Langbeinite-Type Cubic Dithallium Dicadmium Sulfate

Abstract

The structure of the cubic phase of langbeinite-type T12Cd2(SO4)3 at 295 K has been refined from X-ray diffraction data. The bond-valence sums for the monovalent cations show anomalously low values, indicating that some T1 atoms are very loosely bonded. The possible relevance of this result in the phase-transition scheme of compounds similar to T12fd2(SO4)3 is discussed.

X-ray structure determination of the monoclinic (121 K) and orthorhombic (85 K) phases of langbeinite-type dithallium dicadmium sulfate

“…The arrows indicate the polarization vector direction in the six orientational states. monoclinic phase and to the Tl1 polyhedron in the orthorhombic structure are closer to their chemical valence value, since in the cubic phase Tl1 atoms are loosely bonded (Guelylah, Madariaga & Breczewski, 1996). The values of the bond-valence sums calculated for the atoms forming the monoclinic and the orthorhombic structures are given in Table 9.…”

“…The structure of the high-temperature phase of TCdS was previously re®ned using X-ray diffraction measurements (Guelylah, Madariaga & Breczewski, 1996). In the monoclinic structure the heavy atoms (Tl and Cd) are slightly displaced from their cubic positions.…”

“…Starting structural models for the (monoclinic and orthorhombic) low-temperature phases were extrapolated from the room-temperature cubic structure (Guelylah, Madariaga & Breczewski, 1996). In the monoclinic phase the re®nement was carried out in the space group P112 1 .…”

“…The displacement ®eld u lY has been determined from the comparison of the ferroelectric phase and the high-temperature cubic phase, as described by Guelylah, Madariaga & Breczewski (1996). Owing to the polar character of the P2 1 space group the origin was ®xed arbitrarily along the monoclinic z axis.…”

“…The structure of the mineral langbeinite K 2 Mg 2 (SO 4 ) 3 (KMgS) was ®rst solved by Zemann & Zemann (1957) and was found to be cubic (P2 1 3) at room temperature. Other cubic langbeinites whose structures have been determined are those with K + as the monovalent cation, K 2 B 2 (SO 4 ) 3 [B = Cd (Abrahams et al, 1978), Mn (Yamada et al, 1981), Mg, Ni, Ca, Co and Zn (Speer & Salje, 1986)], (NH 4 )CdS (Ng & Calvo, 1975) and TCdS (Cao et al, 1993;Guelylah, Madariaga & Breczewski, 1996). Full structure determinations have been carried out on three orthorhombic K + type II langbeinite KCdS (Abrahams et al, 1978), KMnS (Yamada et al, 1981) and KCaS (Speer & Salje, 1986).…”