Disorder In Sublattice Research Articles

Growth of ZnSnN2 by Molecular Beam Epitaxy

The Zn-IV-N2 family of materials represents a potential earth abundant element alternative to conventional compound semiconductor materials that are based on gallium and indium. While both ZnSiN2 and ZnGeN2 have been studied to some degree, very little is known about the narrow-gap member ZnSnN2. Here, we investigate the growth dynamics of crystalline ZnSnN2 through plasma-assisted molecular beam epitaxy. All films exhibit some degree of crystalline order regardless of growth conditions, although significant tin coverage was observed for films grown with low Zn:Sn flux ratio; Zn flux in particular became increasingly problematic at increased substrate temperatures designed to improve crystallinity. Single-crystal material was achieved through careful optimization of growth parameters. Regardless of deposition conditions or substrate choice, however, all films exhibit a monoclinic structure as opposed to the predicted orthorhombic lattice; this can be directly attributed to sublattice disorder.

Disorder-driven non-Fermi liquid behavior in single-crystalline Ce2Co0.8Si3.2

A single crystal of the Ce-based ternary silicide Ce2Co0.8Si3.2, which crystallizes with a hexagonal AlB2-type related structure, was studied by means of magnetization, resistivity and heat capacity measurements. The compound was characterized as a Kondo paramagnet down to 0.4 K. Its low-temperature behavior is dominated by distinct non-Fermi liquid features, most likely arising due to structural disorder in the nonmagnetic-atom sublattice.

Influence of the Lanthanide Oxides on the Catalytic Behavior of Au/Al<SUB>2</SUB>O<SUB>3</SUB> Catalysts for Total and Preferential CO Oxidation

In this work the influence of the lanthanide series oxide addition to gold supported alumina catalyst is discussed. A clear promoting effect was observed no matter the employed reaction. Nevertheless, the presence of hydrogen in the oxidation mixture reveals interesting dissimilarities within the series of studied oxides. The differences in the catalytic behaviour of the samples are correlated to the crystal structure variations, oxygen sub-lattice disorder, gold presence and oxide's ability to undergo hydration/dehydration reactions.

Molecular dynamics simulation of dislocations in uranium dioxide

The plasticity of the fluorite structure in UO2 is investigated with molecular dynamics simulation and empirical potential. The stacking fault energies and the dislocation core structures with Burgers vector a2〈110〉 are systematically calculated. All dislocation core structures show a significant increase of the oxygen sub-lattice disorder at temperatures higher than 1500K. The threshold stress for dislocation glide is found to decrease with increasing temperature but its values is always very high, several GPa at 0K and several hundred of MPa at 2000K. A relation between the dislocation mobility dependence with temperature and the increase of the oxygen sub-lattice disorder in the dislocation cores is established.

Growth, disorder, and physical properties of ZnSnN2

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

Thermal Variation of Structure and Electrical Conductivity in Bi4YbO7.5

The thermal behavior of the oxide ion-conducting solid electrolyte Bi4YbO7.5 was investigated using a combination of variable temperature X-ray and neutron powder diffraction, thermal analysis (DTA and TGA), and ac impedance spectroscopy. The title compound shows a fluorite-type structure throughout the measured temperature range (20-850 degrees C), with a phase separation at ca. 600 degrees C into a cubic delta-type phase and an orthorhombic phase of assumed stoichiometry Bi17Yb7O36. This type of transition is a relatively common feature in bismuth oxide-based systems and can limit their practical application. Here, the transition was carefully studied using isothermal measurements, which showed that it is accompanied by changes in oxide-ion stoichiometry, as well as significant disorder in the oxide ion sublattice in the delta-type phase. These results correlate with the observed electrical behavior. Analysis of the total neutron scattering through reverse Monte Carlo (RMC) modeling reveals details of the coordination environments for both cations. The oxide-ion vacancy distribution seems to be consistent with a favoring of vacancy pairs, although vacancy pairs exhibit the highest frequency as they have the maximum likelihood. A vacancy ordering model based on three vacancies per cell is presented.

Ионная проводимость и диффузия в суперионных проводниках СuСrS2 - АgСrS2

The dependence of the ionic conductivity of СuСrS2, АgСrS2 compounds and their alloys on temperature is measured. Structural studied carried out earlier showed that the CuxAg1-xCrS2 alloys exist as a mixture of СuСrS2 and АgСrS2 phases at temperatures below 673 K and as a solid solution on the basis of a matrix structure at temperatures above 673 K. It is established that for all systems the activation energy for ionic conductivity has a jump at the phase transition point, which corresponds to the temperature of complete disordering of monovalent ionic sublattices. Chemical diffusion coefficients and ionic transport numbers for all tested compounds were determined from the build-up and decay curves of concentration polarization.

Synthesis, structure and photoluminescence properties of europium-, terbium-, and thulium-doped Ca3Bi(PO4)3 phosphors

A series of Eu(3+), Tb(3+), Tm(3+) singly and triply doped Ca3Bi(PO4)3 (CBP) phosphors were synthesized by solid-state reaction. Their structure and photoluminescence (PL) properties were first investigated in detail. Rietveld refinement analysis confirmed that the CBP has a cubic unit cell. Its space group was determined to be I4[combining macron]3d with cell parameters a = b = c = 9.941 Å and Z = 4. In addition, the Ca3Bi(PO4)3 shows both cation disorder and oxygen sublattice disorder. For the CBP:Eu(3+) phosphor, the charge transfer bands in the PLE spectra were different at 293 K and 4.3 K, and a model was presented to give a possible explanation for this phenomenon. CBP:Eu(3+) shows intense red emission due to (5)D0-(7)F2 transition and its integral emission intensity and quantum efficiency are higher than the commercial Y2O3:Eu(3+) phosphor under irradiation of 397 nm, indicating that the CBP:Eu(3+) phosphor might have potential application in the NUV range for solid-state lighting. The CBP:Tb(3+) and CBP:Tm(3+) phosphors show intense green and blue emission due to (5)D4-(7)F2 transition of the Tb(3+) ions and the (1)D2-(3)F4 transition of the Tm(3+) ions, respectively. The energy transfer from the Tb(3+) to Eu(3+) ions was also validated by the spectra and decay curves of the Tb(3+) ions. Tunable emission colors were obtained by triply doping Eu(3+), Tb(3+) and Tm(3+) activators in a single host and adjusting their relative ratio.

Temperature-dependent Raman spectroscopy studies of phase transformations in the K2WO4 and the MgMoO4 crystals

Temperature-dependent Raman spectroscopy studies of K2WO4 and MgMoO4 polycrystals were performed in order to obtain information about vibrational and structural changes in these materials as a function of temperature. The stability of the monoclinic phase for both K2WO4 and MgMoO4 samples was assessed and our results indicated that this phase is stable in the 295–723K and 300–770K ranges for K2WO4 and MgMoO4, respectively. It was observed that both samples underwent two phase transformations above room temperature. The first phase transformations which occur at about 633K and 640K for K2WO4 and MgMoO4, respectively, is most likely connected with weak tilting and/or rotations of WO4/MoO4 tetrahedral units that lead to a disorder in the oxygen sublattice. Raman spectroscopy data also indicated that K2WO4 and MgMoO4 exhibited a first-order phase transition at around 723K and 770K, respectively, changing from monoclinic to hexagonal symmetry.

Local Disorder in Lithium Imide from Density Functional Simulation and NMR Spectroscopy

Born–Oppenheimer molecular dynamics simulations in combination with calculations of 1H, 7Li, and 15N NMR chemical shifts are used to characterize lithium imide structures at different temperatures. Indications of the onset of local disorder in the lithium sublattice, leading eventually to superionicity, are recognized already at low temperature (100 K). Between 100 and 400 K, a new structure could be stabilized, which presents features that are intermediate between the previously reported Fm3m̅ and the Fd3m̅ structures. The disorder in the Li positions is associated with the reorientation of the NH bonds, which preferentially point toward Li-vacant sites. Clear signatures of such structural rearrangements are visible in the simulated NMR spectra, where smoother profiles are associated with a reduced amount of Li interstitials and a higher occupation probability of the antifluorite sites.

Synthesize 2-methylpyrazine using aqueous glycerol and ethylenediamine over zinc oxide–zinc chromite catalysts: Structure–activity relationship

ZnO–ZnCr2O4 mixed oxides (Zn–Cr–O) were obtained by decomposition of Zn–Cr hydrotalcite precursors synthesized by co-precipitation method at pH∼7 and 9, with a constant Zn/Cr ratio of 2. Based on the product distribution, mechanisms were proposed. The differences in dehydrocyclization activity of Zn–Cr–O mixed oxides were established based on adsorption and spectroscopic data that emphasized changes in the structural properties of Zn–Cr–O obtained at different pHs. X-ray photoelectron (XPS) lines indicated Cr6+ and these species were reduced to Cr3+ during H2-temperature programmed reduction (H2-TPR) analysis. The higher NH3 and O2 uptakes of Zn–Cr–O (pH∼7) than Zn–Cr–O (pH∼9) are attributed to the presence of Cr6+ ions on the catalyst surface. Disorder in the cubic ZnCr2O4 sub-lattice may result in multiple Raman bands appeared in the Zn–Cr–O samples. The characteristic bands of Lewis and Bronsted acid sites at 1450 and 1540cm−1 assigned by FT-IR spectra of pyridine adsorbed on Zn–Cr–O samples, and their relative peak intensities corresponding to Lewis and Bronsted suggested higher ratios for Zn–Cr–O (pH∼7) than for Zn–Cr–O (pH∼9). It is demonstrated that the dehydrocyclization reaction proceeds on ZnO and/or ZnCr2O4 resulting in 2MP synthesis, and catalysts that possess mainly acid sites show intermolecular cyclization of EDA leading to high selectivity of pyrazine.

Melting and superionic transition of Gd-doped ceria nanocrystals: Molecular dynamics study

Molecular dynamics program for simulation of nanocrystals in vacuum, with ionic model approximation, was developed. The use of graphics processor allows us to speed up calculation by two orders of magnitude in comparison with CPU realization. For Ce1−XGdXO2−X/2 system by fitting to experimental data pair interaction potentials were obtained, comparison with existing potential set was carried out. Melting temperature of CeO2 nanocrystal depends on its octahedron diagonal size as Тm(L,nm)=Тm(∞)−(5534±153)⋅L−2.Structural disorder of Ce1−XGdXO2−X/2 nanocrystals (X=0–0.3) in superionic transition and melting regimes was investigated. Dependences of melting temperature and specific volume change at melting from gadolinium dopant quantity have downward trend. It is shown, that oxygen sublattice disorder (“melting”) in crystallites takes place at about 0.7Tm. The contribution of intrinsic disorder to anion diffusion coefficient in systems with gadolinium dopant was extracted and the presence of superionic transition is shown. Comparison with high-temperature ionic conductivity data was carried out.

Vibrational properties of stannite and kesterite type compounds: Raman scattering analysis of Cu2(Fe,Zn)SnS4

This work reports the analysis of the vibrational properties of stannite–kesterite Cu2(Fe,Zn)SnS4 compounds that has been performed by Raman scattering measurements. The detailed analysis of the experimental spectra has allowed determining the frequency and symmetry assignment of the main and weaker peaks from both stannite Cu2FeSnS4 (CFTS) and kesterite Cu2ZnSnS4 (CZTS) phases. The measurements performed in the kesterite CZTS samples have also revealed the presence of local inhomogeneities that are characterised by an additional peak in the spectra at about 331cm−1. This peak has been related to the presence in these local regions of a high degree of disorder in the cation sublattice, in agreement with previous neutron diffraction analysis in similar samples. Finally, the spectra from the solid solution alloys show a one-mode behaviour of the main A/A1 peak with the chemical composition.

Local Oxygen-Vacancy Ordering and Twinned Octahedral Tilting Pattern in the Bi0.81Pb0.19FeO2.905 Cubic Perovskite

The structure of Bi0.81Pb0.19FeO2.905 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mo spectros- copy. In the 80−300 K temperature range, the average crystal structure of Bi0.81Pb0.19- FeO2.905 is a cubic Pm3 perovskite with a = 3.95368(3) A at T = 300 K. The (Pb 2+ , Bi 3+ ) cations and O 2− anions are randomly displaced along the ⟨110⟩ cubic directions, indicating the steric activity of the lone pair on the Pb 2+ and Bi 3+ cations and a tilting distortion of the perovskite framework. The charge imbalance induced by the hetero- valent Bi 3+ → Pb 2+ substitution is compensated by the formation of oxygen vacancies preserving the trivalent state of the Fe cations. On a short scale, oxygen vacancies are located in anion-deficient (FeO1.25) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO4 tetrahedra and FeO5 tetragonal pyramids. The anion-deficient layers act as twin planes for the octahedral tilting pattern of adjacent perovskite blocks. They effectively randomize the octahedral tilting and prevent the cooperative distortion of the perovskite framework. The disorder in the anion sublattice impedes cooperative interactions of the local dipoles induced by the off-center displacements of the Pb and Bi cations. Magnetic susceptibility measurements evidence the antiferromagnetic ordering in Bi0.81Pb0.19FeO2.905 at low temperatures.

Fe-spin reorientation in PrFeAsO: Evidences from resistivity and specific heat studies

We report the magnetic field dependence of resistivity (ρ) and specific heat (C) for the non-superconducting PrFeAsO compound. Our study shows a hitherto unobserved anomaly at TSR in the resistivity and specific heat data, which arises as a result of the interplay of antiferromagnetic (AFM) Pr and Fe sublattices. Below the AFM transition temperature (TNPr), Pr moment orders along the crystallographic c axis and its effect on the iron subsystem causes a reorientation of the ordered in-plane Fe moments in a direction out of the ab plane. Application of magnetic field introduces disorder in the AFM Pr sublattice, which, in turn, reduces the out-of-plane Pr-Fe exchange interaction responsible for Fe spin reorientation. Both in ρ(T) and d(C/T)/dT curves, the peak at TSR broadens with the increase of H due to the introduction of the disorder in the AFM Pr sublattice by magnetic field. In the ρ(T) curve, the peak shifts toward lower temperature with H and disappears above 6 T, while in the d(C/T)/dT curve, the peak remains visible up to 14 T. The broadening of the anomaly at TNPr in C(T) with increasing H further confirms that magnetic field induces disorder in the AFM Pr sublattice.

Atomic structure of surface defects in alumina studied by dynamic force microscopy: strain-relief-, translation- and reflection-related boundaries, including their junctions

We present an extensive atomic resolution frequency modulation dynamic force microscopy study of ultrathin aluminium oxide on a single crystalline NiAl(110) surface. One-dimensional surface defects produced by domain boundaries have been resolved. Images are presented for reflection domain boundaries (RDBs), four different types of antiphase domain boundaries, a nucleation-related translation domain boundary and also domain boundary junctions. New structures and aspects of the boundaries and their network are revealed and merged into a comprehensive picture of the defect arrangements. The alumina film also covers the substrate completely at the boundaries and their junctions and follows the structural building principles found in its unit cell. This encompasses square and rectangular groups of surface oxygen sites. The observed structural elements can be related to the electronic signature of the boundaries and therefore to the electronic defects associated with the boundaries. A coincidence site lattice predicted for the RDBs is in good agreement with experimental data. With Σ = 19 it can be considered to be of low-sigma type, which frequently coincides with special boundary properties. Images of asymmetric RDBs show points of good contact alternating with regions of nearly amorphous disorder in the oxygen sublattice.

The fluorite–pyrochlore transformation of Ho 2− yNd yZr 2O 7

Twelve members of the Ho 2− y Nd y Zr 2O 7 series, prepared using conventional solid state methods, have been characterised by neutron powder diffraction. Ho 2Zr 2O 7 has a defect fluorite structure whereas Nd 2Zr 2O 7 is found to adopt the ordered pyrochlore structure with the composition induced fluorite–pyrochlore transformation occurring near y=1. Rietveld analysis on the neutron data for all the compositions reveals an increase in lattice parameter as a function of y across the entire series, with a small discontinuity associated with the transformation. The neutron profile results suggest that domains of pyrochlore-type initially begin to form before crystallising into a separate phase, and therefore that anion and cation ordering processes are distinct. There is a strong correlation between the extent of disorder in the anion sublattice and the x-parameter of 48 f oxygen. These results point the way to a better understanding of the stability observed in pyrochlore structures.

Strontium migration assisted by oxygen vacancies in SrTiO3from classical and quantum mechanical simulations

We present the pathways for strontium ion migration in SrTiO3, which are based on an exploration of the potential energy landscape through a combination of classical and quantum mechanical techniques. Sr ion migration is enhanced by interaction with an anion vacancy: In the bulk material, Sr cations migrate linearly between adjacent lattice sites, through the center of a square formed by four oxygen ions; however, the activation barrier is substantially reduced, and the path curved, in the presence of an oxygen vacancy. The contribution of partial Schottky disorder in the SrO sublattice to ion migration explains the wide spread of experimental results to date, with direct implications for diffusion processes at highly doped surfaces and interfaces of SrTiO3 as well as other perovskite materials.

Damage recovery in the oxygen sublattice of ZnO by post-implantation annealing

Hydrothermally grown zinc oxide bulk samples were implanted with 200 keV-Co ions with a fluence of 4.5 × 10 16 cm −2 and then annealed in air during 30 min at different temperatures up to 900 °C. After the implantation and each annealing step, the samples were analyzed using the nuclear reaction 16O(α,α) 16O at 3.045 MeV He in random and channeling directions to follow the annealing of the disorder profile in the O sublattice. For comparison, the disorder in the Zn sublattice was also observed by Rutherford backscattering spectrometry (RBS) in random and channeling directions. The results reveal that the disorder created during the Co implantation is slightly higher in the O sublattice than in the Zn sublattice. The disorder recovery induced by the thermal treatments, starts at 500 °C in the O sublattice and at 700 °C in the Zn sublattice. Although, the most part of the disorder recovery occurs between 700 and 800 °C in both sublattices.

Temperature‐dependent Raman scattering studies on Na2Mo2O7 disodium dimolybdate

AbstractTemperature–dependent Raman studies of disodium dimolybdate (Na2Mo2O7) crystal are reported. Lattice dynamical calculation was used to predict both wavenumbers and atomic displacements (eigenvectors) for the vibrational modes. These calculations were based on the classical rigid‐ion model. The high‐temperature Raman scattering study of the crystal showed that it remains in the orthorhombic structure in the 8–848 K range and undergoes a structural phase transition between 848 and 854 K. This phase transition is most likely connected with weak tiltings and/or rotations of both MoO4 (tetrahedra) and MoO6 (octahedra) units, which lead to a disorder in the oxygen sublattice. Copyright © 2010 John Wiley & Sons, Ltd.