X-Ray Study of the Phase Transition in K₂Mn₂(SO₄)₃

Langbeinite-Type Cubic Dithallium Dicadmium Sulfate

“…The so-called type II compounds [e.g. K2Mn2(SO4)3 and K2Cd2(SO4)3] show only a single phase transition to an orthorhombic structure (P212121) as temperature is lowered (Abrahams, Lissalde & Bernstein, 1978;Hikita, Chubachi & Ikeda, 1978;Yamada, Maeda & Adachi, 1981). Based on group theoretical analysis, DvorAk (1972) has shown that a spatially homogeneous phase transition could result only from a phonon instability at the points M and F of the cubic Brillouin zone.…”

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

“…The sequence of phase transitions allows the classi®cation of langbeinites in two distinct types. Crystals that exhibit a series of phase transitions from cubic (P2 1 3) to orthorhombic (P2 1 2 1 2 1 ) across two intermediate, monoclinic (P2 1 ) and triclinic (P1), phases belong to type I, for example Tl 2 Cd 2 (SO 4 ) 3 (TCdS) and Rb 2 Cd 2 (SO 4 ) 3 (RCdS; Brezina & Glogarova, 1972;Ikeda & Yasuda, 1975;Yamada & Kawano, 1977;Hikita et al, 1978). Crystals of type II undergo a single phase transition from the cubic (P2 1 3) to an orthorhombic (P2 1 2 1 2 1 ) phase.…”

“…Crystals of type II undergo a single phase transition from the cubic (P2 1 3) to an orthorhombic (P2 1 2 1 2 1 ) phase. To this type belong compounds such as K 2 Cd 2 (SO 4 ) 3 (KCdS), K 2 Mn 2 (SO 4 ) 3 (KMnS) and K 2 Ca 2 (SO 4 ) 3 (KCaS; Abrahams et al, 1978;Hikita et al, 1978;Yamada et al, 1981;Speer & Salje, 1986). Some of the crystals belonging to type I show only one intermediate monoclinic (P2 1 ) phase [K 2 Co 2 (SO 4 ) 3 (KCoS) and K 2 Zn 2 (SO 4 ) 3 (KZnS; Yamada et al, 1980)], whereas in others such as (NH 4 ) 2 Cd 2 (SO 4 ) 3 [(NH 4 )CdS] the monoclinic phase is stable down to liquid helium temperatures (Glogarova & Fousek, 1973;.…”

Heat capacity studies of phase transitions in langbeinites II. K2Mg2(SO4)3

“…EPR studies Korczak 1986, 1987) of Type 1 compounds and X-ray diffraction studies of a Type 2 compound (Abrahams et al 1978) have led to the conclusion that the freezing of sulfate rotations is responsible for the transition. However, the soft vibrational modes expected on the basis of this References for this table: T1 compounds-Ni, Co, Zn, Ca (Hikita et al 1977b); Mg (Kahrizi and Steinitz 1988); and Cd (Brezina and Glogarova 1972;Ikeda and Yasuda 1975); Rb compounds-Ni, Co, Zn, Mn, Ca (Hikita et al 1977b); Mg (Hikita et al 1977a); and Cd (Hikita et al 1976); NH 4 compounds-Ni, Co, Zn, Ca (Hikita et al 1977b); Mg, Mn (Hikita et al 1977b;KAhrizi and Steinitz 1988);and Cd-(Jonas and Pepinsky 1956;Glogarova and Fousek 1973); K compoundsMg, Zn (Hikita et al 1977a); Ni (Hikita et al 1977b); Co (Hikita et al 1977b;Brezina and Fouskova 1978); Mn (Hikita et al 1978 Speer and Salje (1986). (a) The parameter X is a measure of the displacement of the M s + ion from the midpoint of the segment between the two triangular faces; the distances 26 separates the parallel triangular faces.…”