Order-disorder transformation in RuBr3 and MoBr3:: A two-dimensional Ising model
Order-disorder transformation in RuBr3 and MoBr3:: A two-dimensional Ising model
复制标题
DOI：
10.1021/cm049235q
复制
发表时间：
2004-10-05
影响因子：
8.6
通讯作者：
Toscani, S
中科院分区：
文献类型：
--
作者：
Merlino, S;Labella, L;Toscani, S
元素周期表第 4、5、6 和 8 族元素的几种三卤化物（Cl、Br、I）（例如 TiI3）显示出由沿 c 延伸的平行无限柱构成的共面八面体 [MX6] 的晶体结构。放置在柱内的金属可能会与最近的邻居之一相互作用，因此长和短 MM 距离沿着柱定期交替；柱可能有两种不同的构象，一种是 M-M 对以 z = 1/4 为中心，另一种是 M-M 对以 z = 3/4 为中心。在一定温度以下，两种类型的柱在 a、b 平面中的分布是有序的，形成具有空间群 Pnmm 的斜方晶系。在此温度下，观察到向空间群 P6(3)/mcm 的六方晶型的转变。我们通过在不同温度下进行的 X 射线衍射研究，估计了 RuBr3 和 MoBr3 晶体的转变温度，并表明六方形式也由有序柱组成，该相的较高对称性是由于两种类型的柱在 a、b 平面中的随机分布。我们认为，正如先前作者针对六方相提出的那样，柱中金属的等跨距在任何具有 TiI3 结构类型的化合物中从未实现，并且提出了此类化合物中有序-无序相变的可能机制，并在二维 Ising 模型的框架内进行了讨论。
Several trihalides (Cl, Br, I) of elements of groups 4, 5, 6, and 8 of the periodic table, such as TiI3, show a crystal structure made by parallel infinite columns, running along c, of face-sharing octahedra [MX6]. The metals placed within the columns may interact with one of the nearest neighbors, so that long and short MM distances regularly alternate along the columns; two distinct conformations are possible for the columns, one with M-M pairs centered at z = 1/4, the other with M-M pairs centered at z = 3/4. Below a certain temperature the distribution of the two types of columns in the a,b plane is ordered, giving rise to an orthorhombic form, with space group Pnmm. Over this temperature a transition to a hexagonal form, with space group P6(3)/mcm, is observed. We, through X-ray diffraction studies carried out at different temperatures, estimate this transition temperature for RuBr3 and MoBr3 crystals and show that the hexagonal form too consists of ordered columns, the higher symmetry of that phase being due to a random distribution of the two types of columns in the a,b plane. We suggest that an equal spanning of the metals in the columns, as proposed by previous authors for the hexagonal phase, is never realized in any compound with the TiI3 structure type and present a possible mechanism of the order-disorder phase transition in this class of compounds and discuss it within the frame of a two-dimensional Ising model.