Rocksalt MgO Research Articles

Stabilization of Rock Salt ZnO Nanocrystals by Low-Energy Surfaces and Mg Additions: A First-Principles Study

Whereas bulk zinc oxide (ZnO) exhibits the wurtzite crystal structure, nanoscale ZnO was recently synthesized in the rock salt structure by addition of Mg. Using first-principles methods, we investigated two stabilization routes for accessing rock salt ZnO. The first route is stabilization by Mg addition, which was investigated by considering ZnO–MgO mixed phases. The second route is through size effects, as surface energies become dominant for small nanocrystal sizes. We discovered that the surface energy of rock salt ZnO is surprisingly low at 0.63 J m–2, which is lower than those of wurtzite and zinc blende ZnO and lower than that of rock salt MgO. We predict that pure rock salt ZnO is stable for nanocrystals smaller than 1.6 nm, and that Mg additions can greatly extend the size range in which the rock salt phase is stable. Both mixed-phase and core–shell models were considered in the calculations. The present approach could be applied to predict the stabilization of many other nanocrystal phases in de...

Overcoming the polarization catastrophe in the rocksalt oxides MgO and CaO

Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.

Interstitial Fe in MgO

Isolated 57Fe atoms were studied in MgO single-crystals by emission Mössbauer spectroscopy following implantation of 57Mn decaying to 57Fe. Four Mössbauer spectral components were found corresponding to different Fe lattice positions and/or charge states. Two components represent Fe atoms substituting Mg as Fe2+ and Fe3+, respectively; a third component is due to Fe in a strongly implantation-induced disturbed region. The fourth component, which is the focus of this paper, can be assigned to Fe at an interstitial site. Comparison of its measured isomer shift with ab initio calculations suggests that the interstitial Fe is located on, or close to, the face of the rock-salt MgO structure. To harmonize such an assignment with the measured near-zero quadrupole interaction a local motion process (cage motion) of the Fe has to be stipulated. The relation of such a local motion as a starting point for long range diffusion is discussed.

Oxygen polarity and interfacial atomic arrangement in an MgxZn1−xO/C-MgO/sapphire heterostructure

Oxygen polarity and interfacial structure of an Mg0.55Zn0.45O/Mg0.17Zn0.83O/MgO/sapphire heterostructure were determined by electron energy-loss spectroscopy and aberration-corrected scanning transmission electron microscopy. It was found that O-polarity occurs by an uncommon mechanism in the MgxZn1−xO film with a 0.5 nm-thick compressively strained rocksalt MgO buffer; and that the ∼18% lattice mismatch between Mg0.17Zn0.83O and sapphire is mostly accommodated by misfit dislocations at the Mg0.17Zn0.83O/MgO interface. Density functional theory calculations indicate that the MgO buffer strongly tends to manifest a rocksalt structure rather than wurtzite. Zn substitution of the topmost Mg atoms in the MgO buffer is likely to induce such O-polarity.

Energy levels and deformation potentials for rocksalt MgO

We have studied the high-pressure behavior of the electronic properties of periclase (MgO) using ab initio total energy calculations within the full-potential linearized augmented plane wave method in the framework of the density functional theory. We predict that at zero pressure MgO in the rocksalt structure is a semiconductor with a direct band gap of 5.35eV We show that the material remains a direct band gap semiconductor under pressures up to 100GPa. We report the pressure coefficients and deformation potentials of the three transitions Γ–Γ, Γ–X and Γ–L for the material in question. We also find that the valence band width increases monotonically with increasing pressure suggesting that the rocksalt MgO becomes less ionic under pressure.

Heteroepitaxial growth of MgO(111) thin films on Al2O3(0001): Evidence of a wurtzite to rocksalt transformation

We report on a growth study of MgO films deposited on Al${}_{2}$O${}_{3}$(0001) substrates by magnetron sputtering. The films exhibited a preferred rocksalt MgO(111) orientation. Surprisingly, depending on the O${}_{2}$ gas flow ratio, a structure of graphiticlike wurtzite MgO(0001) has been revealed. The observed Mg-O perpendicular bond length reduction is accompanied by an atomically flat surface morphology for the development of MgO(111) films; the transition to the bulk rocksalt structure occurs in the 3--6 nm coverage range. Previously, relaxation of the electrostatic instability of MgO(111) films accompanied by an in-plane lattice increase has been suggested theoretically [Phys. Rev. Lett. 98, 205701 (2007)]. Here, relying on ab initio calculations, we infer that Mg vacancies facilitate the lattice match with the substrate. This mechanism suggests methods to engineer oxide heterostructures.

Nanoscale thermal stabilization via permutational premelting

Anomalous higher-than-bulk melting in some metal clusters has been previously ascribed to particularly compact cluster structures and emergent covalent bonding at small sizes. Using both classical and ab initio molecular dynamics incorporating realistic interacting atomic environments, we show a MgO cluster which, although structurally flexible and with bulklike ionic bonding, melts at a higher temperature than rock salt MgO. We propose that this unexpectedly high thermal stability is due to the premelted dynamics of a bi-isomeric fluxional state allowing facile permutation of ion positions, raising the configurational entropy and lowering the free energy. We argue that this entropic stabilizing effect could also occur in other (nano)systems which allow for atomic permutations with relatively little structural change.

Effects of In-Plane Magnetization Orientation on Magnetic and Electronic Properties in a Bcc Co (001)/Rock Salt MgO (001)/Bcc Co (001) Magnetic Tunnel Junction System: <I>Ab Initio</I> Calculations

Ab initio calculations were performed on a fully epitaxial bcc Co (001)/rock salt MgO (001)/bcc Co (001) magnetic tunnel junction system for two cases where the magnetization is parallel to bcc Co [100] and to bcc Co [110]. Structural optimization reveals that the two cases are equivalent systems and that the Co electrodes contract in the z-direction whereas the MgO insulating barrier expands. The magnetic moments of each monolayer vary slightly in each case; furthermore, only the magnetic moment at the surface of the Co atom shows any enhancement (12%). The layer decomposed density of states profiles reveals that the bonding character of the junction interface is derived mainly from the 2p-3d hybridization of the MgO and Co interfacial atoms.

Interface structure governed by plastic and structural dissimilarity in perovskite La0.7Sr0.3MnO3 nanodots on rock-salt MgO substrates

Self-assembled La0.7Sr0.3MnO3 perovskite nanodots have been grown on highly mismatched rock-salt type MgO substrates by chemical solution deposition. The interfacial dislocation structure indicates that the relaxation mechanism is governed by the easy glide of MgO 1/2〈101〉{101} dislocations towards the interface, where they dissociate into misfit-relieving and tilting components. The latter induce a 4-domain tilt pattern superimposed to the main cube-on-cube epitaxial orientation. It is shown that the inheritance of the rotational component of the Burgers vector has detrimental consequences on the interfacial quality, evidencing the impact of structural and plastic dissimilarity on the design of self-assembled nanostructures.

Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (αV) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for αV of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained αV is in good agreement with experiment. The values of αV for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.

Structural studies of magnesium nitride fluorides by powder neutron diffraction

Samples of ternary nitride fluorides, Mg3NF3 and Mg2NF have been prepared by solid state reaction of Mg3N2 and MgF2 at 1323–1423K and investigated by powder X-ray and powder neutron diffraction techniques. Mg3NF3 is cubic (space group: Pm3m) and has a structure related to rock-salt MgO, but with one cation site vacant. Mg2NF is tetragonal (space group: I41/amd) and has an anti-LiFeO2 related structure. Both compounds are essentially ionic and form structures in which nitride and fluoride anions are crystallographically ordered. The nitride fluorides show temperature independent paramagnetic behaviour between 5 and 300K.

A robust approach for the growth of epitaxial spinel ferrite films

Heteroepitaxial spinel ferrites NiFe2O4 and CoFe2O4 films have been prepared by pulsed laser deposition at various temperatures (175–690 °C) under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of ablated species at low pressure and enhanced oxidation power of ozone, epitaxy has been achieved at significantly lower temperatures than previously reported. Films grown at temperature below 550 °C show a novel growth mode, which we term “vertical step-flow” growth mode. Epitaxial spinel ferrite films with atomically flat surface over large areas and enhanced magnetic moment can be routinely obtained. Interestingly, the growth mode is independent of the nature of substrate (spinel MgAl2O4, perovskite SrTiO3, and rock-salt MgO) and film thickness. The underlying growth mechanism is discussed.

Large constriction of lattice constant in epitaxial magnesium oxide thin film: Effect of point defects on lattice constant

Epitaxial thin films prepared using an MgO target on silicon substrate often show constriction of lattice constant (a∼4.1 Å). Detailed investigation of the crystal structure excluded the possibility that the epitaxial films are either cubic spinel MgO or magnesium silicate (a/2∼4.1 Å). With such a constriction in rock salt MgO structure point defects must be induced into structure. An ab initio method with semicore pseudopotentials predicted such constrictions on configurations of Schottky type defects. The Schottky defects with random distribution throughout lattice cites satisfied constricted lattice constant, mass density, and crystallography experimentally observed on the epitaxial MgO films.

Controlled growth of Zn-polar ZnO film on MgAl2O4 (1 1 1) substrate using MgO buffer layer

A pure rocksalt MgO buffer layer was used to modify the surface structure of MgAl2O4 (1 1 1) substrates to achieve growth of a Zn-polar ZnO film by radio frequency plasma-assisted molecular beam epitaxy. It is found that this pure rocksalt MgO buffer layer plays a crucial role in 30° rotation domain elimination, surface morphology improvement and Zn-polarity control of the ZnO film, as demonstrated by in situ reflection high-energy electron diffraction and ex situ transmission electron microscopy. Atomic force microscopy observation also illustrates a smooth surface for the ZnO film.

First-principles prediction of the magnetism of 3 d transition-metal-doped Rocksalt MgO

Using first-principles band structure methods, we have systematically studied the electronic structures, magnetic stabilities, and half-metal properties of 3 d transition-metal ( TM) doped Rocksalt MgO compounds TMMg 3O 4 ( TM = V , Cr, Mn, Fe, Co, and Ni). The calculations reveal that only CrMg 3O 4 has a ferromagnetic stability among the six compounds, which is explained by double-exchange mechanism. The magnetic stability is affected by the doping concentration of TM if the top valance band is composed of partially occupied t 2 g states. In addition, CrMg 3O 4 is a half-metallic ferromagnet. The origins of half-metallic and ferromagnetic properties are explored. The Curie temperature ( T c ) of CrMg 3O 4 is 182 K. And it is hard for CrMg 3O 4 to deform due to the large bulk modulus and shear modulus, so it is a promising spintronic material. Our calculations provide the first available information on the magnetic properties of 3 d TM-doped MgO.

Metastable rocksalt ZnO interfacial layer and its influence on polarity selection of Zn-polar ZnO films

By a metastable rocksalt ZnO interfacial structure and a resulting rocksalt-to-wurtzite structure transition in ZnO buffer layer, polarity control of Zn-polar ZnO films was realized on rocksalt MgO (1 1 1) template by plasma-assisted molecular beam epitaxy. The metastable rocksalt ZnO and the phase transition from octahedrally bonded rocksalt to tetrahedrally bonded wurtzite structure during the growth of ZnO buffer layer were established by the combination of in situ reflection high-energy electron diffraction and ex situ high-resolution transmission electron microscopy. Such a metastable rocksalt ZnO has a lattice constant a=4.16 Å and a typical … abcabc… atom stacking along the growth direction. Polarity selection of Zn-polar ZnO films was found greatly depending on the bonding configuration of the epitaxial ZnO interfacial layer, and an atomic model of the structure transition was proposed to explain the mechanism of ZnO polarity selection.

First-principles determination of structural properties of MgO

We present an ab initio calculation of structural properties of MgO in the rocksalt and CsCl structures. We employ the full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the generalized gradient approximation (GGA). The agreement between our results and experiment concerning the lattice constant, bulk modulus and pressure derivative of bulk modulus for rocksalt MgO is better than 2, 9 and 2%, respectively. Our results for CsCl MgO are predictions. The pressure for the rocksalt to CsCl transition is found to be 611.12 GPa showing thus a remarkable stability range of MgO in the rocksalt structure.

Pseudopotential band structures of rocksalt MgO, ZnO, and Mg1−xZnxO

The electronic properties of the rocksalt group-II oxides MgO and ZnO are investigated by means of the empirical pseudopotential method. Using a simple empty core model potential and experimentally known low-temperature transition energies of rocksalt MgO and wurtzite ZnO, we obtained cationic model potential parameters for Mg and Zn atoms, respectively. Making use of the transferability of ionic model potential parameters, we obtained one single set of anionic model potential parameters for the O atom. The electronic properties of the Mg1−xZnxO alloy system are investigated by means of the virtual crystal approximation for x<0.5 in the rocksalt phase.

Oxygen vacancies and peroxo groups on regular and low-coordinated sites of MgO, CaO, SrO, and BaO surfaces

The formation of an oxygen vacancy and the simultaneous re-adsorption of an oxygen atom on regular and low-coordinated (LC) sites (edges and corners) of the surface of alkaline-earth oxides with cubic rock salt structure, MgO, CaO, SrO, and BaO, has been investigated using DFT cluster model calculations. The process corresponds to the formation of a surface Frenkel defect when the vacancy formation energy is partially compensated by the energy gained in the formation of a peroxo group. The structural and electronic properties of vacancies and peroxo groups along the series of alkaline-earth oxides have been analyzed. We found that the role of low-coordinated sites on the surface chemistry of alkaline-earth oxides is of crucial importance for MgO, but decreases for the heavier members. For instance, on BaO the formation of a peroxo group is practically site-insensitive. This is not the case of the vacancy formation, which is always favored on the low-coordinated sites.

Self-organized growth of pyramidal clusters in epitaxial spinel CoCr2O4 films on rock salt MgO(001) substrates

Ferrimagnetic CoCr2O4 epitaxial thin films have been grown on MgO(001) substrates. The surface morphology of the films shows self-organized nanometric pyramids and hut clusters. The origins of this morphology are found in the anisotropy of the surface energy of CoCr2O4 and the epitaxial growth. A comparison with the growth on spinel MgAl2O4(001) substrates is done. It turns out that pyramids on MgO are typically larger and with a higher surface coverage than on MgAl2O4. These results open the possibility of fabricating hybrid structures of ferrimagnetic self-organized nanometric high aspect ratio objects on substrates of selected functionality. Tailoring the size of the objects can be achieved not only by deposition temperature and film thickness but also by appropriate selection of substrates.