Structure of the solid solution [N(CH3)4]2ZnCl1.8Br2.2 in the high‐ and in a new low‐temperature phase

[N(CH3)4]2ZnCII.sBr2. 2, Mr=453 .3 , hereafter [TMA]EZnHal 4. At 293K, orthorhombic, Pnma, a = 1 2 . 5 3 ( 1 ) , b = 9 . 1 4 ( 3 ) , c = 1 5 . 8 3 ( 4 ) A , V = 1812.9 (7)/~3, Z = 4, D x = 1.66 Mg m -3, g = 6.76 mm -~, F(000) = 894-4, R = 0.066 (wR -0.072) for 1147 observed reflexions on a four-circle diffractometer, 2(MoKa)=0.7107/~, . The TMA groups exhibit large oscillations. At 198K, monoclinic, P121/al, a=24.84(5) , b=9.09(6) , c=15.64 (5)A, f l=89 .43 (4 ) °, v = a 5 3 1 . a ( 4 ) A 3, Z = 8 , Dx= 1.70 Mg m -3, g = 6.89 mm -~, F(000) = 1788.8, R = 0.074 (wR = 0.088) for 1169 observed reflexions. Domains were encountered in the crystal and a large decrease of the temperature factor was observed. The phases studied are separated by two other phases: an incommensurate one and a monoclinic one with a threefold unit cell (Z = 12). The ZnHal 4 tetrahedra drive the transitions, rotating principally around the a axis (11 °) and shifting their centre along the b axis (0.42/~). Their small distortion is probably due to the crystal field effect (Hal-Zn-Hal angles range from 108.7 to 112.2°). The mean bond length Zn-Hal is 2.352/~ for Pnma and 2.360/~ for P12Jal. Tetrahedra in the same bc plane rotate in the same sense. Introduction. Recently the (x,T) phase diagram of the solid solution [TMA]2ZnC14_.eBrx was determined by means of DSC and X-ray measurements (Colla, Muralt, Arend, Ehrensperger, Perret, Godefroy & Dumas, 1984). The system shows complete miscibility over the whole concentration range with common high-temperature symmetry Pnma. The phases found are I (Pnma, a = a 0, Z = 4), II and II' (incommensurate, 2/5 + 6 and 1/3 + 6, along x), III (Pn2~a, ferroelectric, a=5a0) , IV (P21/nll, a=3a0) , V (P112Ja, a -a 0) in two different regions, VI (P2~2~21, a=3ao), and VII (P12~/al, a=2a0) . In order to clarify the mechanism of the successive phase transitions of this system, details of the structure at each phase are desired. The structures of [TMA]2ZnBr 4 at room temperature (phase I) and in the low-temperature 0108-2701/87/061070-04501.50 configuration (phase IV) have already been determined by Trou6lan, Lefebvre & Derollez (1984). Phases IV, V and VII undergo proper or improper ferroelastic phase transitions and are usually twinned. We report the structure of a solid-solution system ( x = 2.2) in phase I (293 K) and in the new phase VII (198K) which does not occur in the pure compounds. It is stable down to T = 20 K and exists for x ranging from 0.4 up to 3.0. An efficient method of coping with ferroelastic domainproblems is shown. Experimental. Homogeneous solid-solution crystals were obtained by a growth procedure based on a temperature-difference growth technique with thermally enforced convection and the use of equilibrated solid and aqueous solutions (Arend, Perret, Wriest & Kerkoc, 1986). The composition was determined chemically. A specimen of nearly spherical shape (0.4 mm diameter) was fixed on a capillary tube and coated with epoxy-type resin (Araldite). The temperature of the specimen was controlled at 293.0 (5) K and 198.0 (5) K by a regulated stream of nitrogen gas. Data were collected on an automatic Syntex P2~ four-circle diffractometer with graphite-monochromated Mo Ks radiation. The orientation matrix and the cell parameters were refined by least-squares calculations based on 25 reflexions in the range 6 3tr(/) in the range 0 , 0 , 0 3a(/) in the range 0,0,10 < hkl < 13,6,10. The weights were calculated as 1/tr(1). The XRA Y72 crystallographic programs (Stewart, Kruger, Ammon, Dickinson & Hall, 1972) and MULTAN80 (Main, Fiske, Hull, Lessinger, Germain, Declercq & Woolfson, 1980) were used to determine the structure on a CDC computer; H-atom positions not found; scattering factors from Cromer & Mann (1968); anomalous-dispersion terms from Cromer (1965); no absorption or extinction corrections; anisotropic LS refinement based on F values; estimated agreement factors S = 1-75 and 2.32, ratio of max. shift to e.s.d. 1.1 and 0.9, max. heights in final difference Fourier synthesis smaller than 1.6 and 0.8 e A -3 for the highand low-temperature phases