Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

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.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

AC and DC conductivity study of LiH<Subscript>2</Subscript>PO<Subscript>4</Subscript> compound using impedance spectroscopy

The lithium dihydrogen phosphate LiH₂PO₄ has been investigated by X-ray powder diffraction, scanning electron microscopy (SEM), and electrical impedance spectroscopy. The Rietveld refinements based on the XRD patterns show that the compound is crystallized in the orthorhombic system with Pna2₁ space group, and the refined unit cell parameters are a = 6.2428 Å, b = 7.6445 Å, and c = 6.873 Å. The electrical properties were studied using complex impedance spectroscopy as a function of frequency (10⁴–10⁷ Hz) at various temperatures (300–400 K). The Nyquist plots are well fitted to an equivalent circuit consisting of a series of combination of grains and inhomogeneous electrode surface effect. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. Moreover, the near value of the activation energies obtained from the equivalent circuit and analysis of M″ confirms that the transport is through ion hopping mechanism dominated by the motion of the proton in the structure of the investigated material.     

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.     

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 new one-dimensional hybrid material lattice: AC conductivity and structural characterization of [C<Subscript>7</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>][CdCl<Subscript>4</Subscript>]

The present paper reports the synthesis, crystal structure, ¹³C and ¹¹¹Cd cross-polarization magic-angle spinning nuclear magnetic resonance(CP-MAS-NMR) analysis and ac conductivity for a new organic–inorganic hybrid salt, [C₇H₁₂N₂][CdCl₄]. The compound crystallizes in the triclinic system, space group P\( \overline 1 \), with unit cell dimensions: a?=?7.1050(3) Å, b?=?8.9579(3) Å, c?=?9.4482(3) Å, α?=?81.415(1)°, β?=?89.710(2)°, γ?=?85.765(1)°, V?=?592.97(4) Å³, and Z?=?2. The asymmetric unit is composed of one-2,4-diammonium toluene cation and one [CdCl₄]^2? anion. The Cd atom is in a slightly distorted octahedra coordination environment. Its structure can be described by infinite chains of CdCl₆ octahedron linked to organic cations by a strong charge-assisted N–H???Cl interactions in order to build organic–inorganic layers staked along \( \left[ {0\overline 1 1} \right] \) direction. The solid state ¹³C CP-MAS-NMR spectra has shown seven isotropic resonances, confirming the existence of seven non-equivalent carbon atoms, which is consistent with crystal structure determined by X-ray diffraction. As for ¹¹¹Cd MAS-NMR, it has shown one cadmium site with isotropic chemical shift observed at 167.2 ppm. The complex impedance of the compound has been investigated in the temperature range of 403–460 K and in the frequency range of 200 Hz–5 MHz. The impedance plots have shown semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements.     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Substitution effects on the bulk and surface properties of (Li,Ni)Mn<Subscript>2</Subscript>O<Subscript>4</Subscript>

Manganese oxides of spinel structure, LiMn₂O₄, Li_1-x Ni _x Mn₂O₄ (0.25 ≤ x≤ 0.75), and NiMn₂O₄, were studied by EDS, XRD, SEM, magnetic (M-H, M-T), and XPS measurements. The samples were synthesized by an ultrasound-assisted sol-gel method. EDS analysis showed good agreement with the formulations of the oxides. XRD and Rietveld refinement of X-ray data indicate that all samples crystallize in the Fd3m space group characteristic of the cubic spinel structure. The a-cell parameter ranges from a = 8.2276 Å (x = 0) to a = 8.3980 Å (x = 1). SEM results showed particle agglomerates ranging in size from 2.3 μm (x = 0) down to 0.8 μm (x = 1). Hysteresis magnetization vs. applied field curves in the 5–300K range was recorded. ZFC-FC measurements indicate the presence of two magnetic paramagnetic-ferrimagnetic transitions. The experimental Curie constant was found to vary from 5 to 7.1 cm³ K mol^?1 for the range of compositions studied (0 ≤ x ≤ 1). XPS studies of these oxides revealed the presence of Ni²⁺, Mn³⁺, and Mn⁴⁺. The experimental Ni/Mn atomic ratios obtained by XPS were in good agreement with the nominal values. A linear relationship of the average oxidation state of Mn with Ni content was observed. The oxide’s cation distributions as a function of Ni content from x = 0 ?Li⁺[Mn³⁺Mn⁴⁺]O₄ to x = 1 \( {\mathrm{Ni}}_{0.35}^{2+}{\mathrm{Mn}}_{0.65}^{3+}\left[{\mathrm{Ni}}_{0.65}^{2+}\right.\left.{\mathrm{Mn}}_{1.35}^{3+}\right]{\mathrm{O}}_4 \) were proposed.     

Superconductivity in Ti-doped iron-arsenide compound Sr<Subscript>4</Subscript>Cr<Subscript>0.8</Subscript>Ti<Subscript>1.2</Subscript>O<Subscript>6</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr₄Cr_0.8Ti_1.2O₆Fe₂As₂ (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.     

Phase transition in a KPb<Subscript>2</Subscript>Br<Subscript>5</Subscript> crystal

Crystals of the KPb₂Br₅compound are investigated using polarized light microscopy and calorimetry. The birefringence and the angle of rotation of the optical indicatrix are measured in the temperature range 270–620 K. It is found that the KPb₂Br₅ crystal undergoes a first-order ferroelastic phase transition at temperatures T_0↑ = 519.5 K and T_0↓ = 518.5 K with a change in the enthalpy ΔH = 1300 ± 200 J/mol. This transition is accompanied by both twinning and the symmetry change mmm ? P2₁/c. It is revealed that the angle of rotation of the optical indicatrix exhibits an unusual behavior under variations in the temperature due to a strong temperature dependence of the birefringence.     

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.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

An ab initio study of ion-pair states of the Br<Subscript>2</Subscript> molecule

This contribution reports an ab initio study of 18 ion-pair states of the Br₂ molecule correlating with the Br⁺(³P_J=2,1,0,¹D₂) +Br^–(¹S₀) dissociation asymptotes. Calculations were performed on the CASSCF/CASPT2 level of theory taking into account electron correlations and with inclusion of the spinorbit coupling. Ab initio results are compared with the experiment. For gerade and ungerade ion-pair potentials, the difference between calculated and experimental values of the equilibrium internuclear distance does not exceed ΔR_e = ±0.01 and ±0.05 Å, respectively. For the states correlating to the lowest dissociation asymptote Br⁺(³P₂) + Br^–(¹S₀), the accuracy of relative energies is postilions ΔT_e ± 0.02 eV. The dipole moment functions for some transitions between the ion-pair and valence states were calculated as well.     

The magnetization reversal in CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/CoFe<Subscript>2</Subscript> granular systems

The temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., M _FC and M _ZFC, measured under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange–spring CoFe₂O₄/CoFe₂ composites with different relative content of CoFe₂. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature T _irr. For the sample with less CoFe₂, the curves of ?d(M _FC ? M _ZFC)/dT versus temperature T exhibit a broad peak at an intermediate temperature T ₂ below T _irr , and the moments are suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the ?d(M _FC ? M _ZFC)/dT curves of the sample with more CoFe₂, besides a broad peat at an intermediate temperature T ₂, a rapid rise around the low temperature T ₁~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T ₂ and T _irr towards a lower temperature, and the shift of T ₂ is attributable to the moment reversal of CoFe₂O₄.

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Graphical abstract CoFe₂O₄/CoFe₂ composites with different relative content of CoFe₂ were prepared by reducing CoFe₂O₄ in H₂ for 4 h (S4H) and 8 h (S8H). The temperature-dependent FC and ZFC magnetizations, i.e., M _FC and M _ZFC, under different magnetic fields from 500 Oe to 20 kOe have been investigated. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at field-dependent irreversible temperature T _irr. For the S4H sample, the curves of ?d(M _FC ? M _ZFC)/dT versus temperature T exhibit a broad and field-dependent relaxing peak at T ₂ below T _irr (figure a), and the moments were suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the S8H sample, it exhibits the reentrant spin-glass state around 50 K, as evidenced by a peak in the M _FC curve (inset in figure b) and as a result of the cooperative effects of the random anisotropy of CoFe₂O₄, exchange–spring occurring at the interface of CoFe₂O₄ and CoFe₂ together with the inter-particle dipolar interaction (figure c); in ?d(M _FC ? M _ZFC)/dT curves, besides a broad relaxing peat at T ₂, a rapid rise around the low-temperature T ₁~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T ₂ and T _irr towards a lower temperature, and the shift of T ₂ is attributable to the moment reversal of CoFe₂O₄.

     

Phase separation of the spin system in the La<Subscript>0.93</Subscript>Sr<Subscript>0.07</Subscript>MnO<Subscript>3</Subscript> crystal

The magnetic state of the manganite La_0.93Sr_0.07MnO₃ in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La_1?xSr_xMnO₃ solid-solution system, in which the antiferromagnetic type of order (LaMnO₃, T_N=139.5 K) switches to ferromagnetic ordering (La_0.9Sr_0.1MnO₃, T_C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 Å, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La_0.93Sr_0.07MnO₃ spin system being accounted for by Mn⁴⁺ ion ordering.     

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.     

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.     

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.     

Improved Scaling of the Magnetic Heat Capacity in La<Subscript>0.85</Subscript>Ag<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> Manganite

The heat capacity in a La_0.8 Ag_0.15 MnO₃ manganite has been measured near the Curie temperature T _C in applied magnetic fields up to 26 kOe to study the scaling critical behavior and to obtain the universality class. The conventional scaling fails in application to the manganites with a hysteresis and the strong sensitivity of T _C to a magnetic field. However, the application of the improved scaling procedure designed by us allows yielding the good scaling the magnetic heat =0.23 capacity in La_0.85Ag_0.15MnO₃, which may belong to a new universality class for systems with the strong spin-orbital coupling of t _2g -electrons, namely, double -Heisenberg with the critical exponent of the heat capacity α = ?0.23 and the critical exponent of the correlation radius v=0.7433. This new universality class is consistent with the crystal, magnetic and orbital symmetries for the La_0.85Ag_0.15MnO₃. Scaling failure in the vicinity of T _C in the range of t/H ^1/2ν ≈ [?0.033;0.024] is understood by finite-size and other disordering effects when T →T _C. It is remarkable that finite-size effect is consistent with grain size, L ≈ 50 μm, in the La_0.85Ag_0.15MnO₃. The correlation radius, Lt ^ν ≈ 30.28 Å, estimated from the finite-size effect is of the same order of magnitude with the sizes of the ferromagnetic fluctuations and drops in manganites.