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1999 (1)
1998 (1)
1Author    Wolfgang Jeitschko, HorstA. Mons, Ute Ch, ManfredH. Rodewald, MöllerRequires cookie*
 Title    The Crystal Structure of the Potential Ferroelectric Calcium Rhenate(VI, VII) CanRe4 0 24 and its Relation to the Structure of S rn 0 s40 24  
 Abstract    The title compound was prepared in well-crystallized form by thermal decomposition of Ca5Re20i2. Its structure was determined from single-crystal X-ray diffractometer data: /4 j, 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 n exp = 1.15(±0.10) Pb per Re(VI) atom. The structure is closely related to that of Srn0s4024 which, however, crystallizes in the space group I2la. 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 CanRe4024 crystallizes in the pyroelectric class 4 this compound is expected to be ferroelectric. 
  Reference    (Z. Naturforsch. 53b, 31—36 [1998]; received October 17 1997) 
  Published    1998 
  Keywords    Crystal Structure, Ferroelectrics, Mixed Valence, Rhenates, Perrhenates 
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 TEI-XML for    default:Reihe_B/53/ZNB-1998-53b-0031.pdf 
 Identifier    ZNB-1998-53b-0031 
 Volume    53 
2Author    Wolfgang Jeitschko, HorstA. Mons, Ute Ch, RodewaldRequires cookie*
 Title    Preparation and Crystal Structure of the Calcium Rhenate(VI, VII) Ca5Re30 15.x  
 Abstract    The title compound was prepared by reaction of elemental calcium with the calcium metaper-rhenate Ca(ReC>4)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 Ca5Re3 0 i4.75. A test for superconductivity of this black compound down to 1.5 K was negative. 
  Reference    Z. Naturforsch. 54b, 1483—1488 (1999); received August 16 1999 
  Published    1999 
  Keywords    Crystal Structure, Mixed Valence, Rhenates, Perrhenates 
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 TEI-XML for    default:Reihe_B/54/ZNB-1999-54b-1483.pdf 
 Identifier    ZNB-1999-54b-1483 
 Volume    54