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

Abstract

The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl 2 Cd 2 (SO 4 ) 3 have been determined at 121 and 85 K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO 4 tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO 4 groups.

Proper ferroelectric transition in the multiferroic YMnO3

“…Improper FE exhibit a saturation polarization that is much smaller than for proper FE. BaAl 2 O 4 (P s ~ 0.08 µC/cm 2 ) [16], Mg 3 B 7 O 13 Cl (P s ~ 0.08 µC/cm 2 ) [18] and Tl 2 Cd 2 (SO 4 )3 (Ps ~ 0.07µC/cm 2 ) [19] have significantly smaller polarization than YMnO 3 (P s ~ 5.5 µC/cm 2 ) [20]. We conclude that the space-group of the IP is P6 3 /mcm, where the K 1 mode causes a tripling of the cell at T IP , and the polarization mode Γ 2 yields to a proper ferroelectric state below T FE .…”

X-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinites

“…However, structural investigations of the lowtemperature phases on type I langbeinite have not been carried out owing to the experimental dif®culties arising from the presence of a high number of twin domains. Only recently have the structure of the monoclinic and orthorhombic phases in TCdS been solved (Guelylah et al, 2000). This work revealed some mechanisms, responsible for the phase transitions sequence, which are different from those observed in type II langbeinite.…”

“…Our next step was to perform a chain-adapted symmetry mode analysis to separate the global structural distortion relating the monoclinic and the cubic structures in primary (which trigger the transition) and secondary modes (Aroyo & Pe  rez-Mato, 1998). Examples of this type of analysis can be found in Aroyo & Pe  rez-Mato (1998), Rae et al (1990) and Guelylah et al (2000). In such an analysis the atomic displacement is decomposed in displacements whose symmetries appear in the group±subgroup chain (Fig.…”

Two new phosphate langbeinites, Rb2YbTi(PO4)3and Rb2Yb0.32Ti1.68(PO4)3, investigated at 293 and 150 K

“…Below room temperature, most of them undergo one or several phase transitions, with two possible transitions paths. The ®rst path includes several steps from P2 1 3 to the ®nal orthorhombic space group P2 1 2 1 2 1 , as exempli®ed by the phase transitions of Tl 2 Cd 2 (SO 4 ) 3 (Brezina & Glogarova, 1972;Guelylah et al, 2000): P2 1 3 (RT) 3 P2 1 (128 K) 3 P1 (119 K) 3 P2 1 2 1 2 1 (98 K).…”