THE COVERT WATER IN THE STRUCTURE OF SR(REO4)(2)CENTER-DOT-2H(2)O - AREVISION OF THE SESQUIHYDRATE MODEL

The structure of hydrated strontium perrhenate previously reported by Varfolomeev, Samarj, Fuchs & Lunk [J. Alloy. Compd. (1993), 201, 261-2 65] as the sesquihydrate has been redetermined. A crystal-chemical ana lysis discloses that the composition of this particular compound obtai ned in identical crystallization conditions is Sr(ReO4)(2).2H(2)O {str ontium bis[tetraoxorhenate(1-)] dihydrate}. The structure contains two independent formula units and one disordered ReO4 group. This disorde red group has the form of a trigonal bipyramid with three essentially ordered equatorial O atoms and two partially occupied Re positions (wi th site occupancies of 0.58 and 0.42) at the tetrahedron centres. The corresponding axial sites are occupied simultaneously by O atoms (with site occupancies of 0.58 and 0.42) and water molecules (complementing the unit occupancies). The crystallographic evidence for the existenc e of strontium perrhenate dihydrate is supported by the results of the rmal analysis.