English

• 0   Introduction

  In the early studies on the defects, more attentions have been paid on the defects in alkali halides, such as sodium vacancy V_Na^× and chlorine vacancy V_Cl^×[1]. Since these defects caused the colorless crystals colored, they were named as color centers (or F-centers). In fact, the defects are very important for material properties. As we know, the blue-greenish luminescence of ZnO originates from the zinc vacancy V_Zn″ and oxygen vacancy V_O^··[2] and the conductivity and dielectric loss of some titanate microwave ceramics are affected by the oxygen vacancy V_O^·· and trivalent titanium defect Ti_Ti′^[3]. In recent years, a novel material——solid electride attracted many research interests^[4]. A typical solid electride material is [Ca₁₂Al₁₄O₃₂]²⁺:[2e^-], which was prepared by heating its host material Ca₁₂Al₁₄O₃₃ in reductive atmospheres, such as CO/CO₂ and in which the [2e^-] plays a role as an electron anion. Actually the [2e^-] anion is another expression of the electron captured in the oxygen vacancy V_O^× (F-center), thus this electride material can be considered as the compound with high concentration of V_O^×. Since V_O^× can contribute high concentration of loosely-bonded electrons, after reduction, this material changes from an insulator to a conductor, consequently it can be potentially used as a transparent electrode, low-temperature electron emitter, high-density optical storage^[4-5]. These materials were also used as catalysts for the NH₃ syntheses and CO oxidation^[6-7].

  When previously studied the ternary phase diagram SrO-Y₂O₃-Al₂O₃ in our group^[8], we found that YAlO₃ powder prepared by high temperature reaction always showed light-brown body-color. As we know, YAlO₃ is composed of Y³⁺, Al³⁺ and O^2-, all of which have 8-electron full-shell configurations. Accordingly, this material should have a wide bandgap (E_g≈ 7.1 eV^[9]) and should not have absorption in visible range, so that it should be a colorless crystal or white powder. Therefore, it can be deduced the body-color of this material must arise from some defects. In this study, we attempt to know what defects cause the body-color in this material and how the body-color can be adjusted. Furthermore whether it is possibly to modify YAlO₃ as a material with high defect concentration, so that we can obtain another electride. On the other hand, YAlO₃ is a host material of a laser crystal. Some researches have been reported on growing single crystals of YAlO₃^[10-13] It was found that the grown crystals also had light-brown body-color, which caused the crystal quality detriment. When the crystals were annealed in reductive atmospheres, their body-color was reduced. However, in the previously reported work, the mechanism of the body-color production was not properly discussed. If we could understand the generating mechanism of the body-color and find ways to reduce it, we would improve the quality of the crystal.

  The YAlO₃ powder samples were prepared by solid state reactions at high temperature. X-ray diffraction indicated that besides a few percent of impurity phase Y₄A_l2O₉, the main phase in the samples was the expected YAlO₃, which showed light-brown body-color. The absorption spectra and conduction properties of the samples were characterized, and the formation energies of possible defects were calculated by the first-principle calculations. By analyzing the above data, it is indicated that the main defects in YAlO₃ are probably cation vacancies V_Al^×, which cause its light-brown body-color and the p-type conduction.

  1   Experimental and calculations

  1.1   Sample preparation

  Y₂O₃(99.99 %) and γ-Al₂O₃(luminescent pure) were used as raw materials. The raw materials in 1:1 molar ratio were weighed and thoroughly ground in an agate mortar with pestle. In order to get homogeneous powder mixtures, a few drops of ethanol were added during the grindings. After grindings, the powder mixture samples were put in alumina crucibles, and calcined at 1 450 ℃ for 12 h in a muffle furnace. Intermediate grindings were applied during the calcination for improving the phase purity of the samples. After calcination, the samples were ground into powders.

  For conductivity measurements, sintered pellet samples were prepared. The powder samples were mixed with a few drops of 5%(w/w) polyvinyl alcohol solution as a binder, and then they were pressed into pellets with 10 MPa pressure in a stainless steel die. Finally, the as-pressed pellets were sintered at 1 450 ℃ for 12 h. The obtained sintered pellets had the dimension ~1.3 cm in diameter and ~0.2 cm in thickness with relative density ~88%. To investigate the influences of atmospheres to the body-color and the profile of the reflection spectrum, the powder samples were annealed in a tube furnace with O₂, air and N₂ atmospheres, respectively, at 700 ℃ or 1 450 ℃ for 4 h.

  1.2   Characterizations

  The phase purity of samples were determined by X-ray diffraction (XRD) using a Rigaku Dmax2000 X-ray powder diffractometer (Japan) with Cu Kα radiation (λ=0.154 18 nm) at 40 kV and 100 mA. The XRD patterns were collected in the 2θ range 10°~60° with the scanning rate 8°·min^-1.

  The diffusive reflection spectra were measured by using an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer (Shimadzu UV-3100, Japan) in the range 240~1 000 nm and the BaSO4 powder was used as reference.

  The electrical properties of the samples were characterized by AC impedance spectroscopy using a Wayne Kerr-6500B impedance analyzer (Wayne kerr electronics Ltd, UK) in the frequency range 1 Hz to 10⁷ Hz and the temperature range 200~800 ℃ with the temperature interval 20 ℃/waiting time 20 min for each temperature point. Prior to the measurements, the Pt electrodes on the two opposite surfaces of the pellet samples were prepared by coating platinum paste with two Pt wires followed by heating at 800 ℃ for 10 min to burn off the organic components in the paste. The measured AC impedance data were analyzed using the Zview software^[14].

  1.3   First principle calculations

  To acquire the defect formation energy, the energies (E_L) of the ideal structure of YAlO₃ and the structures with various defects were calculated using CASTEP (Cambridge Serial Total Energy Package, Accelrys Inc.) in Materials Studio 5.0^[15]. The defect formation energy (E_V) is the energy difference (E_L) between the defective and the ideal structures. In these calculations, the density functional theory (DFT) with local-density approximation (LDA) was employed and the wave functions of valence electrons were expanded in an ultrasoft pseudopotential plane-wave basis sets with the specific cut-off energy E_cut. For the calculations, a 2×2×2 supercell (160 atoms) was chosen according to the ICSD file 99419. The initial supercell are a^*=2a=1.036 05 nm, b^*=2b=1.065 90 nm and c^*=2c=1.474 12 nm. Based on this supercell structure, the two defective structure modes were built by removing out one Y atom near the center in the supercell for V_Y^× and one Al atom for V_Al^×, respectively. Before the energy calculations, the geometry optimization for each mode was performed with Broyden-Fletcher-Goldfarb-Shannon (BFGS) alogrithm^[16]. Both E_cut and k-point mesh were tested and set as 260 eV and 3×3×2 Monkhorst-Pack grid, respectively, which are enough for energy convergence. The modes were converged when the force on each atom was less than 1 eV·nm^-1 with a maximum displacement of 0.000 5 nm and a convergence in the total energy of about 5×10^-5 eV·atom^-1.

  2   Results and discussion

  2.1   Phase analysis

  The prepared YAlO₃ powder samples had light-brown body-color. The XRD pattern of YAlO₃ is shown in Fig. 1. The pattern is in accordance with JCPDS file 33-41, except for two weak peaks appearing at 2θ=29.18° and 31.26°, which are from the impurity phase Y₄Al₂O₉(JCPDS file 14-0475). YAlO₃ has orthorhombic unit cell and the cell parameter calculated from the pattern in Figure 1 are as follows: a=0.518 8(1) nm, b=0.525 5(1) nm and c=0.738 5(1) nm. The material had a twisted perovskite structure with the space group Pbnm (No.62), which contains one Y site and one Al site (ICSD file 99419, shown in Fig. 2). In fact it was quite hard to obtain pure-phase YAlO₃ samples. Various preparation conditions were tested, such as modifying raw material ratio Y₂O₃:Al₂O₃, as well as enhancing the heating temperature and prolonging the heating time, the two weak XRD peaks of Y₄Al₂O₉ did not disappear. For comparison, a pure phase sample of Y₄Al₂O₉ was prepared by heating Y₂O₃ and Al₂O₃ mixture with the Y₂O₃:Al₂O₃ ratio 2:1 at 1 100 ℃ for 12 h. The XRD pattern of the prepared sample is in accordance with the JCPDS file 14-0475 and the body-color of the sample is white, thus we believe that the light-brown color of the prepared YAlO₃ sample is caused by YAlO₃ itself rather than Y₄Al₂O₉ and small amount of impurity phase Y₄Al₂O₉ may not seriously affect our results.

  Figure 1.  XRD patterns of as-prepared YAlO₃ (a) and JCPDS file 33-41 (b)

  图选项

  下载全尺寸图像

  下载幻灯片

  Figure 2.  YAlO₃ perovskite structure

  图选项

  下载全尺寸图像

  下载幻灯片

  2.2   Diffusive reflection spectra

  Fig. 3 illustrates the diffusive reflection spectra of the YAlO₃ samples annealed in various atmospheres at 700 ℃ or 1 450 ℃. All the samples have absorption in the visible range 400~800 nm. The absorption in the blue range (e.g.~450 nm) is stronger than that in the red range (e.g.~650 nm), as the result, the samples shows light-brown body-color. The samples annealed in air and O₂ have much stronger absorptions (showing deeper body-color) than the samples annealed in N₂(showing lighter body-color), these imply that the body color of YAlO₃ is caused by some defects, which can be adjusted by atmospheres. According to the correlation between the body-color and the atmospheres, it can be deduced that the defects should be anion excess defects, e.g. interstitial oxygen Oi or cation vacancies V_M^× (V_Y^× or V_Al^×). The defect equilibria can be represented as the following reactions:

  Figure 3.  Diffusive reflection spectra of the YAlO₃ samples annealed in air (a), O₂(b) and N₂(c) at 700 ℃ and annealed in N₂ at 1 450 ℃(d)

  图选项

  下载全尺寸图像

  下载幻灯片

  When the electron transitions occur from valence band to the defects O_i^× and V_M^×, they may be changed to O_i" and V_M′ and the electron holes h^· are produced in the valence band. V_M′ may be further ionized to V_M″ and V_M″′, and more h^. are produced. In these processes, visible lights are absorbed by the material and the body-color appears. It is known that the YAlO₃ perovskite structure is stacked by the closely packed YO₃ layers in the cubic close packing sequence and the Al atoms are located at the octahedral interstitial sites constructed solely by the O atoms. In this kind of the structure, probably there is no space to accommodate O_i, so that we believe the main defects in YAlO₃ should be cation vacancies.

  Further we have a discussion about which cation vacancy, V_Y^× or V_Al^× is the dominant defect. Fig. 4 schematically represents the formation of a cation vacancy. In the atmosphere with high O₂ partial pressure, O₂ can be decomposed into O atoms and absorbed on the surface of the material, e.g. bonded to the Y atoms (two a atoms). At the same time, a cation vacancy V forms on the surface. Due to the thermal diffusion, both the Y atom and Al atom can jump from their own sites to the surface vacancy V and the cation vacancy may diffuse into the lattice. Here we use the ionic potential Z/r to evaluate the attracting strength of a cation to its surrounding oxygen atoms (Z is the valence and r is the radius of the cation). The larger the Z/r ratio, the stronger the attracting strength is. As we know, r_Y³⁺=0.090 nm (CN=6), r_Al³⁺=0.053 5 nm (CN=6)^[17], as the results, the Z/r ratio for Y³⁺(3.33) is less than that for Al³⁺(5.61). This implies that Y³⁺ is easier to jump into the V site than Al³⁺, thus it seems that V_Y^× is the dominant defect. However, the defect formation energy E_V of V_Y^× and V_Al^×, calculated by the first-principle calculations, gives different result. E_L represents the optimized energies for different structural modes, including ideal supercell and the supercell with various cation defects. The defect formation energy E_V is the difference of E_L between the defective cell and the ideal cell. For the calculation, 2×2×2 supercell was used and the results are shown in Table 1. The data indicate that V_Al^× has much lower E_V than V_Y^×, thus V_Al^× should be the dominant defect. As we know, Y³⁺ has much larger size than Al³⁺, and the V_Y^× formation may cause larger lattice distortion and enhance the lattice energy, thus the first-principle calculation indicates that V_Al^× is the main defect in YAlO₃.

  Figure 4.  Schematic diagram for the defect formation

  图选项

  下载全尺寸图像

  下载幻灯片

  Table 1.  Optimized energies (E_L) of the unit cells and the defect formation energies (EV)

  表选项

  导出Excel

  下载全尺寸表图像

  	Ideal cell	VY×	VAl×
  EL/eV	-9 717	-8652	-9 646
  EV/eV		1 065	71

  In the higher O₂ partial pressure (e.g. in O₂ atmosphere), Equilibria (3) and (4) shift to right, so that the concentration of V_Al^× increases and the body-color become deeper; while in N₂ atmosphere, the O₂ partial pressure is low, Equilibria (3) and (4) shift to left, consequently the concentration of V_Al^× decreases and the body-color become lighter. Equilibria (3) and (4) also show during the formation of V_Al^×, gaseous O₂ is changed to solidus O^2-, which can be considered as an oxidezing reaction, thus probably it is an exothermal process (the enthalpy decreases, ΔH < 0) with the reduction of entropy (ΔS < 0). According to the Gibbs free energy equation ΔG=ΔH-TΔS, in order to reduce the concentration of V_Al^× in YAlO₃, i.e. to push Equilibrium (3) and (4) shift to left, besides annealing in N₂ atmo-sphere, enhancing the annealing temperature is also helpful. Therefore the sample annealed in N₂ at 1 450 ℃ has weaker absorption (Fig. 3(d)) than other samples.

  In order to obtain energy levels of the defects in YAlO₃, we further analyzed the reflection spectrum of the sample annealed in O₂ at 700 ℃, which has higher defect concentration. The spectrum is represented as the Absorption-hν form (Fig. 5), (h is Planck constant, and ν is the frequency of light). In the range 1.0 eV to 5.0 eV, three absorption peaks are observed, which correspond to three levels of the defects. The values of the energy levels were taken from the lower absorption edges of the corresponding absorption bands: 1.4, 1.6 and 2.6 eV. Yet at this stage we can not give exact assignments to these three levels, maybe they are related to the electron transitions from valence band to different ionized states of the Al vacancies: V_Al^×, V_Al′ and V_Al″. The spectrum also shows the absorption above 5 eV, but we could not give further interpretation about this absorption because it is near the limit of the measurement. The Ref.[9] indicates the bandgap of YAlO₃ is about 7.1 eV, thus this absorption is not related to the band transition. Based on the above discussions, the band structure with the defect levels is illustrated in Fig. 6.

  Figure 5.  Reflection spectrum in the Absorption-hν form for the YAlO₃ sample annealed in O₂ atmosphere at 700 ℃

  图选项

  下载全尺寸图像

  下载幻灯片

  Figure 6.  Band structure with defect levels of YAlO₃ Inset is the enlarged view for the high frequency data

  图选项

  下载全尺寸图像

  下载幻灯片

  Fig. 6 shows that the bandgap of YAlO₃ is large, E_g=7.1 eV^[9], thus the band transition may cause absorption in the vacuum ultraviolet (VUV) range, rather than in the visible range, which cannot lead to body-color appearance.

  The cation vacancies produce three levels in the bandgap (although we can not give exact assignments for these levels at this stage). The electron transitions from the valence band to these levels (i.e. the electrons are captured at these levels from the valence band and the holes are produced in the valence band) cause absorptions in the visible to near infrared (near-IR) range, which may result in the body-color. The body-color of YAlO₃ has been previously reported by some authors, but they did not give clear explanation about this phenomenon^[10-13]. We believe our above interpretation about the body-color of YAlO₃ is reasonable.

  2.3   AC impedance analysis

  It is mentioned above that the electron transitions from the valence band to the defect levels may cause the body-color of YAlO₃. On the other hand, these transitions also produce the holes in the valence band, resulting in p-type conductions, thus it is an effective way to analyze the defects by characterizing the conductance properties of the materials. In this work, the electrical conductivity of YAlO₃ was measured at various temperatures and in various atmospheres by AC impedance spectroscopy.

  Fig. 7 shows the typical Z″/Z′ plot of YAlO₃ (at 560 ℃) and the simplified equivalent circuit. The large semicircle has the capacitivity 3.28×10^-11 F· cm^-1, which is assigned as the grain boundary response, and the grain boundary resistivity (R_gb) can be extracted from the intercept of the grain boundary semicircle on the Z′ axis. The grain boundary resistivity is much larger than the grain resistivity, thus the grain response is merged in the grain boundary response, and it can only be observed by enlarging the high frequency plot. The grain semicircle is not well resolved and the approximate value of the grain resistivity can be taken, as is shown, by the arrow in the inset.^[18] The values of the grain resistivity at other temperatures were attracted by using the similar method.

  Figure 7.  Typical AC impedance spectrum (Z″-Z′ plot) of YAlO₃ at 560 ℃

  图选项

  下载全尺寸图像

  下载幻灯片

  Fig. 8 illustrates the Arrhenius plots of the conductivity for YAlO₃ in the heating-cooling cycle. At high temperatures (above 580 ℃), the defect reactions (3) and (4) are easier to approach to equilibrium. Whether in the heating or in the cooling process, at a certain temperature, the material has the same defect concentration, thus conductivities for both heating and cooling processes are almost identical. While at low temperatures (below 560 ℃), in the heating process since the YAlO₃ sample was previously quenched from 1 100 ℃, the defects formed during high temperature heating (i.e. 1 100 ℃) was "frozen" in the lattice at low temperatures (i.e. Equilibrium (3) was "frozen"). As the discussion in the 3.2 section, the defect reaction (3) is the entropy reducing process, thus the defect concentration of the sample quenched from high temperature is lower than that of the sample annealed in slow cooling process. Therefore, the conductivities in the heating process is lower than that in cooling process. Theoretically although the conductivities for the heating and cooling processes are different, their slopes in the Arrhenius plots should be identical. However, Fig. 8 shows that the slopes of the plots at low temperatures are slightly different, possibly because at low temperatures, Equilibrium (3) is not completely "frozen".

  Figure 8.  Arrhenius plots of the conductivity for YAlO₃ in the heating-cooling cycle

  图选项

  下载全尺寸图像

  下载幻灯片

  By the Arrhenius plots, the activation energies for both low (E_{a (1)}) and high (E_{a (2)}) temperature ranges were calculated shown in Fig. 8(the data are the average values for the heating and cooling processes). E_{a (1)} corresponds to the hole formation (Equilibrium (4), electron transition from the valence band to V_Al^×). E_{a (2)} corresponds to the two processes: the V_Al^× formation (correlated to Equilibrium (3) with the enthalpy change of ΔH) and the hole formation (correlated to E_{a (1)}), thus we have E_{a (2)}=ΔH+E_{a (1)}. E_{a (1)}(≈1.52 eV) is very close to E₁(≈1.4 eV), indicating that the AC impedance data agree with the optical data well. Based on the above discussions, the enthalpy change of Equilibrium (3) can be calculated: ΔH=0.52 eV[E_{a (2)}]^-1.52 eV[E_{a (1)}]=^-1.00 eV=-96 kJ·mol^-1.

  For further understanding the defect equilibrium, the conductivity variation of YAlO₃ with atmospheres was investigated. The measurements were conducted at 720 ℃ and the data are represented in Fig. 9. In N₂ atmosphere, the conductivity decreases rapidly in the first 75 min, and then it is almost constant in the following hours. Whereas when the atmosphere is switched to O₂, the conductivity increases rapidly. The data in Fig. 9 further support that Equilibria (3) and (4) controls the defect concentration and the conductivity of YAlO₃.

  Figure 9.  Variation of the bulk conductivity σ (at 720 ℃) with annealing time in atmospheres switched in the sequence: N₂→O₂

  图选项

  下载全尺寸图像

  下载幻灯片

  Above results indicates that the defects in YAlO₃ are cation vacnacies, showing p-type conduction, thus this material is difficut to be modified to an electride as [Ca₁₂Al₁₄O₃₂]²⁺:[2e^-], which has n-type conduction. In fact, the conductivity of YAlO₃ is not high: at 600 ℃, its buck conductivity is only about 1.1×10^-4 Ω^-1·cm^-1, much lower than that of [Ca₁₂Al₁₄O₃₂]²⁺:[2e^-](10² Ω^-1 ·cm^-1 at room temperature).

  3   Conclusions

  The YAlO₃ samples with various defect concentr-ations were prepared by the solid state reactions at 1 450 ℃, followed by annealing in various atmospheres and at various temperatures. The defect levels and the defect equilibria were analyzed by the reflection spectra and AC impedance data. The results indicate that the light-brown body-color of YAlO₃ is caused by cation vacancies and the material has p-type conduction. The first-principle calculations show that the Al vacancies V_Al^× may be the domonent defect in YAlO₃, which may cause less lattice distortion. Annealing YAlO₃ in N₂ at high temperature may reduce the V_Al^× concentration and the body-color, which may supply a method to improve the quality of the YAlO₃ crystals.

• 1. [1]

     Mott N F, Gurney R W. Electronic Processes in Ionic Crystals. 2nd Ed. Oxford: Clarendon Press, 1948.

  2. [2]

     Liu Z S, Jing X P, Wang L X, et al. J. Electrochem. Soc., 2006, 153(12):G1035-G1038 doi: 10.1149/1.2354455

  3. [3]

     Zhu H, Kuang X J, Jing X P, et al. Jpn. J. Appl. Phys., 2011, 50:065806 doi: 10.7567/JJAP.50.065806

  4. [4]

     Matsuishi S, Toda Y, Hosono H, et al. Science, 2003, 301: 626-629 doi: 10.1126/science.1083842

  5. [5]

     Kim S W, Shimoyama T, Hosono H, et al. Science, 2011, 333:71-74 doi: 10.1126/science.1204394

  6. [6]

     Inoue Y, Kitano M, Hosono H, et al. ACS Catal., 2014, 4: 674-680 doi: 10.1021/cs401044a

  7. [7]

     Sharif M J, Kitano M, Hosono H, et al. J. Phys. Chem. C, 2015, 119:11725-11731 doi: 10.1021/acs.jpcc.5b02342

  8. [8]

     Wang C H, Lin J H, Jing X P, et al. J. Solid State Chem., 2012, 192:195-200 doi: 10.1016/j.jssc.2012.04.018

  9. [9]

     Tomiki T, Kaminao M, Fukudome F, et al. J. Phys. Soc. Jpn., 1991, 60(5):1799-1813 doi: 10.1143/JPSJ.60.1799

  10. [10]

      李涛, 赵广军, 潘守夔, 等.人工晶体学报, 2002, 31(5): 456-459 http://www.cnki.com.cn/Article/CJFDTotal-RGJT200205007.htmLI Tao, ZHAO Guang-Jun, PAN Shou-Kui, et al. J. Synth. Cryst., 2002, 31(5): 456-459 http://www.cnki.com.cn/Article/CJFDTotal-RGJT200205007.htm

  11. [11]

      Weber M J, Bass M, Comperchio E, et al. Appl. Phys. Lett., 1969, 15:342-345 doi: 10.1063/1.1652851

  12. [12]

      Bernhardt H J. Phys. Stattus Solidi A, 1974, 21:95-98 doi: 10.1002/(ISSN)1521-396X

  13. [13]

      Chen J Y, Zhao G J, Cao D H, et al. Current Appl. Phys., 2010, 10:468-470 doi: 10.1016/j.cap.2009.07.006

  14. [14]

      Zview for Windows, Impedance/Gain Phase Graphing and Analysis Software, Version 1. 4b, Scribner Associates Inc. , Char-lottesville, Virginia, 1996.

  15. [15]

      Clark S J, Segall M D, Payne M C, et al. Z. Kristallogr., 2005, 220:567-570 https://www.degruyter.com/view/j/zkri.2005.220.issue-5-6/zkri.220.5.567.65075/zkri.220.5.567.65075.xml

  16. [16]

      Pfrommer B G, Louie S G, Cohen M L. J. Comput. Phys., 1997, 131:233-240 doi: 10.1006/jcph.1996.5612

  17. [17]

      Shannon R D. Acta Crystallogr., 1976, A32:751-767 https://www.scienceopen.com/document?vid=b136c4d0-2fe4-4ee9-9c7a-48e4bf013280

  18. [18]

      Irvin J T S, Sinclair D C, West A R. Adv. Mater., 1990, 2(3): 132-138 doi: 10.1002/(ISSN)1521-4095

• 

  Figure 1  XRD patterns of as-prepared YAlO₃ (a) and JCPDS file 33-41 (b)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  YAlO₃ perovskite structure

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Diffusive reflection spectra of the YAlO₃ samples annealed in air (a), O₂(b) and N₂(c) at 700 ℃ and annealed in N₂ at 1 450 ℃(d)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Schematic diagram for the defect formation

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Reflection spectrum in the Absorption-hν form for the YAlO₃ sample annealed in O₂ atmosphere at 700 ℃

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Band structure with defect levels of YAlO₃ Inset is the enlarged view for the high frequency data

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Typical AC impedance spectrum (Z″-Z′ plot) of YAlO₃ at 560 ℃

  Inset is the enlarged view for the high frequency data

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Arrhenius plots of the conductivity for YAlO₃ in the heating-cooling cycle

  Prior to the measurements, the sample was quenched from 1 100 ℃

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Variation of the bulk conductivity σ (at 720 ℃) with annealing time in atmospheres switched in the sequence: N₂→O₂

  下载: 全尺寸图片 幻灯片

  Table 1.  Optimized energies (E_L) of the unit cells and the defect formation energies (EV)

  	Ideal cell	VY×	VAl×
  EL/eV	-9 717	-8652	-9 646
  EV/eV		1 065	71

  下载: 导出CSV
•