Thermal Expansion of Two Copper Vanadates MCu(VO3)2Cl (M = Cs, Rb): Structure-Based and Unit-Cell-Based Approaches

Two copper vanadates, MCu(VO3)2Cl (M = Cs, Rb), were synthesized and studied by variable-temperature single-crystal X-ray diffraction in the range up to 700 and 675 K for the Cs- and Rb-containing phases, respectively. The structures are composed of [VO3] chains of edge-sharing VO5 polyhedra and [CuO4Cl] chains of CuO4Cl2 octahedra assembled into a three-dimensional framework. Despite close structural similarity, both compounds differ substantially in thermal behavior: the Cs phase exhibits stronger anisotropy compared to the Rb-containing phase. To interpret the structural dynamics, observed bond lengths were compared with values corrected using the simple rigid-body motion model and with bond lengths obtained from TLS analysis of VO5 polyhedra. It is shown that the observed V–O bond lengths can yield misleading trends, including apparent bond shortening upon heating, whereas rigid-body-corrected values provide a more crystal-chemically consistent picture. In particular, comparison of the observed, SRBM-corrected, and TLS-corrected bond lengths shows that the thermal expansion along the direction of the [VO3] chains is poorly reflected by the observed bond lengths, whereas the corrected bond lengths exhibit much closer agreement with the thermal expansion derived from the unit-cell parameters. For the other directions, such a comparison is hindered by the geometry of the bridging linkages and by the ambiguity associated with the TLS treatment. The results obtained for MCu(VO3)2Cl (M = Cs, Rb) therefore demonstrate the strong influence of bond-length evaluation on the interpretation of structural dynamics.