Lattice Specific Heat Research Articles

Phonon dynamics of plutonium chalcogenides and pnictides

We have investigated the phonon dynamics of the plutonium compounds (PuX; X = S, Se, Te, As, and Sb) by using rigid ion (RIM) and breathing shell models (BSM), later includes breathing motion of the electrons of the Pu-atoms due to f– d hybridization. We discuss the significance of these two approaches in predicting the phonon dispersion curves of PuX compounds and examine the role of electron–phonon interactions. Dominant ionic nature of bonding has been predicted for PuX compounds from the large LO–TO phonon splitting at zone center. We also report the one phonon density of states and first time calculated the lattice specific heat for these compounds.

Field-dependent specific heat ofYb4As3: Agreement between a spin-12model and experiment

We report the low-temperature specific-heat measurements on polydomain ${\text{Yb}}_{4}{\text{As}}_{3}$ at magnetic fields up to 20 T. Taking into account the Bethe ansatz results, the zero-field data have been used for the estimation of the lattice specific heat, resulting in a value of the exchange integral for the Heisenberg model of the antiferromagnetic spin $S=\frac{1}{2}$ chain of $J/{k}_{B}=\ensuremath{-}28\text{ }\text{K}$. A quantitative agreement has been achieved between the experimental magnetic specific-heat data in magnetic field and the numerical results obtained by the quantum transfer-matrix (QTM) simulation technique. The finite-size QTM approximants have been analyzed and an extrapolation procedure recovering the known density matrix renormalization group (DMRG) results down to very low temperature has been proposed. On the basis of the data in magnetic field and using the earlier DMRG results, the energy-gap size $\ensuremath{\Delta}$ has been analyzed as a function of the applied magnetic field $B$, leading to an experimental verification of the scaling law $\ensuremath{\Delta}\ensuremath{\propto}{B}^{2/3}$ following from the sine-Gordon model.

The role of lattice distortions in determining the thermal properties of electron dopedCaMnO3

We have investigated the thermal properties of electron doped perovskite manganiteCaMnO3, the end member () of the Ruddlesden–Popper (RP) calcium manganates series with cationdoping at the A-site. In this paper the functional relation between the latticedistortions and the thermal properties is determined and compared toavailable reports. The temperature dependence of the lattice specific heat(Cv(lattice))of Ca1−xLnxMnO3 (x = 0.05, 0.10, 0.15,0.20) with Ln(= La, Ce,Pr, Nd, Th, Bi) doping at the A-site has been studied as a function of temperature(10 K≤T≤500 K) by means of a rigid ion model (RIM) after modifying its framework to incorporate thevan der Waals interactions. Strong electron–phonon interactions are present in thesecompounds, which are responsible for the variation of the lattice specific heat with cationdoping of varying size and valency. We have found that the calculated thermal propertiesreproduce well the corresponding experimental data, implying that modified RIMrepresents properly the nature of these perovskite manganite systems. We demonstrate thatthe electron concentration, size mismatch and Jahn–Teller (JT) effects are thedominant factors, whereas charge mismatch and buckling of Mn–O–Mn angleinfluence the thermal properties to a lesser degree in the ferromagnetic state.In the insulating paramagnetic state, JT distortions vary linearly and influencethe thermal properties. These specific heat results can be further improved byincluding the ferromagnetic spin wave and charge order contributions to the specificheat.

Magnetic refrigerator for hydrogen liquefaction

Magnetic refrigeration which is based on the magnetocaloric effect of solids has the potential to achieve high thermal efficiency for hydrogen liquefaction. We have been developing a magnetic refrigerator for hydrogen liquefaction which cools down hydrogen gas from liquid natural gas temperature and liquefies at 20 K. The magnetic liquefaction system consists of two magnetic refrigerators: Carnot magnetic refrigerator (CMR) and active magnetic regenerator (AMR) device. CMR with Carnot cycle succeeded in liquefying hydrogen at 20K. Above liquefaction temperature, a regenerative refrigeration cycle should be necessary to precool hydrogen gas, because adiabatic temperature change of magnetic material is reduced due to a large lattice specific heat of magnetic materials. We have tested an AMR device as the precooling stage. It was confirmed for the first time that AMR cycle worked around 20 K.

Generalized density-of-states and anharmonicity of the low-energy phonon bands from coherent inelastic neutron scattering response in the pyrochlore osmatesAOs2O6(A=K,Rb,Cs)

Inelastic neutron scattering on powder samples has been employed to investigate the lattice dynamics of alkali-metal osmates. We have obtained the neutron weighted generalized phonon density-of-states from momentum-transfer averaged coherent inelastic response. We have paid special attention to the low-energy modes that have been assigned to the Einstein term of the lattice specific heat and from which a ``rattling'' behavior of the alkali atoms has been inferred. In all of the three compounds our results reveal a split band with two prominent peaks in the low-energy range associated with the vibrational modes of $A$ atoms. The temperature dependence of these bands has been followed in the range from $1.5\text{ }\text{K}<T<300\text{ }\text{K}$. The strongest temperature effect is visible in the case of ${\text{KOs}}_{2}{\text{O}}_{6}$, the split band peaking at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}5.5\text{ }\text{meV}$, and 6.8 meV at $T=300\text{ }\text{K}$ softens on cooling resulting in a main band at about $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}3.4\text{ }\text{meV}$ at $T\ensuremath{\lesssim}50\text{ }\text{K}$, while a smaller softening of $\ensuremath{\Delta}(\ensuremath{\hbar}\ensuremath{\omega})<1\text{ }\text{meV}$ is seen in the other two compounds. In agreement with earlier work the temperature dependent modifications of the phonon spectrum can be attributed to the weak potential felt by the $A$ atom in the oversized cage where it is located. At low $T$ the characteristic energies for each of the compounds are well correlated with the specific-heat observations published earlier. The powder averaged phonon response shows $Q$ dependence indicating coherent coupling of the alkali atom vibrations with the rest of the lattice.

Lattice specific heat of carbon nanotubes

The lattice specific heat in carbon nanotubes is evaluated within the microscopic model proposed by Mahan and Jeon, published in the Physical Review B, in 2004. Phonons are considered for single wall carbon nanotubes in armchair configuration. As expected, low temperature and high temperature regions show different behaviour of specific heat. Carbon nanotubes are also displaying a very interesting lattice transport depending on the tube diameter, with high thermal conductivities for small diameters.

Diamond’s elastic stiffnesses from 322 K to 10 K

Using resonant-ultrasound spectroscopy, we measured diamond’s monocrystal elastic-stiffness coefficients C11, C12, and C44, between 322 and 10 K. Changes are small and smooth: The bulk modulus B=(C11+2C12)/3 increases about 1 part in 1000, describable by a quasiharmonic Einstein-oscillator model. Zero-temperature Cij correspond to a 2244-K Debye characteristic temperature. Using a low-temperature form of the Grüneisen–Debye model, we calculated an overall thermodynamic Grüneisen parameter of γ=1.26; using a high-temperature form we calculated 0.71; the lattice specific heat yields γ=1.10.

Temperature memory effect in Cu–Al–Ni shape memory alloys studied by adiabatic calorimetry

The temperature memory effect exhibited by Cu–Al–Ni shape memory alloys was studied by means of adiabatic calorimetry and microscopic observations. The harmonic, anharmonic and electronic contributions to the lattice specific heat were estimated by using the experimental data of the metallic components. The obtained results provide an accurate baseline for the quantitative study of the martensitic phase transformations as a function of the thermal history in these alloys. The specific heat of a Cu–Al–Ni sample was measured from 140 to 350 K throughout the phase transition region, and the temperature memory effect was carefully studied. These results are in good agreement with the optical observations as a function of temperature. The global behaviour of the martensitic transformation as regards the temperature memory effect is discussed and interpreted in terms of the microscopic mechanisms of nucleation and motion of the martensite plates.

Specific Heat Behaviour in Real Ferroelectric Crystals

We discuss the relationship between the Landau theory predictions about the temperature dependence of specific heat of “ideal” ferroelectric crystals and the real “nonclassical” thermal properties of ferroelectrics, caused by the structural defects, domains, non-uniform biasing field etc. Using the experimental data for the series of model ferroelectric crystals (KDP, GMO, TMO) and the advanced method of the lattice background specific heat calculation, we have reconsidered and specified the earlier received results.

Specific heat in NpNiSn and NpIrSn

Abstract We present the specific-heat study of the electronic properties of NpIrSn and NpNiSn, crystallizing in the ZrNiAl-type hexagonal and TiNiSi-type orthorhombic structure, respectively. NpIrSn orders antiferromagnetically below T N = 38 K and undergoes a further magnetic phase transition at T 1 = 23.5 K. The γ -coefficient of the electronic specific heat is only moderately enhanced to 75 mJ mol −1 K −2 . ThIrSn, crystallizing in the same structure, is used to estimate the lattice specific heat to analyze the magnetic part. NpNiSn orders antiferromagnetically below T N = 35.5 K. In both compounds the antiferromagnetic order is not affected by magnetic fields up to 14 T.

Comparison of interatomic potentials for UO 2: Part II: Molecular dynamics simulations

An improved knowledge of nuclear fuel can be gained from a better description of atomic-scale processes such as point defects behaviour under irradiation. In these perspectives, computer simulation techniques involving semi-empirical potentials can play a major role as they allow studying some of these processes separately. The range of applicability in static calculations of the available interatomic potentials for UO 2 has been previously assessed by the authors. This study complements the static calculations by including dynamical simulations of the temperature evolution of different elastic properties (lattice parameter, specific heat, bulk modulus and Gruneisen parameter) and by calculations of bulk melting temperature.

The Lattice Specific Heat of RENi5 Compounds at Low Temperatures

The Lattice Specific Heat of RENi5 Compounds at Low Temperatures

Magnetic Properties and Crystalline Electric Field Scheme in RRhIn5 (R: Rare Earth)

A series of ternary compounds RRhIn 5 (R: rare earth) has been grown in the single crystalline form by means of the flux method. The magnetic properties of these compounds were investigated by measuring the lattice parameter, electrical resistivity, specific heat, magnetic susceptibility and magnetization. All the compounds crystallize in the tetragonal HoCoGa 5 -type structure. Most of these compounds order antiferromagnetically at low temperatures, except when R=Y, La, Pr, and Yb. The observed temperature dependence of the specific heat and the anisotropic features in the magnetic susceptibility and magnetization have been analyzed on the basis of the crystalline electric field (CEF) model. The overall energy splitting of the CEF scheme in RRhIn 5 is estimated to be in the range from 44 to 330 K. The magnetic properties are discussed on the basis of the sign of the CEF parameter.

High-temperature X-ray diffraction and specific heat studies on GdAlO 3, Gd 3Al 5O 12 and Gd 4Al 2O 9

Gadolinium aluminates, GdAlO 3, Gd 3Al 5O 12 and Gd 4Al 2O 9 were synthesized by the solution combustion method. Very fine particles in the nanoparticle range of ∼10–20 nm could be prepared by this method as evidenced by surface area measurement by multipoint BET method. Thermal studies on these compounds were carried out using high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC) methods. The thermal expansion coefficients of GdAlO 3, Gd 3Al 5O 12 and Gd 4Al 2O 9 were calculated from the lattice parameter data and specific heats were calculated from DSC data. The lattice parameters of GdAlO 3 and Gd 3Al 5O 12 were found to increase linearly with temperature whereas Gd 4Al 2O 9 did not show a linear trend. The specific heats of these compounds show an increasing trend with increase in aluminum atom fraction. Based on the thermodynamic data available in the literature and the specific heat data obtained in this study, oxygen potential diagram was constructed at 1000 K.

THERMAL EXPANSION STUDY OF A UVAROVITE RICH GARNET

Thermal expansion measurements have been performed on a uvarovite rich garnet sample for the first time and compared with the expansion data on grossular and pyrope-rich garnets reported in the literature. A semiclassical model has been used to analyze the data and to obtain various thermodynamic parameters. Using these parameters, the lattice specific heat and the corresponding entropy have also been calculated.

Synthesis and characterization of a new superconductorNb1−xMgxB2

Synthesis, lattice parameter changes, grain morphology, superconductivity and specific heat of a newsuperconductor Nb1−xMgxB2 (0.0≤x≤0.40) serieswere reported. Magnetization and resistance measurements indicate that the superconducting transitiontemperature TC of Nb1−xMgxB2 with x = 0.10–0.40 is ∼5 K. A large Meissner signal and the result of specific heat measurements indicatethat the superconductors have a bulk superconductivity. The Debye temperatureθD determined by the measurement of specific heat is419(± 10) Kfor Nb0.75Mg0.25B2.

LOW TEMPERATURE SPECIFIC HEAT ANALYSIS OF LaMnO3+δ MANGANITES

We report a theoretical analysis for the experimental specific heat C(T) data of the perovskite manganites LaMnO 3+δ, with δ=0.11, 0.15 and 0.26, in the temperature domain 4≤T≤10 K . Calculations of C(T) have been made within the two-component scheme: one is the Fermionic and the other is Bosonic (phonon or magnon) contribution. Lattice specific heat is well estimated from the Debye temperature for lanthanum manganites with different δ obtained following an overlap repulsive potential. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations. Later on, following double exchange mechanism the role of magnons is assessed toward specific heat and is found that at low temperatures, specific heat shows almost T3/2 dependence on the temperature. We note that the lattice specific heat is smaller for δ=0.11 when compared with that of magnon specific heat below 6 K, while the lattice contribution is larger with the magnon contribution for δ=0.15 and 0.26. It is further noticed that in the ferromagnetic phase, deduced electronic specific heat is smaller in comparison with reported large electronic term in low temperature domain. The present investigations allow us to stress that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in pure LaMnO 3+δ (δ=0.11, 0.15 and 0.26). These findings express that the large Coulomb interaction U suppresses the double occupancies of eg electrons and enhanced electronic specific heat, while there is a decrease of T3/2-term with δ from 0.26 to 0.11. The present numerical analysis of specific heat shows similar results as those revealed from experiments.

Sm-based filled skutterudite [formula omitted] studied by specific heat

We report the specific heat C ( T ) of filled skutterudites SmOs 4 Sb 12 and LaOs 4 Sb 12 . The lattice specific heat of LaOs 4 Sb 12 has a contribution from characteristic low-energy phonon modes centered around ∼ 60 K . For SmOs 4 Sb 12 , considering the influence of a significant shift of the phonon modes toward low energy, we estimated the true electronic contribution in specific heat Δ C el . The obtained Δ C el / T in SmOs 4 Sb 12 steeply increases below 20 K without indicating a Schottky anomaly.

Pressure-dependent phonon properties ofLa0.7Sr0.3MnO3

In this work, we present the calculated results on the pressure dependent phonon properties of rhombohedral $(R\overline{3}c)$ ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ manganite by using a lattice dynamical model theory. The effect of internal pressure determined by the average $A$-site atomic radius is also investigated and compared with the effect of applied pressure. The computed zone center phonon frequencies at ambient pressure agree fairly well with the experimental results and the modes related to the octahedral distortion exhibit hardening with the increase in pressure. The Raman active ${A}_{1g}$ mode is most sensitive to pressure with a slope of $1.0\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$ and shows unusual couple of slope change. Both internal and external pressures harden the ${A}_{1g}$ phonon mode and reduce the electron-phonon interactions. Change in the frequency of the Raman active ${E}_{g}$ modes with frequency is also observed. Different effect is observed for the low and high average $A$-site atomic radius. The phonon dispersion curves in high symmetry directions of the Brillouin zone and phonon density of states are also calculated. The pronounced shift of the peak positions in phonon DOS is observed with the increase in pressure. The analysis of the reduction of the high frequency phonon peak width and effective mode-Gr\"uneisen parameter allows us to draw a conclusion for the decrease in lattice distortion and to some extent Jahn-Teller (JT) distortion, a signature of rhombohedral structure for the manganites. The role of pressure on the lattice specific heat is also discussed.

ANALYSIS OF HEAT CAPACITY OFYBa2Cu3O7-δSUPERCONDUCTORS: ROLE OF LATTICE AND ELECTRON CONTRIBUTION

Observed temperature-dependent heat capacity C (T) behavior of high-TcYBa2Cu3O7-δcuprate superconductors has been theoretically analyzed in the temperature domain 70 ≤ T ≤ 110 K . Calculations of C (T) have been made within the two component scheme: one is the Fermionic term and the other the Bosonic (phonon) contribution. While estimating the electronic term, we use a mean field step and follow two-fluid model below and above Tc. Later on, the lattice heat capacity is estimated within harmonic approximation for high temperature expansion (T > θ/2π), the model has only one free parameter, the moments of phonon density of states. Within the two-fluid model for electronic specific heat along with reported γ value leads to a sharp discontinuity at Tc. The Coulomb correlations and electron-phonon coupling strength have significant implications on the γ. Henceforth, the present numerical analysis of specific heat from the present model shows similar results as those revealed from experiments. The accurate fitting of the specific heat data reveals that it is possible to decompose the documented specific heat into dominant lattice contribution and electronic channel. However, the specific heat from electronic term is only a fraction of lattice specific heat in YBa2Cu3O7-δhigh-Tcsuperconductors.