Effect of Hydration and Ammonization on the Thermal Expansion Behaviour of ZrW2O8: Ab-initio Lattice Dynamical Perspective

TL;DR

This study uses ab-initio calculations to explore how hydration and ammonization alter the phonon modes in ZrW2O8, thereby changing its thermal expansion from negative to positive.

Contribution

It provides a detailed lattice dynamical analysis explaining how hydration and ammonization influence thermal expansion in ZrW2O8, offering insights for engineering thermal properties.

Findings

Hydration inhibits phonon modes responsible for negative thermal expansion.

Ammonization preserves phonon modes leading to negative thermal expansion.

Thermal expansion can be tailored by manipulating phonon dynamics.

Abstract

The hydration and ammonization of ZrW2O8 is known to lead to positive and negative thermal expansion behaviour respectively. We report ab-initio calculations to understand this anomalous behaviour. We identify the crucial low energy phonon modes involving translations, rotations and distortions of WO4 and ZrO6 polyhedra, which lead to NTE in ZrW2O8 in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coefficient could be tailored by engineering the phonon dynamics of a material.