The Covert Water in the Structure of Sr(ReO₄)₂.2H₂O: a Revision of the Sesquihydrate Model

The structure of hydrated strontium perrhenate previously reported by Varfolomeev, Samarj, Fuchs & Lunk [J. Alloy. Compd. (1993), 201, 261-265] as the sesquihydrate has been redetermined. A crystal chemical analysis discloses that the composition of this particular compound obtained in identical crystallization conditions is Sr(ReO₄)₂.2H₂O. The structure contains two independent formula units and one disordered ReO₄ group. This disordered group represents a trigonal bipyramid with three essentially ordered equatorial O atoms and two partially occupied Re positions (occupation numbers of 0.58 and 0.42, respectively) at the tetrahedron centres. The corresponding axial sites are occupied simultaneously by oxygen atoms (occupation numbers of 0.58 and 0.42, respectively) and water molecules (complementing the unit occupancies). The crystallographic evidence of the existence of strontium perrhenate dihydrate is supported by the results of thermal analysis.

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