Polymorphism of strontium monogallate: The framework structures of beta-SrGa2O4 and ABW-type gamma-SrGa2O4

The crystal structures of two modifications of strontium monogallate have been investigated by single-crystal X-ray diffraction. A beta-phase corresponding to the stable modification at room temperature and gamma-SrGa2O4, a quenched high-pressure polymorph prepared at 1250 degrees C and 2.5 GPa. beta-SrGa2O4 is monoclinic, space group P12(1)/c1, a = 8.3770(3) Angstrom, b = 8.9940(2) Angstrom, c = 10.6800(4), beta = 93.932(2)degrees, V = 802.8(5) Angstrom(3), Z = 8, D-calc = 4.82 g cm(-3), wR2 = 0.083 for 2351 reflections; gamma-SrGa2O4 is monoclinic as well, space group P12(1)/n1, a = 8.1100(2) Angstrom, b = 10.7610(3) Angstrom, 9.0570(2) Angstrom, beta = 91.569(1)degrees, V = 790.1(3) Angstrom(3), Z = 8, D-calc = 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 gamma-SrGa2O4 and UUDUDD for beta-SrGa2O4, respectively. gamma-SrGa2O4 belongs to the zeolite ABW-type structures and is apparently the only member with space group symmetry P12(1)/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. (C) 2000 Academic Press.