AB INITIO CALCULATIONS OF OPTICAL PHONON DEFORMATION POTENTIALS IN DIAMOND AND ZINC-BLENDE SEMICONDUCTORS

The behavior of the internal lattice vibrational modes in (NH₄)₂Te_xSn_1-xCl₆ mixed crystals has been investigated by Raman scattering. All of the Raman-active [MX₄]^2- internal modes show two-mode property in the mixed crystals with compositions from x =0 to 1, and apart from the sample with x = 0.5, their energy positions are composition-independent, Space group calculations including site symmetry considera-tions and frequency fittings describe a good force field model. It has been demonstrated that the composition-independent two-mode behavior may come from the strong clustering effect, rather than from the character of the gap and local modes of impurities in mixed crystals. For the sample with x=0.5, the two component elements seem to form a pseudo-periodic lattice structure.     

Raman Spectrum Study on the Doping Effect in LiNbO3: MgO Crystals

Raman spectra of LiNbO₃: MgO crystals have been obtained and compared with Raman spectra of pure LiNbO₃ crystals. For both of the two kinds of Raman active modes (4A₁+9E), no changes of the numbers, frequency-shifts, and relative intensities of the Raman spectral lines are found. However, it is observed that some modes coupled to each other at room temperature. The observed modes coupling phenomena, disappeared at low temperature. It is shown from these results that the Raman spectral lines of A₁ (TO) and E (TO) vibrational modes are mainly determined by the (NbO₆) oxygen octahedra characteristic groups of LiNbO₃ crystals.     

Raman scattering from GaSxSe1-x

The non resonant Raman scattering spectra of GaSe, GaS and of eight different mixed crystals GaS_xSe_1-x are presented. The frequencies of some vibrational modes shift continuosly from GaSe to GaS indicating that the mixed crystals are homogeneous. The discontinuous shift of some other modes is explained either by the change of the stacking of the layers, or by a ‘two mode’ behaviour as previously observed in other types of mixed crystals. Moreover, local vibrational modes are observed.     

Polarized Raman Study On LiNbO3

Polarized Raman was used to study the lattice vibrations in LiNbO₃ single crystals which temperature range from 291K to 11K. The 13 vibrational modes expected from group-theoretical analysis were obtained. The nine internal modes and four external modes of NbO₆ octahedra have been assigned for the first time.     

Structural characterization of Lu0.7Y0.3AlO3 single crystal by Raman spectroscopy

Lutetium yttrium orthoaluminate perovskite [Lu_0.7Y_0.3AlO₃ (LuYAP)] crystals show great potential as fast response and high‐efficiency scintillators as an alternative to LuAlO₃ (LuAP) crystals. This paper aims to offer a complete analysis of the vibrational modes of the LuYAP crystal by means of polarized Raman spectroscopy in the temperature range of 20–290 K and confronting the experimental results with numerical methods in order to characterize the crystal structure. Furthermore, an interpretation of the data in terms of vibrating molecular structures is suggested and the results are successfully confronted with the vibrational modes of similar perovskite crystals. Copyright © 2009 John Wiley & Sons, Ltd.     

Die Eigenschwingungen des Metall-Hydratkomplexes in BeSO4·4H2O und BeSO4·4D2O

Infrared reflection spectra of single crystals of BeSO₄·4H₂O and BeSO₄·4D₂O have been obtained in polarized light at 300°K and at 14°K in the region between 4000 cm^?1 and 300 cm^?1. By a Kronig-Kramers analysis, the frequencies of the infrared active transitions have been calculated. These transitions are attributed to internal vibrations of the water molecules and sulfate ions and, in the region between 1000 cm^?1 and 300 cm^?1, especially to internal and external vibrations of the tetrahedral Be⁺⁺·4aqu-complexes. The vibrational modes of these complexes consist of a superposition of translational and librational modes of the water molecules and translational modes of the central Be⁺⁺-ion. The vibrational frequencies and normal modes of this complex have been calculated in a central-force model, and force-constants have been determined by fitting the calculated frequencies to the observed spectra. The calculations have shown that the modes, which comprise mainly translational motions of the water molecules, are strongly coupled with librational motions of the water molecules. On the other hand, there exist pure librational modes with practically no admixture of translational motions. The optimum sets of force constants for the BeSO₄·4H₂O crystal and the BeSO₄·4D₂O crystal differ in a manner which can be understood under the assumption that the dimensions of the Be(D₂O)₄ complex are about 0.1 Å larger than those of the Be(H₂O)₄ complex. Some arguments supporting this conclusion will be discussed.     

Long wavelength lattice vibrations in GaS and GaSe

The first order Raman spectra and far infrared absorption spectra of GaS and GaSe have been investigated and interpreted. A correspondence is drawn between the observed vibrational modes of the two crystals and a Raman active polar longitudinal mode has been identified in GaSe. The results are consistent with the existence of GaS in a β-polytype (D_6h) and GaSe in an ?-polytype (D_3h).     

The optical spectroscopy of chromium in ed-2 silicate glass

The technique of fluorescence line narrowing is used to probe the inhomogeneously broadened ²E-⁴A₂ transition of Cr³⁺ in a silicate glass. The resultant sharp zero-phonon line is accompanied by a vibrational sideband which carries information about the range of vibrational modes in the glass and about the strength of the orbit-lattice interaction. The homogeneous broadening of the zero-phonon line is measured as a function of temperature and compared with the analogous broadening in Cr-doped crystals. The broadening of the zero-phonon line in the glass at high temperatures is attributed to a Raman relaxation mechanism such as is observed in crystals. At low temperatures, however, there is an additional broadening mechanism which has an approximately linear dependence on temperature and which has no analogue in crystalline materials.     

Micro-channel fabrication by femtosecond laser to arrange neuronal cells on multi-electrode arrays

The surface of a single-crystal germanium wafer was transformed to crystals of germanium fluorides and oxides upon exposure to a vapor of HF and HNO₃ chemical mixture. Structure analysis indicates that the transformation results in a germanate polycrystalline layer consisting of germanium oxide and ammonium fluogermanate with preferential crystal growth orientation in 〈101〉 direction. Local vibrational mode analysis confirms the presence of N–H and Ge–F vibrational modes in addition to Ge–O stretching modes. Energy dispersive studies reveal the presence of hexagonal α-phase GeO₂ crystal clusters and ammonium fluogermanates around these clusters in addition to a surface oxide layer. Electronic band structure as probed by ellipsometry has been associated with the germanium oxide crystals and disorder-induced band tailing effects at the interface of the germanate layer and the bulk Ge wafer. The acid vapor exposure causes Ge surface to emit yellow photoluminescence at room temperature.     

Raman spectroscopic investigation of pure and ytterbium‐doped rare earth silicate crystals

Raman spectroscopy was used to study the molecular structure of a series of selected rare earth (RE) silicate crystals including Y₂SiO₅ (YSO), Lu₂SiO₅ (LSO), (Lu_0.5Y_0.5)₂SiO₅ (LYSO) and their ytterbium‐doped samples. Raman spectra show resolved bands below 500 cm⁻¹ region assigned to the modes of SiO₄ and oxygen vibrations. Multiple bands indicate the nonequivalence of the RE O bonds and the lifting of the degeneracy of the RE ion vibration. Low intensity bands below 500 cm⁻¹ are an indication of impurities. The (SiO₄)⁴⁻ tetrahedra are characterized by bands near 200 cm⁻¹ which show a separation of the components of ν₄ and ν₂, in the 500–700 cm⁻¹ region which are attributed to the distorting bending vibration and in the 880–1000 cm⁻¹ region which are attributed to the symmetric and antisymmetric stretching vibrational modes. The majority of the bands in the 300–610 cm⁻¹ region of Re₂SiO₅ were found to arise from vibrations involving both Si and RE ions, indicating that there is considerable mixing of Si displacements with Si O bending modes and RE O stretching modes. The Raman spectra of RE silicate crystals were analyzed in terms of the molecular structure of the crystals, which enabled separation of the bands attributed to distinct vibrational units. Copyright © 2007 John Wiley & Sons, Ltd.     

Temperature-Dependent Raman Study on Vibrational Modes in LiTaO3

In this paper the results of temperature-dependent Raman study on the vibrational modes in LiTaO₃ crystals are presented. It is found that the two low-frequency A₁ modes show significant changes during the temperature changing process. “Modes softening” and “frequency-repulsion” effect was observed and dicussed.     

Pressure and temperature dependences of the dielectric constant,Raman spectra and lattice constant of SnI42

The pressure and temperature dependences of the Raman frequencies, static dielectric constant and lattice constant of the molecular crystal SnI₄, were investigated. These results are combined to evaluate the pure volume and pure temperature (i.e. volume-independent) dependences of the molecular polarizability and of the Raman-active phonon self-energies. The pressure results also allow the separation of the Raman modes into external and internal modes of the crystal; the external modes exhibit a much stronger pressure dependence, and thereby larger Grüneisen parameters, than do the internal modes. The Raman frequencies increase (decrease) with increasing pressure (temperature) whereas the dielectric constant increases with both increasing pressure and temperature, emphasizing the importance of non-volume effects. In fact, it is found that the isobaric temperature dependence of the dielectric constant is dominated by the pure temperature dependence of the molecular polarizability. The pure volume dependence of the polarizability, on the other hand, is relatively small as is perhaps typical of most molecular crystals.     

Structural Stability,Tunable Electronic,and Optical Properties of XSi2N4/Cs2SnI6 (X = Cr,Mo, W) Heterostructures: First-Principles Studies

Novel optoelectronic and photovoltaic devices are promising, exploiting MoSi₂N₄-based van der Waals (vdW) heterostructures. Herein, six vertical XSi₂N₄/Cs₂SnI₆ (X = Cr, Mo, W) heterostructures are constructed and the atomic structure, stability, and optoelectronic properties via first-principles calculations are investigated. The results of binding energies indicate that XSi₂N₄/SnI₄ is energetically favorable to be established compared to the XSi₂N₄/CsI. Computed charge density differences show that at the XSi₂N₄/SnI₄ interface, there is no significant charge migration or rearrangement, making it unsuitable for use in charge transport devices. It is worth noticing that the built-in electric field induced by electron transfer from CsI to the XSi₂N₄ layer prevents light-induced electron and hole recombination, thereby improving carrier lifetime. Furthermore, the CrSi₂N₄/CsI heterostructure exhibits a wider range of visible light absorption, demonstrating its potential for applications in photoelectronic devices. The electronic and optical properties of XSi₂N₄/Cs₂SnI₆ can be tuned through element substitution. The findings could provide useful guidance for designing XSi₂N₄/Cs₂SnI₆ photoelectronic and photovoltaic devices.     

Vibrational spectra of Y3Al5O12 crystals grown from Ba- and Pb-based flux systems

The IR and Raman vibrational spectra of yttrium aluminum garnet (YAG) single crystals grown from Ba- and Pb-based flux systems have been recorded. The existence of AlO₄ and AlO₆ characteristic vibrational bands confirms the garnet structure. The spectra of the two samples are compared with the spectra recorded for YAG crystals grown from pure melt and the results are discussed.     

The inelastic neutron scattering study of low temperature lattice dynamics of K1−x (NH4) x H2PO4 mixed crystals

The low-temperature lattice dynamics of K_1–x (NH₄) _x H₂PO₄ mixed crystals have been studied using high-resolution inelastic-neutron-scattering; on powder samples for concentrationsx=0.0; 0.06; 0.1; 0.18; 0.3; 0.6 and 0.9; and on a single crystal forx=0.4. The modes have been assigned and are compared with optical spectroscopy results. It was found that the dynamics of acid protons in the hydrogen-bonds is decoupled from the heavy atom lattice. The displacement vectors for the bending modes are inclined by ca 30° with respect to the hydrogen-bond planes. The influence of ammonium ion concentration on the lattice dynamics has been analysed and the implications for the dipole-glass state are discussed. A vibrational mode at 16.8 meV cannot be distinguished in the glass composition spectra. The inhomogeneous piezoelectric coupling constants in these glasses broaden 16.8 meV band so much that it disappears into the noise. The glass-composition samples show line broadening for the NH₄ whole body motions due to the random hydrogen-bond fields experienced by these ions.     

Optical phonon spectra of PbF2 single crystals

The spectra of dipole-active optical phonons are measured for the cubic and orthorhombic phases of PbF₂ single crystals. The frequencies and eigenvectors of normal modes in the Pnma orthorhombic phase are calculated. It is found that the spectrum of the cubic phase exhibits excess vibrational modes of the PbF₂ orthorhombic phase.     

Vibrational kinetics of the active media of CW CO<Subscript>2</Subscript> lasers

The vibrational kinetics of CW CO₂ lasers has been analyzed within the framework of a temperature model. The necessity of taking into account the coupling of the vibrational modes of the CO₂ molecule in determining the occupation numbers and the store of vibrational energy in individual modes is shown. Expressions that connect vibrational temperatures with the rates of excitation and relaxation of the lower vibrational levels of modes have been obtained. The ratios between the vibrational temperatures on selective excitation of the 00° 1 level and on excitation of CO₂ molecules in an electric discharge as well as the character of the dependences of vibrational temperatures on the pumping-energy value are discussed.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 72–79, January–February, 2005.     

Gap discrete breathers in two-component three-dimensional and two-dimensional crystals with Morse interatomic potentials

The properties of gap discrete breathers in three-dimensional and two-dimensional crystals of the composition A ₃ B with interatomic bonds described by the Morse potential have been investigated by the molecular dynamics method for different ratios between the masses of components m _A /m _B . The transition to a thermal equilibrium from a state far from equilibrium has been studied for the two-dimensional crystal. In this case, a short-wavelength phonon vibrational mode is excited in the crystal. This mode appears to be modulationally unstable for not too small amplitudes. During the transition to the state characterized by a uniform energy distribution between all vibrational modes of the crystal, the energy is localized in the form of gap discrete breathers, which exist in time intervals that exceed their oscillation period by several orders of magnitude.     

Vibrational modes of acid hydrogen in CsH<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> and NaH<Subscript>5</Subscript>P<Subscript>2</Subscript>O<Subscript>6</Subscript>: inelastic neutron scattering

The vibrational modes of hydrogen bonds in CsH₅(PO₄)₂ and NaH₅P₂O₆ compounds are analyzed via inelastic incoherent neutron scattering in a wide range of temperatures and Raman light scattering at room temperature. The energy ranges have been determined for the γ, jg, and ? bands of the vibrational modes of acid hydrogens of these compounds. The dependences of the energy on the length of hydrogen bonds in CsH₅(PO₄)₂ have been revealed.