The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): Structural and thermodynamic behavior
The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): Structural and thermodynamic behavior
复制标题
无铁辉石 (Ca0.15Mg1.85Si2O6) 中的高温 P21/c-C2/c 相变：结构和热力学行为
DOI：
10.2138/am-2002-5-607
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发表时间：
2002
影响因子：
3.1
通讯作者：
M. Domeneghetti
中科院分区：
文献类型：
--
作者：
M. Tribaudino;F. Nestola;F. Cámara;M. Domeneghetti
摘要：对两种成分为 Ca0.15Mg1.85Si2O6 的无铁 P21/c 单斜辉石晶体进行了从室温到 1150 °C 的高温原位单晶 X 射线衍射研究 [室温下的晶胞参数：a = 9.651(2) Å, b = 8.846(2) Å, c = 5.202(1) Å, b = 108.38(2)°, V = 421.4 (2) Å3] 通过在 T = 1370 °C、P = 1 atm 等温退火 624 小时合成。在略低于 1000 °C [Tc = 926(39) °C] 时发现一级 P21/c-C2/c 相变。这种转变是通过强度和细胞参数的不连续变化来揭示的。长时间高温加热导致相变温度不可逆升高至 1150 °C 以上，而相变顺序没有明显变化。有人认为，与以前几乎无缺陷的样品中的初始外溶引起的应变耦合是导致 Tc 增加的原因。来自相同起始材料的样品在 T = 1050 °C、P = 1 atm 下进一步退火 72 小时后的 TEM 观察结果与提出的初始外溶模型一致。发现退火会诱导形成平行于（101）方向的斑驳织构。在 T = 25、500、650、800 和 1000 °C 时收集的数据的结构精修结果表明，链构型仅发生微小变化，而在 1000 °C 之前链构型高度分化，证实了转变的强烈一级特征。
Abstract A high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 °C on two crystals of Fe-free P21/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 [cell parameters at room T: a = 9.651(2) Å, b = 8.846(2) Å, c = 5.202(1) Å, b = 108.38(2)°, V = 421.4 (2) Å3] synthesized by isothermal annealing for 624 h at T = 1370 °C, P = 1 atm. A first order P21/c-C2/c phase transition was found slightly below 1000 °C [Tc = 926(39) °C]. The transition was revealed by discontinuous changes in intensities and cell parameters. Prolonged heating at high temperature induced a non-reversible increase in the transition temperature up to more than 1150 °C, without apparent changes in the order of the phase transition. Coupling with strain due to incipient exsolution in a formerly almost defect-free sample is suggested to be responsible for increase in Tc. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 °C, P = 1 atm are consistent with the proposed incipient exsolution model. Annealing was found to induce the formation of a mottled texture oriented parallel to (101). Results from structure refinement of data collected below the transition at T = 25, 500, 650, 800, and 1000 °C showed only minor changes in the chain configurations, which are highly differentiated up to 1000 °C, confirming the strong first-order character of the transition.