Miscibility and ordered structures of MgO-ZnO alloys under high pressure

OPEN SUBJECT AREAS: CONDENSED-MATTER PHYSICS Miscibility and ordered structures of MgO-ZnO alloys under high pressure Fubo Tian, Defang Duan, Da Li, Changbo Chen, Xiaojing Sha, Zhonglong Zhao, Bingbing Liu & Tian Cui THEORY AND COMPUTATION State Key Laboratory of Superhard Materials, College of physics, Jilin University, Changchun 130012, P. R. China. Received 26 March 2014 Accepted 1 July 2014 Published 21 July 2014 The MgxZn12xO alloy system may provide an optically tunable family of wide band gap materials that can be used in various UV luminescences, absorption, lighting, and display applications. A systematic investigation of the MgO-ZnO system using ab initio evolutionary simulations shows that MgxZn12xO alloys exist in ordered ground-state structures at pressures above about 6.5 GPa. Detailed enthalpy calculations for the most stable structures allowed us to construct the pressure-composition phase diagram. In the entire composition, no phase transition from wurzite to rock-salt takes place with increasing Mg content. We also found two different slops occur at near x 5 0.75 of Eg-x curves for different pressures, and the band gaps of high pressure ground-state MgxZn12xO alloys at the Mg concentration of x . 0.75 increase more rapidly than x , 0.75. Correspondence and requests for materials should be addressed to T.C. (. cn) T o realize high-performance ZnO-based optoelectronic devices, two important requirements are necessary: one is p-type doping of ZnO, and the other is modulation of the band gap (Eg)1,2. ZnO can alloy with MgO to form the ternary compound MgxZn12xO to extend the energy band gap, and therefore the detection spectrum into the shorter wavelength region. While p-type doping of ZnO is studied intensively, the latter has been demonstrated by the development research on MgxZn12xO allowing modulation of band gap in a wide range, from 3.34 to 7.8 eV3. Although the topic of ZnO has been extensively researched, less is known concerning the detailed structures of the MgxZn12xO alloy system. Fabrication and characterization of MgxZn12xO alloy are important from the viewpoint of band gap modulation as well as of p–n junction. MgxZn12xO thin films and nanostructures that include nanocrystals as well as core-shell structures have recently been studied with the objective of achieving a viable alloy family with tunable band gap and luminescence at the UV range4–21. Sans et al.4 investigated the dependence of the phase transition on the hydrostatic pressure and found that the wurtzite to rock-salt transition pressure is observed to decrease from 9.5 6 0.2 pure (ZnO) to 7.0 6 0.2 GPa (for x 5 0.13), with an almost linear dependence on the Mg content. Difficulties for the growth of high quality MgxZn12xO films are partly from the fact that MgO (cubic) and ZnO (hexagonal) have different crystal structures at normal conditions. In the last decade, there are numerous theoretical studies on the MgO-ZnO alloys22–40. Sanati et al.22 showed that isostructural MgO-ZnO alloys are stable under certain conditions using the cluster expansion (CE) method, but they also concluded that if MgO and ZnO can adopt their own crystal structures (B1 and B4, respectively), the alloy is predicted to phase separate. This means that MgxZn12xO alloys are not thermodynamically stable, consistent with a rather low observed solid solubility limit for Mg in ZnO. The cluster-expansion method was also used by Seko et al.23 for investigating phase transitions, including vibrational effects through lattice dynamics calculations. Very recently, combining the CASP-CE with a systematic set of first-principles total energies, Liu et al.24 exhaustively searched for the ground-state structures of MgO-ZnO alloys, and found a few structures as yet unreported. Several groups25–29 reported their theoretical studies in which the random MgxZn12xO alloy have been simulated using special quasirandom structures (SQS) approach41,42. There exist other theoretical studies30–35, in which alloying of ZnO and MgO proceeds by substituting Mg atoms by Zn atoms in the cubic rock-salt structure or vice versa in the hexagonal wurtzite structure, and also exist several theoretical reports36–38 on MgZnO2 in various model systems. An extensive systematic study of structural properties of MgxZn12xO was performed by Fan et al.39 using a supercell approach within the local-density approximation. In another theoretical study40, alloying of MgO and ZnO is described within coherent-potential approximation (CPA)43. Bulk and undoped ZnO prefers the hexagonal wurtzite (B4) structure under normal conditions and transforms into a cubic rocksalt (B1) structure at a pressure in the vicinity of 9 GPa reported by some experiments44–46, while MgO adopts the cubic rocksalt structure up to the highest pressure of 227 GPa47. Generally, a large crystal structure difference between the wurtzite-hexagonal ZnO (B4) and the rock-salt-cubic MgO (B1) can cause SCIENTIFIC REPORTS | 4 : 5759 | DOI: 10.1038/srep05759 1 www.nature.com/scientificreports Figure 1 | Thermal stability of MgO-ZnO alloys. a,b Predicted formation enthalpy of MgO-ZnO alloys with respect to decomposition into B1-MgO and ZnO (B4, B3, or B1) at (a) 0 GPa and (b) 8 GPa. c,d Predicted formation enthalpy of MgZn3O4, MgZnO2, and Mg3ZnO4 with respect to decomposition into B1-MgO and (c)B4-ZnO or (d) B3-ZnO below 8 GPa. unstable phase mixing6,48, as reported by Sanati et al.22. The same conclusions are made by Seko et al.23 and Malashevich et al.30, who both get that the formation energy of MgxZn12xO is positive. Are there any stable structures of MgxZn12xO alloy when ZnO and MgO adopt the same structure (B1) at pressures greater than 9 GPa? However, for the ordered structures of MgxZn12xO under high pressure, few theoretical studies have been reported to the best of our knowledge. Therefore, a first-principles calculation is very interesting to investigate the high pressure stable structures of MgxZn1 2 xO, because they are the basis of all properties in theoretical studies and can also provide valuable information for experimental synthesis. Here, we investigate the possible stability of MgO-ZnO alloys under high pressure using the first-principles calculations, and the structures are obtained from a recently developed evolutionary algorithm for the prediction of crystal structures49,50. We also analyse the band gaps of these alloys. Results In this work, we report a few stable ground state structures of MgxZn1 2 xO. The phase stabilities of MgO-ZnO systems are investigated by calculating the formation enthalpy of various MgxZn12xO alloys at different pressures. The formation enthalpy of MgxZn12xO is calculated by using fractional representation MgxZn12xO (0 # x # 1) with respect to the decomposition into MgO and ZnO, as

DH Mgx Zn1{x O ~H Mgx Zn1{x O {½xHB1 (MgO)z(1{x)HB3,B4,B1 (ZnO), where the enthalpies H for MgxZn12xO are obtained for the most stable structures as (...truncated)