Phase transitions in langasite family crystals

Possible structural changes described by the group-subgroup relationships in the Ca₃Ga₂Ge₄O₁₄-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 type can take place. The change of the trigonal symmetry to monoclinic is revealed in La ₃SbZn₃Ge₂O₁₄, whose averaged crystal structure is refined as a derivative structure of the Ca₃Ga ₂Ge₄O₁₄ structure type within the sp. gr. A2 (C2). At ∼250°C, La₃SbZn₃Ge₂O ₁₄ undergoes a reversible phase transition with symmetry rise, A2 ⇔ P312. Phase transitions, P321 ⇔ A2, are observed in La ₃Nb_0.5Ga_5.5O₁₄ and La ₃Ta_0.5Ga_5.5O₁₄ 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.