THE LOW-TEMPERATURE PHASE-TRANSITION SEQUENCE OF THE HALIDE PEROVSKITE TETRAMETHYLAMMONIUM TRICHLOROGERMANATE(II) AND THE STRUCTURE OF ITS INCOMMENSURATELY MODULATED DELTA-PHASE

Tetramethylammonium trichlorogermanate(II), N(CH3)(4)-GeCl3, 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 delta-phase, with primary modulation wavevector q approximate to 0.14 b*. This phase is stable in the temperature range 170 less than or equal to T less than or equal to 200 K and occurs between the room-temperature phase gamma and a lower-temperature phase epsilon. The structure of delta TGC was refined in the (3+1)-dimensional superspace group Pnam(0 beta 0)00s. The average structure of delta-TGC with lattice parameters 13.083 (6) x 8.841 (4) x 9.027(4) Angstrom(3) (T = 190 K) is isomorphous to the structure of the normal room-temperature phase gamma. The latter is of distorted perovskite-type [Depmeier, Moller & Klaska (1980). Acta Cryst. B36, 803-807]. The modulation in the case of the delta-phase is purely displacive, the dominant modulation wave being polarized along the c-axis and of magnitude 0.42 Angstrom. The low-temperature phase epsilon is monoclinic with lattice parameters 13.068 (6) x 8.808 (5) x 8.962(6)Angstrom(3), alpha = 93.9(1)degrees at T = 160 K. The intensity of the incommensurate satellite reflections vanishes at the delta-epsilon transition. The observed diffraction patterns suggest that the delta-epsilon transition at 170 K is also associated with the partial reappearance of the orthorhombic lattice of the gamma-phase. In fact, both single-crystal and powder diffraction display the super-position of a monoclinic and an orthorhombic reciprocal lattice, which persists from T-delta-epsilon = 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 < 160 K.