Specific heat of vitreous systems: Temperature dependence near absolute zero

Unlike the low temperature heat capacities (L.T. C_V) of crystals, those of vitreous substances near T = 0 K require the formula C_V/3R = C₁T+C₂T²+C₃T³+C₅T⁵+…, where the constants C₁ and C₂ represent the anomalous excess characteristic of vitreous systems. The C₃ and C₅ represent, respectively, Debye and dispersion terms as for crystals. Recent literature confirms C₁ ≠ 0 for various glasses and offers diverse theoretical justifications for this. The C₂T² term has been proposed but not evaluated by Leadbetter. As shown here, it is required to represent the experimental C_V of vitreous silica at T < 2 K, is inferred from its Raman spectrum and is supported by theoretical calculations.     

Excess specific heat of a glassy Pd0.775Cu0.06Si0.165 alloy at low temperature

The specific heat of a Pd_0.775Cu_0.06Si_0.165 alloy in both glassy and crystalline phase has been measured from 2.0 to 18.0°K. It was found that the electronic specific heat of the glassy phase, C_e^staggeredA=0.4×10⁻⁴T cal/mole°K, is very small as compared with the value evaluated by assuming electrons being free. In the region T>7°K the lattice specific heat of glassy alloy can be described approximately by the addition of two Einstein terms, with characteristic temperatures of 30 and 50°K, to that of the crystalline phase. The number of modes in these peaks is 0.30% for the former and 1.25% for the latter.     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Low temperature thermal conductivity and specific heat of elastomers

The specific heat C and thermal conductivity κ of polybutadiene are characteristic of all non-crystalline materials at temperatures below ≈ K, reflecting the presence of localized excitations. The changes in C and κ with variation in crosslinking suggest that the relaxation times of the localized excitations may vary with crosslink density. Extension of an elastomer does not reveal a change in density of localized excitations as monitored by measuring κ, possibly because other phonon scattering mechanisms mask the effect. For T ? 10 K the phonon mean-free-path is independent of the microscopic anisotropy of the elastomer.     

Effect of hydrostatic pressure on the elastic moduli of As2S3 and As2Se3 glasses at 195 and 296 K

Velocities of 30 MHz longitudinal and shear ultrasonic waves have been measured in As₂S₃ and As₂Se₃ glasses as a function of hydrostatic pressure up to 1.5 kbar at 195 K and 3 kbar at 296 K. The elastic stiffness moduli are found to have relatively large, positive, pressure dependences which are about the same at both temperatures for both glasses. This behavior is attributed to the weakness of bonding between layers comprised of AsS₃ and AsS₃ pyramids.Inspection of data for a variety of glasses reveals a correlation between the value of C_L/3C_T and whether the elastic moduli are increased or decreased by pressure. (C_L is the longitudinal modulus and C_T the shear modulus.)Using the pressure dependences of the elastic moduli obtained in the present work, it is found that volume change is responsible for most of the temperature dependences of the moduli. In addition elastic gammas are obtained which are consistent with thermal Grüneisen gammas at 12 K. The pressure dependence of the volume of As₂S₃ glass at 296 K is calculated using the present results in the Murnagham equation. Agreement with volumetric data of Weir is obtained.     

Pressure-induced suppression of ferromagnetic phase in LaCoO3 nanoparticles

Magnetic and structural properties of nanocrystalline LaCoO₃ with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below T_C ≈ 85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas T_C remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO₃ nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ～11 kbar. Remarkably, the T_C does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO₃ nanoparticles, likely related to the intermediate-spin (IS) Co³⁺ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co–O bonds.     

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.     

Thermal properties of vitreous silica with He impurities

The specific heat and thermal conductivity of vitreous silica (Suprasil W) doped with He at 420°C and 200 bar has been measured at low temperatures (0.5 K to 8 K). While with respect to an undoped sample the specific heat shows an excess contribution which varies as T^1.7, no effect of doping on the thermal conductivity is observed. This shows that the coupling to phonons of the excitations introduced by He doping is weak compared to that of intrinsic low energy excitations common to glasses. The excess specific heat can be attributed to excitations of He atoms in cavities.     

Heat capacity of LiInS2, LiInSe2 and LiInTe2 between 200 and 550 K

The molar heat capacity at constant pressure of LiInS₂, LiInSe₂ and LiInTe₂ was measured in the temperature range from about 200 K to 550 K. An analysis of the experimental data showed that the anharmonic contribution to the heat capacity can be described by a polynomial of fourth order in the temperature. A comparison of the results for the LiInC₂^VI compounds with those for the AgB^IIIC₂^VI and A^IIB^IVC₂^V chalcopyrite compounds showed that the lattice anharmonicity effects are essentially influenced by the specific nature of the Li C^VI bond.     

Growth from gas cavities in a molten salt and properties of superconducting Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+δ</Subscript> single crystals

Perfect single crystals of the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+δ with the superconducting transition temperature T_C = 72–85 K (depending on the crystallization conditions) are obtained by the method of free growth in gas cavities formed in a KCl solution-melt. The specific features of the growth process are in the formation of an enclosed growth gas cavity in a (previously synthesized) blend of a specified phase composition dissolved in KCl and the free crystal growth in this cavity. The combination of growth and high-temperature annealing in the same process made it possible to obtain uniform (ΔT_C = 1.5 K) single crystals with stable superconducting properties. Annealing of the grown single crystals in oxygen or in air in the temperature range 400–850°C confirmed that the crystals with maximum values of T_C are optimally doped.     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Pressure and heating rate dependence of the crystallization temperature for amorphous (Fe65Ni35)75P16B6Al3 and Ti50Be40Zr10

The crystallization temperature, T_x, was determined at constant heating rate, $\text{R =}\text{T}\text{?}\text{? 7}\text{K min}{}^{\mathrm{?1}}$, by monitoring the electrical resistance. Such experiments were carried out under pressures up to 2.5 GPa, and the resulting dT_x/dP was 15.9 K GPa^?1 for (Fe₆₅Ni₃₅)₇₅P₁₆B₆Al₃ and 8.7 K GPa^?1, 8.1 K GPa^?1 for the two crystallization processes in Ti₅₀Be₄₀Zr₁₀. The activation energies of crystallization under atmospheric pressure were obtained from measurements of T_x at rates from 0.05 K min^?1 ?55 K min^?1, analysed by plotting ln(T_x²R^?1) versus T_x^?1.     

New family of LnBaMn2O6 − γ manganites (Ln = Nd, Sm, and Gd)

A new family of manganites, LnBaMn₂O_{6 ? γ}(Ln = Nd, Sm, and Gd), is obtained by the method of topotactic reactions. In these compounds, rare-earth and barium ions are ordered, which results in a dramatic increase in the temperature of transition to the paramagnetic state. Thus, a SmBaMn₂O₆ compound with a disordered arrangement of Sm and Ba ions is a spin glass with the freezing temperature of magnetic moments T _f = 40 K, whereas in the state with an ordered Sm and Ba arrangement, the Curie temperature T _C, is ～ 280 K. Below T _C, a maximum in resistivity and magnetoresistance is observed. The possible models of magnetic ordering are discussed.     

Chemical ordering and fragility minimum in Cu–As–Se glasses

Modulated Differential Scanning Calorimetric (MDSC) studies have been performed on Cu_xAs₄₀Se_60−x glasses with x ranging between 0 and 30 at.%. The thermal parameters, namely the glass transition temperature (T_g), the heat capacity (C_p) and the jump in heat capacity at the glass transition (ΔC_p) are measured. The composition dependence of these thermal parameters has been found to exhibit interesting variations particularly at x = 20. Thermal crystallization studies show that the structural network is mainly characterized by the ternary Cu₃AsSe₄ units with As₂Se₃ and Cu₂As₃ units depending on the concentration of Cu. These structural differences arising as a function of composition are reflected in the T_g, C_p and ΔC_p. The observed transitions are also understood on the basis of Formal Valence Shell (FVS) model and the classification of strong and fragile liquids.     

EPR study of kinetic properties of a weak polar ferroelectric Li2Ge7O15

The EPR spectra of Mn²⁺ ions have been studied in the vicinity of the phase transition to the ferroelectric phase in lithium heptagermanate crystals. The anomalous broadening of the resonance lines observed in the vicinity of the transition temperature T _C was attributed to an increase of the contribution of low-frequency (ω < 10⁷ Hz) fluctuations of the local order parameter. The critical index of the correlation length is determined to be ν ≈ 0.63, which indicates the Ising-type critical behavior of the crystal. The experimental data obtained in the vicinity of T _C were interpreted with due regard for long-range dipole interactions. According to our estimates, the forces of dipole interactions are prevalent only in the close vicinity of the transition point, ¦ T-T _C¦ ? 0.1 K.     

Low temperature specific heat of doped and undoped glasses

In this work we have measured the specific heat, c_p, of several glasses between 2 and 160 K. The experiments were performed in low silica calcium aluminosilicate (prepared under vacuum and room atmosphere conditions), in silicate and in fluoride glasses. The influence of neodymium, iron and cobalt in c_p values at low temperature was also investigated. The scaling proposed by Liu and Löhneysen was used to analyze the experimental data. The temperatures in which the maxima in c_p/T³, the so called boson peak, occur are discussed in terms of the Hrubý coefficient, which provides information about the glass forming ability.     

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.     

Low-temperature specific heat of amorphous and crystalline Te0.81Ge0.15As0.04

We have measured the specific heats of amorphous and crystalline specimens of Te_0.81Ge_0.15As_0.04 between 0.2 and 20 K, and of crystalline Te_0.93As_0.07 between 1 and 20 K. Amorphous Te_0.81Ge_0.15As_0.04 shows a low-temperature linear specific heat anomaly whose magnitude, 0.027 mJ/mol-K², is similar to that of other amorphous insulators. Crystalline Te_0.81Ge_0.15As_0.04 exists as a two-phase material comprised of GeTe and As-doped Te. The specific heat of this material is analyzed in terms of a weighted average of the properties of its two constituents.     

Anomalous High Pressure Properties in Fullerene Superconductors

Abstract

Correlations among lattice parameter, external pressure, and critical temperature T_c has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dT_c/dP for Na₂Rb_0.5Cs_0.5C₆₀ and Li₃CsC₆₀, having small interfullerene separations, is about a few times lager than that of K₃C₆₀. In contrast, small dT_c/dP values are found in fullerides with expanded unit cells, such as (NH₃)_xNaAA^?C₆₀ (A, A^? = K, Rb and Cs) and A₃Ba₃C₆₀. Interestingly, the chemical and physical pressure effects on T_c are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded.