The low-temperature phase transition sequence of the halide perovskite tetramethylammonium trichlorogermanate(II) and the structure of its incommensurately modulated δ-phase

Tetramethylammonium trichlorogermanate(II), N(CH 3 ) 4 GeCl 3 , abbreviated as TGC, has been investigated by temperature-dependent X-ray powder and single-crystal diffraction. It is shown that TGC has a one-dimensional incommensurately modulated phase, called the δ-phase, with primary modulation wavevector q≃0.14b*. This phase is stable in the temperature range 170 ≤ T ≤ 200 K and occurs between the room-temperature phase γ and a lower-temperature phase e. The structure of δ-TGC was refined in the (3+1)-dimensional superspace group Pnam(0β0)00s. The average structure of δ-TGC with lattice parameters 13.083 (6) x 8.841(4) x 9.027 (4) A 3 (T = 190K) is isomorphous to the structure of the normal room-temperature phase γ. The latter is of distorted perovskite-type [Depmeier, Moller & Klaska (1980). Acta Cryst. B36, 803-807]. The modulation in the case of the δ-phase is purely displacive, the dominant modulation wave being polarized along the c-axis and of magnitude 0.42 A. The low-temperature phase e is monoclinic with lattice parameters 13.068 (6) x 8.808 (5) x 8.962(6)A 3 , α = 93.9 (1)° at T = 160 K. The intensity of the incommensurate satellite reflections vanishes at the δ-e transition. The observed diffraction patterns suggest that the δ-e transition at 170K is also associated with the partial reappearance of the orthorhombic lattice of the γ-phase. In fact, both single-crystal and powder diffraction display the superposition of a monoclinic and an orthorhombic reciprocal lattice, which persists from T δ-e = 170 K down to at least 130 K. The intensity ratio of monoclinic to orthorhombic reflections is ca 5:1 and is essentially constant for T < 160K.