Search results for "PEROVSKITE"
showing 10 items of 458 documents
Semi-empirical calculations of the Nb-ion positions in doped crystals
1998
The atomic and electronic structures of Nb impurities in doped perovskite KTaO3 crystals are calculated using the semi-empirical quantum chemical method of the intermediate neglect of the differential overlap (INDO) and a supercell model. When seven Ta ions are replaced by seven Nb ions, the latter clearly demonstrate self-ordering effects which are related to the experimentally observed impurity-induced phase transition. A single Nb impurity reveals an off-centre displacement which is very close to that found in XAFS experiments. The relevant energy gain is very small, approximately 0.0375 eV, which is much smaller than the Nb-clustering energy gain (0.12 eV). These results led us to the c…
Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems
1999
Abstract A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree–Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO 3 . This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phas…
Self-Ordered Second-Component Nb Clusters in KNbxTa1?xO3 Solid Solutions and Their Physical Properties
1999
Semi-empirical Hartree-Fock calculations using the intermediate neglect of the differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO 3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that a seven-particle cluster in KTaO 3 :Nb has two types of states with different nature. Namely, a state with a dilatation combined with an off-center displacement of the central Nb ion in [111] directions in the cluster, and a state with a full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-…
Cation Environment of BaCeO3−Based Protonic Conductors II: New Computational Models
2011
Quantum chemical calculations have been carried out to simulate Y-doped BaCeO(3) derivatives. Hartree-Fock energy functional was used to study octahedral site environments embedded in a Pmcn orthorhombic framework, showing local arrangement characterized by Ce-O-Ce, Ce-O-Y, and Y-O-Y (Z-O-Ξ) configurations and including or not hydrogen close to the moieties encompassing those configurations. The latter are, in fact, representative of - and, in our modeling approach, were treated as - local arrangements that could be found in Y:BaCeO(3)-doped materials. The geometrical optimizations performed on the structural models and a detailed orbital analysis of these systems allowed us to confirm and …
Dielectric Properties and Conductivity of Ferroelectric LixNa1-xTa 0.1Nb0.9O3 Solid Solutions
2011
Results of a study of dielectric properties and conductivity of ferroelectric perovskite solid solutions of the LixNa1-xTa0.1Nb0.9O3 (х = 0.03–0.135) series with the 290–700 K thermal range and frequencies from 25 to 106 Hz are reported. The charge in this kind of materials is shown to be transferred by Li+ ions the contribution from bulk transport of ions dominating. A first order phase transition close to the second order transitions proceeds in LixNa1-xTa0.1Nb0.9O3 solid solutions within the thermal range examined. The increase of Li concentration enhances the features of the second order transition.
Molecular dynamics study of high-pressure alumina polymorphs with a tight-binding variable-charge model
2016
Abstract A tight-binding variable-charge model aimed at performing large-scale realistic simulations of bulk, surfaces and interfaces of aluminum oxides have been developed. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono–covalent character of the metal–oxygen bond by means of a tight-binding analytical approach in the second-moment approximation of the electronic structure. The parameters of the model were optimized to reproduce structural and energetic properties of the α-Al2O3 corundum structure at room temperature and pressure. The model exhibits a good transferability between five alumina polymorphs: corundum, Rh2O3(II)-type…
Cyano-bridged perovskite [(CH3)3NOH]2[KM(CN)6],[M: Fe(III), Co(III)] for high-temperature multi-axial ferroelectric applications with enhanced therma…
2020
Highly stable ferroelectrics with reversible high-temperature phase transitions and switchable nonlinear optical behaviour are much coveted targets for emerging optoelectronic applications. Here, we demonstrate a cyano-bridged perovskite [(CH3)3NOH]2[KCo(CN)6] (TMAO-Co), a new analogue of the multi-axial ferroelectric [(CH3)3NOH]2[KFe(CN)6] (TMAO-Fe) with improved thermal stability and enhanced second-order nonlinear optical response. Indeed, for TMAO-Co the Curie temperature (Tc) is shifted to a higher value of ca. 416 K (improvement by ca. 10 K versusTMAO-Fe); the separation between Tc and the decomposition threshold is 46 K. TMAO-Co is a biaxial ferroelectric as revealed by P(E) hysteres…
Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites
2019
Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films…
Dielectric and Polarization Properties of Na0.5Bi0.5TiO3-BaTiO3Solid Solutions with Na and K Niobates
2015
The role of substitution in A- and B-sites of perovskite ABO3 structure in phase transitions is evaluated, comparing (1-y)(0.939Na0.5Bi0.5TiO3-0.061BaTiO3)-yKxNa1-xNbO3 solid solutions with x = 0.0, x = 0.5 and x = 1.0. The influence of deviation from a stoichiometric Na/Bi ratio is also studied. The influence of K0.5Na0.5NbO3, NaNbO3 or KNbO3 reduces mainly to suppressing of ferroelectric properties, like in other perovskite ferroelectrics where Ti is replaced by Nb, and development of the relaxor state. Besides the frequency-dependent dielectric permittivity, the relaxor state in the studied compositions is characterized also by a temperature dependence of critical electric fields, corres…
Raman Studies of Structural Phase Transitions in Perovskite Ferroelectric Sodium Niobate Solid Solutions
2008
Raman spectra are sensitive to interactions between structural units and, consequently, may reflect fine rearrangements of the units of cation sublattice occuring at compositional changes affecting the phase equlibrium and ferroelectric properties of the crystal. Obtaining Raman spectra show the information on disordering of structural units depending on composition of the cation sublattice in real Li x Na 1 − x Ta y Nb 1 − y O 3 ferroelectrics. On the basis of experimental studies shown, that not only the frequency but also intensity of some “rigid” bands may serve as the function of order parameter of phase transition in Landau's theory of the second order phase transition.