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The title compound was prepared in well-crystallized form by thermal decomposition of Ca5Re2O12. Its structure was determined from single-crystal X-ray diffractometer data: I41, a = 1107.0(1), c = 1609.3(1) pm, Z = 4, R = 0.056 for 4565 structure factors and 119 variable parameters. The calcium atoms occupy seven different sites with 8, 9, or 10 oxygen neighbors. The two different rhenium atoms are octahedrally coordinated by oxygen atoms with average Re-O distances of 193.1 and 187.7 pm for the six- and seven-valent rhenium atoms, respectively. The compound shows Curie-Weiss behavior with a magnetic moment of μexp = 1.15(±0.10) μB per Re(VI) atom. The structure is closely related to that of Sr11Os4O24 which, however, crystallizes in the space group I2/a. The difference between the two structures arises through the higher coordination numbers of the strontium atoms. It is suggested that at high temperature both compounds crystallize in the common higher symmetry space group I4/a. Since Ca11Re4O24 crystallizes in the pyroelectric class 4 this compound is expected to be ferroelectric.

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structure structure (solids and liquids) D 2000 -012The Crystal Structure of the Potential Ferroelectric Calcium Rhenate(VI, VII) Ca 11 Re 4 O 24 and Its Relation to the Structure of Sr 11 Os 4 O 24 .-The structure of the title compound, obtained by thermal decomposition of Ca 5 Re 2 O 12 in a dynamic vacuum, is determined by single crystal X-ray data (space group I4 1 , Z = 4). The Re atoms occupy two crystallographic sites, both with octahedral oxygen coordination and the Ca atoms occupy seven different sites with 8, 9, or 10 oxygen neighbors. The structure is closely related to that of Sr 11 Os 4 O 24 , which crystallizes in the centrosymmetric, monoclinic space group I2/a. It is suggested that both compounds crystallize in the common higher symmetry group I4 1 /a at higher temperatures. The title rhenate shows Curie-Weiss behavior but no superconducting transition down to 2 K. -(JEITSCHKO, W.; MONS, H. A.; RODEWALD, U. CH.; MOELLER, M. H.

The title compound was prepared by reaction of elemental calcium with the calcium metaperrhenate Ca(ReO4)2. 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 Ca5Re3O14.75. A test for superconductivity of this black compound down to 1.5 K was negative.