- Email: [email protected]
Enhanced thermoelectric properties of Sr2Nb2O7 ceramics with barium substitution
Materials Letters 211 (2018) 270–272
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced thermoelectric properties of Sr2Nb2O7 ceramics with barium substitution Yi-Cheng Liou a,b,⇑, Wen-Chou Tsai a,b, Chih-Fu Kuo a, Hsiao-Chun Tsai a a b
Department of Electronic Engineering, Kun Shan University, Tainan City 71070, Taiwan, ROC Nano Technology R&D Center, Kun Shan University, Tainan City 71070, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 21 June 2017 Received in revised form 5 October 2017 Accepted 7 October 2017 Available online 9 October 2017 Keywords: Electroceramics Sintering Sr2Nb2O7 Thermoelectric Reaction–sintering
a b s t r a c t Properties of Sr1.9 xBa0.1LaxNb2O7 (SBNL; x = 0.1–0.3) thermoelectric ceramics were investigated in this study. SBNL ceramics were prepared by a reaction–sintering process with the calcinations and subsequent pulverization stages bypassed. Resistivity log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 Xcm at 700 °C were observed. Addition of Ba into Sr2Nb2O7 ceramics dramatically enhances the Seebeck coefficient S. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for Sr1.6Ba0.1La0.3Nb2O7. j values at 500 °C are 0.91–1.215 W m 1 K 1 for SBNL ceramics. Enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric ceramics had received much interest due to applications in power generation and cooling. Through the Seebeck effect, thermoelectric power generation directly converts thermal energy into electrical energy. Oxides with excellent thermal and oxidation resistance are generally advantageous for hightemperature operation in air. The figure of merit Z = S2r/j is used to estimate the performance of thermoelectric materials. S, r, and j are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Reducing j of a thermoelectric ceramic could result in a increased Z. Sparks et al. prepared 0.5 mol% Ladoped polycrystalline Sr2Nb2O7 and found low j 1.5–1.7 W m 1 K 1 at 100–1000 °C [1]. Sr2Nb2O7 (distrontium diniobate) ceramics are perovskite-like layered structure ferroelectrics [2,3]. Sr2Nb2O7 ceramics with high Curie point 1327 ± 5 °C are reported as good candidates for high-temperature piezoelectric applications [3,4]. Sr2Nb2O7 was also reported useful for applications in photocatalyst (for splitting water into H2 and O2) [5,6] and photoluminescence [7]. Recently we reported thermoelectric properties for La- and Ti-doped Sr2Nb2O7 ceramics [8,9]. Sr2Nb2O7 is a p-type thermo-
⇑ Corresponding author at: Department of Electronic Engineering, Kun Shan University, Tainan City 71070, Taiwan, ROC. E-mail address: [email protected] (Y.-C. Liou). https://doi.org/10.1016/j.matlet.2017.10.022 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
electric ceramic and S = 26.93 lV K 1 at 200 °C and S = 57.15 lV K 1 at 500 °C. Sr2 xLaxNb2O7 is an n-type thermoelectric ceramic. S = 30.73 lV K 1 at 200 °C and S = 60.75 lV K 1 at 500 °C for Sr1.9La0.1Nb2O7 [8]. Sr2Nb2 xTixO7 is a p-type thermoelectric ceramic. S = 25.57 lV K 1 at 200 °C for Sr2Nb1.97Ti0.03O7 and it increased to 50.21 lV K 1 at 500 °C [9]. In this study, Sr1.9Ba0.1Nb2 O7 n-type thermoelectric ceramics were prepared by a reaction–sintering process. Effects of barium substitution were investigated.
2. Experimental procedures Experimental procedures are similar to those described in our previous report about Sr2 xLaxNb2O7 [8]. Sr1.9 xBa0.1LaxNb2O7 (SBNL; x = 0.1, 0.2 and 0.3; denoted as SBNL10, SBNL20, and SBNL30) in this study were prepared from reagent–grade powders: SrCO3 (99%, Alfa Aesar, USA), BaCO3 (99%, SHOWA, Japan) Nb2O5 (99.8%, High Purity Chemicals, Japan), and La2O3 (99.99%, SHOWA, Japan). The SBNL pellets containing mixed raw powders were directly sintered at 1400 °C at a rate 10 °C/min in a covered alumina crucible in air. Microstructures were analyzed by scanning electron microscopy (SEM). The density of the sintered pellets was measured using the Archimedes method. j values were calculated using j = qCpD, where q is the density, Cp is the heat capacity and D is the thermal diffusivity. NETZSCH LFA 457 MicroFlash (Germany) was used.
Y.-C. Liou et al. / Materials Letters 211 (2018) 270–272
271
3. Results and discussion Well sintering SBNL10 (5.08 g/cm3) and SBNL20 (5.27 g/cm3) were obtained after 1400 °C/6 h sintering. It is noted that a low density of 3.11 g/cm3 was obtained for 1400 °C/6 h sintering SBNL30. In our previous study, 5.15 and 5.28 g/cm3 were observed for 1400 °C/6 h sintering Sr2 xLaxNb2O7 ceramics with x = 0 and 0.1 (SN and SNL10), respectively [8]. The reaction–sintering process is proven a simple and effective process to obtain dense SBNL ceramics. Reflections of XRD profiles of SBNL ceramics with x = 0.1–0.3 are shown in Fig. 1. Along with Sr2Nb2O7 in ICDD PDF # 01-0700114 (SN1), phases match with Sr2Nb2O7 in ICDD PDF # 00-0281246 (SN2) with the major peak at 2h 27° appeared and became the major phase. Orthorhombic SN2 belongs to A21am (36) space group with a = 5.701, b = 26.780, c = 3.955 Å, a = 90°, b = 90°, c = 90° and has a theoretical density 5.204 g/cm3. SEM photographs of as-fired SBNL ceramics sintered at 1400 °C/6 h were presented in Fig. 2. Grains >10 lm and >15 lm are seen in SBNL10 and SBNL 20 pellets, respectively. Smaller grains are found in SBNL30 pellets. This implies the grain growth was inhibited by a high La addition. Resistivity log q for SBNL ceramics sintered at 1400 °C/6 h are shown in Fig. 3. The resistivity of all the samples decreases with increasing temperature indicating a semiconducting characteristic. Log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 X cm at 700 °C were observed for SBNL ceramics. In our previous study, log q = 6. 48 X cm at 400 °C and log q = 4.68 X cm at 700 °C were observed for SN. Log q = 7.59 X cm at 400 °C and log q = 6.13 X cm at 700 °C were observed for SNL10 ceramics [8]. Log q of SNL ceramics increased as part of Sr was substituted by Ba. Gao et al. found the DC resistivity of Sr2 xBaxNb2O7 (x = 0.1–0.4) are 1011 X cm at 200 °C and decrease gradually with the same trend to 105 X cm at 900 °C [4]. The unit cell volume of Sr2 xBaxNb2O7 (x < 1) increases with Ba addition [4]. Oxygen ions become to migrate easily as the unit cell volume increased [10] and the activation energy decreases with increasing Ba substitution [4]. These result in increased holes concentration and conductivity. As Sr2 xBaxNb2O7 ceramics are p-type materials, more electrons contributed by La substitution compensated the holes contributed by oxygen vacancies for SBNL ceramics. It is reasonable that log q of SBNL10 is higher than log q of SNL10 in Fig. 3. S for SBNL ceramics sintered at 1400 °C/6 h is shown in Fig. 4. SBNL ceramics with negative S show n-type thermoelectric properties and the magnitude of S increased with temperature. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for SBNL30. In our previous study, S = 26.9 lV K 1 at 200 °C and S = 5 7.2 lV K 1 at 500 °C were observed for SN. The magnitude of S increased with La content. S = 30.7 lV K 1 at 200 °C and S = 6
Fig. 2. SEM photographs of as-fired (A) SBNL10, (B) SBNL20 and (C) SBNL30 ceramics sintered at 1400 °C/6 h.
Fig. 1. XRD profiles of 1400 °C sintering SBNL ceramics. SN1: ICDD PDF # 01-0700114, SN2: ICDD PDF # 00-028-1246.
0.8 lV K 1 at 500 °C for SNL10 ceramics [8]. Dramatically enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Substitution of Ba decreases the spontaneous polarization and decreases the electric field needed to switch the ferroelectric polarization [4]. We thought decreased spontaneous polarization may be one reason why S of SBNL is higher than S of SNL ceramics. j values measured at 500 °C are 1.141, 1.215 and 0.91 W m 1 K 1 for SBNL10, SBNL20 and SBNL30, respectively. These values are lower than j 1.7 W m 1 K 1 at 500 °C for 0.5 mol% La-doped polycrystalline Sr2Nb2O7 [1]. Usually a higher electrons concentration and a lower resistivity result in a higher j. As SBNL ceramics show a higher log q than
272
Y.-C. Liou et al. / Materials Letters 211 (2018) 270–272
SNL ceramics, it is reasonable for lower j values measured in SBNL ceramics. 0.91 W m 1 K 1 for SBNL30 may be caused by the low density of 3.11 g/cm3. Because j of pores are much lower than j of SBNL grains. We thought the thermoelectric properties could be improved even further if the amount of La is increased. However densification for these ceramics may be a problem to be overcome firstly. 4. Conclusions
Fig. 3. Resistivity for SBNL ceramics sintered at 1400 °C/6 h. Resistivity for Sr2Nb2 O7 (SN) and SNL10 (Sr1.9La0.1Nb2O7) in Ref. [8] are included for comparison.
Sr1.9 xBa0.1LaxNb2O7 thermoelectric ceramics were obtained by a reaction–sintering process with the calcinations and subsequent pulverization stages bypassed. Well sintering SBNL10 with 5.08 g/cm3 and SBNL20 with 5.27 g/cm3 were obtained after 1400 °C/6 h sintering. Log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 X c m at 700 °C were observed for SBNL ceramics. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for SBNL30. The maximum S = 182 lV K 1 appears at 472 °C for Sr1.6Ba0.1La0.3Nb2O7. Dramatically enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Acknowledgements This study was supported by the Ministry of Science and Technology of the Republic of China under contract MOST 103-2221-E168-030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Fig. 4. Seebeck coefficients for SBNL ceramics sintered at 1400 °C/6 h. S for Sr2Nb2O7 (SN) and SNL10 in Ref. [8] are included for comparison.
T.D. Sparks, P.A. Fuierer, D.R. Clarke, J. Am. Ceram. Soc. 93 (2010) 1136–1141. V.A. Isupov, Ferroelectrics 220 (1999) 79–103. H. Ning, H. Yan, M.J. Reece, J. Am. Ceram. Soc. 93 (2010) 1409–1413. Z. Gao, H. Ning, C. Chen, R. Wilson, B. Shi, H. Ye, H. Yan, M.J. Reece, J. Am. Ceram. Soc. 96 (2013) 1163–1170. H. Kato, A. Kudo, J. Photochem. Photobiol. A-Chem. 145 (2001) 129–133. H.G. Kim, D.W. Hwang, J. Kim, Y.G. Kim, J.S. Lee, Chem. Commun. (1999) 1077– 1078. J. Geng, Y. Chen, G. Gu, L. Tian, Opt. Mater. 36 (2014) 1093–1096. Y.C. Liou, W.C. Tsai, J.Y. Yu, J. Electron. Mater. 44 (2015) 4857–4863. Y.C. Liou, W.C. Tsai, J.Y. Yu, H.C. Tsai, Ceram. Int. 41 Part B (2015) 7036–7041. M. Cherry, M.S. Islam, C.R.A. Catlow, J. Solid State Chem. 118 (1995) 125–132.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced thermoelectric properties of Sr2Nb2O7 ceramics with barium substitution Yi-Cheng Liou a,b,⇑, Wen-Chou Tsai a,b, Chih-Fu Kuo a, Hsiao-Chun Tsai a a b
Department of Electronic Engineering, Kun Shan University, Tainan City 71070, Taiwan, ROC Nano Technology R&D Center, Kun Shan University, Tainan City 71070, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 21 June 2017 Received in revised form 5 October 2017 Accepted 7 October 2017 Available online 9 October 2017 Keywords: Electroceramics Sintering Sr2Nb2O7 Thermoelectric Reaction–sintering
a b s t r a c t Properties of Sr1.9 xBa0.1LaxNb2O7 (SBNL; x = 0.1–0.3) thermoelectric ceramics were investigated in this study. SBNL ceramics were prepared by a reaction–sintering process with the calcinations and subsequent pulverization stages bypassed. Resistivity log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 Xcm at 700 °C were observed. Addition of Ba into Sr2Nb2O7 ceramics dramatically enhances the Seebeck coefficient S. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for Sr1.6Ba0.1La0.3Nb2O7. j values at 500 °C are 0.91–1.215 W m 1 K 1 for SBNL ceramics. Enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric ceramics had received much interest due to applications in power generation and cooling. Through the Seebeck effect, thermoelectric power generation directly converts thermal energy into electrical energy. Oxides with excellent thermal and oxidation resistance are generally advantageous for hightemperature operation in air. The figure of merit Z = S2r/j is used to estimate the performance of thermoelectric materials. S, r, and j are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Reducing j of a thermoelectric ceramic could result in a increased Z. Sparks et al. prepared 0.5 mol% Ladoped polycrystalline Sr2Nb2O7 and found low j 1.5–1.7 W m 1 K 1 at 100–1000 °C [1]. Sr2Nb2O7 (distrontium diniobate) ceramics are perovskite-like layered structure ferroelectrics [2,3]. Sr2Nb2O7 ceramics with high Curie point 1327 ± 5 °C are reported as good candidates for high-temperature piezoelectric applications [3,4]. Sr2Nb2O7 was also reported useful for applications in photocatalyst (for splitting water into H2 and O2) [5,6] and photoluminescence [7]. Recently we reported thermoelectric properties for La- and Ti-doped Sr2Nb2O7 ceramics [8,9]. Sr2Nb2O7 is a p-type thermo-
⇑ Corresponding author at: Department of Electronic Engineering, Kun Shan University, Tainan City 71070, Taiwan, ROC. E-mail address: [email protected] (Y.-C. Liou). https://doi.org/10.1016/j.matlet.2017.10.022 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
electric ceramic and S = 26.93 lV K 1 at 200 °C and S = 57.15 lV K 1 at 500 °C. Sr2 xLaxNb2O7 is an n-type thermoelectric ceramic. S = 30.73 lV K 1 at 200 °C and S = 60.75 lV K 1 at 500 °C for Sr1.9La0.1Nb2O7 [8]. Sr2Nb2 xTixO7 is a p-type thermoelectric ceramic. S = 25.57 lV K 1 at 200 °C for Sr2Nb1.97Ti0.03O7 and it increased to 50.21 lV K 1 at 500 °C [9]. In this study, Sr1.9Ba0.1Nb2 O7 n-type thermoelectric ceramics were prepared by a reaction–sintering process. Effects of barium substitution were investigated.
2. Experimental procedures Experimental procedures are similar to those described in our previous report about Sr2 xLaxNb2O7 [8]. Sr1.9 xBa0.1LaxNb2O7 (SBNL; x = 0.1, 0.2 and 0.3; denoted as SBNL10, SBNL20, and SBNL30) in this study were prepared from reagent–grade powders: SrCO3 (99%, Alfa Aesar, USA), BaCO3 (99%, SHOWA, Japan) Nb2O5 (99.8%, High Purity Chemicals, Japan), and La2O3 (99.99%, SHOWA, Japan). The SBNL pellets containing mixed raw powders were directly sintered at 1400 °C at a rate 10 °C/min in a covered alumina crucible in air. Microstructures were analyzed by scanning electron microscopy (SEM). The density of the sintered pellets was measured using the Archimedes method. j values were calculated using j = qCpD, where q is the density, Cp is the heat capacity and D is the thermal diffusivity. NETZSCH LFA 457 MicroFlash (Germany) was used.
Y.-C. Liou et al. / Materials Letters 211 (2018) 270–272
271
3. Results and discussion Well sintering SBNL10 (5.08 g/cm3) and SBNL20 (5.27 g/cm3) were obtained after 1400 °C/6 h sintering. It is noted that a low density of 3.11 g/cm3 was obtained for 1400 °C/6 h sintering SBNL30. In our previous study, 5.15 and 5.28 g/cm3 were observed for 1400 °C/6 h sintering Sr2 xLaxNb2O7 ceramics with x = 0 and 0.1 (SN and SNL10), respectively [8]. The reaction–sintering process is proven a simple and effective process to obtain dense SBNL ceramics. Reflections of XRD profiles of SBNL ceramics with x = 0.1–0.3 are shown in Fig. 1. Along with Sr2Nb2O7 in ICDD PDF # 01-0700114 (SN1), phases match with Sr2Nb2O7 in ICDD PDF # 00-0281246 (SN2) with the major peak at 2h 27° appeared and became the major phase. Orthorhombic SN2 belongs to A21am (36) space group with a = 5.701, b = 26.780, c = 3.955 Å, a = 90°, b = 90°, c = 90° and has a theoretical density 5.204 g/cm3. SEM photographs of as-fired SBNL ceramics sintered at 1400 °C/6 h were presented in Fig. 2. Grains >10 lm and >15 lm are seen in SBNL10 and SBNL 20 pellets, respectively. Smaller grains are found in SBNL30 pellets. This implies the grain growth was inhibited by a high La addition. Resistivity log q for SBNL ceramics sintered at 1400 °C/6 h are shown in Fig. 3. The resistivity of all the samples decreases with increasing temperature indicating a semiconducting characteristic. Log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 X cm at 700 °C were observed for SBNL ceramics. In our previous study, log q = 6. 48 X cm at 400 °C and log q = 4.68 X cm at 700 °C were observed for SN. Log q = 7.59 X cm at 400 °C and log q = 6.13 X cm at 700 °C were observed for SNL10 ceramics [8]. Log q of SNL ceramics increased as part of Sr was substituted by Ba. Gao et al. found the DC resistivity of Sr2 xBaxNb2O7 (x = 0.1–0.4) are 1011 X cm at 200 °C and decrease gradually with the same trend to 105 X cm at 900 °C [4]. The unit cell volume of Sr2 xBaxNb2O7 (x < 1) increases with Ba addition [4]. Oxygen ions become to migrate easily as the unit cell volume increased [10] and the activation energy decreases with increasing Ba substitution [4]. These result in increased holes concentration and conductivity. As Sr2 xBaxNb2O7 ceramics are p-type materials, more electrons contributed by La substitution compensated the holes contributed by oxygen vacancies for SBNL ceramics. It is reasonable that log q of SBNL10 is higher than log q of SNL10 in Fig. 3. S for SBNL ceramics sintered at 1400 °C/6 h is shown in Fig. 4. SBNL ceramics with negative S show n-type thermoelectric properties and the magnitude of S increased with temperature. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for SBNL30. In our previous study, S = 26.9 lV K 1 at 200 °C and S = 5 7.2 lV K 1 at 500 °C were observed for SN. The magnitude of S increased with La content. S = 30.7 lV K 1 at 200 °C and S = 6
Fig. 2. SEM photographs of as-fired (A) SBNL10, (B) SBNL20 and (C) SBNL30 ceramics sintered at 1400 °C/6 h.
Fig. 1. XRD profiles of 1400 °C sintering SBNL ceramics. SN1: ICDD PDF # 01-0700114, SN2: ICDD PDF # 00-028-1246.
0.8 lV K 1 at 500 °C for SNL10 ceramics [8]. Dramatically enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Substitution of Ba decreases the spontaneous polarization and decreases the electric field needed to switch the ferroelectric polarization [4]. We thought decreased spontaneous polarization may be one reason why S of SBNL is higher than S of SNL ceramics. j values measured at 500 °C are 1.141, 1.215 and 0.91 W m 1 K 1 for SBNL10, SBNL20 and SBNL30, respectively. These values are lower than j 1.7 W m 1 K 1 at 500 °C for 0.5 mol% La-doped polycrystalline Sr2Nb2O7 [1]. Usually a higher electrons concentration and a lower resistivity result in a higher j. As SBNL ceramics show a higher log q than
272
Y.-C. Liou et al. / Materials Letters 211 (2018) 270–272
SNL ceramics, it is reasonable for lower j values measured in SBNL ceramics. 0.91 W m 1 K 1 for SBNL30 may be caused by the low density of 3.11 g/cm3. Because j of pores are much lower than j of SBNL grains. We thought the thermoelectric properties could be improved even further if the amount of La is increased. However densification for these ceramics may be a problem to be overcome firstly. 4. Conclusions
Fig. 3. Resistivity for SBNL ceramics sintered at 1400 °C/6 h. Resistivity for Sr2Nb2 O7 (SN) and SNL10 (Sr1.9La0.1Nb2O7) in Ref. [8] are included for comparison.
Sr1.9 xBa0.1LaxNb2O7 thermoelectric ceramics were obtained by a reaction–sintering process with the calcinations and subsequent pulverization stages bypassed. Well sintering SBNL10 with 5.08 g/cm3 and SBNL20 with 5.27 g/cm3 were obtained after 1400 °C/6 h sintering. Log q = 8.5–9.13 X cm at 450 °C and log q = 6.8–7.1 X c m at 700 °C were observed for SBNL ceramics. S = 66 to 58 lV K 1 at 200 °C and S = 182 to 80 lV K 1 at temperatures around 470 °C for SBNL ceramics. The maximum S = 182 lV K 1 appears at 472 °C for SBNL30. The maximum S = 182 lV K 1 appears at 472 °C for Sr1.6Ba0.1La0.3Nb2O7. Dramatically enhanced thermoelectric properties of Sr2Nb2O7 ceramics could be obtained with barium substitution. Acknowledgements This study was supported by the Ministry of Science and Technology of the Republic of China under contract MOST 103-2221-E168-030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Fig. 4. Seebeck coefficients for SBNL ceramics sintered at 1400 °C/6 h. S for Sr2Nb2O7 (SN) and SNL10 in Ref. [8] are included for comparison.
T.D. Sparks, P.A. Fuierer, D.R. Clarke, J. Am. Ceram. Soc. 93 (2010) 1136–1141. V.A. Isupov, Ferroelectrics 220 (1999) 79–103. H. Ning, H. Yan, M.J. Reece, J. Am. Ceram. Soc. 93 (2010) 1409–1413. Z. Gao, H. Ning, C. Chen, R. Wilson, B. Shi, H. Ye, H. Yan, M.J. Reece, J. Am. Ceram. Soc. 96 (2013) 1163–1170. H. Kato, A. Kudo, J. Photochem. Photobiol. A-Chem. 145 (2001) 129–133. H.G. Kim, D.W. Hwang, J. Kim, Y.G. Kim, J.S. Lee, Chem. Commun. (1999) 1077– 1078. J. Geng, Y. Chen, G. Gu, L. Tian, Opt. Mater. 36 (2014) 1093–1096. Y.C. Liou, W.C. Tsai, J.Y. Yu, J. Electron. Mater. 44 (2015) 4857–4863. Y.C. Liou, W.C. Tsai, J.Y. Yu, H.C. Tsai, Ceram. Int. 41 Part B (2015) 7036–7041. M. Cherry, M.S. Islam, C.R.A. Catlow, J. Solid State Chem. 118 (1995) 125–132.