Orthosilicates with glaserite-type crystal structures: Na2BaZr[SiO4]2 and Na2BaHf[SiO4]2

The two title oxides are new isotypic silicates, crystallizing in the non-centrosymmetric space group P3. The crystal structures were analysed with two twin components in each case.

Single crystal particles of Na₂BaZr[SiO₄]₂ [systematic name: disodium barium zirconium bis­(orthosilicate)] and Na₂BaHf[SiO₄]₂ [disodium barium hafnium bis­(orthosilicate)] were extracted from grain-grown polycrystals obtained by heating compacts of binary oxide mixtures at 1473 K. Single crystal X-ray diffraction analysis revealed that these are isostructural orthosilicates with a glaserite-type crystal structure, in which all sites of X, Y, M, and T in the general formula XY₂[M(TO₄)₂] are fully occupied by atoms of different elements. The crystal structures of the title compounds were refined in space group P3 under consideration of a two-component twin model. The SiO₄ tetra­hedra are rotated approximately ±10.2° from the mirror plane of space group P3m around an axis parallel to [001].