Compositional behaviour of Urbach absorption edge and exciton-phonon interaction parameters in Cu6PS5I1−xBrx superionic mixed crystals

Temperature behaviour of optical absorption edge in Cu₆PS₅I_1−xBr_x mixed crystals is studied in the interval 77-325 K. It is shown that the absorption edge has Urbach shape in the 215-325 K temperature interval. The influence of temperature and compositional disorder on the Urbach absorption edge parameters is presented. The mechanism of the Urbach bundle formation and the effect of I→Br anionic substitution on the exciton-phonon interaction parameters is elucidated.     

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Cu₆PS₅Br superionic crystals were implanted by sulphur ions. The effect of ion implantation on the phase transition (PT) temperatures in Cu₆PS₅Br is observed from isoabsorption studies of the optical absorption edge. The dependences of the ferroelastic PT temperature on the ion fluence are analysed.     

Temperature variation of electrical conductivity and absorption edge in Cu7GeSe5I advanced superionic conductor

Single crystals of Cu₇GeSe₅I superionic conductor were grown by chemical transport. Their electrical conductivity in the frequency range 1.0×10⁶–1.2×10⁹ Hz and in the temperature range 196–295 K was measured. Cu₇GeSe₅I crystal is shown to exhibit a rather high electrical conductivity (σ₂₉₅=64.0 S/m at 295 K) and a low activation energy (ΔE_a=0.125 eV). Optical absorption edge of Cu₇GeSe₅I crystals in the temperature range 77–300 K was studied, the temperature dependences of the optical pseudogap and Urbach energy being obtained. The effect of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters was investigated.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Isoabsorption and spectrometric studies of optical absorption edge in Cu6AsS5I superionic crystal

Cu₆AsS₅I single crystals were grown using chemical vapour transport method. Two low-temperature phase transitions (PT) are observed from isoabsorption studies: a first-order PT at Т_?=153±1 K and a second-order PT in the temperature interval T_?I=260–280 K. At low temperatures and high absorption levels an excitonic absorption band was revealed in the range of direct optical transitions. At Т>Т_?, the absorption edge has an exponential shape and a characteristic Urbach bundle is observed. The influence of the cationic P→As substitution on the parameters of the Urbach absorption edge, parameters of exciton–phonon interaction, and phase transitions temperatures are studied.     

Optical absorption and exciton-phonon interaction in solid solutions Cu<Subscript>6</Subscript>PS<Subscript>5</Subscript>I<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

We have studied the optical absorption edge of solid solutions Cu₆PS₅I_{1 − x} Cl _x at high absorption levels in the temperature range of 77–320 K. In a superionic state, we have revealed the Urbach behavior of the absorption edge, determined its parameters, and studied their temperature and concentration variations. We consider exciton-phonon interaction as a mechanism by which the Urbach bundle is formed and show that this bundle can be described in terms of the Dow-Redfield model.     

The effect of temperature and pressure on the optical absorption edge in Cu6PS5X (X=Cl,Br,I) crystals

The absorption edge of monocrystalline Cu₆PS₅X (X=Cl,Br,I) superionic ferroelastics was studied in the 77–303 K temperature range and the mechanisms of exciton–phonon interactions resulting in exponential absorption edges and Urbach tails were analysed. Various types of disordering, affecting the optical absorption tails, were studied. The investigation of the influence of hydrostatic pressure (up to 400 MPa) on optical transitions of Cu₆PS₅X at 295 K revealed that the increase of pressure causes an increase of the optical pseudogap as well as the increase of the slope of the absorption edge indicating pronounced ordering in most disordered crystals. Data obtained from the pressure studies confirmed the predominant contribution of exciton–phonon interaction to the temperature shift of the Urbach absorption edge.     

Concentration variations in the optical pseudogap and refractive index of crystals of Cu6PS5I1–xClx solid solutions

The effect of anionic substitution of halogen atoms on the spectral position and shape of the optical absorption edge and the dispersion of the refractive index of crystals of Cu₆PS₅I_1–xCl_x solid solutions are studied. The concentration dependences of the width of the optical pseudogap, refractive index, and lattice parameters are shown to be interrelated in these solid solution crystals.     

Electrical and optical absorption studies of Cu7GeS5I fast-ion conductor

Electrical conductivity and fundamental absorption spectra of monocrystalline Cu₇GeS₅I were measured in the temperature ranges 95-370 and 77-373 K, respectively. A rather high electrical conductivity (σ_t=6.98×10⁻³Ω⁻¹ cm⁻¹ at 300 K) and low activation energy (ΔE_a=0.183 eV) was found. The influence of different types of disordering on the Urbach absorption edge and electron-phonon interaction parameters were calculated, discussed and compared with the same parameters in Cu₇GeS₅I, Cu₆PX₅I (X=S,Se) and Ag₇GeX₅I (X=S,Se) compounds. We have concluded that the P→Ge and Cu→Ag cation substitution results in an increase of the electrical conductivity and a decrease of the activation energy. Besides, P→Ge substitution, results in complete smearing and disappearance of the exciton absorption bands and in blue shift of the Urbach absorption edge, an increase of the edge energy width and an electron-phonon-interaction enhancement.     

Phase transitions and processes of disordering in crystals of the Cu6PS5I1 − x Cl x solid solutions

The temperatures of phase transitions in crystals of the Cu₆PS₅I_{1 ? x} Cl _x solid solutions have been determined and the x-T phase diagram of the crystals has been constructed using isoabsorption and polarization-optical measurements. The absorption edge of the Cu₆PS₅I_{1 ? x} Cl _x crystals has been investigated in the temperature range 77–320 K and at high absorption levels. The parameters of the Urbach absorption edge and exciton-phonon interaction in the Cu₆PS₅I_{1 ? x} Cl _x crystals have been determined, and the influence of the compositional disorder on these crystals has been analyzed.     

Optical absorption spectrum of Cu2O-CaO-P2O5 glasses

Homogeneous CaO-P₂O₅ and Cu₂O-CaO-P₂O₅ glasses were prepared using a melt-quenched method under controlled conditions. The binary glasses were found to be colourless and transparent while the ternary glasses changed from light green to dark green as the Cu₂O content increased. From the absorption edge studies, the values of the optical band gap, E_opt and Urbach energy, ΔE were evaluated. The position of the absorption edge and hence the optical band gap were found to depend on the glass composition. Analysis of the optical band gap shows that for the binary glasses, the value increases as the content of CaO decreases, while for the ternary glasses, the value of the optical band gap increases as the content of the Cu₂O decreases. The density of the glasses was also measured and was found to increase with the increase in CaO and Cu₂O contents.     

Role of exciton-phonon interactions and disordering processes in the formation of the absorption edge in Cu<Subscript>6</Subscript>P(S<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>x</Subscript>)<Subscript>5</Subscript>Br crystals

The absorption edge in Cu₆P(S_1?x Se_x)⁵Br crystals has been studied for strong absorption in the temperature range of 77–330 K. The parameters of the Urbach absorption edge and exciton-phonon interactions in Cu₆P(S_1-x Se_x)₅Br crystals are determined and their effect on the composition disorder is studied.     

Influence of anionic substitution on phase transitions in Cu6PS5I1− x Cl x mixed crystals

Cu₆PS₅I_{1? ?} ? _x Cl _x mixed crystals were grown using chemical vapour transport. Isoabsorption studies of optical absorption edge and optical polarization measurements were performed in the temperature range 77–320?K. The influence of anionic I?→?Cl substitution on the phase transitions in Cu₆PS₅I_{1? ?} ? _x Cl _x mixed crystals is studied. Compositional dependence of the phase-transition temperatures is obtained and the x,T-diagram for Cu₆PS₅I_{1? ?} ? _x Cl _x mixed crystals is constructed.     

Electrical conductivities of the Ag6PS5X and the Cu6PSe5X (X=Br, I) argyrodites

The Ag₆PS₅Br and Ag₆PS₅I argyrodites crystallize in a face-centered-cubic lattice at room temperature. Both compounds exhibit purely Arrhenius behavior throughout the temperature range 150-400 K with similar activation energies of about 0.23 eV. Cu₆PSe₅Br and Cu₆PSe₅I also crystallize in a face-centered-cubic structure at room temperature. Cu₆PSe₅Br exhibits a distinctive anomaly in electrical conductivity near 286 K while Cu₆PSe₅I undergoes a first-order electrical phase transition near 265 K. Their activation energies above room temperature are 0.13 and 0.30 eV, respectively.     

Internal friction in Cu6PS5Br superionic crystals and related composites

Mechanical properties of Cu₆PS₅Br single crystals and composites based on them have been investigated by the internal friction method. The measurements of the internal friction and the shear modulus have been performed in the temperature range of 80–300 K at deformation frequencies of 10–100 mHz in a mode of forced torsional vibrations. The maxima caused by the superionic and ferroelastic phase transitions have been found in temperature dependences of the internal friction. It has been shown that a more than two-fold decrease in the shear modulus with increasing temperature in the range of 150–230 K is caused by mobility unfreezing in the cation sublattice of the Cu₆PS₅Br single crystal during the superionic phase transition. An abrupt (more than threefold) increase in the shear modulus upon heating in the range of 260–270 K is caused by the ferroelastic phase transition of the Cu₆PS₅Br single crystal. Parameters of the internal friction of this single crystal in the course of mentioned phase transitions have been determined.     

Influence of S→Se substitution on chemical and physical properties of Cu7Ge(S1−xSex)5I superionic solid solutions

Phase diagram of Cu₇GeS₅I-Cu₇GeSe₅I superionic system was investigated by the differential thermal analysis (DTA). Chemical interaction in Cu₇GeS₅I-Cu₇GeSe₅I system was studied by X-ray phase analysis (XPA), microstructural analysis, and density determination. Compositional behaviour of diffuse reflection spectra and energy gap due to S→Se anion substitution in Cu₇Ge(S_1−xSe_x)₅I solid solutions are discussed.     

Optical and acoustic properties of TeO2/WO3 glasses with small amount of additive ZrO2

The optical and acoustic properties of tellurite glasses in the system TeO₂/ZrO₂/WO₃ have been investigated. The refractive index at different wavelengths and the optical spectra of the glasses have been measured. From the refractive index and absorption edge studies for prepared glasses, the optical parameter viz; optical band gap (E_opt), Urbach energy, (ΔE), dispersion energy, E_d, and the average oscillator energy, E₀, have been calculated. Sound velocities were measured by pulse echo technique. From these velocities and densities values, various elastic moduli were calculated. The variations in the refractive index, optical energy gap and elastic moduli with WO₃ content have been discussed in terms of the glass structure. Quantitatively, we used the bond compression model for analyzing the room temperature elastic moduli data. By calculating the number of bonds per unit volume, the average stretching force constant, and the average ring size we can extract valuable information about the structure of the present glasses.     

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi₂O₅ glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.     

Dynamical structure of α-Ag0.99Cu0.01I crystal by Cu NMR chemical shift, spin-lattice relaxation time and molecular dynamics simulation

Solid ⁶³Cu NMR and Molecular Dynamics (MD) methods have been used to investigate the dynamical structure of Ag_0.99Cu_0.01I crystal, through the viewpoint of the chemical shift and the spin-lattice relaxation. In the superionic phase (α-phase), we obtained the temperature variation of the chemical shift as −0.2 ppm/K, and the activation energy of the Cu⁺ ion diffusion as 11 kJ/mol. The temperature dependence of the chemical shift was explained by the calculated chemical shielding surface based on ab initio MO calculation, and by the probability density of Cu⁺ ion estimated by MD simulation. The spin-lattice relaxation was also analyzed by using the MD method in which we assumed two jumping models as the diffusion process of the mobile cations. It is concluded that the temperature dependence of the chemical shift is dominated by the shielding in the vicinity of the 24(h) site, and the observed activation energy is due to the diffusion process of the mobile cations jumping from the 6(b) intrasub-lattice to the nearest-neighbor 6(b) sub-lattice.     

Temperature dependence of the Urbach energy in ordered defect compounds Cu-III3-VI5 and Cu-III5-VI8

Two previous models used with success in Cu-III-VI₂ semiconductors have been employed to study the temperature dependence of the Urbach energy in ordered compounds Cu-III₃-VI₅ and Cu-III₅-VI₈. The model which contains two variable parameters seems to explain better the data over the whole temperature range studied. However, the ordered vacancy or the donor acceptor defect pair in the cation sublattice provides new features in these compounds that need further study.