Ferroelectric Phase Transition in Rb₂Cb₂(SO₄)₃

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.…”

EPR study of structural phase transitions in (NH4)2Cd2(SO4)3

“…ACS and TCS crystaliize in the cubic space group P2,3(T4) at room temperature with lattice constants 10.35 and 10.38 A respectively (Hikita et al 1976). They are isomorphous to the mineral langbeinite, K,Mg,(SO,),, whose X-ray crystal structure (Zeemann and Zeemann 1957) revealed that the K' and Mg2+ lie on three fold axes, and the SO, tetrahedra are in general positions.…”

“…The successive structural phase transitions which the langbeinites go through on cooling seem to have the sequence P2,3 + P2, + P1 -, P2,2,2, (Hikita et al 1976). …”

Optical birefringence imaging of the phase transition of K2Mn2(SO4)3

“…I jT 1 j P2 1 3 jZ 1 j paraelectricY paraelasticY II jT 2`T1 j P2 1 jZ 2 j ferroelectricY III jT 3`T2 j P1 jZ 3 j ferroelectricY IVjT 4`T3 j P2 1 2 1 2 1 jZ 4 j ferroelasticX Group 2 has the transition scheme: I jT 1 j P2 1 3 jZ 1 j paraelasticY IV jT 4`T1 j P2 1 2 1 2 1 jZ 4 j ferroelasticX Examples of the ®rst group are Tl 2 Cd 2 (SO 4 ) 3 (Ikeda & Yasuda, 1975), (NH 4 ) 2 Cd 2 (SO 4 ) 3 (Glogarova & Fousek, 1973), Rb 2 Cd 2 (SO 4 ) 3 (Hikita et al, 1976) and K 2 Zn 2 (SO 4 ) 3 (Hikita et al, 1980); an example of the second group is K 2 Cd 2 (SO 4 ) 3 (Abrahams & Bernstein, 1977). KMS undergoes a cubic to orthorhombic transition at 193 K (Hikita et al, 1976) and thus belongs to the second group. Based on group theoretical considerations, Dvor Ïa  k (1972) concluded that if the high symmetry of all langbeinites is indeed P2 1 3, then in these crystals we can expect only four spatially homogeneous phase transitions with a single homogeneous order parameter.…”