S. S. Mitra

Pressure and Stress Dependence of the Refractive Index of Transparent Crystals

The pressure derivative of the refractive index (dn/dP) and the elastooptic constants (P(ij)) in the transparent frequency regime of semiconducting and ionic crystals are investigated theoretically. The electronic contribution to dn/dP of semiconductors is obtained by carrying out pseudopotential calculations of the band structure as a function of hydrostatic pressure, and the results compared with experiment. The lattice contribution to dn/dP is obtained by relating dn/dP to changes in the effective ionic charge and the phonon spectrum as functions of pressure. As for the P(ij), we perform a detailed application of the theory of Humphreys and Maradudin to calculate these for a variety of cubic crystals as functions of frequency in the transparent regime. The parameters required in the calculation are determined from improved prescriptions, which relate various microscopic functions to experimental data on the pressure dependence of phonon frequencies. The theoretical results are checked employing a relatio between dn/dP and the P(ij). Overall, one finds that frequency dispersion is most important for the ionic materials and is generally negligible for the more highly covalent materials.

Line shape and temperature dependence of the first order Raman spectrum of diamond

Abstract It is shown that a modified Voigt profile best represents the first order Raman line shape of diamond. The temperature dependence of the peak position and the width of this spectrum has been measured in the temperature range of 25–1000 K. The shift of the peak position can be almost fully accounted for by thermal expansion alone. A small contribution due to anharmonic effects becomes evident for temperatures above 700 K. The half-width data agree well with a two-phonon decay process.

Infrared and Raman Spectra Due to Lattice Vibrations

The infrared and Raman spectra of a solid may arise from a number of causes. We shall, however, limit our discussion here to the infrared absorption and reflection and Raman scattering in a crystal due to the interaction of the incident electromagnetic fields with its vibrational modes. Related topics such as infrared spectra due to the transitions among the low-lying electronic levels of a solid, spectra due to impurities or free carriers in a semiconductor, the absorption of radiation due to the excitation of a local mode around an impurity center in a solid, the various magneto-optic effects in semiconductors observed usually in the far infrared, and electronic Raman spectra of solids shall not be included in the present article.

Pressure dependence of Raman spectra of solids. Phase transition in TlI

Abstract A hydrostatic pressure cell that can be used for the study of laser excited Raman spectra of solids as functions of pressure (up to 10 kbar) is described. Results on pressure induced phase transition and long-wavelength phonon frequency shifts of TlI are reported.

Mode‐Grüneisen Parameters and Thermal‐Expansion Coefficient of NaCl, CsCl, and Zinc‐Blende‐Type Crystals

The temperature dependence of the coefficient of thermal expansion of a number of ionic and partially ionic crystals is calculated using a modified rigid ion model and a shell model of lattice dynamics. The model parameters are obtained from long‐wavelength optic mode frequencies, elastic constants and the high‐and low‐frequency dielectric constants. Using the pressure derivatives of these quantities, mode‐Gruneisen parameters as functions of wave vectors are next obtained. Thermal expansion coefficient is calculated from the Gruneisen parameters in the quasiharmonic approximation. The calculated values compare well with experimental data.

Lattice dynamics of cubic zinc sulfide

Abstract The dispersion curves for the lattice vibrations in cubic ZnS have been calculated using a rigid ion model. The TA (L) branch compares well with the same determined from Raman measurements on polytypes. Also reasonable agreement is obtained with the neutron scattering results of Feldkamp et/al. for the acoustic branches in the [100] and [111] directions.

Properties of RF sputtered hydrogenated amorphous germanium-silicon alloys

Abstract Amorphous GeSi:H alloys have been prepared by rf sputtering in an argon-hydrogen atmosphere of varying hydrogen partial pressures. Electrical transport properties in the temperature range of 80 K to 373 K and optical and structural properties of these films have been measured. It is found that for a fixed GeSi composition, the hydrogen content strongly affects the electrical conductivity. The absorption edge has been seen to suffer an initial blue shift yielding to a small red shift at higher hydrogen concentrations and is consistent with the conductivity data. The optical pseudo-gap obtained approaches closer to twice the corresponding mobility gap at higher hydrogen concentrations. Structural studies by Raman scattering confirm the amorphous nature of the films. Raman spectra also reveals the presence of various hydrogenic and lattice modes attributable to siliconhydrogen, germaniumhydrogen, SiSi and GeGe interactions. The depolarization ratio ϱ, measured for both unannealed annealed samples, indicates stabilization, due to annealing at selected temperatures. In view of the development of possible material for photovoltaic conversion of solar energy, it is suggested that by an appropriate choice of a fixed GeSi composition and varying the hydrogen content it will be possible to optimize the electrical conductivity and optical band gap in sputtered hydrogenated a-GeSi alloys.

Vibrational spectra of fluorohafnate glass

Abstract We report the first detailed measurements of fundamental vibrational spectra in fluorohafnate glass. The Raman spectrum is dominated by a single relatively broad peak in the vicinity of 570–590 cm −1 attributed to Hf-F stretching modes, while the infrared spectrum displays two prominent broad peaks. The location of the high frequency peaks is shown to be consistent with the observed position of the infrared absorption edge.

Temperature dependence of the i.r. spectra of liquid crystalline p-heptoxybenzoic acid and p-octoxybenzoic acid

Abstract The infrared spectra of liquid crystalline p -heptoxybenzoic acid (HOBA) and p -octoxybenzoic acid (OBA) were measured as a function of temperature and a direct correlation was obtained between the phases and the type and extent of hydrogen bonding. New bands were observed at ∼3550 and 1730 cm −1 at the smectic—nematic transition; these were assigned to the monomer. Bands due to a cyclic dimer wore observed at ∼2900 and 1675 cm −1 in all phases, whereas bands due to an open dimer were observed at ∼3300 and ∼1680 cm −1 in the spectra of the nematic and isotropic phases. The spectra of both liquid crystals in dilute CCl 4 solution were measured as a function of concentration and temperature. Bands due to the OH and CO stretching vibrations of the monomer and cyclic dimer were observed in the spectra of the dilute solutions. In addition, Δ H ∘ and Δ S ∘ values were obtained from the solution spectra.

Applicability of a rigid-ion model to zinc-blende crystals☆

Abstract The applicability of the rigid-ion model to the phonon dispersion in zinc-blende type crystals is discussed. An invariance relation has been obtained connecting the elastic constants C 11 , C 12 and C 44 and the long wavelength longitudinal and transverse optic mode frequencies. This relation is found to apply well in the case of a number of II–VI and III–V compounds. An empirical relation connecting the ratio of the zone boundary (at X C.P.) LO and LA phonon frequencies with the mass ratios for diatomic cubic crystals is confirmed by the present model. The available neutron scattering and specific heat data on zinc-blende crystals is discussed in the light of the present model.