Polarization mechanism of ferroelectric lattice instability in crystals

It is shown that the intercell dipole-dipole (polarization) interaction lowers the frequencies of the polar transverse-optical (TO) lattice vibrational modes (to a greater extent, the larger the corresponding dipole oscillator strengths) and can be a cause of ferroelectric instability in crystals. A model-independent method for determining the frequencies, eigenvectors, and dipole oscillator strengths of all the polar TO modes for a crystal with a suppressed polarization interaction from the known values of these quantities for the corresponding real crystal is proposed. The results of the corresponding calculations for several compounds with perovskite structure (SrTiO₃, BaTiO₃, KTaO₃, KNbO₃, and KCoF₃) and rutile structure (TiO₂, SnO₂, SiO₂, CoF₂, and NiF₂) are presented, and the nature of the ferroelectric soft mode in oxides with crystal structures of these two types is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 687–693 (April 1997)     

Growth of LuAP/LuYAP single crystals for pet applications

The light yield and decay properties are the most crucial parameters of cerium-doped lutetium orthoaluminate (LuAlO 3 -Ce, LuAP) and yttrium containing solid solution (Lu 1 m x Y x AlO 3 -Ce, LuYAP), which will be used in Positron Emission Tomographs (PET). Both characteristics strongly depend on the optical quality and cerium concentration introduced into the crystal. Factors governing the crystal quality are commented on. A clear improvement of scintillation properties with increasing the doping concentration is demonstrated in a series of LuYAP (Lu=80 v at.%) crystals scanning the concentration range from 0.1 to 0.4 v at.%. Testing of relations is performed between the crystal doping level and low-temperature thermoluminescence spectra, as well as between the doping level and absorption spectra in UV range.     

Vibrational modes symmetry and raman scattering in SnI4 crystals

A group-theoretical vibrational modes symmetry analysis of SnI₄ crystals has been made for the Brillouin zone center and polarized Raman scattering study in these crystals has been carried out. Correlation diagram showing the correspondence between the crystal vibrational modes and the free SnI₄ molecule modes has been built. The nature and peculiarities of the low frequencies spectrum have been considered.     

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.     

Lattice dynamics of K2NaAlF6, K3AlF6, and Na3AlF6 crystals with the elpasolite structure

This paper presents the results of a nonempirical calculation of the static and dynamic properties of K₂NaAlF₆, K₃AlF₆, and Na₃AlF₆ crystals with the elpasolite structure. The calculation is based on a microscopic model of an ionic crystal that allows for the deformability and polarizability of the ions. The deformability parameters of the ions are determined by minimizing the total energy of the crystal. The total energy is regarded as a functional of the electron density, using the local Thomas-Fermi approximation and taking into account exchange (correlation) effects. The results of the calculations of the equilibrium lattice parameters and of the permittivities are in good agreement with the experimental data. Unstable vibrational modes are found in the spectrum of the lattice vibrations, with these modes occupying the phase space in the entire Brillouin zone. Zh. éksp. Teor. Fiz. 114, 1742–1756 (November 1998)     

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.     

Theoretical and experimental analysis on spectral,hyperpolarizability and dielectric properties of semi-organic nonlinear optical crystal: Potassium boromalate hydrate (PBH)

Semi-organic potassium boromalate hydrate KBC₈H₈O₁₀·H₂O (PBH) salt was synthesized by reflux method and grown as single crystals by slow evaporation technique. The single crystal X-ray diffraction, Fourier transform infra red (FT-IR) and Fourier transform Raman spectral analyses were carried out to confirm the grown crystals. The factor group analysis was performed and the possible vibrational modes were predicted for PBH. The first order hyperpolarizability value calculated by quantum chemical calculations is well supported by the results obtained from Kurtz–Perry powder technique. The transparency in the entire blue, green and IR region makes it a valid candidate for the NLO applications. From the dielectric studies it is inferred that conductance predominates at low frequency region due to the exchange of electrical charges at crystal surface which results in the local displacement of electrons in the direction of the applied field.     

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.     

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.     

Synthesis, growth and optical spectroscopy studies of BaBiBO4 and CaBi2B2O7 crystals

Single-crystalline species of the novel compounds BaBiBO₄ and CaBi₂B₂O₇ with Pna2₁ structure were successfully grown by the top-seed solution growth (TSSG) method and solid-state recrystallization, respectively. X-ray diffraction methods were used to confirm the symmetry of the crystal structure and to determine unit cell parameters. Normal-mode classification and assignment were performed from the results of polarized Raman and IR absorption measurements on the obtained crystals. The high-frequency part of the vibrational spectra of both materials (580–1400 cm^?1) is reminiscent of the normal modes of the isolated [BO₃]^3? group. The expected crystal-field induced changes in these modes from group-theoretical considerations provide important clues for the normal-mode assignment of the high-frequency spectral lines.     

Study on the stability of organic–inorganic perovskite solar cell materials based on first principle

ABSTRACT

In order to better understand and elucidate the structural stability of perovskite materials, the lattice parameters and tolerance factors of three crystal structures of perovskite materials are calculated based on the first principle of density functional theory. We find that the perovskite crystal structures are relatively stable and is consistent with the experimental facts as the tolerance factor 0.81?<?T?<?1.11. The elastic modulus of three crystal structures of MAPbI₃, FAPbI₃ and the elastic modulus of FA_0.75Cs_0.25Sn_0.5PB_0.5I₃ are studied. By Voigt-Reuss-Hill approximation, the elastic properties such as bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are obtained. From the elastic modulus C_ij, we can find that the other six kinds of crystal structures are relatively stable except for the orthogonal structure of MAPbI₃ (c). The ductility and brittle toughness of the material are also discussed by B/G and Poisson’s ratio. It is found that MAPbI₃ (a) is the hardest and FAPBI₃ (a) the weakest. Form the three-dimensional surface view of Young's modulus it is found that their dependence in three-dimensional direction is spherical for an isotropic system. The degree of deviation of the Young's modulus sphere reflects the anisotropy of crystal structures. The degree of elastic anisotropy of organic–inorganic perovskite materials follows the order of FAPbI₃(c)?>?MAPbI₃(a)?>?FA_0.75?Cs_0.25?Sn_0.5Pb_0.5I₃?>?FAPbI₃(a)?>?MAPbI₃(b)?>?MAPbI₃(c)?>FAPbI₃(b). Furthermore, by the adsorption energies and density of states (DOS) of these seven crystals for water molecules, the reasons why perovskite materials are easily denatured in high humidity environment were explored. The results show that perovskite materials are easy to denaturate in high humidity environment.     

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.     

Study of the <Emphasis Type="Italic">A</Emphasis> ion effect on the chemical bond of Ti and O ions in <Emphasis Type="Italic">A</Emphasis>TiO<Subscript>3</Subscript> (<Emphasis Type="Italic">A</Emphasis> = Sr,Ba, Ca,Pb) crystals with perovskite structure

The local Ti d densities of states in the valence band have been calculated in clusters of ATiO₃ (A = Ca, Sr, Ba, Pb) crystals with perovskite structure by the full multiple scattering method. The d density of states in PbTiO₃ crystal is found to significantly differ from that in other crystals. This fact indicates a significant effect of lead ions on the chemical bond between Ti and O ions in this crystal.     

High-pressure single-crystal studies of MnTiO3

Abstract

Recent high-pressure single-crystal x-ray diffraction experiments have provided new information on the crystal chemistry of MnTiO₃ and have provided insight into polymorphic transitions among phases of this composition. The stable polymorph of MnTiO₃ at room P and T has the ilmenite structure. At high P and T, MnTiO₃ ilmenite (MnTiO₃ II) transforms to a LiNbO₃ structure through a cation reordering process, and the quenched LiNbO₃-structure phase transforms to a perovskite structure as the pressure is again increased. This transition is unique in that twinned MnTiO₃ II crystals transform under pressure to untwinned crystals having the perovskite structure. The back-transformation of perovskite to the LiNbO₃ structure as pressure is released is similar to that observed previously for the rutile-type dioxides TiO₂ and SnO₂.     

Lattice dynamics and the phase transition from the cubic phase to the tetragonal phase in the LaMnO<Subscript>3</Subscript> crystal within the polarizable-ion model

The paper reports on the results of ab initio calculations of the static and dynamic properties of the LaMnO₃ crystal with a perovskite structure in the cubic, rhombohedral, and orthorhombic phases. The calculations are performed within the ionic crystal model, which takes into account the deformability and polarizability of the ions. It is revealed that the spectrum of lattice vibrations in the cubic phase contains unstable vibrational modes, which occupy the phase space in the entire Brillouin zone. The eigenvectors of the softest mode at the boundary point R of the Brillouin zone are associated with the displacements of the oxygen ions and correspond to the “rotation” of the MnO₆ octahedron. The condensation of one, two, and three components of this mode leads to the tetragonal, orthorhombic, and rhombohedral distortions of the structure. The structural phase transition is described in terms of the local mode approximation with the use of the double perovskite unit cell, in which the MnO₆ octahedron is explicitly separated. The parameters of the model Hamiltonian are determined. The static properties are investigated by the Monte Carlo method. The calculated temperature of the phase transition from the cubic phase (9800 K) is considerably higher than the melting temperature of the crystal under investigation. The calculated frequencies of long-wavelength lattice vibrations in the experimentally observed orthorhombic and rhombohedral phases are in reasonable agreement with experimental data.     

Electronic structure,lattice dynamics,and magnetoelectric properties of double perovskite La<Subscript>2</Subscript>CuTiO<Subscript>6</Subscript>

The results of ab initio calculations of the electronic structure, vibrational properties, and the magnetoelectric effect in the La₂CuTiO₆ crystal with double perovskite structure are presented. The lattice dynamics calculation shows the presence of unstable modes in the phonon spectrum of the high-symmetry cubic phase with space group \(Fm\overline 3 m\). Condensation of two most unstable modes belonging to the center and the boundary point X of the Brillouin zone leads to the formation of a nonpolar stable phase with space group P2₁/n. The calculation taking into account spin polarization shows that the magnetic ground state is E*-type antiferromagnetic with doubled magnetic cell and with the two spin-up and two spin-down configuration of magnetic moments of copper ions along the [010] crystallographic direction. Such ordering of magnetic moments leads to polar space group and polarization formation. The polarization magnitude is estimated as 71 μC/m².     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

‘Dirty’ displacive ferroelectrics

Using the powder Raman technique we have measured most of the lattice vibrational modes in the Pb_1?xLa_xTi_1?x/4O₃ perovskite solid solution series. From the data for these disordered systems the clamped dielectric constant at zero frequency, ?_m, was determined and compared with the clamped dielectric constant, ?_cap, measured by standard capitance techniques. We find that ?_cap/?_m exceeds one, and increases as the transition temperature is approached from below. Strikingly, we also find similar behavior for the ‘classical’ ferroelectrics BaTiO₃, LiNbO₃ and LiTaO₃, as well as for ferroelectrics with the tungsten bronze crystal structure.     

Theoretical strength of 2D hexagonal crystals: application to bubble raft indentation

By means of lattice and molecular dynamics we study the theoretical strength of homogeneously strained, defect-free 2D crystals whose atoms interact via pair potentials with short- and longer-ranged interactions, respectively. We calculate the instability surface, i.e. the boundary in the 3D homogeneous strain space (ε _xx ,?ε _yy ,?ε _xy ), at which the first vanishing of the frequency of a vibrational mode occurs, taking into account all 2(N???1)?+?3 modes of a 2D periodic system of N atoms. We also compute the strain energies of the crystal on the instability surface, thus defining the most dangerous direction(s) of strain where the critical energy density is small. A theory is developed to incorporate the effect of loading device–sample interactions in the lattice instability criterion. The results are applied to the model problem of bubble raft indentation. We analyse the distribution of the unstable phonon modes in the first Brillouin zone as a function of the loading parameter, and discuss the post-critical behaviour of the lattice in the presence of strain gradients as in nano-indentation experiments.