1. Introduction
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 crystal lattice of the most common ZnO phase belongs to the wurtzite type and is strongly anisotropic [8,9,10], giving origin to its piezoelectric and pyroelectric properties [11,12,13,14,15,16].
- CALCIUM BISULFITE
- Cl2 + NaOH → NaCl + NaClO + H2O | Cl2 ra NaCl | Cl2 ra NaClO | NaOH ra NaCl | NaOH ra NaClO
- Ôn thi tốt nghiệp môn Hóa: Phương pháp đếm số đồng phân Este siêu nhanh
- Toluen là gì? Ứng dụng của Toluen trong công nghiệp
- Deutsche Gesellschaft für Hals-Nasen-Ohren-Heilkunde, Kopf- und Hals-Chirurgie e.V., Bonn
The unit cell of the wurtzite-type ZnO (w-ZnO) is hexagonal with the space group P63mc (186). Each type of atom (Zn or O) forms a hexagonal close-packed sublattice displaced relative to each other along the third-order c axis by the parameter u [1]. The Wyckoff positions (2b) of the Zn and O atoms in the unit cell are Zn (0, 0, 0), (1/3, 2/3, 1/2) and O (0, 0, u), (1/3, 2/3, 1/2+u). The coordination of each atom in w-ZnO is tetrahedral by four atoms of the other type, and the parameter u determines the distortion of the ZnO4 tetrahedra. The anisotropic structure of ZnO affects its lattice dynamics. In particular, the anisotropy of ZnO thermal expansion and atom thermal vibrations (thermal ellipsoids) along the a and c axes directions was observed by X-ray diffraction [17,18,19].
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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 thus limits the use of ZnO in power electronics applications [21]. Therefore, an understanding of the phonon dynamics in the high-temperature regime is important for the design of ZnO-based devices.
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The lattice dynamics of wurtzite-type ZnO was studied in the past by the Zn K-edge X-ray absorption spectroscopy at low temperatures (10-300 K) [22,23]. The information on the thermal disorder and anisotropy effects was extracted using two different simulation approaches, such as classical molecular dynamics and reverse Monte Carlo (RMC), which were both combined with ab initio multiple-scattering (MS) theory. 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. It was found that the existing force-field models cannot accurately describe the correlated atomic motion. At the same time, a more accurate solution was obtained from EXAFS data with the RMC method. In particular, two non-equivalent groups of atoms were resolved in both the first and second coordination shells of the absorbing Zn atom as a result of the ZnO structure anisotropy. An increase in temperature leads to the fact that the structure of ZnO becomes even more anisotropic, which is reflected in a change in the parameter u [22]. As a result, oxygen atoms displace along the c axis, and the Zn-O bond lengths vary [22].
Ab initio molecular dynamics (AIMD) provides an alternative, although computationally more expensive, approach to describe the lattice dynamics in zinc oxide. In this study, 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.
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