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International audienceX-ray diffraction measurements on MnGa2Se4, a II-III2-VI4 magnetic semiconductor compound, are made as a function of pressure up to 25 GPa. It is found that in the range of 0P12 GPa the structure is a defect tetragonal; in the range between 12 and 14 GPa a two phase field +NaCl type exists. It is found that at about 14 GPa a transition to the cubic NaCl-type structure occurs and it appeared that this transition is irreversible. From the analysis of the x-ray diffraction lines, lattice parameter values are determined as a function of pressure. These results allow the evaluation of the compressibility coefficients of the corresponding parameters. The observed behavior of , a, and c with pressure are explained by using the relations proposed by Abrahams and Bernstein J. Chem. Phys. 52, 5607 1970. It is found that at about 3 GPa, a symmetry change, from an ordered tetragonal to a disordered tetragonal structure, occur

From the study of the temperature dependence of the optical absorption spectra, the energy gap EG of CuIn3Se5 between 10 and 300 K are calculated using the model proposed by Elliot. This variation is compared to the semiempirical relation suggested by Manoogian–Woolley. The Debye temperature ΘD, the dielectric constant ε0, and the effective masses of free excitons mex, electrons me, and holes mh are estimated from the analysis of the adjustable parameters of these models.

This paper reports on the pressure dependence at room temperature of the absorption coefficient of the layered semiconductor gallium telluride. The absorption edge in the explored pressure range ͑up to 6.1 GPa͒ can be accounted for through the superposition and interaction of a direct gap and an indirect gap. The pressure shift of the direct gap is strongly nonlinear, starting at low pressure with a coefficient of Ϫ85.7 Ϯ0.4 meV/GPa and exhibiting a minimum at around 2.9 GPa. The exciton binding energy decreases under pressure, which can be related to the increase of the dielectric constant. The broadening of the exciton line is explained through phonon-assisted intervalley scattering of conduction-band electrons after the crossover between direct and indirect gaps, with an estimation of the related intervalley deformation potential. ͓S0163-1829͑99͒16035-1͔

The temperature dependence of the energy gap EG and the binding energy Rx of free excitons in single crystals of CuInTe2 have been calculated using Elliot’s model. The samples were prepared by the method of tellurization of stoichiometric Cu and In in liquid phase and the vertical Bridgman technique. The value of Rx around 4 meV agrees quite well with that deduced from the effective mass approximation. The variation of EG with temperature is compared with the empirical model proposed by A. Mannogian and J. C. Woolley [Can. J. Phys. 62, 285 (1984)]. The estimated value of the Debye temperature is in agreement with ΘD=191.4 K reported from specific heat measurements.

A comparative study of the Raman spectra of Cu2BIICIVS4VI and Cu2BIICIVSe4VI(where B = Mn or Fe) magnetic quaternary semiconductor compounds with stannite-type structure (I4¯2m) has been done. Most of the fourteen Raman lines expected for these materials were observed in the spectra. The two strongest lines observed have been assigned to the IR inactive A11 and A12 stannite modes that originated from the motion of the S or Se anion around the Cu and CIV cations remaining at rest. The shift in the frequency of these two lines of about 150 cm−1 to lower energies observed in Cu2BIICIVSe4VI compounds as compared to those in Cu2BIICIVS4VI ones, can then be explained as due to the anion mass effect. Based on the fact that values of these frequencies depend mainly on anion mass and bond-stretching forces between nearest-neighbor atoms, the vibrational frequencies v¯(A12) and v¯(A12) of both modes for several Cu2BIICIVX4VI stannite compounds (where X = S, Se, or Te) very close to the experimental data reported for these materials were calculated from a simple model that relates these stretching forces to the anion-cation bond-distances.

We have examined several sets of heterogeneous double wall carbon nanotubes and analysed the frequency in a wide range of values. We have found that a statistical correlation exists between the D band intensity and the RBMs intensity. All the sample sets show a correlation of the RBM and D Raman intensity which is monotonic and non-linear using the 514nm laser excitation. Our results suggest that a similar mechanism is at the origin of the RBM and D bands. These results are corroborated by observations on carbon nanotubes embedded in a polymer matrix. We derive the phonon potential deformation for the D band and extend the excitation wavelength dependence of the D band to the UV. The intensity of the D band decreases with increasing hydrostatic pressure and fanishes at a pressure comparable to what has been observed earlier for the RBM and D* band.

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