Phase transitions in oxyfluoride (NH4)2WO2F4

(NH₄)₂WO₂F₄ single crystals are grown, and their polarization-optical, calorimetric, and birefringence properties are studied in the temperature range 90–350 K. A first-order structural phase transition is found to occur at T ₀₁↑ = 202 K with thermal hysteresis of ΔT ₀₁ ≈ 6–12 K. The phase transition is accompanied by twinning and modification of the symmetry . An additional weak anomaly in the differential scanning calorimeter signal is found at T ₀₂ ≈ 170 K. The total thermal effect of both anomalies is ∑ΔH _i = 3200 ± 400 J/mol and ∑ΔS _i = 16.5 ± 2.0 J/mol K. The phase transition at T ₀₁ is of the order-disorder type.     

Optical studies of phase transitions in oxyfluoride (NH4)2NbOF5

Polarization-optical studies and measurements of the birefringence Δn and the angle of rotation of the optical indicatrix for the (NH₄)₂NbOF₅ crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures T ₀₁ = 258 K and T ₀₂ ≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic 1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]_or. The effect of the uniaxial compression along [011]_or and the electric field E ≈ 25 kV/cm along [100]_or on the twin structure has been studied. The ferroelastic phase transition at T ₀₁ is due to the appearance of the shear deformation x ₄(T) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence Δn(T) and in the angle of rotation of the indicatrix φ(T) at T ₀₁.     

Structural phase transition in elpasolite-like (NH4)2KWO3F3

A new chemical compound, (NH₄)₂KWO₃F₃, was synthesized. The Rietveld-refined crystal structure was found to be cubic at room temperature and to belong to the elpasolite family (space group ). The heat capacity and unit cell parameters were studied within a broad temperature range. A second-order phase transition was found to occur at 235.4 K and to be well described in terms of phenomenological theory. Hydrostatic pressure broadens the temperature interval of stability of the cubic phase (dT₀/dp = −10.8 K GPa⁻¹). A possible model of structural ordering based on a comparison of the entropy parameters and electron density distribution in oxygen and fluorine atoms is discussed.     

Effect of cation substitution in fluorine-oxygen molybdates (NH4)2 ? xAxMoO2F4

Thermophysical and structural studies of (NH₄)_{2 − x} A _x MoO₂F₄ (A = Cs, Rb, K) solid solutions of oxyfluorides have been performed. The character of the influence of cation substitutions on the stability of the initial phase (space group Cmcm) and on the mechanism of phase transitions has been elucidated.     

The phase diagram of (NH4I)x(KI)1−x

The χ,T phase diagram of (NH₄I)_x(KI)_1−x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χ_c≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational order. Close to χ_c our experiments reveal evidence for two subsequent glass transitions.     

1H relaxation study in (NH4)2SbF5

¹H spin-lattice relaxation rate (T ₁ ⁻¹ ) has been measured using inversion recovery technique in polycrystalline (NH₄)₂SbF₅ system in the temperature range 140–400 K. From the plot of log (M ₀−M) againstτ, we have estimated two differentT ₁ corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT ₁ in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.     

Mechanism and nature of phase transitions in the (NH4)3MoO3F3 oxyfluoride

The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH₄)₃MoO₃F₃ cryolite (space group Fm $ \overline 3 The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH₄)₃MoO₃F₃ cryolite (space group Fm m) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A ₂ A′MO₃ (A,A′ = NH₄, K; M = Mo, W). Original Russian Text ? I.N. Flerov, V.D. Fokina, A.F. Bovina, E.V. Bogdanov, M.S. Molokeev, A.G. Kocharova, E.I. Pogorel’tsev, N.M. Laptash, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 3, pp. 497–506.     

Calorimetric investigations of phase transitions in the cryolites (NH4)3Ga1−x ScxF6(x=1.0,0.1,0)

The specific heat of the cryolites (NH₄)₃Ga_1−x Sc_xF₆ with x=1.0, 0.1, and 0 was measured. The thermodynamic parameters of the phase transitions were determined. A previously unknown phase transition was found in the scandium compound at T=243 K. Fiz. Tverd. Tela (St. Petersburg) 41, 523–528 (March 1998)     

Electron Raman scattering in Tl2Ba2CuO6+x crystals

It is inferred from the position of the 2Δ peak in the electron light scattering spectra that in a sample with high T _c ≈90 K the ratio 2Δ/T _c is approximately equal to 6 and 7 for different polarizations, while in overdoped samples with T _c ≈40 and 35 K the 2Δ peak is observed only in x′y′ polarization with a substantially lower ratio 2Δ/T _c ≈3.2. With decreasing T _c in Tl₂Ba₂CuO_6+x crystals, a transition occurs from strong to weak coupling; the superconducting gap remains anisotropic at different doping levels. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 760–765 (10 December 1996)     

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> (R = Y,Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E ₀ ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature T _s and hysteresis ΔT _s increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.     

Thermo-optical properties of pure and Yb-doped monoclinic KY(WO4)2 crystals

The thermo-optic coefficients, dn/dT, were determined for pure and Yb(20 at.%)-doped monoclinic KY(WO₄)₂ crystals for light polarized along the optical indicatrix axes (N _p,N _m and N _g) in the wavelength range of 0.36–1.06 μm by a laser beam deviation method. The absolute values of thermo-optic coefficients satisfy the relation |dn _p/dT|>|dn _g/dT|>|dn _m/dT| and increase with the wavelength increasing. In the long-wavelength range, all the dn/dT values are negative: dn _p/dT=−14.6, dn _m/dT=−8.9, dn _g/dT=−12.4 [10⁻⁶ K⁻¹] for pure KY(WO₄)₂ at 1.06 μm. The dependency of thermo-optic coefficients on the wavelength was modeled using an approach that takes into account contribution of volumetric thermal expansion and change of electronic bandgap with temperature. Large volumetric expansion of KY(WO₄)₂ plays a key role in the observed negative dn/dT values. Electronic bandgap and its temperature coefficient were determined for KY(WO₄)₂ crystals from thermo-optic dispersion curves as E _g=4.8–5.0 eV and −dE _g/dT=0.7–1.1×10⁻⁴ eV/K. Athermal propagation directions were calculated for KY(WO₄)₂ crystals at the wavelength of 1.06 μm for light polarizations E∥N _m and N _p.     

Inelastic neutron scattering study of the specific features of the phase transitions in (NH4)2WO2F4

Oxyfluoride (NH₄)₂WO₂F₄ has been studied by the inelastic neutron scattering method over a wide temperature range 10–300 K at two initial neutron energies of 15 and 60 meV. The role of tetrahedral ammonium groups in the mechanism of sequential phase transitions at T ₁ = 201 K and T ₂ = 160 K has been discussed.     

Electrical relaxation in proton conductor composites based on (NH4)H2PO4/TiO2

We report on electrical relaxation measurements of (1 − x)NH₄H₂PO₄-xTiO₂ (x = 0.1) composites by admittance spectroscopy, in the 40-Hz–5-MHz frequency range and at temperatures between 303 and 563 K. Simultaneous thermal and electrical measurements on the composites identify a stable crystalline phase between 373 and 463 K. The real part of the conductivity, σ’, shows a power-law frequency dependence below 523 K, which is well described by Jonscher’s expression , where σ ₀ is the dc conductivity, ω _p /2π = f _p is a characteristic relaxation frequency, and n is a fractional exponent between 0 and 1. Both σ ₀ and f _p are thermally activated with nearly the same activation energy in the II region, indicating that the dispersive conductivity originates from the migration of protons. However, activation energies decrease from 0.55 to 0.35 eV and n increases toward 1.0, as the concentration of TiO₂ nanoparticles increases, thus, enhancing cooperative correlation among moving ions. The highest dc conductivity is obtained for the composite x = 0.05 concentration, with values above room temperature about three orders of magnitude higher than that of crystalline NH₄H₂PO₄ (ADP), reaching values on the order of 0.1 (Ω cm)^− 1 above 543 K.     

Spectroscopy of quasiparticle excitations of superconducting bismuth cuprate at high pressures

We study the energy spectrum of Bi2223 (Bi_1.6Pb_0.4Sr_1.8Ca_2.2Cu₃O_x) at high hydrostatic pressures by Andreev-and tunneling-spectroscopy methods. We determine the gap anisotropy in the basal ab plane and find the following values for the parameters Δ(ϕ): Δ_max=42 mV, and Δ_min=19.5 mV (T _c =110 K and dT _c /dP=0.16 K/kbar). We detect an increase in the ratio R=2Δ_max/kT _c with pressure P; for Bi2223 cuprate, dR/dP≈0.017 kbar⁻¹. In the phonon-frequency region we detect a “softening,” due to pressure, of the high-frequency part of the phonon spectrum corresponding to “breathing” modes of oxygen, as well as other optical modes of Cu-O. The characteristic frequencies of the spectrum for ℏΩ>60 mV are found to decrease, with increasing pressure, at a rate d ln(ℏΩ)/dP≈−6.5±0.5×10⁻³ kbar⁻¹. This result explains the observed increase in the ratio 2Δ/kT _c (P) in the model of strong electron-phonon interaction. Zh. éksp. Teor. Fiz. 113, 1397–1410 (April 1998)     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Magnetism,exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS₃ is a layered antiferromagnet (T _N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate⁵ T _2g ground state of the Fe²⁺ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σ_i [δ(L _iz ² −2)+|λ|L _i .S _i ]−Σ _ij J _ij S _i .S _j . The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,J _ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;J _nn k=27.7 K; andJ _nnn k=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT _N=122 K for FePS₃, which is remarkably close to the observed value of theT _N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS₃, for which the high temperature series expansion susceptibility is available.     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

Magnetic properties of the new organic superconductor λ-(BETS)2GaCl4

This papr discusses the results of the first investigation of the magnetic properties of the organic superconductor λ-(BETS)₂GaCl₄. It is shown that the transition to the superconducting state begins at T _c≈7 K, which is considerably lower than the value T _c≈10 K determined from resistive measurements. The estimated value of the critical current density turns out to be two orders of magnitude lower than in superconductors of the family κ-(ET)₂ X. Zh. éksp. Teor. Fiz. 112, 760–762 (August 1997)     

Intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ single crystals

We report on thec-axis superconducting energy gap parameter Δ _c (T) of intrinsic Josephson tunnel junctions inBi ₂ Sr ₂ CaCu ₂ O _8+δ (Bi2212) single crystals. Δ _c (4.2K)≈10−13 meV, which is approximately a factor of two smaller than reported in the majority of tunneling experiments. Δ _c (T) deviates strongly from the BCS temperature dependence. These observations may be explained by a multilayer model of Bi2212 which assumes that theBi−O layers are superconducting due to the proximity effects. The Josephson tunneling then takes place between adjacentBi−O layers while there is a strong proximity coupling betweenBi−O andCu−O layers. The work is supported by Swedish Supercon-ductivity Consortium and NUTEK, and, in part, by Russian Foundation for Basic Research, grant #95-02-04307     

Temperature behavior of conductivity in a La2CuO4 + δ single crystal upon the paramagnetic-antiferromagnetic transition

The temperature dependence of the resistance of a La₂CuO_{4 + δ} (δ ≈ 0.05) single crystal with the Neel temperature T _N ≈ 205 K was investigated in order to establish the correlation between the transport and magnetic properties of the crystal. The R(T) dependence near T _N reveals a kink related to the enhancement of sample’s conductivity upon the transition from the antiferromagnetic to paramagnetic state. With an increase in temperature far above T _N, the transition from the dielectric (dR/dT < 0) to metal (dR/dT > 0) occurs. The observed behavior of resistance is attributed to delocalization of carriers above T _N.