Specific heat of the [NH<Subscript>2</Subscript>(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystal in the temperature region 80–300 K

The specific heat of [NH₂(CH₃)₂]₂ZnCl₄ was measured calorimetrically in the temperature region 80–300 K. As the temperature T decreases, the C _p (T) dependence indicates a phase transition sequence, with the phase transition at T₆=151 K observed for the first time. The thermodynamic characteristics of the crystal were refined. The transformation occurring at T₂=298.3 K is shown to be an incommensurate-commensurate phase transition.     

<Superscript>2</Superscript>H NMR investigation of the transition to the proton glass state in the Cs<Subscript>5</Subscript>H<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript> · 0.5H<Subscript>2</Subscript>O crystal

A crystal of the Cs₅H₃(SO₄)₄ · xH₂O (x ≈ 0.5) (PCHS) compound, which belongs to the family of proton conductors with a complex system of hydrogen bonds, is investigated by ²H NMR spectroscopy. The temperature and orientation dependences of the ²H NMR spectra are measured and analyzed. It is established that, upon transition to the glassy phase at the temperature T _g = 260 K, the parameters characterizing the proton exchange between positions in hydrogen bonds remain unchanged to within the limits of experimental error. The protons in the two-dimensional network of hydrogen bonds in the (001) plane are dynamically disordered over possible positions down to temperatures considerably lower than the glass transition point T _g . However, water molecules are fixed at particular structural positions in the phase transition range. In PCHS crystals with a nonstoichiometric water content, this circumstance can be responsible for the frustration that leads to the formation of the glassy state.     

Origin of the structural local transition in the molecular impurity ion MnO<Stack><Subscript>4</Subscript><Superscript>2−</Superscript></Stack> in the K<Subscript>3</Subscript>Na(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript> ferroelastic

A new model is proposed for a local transition in a Jahn-Teller impurity center in a crystal with a ferroelastic (ferroelectric) phase transition. This model is based on direct interaction of the order parameter of the phase transition in the matrix with the Jahn-Teller impurity degrees of freedom. It is shown that, under these conditions, the order parameter field can induce lifting of degeneracy of the electronic states active in the Jahn-Teller effect, which is accompanied by a transition from the Jahn-Teller effect to the pseudo-Jahn-Teller effect with its subsequent suppression. As a result, a decrease in temperature gives rise to a structural local transition in the region of the low-symmetry ferroelastic (ferroelectric) matrix phase from the many-well local adiabatic to a single-well potential. The model proposed allows interpretation of experimental data obtained in an EPR study of the molecular impurity ion MnO ₄ ^2? in the K₃Na(CrO₄)₂ ferroelastic.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Heat capacity of the [N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CdBr<Subscript>4</Subscript> crystal in the temperature range 80–300 K

The heat capacity of the [[N(C₂H₅)₄]₂CdBr₄ crystal is measured by the calorimetric method in the temperature range from 80 to 300 K. It is revealed for the first time that the temperature dependence of the heat capacity C _p (T) exhibits an anomaly associated with the first-order phase transition occurring at the temperature T ₁ = 226.5 K. A long relaxation of the temperature of the crystal is observed in the temperature range 150–165 K.     

O<Subscript>2</Subscript> photodesorption from AuO<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> and Au<Subscript>2</Subscript>O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>

Using time-resolved photoelectron spectroscopy, the decay channels of AuO₂⁻ and Au₂O₂⁻ following photoexcitation with 3.1-eV photons have been studied. For AuO₂⁻, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au₂O₂⁻, the spectra indicate O₂ desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au _n O₂⁻ with analogous data on Ag _n O₂⁻ clusters, a discernible pattern emerges: for dissociatively bound O₂(AuO₂⁻, Ag₃O₂⁻), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au₂O₂⁻, Ag₂O₂⁻, Ag₄O₂⁻, Ag₈O₂⁻), the 3.1-eV photoexcitation triggers fast O₂ desorption with a high quantum yield.     

Anomalous states of the structure of [Rb<Subscript>0.7</Subscript>(NH<Subscript>4</Subscript>)<Subscript>0.3</Subscript>]<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals in the temperature range 4.2–300 K

Crystals of [Rb_0.7(NH₄)_0.3]₂SO₄ solid solutions are studied using x-ray diffractometry in the temperature range 4.2–300 K. No anomalies are revealed in the temperature dependences of the lattice parameters and the volume of the host unit cell. A series of superstructure reflections observed along the basis axes corresponds to the guest lattice formed in the matrix of the host structure. From analyzing the axial ratio of these structures and their temperature dependences, it is concluded that the structure of the crystal has the form of an incommensurate composite. The guest structure of the composite at room temperature can be considered a set of chains that are not correlated along the b direction. In the plane perpendicular to the chain axes, these chains form a regular framework that is also incommensurate to the host lattice.     

Fractal magnetic microstructure in the (Co<Subscript>41</Subscript>Fe<Subscript>39</Subscript>B<Subscript>20</Subscript>)<Subscript>x</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite films

Magnetostructural methods are applied to determine the exchange bond percolation limit in (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_1?x nanocomposites (x _c = 0.30 ± 0.02), which separates the phase plane along the metal concentration axis into a superparamagnetic region and a ferromagnetic region. It is shown that, with respect to the singularities of the magnetization up to the magnetization saturation curves, the ferromagnetic region is further subdivided into three regions differing in the character of the spatial propagation of the magnetization ripples or in the magnetic correlation function characteristics. The fractal dimension of the nanocomposite magnetic microstructure near the percolation threshold is determined.     

Calculation of refractive indices for the (NH<Subscript>2</Subscript>CH<Subscript>2</Subscript>COOH)<Subscript>2</Subscript> · HNO<Subscript>3</Subscript> crystal

The refraction R of the diglycine nitrate (DGN) crystal, (NH₂CH₂COOH)₂ · HNO₃, in the para-and ferroelectric phases has been calculated in the model of noninteracting diatomic chemical bonds of the elementary unit cell of the crystal on the basis of the longitudinal and transversal polarizabilities of these bonds. The calculated magnitudes of the principal refractive indices n _p , n _m , and n _g and the orientations of the optical indicatrix of the crystal agree satisfactorily with experimentally observed values. Introducing the coefficient of Lorenz-Lorentz interaction x into the corresponding formula permits better agreement of the calculated and experimental refractive indices of DGN crystal to be obtained. The temperature changes of these x coefficients upon the ferroelectric phase transition in the DGN crystal have been analyzed.     

Mössbauer and magnetic studies of the phase state of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/La<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> composites

The formation of an intermediate phase in SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composites was demonstrated for the first time using only Mössbauer spectroscopy. The SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composite was prepared by the two-stage (sol–gel and hydrothermal) synthesis with varying initial conditions. The X-ray diffraction studies showed that the composite consisted of two phases: well-formed structures of manganite La_0.9Ca_0.1MnO₃ and hexagonal ferrite SrFe₁₂O₁₉. It was found that nanocrystalline La_0.9Ca_0.1MnO₃ particles with size d ? 150 nm formed in the composites at the surface of plate-like SrFe₁₂O₁₉ crystallites. The Mössbauer studies showed that the composite contained additional (intermediate) phase La_0.9Ca_0.1Mn(Fe)O₃ that formed at the interface between SrFe12O19 and La_0.9Ca_0.1MnO₃ phases. The intermediate phase concentration increased with the molar content of La_0.9Ca_0.1MnO₃; in this case, the fraction of the surface of SrFe₁₂O₁₉ crystallites coated with La_0.9Ca_0.1MnO₃ increased, which led to the increase in the total area of the interface surface and the intermediate phase concentration.     

Phase formation in the BaCO<Subscript>3</Subscript>–PbO–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Features of the formation of lead-ferroniobate compounds in the xBaCO₃–(1 – x)PbO–Fe₂O₃–Nb₂O₅ system by solid-phase synthesis are investigated. For perovskite-type lead-ferroniobate solid solution, a single-phase concentration region is revealed at 1233 K. The crystalline structures of the synthesized compounds are refined using Rietveld analysis and the Pm3?m and R3m space groups. Ceramic samples of lead ferroniobate are studied by scanning electron microscopy.     

Optical investigations of the clathrate α-Eu<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Ge<Subscript>30</Subscript>

We performed measurements of the optical reflectivity in the energy range 0.007–30 eV on the clathrate-VIII type compound α-Eu₈Ga_{16- x}Ge_{30 x} in order to investigate its electronic band structure. The very low charge carrier concentration as well as ferromagnetic ordering of the divalent Eu ions below 10.5 K characterize the spectra at photon energies below ≃0.4 eV in accordance with the results of band structure calculations. Disorder induced bound states have been identified to affect the optical conductivity at energies between 10 and 100 meV.     

Internal friction and magnetoelectric response in two-layer composites Tb<Subscript>0.12</Subscript>Dy<Subscript>0.2</Subscript>Fe<Subscript>0.68</Subscript>–PbZr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>O<Subscript>3</Subscript>

The inverse magnetoelectric effect and internal friction in two-layer composites based on ferromagnetic Tb_0.12Dy_0.2Fe_0.68 and piezoelectric PbZr_0.53Ti_0.47O₃ are studied in an ac electrical field in the frequency range of 52–213 kHz at temperatures of 293 to 400 K. A correlation is found between the internal friction and the efficiency of the inverse magnetoelectric transformation at resonant frequencies.     

μSR study of Eu<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> and EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

A comparative μSR study of ceramic samples of the EuMn₂O₅ and Eu_0.8Ce_0.2Mn₂O₅ multiferroics is performed in the temperature range from 15 to 300 K. It is found that the Ce doping of the EuMn₂O₅ sample slightly reduces the temperature of the magnetic phase transition from T _N = 45 K for the EuMn₂O₅ sample to T _N = 42.5 K for the Eu_0.8Ce_0.2Mn₂O₅ sample. Below the temperature T _N for both samples, there are two types of localization of a thermalized muon with different temperature dependences of the precession frequency of the magnetic moment of the muon in an internal magnetic field. The higher frequency in both samples refers to the initial antiferromagnetic matrix. The behavior of this frequency in Eu_0.8Ce_0.2Mn₂O₅ follows the Curie–Weiss law with the exponent β = 0.29 ± 0.02, which differs from the value β = 0.39 standard for 3D Heisenberg magnetics and is observed in EuMn₂O₅, because of the strong frustration of the doped sample. The temperature-independent low frequency is due to the presence of Mn³⁺–Mn⁴⁺ ferromagnetic pairs located along the b axis of the antiferromagnetic matrix and in the regions of phase separation, which contain such ion pairs and e _g electrons recharging them. In both samples, polarization losses are the same (about 20%) and are associated with the formation of Mn⁴⁺–Mn⁴⁺ + Mu complexes near Mn³⁺–Mn⁴⁺ ferromagnetic pairs. In the temperature interval from 25 to 45 K, the separation of the Eu_0.8Ce_0.2Mn₂O₅ structure into two fractions where the relaxation rates of polarization of muons differ by an order of magnitude is revealed. This effect is due to a change in the state of regions of phase separation (1D superlattices) at the indicated temperatures. Such effect in EuMn₂O₅ is significantly weaker.     

High-Temperature Heat Capacity of Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>–Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> Solid Solutions

Differential scanning calorimetry has been used to study the influence of temperature on the heat capacity of synthesized vanadates Zn₂V₂O₇, (Cu_0.56Zn_1.44)V₂O₇, and (Cu_1.0Zn_1.0)V₂O₇. It is found that dependences C_p = f(T) have extremes. The thermodynamic properties of Zn₂V₂O₇ have been determined.     

Synthesis and characterization of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>·LiMn<Subscript>0.33</Subscript>Fe<Subscript>0.67</Subscript>PO<Subscript>4</Subscript>/C cathode materials

A new polyanionic cathode material, Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C for lithium-ion batteries, was synthesized using a sol-gel method and with N,N-dimethyl formamide as a dispersion agent. The analysis of electron transmission spectroscopy and X-ray diffraction revealed that the composite contained two phases. The material has high crystallinity with a grain size of 20–50 nm. The valence states of Mn, V, and Fe in the composite were analyzed by X-ray photoelectron spectroscopy. The electrochemical kinetics in Li₃V₂(PO₄)₃ is effectively enhanced by the incorporation of LiMnPO₄ and LiFePO₄, via structure modification and reduced Li diffusion length. The Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C materials displayed high rate capacity and steady cycle performance with discharge capacity remained 148 mAh g^?1 after 50 cycles at the rate of 0.2C. In particular, the composite exhibited excellent reversible capacities, with the values of 157, 134, 120, 102, and 94 mAh g^?1 at charge/discharge 0.2, 0.5, 1, 2, and 5C rates, respectively.     

Cluster spin glass and superparamagnetism in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10-</Subscript><Subscript>δ</Subscript>

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr₂Eu_1.5Ce_0.5Cu₂O_{10- δ} (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Preparation and characterization of organic–inorganic hybrid compound [N(C<Subscript>4</Subscript>H<Subscript>9</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>Cu<Subscript>2</Subscript>Cl<Subscript>6</Subscript>

Organic–inorganic hybrid sample [N(C₄H₉)₄]₂Cu₂Cl₆ was prepared via the reaction between copper chloride and tetrabutylammonium chloride. The compound was characterized by X-ray powder diffraction, IR, Raman, differential scanning calorimetry (DSC), DTA-TGA analysis and electrical impedance spectroscopy. DSC studies indicate a presence of one-phase transition at 343 K. The complex impedance of compound [N(C₄H₉)₄]₂Cu₂Cl₆ have been investigated in temperature and frequency ranges 300–380 K and 200 Hz–5 MHz, respectively. The Z′ and Z″ versus frequency plots are well fitted to an equivalent circuit model. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The frequency dependence of the conductivity is interpreted in term of Jonscher's law: s(w) = s_dc + Awⁿ \sigma (\omega ){ } = {\sigma_{\rm{dc}}} + { }A{\omega^n} . The conductivity follows the Arrhenius relation. The variation of the value of these elements with temperatures confirmed the availability of the phase transition at 343 K detected by DSC and electrical measurements.