Coexistence of superfluid and solid helium in aerogel

The results of recent neutron scattering studies of solid helium in silica aerogel are discussed. Previously I.V. Kalinin et al., Pis’ma Zh. éksp. Teor. Fiz. 87 (1), 743 (2008) [JETP Lett. 87 (1), 645 (2008)], we detected the existence of a superfluid phase in solid helium at a temperature below 0.6 K and a pressure of 51 bar, although, according to the phase diagram, helium should be in the solid state under these conditions. This work is a continuation of the above studies whose main goal was to examine the detected phenomenon and to establish basic parameters of the existence of a superfluid phase. We have determined the temperature of the superfluid transition from solid to superfluid helium, T _C = 1.3 K, by analyzing experimental data. The superfluid phase excitation parameters (lifetime, intensity, and energy) have a temperature dependence similar to that of bulk helium. The superfluid phase coexists with the solid phase in the entire measured temperature range from T = 0.05 K to T _C and is a nonequilibrium one and disappears at T _C.     

Infrared spectra of solid germane

An IR spectroscopic study has shown that GeH₄ molecules are situated on one set of sites of C, symmetry in phase IV. The III–IV transition at 63 K is apparently a second-order phase transition. In phases II and III the molecules are on sites of C_3v. or C₃ symmetry. The II–III phase transition was observed at 67 K. In solid GeD₄, phase transitions were observed at 68.5 and 77 K. In phases II and III the site symmetry is C_I. The II–III phase transition in GeD₄ is apparently second order. There is evidence that the ν₁, vibration of GeD₄ is IR active in the solid state.     

Radiation-induced phase transition in a film of niobium-tin solid solution

A 5-μm-thick film of a niobium-tin solid solution (18.3 at % Sn) grown by ion-plasma sputtering with subsequent codeposition of ultrafine metal particles is irradiated by a fast proton flux with a fluence of 10¹⁹ p ⁺/cm². X-ray diffraction analysis carried out using CuK _α (λ = 0.154178 nm), CoK _α (λ = 0.179021 nm), and MoK _α (λ = 0.071069 nm) radiations shows the presence of a radiation-induced stannide niobium (Nb₃Sn) phase in the region of proton energy dissipation (at a depth of 2.5 μm from the surface of the solid solution film). It is found that at the end of the particle range, nonlocal interaction between the protons and the concentrated matrix solid solution takes place. When interacting with the supersaturated solid solution, a bombarding particle covers a tangible area of the solution, so that an intermetallic phase greater than critical in size nucleates. The feasibility is demonstrated of using a fast particle flux to produce an intermetallic (superconducting) phase inside a solid solution layer with a composition close to the stoichiometric composition of the intermetallic.     

Calculation of phase diagrams for solid solutions of ferroelectrics

The integral equations for calculating ferroelectric and antiferroelectric phase transition temperatures, order parameters, and critical concentrations of solid solution components are derived. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is constructed taking into account the contribution of nonlinear effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature on the composition of a binary solid solution in the ferroelectric-antiferroelectric and ferroelectric-paraelectric systems is calculated. Numerical calculations are carried out for the PbTi_xZr_1?x O₃ and BaZr_xTi_1?x O₃ solid solutions. The results obtained are in good agreement with the experimental phase diagrams of these systems. Analysis of the results indicates that any solid solution containing ferroelectric (antiferroelectric) and paraelectric components transforms into a relaxor state at sufficiently high concentrations of the paraelectric component.     

Formation of NI80P20 solid solution phase under pressure

A Ni-P solid solution phase was obtained by quenching of melts under a pressure of 4.5 GPa. This was considered as a metastable high pressure phase. Despite the lack of thermodynamic parameters for Ni₈₀, P₂₀ alloy under pressure, the degree of undercooling, nucleation frequency and crystal growth velocity were calculated. We conclude that metastable phases with the same composition as the melting phase, such as supersaturated solid solution phase and amorphous phase, are easily prepared by high-pressure quenching.     

Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of Lu _x Y_{1 ? x} BO₃ solid solutions doped with Ce³⁺ or Eu⁺³ have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration x < 0.5. For a higher lutetium concentration x, the solid solutions contain an additional calcite phase with a content less than 5 wt %. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the Lu _x Y_{1 ? x} BO₃: Ce³⁺ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce³⁺ 5d(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the Lu _x Y_{1 ? x} BO₃: Eu³⁺ solid solutions with intermediate concentrations x, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.     

The Auger spectrum of solid ammonia

The Auger electron spectrum of solid ammonia has been measured using Al K_α radiation as an excitation source. The spectral profile has been analysed in terms of seven main peaks, as for the free molecule spectrum, along with an additional satellite peak at higher kinetic energy. The major change from the gas phase to the solid state spectrum is a significant lifetime broadening of the peaks involving the 3a₁-type molecular orbital. In the free molecule the 3a₁ orbital is occupied by a lone pair of electrons, whereas, in the solid these electrons are primarily responsible for the hydrogen bonding between the NH₃ entities forming the molecular crystal.     

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi_{1 ? x} Pr _x FeO₃ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi_{1 ? x} Pr _x FeO₃ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO₃ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi_{1 ? x} Pr _x FeO₃ solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Evidence for a solid phase of dodecahedral C 20

Evidence is presented for the formation of a solid phase based on the smallest fullerene, C₂₀, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10^-4 torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C₂₀, C₂₁ and C₂₂ species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C₂₀ cages as building blocks. Unlike solid C₆₀ and fully protonated C₂₀, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C₂₀ dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell. Received 10 October 2002 / Received in final form 24 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: zafar.iqbal@njit.edu     

Fine structure in the proton magnetic resonance spectrum of solid benzene and solid benzene-cyclohexane mixtures

The PMR spectrum of solid benzene in the vicinity of the melting point consists of broad and narrow lines. New narrow lines in the spectrum of benzene containing some impurity appear at the top of the broad line. Intensities of narrow lines increase with rising of temperature. The temperature dependence of line width and relaxation timesT ₁ andT ₂ have been measured in pure solid benzene an benzene-cyclohexane mixtures. It is assumed that narrow lines in spectra of pure benzene and its mixtures are caused by appearance of a phase with mobile molecules located in the crystallite joint or on its surface. The spectrum of imperfect monocrystalline benzene have been studied also. The narrow line of this sample is split into several narrow components. This is explained by demagnetization fields the distribution of which has a discrete character,     

Specific heat of solid deuterium in the region of the proposed rotational glass phase

The specific heat of solid deuterium of 33% paradeuterium concentration has been measured for 1 K > T > 0.115K by a thermal relaxation technique. No evidence of a phase transition was found in the samples in the temperature range associated with the molecular rotational glass phase. No thermal remanence effects were observed for characteristic measurements times greater than 30 seconds. Integration of the specific heat indicates that rotational entropy of the solid below 0.35 Kelvin equals about 10% of the free rotator entropy, in agreement with ?P/?T)_V studies of solid H₂.     

Experimental determination of interfacial energies for Ag2Al solid solution in the CuAl2--Ag2Al system

The equilibrated grain boundary groove shapes of solid solution Ag2Al in equilibrium with an Al-Cu-Ag liquid were observed from a quenched sample with a radial heat flow apparatus.The Gibbs-Thomson coefficient,solid-liquid interfacial energy and grain boundary energy of the solid solution Ag2Al have been determined from the observed grain boundary groove shapes.The thermal conductivity of the solid phase and the thermal conductivity ratio of the liquid phase to solid phase for Ag2Al-28.3 at the %CuAl2 alloy at the melting temperature have also been measured with a radial heat flow apparatus and Bridgman type growth apparatus,separately.     

Ceria co-doped with calcium (Ca) and strontium (Sr): a potential candidate as a solid electrolyte for intermediate temperature solid oxide fuel cells

Co-doped samples of Ce_0.95?x Ca_0.05Sr _x O_1.95?x , where (x?=?0.00, 0.01, 0.02, and 0.03), have been prepared by auto-combustion method and characterized to explore their use as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Crystal structure, microstructure, and ionic conductivity have been characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy, respectively. All the compositions have been found to be single phase. Results show that the samples co-doped with Ca and Sr exhibit higher ionic conductivity than the samples singly doped with Ca in the intermediate temperature range. Ce_0.93Ca_0.05Sr_0.02O_2?δ exhibits maximum conductivity among all the compositions. This may be a potential candidate as a solid electrolyte for IT-SOFCs.     

Formation of BaTiO3 thin films from (110) TiO2 rutile single crystals and BaCO3 by solid state reactions

The formation of BaTiO₃ thin films from (110) TiO₂ rutile single crystals and BaCO₃ was investigated experimentally by solid–solid and gas–solid reactions in vacuum. X-ray diffraction revealed the formation of an intermediate Ba₂TiO₄ phase before BaTiO₃ is formed. According to our calculations the formation of Ba₂TiO₄ is associated with a maximum decrease in the Gibbs energy at a CO₂ pressure lower than 10^− 4 mbar. Reactions at 600–900 °C showed different processes to occur in the solid–solid and gas–solid reactions. The observations are interpreted in terms of the different mass transport mechanisms involved. The results shed new light on the phase sequence during BaTiO₃ formation; in particular a dissociation of BaCO₃ prior to its participation in the reaction has become rather unlikely.     

Vibrational modes and phase transition of Si-Ge solid solution

We present the simplified treatment where the lattice vibrations of Si or Ge atoms in the Si-Ge solid solution are replaced with that of pure Si or Ge crystal at lattice constants of the alloy. Considering the volume effect on the force constants of the pure constituent, we obtain the phonon dispersion curves of the local and band modes for Si_0.91Ge_0.09 and Si_0.11Ge_0.89 systems and the concentration x-dependence of the local and band modes frequencies in the Si_1?xGe_x solid solutions. Then, from the calculation of the effective mode Grüneisen parameter γ_i for the average phonon modes in the Si_1?xGe_x systems, we obtain the predominant correlation between TA mode Grüneisen parameter γ^X_TA at the point X and the phase transition pressure P_t, and the softening of TA modes is related to the pressure-induced phase transition of the Si-Ge solid solution.     

Effect of phase transitions on thermal desorption from a solid surface

Thermal desorption from the surfaces of films and polycrystals undergoing semiconductor-metal (VO₂), ferroelectric-paraelectric (Ba_0.9Sr_0.1TiO₃ and TsTS-19), ferromagnet-paramagnet (Ni) phase transitions has been investigated. A sharp increase in the desorption activity of a surface was observed near a phase transition. The increase in the thermal desorption signal is caused by local deformations which arise in a solid at a phase transition. Fiz. Tverd. Tela (St. Petersburg) 39, 573–576 (March 1997)     

Positron annihilation in solid,liquid and undercooled melts of Co80Pd20

Positron annihilation measurements have been performed in solid, liquid and undercooled Co₈₀Pd₂₀ alloy using electromagnetic levitation as containerless processing method. The formation enthalpy for a single vacancy is H_1v = (1.7 ± 0.1) eV. In the melt, the thermal expansion continues linear in the undercooled phase and is larger than that of the solid alloy. The mean free volume in the liquid phase is slightly larger than the volume of a single vacancy. At the Curie temperatures of both solid and liquid phase, the S-parameter indicates no effect on the atomic structure. Different H₂ concentrations in the processing gas atmosphere have no detectable influence on the data.     

NTCR behavior of Sm-substituted barium zirconium titanate nanocrystalline solid solution

Ceramic solid solution of nanocrystalline barium zirconium titanate in the form of Ba(Zr_0.52Ti_0.48)O₃ substituted by samarium (Sm³⁺) was prepared using the conventional solid state reaction method. The phase assemblage analyzed by the X-ray diffraction technique was fitted for cubic-crystal-symmetry. The change in the grain size depicted the influence of Sm³⁺ ions on the microstructure. The electrical behavior was studied in the temperature range from 323 to 773 K. The sintered samples exhibited a negative temperature coefficient of resistance (NTCR) and superior semiconducting behavior above 513 K. Addition of Sm³⁺ increased the room temperature resistivity of Ba(Zr_0.52Ti_0.48)O₃ solid solution. The results obtained from the thermoelectric power measurement confirm electrons as the majority charge carriers.     

New measurements of the infrared spectrum of solid SO2: Applications to IO

We present the results of additional studies of the 6–10 μm spectrum of solid SO₂ formed under a variety of deposition temperatures and deposition rates. Our and other published spectra support the suggestion that absorption by solid SO₂ is a plausible way to reconcile Voyager IRIS observations of Io with models of gas phase absorption by SO₂. Our laboratory spectra also suggest that such solid particles, resulting from condensation of volcanically emitted gaseous SO₂, are formed at a temperature below 110K, although this upper limit apparently conflicts with the work of Savoie and Campbell.