Correlation between upconversion photoluminescence and dielectric response in Ba-substituted (Sr
12xBa
x)
4(La
0.85Ho
0.025Yb
0.125)
2Ti
4Nb
6O
30T. Wei,1,2,a)X. D. Wang,1C. Z. Zhao,3M. F. Liu,2and J. M. Liu2,b)
1College of Science, Civil Aviation University of China, Tianjin 300300, China
2Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
3School of Electronics and Information Engineering, Tianjin Polytechnic University, Tianjin 300160, China
(Received 14 December 2013; accepted 23 June 2014; published online 1 July 2014)
The filled tetragonal tungsten bronze (Sr1xBax)4(La0.85Ho0.025Yb0.125)2Ti4Nb6O30 (SBLTNx:
Ho-Yb) ceramics with different Ba substitution levels (x) are prepared. The upconversion photoluminescence (UC-PL) and dielectric permittivity are investigated. The substitution of Sr2þ ions at the A2-sites by larger Ba2þions results in substantial variation of the UC-PL intensity as a function of substitution levelx. Furthermore, the dielectric response to the substitution of Sr2þby Ba2þ suggests a close correlation between the UC-PL intensity and dielectric permittivity. The origin for this correlation is discussed based on the random stress field (RSF) model.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4887001]
Filled tetragonal tungsten bronze (TTB) oxides with a general chemical formula (A2)4(A1)2(C)4(B1)2(B2)8O30, in which the A1-, A2-, B1-, and B2-sites are filled while the C-sites are unoccupied, have attracted considerable attention owing to the particular dielectric and ferroelectric proper- ties.1–5 The filled TTB structure consists of layers with a complex array of (B1/B2)O6 octahedra sharing corners in such a way that three different types of interstices (square A1, pentagonal A2, and trigonal C) are available, as sche- matically shown in Fig.1.1–4This big class of oxides have found a lot of application potentials in electronic storages, microwave devices, super-capacitors, and so on.
Along this line, several filled TTB materials with rare earth (Re) ions at the A1-sites as activators may be explored for the upconversion photoluminescence (UC-PL) effect.6It is believed that the inner shell electronic transitions of Re ions depend substantially on the crystal structure symmetry surrounding the Re ions.7Thus, the UC-PL efficiency should be closely related to the interaction between the Re ions and crystal field environment.8It is noted that the A2-sites ionic occupation and distribution have major influence on the local lattice distortion (LLD),1–5thus enabling the dependence of the UC-PL performance on the ionic occupation and distri- bution. Tailoring the local lattice structure around the Re ions at the A2-sites seems to be an appropriate strategy to modify the UC-PL performance.1–5,7
Unfortunately, so far our knowledge on the correlation between the A2-sites occupation/distribution and the UC-PL performance has been very limited. Furthermore, as one kind of novel microwave materials,9the dielectric behavior of the filled TTB oxides and its dependence on the local lattice distortion have been extensively investigated, including the effects of the A2-site and A1-site cations occupation and dis- tribution, (B1/B2)O6 octahedral commensurate and incom- mensurate tilting, and polar nano-clusters.1–5,10,11Keeping in
mind the fact that the dielectric permittivity may be a rela- tively sensitive prober of the local lattice structure, one is able to predict the probably remarkable dependence of the UC-PL performance on the dielectric behavior. In addition, an understanding of the correspondence between the UC-PL properties and dielectric behavior would be highly beneficial to materials design and synthesis for future integrated optoe- lectronic devices.12
Herein, we may choose the Sr4La2Ti4Nb6O30with rela- tively low phonon energy as the host because it has been regarded as a model system for investigating the structural, dielectric, and ferroelectric properties of filled TTB com- pounds.1–5,10,11,13 Moreover, Ho3þand Yb3þions are taken as the UC-PL activator and sensitizer, respectively, owing to the availability of cheap pump sources for Yb3þ excita- tion.7,14In this work, we pay attention to the influence of the A2-site ionic occupation and distribution on the UC-PL spec- troscopic and dielectric behavior by addressing the Ba2þ substituted Sr4La2Ti4Nb6O30ceramics with fixed Ho3þ and Yb3þconcentrations.
The polycrystalline Ba-substituted (Sr1xBax)4 (La0.85Ho0.025Yb0.125)2Ti4Nb6O30 samples, abbreviated as
FIG. 1. A schematic diagram of the filled TTB structure. The A1-, A2-, B1-, B2-, and C-sites are labeled.
a)Author to whom correspondence should be addressed. Electronic mail:
b)E-mail: [email protected]
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SBLTNx: Ho-Yb, with different substitution levels (x) were prepared by the solid state processing using highly purified element oxides and carbonates. Details of the sample prepa- ration can be found elsewhere.3,4,15 The crystal structure of SBLTNx: Ho-Yb samples was checked using the X-ray diffraction (XRD) technique (X’pert-MPD, Philips) with the Cu Ka radiation. The operating current and voltage are 40 mA and 40 kV, respectively. The general structure analy- sis system (GSAS) program was used for the Rietveld structural refinement of the XRD data.16 For the UC-PL measurements, a 980 nm power-controllable infrared (IR) diode laser (Hi-Tech Optoelectronics Co., Ltd.) with a maxi- mum power of 2 W was used to pump the samples. The emit- ted UC fluorescence was collected by a Zolix SBP300 spectrofluorometer (SBP300, Zolix Instruments Co., Ltd.) with multiplier phototubes as detectors. The resolution of detectors is within61 nm. The silver electrodes were depos- ited on the two surfaces of the disk-like samples in order to perform electrical measurements. The dielectric characteris- tics were probed using the dielectric spectrometer (TH2828S, Tonghui electronic Co., Ltd.). All the optical and electrical measurements were carried out at room temperature.
Fig.2(a)presents the measured XRD patterns of several representative samples and the diffraction intensity is scaled logarithmically for a clear identification of those weak reflec- tions if any. All the XRD patterns can be well indexed by the TTB structure in space groupP4/mbm.2–4,11No other impu- rity phase is detected within the apparatus resolution limit.
To show the effect of Ba-substitution and to provide more evidence with the single-phase structure and good crystallin- ity, we re-plot the XRD patterns ranging from 2h¼23 to 37in Fig.2(b). It is observed that the diffraction peaks shift gradually to the low angle side with increasingx. For exam- ple, the main diffraction peak (311) for samplex¼0 is at 32.64, while it shifts to 32.34 for sample x¼0.6. This shifting effect is qualitatively understandable because the Ba2þ ion is larger than Sr2þ ion, resulting in the lattice expansion. However, to this stage, one needs to check whether the Ba2þsubstitution of the Sr2þsites does occur.
Subsequently, we consult to the Rietveld structural refinement of the XRD data. Here, we take the refinement for samplex¼0.6 as an example and the results are shown in Fig. 2(c). The reliability factors of Rp and Rwp for the Rietveld refinement are 4.61% and 6.34%, respectively. The Rietveld refined data suggest that high fitting reliability is obtained given that the Ba2þand Sr2þions fully occupy the A2-sites (pentagonal interstices) in the TTB structure, while the A1-sites (tetragonal interstices) are occupied by La3þ, Yb3þ, and Ho3þions in random order. For other samples, the Rietveld refinements give the same conclusion. In fact, simi- lar results were also reported by Chenet al.in (BaxSr1x)4
Nd2Ti4Nb6O30ceramics.11
For pure TTB-type Sr4La2Ti4Nb6O30, the A2-sites are fully and orderly filled by single Sr2þ ions. However, for SBLTN x: Ho-Yb, the Ba2þ substitution and the random occupation at the A2-sites will inevitably generate disorder effect. On the other hand, referring to the effective ionic radii of Ba2þ(1.61 A˚ , CN12) and Sr2þ(1.44 A˚ , CN12) ions,17one believes that the LLD will be considerable owing to the rela- tively large size mismatch at the A2-sites (Ba2þ and Sr2þ).
For example, as generated by the Rietveld fitting, the Ti(1)- O(1) bond length is 1.9343(8) A˚ for sample x¼0 and 1.9662(20) A˚ for sample x¼0.6, indicating the remarkable local lattice expansion. In addition, the O(1)-Ti(1)-O(1) bond angle changes from 174.3(10)0 for sample x¼0 to 166.0(10)0for samplex¼0.6, suggesting again the remark- able LLD, induced by the Ba2þsubstitution at the Sr2þsites.
Now we look at the UC-PL spectra under the 980 nm IR excitation, and the data for several samples are shown in Fig.3. For samplex¼0, three distinct emission bands aris- ing from the intra f-f transition of Ho3þ, located, respec- tively, around 548 nm (5S2!5I8), 660 nm (5F5!5I8), and 758 nm (5S2! 5I7), can be observed.18,19 The intense UC green luminescence can be easily identified by the naked eyes at room temperature, as shown in the inset of Fig. 3.
Similar results were reported in literature. For example, Ramasamy et al.reported that the breaking of local crystal field symmetry in Fe3þ doped b-NaGdF4: Er-Yb materials
FIG. 2. (a) Measured XRD patterns for a series of SBLTNx: Ho-Yb samples, x¼0.0 (A), 0.3 (B), 0.6 (C), 0.7 (D), 0.9 (E), and 1.0 (F). Bragg positions of the Sr4La2Ti4Nb6O30are labeled (S). (b) The local XRD patterns ranging from 23to 37. (c) XRD pattern of samplex¼0.6 (open circle dot, meas- ured) and evaluated pattern (solid line, Rietveld refined) with reliability fac- tors Rp¼4.61% and Rwp¼6.34%. The dashed line shows the difference between the measured data and Rietveld refined data. The short vertical solid lines are the Bragg positions.
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can greatly enhance the UC-PL intensity.20 Chen et al.
observed such UC luminescence in Liþ-doped Y2O3: Er-Yb system.21 For the present SBLTNx: Ho-Yb samples, the LLD induced by the larger Ba2þ ions at the A2-sites may reduce the local crystal field symmetry too, leading to an enhancement of the UC-PL intensity (I) according to the Laporte selection rules. To demonstrate this substitution induced effect, the measured UC-PL spectra for samples x¼0.3, 0.7, 0.9, and 1.0, respectively, are shown in Fig.3. It can be seen that the positions of the UC-PL emission bands are not altered by the incorporation of Ba2þ. However, the intensity of UC-PL emission bands displays evident variation with increasingx. For a clearer illustration of this effect, we re-plot the UC-PL intensities, I (5S2! 5I8), I (5F5 !5I8), and I (5S2!5I7), as a function ofx, respectively, in Fig.4. It is surprisingly found that these intensities first decay down a minimal and then rebounce upward. In details, these inten- sities decay with increasingxup to0.7, reaching the mini- mal, and then they are enhanced rapidly at x>0.7. These behaviors obviously do not follow the above prediction and observations, i.e., the UC-PL intensity may be enhanced due to the breaking of local crystal field symmetry,3,4,18,19 although we do not have sufficient evidence to support that the Ba2þsubstitution breaks the local crystal field symmetry or not. Nevertheless, the data show that the Ba2þsubstitution does have distinct impact on the UC-PL intensity and the
highest UC-PL intensities are obtained atx¼0.0 and 1.0. It is noted that for the two cases, the A2-sites (pentagonal inter- stices) are orderly occupied by Sr2þor Ba2þcations, respec- tively. In contrast, for the other cases the A2-sites are randomly occupied by Sr2þand Ba2þ. These results seem to suggest that the occupation disordering at the A2-sites does not benefit to the UC-PL intensity enhancement, and the minimal of the UC-PL intensity appears atx0.7.
In prior to discuss the effect of the Ba2þsubstitution on the UC-PL intensity, we first come to the UC-PL mecha- nism. Fig. 5(a) illustrates the dependence of the UC-PL intensity (I) as a function of the excitation power (P).
Well defined linear relationships between Log(I) versus Log(P) can be observed. The fitted linear slopes for the I (5S2!5I8), I (5F5!5I8), and I (5S2!5I7) transitions are 1.9060.02, 1.9660.03, and 1.7260.02, respectively, sug- gesting the two-photon process in the UC-PL excitation for the present SBLTN x: Ho-Yb.7,8,18,19 Moreover, Fig. 5(b) gives a schematic diagram of the population and UC-PL processes. First, the Yb3þabsorbs the excitation energy (hv) by the ground state absorption (GSA) (Yb3þ(2F7/2)þhv! Yb3þ(2F5/2)) and transfers it to Ho3þion by energy transfer (ET) (Ho3þ(5I8)þYb3þ(2F5/2) ! Ho3þ(5I6)þYb3þ(2F7/2)).
Then, some Ho3þ ions in the 5I6 state may relax non- radiatively to the5I7level by multiphonon assisted relaxation (MAR). Second, Ho3þ(5S2) and Ho3þ (5F5) could be popu- lated through the ET (Ho3þ(5I6)þYb3þ(2F5/2)!Ho3þ(5S2) þYb3þ(2F7/2), Ho3þ(5I7)þYb3þ(2F5/2)!Ho3þ(5F5) þYb3þ(2F7/2)), and then relaxes radiatively to the5I8and5I7 to produce green and red emissions.7,8,18,19
FIG. 3. Measured UC-PL spectra of the SBLTNx: Ho-Yb sample,x¼0.0 (a), 0.3 (b), 0.7 (c), 0.9 (d), and 1.0 (e), pumped by the 980 nm IR excitation.
The inset shows the typical photograph of the photoluminescence for sample x¼0.
FIG. 4. The measured variation of the UC-PL intensities, i.e., I (5S2!5I8) (a), I (5F5!5I8) (b), and I (5S2!5I7) (c), as a function ofx, respectively.
FIG. 5. (a) The UC-PL intensities I (5S2!5I8), I (5F5!5I8), and I (5S2!5I7), as a function of powerP. The solid lines give the linear fitting results. (b) A schematic diagram of the Ho3þand Yb3þenergy levels for the UC emission.
Finally, we look at the effect of the Ba2þsubstitution on the dielectric permittivity. We measure the dielectric fre- quency spectrum of these samples. Fig. 6 presents the dependences of the room temperature dielectric permittivity (er) on x at three frequencies of the ac electric field (f¼10 kHz (a), 100 kHz (b), 1.0 MHz (c)). It is seen that the er decreases slightly with increasing f for all the samples, owing to the well known frequency dispersion mechanism (suppression of active polarizations).22 More interestingly, we observe that for each frequency, theerfirst decreases and then increases with increasingx, as illustrated in Fig.6. The minimal appears atx0.6, roughly coinciding with the min- imal of the UC-PL intensity as a functionx. The dependence is also surprisingly similar to that of the UC-PL intensity I(x), suggesting the intimate correlation between the UC-PL and the dielectric permittivity. In the other words, the x-dependences of the two processes may have the similar structural origins.
For the SBLTNx: Ho-Yb, the Ba2þsubstitution of Sr2þ at the A2-sites results in the random ionic occupation. On the other hand, the relatively large size mismatch at the A2-sites (Ba2þand Sr2þ) induces the LLD. Both of them are believed to have significant influence on the UC-PL intensity and dielectric permittivity (er). The underlying mechanism can be understood in the random stress field (RSF) framework.
The LLD can tailor the local lattice structure around the Ho3þ and Yb3þions. As well known, the electronic transi- tion probabilities of rare earth (Re) ions are tightly related to the local crystal environment.18–21It is thus expected that the UC-PL intensity can be tuned by the RSF. First, the RSF can affect the spatial distribution of Ho3þand Yb3þat the A1-sites and induce the concentration quenching, thus reduc- ing the UC-PL intensity.20Forx¼0.0 and 1.0, owing to the A2-sites ordered alignment, no RSF exists and thus high UC- PL intensity is observed. For the partial substitution cases, however, the RSF is gradually enhanced with increasing x fromx¼0.0, and then suppressed whenxis close tox¼1.0.
The enhanced RSF is definitely unfavored for the UC-PL process. The lowest UC-PL intensity appearing atx0.7 fits the above scenario. Here, it should be mentioned that the RSF can also influence the tilting and rotation of the (Ti/Nb)O6 octahedra around the A1-sites (La3þ, Ho3þ, and Yb3þ). The ET processes between Ho3þand Yb3þions and the radiation parameters of Ho3þ and Yb3þ ions will be
affected. The details of these consequences need further study in the future.
Along the other line, it is not strange that the dielectric behavior depends on the A2-sites related RSF too. Different types of polarizations (i.e., electronic, ionic, dipolar, and space charge) contribute to the dielectric permittivity.23The response frequency for electronic and ionic polarizations is about 1016and 1013Hz, respectively. When the frequency is over 10 kHz, the contribution from space charge polarization can be negligible.23,24Given the frequency range covered by the present experiments (10 kHz–1.0 MHz), it seems that the dielectric permittivity is mainly contributed by dipolar polar- izations which will vibrate in response to theacelectric field.
The RSF exerts effect on the octahedral (Ti/Nb)O6 and suppresses the Ti/Nb dipolar ion orientation along the ac electric field, resulting in the reduction of the dielectric permittivity.25
In conclusion, the filled TTB structure SBLTNx: Ho-Yb ceramics have been synthesized by the solid state processing.
Given the 980 nm IR excitation, the bright UC green and weak red emissions, corresponding to the (5S2/5F5) ! 5I8 and5S2!5I7transitions of Ho3þ, are confirmed to be a two- photon ET process. More importantly, it has been revealed that the A2-site ionic distribution in the SBLTNx: Ho-Yb ceramics has significant influences on both the UC-PL inten- sity and dielectric permittivity. The RSF model is used to explain the responses of the UC-PL intensity and dielectric permittivity to the Ba2þsubstitution levelx.
This work was supported by the Natural Science Foundation of China (Nos. 51102277, 51332006 and 11234005) and the Tianjin Research Program of Application Foundation and Advanced Technology (No.
14JCQNJC03700).
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FIG. 6. The measured variation of the dielectric permittivityer, as a function ofxunder three different frequencies, 10 kHz (a), 100 kHz (b), 1.0 MHz (c).
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