Rocksalt-type Structure Research Articles

Synthesis and Crystal Structure of a New Complex Uranium Oxide, Na4.5Nd0.5UO6

Single crystals of Na4.5Nd0.5UO6 were grown out of a molten hydroxide flux and characterized by single crystal X-ray diffraction. Na4.5Nd0.5UO6 crystallizes in the orthorhombic space group Fddd with a = 6.6574(5) A, b = 9.7031(6) A, and c = 20.1571(13) A. The material exhibits an ordered rock salt structure type. The uranium atoms are located in a distorted UO6 octahedral coordination environment containing two short, 2.056(4) A, and four long, 2.116(3) A, U–O bonds. The short bonds are significantly longer compared to what is typically observed for the uranyl cation, UO2 2+. The synthesis and crystal structure of a new complex uranium oxide, Na4.5Nd0.5UO6 are reported.

On the structural stability of CeN at high pressures: Ab initio calculations

The structural stability of CeN under hydrostatic compression has been analyzed theoretically. The comparison of enthalpies calculated as a function of hydrostatic compression for rocksalt type (B1), tetragonal (B10), and CsCl type (B2) structures suggests that the B1 phase will transform to B10 structure at ∼53 GPa, which upon further compression will transform to B2 phase at ∼200 GPa. However, the static high pressure energy dispersive x-ray diffraction measurements on CeN by Olsen et al. [J. Alloys Compd. 533, 29 (2012)] report that the B1 phase transforms directly to B2 phase at ∼65 GPa. To resolve the discrepancy between our calculations and experimental results, we have performed lattice dynamic calculations on these structures. The phonon spectra calculated at zero pressure correctly show B1 phase to be dynamically stable, and B10 and B2 to be unstable. At 60 GPa, the B1 phase becomes dynamically unstable and the B10 structure emerges as a dynamically stable phase whereas B2 still remains unstable. At still higher pressure of ≥200 GPa, the B2 phase becomes not only the lowest enthalpy structure but also dynamically stable. These findings support the results of our static lattice calculations. Further, our calculated angle dispersive x-ray diffraction pattern of B1, B10, and B2 phases shows that most of the diffraction peaks of B10 phase except few weak peaks coincide with the peaks of either B1 or B2 phase; which may pose a difficulty in unambiguously identifying the high pressure phase until a sufficient amount of B1 phase is transformed to the new structure so that the weak peaks, if present, are also visible.

Synthesis and Electrochemical Properties of Novel Co2+/Co3+ Layered Ordered Rock-Salt Structure Type Li0.75Ca0.25CoO¬2

New layered transition metal oxide cathode material, Li0.75Ca0.25CoO2 (Space Group = P63/mmc; a = 2.817 Å, b = 2.817 Å, c = 10.79 Å ß = 120{degree sign}) was prepared by a multistep synthetic technique. Electrochemical cycling of 0.75 Li+ per formula unit utilizes both the Co3+/Co4+ and Co2+/Co3+ redox couples. The presence of Ca2+ between CoO2 layers results in low volume difference (~0.8%) between fully lithiated and delithiated materials.

Synthesis and electrochemical properties of layered Li1.0Mn0.82Ni0.10Ti0.08O2 prepared by chemical lithium insertion

The novel LixMnyNizTi1−y−zO2 compounds were prepared by Na+/Li+ ion-exchange and chemical lithium insertion. The eight products were characterized using X-ray diffraction, ICP-AES, SEM, and electrochemical measurements. All the samples were adopted the layered rock-salt type structure. The electrochemical properties of Li0.6Mn0.82Ni0.10Ti0.08O2 sample exhibited the best capacity retention of 87% at 20th charge–discharge test in the voltage range of 1.5 and 5.0 V of all the present samples. The extra lithium was added by chemical lithium insertion using LiI and then fully occupied Li1.0Mn0.82Ni0.10Ti0.08O2 in lithium site was obtained. The initial charge and discharge capacities of Li/Li1.0Mn0.82Ni0.10Ti0.08O2 cell were 250 mAh g−1 and 244 mAh g−1, respectively, with the first coulombic efficiency of 98% between 2.0 and 4.8 V. The graphite/Li1.0Mn0.82Ni0.10Ti0.08O2 cell indicated the first discharge capacity of 208 mAh g−1 at 1.8 V after charging up to 4.7 V. The irreversible capacity loss of 39 mAh g−1 was ascribed to the formation of a solid–electrolyte interface (SEI). The Li1.0Mn0.82Ni0.10Ti0.08O2 is one of the promising materials for positive electrode material in lithium ion batteries for the view point of high capacity and coulombic efficiency.

Electron microscopy study of CVT grown Fe-sulphides

Recent studies of {110} twin boundaries in natural pyrite (FeS2) indicated that twinning is triggered by the presence of Cu during crystal growth. Here we investigated the formation sequence of Fe-sulphides produced by the chemical vapor transport (CVT) method at 600°C in an evacuated quartz tube using Fe- and Cu-halides and elementary S as the reaction precursors. Depending on the mobility of elements different Fe-sulphides crystallized through gradually decreasing temperature zones. At the higher temperature zones, where metal ions are present in abundance, the main reaction product is FeS in form of pyrrhotite-3T. The main characteristic of this pyrrhotite is the presence of {111} layers cubic FeS coherently intergrown with the {0001} layers of the hosting hexagonal FeS. While the hosting structure corresponds to distorted troilite (nickeline-type structure) with octahedrally coordinated Fe2+ ions, iron in cubic FeS either remains in octahedral coordination (rocksalt-type structure) or is translated to tetrahedral interstices (sphalerite-type structure). HRTEM analysis suggests that Fe2+ ions in the cubic sequences remain in the octahedral sites, suggesting a new, cubic close-packed, rocksalt-type structure of FeS. Cubic FeS shows a high density of twins and SFs in the {111} planes. Pyrite crystallized in the temperature zone between 500 and 450°C, where the concentration of metal ions is depleted. With a decreasing temperature its morphology changes from {111}→{210}→{100}. Doping with copper did not result in twinning of pyrite, suggesting that different thermodynamic conditions are present in the natural environment. Using the natural seed crystal we showed that the twin boundaries are continued across the interface into the CVT grown epitaxial pyrite, whereas the marcasite-type SFs terminate at the interface.

Ab initio calculations on structural, elastic and dynamic stability of CdO at high pressures

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability, of CdO under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type (B2) structures determined as a function of compression suggests the B1 → B2 transition at ∼87 GPa, in good agreement with experimental value of 90.6 GPa [Liu et al. Phys. Rev. B 70, 0941141 (2004)]. Various physical quantities, such as zero pressure equilibrium volume, bulk modulus, pressure derivative of bulk modulus, Gruneisen parameter, and Debye temperature have been derived from the theoretically determined equation of state. All these physical quantities show a reasonably good agreement with the available experimental data. Additionally, employing the theoretically determined thermal equation of state in conjunction with Rankine Hugoniot relation, we have predicted the Hugoniot of B1 phase of this material. The single crystal elastic constants of B1 phase calculated up to the pressure of 166 GPa show that though, the shear elastic stability of B1 phase decreases monotonically with compression, it still remains stable elastically even at transition point. Our lattice dynamic calculations show that the B1 phase remains stable dynamically also for pressures <118 GPa. For B2 phase, our calculations demonstrate that this structure will be elastically stable beyond ∼38 GPa. However, to stabilize this phase dynamically a still higher pressure of ∼55 GPa will be required, which is less than B1 → B2 transition pressure.

Ab initio investigations of structural, elastic and electronic properties of ZnSiP2: Pressure effect

In this work, we present ab initio investigations of the pressure effect on the structural, elastic and electronic properties of ZnSiP2 by employing the plane wave pseudo-potential method (PP-PW) within the generalized gradient approximation (GGA-PW91). The calculated equilibrium structural parameters are in excellent agreement with available experimental and theoretical results. We have found that ZnSiP2 undergoes a structural phase transition under pressure from chalcopyrite to rocksalt type structure at 35GPa. Single-crystal and polycrystalline elastic constants and some related properties under pressure effect in both chalcopyrite and rocksalt phases have been predicted. The analysis of the bulk modulus to shear modulus (B/G) ratio shows that ZnSiP2 must be classified as brittle material. Electronic properties and chemical bonding nature have been studied throughout the band structure, density of states and charge distribution analyses. It is found that the studied compound is a direct band gap (Γ−Γ) semiconductor (Eg=1.34eV) in chalcopyrite structure, and is a conductor in the rock-salt structure. The chemical bonding of ZnSiP2 has a mixture of ionic–covalent and ionic–covalent–metallic character, respectively in chalcopyrite and rocksalt type structure.

Enhancement of electrical transport through the anisotropic nanostructure performance of heavily Yb-doped PbSe0.2Te0.8 thin films

Solid-state microwave-assisted plasma technique was used to synthesize high-purity Pb1−xYbxSe0.2Te0.8 from initial components to obtain polycrystalline ingots with large grains. Pb1−xYbxSe0.2Te0.8 thin films were then deposited onto glass substrates using thermal evaporation in vacuum. The films have the polycrystalline rock salt-type (NaCl) structure. The crystal lattice parameters for the powders and thin films obtained from X-ray diffraction (XRD) patterns showed that the values of the lattice parameters increased with increasing Yb content. The field emission scanning electron microscopy images reveal that the Pb1−xYbxSe0.2Te0.8 thin films have uniform crystal grain sizes and dense nanostructures. The electrical transport properties of the thin films were measured in the temperature range of 298 K–523 K. The Seebeck coefficient of the films increased with x when x was in the range of 0.015–0.045, whereas it decreased for x between the range of 0.06 and 0.105. The carrier concentration was 4.3 × 1017 cm−3 for x = 0.015, and the maximum value was 6.34 × 1017 cm−3 at x = 0.075.

Characterization of nanocrystalline PbS thin films prepared using microwave-assisted chemical bath deposition

Nanocrystalline lead sulfide (PbS) thin films were synthesized on glass substrates using microwave-assisted chemical bath deposition (CBD) method. Various deposition periods of time ranging from 30 to 120min were used. Results demonstrated that the thickness of the thin films increased with longer deposition time. X-ray diffraction (XRD) measurement revealed that all thin films have cubic rock salt (NaCl) type structure. The surface morphology studied using scanning electron microscopy (SEM) showed that the films have uniform surface morphology over the entire substrate and were of good quality. AFM images confirm that the films have a smooth surface with good adherence to the substrate, a narrow particle size distribution, and that the surface roughness increased with increasing deposition time. Energy gap Eg decreases as the deposition time increases. Electrical measurements revealed that all films were p-type and that the conductivity decreased as the deposition time increased.

Experimental and Theoretical Evidence for Pressure-Induced Metallization in FeO with Rocksalt-Type Structure

Electrical conductivity of FeO was measured up to 141 GPa and 2480 K in a laser-heated diamond-anvil cell. The results show that rock-salt (B1) type structured FeO metallizes at around 70 GPa and 1900 K without any structural phase transition. We computed fully self-consistently the electronic structure and the electrical conductivity of B1 FeO as a function of pressure and temperature, and found that although insulating as expected at ambient condition, B1 FeO metallizes at high temperatures, consistent with experiments. The observed metallization is related to spin crossover.

On the spinel formation in Co1 − xO/Co2TiO4 composites via reactive sintering, exsolution and oxidation

The formation mechanism and microstructural development of the spinel phases in the Co1−xO/Co2TiO4 composites upon reactive sintering the Co1−xO and TiO2 powders (9:1 molar ratio) at 1450°C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co2TiO4 spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co1−xO grains with a rock salt-type structure during reactive sintering. The submicron sized Co2TiO4 particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co1−xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co2TiO4 nanoparticles were precipitated and the Ti-doped Co1−xO host was partially oxidized as Co3−δO4 spinel by rapid diffusion along the {111} and {100}-decorated interphase interface and the free surface of the composites.

Defect doping and characterization in oxide single crystals using femtosecond laser

ABSTRACTDefects inside oxide ceramics of magnesium oxide (MgO) and sapphire (Al2O3) were formed using femtosecond laser irradiation. The laser irradiance in these ceramics forms various defects classified as dislocations, twins, or nano cracks near the focal point due to the increase of temperature and successive generation of shockwave within several picoseconds. The morphology of defects mainly depends on the crystal structure; dense dislocations inside MgO with a rock-salt type structure, and nanocracks, dislocations, or twins in sapphire with a corundum structure. The TEM analysis revealed that the dense dislocations formed on the two slip planes {110} to form a cross-shaped pattern when the laser beam focused on the (100) plane in MgO. In case of sapphire crystal, cracks propagated from the focal point that has an amorphous structure along R-planes {11-02}, which had the lowest fracture surface energy. The nano crack transformed into the aligned void and dislocation structure during the heat treatment over 1573K, which is one of the crack healing effect.

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.

Full potential calculation of structural, elastic properties and high-pressure phase of binary noble metal carbide: ruthenium carbide

We present in this paper the results of an ab initio theoretical study within the local density approximation (LDA) to determine in rock-salt (B1), cesium chloride (B2), zinc-blende (B3), and tungsten carbide (WC) type structures, the structural, elastic constants, hardness properties and high-pressure phase of the noble metal carbide of ruthenium carbide (RuC). The ground state properties such as the equilibrium lattice constant, elastic constant, the bulk modulus, its pressure derivative, and the hardness in the four phases are determined and compared with available theoretical data. Only for the three phases B1, B3, and WC, is the RuC mechanically stable, while in the B2 phase it is unstable, but in B3 RuC is the most energetically favourable phase with the bulk modulus 263 GPa, and at sufficiently high pressure ( P t =19.2 GPa) the tungsten carbide (WC) structure would be favoured, where ReC–WC is meta-stable. The highest bulk modulus values in the B3, B2, and WC structures and the hardnesses of H(B3)=36.94 GPa, H(B1)=25.21 GPa, and H(WC)=25.30 GPa indicate that the RuC compound is a superhard material in B3, and is not superhard in B1 and WC structures compared with the H(diamond)=96 GPa.

Crystal structure and superconductive characteristics of Nb 0.89Al 0.11 oxynitrides

The crystal structure and superconductive characteristics of the niobium–aluminum oxynitrides were investigated. Single-phase products were successfully obtained starting from a cation ratio of Nb 0.89Al 0.11. The as-nitrided product crystallized in a cation-deficient rock-salt type structure with a chemical formula of (Nb 0.60Al 0.08□ 0 .32)(O 0.21N 0.79), while annealing at 1773 K under a nitrogen pressure of 0.5 MPa led to a highly crystallized product with a simple rock-salt type structure represented as (Nb 0.89Al 0.11)(O 0.17N 0.85). Upon post-annealing, both the critical temperature ( T c) and the superconductive volume fraction ( V SC) of the oxynitride were significantly enhanced from T c≈7 K and V SC=23% for the as-nitrided product to T c=17.3 K and V SC=91% for the post-annealed product.

Structural, electronic, and magnetic properties of nanometer-sized iron-oxide atomic clusters: Comparison between GGA andGGA+Uapproaches

We perform spin-polarized density-functional theory simulations within the generalized gradient approximation (GGA) and $\text{GGA}+\text{U}$ method on nanometer-sized iron-oxide atomic clusters in different stoichiometries. By comparing total energies of structures with different symmetries and selected collinear magnetic configurations we find that low symmetry and, in general, ferrimagnetic structures exhibiting low total magnetic moment are energetically favorable. For the oxygen-rich ${\text{Fe}}_{25}{\text{O}}_{30}$ cluster we obtain a cagelike geometry with a few ions within the cage that seem to stabilize the structure. Considering the ${\text{Fe}}_{33}{\text{O}}_{32}$ cluster of nanometer-size we propose the formation of a rocksalt type structure, which is characteristic of bulk FeO. Based on data of iron $d$ shell electron occupancies, we exclude double exchange from possible magnetic interactions between iron ions, and we point to a competition between direct exchange and superexchange, where the dominant interaction is determined by the cluster topology. For the smaller ${\text{Fe}}_{13}{\text{O}}_{8}$ cluster we find ferromagnetic energetically favorable geometries of lower symmetry than previously reported. Our results demonstrate the importance of going beyond GGA, in particular, physical properties obtained within $\text{GGA}+\text{U}$ description are found to be remarkably different from those using GGA. In order to confirm our theoretical predictions, cluster experiments in this size regime are desirable.

Solvothermal synthesis of group 5 and 6 nitrides via reactions using LiNH2 and ammonia nitrogen sources

Transition metal chlorides are reacted with lithium amide or ammonia under solvothermal conditions in benzene at temperatures up to 550 degrees C. The products are metal nitrides with particle sizes of a few nm. VN, NbN, CrN, MoN and WN form with a cubic rocksalt-type structure, whilst Ta(3)N(5) adopts the known orthorhombic structure. Products often contain carbon due to solvent decomposition, the carbon content is higher when ammonia is the nitrogen source, and varied to some extent from metal to metal. Analytical data shows nitrogen-deficient carbonitride compositions. Most samples crystallise with partially aggregated, regular crystallites. Some crystallise with a nanorod morphology and this was most pronounced in Ta(3)N(5), which forms high aspect ratio, single crystal nanorods when synthesised with ammonia.

Synthesis of a New Modification of Lithium Chloride Confirming Theoretical Predictions

AbstractIn accordance with prior calculations, the new polymorph β‐LiCl (wurtzite structure type) has been synthesised, by the low‐temperature atomic‐beam‐deposition (LT–ABD) technique, in a mixture with α‐LiCl (rock salt structure type) by depositing LiCl vapour (2 to 5.3 × 10–4 mbar) onto a cooled substrate (–30 to –60 °C). The maximum β‐LiCl fraction of 53 % was obtained using a sapphire (0001) substrate at –50 °C and 3.7 × 10–4 mbar LiCl vapour pressure. The proportion of the new polymorph contained in the bulk sample decreases as temperature or vapour pressure deviate from these values, until finally the rock salt type LiCl is found exclusively. When the samples are warmed up to room temperature, β‐LiCl irreversibly transforms to α‐LiCl. The X‐ray diffraction pattern of the two phase LiCl sample measured at –50 °C has been indexed and refined based on a hexagonal unit cell for β‐LiCl with the lattice constants a = 3.852(1) Å and c = 6.118(1) Å and a cubic unit cell for α‐LiCl with the lattice constant a = 5.0630(8) Å. By Rietveld refinement the wurtzite type ofstructure (P63mc, No. 186) was suggested for the new hexagonal modification of LiCl with the Li–Cl distances (2.32 and 2.34 Å) being 8 % smaller than those of α‐LiCl. Moreover, the cell volume decreases as much as 16 % during the transition from β‐LiCl to α‐LiCl. Both the shifts in bond lengths and volume correspond well with the situation encountered for LiBr and LiI. Besides the variation of LiCl vapour pressure and substrate temperature, also different substrate materials were employed for testing their influence on formation of the β‐LiCl polymorph.

Lithium intercalation into transition metal oxides: A route to generate new ordered rock salt type structure

Lithium intercalation in oxides is a topic of capital importance for the development of lithium batteries materials. We review herein the lithium insertion process involved in different structures 3D, 2D and 1D. In most of the cases, starting from an anionic cubic close packed array, the topotactic process of alkaline ion insertion, associated with a biphasic process in the potential-composition electrochemical curve, generates an ordered rock salt type structure for the fully reduced phase. Such a behavior allowed us to imagine other fully lithiated compositions that could show really good electrochemical behavior toward the lithium insertion/deinsertion.

Topotactic Transformation of the Cationic Conductor Li4Mo5O17 into a Rock Salt Type Oxide Li12Mo5O17

Intercalation of lithium in the ribbon structure Li4Mo5O17 has been achieved, using both electrochemistry and soft chemistry. The ab initio structure determination of the “Mo−O” framework of Li12Mo5O17 shows that the [Mo5O17]∞ ribbons keep the same arrangement of edge sharing MoO6 octahedra and the same orientation as in the parent structure but that a topotactic antidistortion of the ribbons appears, as a result of the larger size of Mo4+ in “Li12” compared to Mo6+ in “Li4”. On the basis of bond valence calculations, it is observed that 12 octahedral sites are available for Li+ in the new structure so that an ordered hypothetical rock salt type structure can be proposed for Li12Mo5O17. After the first Li insertion, a stable reversible capacity of 100 mA·h/g is maintained after 20 cycles. A complete structural reversibility leading back to the ribbon type Li4Mo5O17 structure is obtained using a very low rate of C/100. The exploration of the Li mobility in those oxides shows that Li4Mo5O17 is a cationic conductor with σ = 10−3.5 S/cm at 500 °C and Ea = 0.35 eV.