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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Phase transitions in K2Cr2O7 and structural redeterminations of phase II
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Phase transitions in K2Cr2O7 and structural redeterminations of phase II

机译：K2Cr2O7中的相变和II期的结构重新确定

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摘要

Crystals of phase II K2Cr2O7, potassium dichromate, space group P (1) over bar, grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P2(1), at a phase-transition temperature, T-PT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 ( 2) K. The endotherm on subsequent heatings occurs reproducibly at T-PT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.
机译：从水溶液中生长出来的II型K2Cr2O7晶体，重铬酸钾，空间棒（1）在水溶液上生长，经历了从一阶到I相的空间转变，据报道空间相为P2（1）/ n，在相变温度T下-PT，第一次加热时为544（2）K;相应的冷却转变为502（2）K。后续加热的吸热在T-PT = 531（2）K时可再现地发生。大约531和544 K之间的质量损失（确定为包括水）迅速且持续不断。缓慢升高至更高温度，总损失约为0.20％。第一次加热时较高的TPT与过热水的压力升高有关，而过热水占据了夹杂物缺陷。在阶段II中潜在的对角滑行平面可以推断出阶段I的结构。 296 K处的三斜结构已经独立确定。正态概率分析显示了所得原子坐标与先前原子坐标之间的高度一致性，但不确定性减少了大约2倍。较早的不确定性因可比因素而被系统低估了。 Krivovichev等人在大约300 K处确定了相移轴上IIb相的结构，即空间群A2 / a。 [Acta Cryst。 （2000），C56，629-630]。相I和相II之间的一阶跃迁是由于获得或失去n滑移面时每个四面体中末端O原子的相对旋转大约60度而产生的。 IIb和I相之间也可能是一阶的过渡，但II和IIb相之间不可能过渡。在阶段IIb和阶段I之间可能存在一个中间阶段。

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