) 1.889 (16) 1.885 (26) 1.820 (28) 1.854 (24)
5F5–5I8 1.980 (14) 1.979 (19) 1.938 (42) 1.877 (18) 1.867 (25) 1.805 (27) 1.856 (21)
5S2–5I7 2.024 (50) 2.205 (122) 2.143 (105) 1.992 (26) 1.942 (23) 1.8738 (22) 1.963 (20) Fig. 5 A schematic diagram of the Ho3?and Yb3?energy levels for the UC emission (Color figure online)
whereI is the output UC luminescence intensity,P is the power of pumping laser, and n is the absorbed photon numbers per UC emission photon which can be calculated from the slope of the log (I) versus log (P) fitting. Several representative power law plots for sample p=3 and samplep =5 are given in Fig.4. The well-defined linear relationships between log(I) versus log(P) for all the green, red, and NIR emissions can be observed. Table1 presents the fitting evaluated slopes (n values). These results
indicate that the two-photon process is responsible for the
5S2?5I8, 5F5?5I8, and 5S2?5I7 emissions [7–13].
Furthermore, as shown in Table1, the slopes (nvalues) for sample p=3.5 are 2.344(26), 1.979(19), and 2.205(122) for the 5S2?5I8, 5F5?5I8, and 5S2?5I7 emissions, respectively, the largest among all the samples. It is implied that the UC emission origins remain similar, while the relatively hard UC saturation process in samplep=3.5 may be suggested [13,28].
4 m (c)
(a) (b)
(d)
(e) (f)
(g)
4 m
4 m 4 m
4 m 4 m
4 m Fig. 6 Surface morphologies of
all BLTNp:Ho–Yb samples withp=3 (a), 3.5 (b), 4 (c), 4.15 (d), 4.35 (e), 4.5 (f), and 5 (g)
For an overview of the UC emission associated with Ho3?
and Yb3?, we plot in Fig.5the UC luminescence mechanism diagram. As shown in Fig.5, the Yb3?ions can be promoted to the2F5/2state via the ground-state absorption (GSA) of 980 nm laser photon. Then, two energy transfers, i.e., ET1 (Ho3?(5I8)?Yb3?(2F5/2)?Ho3?(5I6)?Yb3?(2F7/2)) and ET2 (Ho3?(5I6)?Yb3?(2F5/2)?Ho3?(5S2)?Yb3?(2F7/
2)) from Yb3?ions can promote the Ho3?ion to the5S2state.
The radiative relaxation of the5S2state of some Ho3?ions will produce the UC green (5S2?5I8) and NIR (5S2?5I7) emissions [16, 25,26]. In addition, with the help of non- radiative processes involving the multi-phonon-assisted relaxation (MAR), the 5F5 state can be populated and the subsequent radiative relaxation from the5F5state to the5I8 state will give the UC red emission (5F5?5I8). Alternatively, the Ho3?ions at the5I6state can non-radiatively relax to the5I7 state, and are further excited to the 5F5 state by ET3 (Ho3?(5I7)?Yb3?(2F5/2)?Ho3?(5F5)?Yb3?(2F7/2)) process which also has contributed to the UC red emission (5F5?5I8) [16,25,26].
To study the surface morphologies of all BLTNp:Ho–
Yb samples, we measured the SEM images of the ceramics with p =3, 3.5, 4, 4.15, 4.35, 4.5, and 5, as shown in Fig.6a–g, respectively. The different surface morphologies
are observed with the increase ofp. Pillar grain morphol- ogy is observed for p=3, 4.15, 4.35, 4.5, and 5. The length of grains is about several micrometers for these samples. For these samples, non-monotonic variation of grain sizes is detected withp. The grain sizes ofp =4.15 are larger than those of p =3. For p=4.35 and 4.5, similar grain sizes are observed which are both smaller than those of p =5. It is worthwhile to mention that the grain sizes show slight variation with the increase ofp(see Fig.6a, d–g). However, for p=3.5 and 4, abnormally large grains are observed as shown in Fig.6b, c which may be attributed to the favorable growth of these compositions at present sintering temperature condition. Generally, lar- ger grain size means good crystallization and will lead to high emission intensity [29]. However, it does not coincide with the obtained UC luminescence results.
Given the diagram shown in Fig.5 and the above dis- cussion, one can look at the physics underlying the great enhancement of the UC emission intensity. We focus on the local crystal field environment around Ln3? ions in response to the perturbations of the occupation and distri- bution of the A- and B-site cations, which can act as an effective path to improve the UC behavior of FTTB-based systems. First, we present the dielectric data of these
(a) (b)
(c) (d)
Fig. 7 Temperature dependence of real dielectric permittivity at a frequency of 1 MHz for all BLTNp:Ho–Yb samples (a). Rietveld structural refinement results for samples p=3 (b), 4 (c), and 5 (d). The open circle dots represent the measured XRD reflections and the solid lines are the Rietveld refined results. The dashed line shows the difference between the measured data and Rietveld refined data. The short vertical solid lines guide for eyes the corresponding Bragg positions (Color figure online)
Table 2 Refined structural parameters and reliability factors for BLTNp:Ho–Yb
p 3 3.5 4 4.15 4.35 4.5 5
a (A˚ ) 12.37787 (12) 12.40650 (7) 12.42652 (7) 12.43434 (9) 12.44638 (10) 12.45659 (9) 12.49336 (9) c (A˚ ) 3.91207 (6) 3.91633 (4) 3.92337 (4) 3.92821 (4) 3.93383 (5) 3.93855 (4) 3.95695 (4) Vunit(A˚3) 599.374 (14) 602.807 (9) 605.840 (9) 607.352 (11) 609.400 (11) 611.131 (10) 617.616 (10)
Rp(%) 5.03 5.43 5.35 5.00 4.77 4.75 4.94
Rwp(%) 6.60 7.25 7.13 6.71 6.41 6.41 6.89
samples as an additional evidence for the above discussion.
Figure7a shows the temperature (T) dependence of the real part of the dielectric permittivity at a frequency of 1 MHz.
A monotonous decrease of the dielectric permittivity with increasingT is observed, confirming that the samples are all of paraelectric nature, and their crystal structures are the centrosymmetric P4/mbmspace group at room temperature [14–20].
Subsequently, the lattice structures and distortions of these samples are quantitatively characterized using the
Rietveld structural refinement of the high-resolution XRD data [23, 24]. As several examples, the typical Rietveld refinement results for samples p =3, 4, and 5 are pre- sented in Fig.7b, c, and d, respectively. The XRD patterns confirm the P4/mbm space group, and no other impurity phase is observed. The Ho3?and Yb3?ions are believed to dissolve in the FTTB-type host matrix. Very small differ- ence between the measured and refined XRD patterns is shown for each case. The reliability of the Rietveld refinement is demonstrated by the high-quality refinement parameters (Rp and Rwp), as shown in Table2. Further- more, Fig.8 shows the 3D crystallographic structure dia- gram of BLTNp:Ho–Yb samples in which all the ionic sites have been labeled. It will be used as a facile reference for discussions on the local lattice structure.
The Rietveld refined data suggest that the high fitting reliability is obtained, given that the Ba2? and Ln3?ions occupy the A-sites (pentagonal and tetragonal interstices), while the B-sites (center of octahedra) are occupied by Ti4? and Nb5? ions, as schematically shown in Fig.9.
With increasing p, as seen in samples p =3 and 3.5, partial Ba2? ions start to fill the A2-sites (pentagonal interstices) in samples, as shown in Fig. 9a. For sample p =4, the A2-sites (pentagonal interstices) are fully occupied by Ba2? ions, as shown in Fig.9b, and finally the A1-sites (tetragonal interstices) are taken both by Ln3? and Ba2? ions for samplesp=4.15, 4.35, 4.5, and 5.0, as illustrated in Fig.9c [14–20]. Besides, the con- centration of Nb5? ions at the B1- and B2-sites is also enhanced with increasing p.
Ba Ln Ba/La La/Ba Ti/Nb O
(a) (b) (c)
a b
A2 A2 A2
A1 B1 A1 B1 A1 B1
B2 B2 B2
Fig. 9 A schematic diagram of ions sitting in the unit cell of the FTTB-type BLTNp:Ho–Yb. The A1-, A2-, B1-, and B2-sites are labeled. A1-sites are only occupied by Ln3?(La3?, Ho3?, and Yb3?) ions, and A2-sites are mixing filled by Ba2? and La3? ions for samples p=3 and 3.5 (a). A1- and A2-sites are, respectively,
occupied by Ba2?and Ln3?(La3?, Ho3?, and Yb3?) ions for sample p=4 (b). A1-sites are mixing filled by Ln3?(La3?, Ho3?, and Yb3?) and Ba2?, while A2-sites are only occupied by Ba2? for samples p=4.15, 4.35, 4.5, and 5 (c) (Color figure online)
O5
O5 O5
O5 O3
O3
O3 O3 O4
O4 O1
O2 O5O3 O5 O2
O3
O4 O4 O1
O1
O1 O4
O4
O4 O4 O2
O5
O2 O3 O5 O3
O4 O4
O4 O4 O4
O4 O4 O4
O5 O3 O3
O3
O3 O3 O3
O3 O3
O1 O1
O1
O1 O5
O5
O5 A1
A1 A1
A1
A1
A2
A2 A2
A2
B1 B1 B1
B1
B2 B2
B2 B2
B2
B2
B2 B2
a b c
Fig. 8 3D crystallographic structure diagram of BLTNp:Ho–Yb samples in which all the ionic sites have been labeled (Color figure online)
It is believed that the variation of the occupation and distribution of the A- and B-sites ions will inevitably influence the local crystal field environment around Ln3?
(La3?, Ho3?,and Yb3?) ions [7–13]. For this purpose, we list all the lattice parameters of these samples in Table2.
For sample p =3.0, the lattice constants are
a =12.37787(12) A˚ andc=3.91207(6) A˚ , and unit vol- ume is V=599.374(14) A˚3. For sample p=5, these parameters area =12.49336(9) A˚ ,c=3.95695(4) A˚ , and V=617.616(10) A˚3. Obviously, the lattice parameters (a, c, andV) increase continuously with increasing p. This is qualitatively understandable because the effective ionic
3.0 3.5 4.0 4.5 5.0
3.01 3.02 3.03 3.04
3.0 3.5 4.0 4.5 5.0
3.89 3.90 3.91 3.92
3.0 3.5 4.0 4.5 5.0
3.44 3.45 3.46 3.47
3.0 3.5 4.0 4.5 5.0
3.58 3.60 3.62 3.64
3.0 3.5 4.0 4.5 5.0
1.84 1.86 1.88 1.90
Ba1-O1
Bond Length
p
(a)
(c)
Ba1-(Ti/N b)2
Bond Length
p
(b )
(La/Ho/Y b)1-(Ti/N b)2
Bond Length
p
(e)
(Ti/N b)1-Ba1
Bond Length
p
(d )
Bond Length
p
(Ti/N b)2-O4
a b
c Ba1-O1
(La/Ho/Yb)1- (Ti/Nb)2
(f)
a b c Ba1-(Ti/Nb)2
(Ti/Nb)2-O4 (Ti/Nb)1-Ba1
(g)
Fig. 10 The change of Ba1–O1 (a), (La/Ho/Yb)1–(Ti/Nb)2 (b), Ba1–
(Ti/Nb)2 (c), (Ti/Nb)2–O4 (d), and (Ti/Nb)1–Ba1 (e) bond lengths as a function ofp. Chemical bonds of Ba1–O1 (f), (La/Ho/Yb)1–(Ti/
Nb)2 (f), Ba1–(Ti/Nb)2 (g), (Ti/Nb)2–O4 (g), and (Ti/Nb)1–Ba1 (g) are schematically labeled in the diagram of the FTTB-type BLTNp:Ho–Yb (Color figure online)
radii of Ba2? (1.420 A˚ , CN8) and Nb5? (0.640 A˚ , CN6) are bigger than those of La3? (1.160 A˚ , CN8) and Ti4?
(0.605 A˚ , CN6) [30]. It should be noted that the expansion of unit cell is not the origin for the UC luminescence enhancement since larger distance between Ho3?and Yb3?
ions will degrade the efficiency of ET processes from Yb3?
to Ho3?.
We then consult the lattice distortions of these samples in response to varyingp. A set of selected chemical bond lengths and bond angles as a function ofpare presented in Fig.10and Fig.11. The chemical bond lengths and bond angles display obvious variation with p, suggesting remarkable lattice distortion. For example, the lengths of Ba1–O1, Ba1–(Ti/Nb)2, (Ti/Nb)2–O4, and (Ti/Nb)1–Ba1
3.0 3.5 4.0 4.5 5.0
93 94 95 96
3.0 3.5 4.0 4.5 5.0
125 126 127 128 129
3.0 3.5 4.0 4.5 5.0
90 91 92 93
3.0 3.5 4.0 4.5 5.0
146 148 150 152
(b) Ba1-O4-(Ti/Nb)1
Bond Angle
p
(c) Ba1-(Ti/Nb)1-O4
Bond Angle
p
(Ti/Nb)2-O5-(La/Ho/Yb)1
Bond Angle
p
(a)
(d) (Ti/Nb)1-O4-(Ti/Nb)2
Bond Angle
p
a b
c (Ti/Nb)2-O5-(La/Ho/Yb)1
Ba1-O4-(Ti/Nb)1
(e)
a b (f) c
Ba1-(Ti/Nb)1-O4 (Ti/Nb)1-O4-(Ti/Nb)2
Fig. 11 The change of (Ti/Nb)2–O5–(La/Ho/Yb)1 (a), Ba1–O4–(Ti/
Nb)1 (b), Ba1–(Ti/Nb)1–O4 (c), and (Ti/Nb)1–O4–(Ti/Nb)2 (d) bond angles as a function ofp. (Ti/Nb)2–O5–(La/Ho/Yb)1 (e), Ba1–O4–
(Ti/Nb)1 (e), Ba1–(Ti/Nb)1–O4 (f), and (Ti/Nb)1–O4–(Ti/Nb)2 (f) bond angles are schematically marked in the diagram of the FTTB-type BLTNp:Ho–Yb (Color figure online)
bonds show the minimum values (3.0103, 3.8866, 1.837, and 3.5911 A˚ , respectively) at p =3.5. The dramatic changes of these bond lengths occur fromp=3.5 to 5.0. In addition, as shown in Fig.10b, sharp variation of the (La/Ho/Yb)1–(Ti/Nb)2 bond length appears fromp=4.0 to 5.0, although slight change is observed too fromp=3.0 to 4.0. For clarity, the Ba1–O1, (La/Ho/Yb)1–(Ti/Nb)2, Ba1–(Ti/Nb)2, (Ti/Nb)2–O4, and (Ti/Nb)1–Ba1 chemical bonds have been schematically marked in Fig.10f, g.
On the other hand, with increasingp, the (Ti/Nb)2–O5–
(La/Ho/Yb)1, Ba1–O4–(Ti/Nb)1, Ba1–(Ti/Nb)1–O4, and (Ti/Nb)1–O4–(Ti/Nb)2 chemical bond angles also display conspicuous variation, as shown in Fig.11, where these bond angles are marked. The (Ti/Nb)2–O5–(La/Ho/Yb)1, Ba1–O4–(Ti/Nb)1, and Ba1–(Ti/Nb)1–O4 bond angles present similar variation tendency withp. It is interestingly found that the minimum bond angle value (90.1, 93.34, and 125.63, for (Ti/Nb)2–O5–(La/Ho/Yb)1, Ba1–O4–(Ti/
Nb)1, and Ba1–(Ti/Nb)1–O4, respectively) appears again in samplep=3.5. A remarkable increase of the (Ti/Nb)2–
O5–(La/Ho/Yb)1, Ba1–O4–(Ti/Nb)1, and Ba1–(Ti/Nb)1–
O4 bond angles is obtained fromp =3.5 to 5.0, suggesting the enhanced lattice distortion. For (Ti/Nb)1–O4–(Ti/Nb)2 bond angle, the maximum value (152) appears atp =3.5.
Now, one may correlate the dependence of these lattice parameters on quantitypwith that of the UC luminescence intensity onp. It is clearly seen that they are quite sensitive topand the one-to-one correspondence between them can be established from the data in Fig.10 and 11. As well known, the UC luminescence intensity is mainly dependent of the intra-4f electronic transition probabilities of Ln3?
ions [1]. These electronic transitions are parity-forbidden according to the selection rule [1, 31, 32]. Nevertheless, this prohibition can be released if the 4f states inter-mix with higher electronic configurations, given local crystal field in the host matrix [1, 31, 32]. The variation of the local crystal field can lead to a variation of the electronic distribution density of Ln3?ions and may modulate the UC luminescence intensity [33,34]. The highly enhanced UC luminescence intensity for p =5 should be attributed to the serious lattice distortion resulting from the modulated A- and B-sites occupation and distribution [31–34].
Conclusions
FTTB structure of BLTNp:Ho–Yb ceramics with different phas been synthesized by a solid-state processing. Under the 980 nm NIR excitation, bright UC green emission, red emission, and NIR emissions, originating from the two- photon energy-transfer process associated with the
5S2?5I8, 5F5?5I8, and 5S2?5I7 transitions of Ho3?
ions, respectively, were observed. By tuning A- and B-sites
ions occupation and distribution, one order enhancement of the UC luminescence intensity has been confirmed upon a change of p from 3.5 to 5.0. To get the reason for this enhancement, we study the local lattice environment around Ln3? ions. It is interestingly found that the UC luminescence tightly correlates with the crystal lattice distortion based on our Rietveld structural refinement analyses. This work provides a new understanding of UC luminescence from the local lattice aspect.
Acknowledgements The authors gratefully acknowledge financial support from Natural Science Foundation of China (Nos. 51102277, 11234005), and the Tianjin Research Program of Application Foun- dation and Advanced Technology (No. 14JCQNJC03700).
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