Preparation and Crystal Structure of the Calcium Rhenate(VI, VII) Ca5Re3O15-x

The title compound was prepared by reaction of elemental calcium with the calcium metaperrhenate Ca(ReO . Its crystal structure was determined from single-crystal X-ray data: Amm2, a = 560.31 (5)pm, b = 1572.4( 1)pm, c = 719.91 (6)pm ,Z = 2 ,R = 0.033 for 930 structure factors and 46 variable parameters. The calcium atoms occupy three atomic sites, all with seven oxygen neighbors. Of the two different rhenium atoms one has square-pyramidal oxygen coordination with an average oxidation number +6.25. The other rhenium site (oxidation number +7) was refined as a split position with trigonal-bipyramidal (75%) and tetrahedral oxygen coordination (25%). One oxygen site remains unoccupied, whenever the tetrahedral rhenium site is occupied, resulting in the composition Ca Re . A test for superconductivity of this black compound down to 1.5 K was negative.