Theoretical study of local lattice structure of Mn in perovskite fluorides A2MF4 (A=K, Rb; M=Mg, Zn, Cd) series

A theoretical method for investigating the inter-relation between the molecular structure and electronic structure has been established on the basis of the 252×252 complete energy matrices for a 3d⁵ configuration ion in a tetragonal ligand field. By means of this method, which is independent of the X-ray diffraction, the local structure of the paramagnetic Mn²⁺ ion in perovskite fluorides A₂MF₄ (A=K, Rb; M=Zn, Mg, Cd) are determined directly by analyzing the EPR spectrum of octahedral Mn²⁺ center in A₂MF₄ crystals and the optical absorption spectrum of the (MnF₆)⁴⁻ cluster. It is shown that, comparing with the octahedral cubic structure, the local micro-structure in the vicinity of Mn²⁺ displays an elongated distortion when and a compressed distortion when , and ΔR vs. as well as ΔR vs. in the distortion region is, respectively, approximately linear. Simultaneously, the theoretical zero-field-splitting parameters , and are in good agreement with the experimental values.     

Dielectric characteristics and polarization response of lead-free ferroelectric (Bi0.5Na0.5)0.94Ba0.06TiO3-P(VDF-TrFE) 0-3 composites

Bismuth sodium barium titanate/poly(vinylidene fluoride-trifluoroethylene) 70/30 [(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-P(VDF-TrFE)] 0-3 composites were prepared by a hot-press method for different volume fractions of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramic powder in a P(VDF-TrFE) 70/30 copolymer matrix. The relative permittivity and dielectric loss of the composites increased with increase in the volume fraction of the ceramics, which well follows the Bruggeman model. The polarization responses of the composites were strongly dependent on the ceramic volume fraction. The composites with a higher ceramic volume fraction showed an increase in remanent polarization. At room temperature, a 0.3(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-0.7P(VDF-TrFE) composite showed a relative permittivity ε_r=30, remanent polarization and coercive field      

Mott-type hopping conduction in the ordered and disordered phases of LiCoO2

The temperature dependence of dc resistance was observed in the range of for layered-rock-salt (hexagonal structure, ordered distribution between Li and Co) and modified-spinel (cubic, random distribution) phases of LiCoO₂. The results suggest Mott-type hopping conduction arising from the localized Co-3d electrons in the valence band. The densities of states (DOS) at the Fermi energy (E_F) estimated from the slope of the resistance curves were 2.0×10²⁰ and 5.5×10¹⁹ cm⁻³ eV⁻¹ for the ordered and disordered phases, respectively. The relatively low DOS at E_F in the disordered phase suggests that E_F approaches the edge of the valence band as a result of the narrowing of Co-t_2g bands due to the higher lattice symmetry in the disordered phase.     

TiO2-nonstoichiometry dependence on piezoelectric properties and depolarization temperature of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06Ti_xO_1+2x lead-free piezoceramics with x varying from 0.97 to 1.03 were fabricated and characterized in order to investigate the effects of TiO₂-nonstoichiometry on the piezoelectric properties and depolarization temperature of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition. X-ray diffraction (XRD) analysis showed that all samples have a single phase of perovskite structure with rhombohedral symmetry. Piezoelectric and dielectric measurements revealed that deficiency of TiO₂ leads to an increase in the piezoelectric coefficient (d₃₃), free relative permittivity (), and loss tangent (tan δ) besides an increase in the electromechanical coupling coefficient (k_p) within a certain amount, while excess of TiO₂ results in a decrease in k_p, d₃₃, and , but an increase in tan δ. Depolarization temperature (T_d) measurement indicated a decrease and an increase in T_d as a result of increasing TiO₂ deficiency and TiO₂ excess, respectively. This TiO₂-nonstoichiometry also induced changes in the remanent polarization (P_r) and coercive field (E_c) of the ceramics.     

Investigation of nonlinear optical properties of organic dye by Z-scan technique using He-Ne laser

The third-order nonlinear optical properties of Basic Green 1 dye were measured by the Z-scan technique and measurements were carried out at different concentrations and several incident intensities. The results showed that the Basic Green 1 dye exhibited large nonlinear refractive coefficient () and nonlinear absorption coefficient () at the wavelength 632.8 nm of He-Ne. The negative sign of the nonlinear refractive index n₂ indicates that this material exhibits self-defocusing optical nonlinearity. These results show that Basic Green 1 dye has potential applications in nonlinear optics.     

Judd-Ofelt parameters of Er in transparent TeO2-based nanocrystallized glasses

Transparent Er³⁺-doped bulk nanocrystallized (size of nanocrystals: ∼40 nm) glasses of 15K₂O·15Nb₂O₅·70TeO₂·0.5Er₂O₃ and 10BaO·10Gd₂O₃·80TeO₂·0.5Er₂O₃ are prepared, and the Judd-Ofelt parameters, (t=2, 4, 6), of Er³⁺ are evaluated from optical absorption spectra. The change in the molar polarizability due to the nanocrystallization is small in both samples, but a clear decrease in the mean atomic volume due to the nanocrystallization, i.e. more close atom packing, is observed. In both systems, a large decrease is observed in the parameter due to the nanocrystallization, indicating that the degree of the site symmetry of Er³⁺ ions in nanocrystallized glasses is much higher than that in the precursor glasses. The decrease in the and parameters due to the crystallization is small, suggesting that the covalency of Er³⁺-O bonds in nanocrystals is not so different from that in the precursor glasses.     

Interference of resonance luminescence of exciton polaritons in CuGaS2 crystals

The nonmodulated and wavelength-modulated reflection spectra of CuGaS₂ crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons Γ₄ (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10 K have been studied. The states n = 1, 2 and 3 of Γ₅ excitons are found. The main parameters of the A (Γ₄, Γ₅) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ₄ for the states n = 1 and n = 2, the effective masses of electrons and holes are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons Γ₅ in the luminescence spectra excited by the line 4880 Å of Ar⁺ laser. In the luminescence spectra the interference is found.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.