Density of states determination of mixed semiconductors by neutron diffraction

The phonon density of states of mixed crystals has been determined by neutron diffraction in powders. In the “two mode type” mixed crystal ZnS_xSe_1-x, the great features of the host lattice density of states, are not modified up to an “impurity” concentration of 25%. On the other hand, the presence of Te in ZnSe_xTe_1-x with x = 0.25, give rise to a “one-mode type” mixed crystal, the density of states of which is an average of that of the two components.These results justify that the eigenfrequencies of a two modes type mixed crystal can be calculated using the unperturbed host lattice density of states (additivity of Green's function).     

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Ionic conductivity of KBr_xI_1?x(0?x?1) mixed crystals, σ_x, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σ_x, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBr_xI_1?x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σ_x/σ₁ (σ₁ refers to KBr) have been suggested.     

Reststrahlspektrum und Eigenabsorptionskante von CdxCa1−xO

The reflectivity of Cd_xCa_1?xO mixed crystals prepared by sintering has been measured at 300 °K from 0.2 to 47μm. The Reststrahlenbands show the expected “one-mode” behavior. By Kramers-Kronig-analysis, the longitudinal and transversal optical mode frequencies, the static and high frequency dielectric constants, the damping constants and the Szigeti-effective charges were determined in dependence on the relative admixturex of Cadmium. While the bandgap and the optical lattice frequencies vary linearly withx, the effective charge shows a deviation from linearity.     

Structure and dynamics of the pure and mixed fluorites <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Ca,Sr, Ba,and Pb)

The structure and dynamics of the crystal lattice of MeF₂ fluorites (Me = Ca, Sr, Ba, and Pb) under external hydrostatic compression (0–3.5 GPa) are calculated within the shell model in the pair potential approximation. The first-order structural phase transition from the cubic to the orthorhombic phase in these crystals under pressure is investigated. The effect of chemical pressure on the BaF₂ crystal is analyzed by the simulation of mixed crystals, namely, Ba_1?xCa_xF₂ and Ba_1?xSr_xF₂. It is demonstrated that the supercell method, as applied to the simulation of mixed crystals, results in a lower lattice energy per formula unit as compared to the lattice energy obtained by the virtual-crystal method.     

Ionic conductivity in tysonite-type solid solutions La1−xBaxF3−x

The ionic conductivity of single crystals of tysonite-type solid solutions La_1?xBa_xF_3?x(0?x?0.095) has been studied parallel and perpendicular to the crystallographic c axis in the temperature range 293–1300 K. Three regions can be discerned in the compositional dependence of the ionic conductivity: (i) the “pure” crystal, in which at room temperature no exchange occurs between different types of anion sites in the tysonite structure; (ii) an intermediate region(0 < x < 7 × 10^-2) which reveals changes in both the conductivity activation enthalpy and the magnitude of the conductivity; (iii) a concentrated solid-solution region (x > 7 × 10^-2), where fluoride ions interchange easily among the different anion sublattices. Diffusion coefficients calculated from ionic conductivity results, are in good agreement with those calculated from ¹⁹F NMR measurements. Using the present data, along with ¹⁹F NMR data, dielectric relaxation data and structural considerations, mechanisms governing the ionic conductivity are proposed.     

Surface phonon–polaritons in rectangular quantum wires of polar ternary mixed crystals

In this paper, the dispersion relations of surface phonon–polaritons in freestanding rectangular quantum wire systems of polar ternary mixed crystals are derived. The numerical calculations for Al_xGa_1−xAs and Zn_xCd_1−xSe quantum wire systems are performed. The results reveal that the frequencies of surface phonon–polariton modes are sensitive to the geometric structures of the quantum wires, the wave-vectors in z-direction, and the compositions of the ternary mixed crystal materials. The effects of the “two-mode” and “one-mode” behaviors of the ternary mixed crystals on the surface phonon–polariton modes are also discussed.     

Lattice dynamics of MF3 crystals (M = Al, Ga, and In)

The phonon spectra, Born effective charges, and dielectric constants ε_∞ for the □AlF₃, □GaF₃, and □InF₃ crystals (where □ is a vacancy) have been calculated in terms of the generalized Gordon-Kim method. The calculated spectra of lattice vibrations contain no imaginary vibrational frequencies. This suggests the stability of the cubic phase of these compounds but contradicts the observable structural transition from cubic to rhombohedral phase. It is assumed that such a transition in the □AlF₃, □GaF₃, and □InF₃ crystals is brought about by structural defects. The calculated spectrum of lattice vibrations of the “completely defective” crystal M□F₃ (M = Al, Ga, and In) indicates a strong instability of the cubic phase. Within the mean crystal approximation, the cubic phase of M _x M _1?x F₃ crystals appears to be unstable at small x≤0.05.     

Surface and interface phonon-polaritons in freestanding quantum well wire systems of polar ternary mixed crystals

The surface and interface phonon-polaritons in freestanding rectangular quantum well wire systems consisting of polar ternary mixed crystals are investigated in the modified random-element-isodisplacement model and the Born–Huang approximation, based on the Maxwell's equations with the boundary conditions. The numerical results of the surface and interface phonon-polariton frequencies as functions of the wave-vector, geometric structure, and the composition of the ternary mixed crystals in GaAs/Al_xGa_1−xAs and Zn_xCd_1−xSe/ZnSe quantum well wire systems are obtained and discussed. It is shown that there are 10 and 8 branches of surface and interface phonon-polaritons in the two quantum well wire systems respectively. The effects of the “two-mode” and “one-mode” behaviors of the ternary mixed crystals on the surface and interface phonon-polariton modes are shown in the dispersion curves.     

Pulsed EPR spectroscopy of Gd3+ centers in Ca1−x R x F2+x (R=La, Y) mixed crystals

A study of Gd³⁺ centers in Ca_1?x R _x F_2+x (R=La, Y) crystals using pulsed EPR spectroscopy is presented. The echo-induced EPR (ESE-EPR) spectrum shows, besides the signal of slightly perturbed cubic Gd³⁺ centers, a broad signal at g≈2 due to Gd³⁺ centers at low symmetry sites. To describe the effects of R³⁺ ions on the EPR Gd³⁺, a model, including cubic and linear R³⁺?2F _i ^? centers, is developed. Its predictions are compared with the experimental results. The composition dependence of the EPR signal due to slightly perturbed cubic Gd³⁺ centers in mixed Ca_1?x R _x F_2+x crystals is explained taking into account the different clustering tendency in La and Y crystals. Moreover, the formation of mixed clusters involving R³⁺ and Gd³⁺ ions is proposed for both series of samples. A greater clustering trend is found in the Y crystals than in the La ones. Gd³⁺ ions are found to be a “non innocent” paramagnetic probe for structural studies in these mixed crystals.     

Peculiarities of cascade photon emission and energy storage in M1−xPrxF2+x (M=Ca,Sr, Ba,x≈0.35) crystals

Peculiarities of cascade photon emission (CPE) and energy storage in M_1?xPr_xF_2+x (M=Ca, Sr, Ba, x≈0.35) crystals were studied. The investigation of lattice parameters revealed that these solid solutions belong to the fluorite structure type with the lattice constant noticeably different from that of MF₂ crystals. Absorption, emission and excitation spectra of M_0.65Pr_0.35F_2.35 were measured at LHeT and RT. As it turned out the typical for 4f²→4f² transition in Pr³⁺ emission lines are broadened as compared with the PrF₃ crystal. The analysis of the excitation spectra broadening does not allow bringing out the type of the superlattice, which is inherent to the material, but it indicates clearly the simultaneous presence of different types of the Pr centers in mixed crystals. Yet another specific feature is the higher radiation sensitivity of these fluorides relatively PrF₃, MF₂ and Pr-doped MF₂ crystals. Coloration efficiency enhances in direction Ca→Sr→Ba, and the positions of induced absorption band depend on composition of the solid solution. Colorization, thermo-stimulated luminescence and afterglow of the M_1?xPr_xF_2+x crystals denote high radiation sensitivity as compared with M_1?xCe_xF_2+x.     

Unusual behavior of the lattice thermal conductivity and of the Lorenz number in the YbIn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>4+<Emphasis Type="Italic">x</Emphasis></Subscript> system

Samples of various compositions were obtained in the homogeneity range of the Yb-In-Cu system (YbIn_1?xCu_4+x), from stoichiometric (YbInCu₄) to YbIn_0.905Cu_4.095. Their lattice constant (at 300 K and in the range 20–100 K), total thermal conductivity, and electrical resistivity (from 4 to 300 K) were measured. All the compositions studied exhibited an isostructural phase transition at T _v ?40–80 K driven by a change in the Yb ion valence state. It was shown that within the YbIn_1?xCu_4+x homogeneity range, the lattice thermal conductivity κ_ph decreases with increasing x; at T>T _v , κ_ph grows with temperature and the Lorenz number (which enters the Wiedemann-Franz law for the electronic component of thermal conductivity) of the light heavy-fermion system, to which YbIn_1?xCu_4+x belongs for T<T _v , behaves as it does in classical heavy-fermion systems. Thermal cycling performed through T _v generates stresses in the YbIn_1?xCu_4+x lattice, which entails an increase in the electrical resistivity and a decrease in the thermal conductivity. “Soft anneal” (prolonged room-temperature aging of samples) makes the effect disappear. A conclusion is drawn as to the nature of the effects observed.     

Mössbauer studies of the surface and bulk magnetic structure of scandium-substituted Ba-M-type hexaferrites

A direct comparison of the magnetic structures of a surface layer and of the bulk of Ba-M-type hexagonal ferrites with iron ions partially replaced by Sc diamagnetic ions (BaFe_12?x Sc_xO₁₉) has been made by simultaneous Mössbauer spectroscopy with detection of gamma rays, characteristic x-ray emission, and electrons. It has been found that, if the magnetic lattice of a Ba-M-type hexagonal ferrite is weakly diluted by Sc diamagnetic ions, a ～300-nm thick macroscopic layer forms on the surface of a BaFe_11.4Sc_0.6O₁₉ crystal, in which the iron-ion magnetic moments are noncollinear with the moments in the bulk. The noncollinear magnetic structure forms in the near-surface layer of BaFe_12?x Sc_xO₁₉ crystals because the exchange interaction energy is additionally reduced by the presence of such a “defect” as the surface. This is the first observation in ferromagnetic crystals of an anisotropic surface layer whose magnetic properties, as predicted by Néel, differ from those of the bulk.     

Magnetic, electrical, and optical properties of electron-doped Ca1 − x LaxMnO3 − δ(x ≤ 0.12) single crystals

The magnetic, electrical, and optical properties of Ca_{1 ? x} La_xMnO_{3 ? δ}(x ≤ 0.12) manganite single crystals have been studied. The state with a spatially inhomogeneous electron distribution has been found. Interrelations between the electric and magnetic subsystems are analyzed. The obtained magnetic data show evidence for the formation of a G-type antiferromagnetic (G-AFM) phase with a spin-canted structure in the crystal with x = 0.05, for which the Curie and Néel temperatures are T _C = T _N(G) = 115 K. On cooling from the paramagnetic state, the crystals with x = 0.10 and 0.12 exhibit transitions from the paramagnetic to a C-type antiferromagnetic (C-AFM) phase in a part of the volume at T _N(C) = 150 and 200 K, and from the paramagnetic to the G-type antiferromagnetic (G-AFM) phase in the remaining volume at T _N(G) = 110 and 108 K, respectively. The onset of the C-type magnetic phase nucleation in crystals is observed at lower dopant (La) concentrations than in polycrystalline samples, which is explained by the deviation of single crystals from the stoichiometry with respect to oxygen. The magnetic phase transitions are manifested by anomalies in the electric resistance and magnetoresistance of doped crystals. An analysis of the electrical and optical properties of the samples shows evidence of (i) the formation of a charge energy gap in the C-AFM phase with retained paramagnetic metallic regions and (ii) the presence of ferromagnetic “metallic” droplets in the insulating G-AFM phase. The multiphase state of Ca_{1 ? x} La_xMnO_{3 ? δ} manganite single crystals featuring the coexistence of two magnetic phases, the regions with orbital/charge ordering, and the FM “metallic” droplets is related to a competition of exchange interactions by the superexchange and double exchange mechanisms.     

Soliton modulation-widths for Pb1−xSnxTe TO-ponons

Some new TO-phonon width data for Pb_1?xSn_xTe with strong temperature-dependence are explained in terms of a “modulation-effect” due to non-linear lattice modes. Reasonable values of the parameters provide very good agreement with the experimental widths.     

DSC and electrical conductivity studies of (Li1−xKx)2SO4 mixed crystals

Abstract

(Li_1?K_x)₂SO₄ mixed crystals were prepared by the precipitation technique where x = 0.5, 0.7, 0.9 and 0.99. The phase transformations of the mixed crystals have been analyzed by the DSC technique. The DSC curves of (Li_1?xK_x)₂SO₄ mixed crystals reveal that as the potassium content increases the first high temperature phase of the intermediate LiKSO₄ phase at T = 436°C disappears and a two- phase mixture of LiKSO₄ and K₂SO₄ is found for x = 0.7 and 0.9. It is observed from the electrical conductivity measurements of (Li_1?xK_x)₂SO₄ mixed crystals that the electrical conductivity increases as the K₂SO₄ concentration increases with average activation energy of 1.04 eV. The enhancement in the electrical conductivity is primarily a result of the presence of two ionically conducting constituents and the formation of a diffuse space charge layer at the interface region between these two phases.     

Effect of homovalent (I−Br−) ion substitution on the ionic conductivity of LiI1−x Brx systems

The study of electric conductivity in the whole range of compositions of mixed LiI_1?x Br_x crystals has shown that doping of pure lithium halide (LiI or LiBr) by homovalent substitution leads to an increase in conductivity. This enhancement is essentially pronounced in the solid solution domains near the compositions of pure compounds (x < 0.15 and x > 0.90). Maximum conductivity is attained for the two phase composition LiI_0.75Br_0.25 (5 × 10^?7 S cm^?1 at 293 K) compared to those of starting compounds (4 × 10^?8: LiI and 9 × 10^?9: LiBr at 293 K). An Arrhenius behaviour of the conductivity evolution versus the inverse of the temperature shows that variations in conductivity are related to those of the activation energy whose minimum is close to 8.7 Kcal mole^?1. Lattice strain involved by substituting anions of different raddi could be the factor which increase the defect population.     

Austausch und Spinresonanz in den Gadolinium-Pniktiden GdP,GdAs und GdSb

Powder samples of mixed crystals Gd _x Y_1?x P, Gd _x Y_1?x As and Gd _x Y_1?x Sb have been studied by electron-spin-resonance for Gd-concentrations 0.01≦x≦1 between the ordering temperatures and room temperature. All measurements show a single exchange-narrowed and asymmetric absorption line of Lorentzian shape in the centre region. Theg-factors are 1.995±0.008 in agreement with the expected value for a⁸ S _7/2-state of the half-filled 4f-shell. The linewidths are explained by dipolar and exchange interactions. It is shown that the effect of exchange narrowing is proportional to the sum of the absolute values of the exchange parameters. Therefore a new method for studying the magnetic coupling far above the ordering temperature is proposed. The dependence of exchange on changes of the lattice parameters has been evaluated. The observed asymmetry of the resonance line gives a measure for the electric conductivity. In particular the beginning of critical scattering near the ordering temperature is observed.     

Comparative analysis of the twin and heterophase structures in (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 crystals

The specific features of the coexistence of phases in heavily twinned crystals of the (1?x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ system in the vicinity of the morphotropic phase boundary (0.30≤x≤0.40) are investigated. The phase transformations in crystals at x = const during cooling are considered at electric field strengths E=0 and 0.1 MV/m. The conditions of the formation of interphase boundaries (zero net strain planes) are determined for different first-order phase transitions. The results of calculating the tetragonal-monoclinic M _C and monoclinic-monoclinic (M _C –M _A ) phase transitions are represented in the form of “twin state-interphase boundary” diagrams. The effect of a 90° domain (twin) structure of the tetragonal phase on the heterophase state associated with the presence of monoclinic phases is analyzed.     

Ionic conductivity in the single crystals of non-stoichiometric fluorite phases M1−xRxF2+x (M=Ca,Sr, Ba; R=Y,La-Lu)

The ionic conductivity has been measured of single crystals of rare earth fluoride solid solutions in Ca, Sr and Ba fluorides described by the formula M_1?xR_xF_2+x (M=Ca, Sr, Ba; R=Y, La-Lu). The measurements have been done using dc and ac within the temperature range 300–850 K. With the increase of RF₃ concentration up to x=0.05–0.15 (in different systems) conductivity steeply increases. With the further concentration growth the conductivity increases only slightly, reaching saturation value for the solid solution Ba_1?xR_xF_2+x. The dependence of solid solution conductivity on the chemical composition has been studied for isoconcentrates M_0.9R_0.1F_2.1. It has been established that in CaF_2- based series the crystals with R=Gd, Tb possess maximum conductivity and minimum activation energy. For SrF₂ and BaF₂-based series the highest conductivity was observed for crystals containing LaF₃. It has been established that for all the crystals, independent of the chemical composition and defect concentration, the conductivity logarithm at definite temperature linearly depends on the activation energy. Within the investigated class of substances the optimum composition for solid electrolytes is Sr_0.69La_0.31F_2.31.     

Influence of composition nonstoichiometry on the electrical conductivity of LiNaGe4O9 crystals

The electrical conductivity σ of Li_{2 ? x} Na _x Ge₄O₉ (x = 1, 0.5, 0.2) crystals in an alternating-current electric field has been investigated at a frequency of 1 kHz in the temperature range of 300–800 K. A considerable anisotropy of the electrical conductivity has been revealed for crystals with a sodium concentration x = 1 at T > 500 K. It has been shown that the electrical conductivity σ along certain crystallographic directions increases by more than three orders of magnitude with a change in the sodium concentration from x = 1 to x = 0.2. The results have been discussed taking into account the specific features of the structure of the crystals under investigation. Presumably, the major charge carriers are interstitial Li ions migrating along channels of the framework structure of the Li_{2 ? x} Na _x Ge₄O₉ crystals.