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The 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

Wolfgang Jeitschko*, Horst A. Mons, Ute Ch. Rodewald, Manfred H. M¨oller

Anorganisch-Chemisches Institut, Universit¨at M¨unster, Wilhelm-Klemm-Straße 8, D-48149 M¨unster, Germany Z. Naturforsch.53 b,31–36 (1998); received October 17, 1997

Crystal Structure, Ferroelectrics, Mixed Valence, Rhenates, Perrhenates

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 ofexp= 1.15(0.10)

B 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 I41/a. Since Ca11Re4O24

crystallizes in the pyroelectric class 4 this compound is expected to be ferroelectric.

Reprint requests to W. Jeitschko.

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