Thermal conductivity of (VO)2P2O7 single crystals

Thermal conductivity ϰ of single-crystal (VO)₂P₂O₇ has been studied within the 4–300 K range. A break was found in the ϰ(T) relation about 200 K, in the region of the transition from diffuse antiferromagnetic ordering (200–4 K) to a classical paramagnet (T=200–300 K). In the low-temperature domain (4–200 K), one may expect an additional contribution to ϰ(T) from the magnon component of thermal conductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 2093–2094 (November 1998)     

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO₃. It is shown that its phase state is determined by a second-order λ transition at T _tr=520–540 K and a first-order δ transition at T _tr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO₃ have been established. Fiz. Tverd. Tela (St. Petersburg) 40, 2109–2112 (November 1998)     

Effect of intercalation by silver on the charge-density-wave state in TiSe2

The temperature dependence of electrical conductivity σ (77–300 K) and magnetic susceptibility χ (2–300 K) of Ag_xTiSe₂ in the Ag_1/4TiSe₂ phase has been studied in order to determine the possibility of preserving the charge-density-wave state in silver-intercalated TiSe₂. The behavior of χ and σ in this phase is compared with that of the starting compound TiSe₂. Fiz. Tverd. Tela (St. Petersburg) 40, 2165–2167 (December 1998)     

Fluorescence of N,N-di(2-carboxyethyl)-<Emphasis Type="Italic">p</Emphasis>-anisidine in solution and crystalline states

The fluorescence properties of N,N-di(2-carboxyethyl)-p-anisidine (I) in solvents of various nature and in the crystalline state have been studied at room temperature (273 K) and at the boiling point of liquid nitrogen (77 K). Fluorescence in aqueous solutions of I with protonated (λ _ex /λ _fl ^max = 225/290 nm) and unprotonated (λ _ex /λ _fl ^max = 270/380 nm) amino nitrogen has been detected. On going from aqueous solutions to nonaqueous, the fluorescence band of unprotonated I experiences a blue shift and its intensity rises. The fluorescence intensity of the band in aprotic polar solvents is higher than that in protic solvents. A linear dependence of the fluorescence intensity of deprotonated I on Cu(II) concentration (ranging from 1.0 to 5.0 mg/dm³) in aqueous solution has been found. The fluorescence intensity of I in aqueous solutions at 77 K and pH 1–6 has been shown to increase in the presence of Zn(II) (1–170 mg/dm³) and Cd(II) (2–330 mg/dm³) although a similar dependence is not observed at 293 K.     

Influence of magnetic field on the superconducting transition in a Nd1.82Ce0.18CuO4−δ film

The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd_1.82Ce_0.18CuO_4−δ single crystal film (T _c =6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B _c 2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

Pyroelectric properties of β-BaB2O4 in the temperature range from 2.5 to 90 K

Results are presented of experimental studies of how the spontaneous polarization of β-BaB₂O₄ (BBO) varies with temperature in the range from 2.5 to 90 K. Values of the pyroelectric coefficient γ _s are calculated, and effective values of the Debye temperature (Θ=112 K) and the Einstein temperature (Θ_E=28 K) are estimated. It is shown that the dominant contribution to γ _s comes from acoustic lattice modes. A mechanism is proposed to explain why the temperature dependence of the pyroelectric effect in linear pyroelectric materials is different from that in ferroelectrics. Fiz. Tverd. Tela (St. Petersburg) 39, 1631–1633 (September 1997)     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

Effect of doping on the magnetic properties of the low-dimensional antiferromagnet CuO

The effect of doping with Li⁺, Zn²⁺, Ni²⁺, and Ga³⁺ ions on the magnetic susceptibility of the low-dimensional antiferromagnet CuO (T _N=230 K) has been studied within a broad temperature range of 77–600 K. The solubility of impurity ions in the CuO lattice is low, ⩽3%. Impurity ions, similar to intrinsic defects, distort antiferromagnetic coupling and can shift the long-and short-range magnetic-order regions toward lower T. Fiz. Tverd. Tela (St. Petersburg) 40, 1876–1880 (October 1998)     

Anisotropy of the transport properties of single-crystal Bi2 Te3 disordered by electron bombardment

Using single-crystal samples of Bi₂Te₃ bombarded by 5-MeV electrons at a temperature of 250 K, we study the electrical resistivity and the Hall effect in the temperature range 1.7–370 K and the Shubnikov-de Haas effect at T=4.2 K in magnetic fields up to 14 T. We find that electron bombardment of Bi₂Te₃ crystals results in a transition from the metallic p-type state to the metallic state with a Fermi surface. Annealing at 350 K eliminates the radiation defects and restores the p-type metallic conductance. Zh. éksp. Teor. Fiz. 113, 1787–1798 (May 1998)     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)     

Spontaneous photorefractive effect in Sr1−x CaxTiO3 (x=0.014)

The temperature dependence of the principal values of the refractive index in Sr_1−x Ca_xTiO₃ (x=0.014) has been measured in the 17–275 K range under various conditions of sample illumination with 1.96 eV photons. The spontaneous photorefractive contribution δn ^ph to the temperature-induced variation of the refractive index of Sr_1−x Ca_xTiO₃, which appears after illumination of the sample in the ferrophase (transition temperature T _c=32 K) and persists in the paraphase under heating up to 150 K, has been separated. The photoinduced polarization has been estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 711–713 (April 1997)     

Electrically active defects in BiSiO crystals undoped and doped with Cr and Mn ions

The influence of doping of Bi₁₂SiO₂₀ (BSO) with chromium and manganese ions on the thermal depolarization currents (TDCs) is investigated. Measurements are performed in the temperature interval 300–800 K as the preliminary polarization temperature is varied in the range T _p=300–523 K. It is shown that doping significantly alters the structure of the TDC spectra. The Cr and Mn ions produce a set of new peaks over the entire investigated temperature range. The thermal activation energies are 0.85–1.98 eV (BSO:Cr) and 0.58–1.72 eV (BSO:Mn). Another consequence of doping is an increase in the amplitudes of the peaks and the charge accumulated during preliminary polarization. Fiz. Tverd. Tela (St. Petersburg) 40, 472–474 (March 1998)     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Thermal expansion of ScxTi1−x Fe2 itinerant magnets

A study of the thermal expansion coefficient (TEC) of the Sc_xTi_1−x Fe₂ itinerant magnets has been made within the 5–1200 K range at the transition from the TiFe₂ antiferromagnet (T _N=270 K) to the ScFe₂ ferromagnet (T _C=540 K). A negative TEC magnetic contribution α _m(T) has been found, which is associated with the formation of spin-fluctuation-induced local magnetic moments in both the magnetically ordered and the paramagnetic state. The specific features in the α _m(T) dependence are shown to be due to the shape of the density-of-states function near the Fermi level. Fiz. Tverd. Tela (St. Petersburg) 41, 2174–2178 (December 1999)     

Galvanomagnetic properties of the Al1−x Six nonequilibrium substitutional solid solutions

Galvanomagnetic characteristics of the A_1−x Si_x solid solutions (x<12 at. %) have been studied within a broad range of temperatures (1.8–290 K) and magnetic fields (up to 15 T). An anomaly in the concentration dependence of the Hall coefficient R _H (x,T=290 K) has been revealed near the boundary of absolute instability (x<8.5 at. %) of compounds in the Al_1−x Si_x series. The variation of the Hall coefficient and of the magnetoresistance in the A_1−x Si_x series at low (T<77 K) temperatures is analyzed within models taking into account the anisotropy in conduction-band electron scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 3–10 (January 1999)     

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F ⁻ and Cl ₃ ⁻ -center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed. Fiz. Tverd. Tela (St. Petersburg) 41, 433–441 (March 1999)     

Specific heat of the elpasolite Pb2MgWO6

The specific heat of Pb₂MgWO₆ has been measured in the temperature interval 83–370 K. An anomaly in the specific heat associated with the phase transition at T ₀=312.8K has been discovered. The thermodynamic parameters of the structural phase transition Fm3m-Pmcn have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 1686–1688 (September 1999)     

Heat conductivity and the Lorentz number of the Sm1−x GdxS “black” phase

Measurement of the heat conductivity and electrical resistivity of two Sm_1−x Gd_xS compositions with x=0.1 and 0.14 is reported within the 80–300 K interval. An analysis of experimental data on the electronic component of heat conductivity permits a conclusion that the d subband of “heavy” carriers in the conduction band of these materials lies above the s “light”-carrier subband. Fiz. Tverd. Tela (St. Petersburg) 41, 26–29 (January 1999)     

Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS₂ and TlInS₂ single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS₂ and TlInS₂ single crystals in the temperature range 110–150 K. Fiz. Tverd. Tela (St. Petersburg) 40, 612–615 (April 1998)