A. P. Litvinchuk

Raman and infrared-active phonons in hexagonal HoMnO3 single crystals: magnetic ordering effects

Polarized first- and second-order Raman scattering and infrared reflection spectra of hexagonal HoMnO3 single crystals in the temperature range 10–300 K are reported. Based on the symmetry analysis and comparison with the results of lattice dynamics calculations the observed lines are assigned to the lattice eigenmodes. The magnetic ordering of Mn ions, which occurs below TN = 76 K, is shown to affect Raman- and infrared-active phonons, which modulate Mn–O–Mn bonds and, consequently, the Mn–Mn exchange interaction.

Electronic band structure and optical phonons of BaSnO3 and Ba0.97La0.03SnO3 single crystals: Theory and experiment

Optical properties of insulating BaSnO3 (BSO) and conducting Ba0.97La0.03SnO3 (BLSO) single crystals were studied at room temperature in a wide spectral range between 0.01 and 5.9 eV by means of spectroscopic ellipsometry. The far-infrared spectra of the optical phonons in BSO and BLSO were complemented by polarized Raman scattering measurements in BSO. The electronic band structure and the optical response (dielectric function) were calculated using density functional theory, which allowed for the interpretation of the main spectroscopic features such as optical phonons and electronic interband transitions. To reconcile the observed experimental spectra with the theory, a departure from the ideal perovskite structure on the local scale was proposed for BSO.

Optical and electronic properties of metal doped thermoelectric Zn4Sb3

Optical and electronic properties of metal (Pb, Bi, Sn, and In) doped Zn4Sb3 are reported in the temperature range 80–300 K, which covers the β, α, and α′ structural phases of this thermoelectric material. Metal doping alters the subtle balance between Zn disorder and Zn deficiency present in β-Zn4Sb3 and changes its low temperature structural behavior. Analysis of infrared reflection data shows that the formation of ordered α′-Zn4Sb3 is accompanied by a substantial increase in the free charge-carrier concentration. In contrast, for samples where doping suppresses the occurrence of the low temperature α′-phase, the free charge-carrier concentration is only weakly temperature dependent. Different degrees of structural disorder in doped β-Zn4Sb3 and the ordering processes at low temperatures leading to α- and α′-Zn4Sb3 are also recognized in the charge-carrier dynamics.

Structural, transport, magnetic properties and raman spectroscopy of orthorhombic Y1-xCaxMnO3 (0 ≤ x ≤ 0.5)

Orthorhombic Y1−xCaxMnO3 (0≤x≤0.5) was prepared, and the variations with x of its structural, magnetic, and electrical properties and the polarized Raman spectra were investigated. The lattice parameters change systematically with x. Although there are strong indications for increasing disorder above x = 0.20, the average structure remains orthorhombic in the whole substitutional range. Ca doping increases the conductivity, but the temperature dependence of resistivity ρ(T) remains semiconducting for all x. The average magnetic exchange interaction changes from antiferromagnetic for x 0.08. The evolution with x of the Raman spectra provides evidence for an increasingly disordered oxygen sublattice at x≥0.10, presumably due to quasistatic and/or dynamical Jahn–Teller distortions.

Optical phonons in the kesterite Cu2ZnGeS4 semiconductor: polarized Raman spectroscopy and first-principle calculations

A comprehensive vibrational analysis of the kesterite Cu2ZnGeS4 semiconductor (space group I) is reported, which includes experimental in-plane rotation polarized Raman scattering measurements from the (1 0 1)-single crystal facet as well as first principle lattice dynamic calculations. 17 out of the 27 expected vibrational modes of the kesterite structure are unambiguously identified. Raman scattering measurements performed under resonant excitation conditions show a pronounced enhancement of the longitudinal optical components of the polar modes. The appearance of several additional lines in the Raman spectra have been interpreted as being due to the presence of inclusions of Cu2ZnGeS4 polymorph with the P2c lattice symmetry, which differs from the kesterite by cation sublattice arrangement.

Fermi resonance in the phonon spectra of quaternary chalcogenides of the type Cu2ZnGeS4.

The experimental resonant and non-resonant Raman scattering spectra of the kesterite structural modification of Cu2ZnGeS4 single crystals are reported. The results are compared with those calculated theoretically within the density functional perturbation theory. For the majority of lines a good agreement (within 2-5 cm(-1)) is established between experimental and calculated mode frequencies. However, several dominant spectral lines, in particular the two intense fully symmetric modes, are found to deviate from the calculated values by as much as 20 cm(-1). A possible reason for this discrepancy is found to be associated with the Fermi resonant interaction between one and two-phonon vibrational excitations. The modelling of spectra, which takes into account the symmetry of interacting states, allows a qualitative description of the observed experimental findings. Due to the similarity of the vibrational spectra of Cu2A (II) B (IV) S4 (A  =  Zn, Mn, Cd; B  =  Sn, Ge, Si) chalcogenides, Fermi resonance is argued to be a general phenomenon for this class of compounds.

Lattice dynamics and superionic properties of Cd1−xPbxF2 crystals

Abstract Reflectivity in the infrared range, Raman scattering and ionic transport properties in Cd 1− x Pb x F 2 crystals have been investigated. A “one-mode” behaviour of these crystals has been found. The role of the PbF 2 cation sub-lattice in the anion disordering process has been discussed. A correlation has been found between the ionic transport properties and the fluorine-cation interaction coefficient for Cd 1− x Pb x F 2 crystals.

Structural Polymorphism in “Kesterite” Cu2ZnSnS4: Raman Spectroscopy and First-Principles Calculations Analysis

This work presents a comprehensive analysis of the structural and vibrational properties of the kesterite Cu2ZnSnS4 (CZTS, I4̅ space group) as well as its polymorphs with the space groups P4̅2c and P4̅2m, from both experimental and theoretical point of views. Multiwavelength Raman scattering measurements performed on bulk CZTS polycrystalline samples were utilized to experimentally determine properties of the most intense Raman modes expected in these crystalline structures according to group theory analysis. The experimental results compare well with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Vibrational patterns of the most intense fully symmetric modes corresponding to the P4̅2c structure were compared with the corresponding modes in the I4̅ CZTS structure. The results point to the need to look beyond the standard phases (kesterite and stannite) of CZTS while exploring and explaining the electronic and vibrational properties of these materials, as well as the possibility of using Raman spectroscopy as an effective technique for detecting the presence of different crystallographic modifications within the same material.

Optical properties of quaternary kesterite-type Cu2Zn(Sn1−xGex)S4 crystalline alloys: Raman scattering, photoluminescence and first-principle calculations

The transformation of the vibrational spectrum of Cu2Zn(Sn1−xGex)S4 single crystals over the entire composition range (0 ≤ x ≤ 1) is studied experimentally by low-temperature Raman scattering and photoluminescence spectroscopies, as well as theoretically in the framework of density functional theory (DFT). It is shown that unlike “classic” mixed binary II–VI and III–V compounds, which are characterized by either one- or two-mode behavior of spectra transformation upon composition variation, the vibrational modes of the quaternary semiconductor Cu2Zn(Sn1−xGex)S4 exhibit both types of behavior within the same alloy system. DFT calculations reveal that the two-mode transformation is in fact observed for the vibrational modes, which possess a very small dispersion across the Brillouin zone, that is typical for a molecular crystal. These modes are due to the “breathing” motion of sulfur within GeS4 and SnS4 tetrahedra. The effects of structural (positional) disorder of mixed crystals are analyzed based on Raman scattering as well as photoluminescence results.

Raman Spectra of the Half-Metallic Ferromagnet CrO2

Chromium dioxide, CrO2, attracts significant interest as it has a unique electronic band structure, resulting in half-metallic ferromagnetism (TC ≈ 390 K) with completely spin-polarized electrons at EF. In this work we report the polarized Raman spectra of CrO2 between 5 and 420 K, as obtained from (110) and (001) oriented thin films on TiO2 substrates of the same orientation. The Raman lines corresponding to the four Raman allowed phonon modes of A1g, B1g, B2g and Eg symmetry were identified. The two low frequency Raman lines are very narrow, in particular at low temperatures, indicating weak scattering of the corresponding B1g and Eg phonons. The Raman intensities are not strongly affected by the magnetic ordering, whereas the line position versus T dependencies change their slope near TC for the A1g and Eg modes. At low temperatures the variations with T of the phonon linewidths follow the dependence expected for a phonon–phonon scattering. The additional broadening, which appears with approaching TC is tentatively assigned to the scattering of phonons from collective spin fluctuations.