There has been renewed interest in the scintillation properties of Cs2HfCl6 (CHC) primarily due to its favorable characteristics such as nonhygroscopicity, cubic structure, and fairly high light yield without any intentional doping. In this work, we report on its electronic and optical properties using first-principles calculations. As a large gap insulator (band gap ∼ 6 eV), CHC favors the formation of localized charge carriers, viz., V k centers and electron polarons. The [HfCl6]-octahedra play a central role in trapping both types of localized carriers that leads to several low energy excitonic structures or self-trapped excitons (STEs). The observed emission spectrum of CHC is compared with our modeled STE structures and their emission energies. We find that Zr present as an unintentional impurity has high solubility in CHC and may be responsible for a secondary emission peak observed around 480 nm. Finally, we mention the electronic structure of the bromide and iodide analogues of CHC as well as the mixed halide Cs2HfCl3Br3 and speculate on the possibility of higher light yield and faster scintillation.

Defects in semiconductors introduce vibrational modes that are distinct from bulk modes because they are spatially localized in the vicinity of the defect. Light impurities produce high-frequency modes often visible by Fourier-transform infrared absorption or Raman spectroscopy. Their vibrational lifetimes vary by orders of magnitude and sometimes exhibit unexpectedly large isotope effects. Heavy impurities introduce low-frequency modes sometimes visible as phonon replicas in photoluminescence bands. But other defects such as surfaces or interfaces exhibit spatially localized modes (SLMs) as well. All of them can trap phonons, which ultimately decay into lower-frequency bulk phonons. When heat flows through a material containing defects, phonon trapping at localized modes followed by their decay into bulk phonons is usually described in terms of phonon scattering: defects are assumed to be static scattering centers and the properties of the defect-related SLMs modes are ignored. These dynamic properties of defects are important. In this paper, we quantify the concepts of vibrational localization and phonon trapping, distinguish between normal and anomalous decay of localized excitations, discuss the meaning of phonon scattering in real space at the atomic level, and illustrate the importance of phonon trapping in the case of heat flow at Si/Ge and Si/C interfaces. V

We present the details of a method to perform molecular-dynamics (MD) simulations without thermostat and with very small temperature fluctuations ±ΔT starting with MD step 1. It involves preparing the supercell at the time t = 0 in physically correct microstates using the eigenvectors of the dynamical matrix. Each initial microstate corresponds to a different distribution of kinetic and potential energies for each vibrational mode (the total energy of each microstate is the same). Averaging the MD runs over many initial microstates further reduces ΔT. The electronic states are obtained using first-principles theory (density-functional theory in periodic supercells). Three applications are discussed: the lifetime and decay of vibrational excitations, the isotope dependence of thermal conductivities, and the flow of heat at an interface.

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A first-principles method to calculate the thermal conductivity in nanostructures that may contain defects or impurities is described in detail. The method mimics the so-called "laser-flash" technique to measure thermal conductivities. It starts with first-principles density-functional theory and involves the preparation of various regions of a supercell at slightly different temperatures. The temperature fluctuations are minimized without using a thermostat and, after averaging over random initial conditions, temperature changes as small as 5 K can be monitored (from 120 to 125 K). The changes to the phonon density of states and the specific heat induced by several atomic percent of impurities are discussed. The thermal conductivity of Si supercells is calculated as a function of the temperature and of the impurity content. For most impurities, the drop in thermal conductivity is unremarkable. However, there exist narrow ranges of impurity parameters (mass, bond strength, etc.) for which substantial drops in the thermal conductivity are predicted. These drops are isotope dependent and appear to be related to the vibrational lifetime of specific impurity-related modes.