New thermodynamic data for CoTiO3, NiTiO3 and CoCO3 based on low-temperature calorimetric measurements

The low-temperature heat capacities of nickel titanate (NiTiO3), cobalt titanate (CoTiO3), and cobalt carbonate (CoCO3) were measured between 2 and 300 K, and thermochemical functions were derived from the results. Our new data show previously unknown low-temperature lambda-shaped heat capacity anomalies peaking at 37 K for CoTiO3 and 26 K for NiTiO3. From our data we calculate standard molar entropies (298.15 K) for NiTiO3 of 90.9 ± 0.7 J mol-1 K-1 and for CoTiO3 of 94.4 ± 0.8 J mol-1 K-1. For CoCO3, we find only a small broad heat capacity anomaly, peaking at about 31 K. From our data, we suggest a new standard entropy (298.15 K) for CoCO3 of 88.9 ± 0.7 J mol-1 K-1

First-principles determination of phonon lifetimes, mean free paths, and thermal conductivities in crystalline materials:Pure silicon and germanium

The thermal properties of insulating, crystalline materials are essentially determined by their phonon dispersions, the finite-temperature excitations of their phonon populations-treated as a Bose-Einstein gas of harmonic oscillators-and the lifetimes of these excitations. The conceptual foundations of this picture are now a well-established cornerstone in the theory of solids. However, only in recent years our theoretical and algorithmic capabilities have reached the point where we can now determine all these quantities from first-principles, i.e. from a quantum-mechanical description of the system at hand without any empirical input. Such advances have been largely due to the development of density-functional perturbation theory that allows to calculate second-and third-order perturbations of a system of interacting electrons with a cost that is independent of the wavelength of the perturbation. Here we present an extensive case study for the phonon dispersions, phonon lifetimes, phonon mean free paths, and thermal conductivities for isotopically pure silicon and germanium, showing excellent agreement with experimental results, where available, and providing much needed microscopic insight in the fundamental atomistic processes giving rise to thermal conductivity in crystals