High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates:: systematics in enthalpies of formation of A2+B4+O3 perovskites
High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates:: systematics in enthalpies of formation of A2+B4+O3 perovskites
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DOI：
10.1007/s00269-005-0034-1
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发表时间：
2005-12-01
影响因子：
1.4
通讯作者：
Koyama, K
中科院分区：
文献类型：
--
作者：
Akaogi, M;Kojitani, H;Koyama, K
在高达 26 GPa 和 2,073 K 的压力下检查 MgGeO3 和 ZnGeO3 的相变，以确定钛铁矿 - 钙钛矿转变边界。在这两个系统中，钙钛矿相在压力释放时转化为铌酸锂结构。对于 MgGeO3 和 ZnGeO3，钛铁矿钙钛矿边界具有负斜率，分别表示为 P(GPa) = 38.4 - 0.0082T(K) 和 P(GPa) = 27.4 - 0.0032T(K)。 SrGeO3 多晶型物的热函通过高温量热法测量。 298 K 时 SrGeO3 假硅灰石 - 硅灰石和硅灰石 - 钙钛矿转变的焓分别确定为 6.0 +/- 8.6 和 48.9 +/- 5.8 kJ/mol。使用测量的焓数据计算出的 SrGeO3 的转变边界与之前高压实验确定的边界一致。通过量热测量，在 298 K 下由成分氧化物形成的 SrGeO3 钙钛矿的形成焓（Delta H-f 度）为 - 73.6 +/- 5.6 kJ/mol。对 MgGeO3 和 ZnGeO3 中钛铁矿钙钛矿转变边界以及 SrSiO3 钙钛矿形成边界的热力学分析提供了用于估计钙钛矿形成熵的转变焓。组成氧化物的 MgGeO3、ZnGeO3 和 SrSiO3 钙钛矿的 Delta H(f) 度分别为 10.2 +/- 4.5、33.8 +/- 7.2 和 - 3.0 +/- 2.2 kJ/mol。将上述高压钙钛矿的形成焓的现有数据与在大气压和高压下稳定的A(2+) B4+ O-3钙钛矿的已发表数据相结合，探索Delta H(f)度与八倍配位A(2+) (R-A)和六倍配位B4+ (R-B)阳离子的离子半径之间的关系。结果表明，随着R-A和R-B的减小，A(2+) B4+ O-3钙钛矿的形成焓增加。生成焓与耐受因子（t=R-A+R-o）/根2（R-B+R-o），R-o：O2-半径）之间的关系并不简单；然而，在钙钛矿结构中，生成焓与 A(2+) 和 B4+ 半径与理想尺寸的偏差平方和之间存在线性关系。显示钙钛矿形成焓随 A(2+) 和 B4+ 半径变化的图表表明等焓曲线存在系统变化。这些 Delta H(f) 度与 R-A 和 R-B 的关系可用于估计尚未合成的钙钛矿的形成焓。
Phase transitions in MgGeO3 and ZnGeO3 were examined up to 26 GPa and 2,073 K to determine ilmenite - perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite perovskite boundaries have negative slopes and are expressed as P(GPa) = 38.4 - 0.0082T(K) and P(GPa) = 27.4 - 0.0032T( K), respectively, for MgGeO3 and ZnGeO3. Enthalpies of SrGeO3 polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO3 pseudowollasonite - walstromite and walstromite - perovskite transitions at 298 K were determined to be 6.0 +/- 8.6 and 48.9 +/- 5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO3, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (Delta H-f degrees) of SrGeO3 perovskite from the constituent oxides at 298 K was determined to be - 73.6 +/- 5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite perovskite transition boundaries in MgGeO3 and ZnGeO3 and the boundary of formation of SrSiO3 perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The Delta H(f)degrees of MgGeO3, ZnGeO3 and SrSiO3 perovskites from constituent oxides were 10.2 +/- 4.5, 33.8 +/- 7.2 and - 3.0 +/- 2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A(2+) B4+ O-3 perovskites stable at both atmospheric and high pressures to explore the relationship between Delta H(f)degrees and ionic radii of eightfold coordinated A(2+) (R-A) and sixfold coordinated B4+ (R-B) cations. The results show that enthalpy of formation of A(2+) B4+ O-3 perovskite increases with decreasing R-A and R-B. The relationship between the enthalpy of formation and tolerance factor (t = R-A + R-o)/root 2(R-B + R-o), R-o: O2- radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A(2+) and B4+ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A(2+) and B4+ radii indicates a systematic change with equi-enthalpy curves. These relationships of Delta H(f)degrees with R-A and R-B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.