Study of peculiarities of the thermal expansion of zirconium thin films by molecular-dynamics simulation

TL;DR

This study investigates the thermal expansion behavior of zirconium thin films with different crystal structures and orientations using molecular dynamics simulations, revealing anisotropic expansion and negative coefficients in some cases.

Contribution

It provides new insights into the anisotropic and unusual thermal expansion properties of zirconium thin films, including the occurrence of negative expansion coefficients in metastable phases.

Findings

Metastable fct zirconium films exhibit negative thermal expansion in-plane.

Surface atomic layers show distinct vibrational density of states behavior.

Lattice parameters decrease with temperature in directions with vibrational mode softening.

Abstract

The peculiarities of thermal expansion of $bcc$ and $fcc$ zirconium films with (100) and (110) crystallographic orientations are studied at a constant zero pressure by the molecular dynamics (MD) method with a many-body interatomic interaction potential obtained in the embedded atom model. It is shown that after relaxation the cubic lattices become tetragonal ($bct$ and $fct$), and for the metastable $fct$ films the linear coefficients of thermal expansion in the film plane have a negative value in a wide temperature range. The total and local vibrational density of states (VDOS) polarized along the $x,y,z$ axes is calculated for the surface and interior layers of $bct$ and $fct$ Zr films as a function of the temperature. It is shown that the peculiarities of the behavior of the vibrational density of states of surface atomic layers manifest themselves in the anisotropy of the changes of the film lattice parameters with temperature variation. A decrease in the lattice parameters with increasing temperature is observed in the directions where there occurs a softening of the local vibrational modes.