Phase transition and crystal structures of
Phase transition and crystal structures of
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相变和晶体结构
DOI：
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
J. García‐Ruiz
中科院分区：
文献类型：
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作者：
G. Subías;E. Palacios;J. Blasco;J. García‐Ruiz
x射线衍射和差示扫描量热实验表明，该材料在室温下经历了从型结构到低温下单斜晶胞的一级结构相变。跃迁是由四边形单元胞的[110]和[001]方向的耦合倾斜引起的，并且已经在朗道理论的框架内用两个相互作用的阶参数和考虑到六阶项的自由能展开来解释。假设铵离子在高温相中可以绕四重结晶轴自由旋转，解释了相变的大熵含量。
X-ray diffraction and differential scanning calorimetry experiments have made it evident that undergoes a first-order structural phase transition at from a -type structure at room temperature to a monoclinic cell at low temperatures. The transition arises from coupled tilts and around [110] and [001] directions of the tetragonal unit cell and have been explained in the framework of the Landau theory using an expansion of the free energy with two interacting order parameters and , taking into account up to sixth-order terms. The large entropy content of the transition has been explained assuming that the ammonium ion is free to rotate around the fourfold crystallographic axis in the high-temperature phase.