THE LOW-TEMPERATURE PHASE SEQUENCE GAMMA-DELTA-EPSILON IN HALIDE PEROVSKITE TETRAMETHYLAMMONIUM TRICHLOROGERMANATE(II) STUDIED BY X-RAY-DIFFRACTION

The halide perovskite tetramethylammonium trichlorogermanate(II), N(CH 3)(4)GeCl3, undergoes a structural phase transition at 200 K from an o rthorhombic room-temperature phase to an incommensurately modulated or thorhombic phase and at 170 K a further transition to a phase characte rized by the coexistence of a monoclinic and an orthorhombic Bravais l attice. It is proposed that the superposition of the two Bravais latti ces results from the low bulk modulus (9 GPa) in conjunction with the formation of ferroelastic domains. Local stresses implied by adjacent monoclinic domains allow the soft material to form orthorhombic domain boundaries. Two order parameters xi and eta governing the phase trans ition at 170 K can be related to a translational and a rotational part of the displacive modulation of the rigid GeCl3 and tetramethylammoni um molecules. Experimental evidence suggests that the displacive modul ation in the incommensurate phase is related to short-range disorder o bserved in the room-temperature phase.