Phase transitions in langasite family crystals

Abstract

Possible structural changes described by the group-subgroup relationships in the Ca/sub 3/Ga/sub 2/Ge/sub 4/O/sub 14/-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by the symmetry lowering to the maximal non-isomorphic subgroups P3 and C2; transitions to other subgroups are possible also. It is shown that only destructive phase transitions with symmetry rise up to the minimal non-isomorphic supergroups for the given structure hype can take place. The change of the trigonal symmetry to monoclinic is revealed in La/sub 3/SbZn/sub 3/Ge/sub 2/O/sub 14/, whose averaged crystal structure is refined as a derivative structure of the Ca/sub 3/Ga/sub 2/Ge/sub 4/O/sub 14/ structure type within the sp. gr. A2 (C2). At /spl sim/250/spl deg/C, La/sub 3/SbZn/sub 3/Ge/sub 2/O/sub 14/ undergoes a reversible phase transition with symmetry rise,. A2/spl hArr/ P312. Phase transitions, P321/spl hArr/A2, are observed in La/sub 3/Nb/sub 0.5/Ga/sub 5.5/O/sub 14/ and La/sub 3/Ta/sub 0.5/Ga/sub 5.5/O/sub 14/ under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the a-b plane gives rise to a phase transition accompanied by the loss of the 3-fold axis. The search for low-temperature phase transitions for some compositions of the langasite family have not revealed them.