Weakley T J R, Ylvisaker E R, Yager R J, Stephens J E, Wiegel R D, Mengis M, Bauer M R, Wu P, Photinos P, Abrahams S C
Department of Chemistry, University of Oregon, Eugene, OR 97403, USA.
Acta Crystallogr B. 2004 Dec;60(Pt 6):705-15. doi: 10.1107/S010876810402333X. Epub 2004 Nov 11.
Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, 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 TPT = 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 60 degrees 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.
从水溶液中生长出来的II相重铬酸钾（K₂Cr₂O₇）晶体，空间群为P1，在首次加热时，于相变温度TPT = 544 (2) K经历一级相变转变为I相，据报道其空间群为P21/n；冷却时相应的转变温度为502 (2) K。后续加热时的吸热过程在TPT = 531 (2) K可重复出现。在约531至544 K之间的质量损失被确定为包含的水，损失迅速，并且在更高温度下继续缓慢进行，总损失约为0.20%。首次加热时较高的TPT与占据包裹体缺陷的过热水压力增加有关。II相中的潜在对角滑移面使得可以推断出I相的结构。已独立重新测定了296 K时的三斜结构。正态概率分析表明，所得原子坐标与先前的原子坐标高度一致，但不确定性降低了约2倍。早期的不确定性被系统地低估了类似的倍数。Krivovichev等人[《晶体学报》(2000年)，C56，629 - 630]在约300 K时确定了通过轴变换后空间群为A2/a的IIb相的结构。I相和II相之间的一级相变源于每个四面体中末端O原子随着获得或失去n滑移面而发生的约60度相对旋转。IIb相和I相之间也可能存在一级相变，但II相和IIb相之间不存在。IIb相和I相之间可能存在一个中间相。
