Using ab initio calculations we have investigated the thermal conductivity ($\kappa$) of diamond nanowires, unveiling unusual features unique to this system. In sharp contrast with Si, $\kappa \left(T\right)$ of diamond nanowires as thick as 400 nm still increase monotonically with temperature up to 300 K, and room-temperature size effects are stronger than for Si. A marked dependence of $\kappa$ on the crystallographic orientation is predicted, which is apparent even at room temperature. [001] growth direction always possesses the largest $\kappa$ in diamond nanowires. The predicted features point to a potential use of diamond nanowires for the precise control of thermal flow in nanoscale devices.

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li₇La₃Zr₂O₁₂ appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga³⁺, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc³⁺, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li₇La₃Zr₂O₁₂ according to the resulting stoichiometry, Li_7–3x+yGa_xLa₃Zr_2–ySc_yO₁₂. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li_6.65Ga_0.15La₃Zr_1.90Sc_0.10O₁₂, with preferential Ga³⁺ occupation of tetrahedral Li_24d sites over the distorted octahedral Li_96h sites. ⁷Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10^–3 S cm^–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively.

We show that impurities and vacancies affect the thermal conductivity much more strongly than what is predicted by widely accepted models. When local distortions around point defects are strong, standard perturbative approaches fail, and phonon scattering can only be accounted for by an exact Green's function calculation. We apply the theory to the study, from first-principles, of nitrogen and vacancy defects in diamond. The thermal conductivity is computed by solving the linearized Boltzmann transport equation. The Born approximation underestimates the phonon scattering cross sections of nitrogen and vacancies by factors of 3 and 10, respectively. Thermal conductivity calculations are in good agreement with experiment.

It has been highly debated whether the thermal conductivity $\kappa$ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to $\kappa$ values not only lower than the alloy’s, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that $\kappa$ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower $\kappa$ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of $\sim 3\mathrm{THz}$. Our calculated $\kappa$’s quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.