Light scattering spectra have been measured for molten MgCl2, CaCl2, SrCl2 and BaCl2. The temperature dependence of the spectra has been studied for MgCl2 and CaCl2. Anhydrous samples were prepared from hydrated salts by dehydration under high vacuum (CaCl2 and BaCl2), by dehydration in the presence of NH4Cl followed by sublimation (MgCl2) and by dehydration and zone refining (SrCl2). The spectra, while of the same general form as those found for the alkali halides, show one marked difference in that MgCl2 exhibits a well resolved peak in the polarised spectrum. The spectra of CaCl2, SrCl2 and BaCl2 show incompletely resolved shoulders which are more pronounced than those found in the spectra of the molten alkali iodides. A new method of analysis has been applied to the data from the molten alkaline-earth chlorides. Fourier transforms have been computed and the spectra have been examined in the time domain. The subsequent analysis is based on the hypothesis that the spectra are associated with the correlation functions of the charge-rather than the mass-currents. Thus there is little or no influence from Rayleigh-Brillouin scattering but it is anticipated that there will be a three-line spectrum associated with the charge currents. These are shown to be the equivalent of the first-order Raman effect while the second-order Raman effect gives rise to high-frequency exponential tails.

The crystal and magnetic structure of Rb2CrCl4, a compound that exhibits two-dimensional ferromagnetism with nearly easy-plane behaviour, has been investigated using single-crystal neutron diffraction at 5.5, 77 and 300K. Selected Bragg reflections have also been examined at temperatures up to 573K. At all temperatures the structure is described by the space-group Cmca (D2h18), a subgroup of the K2NiF4 structure I4/mmm. There is an antiferrodistortive displacement of the Cl- ions in the basal plane such that the CrCl6 units are tetragonally elongated with principal axes alternately along (011) and (011) of the Cmca unit cell. At 5.5K the nearest-neighbour Cr-Cl distances are 2.700(7) and 2.387(6) AA within the basal plane and 2.371(22) AA parallel to the a axis. The ordered moments are canted alternately to one side and the other of the (010) or (001) axes of the Cmca cell by +or-(5.2+or-1.1 degrees ).

Satellite absorption bands near the 630 nm exciton-magnon combination bands in crystals of the ionic ferromagnet Rb2CrCl4 doped with 1.8% and 3.8% Mn2+ are identified as electronic excitations of Cr2+ coupled to spin flips on isolated Mn2+. The energies and relative intensities may be rationalised with a simple molecular field model and an estimate is made of the Cr-Mn ground state exchange constant.

The long-wavelength spin waves in Rb2CrCl4, a nearly two-dimensional ferromagnet, have been investigated at several temperatures below Tc=52.4K using neutron inelastic scattering techniques. The data have been analysed in terms of a Hartree-Fock theory using matching-matrix elements to give correctly the effects of anisotropy. Values for the parameters in the spin Hamiltonian have been found, and the theory accounts well for the energy renormalisation of the spin waves and for the transition temperature and variation of magnetic moment with temperature. Due to weak uniaxial anisotropy terms the spin wave spectrum has a small energy gap of approximately=1K at zero wavevector, which precludes true XY-type behaviour, and a canting of the spins of +or-1.1 degrees from (110) directions is predicted.

Structural phase transitions in K2CO3have been investigated using light scattering techniques. The Raman spectrum of the high-temperature hexagonal phase consists of a very broad structureless band which suggests that the system is highly anharmonic. Below the hexagonal-monoclinic transition at Tcl=693K the authors observe a soft mode which is assigned to librational motion of the CO3groups. Evidence of a cell doubling transition at Tc2=526K is found in the number of Raman active modes observed at low temperature, although a soft mode is not observed at the transition.

Under irradiation with X-rays at 4K AgCl emits a broad band peaking at 2.52 eV. This luminescence shows magnetic circular polarization with a resonance in low magnetic field. EPR has been detected in the excited state giving rise to the emission. This indicates that the self-trapped exciton is the source of the emission and that self-trapping involves the 4d wavefunction of the silver ion. In AgCl crystals containing about 2 mole per cent of bromine an additional centre emits in the 2.52 eV region and excited-state EPR shows that this is due to electron-hole recombination at silver sites perturbed by bromine impurities. AgBr emits luminescence at 2.14 eV at 4K and EPR of the excited state has been measured.

Using far-infrared absorption spectroscopy, the variation of the frequency of the zone-centre magnon in Fe1-cMgcCl2 has been measured for the complete range of c. The magnon intensity does not vanish at the percolation limit. Instead it gradually becomes an electronic transition of Fe2+ in MgCl2 as c approaches 1. A mean-field model describes the frequency variation of the absorption quite well, but no simple model gives a satisfactory description of the lineshape. The measurements provide evidence for the existence of a local magnetic mode predicted by an ATA calculation for c=0.1.