Spontaneous Frenkel pair formation in Zirconium Carbide

TL;DR

This study uses density functional theory and molecular dynamics to reveal spontaneous Frenkel pair formation in zirconium carbide at high temperatures, analyzing defect structures, energies, and their impact on material properties.

Contribution

It demonstrates spontaneous Frenkel pair formation in ZrC at high temperature and provides detailed analysis of defect structures, energies, and their effects on thermophysical properties.

Findings

Frenkel pairs form spontaneously at 3200 K, below melting point.

Thermal expansion reduces defect formation energy by 0.7-1.5 eV.

Estimated defect concentration at 3000 K is 1.2% in stoichiometric ZrC.

Abstract

With density functional theory we have performed molecular dynamics simulations of ZrC which displayed spontaneous Frenkel pair formation at a temperature of 3200 K, some 500 K below the melting point. To understand this behaviour, rarely seen in equilibrium simulations, we quenched and examined a set of lattices containing a Frenkel pair. Five metastable structures were found, and their formation energies and electronic properties were studied. Their thermal generation was found to be facilitated by a reduction of between 0.7 and 1.5 eV in formation energy due to thermal expansion of the lattice. With input from a quasi-harmonic description of the defect free energy of formation, an ideal solution model was used to estimate lower bounds on their concentration as a function of temperature and stoichiometry. At 3000 K (0.81 of the melting temperature) their concentration was estimated to be 1.2% per mole in a stoichiometric crystal, and 0.3% per mole in a crystal with 10% per mole of constitutional vacancies. Their contribution to heat capacity, thermal expansion and bulk modulus was estimated.