Nonstoichiometric high-pressure phase of Tm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

Tm _x Cu₃V₄O₁₂, a perovskite-like oxide (space group, Im-3; Z = 2; a = 7.279–7.293 Å) containing vacancies in its cationic sublattice, was obtained barothermally (P = 7.0–9.0 GPa, t = 1000–1100°C) for the first time. The temperature dependences on the electrical resistivity (10–300 K) and the magnetic susceptibility (0–300 K) were investigated. It was shown that the oxide Tm _x Cu₃V₄O₁₂ is characterized by metal-type conductivity and paramagnetic properties.     

Specific features of anion transfer in HoF<Subscript>3</Subscript> crystals at high temperatures

The anionic conductivity of HoF₃ single crystals with a β-YF₃ structure (orthorhombic crystal system, space group Pnma) is investigated over a wide range of temperatures (323–1073 K). The unit cell parameters of HoF₃ crystals are as follows: a=0.6384±0.0009 nm, b=0.6844±0.0009 nm, and c=0.4356±0.0005 nm. It is revealed that the conductivity anisotropy of the HoF₃ crystals is insignificant over the entire temperature range covered. The crossover from one mechanism of ion transfer to another mechanism is observed near the critical temperature T_c≈620 K. The activation enthalpy of electrical conduction is found to be ΔH₁=0.744 eV at T<T_c and ΔH₂=0.43 eV at T>T_c. The fluorine vacancies are the most probable charge carriers in HoF₃ crystals. The fluorine ionic conductivities at temperatures of 323, 500, and 1073 K are equal to 5×10^?10, 5×10^?6, and 2×10^?3 S cm^?1, respectively.     

CeAlO<Subscript>3</Subscript> crystals: Preparation and study of their electrical and optical characteristics

Crystals of cerium aluminate with perovskite structure were obtained using the cold-crucible technique. The electrical and optical properties of cerium aluminate were studied in air in the range 300–1300 K. The main characteristics of CeAlO₃ at T=300 K are a follows: electrical conductivity σ=10^?7 S/cm, dielectric permittivity ?=3000–10000 (both measured at a frequency of 1000 Hz), thermal band-gap width ΔE=2.3±0.5 eV, and optical width δE=2.65±0.25 eV, which decreases at a rate of ?0.62×10^?3 eV/K with increasing temperature in the 300-to 1500-K interval.     

Electrical instability of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu₂O₂ phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T₀/T)^1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.     

Synthesis and properties of high-pressure phase [Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>](V<Subscript>4</Subscript>)O<Subscript>12</Subscript>

A new perovskite-like compound Er_0.73Cu₃V₄O₁₂ (space group Im \(\bar 3\), Z = 2, a = 7.266 Å) has been synthesized barothermally (P = 8.0 GPa, t = 1000°C). Its electrical and magnetic properties have been studied. It is found that the temperature dependence of the electrical conductivity (in the range 78–300 K) has of semiconductor type. The behavior of the impedance and admittance has been analyzed at 290 K and frequencies of 200 Hz to 200 kHz under atmospheric pressure and at high (15–42 GPa) pressures.     

Internal friction of Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

This paper reports on the results of measurements of the internal friction Q^?1 and the shear modulus G of Li₂B₄O₇ single crystals along the crystallographic directions [100] and [001] in the temperature range 300–550 K for strain amplitudes of (2–10)×10^?5 at infralow frequencies. The anomalies observed in Q^?1 and G in the temperature range 390–410 K are due to thermal activation of the mobility of lithium cations and their migration from one energetically equivalent position to another. A jump in the internal friction background is revealed in the vicinity of the Q^?1 and G anomalies for the Li₂B₄O₇ crystal. The magnitude of this jump depends on the crystallographic direction.     

Thermopower in the region of hopping conduction in TlNiS<Subscript>2</Subscript>

Samples of the composition TlNiS₂ in the hexagonal system with the unit cell parameters a=12.28 Å, c=19.32 Å, and ρ=6.90 g/cm³ are synthesized. The results of the investigation into the electrical and thermoelectrical properties of TlNiS₂ samples in the temperature range 80–300 K indicate that TlNiS₂ is a p-type semiconductor. It is found that, at temperatures ranging from 110 to 240 K, TlNiS₂ samples in a dc electric field possess variable-range-hopping conduction at the states localized in the vicinity of the Fermi level. The density of localized states near the Fermi level is determined to be N_F=9×10²⁰ eV^?1 cm^?3, and the scatter of the states is estimated as J≈2×10^?2 eV. In the temperature range 80–110 K, TlNiS₂ exhibits activationless hopping conduction. At low temperatures (80–240 K), the thermopower of TlNiS₂ is adequately described by the relationship α(T)=A+BT, which is characteristic of the hopping mechanism of charge transfer. In the case when the temperature increases to the temperature of the onset of intrinsic conduction with the activation energy ΔE=1.0 eV, there arise majority intrinsic charge carriers of both signs. This leads to an increase in the electrical conductivity σ and, at the same time, to a drastic decrease in the thermopower α; in this case, the thermopower is virtually independent of the temperature.     

A theoretical investigation of structural,mechanical, electronic and thermoelectric properties of orthorhombic CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript>

The structural, mechanical, electronic and thermoelectric properties of the low temperature orthorhombic perovskite phase of CH₃NH₃PbI₃ have been investigated using density functional theory (DFT). Elastic parameters bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt–Reuss–Hill averaging scheme. Phonon dispersions of the structure were investigated using a finite displacement method. The relaxed system is dynamically stable, and the equilibrium elastic constants satisfy all the mechanical stability criteria for orthorhombic crystals, showing stability against the influence of external forces. The lattice thermal conductivity was calculated within the single-mode relaxation-time approximation of the Boltzmann equation from first-principles anharmonic lattice dynamics calculations. Our results show that lattice thermal conductivity is anisotropic, and the corresponding lattice thermal conductivity at 150 K was found to be 0.189, 0.138, and 0.530 Wm^?1K^?1 in the a, b, and c directions. Electronic structure calculations demonstrate that this compound has a DFT direct band gap at the gamma point of about 1.57 eV. The electronic transport properties have been calculated by solving the semiclassical Boltzmann transport equation on top of DFT calculations, within the constant relaxation time approximation. The Seebeck coefficient S is almost constant from 50 to 150 K. At temperatures 100 and 150 K, the maximal figure of merit is found to be 0.06 and 0.122 in the direction of the c-axis, respectively.     

Electrical conductivity and permittivity of the one-dimensional superionic conductor LiCuVO<Subscript>4</Subscript>

The electrical conductivity σ^a and permittivities ?^a, ?^b, and ?^c of a LiCuVO₄ single crystal have been measured along the a, b, and c crystallographic axes, respectively, in the temperature range 300–390 K at a frequency of 10³ Hz. The temperature dependences σ(T) and ?(T) were found to be typical for superionics.     

Studies on structural and electrical properties of Mg<Subscript>0. 5+y</Subscript> (Zr<Subscript>2-y</Subscript>Fe<Subscript>y</Subscript>) <Subscript>2</Subscript> (PO<Subscript>4</Subscript>) <Subscript>3</Subscript> ceramic electrolytes

The sample of Mg_{0. 5+y} (Zr_1-y Fe_y) ₂ (PO₄) ₃ (0.0 ≤y ≤0.5) was synthesized using the sol-gel method. The structures of the samples were investigated using X-ray diffraction and Fourier transform infrared spectroscopy measurement. XRD studies showed that samples had a monoclinic structure which was iso-structured with the parent compound, Mg_0.5Zr (PO₄) ₃. The complex impedance spectroscopy was carried out in the frequency range 1–6 MHz and temperature range 303 to 773 K to study the electrical properties of the electrolytes. The substitutions of Fe³⁺ with Zr⁴⁺ in the Mg_0.5Zr (PO₄) ₃ structure was introduced as an extrainterstitial Mg²⁺ ion in the modified structured. The compound of Mg_0.5+y (Zr_1-y Fe_y)₂(PO₄)₃ with y?=?0.4 gives a maximum conductivity value of 1.25?×?10^?5 S cm^?1 at room temperature and 7.18?×?10^?5 S cm^?1 at 773 K. Charge carrier concentration, mobile ion concentration, and ion hopping rate are calculated by fitting the conductance spectra to power law variation, σ _ac (ω)?=?σ _o ? +?Aω ^α . The charge carrier concentration and mobile ion concentration increases with increase of Fe³⁺ inclusion. This implies the increase in conductivity of the compounds was due to extra interstitial Mg²⁺ ions.     

Magnetic and electrical properties of Fe<Subscript>1.91</Subscript>V<Subscript>0.09</Subscript>BO<Subscript>4</Subscript> warwickite

We have performed a complex investigation of the structure and the magnetic and electrical properties of a warwickite single crystal with the composition Fe_1.91V_0.09BO₄. The results of Mössbauer measurements at T=300 K indicate that there exist “localized” (Fe²⁺, Fe³⁺) and “delocalized” (Fe_2.5+) states distributed over two crystallographically nonequivalent positions. The results of magnetic measurements show that warwickite is a P-type ferrimagnet below T=130 K. The material exhibits hopping conductivity involving strongly interacting electrons. The experimental data are analyzed in comparison to the properties of the initial (unsubstituted) Fe₂BO₄ warwickite. The entire body of data on the electric conductivity and magnetization are interpreted on a qualitative basis.     

Crystal-Physical Model of Ion Transport in Nonlinear Optical Crystals of KTiOPO<Subscript>4</Subscript>

The ionic conductivity along the principal axes a, b, and c of the unit cell of the nonlinear-optical high-resistance KTiOPO₄ single crystals (rhombic syngony, space group Pna2₁), which are as-grown and after thermal annealing in vacuum, has been investigated by the method of impedance spectroscopy. The crystals were grown from a solution-melt by the Czochralski method. The as-grown KTiOPO₄ crystals possess a quasi-one-dimensional conductivity along the crystallographic c axis, which is caused by the migration of K⁺ cations: σ_║c = 1.0 × 10^–5 S/cm at 573 K. Wherein the characteristics of the anisotropy of ionic conductivity of the crystals is equal to σ_║c/σ_║a= 3 and σ_║c/σ_║b= 24. The thermal annealing at 1000 K for 10 h in vacuum increases the magnitude of σ_║c of KTiOPO₄ by a factor of 28 and leads to an increase in the ratio σ_║c/σ_║b= 2.1 × 10³ at 573 K. A crystal-physical model of ionic transport in KTiOPO₄ crystals has been proposed.     

Mechanisms of low-temperature high-frequency conductivity in systems with a dense array of Ge<Subscript>0.7</Subscript>Si<Subscript>0.3</Subscript> quantum dots in silicon

High-frequency (HF) conductivity in systems with a dense (with a density of n = 3 × 10¹¹ cm^?2) array of self-organized Ge_0.7Si_0.3 quantum dots in silicon with different boron concentrations n_B is determined by acoustic methods. The measurements of the absorption coefficient and the velocity of surface acoustic waves (SAWs) with frequencies of 30–300 MHz that interact with holes localized in quantum dots are carried out in magnetic fields of up to 18 T in the temperature interval from 1 to 20 K. Using one of the samples (n_B = 8.2 × 10¹¹ cm^?2), it is shown that, at temperatures T ≤ 4 K, the HF conductivity is realized by the hopping of holes between the states localized in different quantum dots and can be explained within a two-site model in the case of

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, where ω is the SAW frequency and τ₀ is the relaxation time of the populations of the sites (quantum dots). For T > 7 K, the HF conductivity has an activation character associated with the diffusion over the states at the mobility threshold. In the interval 4 K < T < 7 K, the HF conductivity is determined by a combination of the hopping and activation mechanisms. The contributions of these mechanisms are distinguished; it is found that the temperature dependence of the hopping HF conductivity approaches saturation at T* ≈ 4.5 K, which points to a τ₀ ≤ 1. A value of τ₀(T*) ≈ 5 × 10^?9 s is determined from the condition ωτ₀(T*) ≈ 1.

     

Conduction through localized states in a single crystal of the TlGa<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> alloy

Layered single crystals of the TlGa_0.5Fe_0.5Se₂ alloy in a dc electric field at temperatures ranging from 128 to 178 K are found to possess variable-range-hopping conduction along natural crystal layers through states localized in the vicinity of the Fermi level. The parameters characterizing the electrical conduction in the TlGa_0.5Fe_0.5Se₂ crystals are estimated as follows: the density of states near the Fermi level N_F = 2.8 × 10¹⁷ eV^?1 cm^?3, the spread in energy of these states ΔE = 0.13 eV, the average hopping length R_av = 233 Å, and the concentration of deep-lying traps N _t = 3.6 × 10¹⁶ cm^?3.     

Thermal conductivity and heat capacity of LuMgCu<Subscript>4</Subscript>

This paper reports on the measurements of the thermal conductivity κ and electrical resistivity ρ in the temperature range 5–300 K and the heat capacity at constant pressure C _p in the range 80–300 K for the metallic nonmagnetic compound LuMgCu₄. The experimental values of κ and C _p for the LuMgCu₄ compound are compared with the corresponding data available in the literature for the light heavy-fermion compound YbMgCu₄. It is shown that, in the low-temperature range (5–20 K), the phonon thermal conductivity κ_ph of YbMgCu₄ is lower than κ_ph of LuMgCu₄ as a result of phonon scattering from magnetic moment fluctuations of the Yb 4f electrons and, conversely, the heat capacity of LuMgCu₄ in the range 80–300 K is lower than that of YbMgCu₄ because the heat capacity of the latter compound has an additional magnetic component.     

Conductivity and spectroscopic studies of Li<Subscript>2</Subscript>O-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

Lithium vanadium-borate glasses with the composition of 0.3Li₂O–(0.7-x)B₂O₃–xV₂O₅ (x?=?0.3, 0.325, 0.35, 0.375, 0.4, 0.425, 0.45, and 0.475) were prepared by melt-quenching method. According to differential scanning calorimetry data, vanadium oxide acts as both glass former and glass modifier, since the thermal stability of glasses decreases with an increase in V₂O₅ concentration. Fourier transform infrared spectroscopy data show that the vibrations of [VO₄] structural units occur at V₂O₅ concentration of 45 mol%. It is established that the concentration of V⁴⁺ ions increases exponentially with the growth of vanadium oxide concentration. Direct and alternative current measurements are carried out to estimate the contribution both electronic and ionic conductivities to the value of total conductivity. It is shown that the electronic conductivity is predominant in the total one. The glass having the composition of 0.3Li₂O-0.275B₂O₃-0.475V₂O₅ shows the highest electrical conductivity that has the value of 7.4?×?10^?5 S cm^?1 at room temperature.     

Divalent rare-earth ions in LaF<Subscript>3</Subscript> crystals

The optical spectra and electric conductivity of LaF₃ crystals doped with 0.01, 0.1, and 0.3 mol % YbF₃, where Yb was partly or completely recharged to the divalent state, are studied. The long-wavelength absorption band of 370 nm is caused by electrons transitioning from state 4f ¹⁴ to the level of anion vacancies. The remaining bands at 300–190 nm are caused by 4f ¹⁴–5d ¹4f ¹³ transitions in Yb²⁺. The bulk electric conductivity and peaks of the dielectric losses of LaF₃–Yb²⁺ crystals are caused by Yb²⁺–anion vacancy dipoles. The activation energy of the reorientation of Yb dipoles is 0.58 eV. The optical and dielectric properties of Yb²⁺ centers are compared to those of Sm²⁺ and Eu²⁺ centers studied earlier in LaF₃ crystals.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Electrophysical parameters of <Emphasis Type="Italic">c</Emphasis>-oriented Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> films with a low concentration of antistructural defects

Epitaxial c-oriented Bi₂Te₃ films 1.2 μm in thickness are grown by the hot wall method for a low supersaturation of the vapor phase over the surface of mica substrates. The hexagonal unit cell parameters a = 4.386 Å and c = 30.452 Å of the grown films almost coincide with the corresponding parameters of stoichiometric bulk Bi₂Te₃ crystals. At T = 100 K, the Hall concentration of electrons in the films is on the order of 8 × 10¹⁸ cm^?3, while the highest values of the thermoelectric coefficient (α ≈ 280 μV K^?1) are observed at temperatures on the order of 260 K. Under impurity conduction conditions, conductivity σ of the films increases upon cooling in inverse proportion to the squared temperature. In the temperature range 100–200 K, thermoelectric power parameter α²σ of Bi₂Te₃ films has values of 80–90 μW cm^?1 K^?2.     

Charge carrier mobility in crystals of the Pb<Subscript>0.67</Subscript>Cd<Subscript>0.33</Subscript>F<Subscript>2</Subscript> superionic conductor

The frequency (ν = 10^?1–10⁷ Hz) dependences σ(ν) of the conductivity of single crystals of the Pb_0.67Cd_0.33F₂ superionic conductor with the fluorite-type structure (CaF₂) in the temperature range of 132–395 K have been studied. The dependences σ(ν) have been discussed in the framework of the hopping relaxation of ionic carriers, which are mobile anions F^?. From experimental curves σ(ν), the direct-current (dc) conductivity σ_dc and the average charge carrier hopping frequency ν_h have been determined. This has made it possible to calculate the charge carrier mobility μ_mob and charge carrier concentration n _mob in these crystals. At room temperature (293 K), the electrical parameters are σ_dc = 1.6 × 10^?4 S/cm, ν_h = 2.7 × 10⁷ Hz, μ_mob = 2.0 × 10^?7 cm²/(s V), and n _mob = 5.1 × 10²¹ cm^?3.