Crystal structure and thermal behaviour of (Rb,Cs)BSi2O6 solid solutions

The crystal chemistry of Rb‐Cs boroleucites has been studied by means of X‐ray powder diffraction at room and elevated temperatures. The cubic I‐43d → cubic Ia3d phase transition was investigated using a series of samples prepared by solid‐state reaction along the pseudobinary system RbBSi₂O₆ ‐ CsBSi₂O₆. The Rietveld refinement of the structures of Rb_1‐xCs_xBSi₂O₆ solid solutions (x = 0.2, 0.4, 0.6, 0.8) demonstrates that the solutions with a high Rb content crystallise in the cubic I‐43d space group, and the boroleucites with a considerable Cs content have Ia3d symmetry. Rb can substitute Cs in a wide range of compositions. Within a narrow range of x = 0.5 ‐ 0.6 immiscibility was revealed. Under Rb‐Cs substitution the cubic lattice parameter, the (Rb,Cs)‐O distances, and the angles between tetrahedra of the I‐43d phase change clearly, while those of the Ia3d phase change slightly. The HTXRD data shows that the I‐43d phase transforms into a Ia3d phase on heating analogously to a change of the composition. As the Cs content increases the transition temperature decreases. The low temperature I‐43d phase shows a higher thermal expansion than the high temperature Ia3d phase. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim