Crystal structure of strontium hilgardite

Abstract

The crystal structure of strontium hilgardite, CaSr[B 5 O 9 ]Cl ⋅ H 2 O, was established by the method of X-ray diffraction analysis (synchrotron radiation; diffractometer equipped with a position-sensitive detector; λ = 0.688 Å; 3191 reflections with F < 4σ( F ); R = 0.045 in the anisotropic approximation). The triclinic unit-cell parameters are as follows: a = 6.5732(6) Å, b = 6.4445(6) Å, c = 6.3693(6) Å, α = 60.995(2)°, β = 61.257(2)°, γ = 77.191(2)°, sp. gr. P 1; Z = 1. The Ca and Sr atoms were found to be disordered over two positions. The structures of strontium hilgardite and hilgardite-1 A differ in the configurations of the seven-vertex Sr-and Ca(2)-polyhedra. The structure solved in this work is consistent with two series of borates studied previously. One of these groups involves pentaborates with different degrees of hydration of borate complexes, and the second group includes Sr-containing borates.