Growth,structure, and magnetic properties of CuFeTe<Subscript>2</Subscript> single crystals

CuFeTe₂ single crystals were grown and the temperature dependence of their magnetic susceptibility in the temperature range 1.8–400 K was investigated. It is found that the magnetic susceptibility shows anomalies at temperatures T _s = 65 and T _N = 125 K. At T > 125 K, the crystal is in the paramagnetic state controlled by Fe²⁺ and Cu²⁺ ions with an effective magnetic moment of 1.44 μB.     

Electrical conductivity of CsCuCl<Subscript>3</Subscript> crystals at structural phase transition

The electrical conductivity σ of CsCuCl₃ single crystals synthesized by the crystallization method from aqueous solutions in the ternary CsCl–CuCl₂–H₂O system has been studied. The σ measurements for CsCuCl₃ crystals have been carried out in the temperature range of 397–455 K, which covers the structural phase transition from the low-temperature (sp. gr. P6₁22, Z = 6) to the high-temperature (sp. gr. P6₃mc, Z = 2) modification at T _tr = 423 ± 8 K. A jump of σ by a factor of ~3 is observed on the σ(T) dependence in the region of the structural transition, which indicates the existence of first-order phase transition. The electric transfer activation enthalpies ΔHσ are found to be 1.0 ± 0.1 eV at T > T _tr and 0.8 ± 0.1 eV at T < T _tr. The σ value for CsCuC_l3 crystals amounts to 7 × 10^–6 S/cm at 455 K.     

Specific features of the crystal structure and magnetic properties of KTaO<Subscript>3</Subscript> produced by electrolysis of melts

The magnetic susceptibility χ(T) at 4.2 K < T < 293 K; the dependence of the magnetic moment on the magnetic field strength, M(H), at 4.2, 77, and 293 K; and the electrical resistivity ρ(T) at 4.2 K < T < 293 K are studied for samples of perovskite-phase KTaO₃ obtained by both solid-phase synthesis (KTaO ₃ ^s ) and deposition on a cathode during electrolysis of melts (KTaO ₃ ^e ). Yellowish white KTaO ₃ ^s powders are diamagnetic and reveal dielectric properties. Dark polycrystalline KTaO ₃ ^e samples with metallic luster are characterized by the dependence ρ(T) typical of metals and additional paramagnetic contribution to the paramagnetic susceptibility as compared with KTaO ₃ ^e . Changes in the properties of KTaO₃ during electrocrystallization are attributed to partial reduction of tantalum. They are revealed in the structural features of KTaO ₃ ^e (excess of tantalum as compared to the stoichiometric composition of KTaO ₃ ^e , deficiency of the oxygen sublattice, and clearly pronounced anharmonicity of atomic vibrations). A change of the cation-anion-cation interactions, occurring owing to the overlapping of oxygen p orbitals with tantalum t_2g orbitals and the formation of impurity levels near the conduction band, leads to the generation of free carriers, which make a paramagnetic contribution to the magnetic susceptibility.     

Separate determination of the amplitude of thermal vibrations and static atomic displacements in titanium carbide by neutron diffraction

The amplitude of thermal (dynamic) atomic vibrations and meansquare static atomic displacements in titanium carbide TiC _x (x = 0.97, 0.88, 0.70) have been separately determined by measuring neutron diffraction patterns at two temperatures (T ₁ = 300 K and T ₂ = 80 K). The static lattice distortions in stoichiometric titanium carbide are experimentally found to be negligible. In the TiC _x homogeneity range, the amplitude \(\sqrt {\overline {u_{dyn}^2 } } \) of thermal atomic vibrations significantly increases with a decrease in the carbon concentration. The Debye temperature has been determined for the first time in the TiC _x homogeneity range at both room and liquid-nitrogen temperatures.     

Structural transformations in Cu<Subscript>1.95</Subscript>Ni<Subscript>0.05</Subscript>S crystals

A solid solution of the Cu_1.95Ni_0.05S composition has been synthesized for the first time due to the partial replacement of Cu with Ni atoms in Cu₂S. The polymorphic transformations in the polycrystalline samples in the temperature range of 300–1400 K have been investigated by X-ray diffraction and differential thermal analysis. It is established that, at room temperature, the synthesized Cu_1.95Ni_0.05S samples have an orthorhombic lattice with unit-cell parameters a = 26.50 Å, b = 15.39 Å, and c = 13.85 Å (sp. gr. Abm2). Heating to T = 379 ± 2 K leads to its transformation into a hexagonal lattice with parameters a = 3.960 Å and c = 6.78 Å (sp. gr. P6₃/mmc). At 750 ± 2 K, the hexagonal modification is transformed into a cubic one with period a = 5.788 Å (sp. gr. Fm\(\bar 3\)m). The phase transition in this crystal is enantiotropic.     

Characteristic of spontaneous polarization in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The temperature behavior of the spontaneous polarization of lead tetragermanate, a uniaxial ferroelectric, is studied in the range from 4.2 to 300 K. The results obtained along with the data from the literature make it possible to reconstruct a complete pattern of the behavior of P _s (T) both in the vicinity of the phase transition and at lower temperatures. In the range from 290 K to T_C, the crystal behavior is found to change from the dipole type (β = 1/2) to the pseudoquadrupole type (β = 1/4). This specific crossover manifests itself in the change in the behavior of P _s ^1/β as a function of (T_C-T). In the low-temperature range, weak anomalies in the dependenceP _s >(T) are found, which point to the occurrence of contributions from the dipole moments of separate structural fragments of Ge₂O₇ and GeO₄, which have internal degrees of freedom and are weakly bound to the dynamics of the crystal lattice.     

Vibrational modes of hydrogen in (NH<Subscript>4</Subscript>)H<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> (Neutron scattering and simulation)

The vibrational modes of hydrogen in (NH₄)H₅(PO₄)₂ have been investigated by inelastic incoherent neutron scattering at temperatures above (220 K) and below (160 and 5 K) the phase-transition point T _c = 180 K. Computer simulation of vibrational modes has been performed for the low-temperature phase (NH₄)H₅(PO₄)₂ at 5 K. The calculated generalized incoherent dynamic factor and the partial incoherent dynamic factors for all hydrogens make it possible to identify the observed modes from acid hydrogens and hydrogens of ammonium ions.     

High-pressure synthesis and magnetic properties of Bi<Subscript>0.5</Subscript>D<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">D</Emphasis> = Pb,Ba) solid solutions

Perovskites Bi_0.5D_0.5MnO₃(D = Pb, Ba) were prepared under high pressure (4 GPa) at 1200–1300°C. According to the X-ray diffraction data, crystalline Bi_0.5Pb_0.5MnO₃ has a tetragonal unit cell with the parameters a = 3.940 Å and c = 3.800 Å, whereas Bi_0.5Ba_0.5MnO₃ crystals are cubic with a = 3.940 Å. It is concluded from magnetic studies that lead-containing manganite is an antiferromagnet with T_N = 120 K, whereas Bi_0.5Ba_0.5MnO₃ is a spin glass with spin-freezing temperature T _f = 38 K. Both compounds are decomposed upon heating in air at temperatures above 500°C. With the use of synthesis in air, Bi_0.5Ca_{0.5 ? x}D _x MnO₃ solid solutions with x as high as 0.25 were obtained.     

Crystal and magnetic structure of the Sm<Subscript>0.45</Subscript>Sr<Subscript>0.55</Subscript>MnO<Subscript>3</Subscript> manganite studied by neutron powder diffraction

Neutron diffraction investigation of the ¹⁵²Sm_0.45Sr_0.55MnO₃ manganite is performed. The diffraction data are compared with the magnetic and transport properties of this compound. The parameters of the crystal and magnetic structures are determined. Manganite belongs to the orthorhombic system (sp. gr. Pnma) and has a perovskite-like structure in the entire temperature range under study (1.5–260 K). The ground state of the ¹⁵²Sm_0.45Sr_0.55MnO₃ manganite at low temperatures is a single-phase A-type antiferromagnetic insulator with T _N ～ 180 K.     

Structural study of new compound Bi<Subscript>2.53</Subscript>Li<Subscript>0.29</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> by the powder neutron diffraction method

A new compound of composition Bi_2.53Li_0.29Nb₂O₉ was synthesized in the course of the search for new materials with high ionic conductivity. Its crystal structure was determined from the neutron diffraction data. The new compound Bi_2.53Li_0.29Nb₂O₉ is crystallized in the orthorhombic system, sp. gr. Cmc2₁, and unit-cell parameters a = 24.849(1) Å, b = 5.4536(3) Å, and c = 5.4619(2) Å at T = 290 K (a = 24.843(2) Å, b = 5.4456(5) Å, and c = 5.4546(5) Å at T = 10 K). Within the temperature range 10–870 K, no structural phase transitions were revealed. The atomic coordinates and the thermal factors in the isotropic approximation were refined by the Rietveld method at 290 and 10 K. The data obtained were analyzed based on the calculated local balance of bond strengths.     

Use of a fluorine-containing solvent in the synthesis of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> single crystals

A laboratory technological procedure has been developed for the synthesis of high-temperature superconducting YBa₂Cu₃O_{7 ? δ} single crystals (T _c ～ 90 K, ΔT _c ～ 1.0 K) up to 0.25 cm² in size from a nonstoichiometric fluorine-containing flux of (YO_1.5)(BaO)_{4 ? x}(BaF₂)_x(CuO)₁₀, where 2 ≥ x ≥ 0, using a combination of enhanced nucleation and directional crystallization by the Czochralski method. Studies using differential thermal analysis demonstrated that the addition of BaF₂ decreased the eutectic-crystallization temperature and increased the crystal-growth rate. The optimum concentration of BaF₂ in the starting melt composition was found (x = 0.2). The single-crystal surface was studied by atomic-force nanoscopy. The morphology of single crystals that have been synthesized from a melt of their own components differs substantially from that of crystals grown from a BaF₂-containing melt.     

Structural features of single crystals of LuB<Subscript>12</Subscript> upon a transition to the cage-glass phase

The unit-cell parameters of dodecaboride LuB₁₂, which undergoes a transition to the cage-glass phase, have been determined for the first time in the temperature range of 50–75 K by X-ray diffraction, and the single-crystal structure of this compound is established at 50 K. Nonlinear changes in the unit-cell parameters correspond to anomalies in the physical properties near the glass-transition temperature T* ~ 50–70 K. This compound has cubic symmetry at room temperature, and it is reduced to tetragonal symmetry at lower temperatures. Based on the X-ray diffraction data and relying on the physical properties of the crystals, the structure model, in which a small part (~15%) of Lu atoms are displaced from the 2a sites at the centers of the B₂₄ cuboctahedra to the 16n sites of sp. gr. I4/mmm, seems preferable.     

Elastic and dielectric properties of A<Superscript>II</Superscript>B<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack> (<Emphasis Type="Italic">A</Emphasis> = Cd or Zn, <Emphasis Type="Italic">B</Emphasis> = P or As) single crystals

Elastic and dielectric properties of CdP₂, ZnP₂, and ZnAs₂ single crystals are investigated at frequencies of 10², 10³, 10⁴, 10⁶, and 10⁷ Hz in the [00l], [h00], and [hk0] directions in the temperature range 78–400 K. The elastic constants, the Gruneisen parameters, and the force constants of the crystals are calculated from the measured ultrasonic velocities. The elastic constants C_ij decrease with an increase in temperature and anomalously change in narrow (ΔT = 10–20 K) temperature ranges. The permittivity sharply increases from ε ≈ 7–14 at 78–150 K to ε ≈ 10²–10³ in the temperature range 175–225 K without any signs of a structural phase transition. The behavior of the temperature-frequency dependences of the complex permittivity ε*(f, T) is typical of relaxation processes. The dielectric relaxation in A^IIB ₂ ^V is considered on the basis of the model of isolated defects. The conuctivity σ of the single crystals under study is a sum of the frequency-dependent (hopping) conductivity σ_h and the conductivity σ_s that is typical of semiconductors. The hopping conductivity increases with an increase in frequency according to the law σ _h ～f^α, where α < 1 at low temperatures and α > 1 at high temperatures.     

Behavior of ammonium ion in β-LiRb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(NH<Subscript>4</Subscript>)<Subscript>x</Subscript>SO<Subscript>4</Subscript> mixed crystals (0.5 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1.0)

The influence of replacement of an ammonium ion by rubidium on the x-T phase diagram in the concentration range 0.50 ≤ x ≤ 1.0 has been studied by X-ray and neutron powder diffraction over a wide temperature range. It is shown that a decrease in the ammonium concentration is accompanied by an increase in the temperature of the II ? I phase transition and stabilization of phase II up to low temperatures (20 K). The changes occurring in the dynamics of mixed crystals are studied by inelastic incoherent neutron scattering. The spectra of the generalized vibrational density of states obtained allowed one to establish the difference in the phonon modes corresponding to phase III of β-LiNH₄SO₄ and phase II of β-LiRb_1?x(NH₄)_xSO₄ mixed crystal with x = 0.91 and x = 0.77 at 20 K. It is shown that a mixed crystal with x = 0.77 at 20 K is in the orientational glass state.     

Conditions for single-crystal formation during bridgman growth

The microstructure of IV–VI crystals grown by varying the shape of the conical bottom of a ampoule, the melt superheating ΔT⁺ with respect to the liquidus temperature of a blend, the axial temperature gradient in a furnace ΔT, and the pulling rate of a ampoule v were studied. It was found that the nucleation of a single-crystal seed is most likely under the following conditions: ΔT ≤ 20 K/cm, ΔT⁺ < 30 K, and v < 0.4 mm/h. The conical bottom of a ampoule should have walls of constant thickness. The principle of geometric selection of a single-crystal seed is not efficient. The obtained results are discussed with regard to data on the structure and behavior of IV–VI melts upon heating and cooling.     

Computer simulation of the self-assembly of paulingite crystal structure from suprapolyhedral nanocluster precursors K6, K16, and K20

A combinatorial and topological analysis of the paulingite crystal structure (a = 35.093 Å, V = 43 217 ų; sp. gr. Im \(\bar 3\) m) has been performed by computational methods using the TOPOS program package. The application of the complete expansion of the 3D factor graph into nonintersecting substructures of a cluster type has revealed three types of nanocluster precursors in the tetrahedral T framework: K6, K16, and K20; they consist of 6T, 16T, and 20T tetrahedra, which are involved in the matrix self-assembly of the crystal structure. The translated cell contains 44 clusters (8 K6+ 24 K16 + 12 K20). None of the clusters have shared T tetrahedra. Three cluster precursors form a crystallochemically complex structure with extraframework Na⁺/Ca²⁺ and K⁺/Ba²⁺ cations which carry out two structural functions as templates (stabilizing nanocluster precursors) and as spacers (filling the voids between precursors). The self-assembly code of the 3D structure from complementary bound nanocluster precursors is completely reconstructed in the form primary chain → microlayer → microframework → … framework.     

Magnetoplastic effect: Basic properties and physical mechanisms

This paper presents a review of the main results of investigations into the magnetoplastic effect, which manifests itself in motion of dislocations in crystals exposed to magnetic fields. The dependences of the mean free path of dislocations on the induction and direction of the magnetic field, the magnetic treatment time, the temperature, and the type and concentration of impurities are studied for NaCl, LiF, CsI, Zn, Al, and InSb crystals. The threshold magnetic field B _c below which the effect is absent, the saturation field B₀ above which the mean free paths of dislocations remain constant with an increase in the magnetic induction B, and the critical frequency v _c of rotation of a sample in the magnetic field above which the effect disappears are examined. The quantities B _c , B₀, and v _c are investigated as functions of the basic physical parameters. It is found that the magnetoplastic effect is highly sensitive to X-ray radiation at low doses and to simultaneous action of an electric field or mechanical loading. The hardening of NaCl(Pb) crystals in the magnetic field is revealed. The theoretical interpretation is proposed for all the findings and dependences observed.     

Low-temperature X-ray diffraction studies of [(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>]<Subscript>5</Subscript>Cd<Subscript>3</Subscript>Cl<Subscript>11</Subscript> crystals

[(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are grown by the method of isothermal evaporation from saturated aqueous solutions containing dimethylamine and cadmium chlorides, [(CH₃)2NH₂]Cl and CdCl_2.5H₂O. The crystal grown are studied by the X-ray diffraction method. It is established that the crystals are orthorhombic with the unit-cell parameters at room temperature a = 18.115 ± 0.004 Å, b = 11.432 ± 0.002 Å, and c = 15.821 ± 0.003 Å. The unit-cell parameters a, b, and c of the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are measured as functions of temperature in the temperature range 100–320 K. The data obtained were used to determine the thermal expansion coefficients along the main crystallographic axes. The temperature curves of the unit-cell parameters and thermal expansion coefficients showed pronounced anomalies in the vicinity of the temperatures T ₁ = 120, T ₂ = 150, and T ₃ = 180 K corresponding to the phase transitions in the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals. The crystals are also characterized by a pronounced anisotropy of thermal expansion.     

Magnetic properties and crystal structure of β-Ta

Polycrystalline samples of β-Ta with Frank-Kasper σ-phase structure prepared by electrolysis are studied. The atomic parameters are determined by the Rietveld method. The magnetic susceptibility is measured in the temperature range 4.2 < T < 273 K and the dependence of the magnetization on the magnetic field strength is measured at 4.2 and 77 K. The analysis of interatomic distances and the results of magnetic measurements indicate that clusters with a localized excess charge exist in β-Ta. The data obtained in this study suggest structural changes in β-Ta at T < 77 K.     

X-ray diffraction study of Ba<Subscript>3</Subscript>TaFe<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> single crystal—a promising langasite-type multiferroic

Ba₃TaFe₃Si₂O₁₄ single crystals (sp. gr. P321, Z = 1), promising langasite-type multiferroics, have been grown by floating zone melting. An accurate X-ray diffraction study of Ba₃TaFe₃Si₂O₁₄ single crystal has been performed using two datasets, obtained independently for two different orientations of the same sample on a diffractometer equipped with a CCD area detector at 295 K. Structure refinement is performed based on an averaged dataset: a = 8.5355(1) Å, c = 5.2332(1) Å, sp. gr. P321, Z = 1; the R factors of model structure refinement were found to be R/wR = 1.02/1.23% for 4552 independent reflections. Disordering asymmetry is revealed for the magnetic Fe ion in the 3f site and the Ba cation in the 3e site.