Order-Disorder Transformation in RuBr3 and MoBr3: A Two-Dimensional Ising Model

详细信息    查看全文

• 作者：Stefano Merlino ; Luca Labella ; Fabio Marchetti ; and Simone Toscani
• 刊名：Chemistry of Materials
• 出版年：2004
• 出版时间：October 5, 2004
• 年：2004
• 卷：16
• 期：20
• 页码：3895 - 3903
• 全文大小：220K
• 年卷期：v.16,no.20(October 5, 2004)
• ISSN：1520-5002

文摘

Several trihalides (Cl, Br, I) of elements of groups 4, 5, 6, and 8 of the periodic table, suchas 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 ofthe nearest neighbors, so that long and short MM distances regularly alternate along thecolumns; two distinct conformations are possible for the columns, one with M-M pairscentered at z = 1/4, the other with M-M pairs centered at z = 3/4. Below a certaintemperature the distribution of the two types of columns in the a,b plane is ordered, givingrise to an orthorhombic form, with space group Pnmm. Over this temperature a transitionto a hexagonal form, with space group P6₃/mcm, is observed. We, through X-ray diffractionstudies carried out at different temperatures, estimate this transition temperature for RuBr₃and MoBr₃ crystals and show that the hexagonal form too consists of ordered columns, thehigher symmetry of that phase being due to a random distribution of the two types of columnsin the a,b plane. We suggest that an equal spanning of the metals in the columns, as proposedby previous authors for the hexagonal phase, is never realized in any compound with theTiI₃ structure type and present a possible mechanism of the order-disorder phase transitionin this class of compounds and discuss it within the frame of a two-dimensional Ising model.