Rock Salt Crystal Structure Research Articles

Impact of thickness on vacuum deposited PbSe thin films

Lead Selenide thin films were prepared by vacuum evaporation technique with different thickness ranges from 50 to 200 nm on glass substrates. The structural studies revealed that the prepared films are strongly oriented on (2 0 0) plane with rock-salt crystal structure. The various structural parameters such as grain size (D), lattice constant (a), micro strain (ε) and dislocation density (δ) were calculated. The surface morphology of the films was also analyzed. The optical absorption of the films starts with visible region and obtained energy gap of the films lies between 1.5 and 1.9 eV. The room temperature Photoluminescence spectrum shows the emission peak at visible region (380–405 nm) and the blue shift was observed with decreasing the film thickness. The electrical mobility, resistivity, carrier concentration and mean free path (L) of the free carriers of the films were studied for all the samples and compared.

Ammonolysis of HTiNbO5–n-Propylamine Intercalation Compound

Abstract The ammonolysis reaction to form (Ti,Nb)N was enhanced by the use of HTiNbO5–n-propylamine intercalation compound as a starting material and compared with that for a simple mixture of TiO2 and Nb2O5. The ammonolysis product formed at 1200 °C was a superconductor (Tc = 15 K) with a rock-salt crystal structure having many structural defects along the stacking direction of the intercalation compound.

Influence of sputtering pressure on band gap of Zn1−xMgxO thin films prepared by radio frequency magnetron sputtering

Zn1−xMgxO thin films were deposited onto quartz glass substrates using a radio frequency magnetron sputtering at various deposition pressures. It was found that the Mg concentration in the Zn1−xMgxO thin films significantly increased by decreasing the deposition pressure from 9 to 1 Pa, which contributed to an increase in the band gap of the ZnMgO films. In addition, the Zn1−xMgxO thin films, which had a hexagonal wurtzite structure when obtained by high pressure deposition (7 and 9 Pa) at x ≤ 0.478, had a cubic rock-salt crystal structure when deposition pressure was decreased to 1–5 Pa at x ≥ 0.482. These results show that the band gap of the ZnMgO thin films could be easily modulated only by adjusting of the deposition pressure during the radio frequency sputtering process.

Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb 1− xMx)(N 1− yO y) where M=Mg, Si, and x≈ y

New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N 2 and the final compositions were (Nb 0.95Mg 0.05)(N 0.92O 0.08) at 1500 °C and (Nb 0.87Si 0.09□ 0.04)(N 0.87O 0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of T c=17.6 K/100% for Mg- and T c=16.2 K/95% for the Si-doped niobium oxynitrides.

Solution Synthesis and Optical Properties of SnTe Nanocrystals

A facile approach has been developed for the synthesis of SnTe nanocrystals using triethanolamine as the stabilizing agent. The size of SnTe nanoparticles can be readily tuned from 2.7 to 32 nm. Powder X-ray diffraction and selective-area electron diffraction (SAED) indicate that the nanoparticles adopt the cubic rock-salt crystal structure. The direct band gap of the resulting nanoparticles can be significantly tuned and blue-shifted relative to the bulk value by changing the nanocrystal size.

High pressure synthesis of LiMeO2–ZnO (Me=Fe 3+, Ti3+) solid solutions with a rock salt structure

Metastable LiMeO2–ZnO (Me=Fe 3+, Ti3+) solid solutions with a rock salt crystal structure have been synthesized by the solid-state reaction of ZnO with LiMeO2 complex oxides at 7.7 GPa and 1350–1450 K. The structure, phase composition, thermal stability and thermal expansion of the recovered samples have been studied by X-ray diffraction with synchrotron radiation. At ambient pressure, rock salt LiMeO2–ZnO solid solutions are kinetically stable up to 670–800 K, depending on the composition.

Positron annihilation study of the microstructure of Co doped ZnO nanocrystals

ZnO nanopowders are mechanically mixed with a certain quantity of Co3O4 nanopowders to obtain 10at.% Co doped Co3O4/ZnO composites. The nanocomposites are annealed in argon atmosphere at different temperatures between 100 ℃ and 1200 ℃. The structure and the grain size of the nanocomposite are investigated by X-ray diffraction 2 scans. With annealing temperature increasing up to 700 ℃, Co3O4 phase gradually disappears, and ZnO grain size begins to increase significantly. After annealing at above 1000 ℃, Co3O4 phase completely disappears, and CoO phase (rock-salt crystal structure) appears. Positron annihilation lifetime measurements reveal a large number of Zn vacancies and vacancy clusters existing in the interface region of the Co3O4 /ZnO nanocomposites. These defects are gradually recovered after annealing at above 700 ℃, and their number is under the detection limit after annealing at 900 ℃. The same conclusion can be drawn from the coincidence Doppler broadening (CDB) measurements.

Nitriding response of ferritic Fe-Ti-Cr alloys*

Abstract Specimens of ferritic iron-based Fe-Ti-Cr alloys, containing a total of 0.30 at.% alloying elements, Ti and Cr with varying Ti/Cr atomic ratio (0.45, 0.87 and 1.90), were nitrided in a NH3/H2 gas atmosphere at 580 °C. The microstructure of the nitrided zone was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nitride platelets (length ≤ 30 nm and thickness ≤ 1 nm) developed in the nitrided zone. Particularly clear for the larger Ti/Cr atomic ratios, a tetragonally distorted ferrite develops next to the cubic ferrite, as evidenced both by XRD and TEM. The absence of separate TiN and CrN diffraction spots in the selected area electron diffraction patterns suggests the development of mixed TixCr1-xN nitride precipitates. The mixed TixCr1-xN nitride precipitates are of rock-salt crystal structure type and obey a Bain orientation relationship with the ferrite matrix.

An interatomic potential for aluminum arsenide: A molecular dynamics study

An effective interatomic potential is proposed for AlAs. Molecular dynamics technique was used to simulate the energetics of zinc-blende, wurtzite, rocksalt and NiAs crystal structures of AlAs. The calculated cohesive energy, bulk modulus and C 11 elastic constant at room temperature and melting temperature agreed very well with experimental values. By constructing a common tangent to the calculated energy versus volume curves for zinc-blende and rocksalt structures of AlAs, the pressure of structural transition between these phases was found to be 10 GPa. Vibrational density of states and specific heat were compared with first-principle calculations, displaying very good agreement. The proposed interatomic potential allows us to predict the temperature dependence of several properties, such as the elastic constants and specific heat.

High-pressure structural and electronic properties of InN

We theoretically study the electronic properties, and,pressure-induced solid-solid phase transformation by InN by using the first-principles pseudopotential method. The wurtzite (B4), rocksalt (B1), zinc-blende (B3), CsCl-type (B2), and Cmcm crystal structures of InN have been considered. The calculations indicate that the phase transitions from B4 phase to B1 phase and B3 structure to B1 structure occur at a transition pressure of 10.2 and 9.6 GPa, respectively. The detailed volume changes during the phase transformations were analyzed. Moreover, the analysis of the band structure indicates that the bandgap of B4 phase is direct, while B1 phase is indirect under high-pressure. The mechanism of these changes of band structures was analyzed. The positive pressure derivative of the indirect and direct gap indicates that it is impossible to make B1 phase of InN metallic up to 200 GPa. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The development of mixed Cr1−xAlxN nitride precipitates upon nitriding iron-based Fe-Cr-Al alloys

Abstract The development of alloying element nitrides upon gaseous nitriding of Fe–1.5 wt.-% Cr–1.5 wt.-% Al (Fe–1.6 at.-% Cr–3.1 at.-% Al) alloy was investigated, employing metallographic methods, X-ray diffractometry, electron probe microanalysis, microhardnesss measurement and (conventional, high resolution and scanning) transmission electron microscopy. It could be shown that Cr and Al do not precipitate in separate nitrides. Instead, mixed Cr1−xAlxN nitride precipitates of cubic rocksalt crystal structure develop. Upon subsequent annealing at elevated temperature, the originally mixed nitride transforms into two binary nitrides, namely CrN (cubic rocksalt crystal structure) and AlN (hexagonal wurtzite crystal structure) by outward diffusion of Al from the initially mixed nitride.

Coherent {001} interfaces between rocksalt and zinc-blende crystal structures

We study coherent, two-dimensional (2D) rocksalt PbTe epilayers in a zinc-blende CdTe matrix. The embedded material transforms during an annealing step into highly symmetric quantum dots with coherent interfaces. The atomic structure of the polar {001} interfaces between the two tellurides is investigated by high-resolution transmission electron microscopy in combination with multislice simulations. Both the original 2D PbTe epilayers and the resulting PbTe quantum dots are terminated by distinctively different cation-terminated [(001)A] and anion-terminated [(001)B] interfaces at opposite faces, whereas for the structurally similar ErAs/InGaAs heterosystem as yet only the (001)A interface was reported in the literature. Ab initio simulations were performed to assess these seeming differences, which were found to be mainly caused by the different polar bonding character of the involved II-VI and III-V compounds.

Contributions from the indirect ionic interactions to the elastic constants in the rock-salt structure crystals

Under the harmonic approximation, the contributions from the indirect ionic interactions to the elastic constants are calculated for the alkali halide and silver halide crystals with the rock-salt structure. The coupling constants of the indirect ionic interactions are calculated by the self-consistent field treatment of the local density approximation and the spherical solid model. The calculated values of the coupling constants are large for the silver ion. The indirect ionic interaction significantly affects the elastic constants. It quantitatively explains the deviation from the Cauchy relation in alkali halide crystals. Moreover, it provides a clear account for the large values of the deviation from the Cauchy relation in AgCl and AgBr.

Triangular Single-Crystalline Nanorings of PbS Formed at the Air/Water Interface

Triangular single-crystalline nanorings of PbS were prepared at the air/water interface and characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV−vis spectroscopy. The nanorings were transformed from triangular PbS nanopyramids that were formed under Langmuir monolayers of arachidic acid (AA) through interfacial reactions between Pb2+ ions in the subphases and H2S in the gaseous phase. The basal plane of the pyramids is a (111) face of PbS crystals with rock salt crystal structure, which is parallel to the air/water interface. The tips of the pyramids beneath the interface dissolved gradually due to selective etching of Pb2+ in the nanocrystals by Cl− ions in the subphase because of their higher surface energy, resulting in the formation of the rings. The size of the rings could be tuned by changing the experimental conditions, and the optical properties changed when the rings formed.

Thermal conductivity of (Zr,W)N/ScN metal/semiconductor multilayers and superlattices

The cross-plane thermal conductivities of metal/semiconductor multilayers and epitaxial superlattices have been measured as a function of period by time-domain thermoreflectance at room temperature. (001)-oriented ZrN (metal)/ScN (semiconductor) multilayers and (Zr,W)N/ScN epitaxial superlattices with the rocksalt crystal structure were grown on (001)MgO substrates by reactive magnetron sputtering. A distinct minimum in thermal conductivity at a period of ∼6 nm is observed for ZrN/ScN multilayers. The minimum thermal conductivity of 5.25 W/m K is a factor of ∼2.7 smaller than the mean of the thermal conductivities (including only the lattice contributions) of the values measured for films of the constituent materials, and approximately equal to the lattice component of the thermal conductivity of a Zr0.65Sc0.35N alloy film (∼5 W/m K). Alloying the ZrN layers with WNx reduces the lattice mismatch, yielding epitaxial (Zr,W)N/ScN superlattices. The addition of WNx also reduces the thermal conductivity to ∼2 W/m K, a value that is sufficiently low to suggest promise for these materials as solid-state thermionic generators.

Synthesis of rock-salt MeO–ZnO solid solutions (Me=Ni 2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

A series of metastable Me1−x Zn x O solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) with the rock-salt (rs) crystal structure have been synthesized from binary oxides by quenching at 7.7 GPa and 1450–1650 K. Phase composition of the samples, as well as structural properties and stoichiometry of synthesized solid solutions, have been studied by X-ray powder diffraction, both conventional and with synchrotron radiation. The widest (0.3 ≤x≤0.8) composition range of the existence of individual rs solid solution has been established for the NiO–ZnO system. The bulk rs-Co1−x Zn x O, rs-Fe1−x Zn x O and rs-Mn1−x Zn x O solid solutions may be quenched down to ambient conditions with only twice as low ZnO content, i.e. x≤0.5, 0.5 and 0.4, respectively; while formation of rs solid solutions in the CdO–ZnO system has not been observed in the entire concentration range.

Structural and electronic properties of ZnO under high pressure

We theoretically study the electronic properties and pressure-induced solid–solid phase transitions of ZnO by using the first-principles pseudopotential method. The wurtzite (B4), rocksalt (B1), zinc blende (B3) and CsCl (B2) crystal structures of ZnO have been considered. It has been shown that the B3 phase is not a stable phase in ZnO over the pressure range of 0–300 GPa. The sequence of these phases is B4 → B1 → B2 with the transition pressure is found to be 8.8 and 263 GPa, respectively. The detailed volume changes during the phase transformation were analyzed. Moreover, the analysis of the electronic properties indicates that the bandgap of B4-ZnO is direct, while B1 and B2 phases become indirect under high pressure. The mechanism of these changes of band structures was analyzed.

Electronic structure and phase stability of MgO, ZnO, CdO, and related ternary alloys

The electronic structure and phase stability of MgO, ZnO, CdO, and related alloys in the rocksalt (B1), zincblende (B3), and wurtzite (B4) crystal structures were examined within first-principles band structure theory; the thermodynamically stable phases are reproduced for each material. The band alignment and band-gap deformation potentials were analyzed, showing an increase in the valence band maximum from Mg to Zn to Cd. Ternary alloy formation was explored through application of the special quasirandom structure method. The B1 structure is stable over all (Mg,Cd)O compositions, as expected from the preferences of the binary oxides. The (Mg,Zn)O alloy undergoes a tetrahedral to octahedral transition above 34% Mg content, in agreement with experiment. For (Zn,Cd)O, a transition is predicted above 62% Cd content. These results imply that band-gap manipulation of ZnO from alloying with Mg (Cd) will be limited to 4.0 eV (1.6 eV), while preserving the tetrahedral coordination of the host.

Computer Simulation of the Phase-change Cycle of GST-225

ABSTRACTWe have simulated for the first time, by ab initio molecular dynamics, the complete phase-transformation cycle (liquid-crystal, liquid-amorphous-crystal) of the phase-change (PC) memory material Ge2Sb2Te5 (GST-225). We have observed that rapid cooling of the simulated melt leads to an amorphous product, whereas slow cooling results in the metastable rocksalt crystal. Furthermore, crystallization to the same structure is observed to occur on annealing the quenched amorphous model to temperatures below the melting temperature. The RDF of the energy-relaxed amorphous GST-225 structure agrees very well with experimental neutron-diffraction data, reproducing the shortening of the Ge-Te bond length relative to that in the rocksalt crystal structure observed experimentally.We have observed crystal-nucleation events in the simulated liquid that have been identified as the creation of connected near-regular square fourfold rings, the basic structural units of the rocksalt structure. These crystal nuclei are invariably found to be quenched into the amorphous state on rapid cooling of the simulated melt. This observation therefore explains why GST materials crystallize so readily and why homogeneous nucleation is so facile.

ZnxNi1-xO Mixed-Metal Oxides by AES

ZnxNi1-xO samples with a compositional range 0 < × ≤ 0.3 were synthesized via thermal dissolution in air and their surface characterized with Auger electron spectroscopy (AES). AES analysis shows the surface composition to be comparable to the bulk composition. These systems are interesting as they place zinc in an octahedral environment which is unusual for zinc normally preferring a tetrahedral environment. This submission contains survey spectra for the homogeneous solid solutions, Zn0.05Ni0.95O, Zn0.1Ni0.9O, Zn0.2Ni0.8O, Zn0.3Ni0.7O and for pure NiO and ZnO. All samples have the rocksalt crystal structure except ZnO which has the Wurtzite crystal structure.