Study of High-Temperature Behaviour of ZnO by Ab Initio Molecular Dynamics Simulations and X-ray Absorption Spectroscopy.

Dmitry Bocharov,Matthias Krack,Inga Pudza,Konstantin Klementiev,Alexei Kuzmin

doi:10.3390/ma14185206

Abstract

Wurtzite-type zinc oxide (w-ZnO) is a widely used material with a pronounced structural anisotropy along the c axis, which affects its lattice dynamics and represents a difficulty for its accurate description using classical models of interatomic interactions. In this study, ab initio molecular dynamics (AIMD) was employed to simulate a bulk w-ZnO phase in the NpT ensemble in the high-temperature range from 300 K to 1200 K. The results of the simulations were validated by comparison with the experimental Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra and known diffraction data. AIMD NpT simulations reproduced well the thermal expansion of the lattice, and the pronounced anharmonicity of Zn–O bonding was observed above 600 K. The values of mean-square relative displacements and mean-square displacements for Zn–O and Zn–Zn atom pairs were obtained as a function of interatomic distance and temperature. They were used to calculate the characteristic Einstein temperatures. The temperature dependences of the O–Zn–O and Zn–O–Zn bond angle distributions were also determined.

Highlights

Zinc oxide (ZnO) is a wide band-gap (Eg = 3.37 eV) semiconductor, which has a wide range of technological applications, making it an extremely popular research topic in recent years [1,2,3,4,5,6,7]
The coordination of each atom in wurtzite-type ZnO (w-ZnO) is tetrahedral by four atoms of the other type, and the parameter u determines the distortion of the ZnO4 tetrahedra
Note that there is some scattering in the experimental values of the lattice parameters of ZnO at 300 K [44]

Summary

Introduction

Phonons control all thermal properties of ZnO such as heat capacity, thermal expansion and thermal conductivity [20] The latter is a key factor responsible for heat dissipation and limits the use of ZnO in power electronics applications [21]. The lattice dynamics of wurtzite-type ZnO was studied in the past by the Zn K-edge. The accuracy of several force-field models [24,25,26] often used for molecular dynamics simulations of bulk and nanocrystalline ZnO was confirmed by comparing the experimental and simulated Zn. K-edge extended X-ray absorption fine structure (EXAFS) spectra. We performed AIMD simulations for bulk wurtzite-type ZnO at high temperatures (300–1200 K) and validated the obtained theoretical results by their direct comparison with the experimental Zn K-edge EXAFS spectra

Experimental Details

Computational Details

Results and Discussion

Conclusions