Phase transitions in leucite: X-ray diffraction studies
Phase transitions in leucite: X-ray diffraction studies
复制标题
白榴石中的相变：X 射线衍射研究
DOI：
10.1007/bf00223322
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发表时间：
1989
影响因子：
1.4
通讯作者：
W. Schmahl
中科院分区：
文献类型：
--
作者：
D. Palmer;E. Salje;W. Schmahl
测量了天然白榴石的晶格参数和选定的X射线反射强度随温度的变化，以表征其相变行为。在低温下，白榴石具有较大的铁弹性变形，但晶格参数的温度演化不能用纯铁弹性相变来解释；特别是，体积随温度的显著变化意味着额外的相变机制，我们将其与K离子在低温相中的偏心以及<111>结构通道的坍塌相关联。因此，转变行为可以根据两种相互竞争的机制来合理化：(I)铁弹性(与M3M→4/mm的变化一致)；(Ii)体积变化(与M3M→4/m一致)。两个序参数QI和QII的耦合产生了中间的4/mm四方相，我们的结果证实了I41/a→I41/Acd相的存在，但200反射在高温下没有消失，这意味着预期的从I41/Acd到Ia3d(立方)对称性的相变并没有发生。我们将其归因于与序参数共轭的残余应变场，这是由于缺陷(可能是Al/Si有序)造成的。然而，在我们的实验分辨率下，晶格在665°C时变成了米立方。
Lattice parameters, and intensities of selected X-ray reflexions, have been measured as a function of temperature for natural leucite, to characterise the phase transformation behaviour. At low temperatures leucite has a large ferroelastic distortion, but the temperature evolution of lattice parameters cannot be explained in terms of a purely ferroelastic phase transition; in particular, the considerable change in volume with temperature implies an additional transition mechanism, which we correlate with off-centring of K-ions in the low-temperature phase, and a collapse of the 〈111〉 structural channels. The transition behavior can therefore be rationalised in terms of two competing mechanisms: (I) Ferroelastic (consistent with the change m3m→ 4/mmm); (II) Volume-changing (consistent with m3m → 4/m). Coupling of the two order parameters QI, and QII gives rise to the intermediate 4/mmm tetragonal phase.Our results confirm the existence of an I41/a→I41/acd transition, but the non-disappearance of the 200 reflexion at high temperatures implies that the expected transition from I41/acd to Ia3d (cubic) symmetry does not occur. We attribute this to a residual strain field conjugated to the order parameter, due to defects (with possible Al/Si order). Nevertheless, within our experimental resolution, the lattice becomes metrically cubic at 665° C.