Crystal formation from glass, crystal structure refinement and thermal behavior of K 1− x Rb x BSi 2 O 6 boroleucite 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 degrees 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 (4) over bar 3d, another to Ia (3) over bard. After heat-treatment at 950 degrees C/51 h, K1-xRbxBSi2O6 solid solutions crystallize in the cubic space group I (4) over bar 3d, 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)O-15 polyhedron, (K,Rb)-(K,Rb) bond lengths, (K,Rb)-O-(K,Rb) angles between (K,Rb)O-15 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 divided by 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.