Heat capacity of α\alpha-GaN: Isotope Effects

Until recently, the heat capacity of GaN had only been measured for polycrystalline powder samples. Semiempirical as well as \textit{first-principles} calculations have appeared within the past few years. We present in this article measurements of the heat capacity of hexagonal single crystals of GaN in the 20-1400K temperature range. We find that our data deviate significantly from the literature values for polycrystalline materials. The dependence of the heat capacity on the isotopic mass has also been investigated recently for monatomic crystals such as diamond, silicon, and germanium. Multi-atomic crystals are expected to exhibit a different dependence of these heat capacities on the masses of each of the isotopes present. These effects have not been investigated in the past. We also present \textit{first-principles} calculations of the dependence of the heat capacities of GaN, as a canonical binary material, on each of the Ga and N masses. We show that they are indeed different, as expected from the fact that the Ga mass affects mainly the acoustic, that of N the optic phonons. It is hoped that these calculations will encourage experimental measurements of the dependence of the heat capacity on isotopic masses in binary and more complex semiconductors.Comment: 12 pages, 5 Figures, submitted to PR

Low-temperature thermal conductivity of solid carbon dioxide

Preliminary results of the thermal conductivity measurements performed in the temperature range 1.5-35 K on pure carbon dioxide are presented. The data below 25 K have been obtained for the first time. The thermal conductivity coefficient reaches very high value, of about 700 W/(m×K), unusual for simple molecular crystal. Straightforward analysis applied to the data indicates the case of large-grained sample

The temperature dependence of magnetic susceptibility of solid oxygen

Temperature dependence of magnetic susceptibility of polycrystalline oxygen samples has been measured in the range from 1 to 54 K. The measurements were performed using our home-designed SQUID-based gradiometer. The sensitivity of the instrument with respect to susceptibility reaches values of the order of 10⁻¹³. The results obtained are compared with earlier literature data

Orientational isotopic effects in the thermal conductivity of CH4/CD4 solid solutions

The thermal conductivity of (CH4)1–c(CD4)c solid solutions with c = 0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 has been measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II) and completely ordered (phase III). The temperature range is 1.3–30 K. It is shown that the thermal conductivity has different temperature dependences k(T) in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in k(T) is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the k(T) curve of phase II approaches the dependence k(T) typical of phase I. There is a hysteresis in the vicinity of the II↔III phase transition. In phase III the impurity effect in k(T) can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH4 and CD4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4–CD4 system

Thermal conductivity of solid parahydrogen with methane admixtures

The thermal conductivity of solid parahydrogen crystal with methane admixtures has been measured in the temperature range 1.5 to 8 K. Solid samples were grown from the gas mixture at 13 K. Concentration of CH₄ admixture molecules in the gas varied from 5 to 570 ppm. A very broad maximum of thermal conductivity with absolute value of about 110 W/(m×K) is observed at 2.6 K. The data are interpreted by Callaway model considering phonons resonant scattering on quasi-local vibrations of CH₄ molecules, phonon-grain boundary and phonon-phonon scattering processes. The increase of grain boundary scattering leads to the decrease of the maximum broadening. The analysis shows that the solid mixture of p-H₂ and CH₄ is a heterogeneous solution for CH₄ concentration higher than 0.1 ppm

Glassy Anomalies in the Low-Temperature Thermal Properties of a Minimally Disordered Crystalline Solid

The low-temperature thermal and transport properties of an unusual kind of crystal exhibiting minimal molecular positional and tilting disorder have been measured. The material, namely, low-dimensional, highly anisotropic pentachloronitrobenzene has a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that low-temperature glassy anomalies can be found in a system with minimal disorder due to the freezing of (mostly in-plane) reorientational jumps of molecules between equivalent crystallographic positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of the above-mentioned universal glassy properties at low temperature.Peer ReviewedPostprint (published version

Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

The thermal conductivity κ(T) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C 6H 11OH) has been measured under equilibrium vapor pressure within the 2-200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C 6H 11CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar. © 2012 American Physical Society.This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and the Catalan Government (Grant No. 2009SGR-1251)Peer Reviewe