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The crystal structures of two K 1Àx Cs x BSi 2 O 6 solid solutions have been refined at room temperature by the Rietveld method:They are isostructural to cubic KBSi 2 O 6 with the space group I 4 43d. The structure is built up from (Si,B)O 4 tetrahedra linked in four-, six-and eightfold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically.Thermal behaviour of KBSi 2 O 6 and K 0.5 Cs 0.5 BSi 2 O 6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi 2 O 6 has been found in situ under heating. The new polymorphic I 4 43d (cubic) -Ia 3 3d (cubic) transition and the new Ia 3 3d cubic polymorphic phase has been proposed for K 1Àx Cs x BSi 2 O 6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.

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