Excess Specific Heat Research Articles

Large low-temperature specific heat in pyrochloreBi2Ti2O7

Both amorphous and crystalline materials frequently exhibit low-temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/{T}^{3}$ versus $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore ${\text{Bi}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, we have measured the specific heat of crystalline ${\text{Bi}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ and related compounds. We find that the peak in $C/{T}^{3}$ versus $T$ in ${\text{Bi}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ falls at a substantially lower temperature than other similar compounds consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline ${\text{Bi}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ is consistent with quenched configurational disorder among Bi lone pairs produced by the geometrical frustration, which could represent a possible realization of ``charge ice.''

Mysterious Paramagnetic States of Fe 3d Spin in Antiferromagnetic Insulator of λ-BETS2FeCl4 System

In this study, we have investigated a quasi-two-dimensional organic conductor, λ-BETS 2 FeCl 4 , that exhibits a phase transition from a paramagnetic metal to an antiferromagnetic (AF) insulator at a critical temperature ( T MI ∼8.3 K). To investigate the mechanism of a notable phase transition, we studied the thermal properties of the conductor for temperatures down to 0.2 K at zero magnetic field. We observed that the specific heat of the conductor showed a sharp peak at T MI and a Schottky-type hump. The temperature dependence of the excess specific heat clarifies that π and 3d spins do not cooperatively form an AF order at T MI . We concluded that a 3d spin system keeps a paramagnetic state even at the lowest studied temperature.

Thermal study of monovalent-divalent phase transition in npBifc-F1TCNQ System

In a new molecular solid composed of di-neopentyl-biferrocene (npBifc) and fluorotetracyanoquinodimethane (F1TCNQ)3, Mochida reported the discovery of a reversible valence transfer that can be regarded as an "ionic(I)-ionic(II)" phase transfer between the monovalent state (D+A-) and the divalent state (D2+A2-). We have studied thermo-dynamical properties of this transformation for this complex using the differential thermal analyses (DTA). We observed a broad excess specific heat with multi-peaks attributed to micro-domain structure over the corresponding temperature range (100–150K) accompanied by temperature hysteresis of 7K. The transition entropy (ΔS) was determined to be 22 ± 2 J/mol-K and almost satisfied a Clausius-Clapeyron relation. These experimental results provide an experimental confirmation of the first order phase transition for the monovalent-divalent transfer. At the transition, we observe that the electronic degrees of freedom remained constant values, while large entropy absorbed crossing from low temperature phase to high temperature one is contributed by the lattice one. We finally estimated the internal energy and concluded that delicate energy valance between Madelung, ionization and affinity energies enable this system to exhibit a temperature induce monovalent-divalent phase transition.

Thermodynamic properties of the Pd 77.5Cu 6Si 16.5 undercooled liquid

Proper understanding of glass formation implies the knowledge of the thermodynamics of the undercooled melts. Specifically, high values of the excess specific heat of the liquid are expected for good glass-formers. Extending the work of Gillessen and Herlach [F. Gillessen, D.M. Herlach, J. Non-Cryst. Solids 117–118 (1990) 555–558], we re-propose a calculation of the temperature dependence of entropy difference between amorphous-liquid and crystal states. An amorphous Pd 77.5Cu 6Si 16.5 alloy has been produced by injection casting in a cylindrical copper mould. DSC measurements in the liquid, amorphous and crystalline states were performed with samples sliced from the cylinder to determine the heat of fusion, of crystallization and the difference in specific heat capacity between amorphous-liquid and crystal phases. These thermodynamic quantities are used to calculate the thermodynamic functions of the liquid-glass with reference to the equilibrium crystal mixture. The data are compared to those of other bulk glass-formers in terms of fragility plots.

Low temperature specific heat of heterocyclic polymer networks: Effect of network density

Measurements of low frequency Raman scattering and low temperature specific heat between 2 and 20 K have been performed in two glassy, heterocyclic polymer networks. The effective network densities of heterocyclic polymer networks are varied by changing the ratio of bi- and mono-functional isocyanate monomers, leaving the overall chemical structure essentially unchanged. A boson peak at ∼ 20 cm−1 characterizes the Raman spectra at room temperature of both samples. Below 10 K, the specific heats deviate from a cubic temperature dependence, as predicted by the Debye theory, and reveal an excess specific heat, having the shape of a well-defined peak in a C p/T 3 plot with a maximum at ∼ 5 K. The increase in effective network density is accompanied by a slight decrease in excess specific heat. These observations have been explained in terms of additional low-energy vibrations associated with the monomers building the polymer networks.

Reduction of the Yb valence inYbAl3nanoparticles

Measurements of specific heat, dc magnetic susceptibility, and Yb ${L}_{\text{II}}$ and ${L}_{\text{III}}$ x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) on ${\text{YbAl}}_{3}$ milled alloys are reported. X-ray diffraction patterns are consistent with a reduction in particle size down to 10 nm and an increase in the lattice strain up to 0.4% for 120 h of milling time. A decrease in the mean valence from 2.86 for the unmilled alloy to 2.70 for 120 h milled ${\text{YbAl}}_{3}$ is obtained from the analysis of XANES spectra. From the analysis of spectra in the EXAFS region, an increase in the mean-square disorder of neighbor distance with milling time is detected in good agreement with the results of x-ray diffraction. Size effects strongly influence the magnetic and thermal properties. The value for the maximum of the magnetic susceptibility decreases around 30% for 120 h milled alloy and an excess specific heat, with a peak around 40 K in the milled samples, is derived. These changes in the physical properties along the milled ${\text{YbAl}}_{3}$ alloys are associated with the reduction in particle size. Such a reduction leads to the existence of a large number of ${\text{Yb}}^{2+}$ atoms at the surface with respect to the bulk affecting the overall electronic state.

Martensitic transformation B2–R in Ni–Ti–Fe: experimental determination of the Landaupotential and quantum saturation of the order parameter

The Landau potential of the martensitic phase transformation inNi46.8Ti50Fe3.2 was determined using high resolution x-ray diffraction to measure the spontaneous strainand calorimetric measurements to determine the excess specific heat of the R phase. Thespontaneous strain is proportional to the square of the order parameter which is tested bythe relation of the excess entropy and the order parameter. The parameters ofthe Landau free energy were determined by fitting the temperature evolution ofthe order parameter and using the scaling between the excess entropy and theorder parameter. The double well potential at absolute zero temperature wascalculated and the interface energy and domain wall thickness were estimated.

Low temperature specific heat of the single molecule magnet Fe 8

We present low temperature specific heat and ac-susceptibility measurements of Fe 8 powdered sample. Below 1.3 K, super-paramagnetic blocking effects and an excess specific heat contribution are observed. The latter is attributed to a splitting of the ground state doublet in an inhomogeneous local field of hyperfine and dipolar origin. The local field contributions are evident at the resonances observed in the field dependent ac-susceptibility and specific heat below 0.5 K. The low temperature Schottky contribution is in agreement with known crystal field parameters of effective spin Hamiltonians proposed to simulate EPR and inelastic neutron scattering experiments.

Low-temperature measurements of the specific heat capacity of a thick ferroelectric copolymer film of vinylidene fluoride and trifluoroethylene

The specific heat capacity ${c}_{p}$ of a $110\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thick and partially crystallized $(\ensuremath{\sim}77%)$ ferroelectric copolymer film of vinylidene fluoride (PVDF) $(\ensuremath{\sim}75\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%)$ and trifluoroethylene $(\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%)$ was measured from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of ${c}_{p}∕{T}^{3}$ reveals a shallow hump, centered at $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, consistent with non-Debye behavior of semicrystalline materials. These ${c}_{p}$ data were used, in conjunction with previous measurements of the pyroelectric coefficient $\ensuremath{\mid}{p}^{\ensuremath{'}}\ensuremath{\mid}$, to infer the temperature profile of the macroscopic Gr\uneisen parameter based on ratios of $\ensuremath{\mid}{p}^{\ensuremath{'}}\ensuremath{\mid}∕{c}_{p}$. Comparisons with similar data for PVDF are consistent with a correlation between anharmonicity of the interaction potential and excess specific heat capacity.

On the excess specific-heat of single-walled carbon-nanotube ropes due to the adsorption of helium atoms in the temperature range 2-20 K

Experimental specific-heat measurements in single-walled carbon-nanotube (SWNT) ropes show marked increase when He-4 is allowed to be adsorbed, the increase in the values varying from 2 to 2.5 times in the temperature range 2-20 K. Beyond 20 K, the effect of adsorption on the specific heat of SWNT ropes is negligible. The phonon frequency distribution function (FDF) of the dynamical modes was extracted by employing the unfolding technique from the observed experimental temperature variation of the specific heat and a trial phonon FDF. The agreement between the observed experimental values of specific heat and the values obtained using the above method is quite good, the error being less than 10% in the temperature range 2-20 K and within 6% for 20 T 300 K at most of the temperature values. The change in the values of the specific heat can be attributed to the fact that the phonon FDF of He-4 adsorbed SWNT ropes shows marked difference from that when there is no adsorption for energies less than 200 K. The sensitivity of specific heat to adsorption at low temperatures by SWNT ropes is highlighted, particularly, by the appearance of large number of dynamical modes at 15 K. This can be experimentally checked using Rayleigh recoilless fraction of Mossbauer -ray photons from the carbon nanotube ropes sample.

Specific heat and related thermophysical properties of liquid Fe-Cu-Mo alloy

The specific heat and related thermophysical properties of liquid Fe77.5Cu13Mo9.5 monotectic alloy were investigated by an electromagnetic levitation drop calorimeter over a wide temperature range from 1482 to 1818 K. A maximum undercooling of 221 K (0.13 T m) was achieved and the specific heat was determined as 44.71 J·mol−1·K−1. The excess specific heat, enthalpy change, entropy change and Gibbs free energy difference of this alloy were calculated on the basis of experimental results. It was found that the calculated results by traditional estimating methods can only describe the solidification process under low undercooling conditions. Only the experimental results can reflect the reality under high undercooling conditions. Meanwhile, the thermal diffusivity, thermal conductivity, and sound speed were derived from the present experimental results. Furthermore, the solidified microstructural morphology was examined, which consists of (Fe) and (Cu) phases. The calculated interface energy was applied to exploring the correlation between competitive nucleation and solidification microstructure within monotectic alloy.

Calorimetric study and Landau analysis of the phase transitions in PbZr1–xSnxO3 single crystals with 0 ≤ x ≤ 0.4

AbstractDifferential scanning calorimetry experiments have been performed for PbZr1–x Snx O3 single crystals where 0 ≤ x ≤ 0.4 at the temperature range of 100–600 K. The temperatures and orders of particular phase transitions have been determined and the phase diagram was compared to dielectric measurements. The first order character of the phase transitions in PbZr1–x Snx O3 was discussed. The total transitions energy and entropy at the phase transitions were calculated. The excess specific heat and excess entropy associated with the phase transitions were analysed using Landau theory. The comparison of calorimetric data with the evolution of the order parameter indicates that the phase transitions in PbZr1–x Snx O3 single crystals seem to follow a mean field Landau potential. The results are compared to the earlier dielectric and birefringence experiments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pressure variation of reentrant transition temperature in liquid crystals

High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T(AN) exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic -- smectic A -- reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (gamma, lambda(eff)) plays an important role. The cubic coupling gamma is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling lambda(eff) is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through gamma and lambda(eff) which justifies the experimental pressure dependence in the reentrant transition temperature [Formula: see text]. The pressure dependence of gamma and lambda(eff) are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.

Low-temperature specific heat in caesium borate glasses

Low-temperature specific heat measurements (1.5–25 K) performed on caesium borate glasses at different concentrations have pointed out the presence of an excess specific heat, C p, over the Debye contribution. In a plot C p/T 3 vs. T, it appears as a bump, temperature location of which is almost unchanged whereas its intensity increases with caesium concentration. A comparison with the specific heat in sodium borate glasses has revealed highly dissimilar low-energy dynamics, despite the same structural units building up the borate matrix. The different behaviours observed have been related to the peculiar structural changes that the inclusion of alkaline cations cause in the mesoscopic length scale where the effects of the medium-range order (MRO) are usually observed. It is believed that the particularly open structure of caesium glasses assures a very low frequency for the local oscillations of Cs+ ions that, consequently, rule the vibrational dynamics in the frequency region of the boson peak by their localized character.

Specific Heat Peculiarities in (NH4)2SbF5

Precise measurements of the specific heat in (NH4)2SbF5 crystal have been performed using an ac-calorimeter. Anomalies previously connected with phase transitions at 293 K, 169 K, and 139 K are detected and discussed. Two second order transitions at 293 K and 169 K are confirmed. Thermal parameters (as the excess enthalpy Δ H, and the excess entropy ΔS) are estimated and possible mechanisms are presented. The anomaly at 139 K is attributed to an overcritical trace of isostructural phase transition. Simplified model calculations of the excess of specific heat are compared with the Δ cp measured experimentally.

Specific heat and critical behavior of Tl 0.3MoO 3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl 0.3MoO 3 has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl 0.3MoO 3 is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.

Thermal Properties on Single Crystals of Ferroelectric Barium Dititanate

Single crystals of ferroelectric BaTi2O5 were grown by a floating zone method. Heat capacities of the crystals were precisely measured over a wide temperature range of 2–800 K. The crystals show successive heat-anomalies consisting of a λ -type anomaly and a broad hump. For the λ -type anomaly, the excess specific heat Δ C p reaches 2.4 Jmol− 1K− 1 at the ferroelectric Curie temperature of T C = 726 K.; the transition enthalpy Δ H and transition entropy Δ S were estimated to be 80 Jmol−1 and 0.11 Jmol− 1K−1, respectively. The broad hump shows a maximum at approximately T g = 540 K and a symmetrical shape spreading over a wide temperature range of 450–650 K. Moreover, a sharp hump was observed in the C p /T 3 vs. T curve at 23 K. Some discussions are presented about these anomalies and causes of the ferroelectric phase transition.

Surface Water and the Origin of the Positive Excess Specific Heat for 7 nm Rutile and Anatase Nanoparticles

Specific heats (15-350 K) have been measured on 7 nm TiO2 anatase and rutile nanoparticles containing significant amounts of surface-adsorbed water. By successively reducing the water content without changing particle size, we observed two types of water behavior. The specific heat of bare 7 nm particles was estimated using the water specific heats. Contrary to previous literature reports, the bare small particle specific heats are the same as those of the bulk, within experimental error.

Phonon spectrum of the ferromagnetic superconductorUGe2and consequences for its specific heat

We report inelastic neutron scattering measurements of the phonon spectrum of the pressure-induced ferromagnetic superconductor UGe$_{2}$. No changes of the spectrum were found on cooling down to low temperature. The phonon contribution to the specific heat was estimated from a fit to our data. The excess specific heat previously noted at around $T_{x} \approx$ 30 K is not due to phonons but is well described by the temperature dependence of the magnetic order parameter at the molecular field level.

Pressure perturbation calorimetry of poly(ethylene) glycol solutions in water

The physico-chemical properties of poly(ethylene) glycol solutions in water have been studied with use of pressure perturbation calorimetry. The three PEGs of average molecular mass (Mr) 6000, 10000, 20000 were used. The concentration of polymers was changed in the range 0–30% mass per volume (w/v%). On the basic of VP-DSC measurements with use of PPC technique the dependencies of thermal expansion coefficient (α) and excess specific heat capacity (Cp,exc) on temperature were determinated for PEG–water solutions.