O. S. Binbrek

Raman and infrared studies of the ferroelectric transition in ammonium sulphate

Abstract The Raman and i.r. spectra of ammonium sulphate and its fully deuterated analogue have been measured at temperatures above and below the ferroelectric transition temperature. No marked changes in the spectra occur at T c , although several small peaks appear below T c and increase in intensity as the temperature is reduced. There are no anomalous changes in either frequencies or peak widths, and the normal mode frequencies of the NH 4 + ions are those expected for a tetrahedral rather than a distorted configuration. The results do not support either of the simple models of the phase transition which have been proposed.

Lattice vibrations of boron trihalide crystals

The infrared and Raman spectra of polycrystalline samples of BCl3, BBr3, and BI3 at 80 and 18 K have been recorded. In the intramolecular region, isotopic and crystalline field splittings are observed in substantial agreement with earlier work. In the lattice region, no far‐infrared absorption has been found, in accordance with predictions from a group theoretical analysis based on the known structures of these isomorphous crystals. The three observed Raman lattice modes are assigned according to these procedures, and their frequencies are compared with those derived from calculations of the lattice statics and dynamics of these crystals. It is found that a four‐parameter model using the rigid molecule approximation and simple pairwise interaction potentials between X–X and B–X atoms is capable of giving acceptable values of the zone center frequencies while simultaneously satisfying the static equilibrium conditions for these crystals.

CRYSTAL STRUCTURES OF DIBROMODICHLOROMETHANE AND BROMOTRICHLOROMETHANE

The neutron powder profiles for CBr2Cl2 and CBrCl3 have been recorded at temperatures ranging from about 260 K to 5 K. The profiles at the highest temperatures are consistent with fcc structures with a = 8.597 (2) A and a = 8.526(2) A respectively. CBrCl3 has a second plastic phase observed at 245 K. The remaining profiles, below 250 K for CBr2Cl2, or below 225 K for CBrCl3, can be analysed in terms of the C2/c space group with Z = 32. This is the same as for the ordered phases of CBr4 and CCl4. Orientational disorder of the molecules leads to the structural similarity of all members of the family. Tests for partial ordering were not successful.

Structure of solid fluoroform

The structure of fluoroform, CHF3, was determined at 4·2, 40 and 70 K using neutron powder profile analysis. There is only one phase with a monoclinic space group P21/c. At the lowest temperature the lattice parameters are: a = 4·7561(1) A, b = 6·6280(2) A, c = 8·5232(2) A β = 122·02(2)°. Starting positions and orientations of the four molecules in the unit cell were determined to high accuracy using an energy minimization process with atom-atom potentials. The structure is made up of weakly hydrogen bonded sheets with adjacent sheets related by centres of symmetry.

Raman and far infrared spectra of the two solid phases of methyl bromide: CH3Br and CD3Br

The phase transition in crystalline methyl bromide: CH3Br and CD3Br, has been studied by Raman spectroscopy. Spectra of the higher temperature α phase are reported here for the first time. The transition in CD3Br is found to occur near 158 K, about 16 K lower than in CH3Br. Raman and far infrared spectra of the low temperature β phase at 20 K are also reported. Additional peaks in the lattice region and new splittings in the υ5 and υ6 regions have been observed. The spectra are assigned by means of a group theoretical analysis based on the known crystal structure of this phase and good agreement is obtained.

The two solid phases of chlorodifluoromethane

The crystal structure of the two phases of chlorodifluoromethane (CHClF2) were determined at 70 K and 10 K using Rietveld refinement of neutron powder profiles. The upper phase is tetragonal P42/n (C4 4 h) with eight molecules in the unit cell occupying general positions. The lattice constants are a = 10•3711(1) A and c = 5•5915(2) A. The lower phase is monoclinic P112/n (C2 4 h) with eight molecules in the unit cell with dimensions a = 10•1106(2), b = 10•4830(2), c = 5•5868(2) A and γ = 90•319(2)°. The asymmetric unit has two molecules in general positions. The phase transition is of a displacive type but there are no large changes in the molecular positions or orientations at the transition.

Crystal structure of chloroiodomethane

The crystal structure of phase III of chloroiodomethane-d 2 was determined at 13 K using Rietveld refinement of neutron powder profiles. The space group is Pnma (D16 2h) and there are four molecules in its orthorhombic unit cell with dimensions a = 6·383(4) A, b = 6·706(1) A and c = 8·867(4) A. The molecules form a ‘herringbone’ pattern in layers perpendicular to the b axis, similar to that in the halogen and the hydrogen halides crystals.

CRYSTAL STRUCTURES OF DIBROMOCHLOROMETHANE AND BROMODICHLOROMETHANE

The crystal structures of CHB2C1 and CHBrC12 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. Whereas CHBr3 has three phases, both CHBr2C1 and CHBrC12 have only one. The two CHBr3 phases which are absent in these compounds require the molecules to have threefold axes. The degree of disorder of the halogen atoms has been examined and the results indicate that the Br and C1 atoms have definite preferences for some sites over others, so that the molecules on average do not have a threefold axis.

Neutron powder-profile study of chlorofluoromethane

The crystal structure of chlorofluoromethane, CH(2)ClF, has been determined at 100 and 30 K using indexing, packing considerations and Rietveld refinement of neutron powder profiles. There is only one phase, in monoclinic space group P2(1) and with two molecules in the unit cell occupying general positions. The structure has close packing in several directions and there are weak hydrogen bonds forming zigzag chains.

CRYSTAL STRUCTURE OF DICHLOROFLUOROMETHANE

The crystal structure of dichlorofluoromethane or freon 21 (CHFCl2) has been determined at temperatures ranging from 5 K to 105 K using neutron powder profile techniques. There is only one phase present at all temperatures, with eight molecules per unit cell positioned on general sites in the space group Pbca. Starting positions and orientations of the molecules were determined to high accuracy using an energy minimization process with atom-atom potentials. Coulombic interactions, as represented using atomic charges, were shown to be unimportant in determining the structure.