Crystal formation from glass, crystal structure refinement and thermal behavior of K1−xRbxBSi2O6boroleucite solid solutions from X-ray powder diffraction data

The crystal chemistry of K1−xRbxBSi2O6 mixed boroleucites has been studied by means of X-ray powder diffraction at room temperature. Boroleucites under study were prepared by crystallization from glass at 700–1000 °C for a few to 1000 hours time. First glass crystallization process starts from formation of two solid solutions: one of them is related to space group I-43d, another to Ia-3d. After heat-treatment at 950 °C/51 h, K1−xRbxBSi2O6 solid solutions crystallize in the cubic space group I-43d, in a wide range of compositions as Rietveld refinement of the structures of solid solutions (nominal composition x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) demonstrated. Within a narrow range of x near 0.2 area of immiscibility was found. Under substitution of K by Rb structural parameters (cubic lattice parameter a, (K,Rb)—O bond lengths in (K,Rb)O15 polyhedron, (K,Rb)—(K,Rb) bond lengths, (K,Rb)—O—(K,Rb) äangles between (K,Rb)O15 polyhedra, and T—O—T angles between tetrahedra) of the phase increased nonlinearly due to cationic size rising. Non-linear behavior of the composition dependence of structural parameters was observed near x = 0.2 ÷ 0.4. A comparative analysis has been made for the K—Rb—Cs substitution in the two sequential boroleucite series KBSi2O6—RbBSi2O6 and RbBSi2O6—CsBSi2O6.