D J Lockwood

Raman spectrum of synthetic zircon (ZrSiO4) and thorite (ThSiO4)

The complete Raman spectra of single-crystal synthetic zircon and thorite are reported for the first time. Results are presented for thorite at 295K and zircon at 295 and 90K. The temperature-dependent study was essential for distinguishing between weak first-order bands and second-order features which can have narrow half-widths. The lattice vibrations in both crystals are notably harmonic. The spectra are analysed in terms of internal and lattice modes of the SiO44- complex with some success. The crystal-field splittings for the internal modes are large. A comparison between the zircon and thorite results enables an assignment to be made of the Raman spectrum of powdered hafnon (HfSiO4) reported by Nicola and Rutt (1974).

Raman scattering study of the three structural phases of KMnF3

The Raman spectra of the three phases of KMnF3 have been measured and a detailed group theoretical analysis has been carried out in order to identify the observed peaks. The results for the intermediate temperature phase are consistent with a tetragonal structure of D4h18 symmetry but the results for the low-temperature phase do not unambiguously determine the crystal structure. Particular attention was paid to the temperature-dependent modes. The soft modes associated with the upper phase transition were fitted to the response functions for a damped simple harmonic oscillator and for a model including damping by phonon density fluctuations. The results favour the latter model and suggest that non-classical critical fluctuations play an important role near the upper phase transition.

A Raman study of the ferroelectric soft mode in lead germanate

The soft mode Raman spectra of ferroelectric Pb5Ge3O11 have been carefully measured as a function of temperature. These new results were analyzed to obtain the parameters of various models for the soft mode spectral response for frequencies larger than 4 cm-1. The modes employed were: (i) a Debye relaxational function for the overdamped spectra; (ii) a classically damped harmonic oscillator at all temperatures and (iii) a model which allows for coupling between the soft mode and phonon density fluctuations. It is found that the low-frequency Raman spectra are well described by the damped harmonic oscillator model for 30K<or approximately=Tc-T<or approximately=80K. Further, the resonance frequency and damping parameters obtained do not show the expected temperature dependence but diverge for Tc-T<30K. It is shown that the self-energy of this simple model does not have the appropriate frequency dependence and that a more suitable form can be obtained when coupling of the soft mode to phonon density fluctuations is considered.

Raman scattering at structural phase transitions

The Raman scattering of light from the ferroelectric fluctuations of the uniaxial ferroelectric lead germanate is analysed and found to be well described by models for which the self-energy has appreciable dispersion at frequencies comparable with the soft-mode linewidth. A model is developed to calculate the self-energy arising from phonon-density fluctuations of the modes belonging to the same branch. The model shows that this contribution to the self-energy of the soft mode is particularly large if the modes with which it is interacting are overdamped. In detail, however, the model suggests that close to Tc there is an appreciable contribution to the Raman scattering from the interference terms between the one- and two-phonon scattering.

Raman spectroscopy and dielectric constants of ferroelectric Rochelle salt and calcium tartrate

The Raman spectrum of Rochelle salt (NaKC4H4O4d6.4H2O) has been measured in greater detail than hitherto, while the Raman spectrum of calcium tartrate (CaC4H4O6.4H4O) has been recorded for the first time; mode assignments for both materials have been made where possible. The spectrum of Rochelle salt shows no important changes on passing through either of the ferroelectric phase transitions,a nd dielectric constant measurements on this material confirm departures from Curie-Weiss behaviour. Dielectric constant measurements on calcium tartrate show no evidence of any phase transition between 250 and 495K.

Raman spectral study of the ferroelectric phase transition in boracites

Abstract The dynamics of the cubic-to-orthorhombic ferroelectric transition in Cr3B7O13Cl have been investigated using Raman spectroscopy. Two soft modes were found in the ferroelectric phase. The integrated intensity of a wing feature in the A 1 spectrum exhibits a temperature dependence that is remarkably similar to the dielectric constant behaviour in other boracites. The Raman spectrum of single-domain orthorhombic Mn3B7O13Cl is reported for comparison purposes.

Raman spectrum of AgGaS2

The first- and second-order Raman spectrum of AgGaS2 has been measured at room temperature. A careful analysis of the data gives phonon frequencies for the various representations at the zone centre which differ considerably from the results of two other investigations, and reasons are given why previous determinations are probably in error. The present results indicate that AgGaS2 cannot be regarded as a small perturbation of a zincblende structure, but they are in accord with the predictions of a recent model calculation by Bettini (1974). A comparison of the optic modes for AgGaS2, CuGaS2 and CuAlS2 shows regularities which are also apparent in the analogous phosphides, and it is inferred that there is an unobserved high-frequency B1 mode in both CuAlS2 and CdSiP2.

Raman study of the ferroelectric phase transition in copper-chlorine boracite

Abstract The dynamics of the improper ferroelectric transition in Cu3B7O13Cl have been investigated using Raman spectroscopy. Details are presented of the cubic phase spectrum and of two soft modes found in the orthorhombic ferroelectric phase. Comparisons are made with results from earlier Raman measurements on Cr3B7O13Cl.

Raman spectrum of Zn4O(BO2)6

The first-order Raman spectrum of cubic zinc metaborate has been measured at room temperature, and the number of q=0 phonon frequencies found exceeds the group theoretical prediction by one. The additional band appears as an interference feature at 122.5 cm-1 and is thought to be attributable to some impurity introduced during crystal growth. The results are in reasonable agreement with an earlier qualitative investigation of the infrared transmission spectrum of this material, and the reported fluorescent properties are verified. A comparison between the Raman results for zinc metaborate and those for Cr3B7O13Cl shows some correlations. A simple force-constant calculation identified four bands in each compound that can be associated with vibrations of BO4 tetrahedra contained within the boron-oxygen framework.

The Raman spectrum of pure and barium doped lead germanate

Abstract Observations of the Raman spectra of Pb5-x Ba x Ge3O11 show that for 0<X ≤ 0.4 while there are small changes in relative intensities and increased broadening of all the modes, the frequencies and number of both the lattice and internal modes do not appear to be substantially altered in comparison to those of the pure material. The temperature dependence of the ferroelectric soft A-mode however is dramatically affected, even for small X and this is accompanied by a much reduced transition temperature.