Raman and far IR studies on Ag2CdI4 and Cu2CdI4 superionic compounds

Raman spectral studies, far infrared reflection and transmission spectra for the superionic conductors Ag₂CdI₄, Cu₂CdI₄ at 300 K are reported. From these studies the TO and LO modes are assigned and optical conductivity in the region 30 cm^-1-150 cm^-1 are calculated. The behaviour of conductivity in this frequency region is explained on the basis of jump diffusion model.     

The excitonic absorption spectrum of thin Ag2ZnI4 films

The electron absorption spectrum of thin films of the Ag₂ZnI₄ complex compound is studied at photon energies of 3–6 eV. It is established that the interband absorption edge corresponds to an allowed direct transitions across the energy gap E _g=3.7 eV. A strong exciton band is adjacent to the absorption edge at E _ex=3.625 eV (80 K); in the 80–390 K range, the temperature behavior of the half-width Γ of this band is determined by the exciton-phonon interaction typical of quasi-one-dimensional excitons. At T≤390 K, a discontinuity in the slope of the E _ex(T) and Γ(T) dependences is observed. This discontinuity is associated with the generation of Frenkel defects and is accompanied by the transfer of Ag ions to the interstitial sites and vacancies of the crystal lattice of the compound.     

Thermal,Electrical and Optical Studies of Phase Transitions in Ag2CdI4 Solid Electrolyte

The article deals with the preparation method of bulk Ag₂CdI₄ superionic material and with investigations of structural phase transitions (PT) which take place in the 100÷450 K temperature range. The phase transitions were studied by thermal, electrical and luminescence methods. The temperature dependencies of thermal expansion, heat capacity, thermal conductivity, electric conductivity and excitation luminescence spectra show that in the temperature range 100 ÷ 450 K Ag₂CdI₄ possesses a sequence of structural changes at 180 ÷190 K, 320 ÷ 330 K, 350 ÷ 370 K and at T>400K. The nature and mechanisms of the observed structural changes are discussed.     

Precursor derived LiMn2O4 thin films as ionic conductor

Thin films of spinel LiMn₂O₄ have been fabricated using a metallorganic precursor. Crystalline films have been deposited on Au substrates to exhibit as the cathode in rechargeable thin film lithium batteries. The nucleation and growth of spinel LiMn₂O₄ crystallites were investigated with heat treatment of the deposited thin films. Film capacity density as high as 22 μAh/cm² was measured for LiMn₂O₄. The film heat treated at 700 °C were cycled electrochemically up to 30 cycles against Li metal without any degradation of the capacity. There were neither open area nor amorphous layers which prevent the Li⁺ions transfer at the boundaries in the LiMn₂O₄ thin film. The microscopic study revealed that (111) planes in the two grains directly bonded at the grain boundary which could proceed the lithium ion intercalation or deintercalation smoothly.     

Photoacoustic determination of low absorption coefficients in CdI2 layered semiconductor

Photoacoustic spectra in thin samples of CdI₂ layered semiconductor have been measured in the region of energies near the fundamental energy gap. By means of this technique it was possible to obtain the spectrum of the low values of the absorption coefficient in the range between 3.12 and 3.26 eV, where the absorption coefficient varied from 5 to 100 cm^-1. The experimental results are sound with respect to the indirect nature of the optical gap of the CdI₂.     

Frequency dependence of interband two-photon absorption mechanisms in ZnS and CdI2

Different transition mechanisms related to two-photon absorption (TPA) processes have been investigated in direct (ZnS) and indirect gap (CdI₂) semiconductors. Experimental results show that, in both direct and indirect TPA processes, the dominant transition mechanism depends on the frequency rather than on material parameters. In particular, it is of an allowed-allowed type near the energy gap and, far from this, it becomes of an allowed-forbidden one.     

The superconductor/ionic conductor interface

This paper deals with the charge transfer across the interface superconductor/ionic conductor — a relatively unknown field at present. Both n-type classical and p- and n-type high T_c superconductors are considered. Different types of solid and liquid electrolytes are used in experiments covering temperature ranges down to 10 and 88 K, respectively. Transient technique, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry represent the electrochemical methods in the study of Faradaic charge transfer processes occurring at that interface. First results with solid electrolytes show a current doubling around T_c. This effect is interpreted as a quantum-electrochemical kinetic effect caused by Cooper pairs as correlated charge carriers.     

Ionic conductivity enhancement during pre-crystallization in amorphous lithium ionic conductor Li2B2O4

It is found from the temperature dependence of the ionic conductivity of amorphous Li₂B₂O₄ that the ion transport process is rather complicated. At temperature below T_k (≈310°C), the ionic conductivity obeys an Arrhenius relation. Above the crystallization temperature T_c (≈410°C) ion transport is dominated by the process in the crystalline state. In between the ionic conductivity is anomalously enhanced. In this pre-crystallization range two distinguishable steps with a transition temperature at T_p (≈380°C) can be identified. At temperature between T_k and T_p, the conductivity increase is due to the redistribution of free volume. For temperature between T_p and T_c the specimen contains a small amount of crystallites but the crystallinity is less than 5%. The ⁷Li NMR spectra show that the line shape of partially crystallized amorphous material is quite similar to that of LiCl containing DSPP but in this case the second-phase particles are crystallites of the parent material.     

Polarization dependence of the two quantum absorption spectrum of intrinsic excitons in ZnO

The intrinsic exciton spectrum of ZnO at 4.2°K has been investigated by two quantum absorption of light polarized parallel and perpendicular to hexagonal c-axis (light direction normal to c-axis). Three lines are observed with intensity and spectral position depending on polarization of the two interacting beams in respect to the c-axis. They are interpreted as n=2 states of the exciton series A, B and C with 2P₀ and 2P₊₁ envelope.     

The IR absorption spectrum of 2 - aminothiazolid - 4 - one

A mechanical model has been used to calculate the frequencies in the vibrational spectrum of the imine and amine tautomeric structures for this compound; the IR spectrum for the crystalline state has been deduced for the range 80–3500 cm^–1, and a discussion is given for the effect of the tautomeric prototropic forms on the vibrational spectrum.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 76–82, September, 1971.     

The Raman spectra of CdI2

Raman scattering experiments on CdI₂ at high sensitivity give evidence of several new lines in addition to well known ones. The measurements performed at different temperatures allow us to distinguish between one phonon spectrum and two phonon one. The assignment of the new lines associated to one phonon scattering is also discussed.     

Fast intrinsic emission in Cs2CdI4 single crystal

The spectral-luminescent studies of the ${\mathrm{Cs}}_2{\mathrm{CdI}}_4$ single crystals are reported. The fast luminescence in the 3.6–2.0 eV energy range with decay time $\tau =4.6\mathrm{ns}$ was revealed in the crystal under high-energy excitation $(E_{\mathrm{exc}}>8\mathrm{eV})$ at 9 K. This luminescence is supposed to be caused by the hole transitions between subbands of the split valence band. The energy-band picture for ${\mathrm{Cs}}_2{\mathrm{CdI}}_4$ crystals was calculated as a proof of the luminescence model.     

Absorption spectrum and excitons in thin films of the solid electrolyte RbCu4Cl3I2

We have investigated the absorption spectrum of thin films of the superionic conductor RbCu₄Cl₃I₂ synthesized on NaCl crystalline substrates. It is shown that the electron and exciton excitations in the energy interval 3–6 eV are associated with optical transitions in the CuHal sublattice, and the edge of the fundamental band is controlled by optical transitions in the Cu(II)Hal sublattice. It is found that the large band gap of this compound (E _g =3.86 eV) in comparison with those of CuCl and CuI is a result of the small number of Cu ions in the second coordination sphere. The temperature dependence of the spectral position and half-width of the low-temperature exciton band reveals features associated with the phase transitions γ→β (T _c1=170 K) and β→α (T _c2=220 K) and with disordering of the cation sublattice attendant to the transition to the superionic state. Fiz. Tverd. Tela (St. Petersburg) 40, 1022–1026 (June 1998)     

快离子导体AgxTiS2中Ag+离子-空位的二维基态结构与能量性质研究

根据Ag 离子-空位的二维有序结构建立了三维晶胞模型.采用局域密度近似下的平面波赝势方法,对有序Agx Tis2(x=0,1/4,1/3,1/2,2/3,3/4,1)系列进行了几何结构优化和总能量计算,并与Lix Tis2系列进行了对比研究.有序Agx Tis2系统的晶格参量增量△a0 和△c0随离子浓度单调增加,与实验结果符合得较好.有序Agx TiS2和Lix Tis2系统的总能量均随插层离子浓度增加线性下降,且前者下降较快.有序Agx TiS2(x=0,1/4,1/3,1/2,2/3,3/4,1)系统的形成能均小于零,表明其基态性质.具有3a0x3a0超结构的Ag1/3 Tis2系统的形成能最低.能量性质的对比显示,有序Agx TiS2系统的形成能较低,有序一无序相变温度较高,离子扩散活化能较高.根据计算结果对有关实验给出了合理解释.     

Time dependence of the luminescence intensity in CdI2-Cd and CdI2-Ag crystals

The decrease in the luminescence intensity (luminescence ??fatigue??) of long-wavelength emission bands with time in CdI₂-Cd (??_max = 700 nm) and CdI₂-Ag (??_max = 680 nm) crystals upon exposure to light in the fundamental absorption region has been studied. The process parameters, i.e., the capture cross section ?? and the fraction ?? of centers disappeared during interband irradiation, have been determined. The model of electronic processes has been proposed, within which experimental data have been consistently explained.     

New compounds with incommensurate phases: Cs2CdI4 and Cs2ZnI4

Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs₂CdI₄ and Cs₂ZnI₄. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P2₁/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs₂ZnI₄ crystal in a broad pre-transition region (T - T _i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).     

Far-infrared absorption by excitons in silicon

We report on the first observation of 1s to 2p absorption by excitons in pure silicon. A group of strong lines at 10.2, 11.4 and 12.0 meV is observed along with a continuum extending to higher frequency. A theoretical model for the excitonic absorption accounts well for the observed structure.     

The visible absorption spectrum of S2O

The visible absorption spectrum of S₂O vapor has been studied by spectrographic and laser optoacoustic techniques. Magnetic rotation spectroscopy was used to establish the triplet-singlet nature of the transition. Vibrational analysis gave ν′₁ = 1089, ν′₂ = 505, and ν′₃ = 332 cm⁻¹.