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(CH₃)₄GeCl₃, undergoes a structural phase transition at 200 K from an orthorhombic room-temperature phase to an incommensurately modulated orthorhombic phase and at 170 K a further transition to a phase characterized by the coexistence of a monoclinic and an orthorhombic Bravais lattice. It is proposed that the superposition of the two Bravais lattices 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 transition at 170 K can be related to a translational and a rotational part of the displacive modulation of the rigid GeCl₃ and tetramethylammonium molecules. Experimental evidence suggests that the displacive modulation in the incommensurate phase is related to short-range disorder observed in the room-temperature phase.