Polymorphism of Strontium Monogallate: The Framework Structures of β-SrGa2O4 and ABW-Type γ-SrGa2O4

Abstract

The crystal structures of two modifications of strontium monogallate have been investigated by single-crystal X-ray diffraction. A β-phase corresponding to the stable modification at room temperature and γ-SrGa2O4, a quenched high-pressure polymorph prepared at 1250°C and 2.5 GPa. β-SrGa2O4 is monoclinic, space group P121/c1, a=8.3770(3) Å, b=8.9940(2) Å, c=10.6800(4), β=93.932(2)°, V=802.8(5) Å3, Z=8, Dcalc=4.82 g cm−3, wR2=0.083 for 2351 reflections; γ-SrGa2O4 is monoclinic as well, space group P121/n1, a=8.1100(2) Å, b=10.7610(3) Å, c=9.0570(2) Å, β=91.569(1)°, V=790.1(3) Å3, Z=8, Dcalc=4.89 g cm−3, wR2=0.068 for 2300 reflections. The structures of both phases are characterized by layers of six-membered rings of GaO4 tetrahedra perpendicular to the a axis. The stacking of the layers parallel to the a axis results in a three-dimensional framework containing channels that are occupied by the Sr cations for charge compensation. The two modifications differ with respect to the relative orientation of up (U) and down (D) pointing apices of adjacent tetrahedra within a single ring. The sequence of directedness is UUUDDD for γ-SrGa2O4 and UUDUDD for β-SrGa2O4, respectively. γ-SrGa2O4 belongs to the zeolite ABW-type structures and is apparently the only member with space group symmetry P121/n1 in combination with a doubled b axis compared to the ABW-aristotype (space group Jmma). It is the first member of this structure family with exclusively trivalent tetrahedral ions of only one chemical species and divalent nonframework cations within the channels. The compound is isotypic with the monclinic modification of CaGa2O4.