Thermal diffusivity of alkali and silver halide crystals as a function of temperature

Abstract

The phonon component of thermal diffusivity (D) for ten synthetic single-crystals (LiF, NaCl, NaI, NaI:Tl, KCl, KBr, CsI, CsI:Tl, AgCl, and AgBr) with the B1 and B2 structures was measured from ambient temperature (T) up to ∼1093 K using contact-free, laser-flash analysis, from which effects of ballistic radiative transfer were removed. We investigated optical flats from different manufacturers as well as pellets made from compressed powders of most of the above chemical compositions plus LiI, NaBr, KI, RbCl, RbBr, RbI, CsCl, CsBr, and AgI. Impurities were characterized using various spectroscopic methods. With increasing T,D decreases such that near melting the derivatives ∂D/∂T are low, −0.0006±0.0004 mm2 s−1 K−1. Our results are ∼16% lower than D298 previously obtained with contact methods, which are elevated by ballistic radiative transfer for these infrared (IR) windows, and are well described by either D−1 following a low order polynomial in T, or by D−1∝T+n, where n ranges from 1.0294 to 1.9429. Inverse correlations were found between D298 and both density and thermal expansivity (α). Primitive lattice constant times compressional velocity correlates directly with D but changes much more slowly with temperature. Instead, D(T) is proportional to (TαL)−1 from ∼0 K up to the limit of measurements, in accord with these physical properties being anharmonic. On average, the damped harmonic oscillator–phonon gas model reproduces D298 based on two physical properties: compressional velocity and the damping coefficient (Γ) from analysis of IR reflectivity data. Given large uncertainties in Γ(T), D−1(T) is reproduced for LiF, NaCl, MgO, and the silver halides, for which IR reflectivity data are available. Our correlations show that optical phonons largely govern heat transport of insulators, and permit prediction of D and thus thermal conductivity for simple, diatomic solids.