Lattice dynamics and negative thermal expansion in layered mercury-based halides

Abstract

Materials with negative thermal expansion are beneficial as they can be appropriately tuned to be compatible for device engineering. Here, we report our first-principles investigation on Hg2X2 and HgX2 (X= Cl, I), which exhibits negative thermal expansion behaviour along c-direction. The first-principles studies reveal an anisotropic thermal expansion behaviour in the investigated compounds over a broad range of temperature. The computed coefficients along the c-axis are found to be negative for Hg2Cl2 and HgCl2. Hg2I2 exhibits negative thermal expansion behaviour up to 100 K, while HgI2 exhibits positive thermal expansion, whose values are qualitatively in good accord with the experimentally reported ones. The Grüneisen parameters along with elastic compliance matrix are used to calculate the thermal expansion coefficients of these materials. A detailed study of vibrational spectra was also done to get a better understanding of thermal expansion.