Cation-anion Bond Length Research Articles

On the atomic ordering in the B-sublattice of high-entropy perovskites

Atomic ordering in perovskites has been the subject of intensive research for decades. With the growing interest in high-entropy perovskites (HEP), the issue of atomic ordering is now reaching a new level of significance. Many multi-cationic perovskites have already been synthesized, and some of them exhibit atomic ordering. Therefore, the problems of estimating the phase transition temperature of atomic ordering, determining the type of emerging atomic order, and the distribution of cations in ordered sublattices need to be addressed. In this work, based on the unstrained elastic cation-anion bonds model, we develop a Monte Carlo calculation scheme of atomic order-disorder phase transitions in the B-sublattice of perovskites with arbitrary composition and apply it to the case of HEP. We find that HEP’s may experience rock-salt type atomic ordering, while preserving considerable configurational entropy and essentially becoming ordered disordered perovskites. For many HEP’s, whose synthesis has been already reported in literature or which await their synthesis, we estimate the phase transition temperature Tod, reduced cubic lattice parameters, and the composition of the ordered sublattices. Finally, we derive a simple relation for the estimation of Tod from normalized standard deviation of the unstrained cation-anion bond lengths of atoms in the B-sublattice, which can be easily calculated.

Effects of Fe occupancy on physical properties of non-magnetic GeGa2O4 spinel oxide

This study investigated the structural, elastic, electronic, thermal, magnetic, and optical behavior of GeGa2O4, FeGa2O4, GeFe2O4, and FeFe2O4 spinel oxides based on ab-initio calculations. The results showed that the non-magnetic spinel oxide GeGa2O4 exhibited remarkable physical properties due to Fe occupancy at different lattice sites within the spinel lattice. The Fe containing spinel oxides (FeGa2O4, GeFe2O4, and FeFe2O4) were found to be stable in the magnetic phase in contrast to the host GeGa2O4 spinel oxide. The spin polarizations of FeGa2O4, GeFe2O4, and FeFe2O4 oxides were calculated as 98.5%, 98.4%, and 100%, respectively. Analysis of the elastic and mechanical properties showed that all of the spinel oxides followed the Born and Huang criteria for mechanical stability. The highest Debye temperature (θD = 663.77 K) was determined for the FeGa2O4 spinel oxide. For GeGa2O4, FeGa2O4, GeFe2O4, and FeFe2O4 oxides, the calculated structural parameters (cation–anion bond lengths and average values of tetrahedral and octahedral radii) predicted the following cation distributions among the A and B atomic sites: (Ge2+)[Ga23+]O4−2, (Fe2+)[Ga23+]O4−2, (Ge4+)[Fe22+]O4−2, and (Fe+2)[Fe23+]O4−2. The calculated electronic structures indicated that the host GeGa2O4 spinel oxide and FeFe2O4 spinel ferrite were semi-conducting and half-metallic ferromagnetic, respectively, whereas FeGa2O4 and GeFe2O4 oxides exhibited semi-metallic behavior. The GeFe2O4 spinel oxide was found to have the highest saturation magnetization (185.79 emu g−1) and total magnetic moment (8.26 ΣBfu−1). The calculated optical properties indicated the suitability of these oxides for use in optical filters and sensors. The electronic and magnetic characteristics of these spinel oxides with excellent thermal and elastic stability suggest that they have potential uses in diluted magnetic semiconductor devices.

Comparison of the different distribution functions in Gd-doped ceria system by molecular dynamics simulations

Cerium dioxide-based materials are among the most studied for applications in the energy and environmental fields and are also of interest in biology and medicine. The fluorite structure of CeO2 is locally distorted by the concomitant presence of doping cations, such as Gd3+ and oxygen vacancies. The cation-anion bond length distribution then becomes increasingly asymmetric with the doping ratio and temperature. In these cases, the MD simulation results indicate that the commonly used maximum of the pair correlation function g(r) first peak can no longer be adopted to estimate the mean bond length. To determine the true cation-anion bond length, the analysis of the radial distribution function R(r) first peak is necessary. Furthermore, the asymmetry of this peak must be accounted for when extracting the mean value of the distribution. The gap between the g(r) maximum and the R(r) mean position derived from the fit using a skewed Gaussian function clearly increases with the doping ratio and temperature, leading to different conclusions concerning bond length evolution. The present study also suggests that care must be taken when the bond length is deduced from the pair distribution functions G pdf(r) as is the case in total scattering experiments (x-ray and neutrons). Finally, relations between the cumulants of the effective distribution of distances as determined in extended x-ray absorption fine structure experiments and the cumulants describing the real distance distribution are proposed considering that both these distributions are modelled by a skewed Gaussian function.

Structural and mechanical properties of zinc aluminoborate glasses with different content of aluminum oxide

The glass samples were prepared according to the following formula: (35 − x) ZnO, 55B2O3, 10Li2O, x Al2O3, where (x = 0.5; 1; 1.5; 2; 2.5; 3; 3.5; 4 mol%) by melt quenching method. Structural parameters such as density, molar volume, cation–anion bond length, and packing density were studied and were correlated to the structural changes. It is found that the density decreases and the molar volume increases with increasing Al2O3 content. FT-IR spectra were measured to investigate the glass structure of these samples and to confirm the formation of non bridging Al–O–B bond with the addition of Al2O3 content. The FT-IR spectra were deconvoluted using curves of Gaussian shape at approximately the same frequencies. The fraction of fourfold coordinated boron atom (N4) was calculated from the deconvoluted data. The longitudinal and transverse ultrasonic wave velocities were measured using the pulse-echo technique. The longitudinal L, shear S, Young’s E and bulk B module were calculated from the measured velocity and density values of the prepared samples and were correlated with the structural changes and all reflects that addition of Al2O3 to present glasses results in de-polymerization of the glass network and formation of non-bridging oxygen (NBOs).

A DFT+UStudy of Strain-Dependent Ionic Migration in Sm-Doped Ceria

Recent experiments on thin films and hetero-structures of ionically conducting oxides have indicated enhancement of oxygen ion conduction typically ascribed to strain-related effects. We performed density functional theory (DFT) calculations with Hubbard U correction to investigate the effects of biaxial lattice strain on the oxygen anion diffusivity in Sm-doped ceria (SDC). Here we found that the migration barriers are strongly affected by the applied strain. The changes in the migration barriers can almost be linearly correlated with cation–anion bond-lengths between the migrating oxygen and its nearest neighbor cations. In this work, we reported various possible dopant configurations for oxygen ion migrations at the vicinity of the oxygen vacancy. Our results indicate that the ionic migration barriers can be lowered by both compressive and tensile strains. The insight gained from this study may enable us to engineer and to improve oxygen ion migration in ceria-based solid electrolyte materials by compression or tension of the lattice parameters.

Neutron diffraction study of Li4Ti5O12 at low temperatures

The crystal structure of the “zero-strain” positive electrode material Li4Ti5O12 was characterized by neutron powder diffraction in the temperature range 3.4 K–300 K. No phase transition was detected, and the thermal evolution of lattice parameters has been evaluated by the 2nd order Grüneisen approximation using the Debye formalism for internal energy and intrinsic anharmonicity contributions. A relatively high Debye temperature θD = 689 ± 71 K was determined. The thermal behavior of cation-anion bond lengths in octahedral and tetrahedral environments is discussed. The lithium diffusion pathway in Li4Ti5O12 was discussed on the basis of bond-valence modeling.

Mn incorporation in CuInS2 chalcopyrites: Structure, magnetism and optical properties

The Mn-substituted CuInS2 compounds (CuIn1–xMnxS2 with x = 0–0.20 and Cu1–yMnyInS2 with y = 0.05–0.10) were synthesized using high-temperature solid–state reactions. Single-phase materials with the chalcopyrite structure persist with up to 10% of the Cu/In sites being replaced with Mn. The introduction of manganese results in a linear expansion of the lattice parameters as a function of Mn concentration, following Vegard's law. Rietveld refinements on a combination of X-ray and neutron powder diffraction data reveal a site preference of Mn for the In site under In-poor conditions and the existence of cation anti-site occupation, CuIn and InCu. The Mn substitution increases the anion displacement, accentuating the cation-anion bond length mismatch. The greater variance in the bond alternation and the addition of Mn d–S p hybridization near the Fermi level result in a decrease in the bandgap by ∼0.1 eV. The CuInS2:Mn compounds display paramagnetic behavior with short-range antiferromagnetic interactions. X-ray photoelectron spectroscopy suggests the presence of Cu+, Mn2+, and In3+ in the samples.

Dependence of Optical Band Gap on Residual Stress in Group IIB Iodide (ZnI2, CdI2, HgI2) Films

Group IIB iodides are layered structured semiconductors and are chosen particularly for studying the residual stress dependence of band gap because of their anisotropy along and across the layers. The development of residual stress with film thickness, substrate temperature, annealing temperature etc. was studied using X-ray diffraction in these films grown by vacuum evaporation. The three compounds show completely different growth and residual stress behaviors. The room temperature optical band gaps of these films were determined by optical absorption measurements. Only ZnI2 exhibits excitonic absorption at room temperature. The optical band gaps of ZnI2 and HgI2 correlate linearly with residual stress while CdI2 shows nonlinear behavior. The magnitude of band gap variation with residual stress can be interpreted as due to the affected cation-cation, cation-anion and anion-anion bond lengths. The ionicity of these three compounds decreases linearly with their cation-anion radii ratio and the band gap increases with ionicity.

Comparative study of structural and morphological properties of CuIn 3S 5 and CuIn 7S 11 materials

CuIn 3S 5 and CuIn 7S 11 powders were prepared by solid-state reaction method using high-purity elemental copper, indium and sulphur. The films prepared from CuIn 3S 5 and CuIn 7S 11 powders were grown by thermal evaporation under vacuum (10 −6 Torr) on glass substrates at different substrate temperature Ts varying from room temperature to 200 °C. The powders and thin films were characterized for their structural properties by using X-ray diffraction (XRD) and energy dispersive X-ray (EDX). Both powders were polycrystalline with chalcopyrite and spinel structure, respectively. From the XRD data, we calculated the lattice parameters of the structure for the compounds. For CuIn 3S 5 powder, we also calculated the cation–anion bond lengths. The effect of substrate temperature Ts on the structural properties of the films, such as crystal phase, preferred orientation and crystallinity was investigated. Indeed, X-ray diffraction analysis revealed that the films deposited at a room temperature (30 °C) are amorphous in nature while those deposited on heated were polycrystalline with a preferred orientation along (1 1 2) of the chalcopyrite phase and (3 1 1) of the spinel phase for CuIn 3S 5 and CuIn 7S 11 films prepared from powders, respectively. The morphology of the films was determined by atomic force microscopy AFM. The surface roughness and the grain size of the films increase on increasing the substrate temperature.

Structural, electrical and optical properties of transparent Zn1 − xMgxO nanocomposite thin films

Zn1−xMgxO thin films of various Mg compositions were deposited on quartz substrates using inexpensive ultrasonic spray pyrolysis technique. The influence of varying Mg composition and substrate temperature on structural, electrical and optical properties of Zn1−xMgxO films were systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70mol%. AFM images of the Zn1−xMgxO films (x=0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zn1−xMgxO. The variation of the cation–anion bond length to Mg content shows that the lattice constant of the hexagonal Zn1−xMgxO decreases with corresponding increase in Mg content, which result in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side.

Synthesis, elaboration and characterization of the new material CuIn3S5 thin films

CuIn3S5 compound was prepared by direct reaction of high-purity elemental copper, indium and sulphur. CuIn3S5 thin films were prepared from powder by thermal evaporation under vacuum (10−6 mbar) onto glass substrates. The glass substrates were heated from 30 to 200 °C. The powder was characterized for their structural and compositional properties by using X-ray diffraction (XRD) and energy dispersive X-ray (EDAX). The XRD studies revealed that the powder exhibiting P-chalcopyrite structure. From the XRD data, we calculated the lattice parameters a and c. Then, the cation–anion bond lengths lAC and lBC are deduced. The films were characterized for their structural, compositional, morphological and optical properties by using XRD, EDAX, atomic force microscopy and optical measurement techniques (transmittance and reflectance). XRD analysis revealed that the films deposited at a room temperature (30 °C) are amorphous in nature, whereas those deposited on heated substrates (≥75 °C) were polycrystalline with a preferred orientation along (112) of the chalcopyrite phase. The surface morphological analysis revealed that the films grown at different substrate temperature had an average roughness between 1.1 and 4.8 nm. From the analysis of the transmission and reflection data, the values of direct and indirect band gap of the films were determined. We found that the optical band gap decreases when the substrate temperature increases.

Investigations of the bond-selective response in a piezoelectric Li2SO4·H2O crystal to an applied external electric field

Piezoelectric lithium sulfate monohydrate, Li2SO4.H2O, was analyzed with respect to the relationship between the static structural properties of the crystal and its response to an external electric field. The static electron density was determined via standard low-temperature X-ray data collection at 90 (5) K using an Enraf-Nonius CAD-4 diffractometer, Mo Kalpha radiation and multipole model refinement. Then a synchrotron-radiation experiment using the D3 beamline at HASYLAB was conducted in order to investigate the structural deformations in Li2SO4.H2O caused by an applied external electric field. In particular, the shifts of Bragg-peak positions induced by the electric field were measured and the piezoelectric constants d211, d222, d233 and d213 of Li2SO4.H2O were obtained from the shifts. With the same experimental setup the variations of more than 100 Bragg intensities were measured under an applied electric field. The data were used to refine the corresponding displacements of individual atoms within the unit cell. The distortions of the cation-anion bond lengths in the LiO4, LiO3(H2O) and SO4 tetrahedra were evaluated and then analyzed in terms of the electron-density-related properties of the Li-O and S-O bonds. The two lithium structural units were found to be strongly deformed by the applied electric field, while the SO(4) tetrahedron changed less. This is in agreement with the low bond strength of the Li-O bonds.

Effect of Mg content on structural, electrical and optical properties of Zn 1−xMg xO nanocomposite thin films

We report on the growth of Zn 1− x Mg x O (ZMO) thin films on quartz substrate using pulsed laser deposition (PLD) technique. The influence of varying Mg composition on structural, electrical and optical properties of ZMO films has been systematically investigated. Increase in Mg content (in the range 0.0⩽ x⩽1.0), reflects the structural phase transition from wurtzite via mixed phase region to cubic one. X-ray diffraction (XRD) studies indicate the hexagonal wurtzite phase at Mg composition ranging from 0% to 30%; mixture of wurtzite and cubic phases for 40% and single cubic phase at Mg content greater than 50%. The variation of the cation–anion bond length to Mg content shows that the lattice constant of the hexagonal ZMO decreases with corresponding increase in Mg content, which result in the structure gradually deviating from the wurtzite structure. The optical measurements reveal a blue shift in absorption edge and increase in transmittance from 75% to 96% with increase in Mg content. Tuning of the band gap has been obtained from 3.41 to 6.58 eV with corresponding increase in Mg content from x=0.0 to 1.0, which demonstrates that the films are useful for window layer of solar cells that improve the overall efficiency by decreasing the absorption loss.

High pressure phase transitions and compressibilities of Er2Zr2O7 and Ho2Zr2O7

Phase stability and compressibility of rare earth zirconates with the defect-fluorite structure were investigated by in situ synchrotron x-ray diffraction. A sluggish defect-fluorite to a cotunnitelike phase transformation occurred at pressures of ∼22 and ∼30GPa for Er2Zr2O7 and Ho2Zr2O7, respectively. Enhanced compressibility was found for the high pressure phase as a result of increasing cation coordination number and cation-anion bond length.

Bimodal Bond-Length Distributions in Cobalt-Doped CdSe, ZnSe, and Cd1-xZnxSe Quantum Dots

Electronic absorption spectroscopy has been used to study changes in Co2+ ligand-field parameters as a function of alloy composition in Co2+-doped Cd(1-x)Zn(x)Se nanocrystals. A shift in the energy of the 4T1(P) excited-state with alloy composition is observed. Analysis reveals that Co2+-Se2- bond lengths change relatively little as the host is varied continuously from CdSe to ZnSe, generating a large difference between microscopic and average cation-anion bond lengths in Co2+-doped CdSe nanocrystals but not in Co2+-doped ZnSe nanocrystals. The bimodal bond-length distributions observed here are shown to cause a diameter-dependent enthalpic destabilization of doped semiconductor nanocrystals.

Ion state of atoms and the properties of perovskite-like compound CaCu3V4O12

The binding energies and valence state of atoms in the perovskite-like compound CaCu3V4O12 have been determined using XPS spectroscopy. The stoichiometry of this phase is formulated as Ca2+Cu2+Cu 2 + (V 2 5+ V 2 4+ O12). Under an air atmosphere, the phase interacts with water vapor and oxygen. As a result, Ca(OH)2 is formed on its surface, the Cu+ and V4+ ion concentrations decrease, and the Cu2+ and V5+ concentrations increase in association. Raman spectra show shortened cation-anion bond lengths and cation-anion-cation bond angles in CaCu3V4O12 compared to perovskite CuVO3; the two structures are alike. The electrical conductivity, magnetic susceptibility, thermal and sensor properties of CaCu3V4O12 in aqueous salt solutions have been studied.

Systematic and design of noncentrosymmetric sulfides and selenides for nonlinear optics

The sets of 412 noncentrosymmetric (NCS) binary and >130 ternary sulfides and 160 binary selenides have been compilled and classified. The elliptical regions on the plane of the shortest chemical bond lengths between cations and sulphur have been defined. The NCS crystals are located only into these regions. Relationship between nonlinear optical susceptibility χ (2) and chemical bonding in sulfides has been considered. The intervals of the shortest cation–anion bond lengths promising for high rank χ (2) have been estimated for the compounds.

Structural variation of cubic and hexagonal MgxZn1−xO layers grown on MgO(111)∕c-sapphire

We report on the structure study of MgxZn1−xO films and, in particular, we will focus on MgxZn1−xO layers with x=0.28 and 0.41 MgxZn1−xO layers with different crystal structures of cubic and wurtzite that have been grown by plasma-assisted molecular-beam epitaxy on MgO∕c-sapphire with Mg∕Zn flux ratio control. The MgxZn1−xO films have been characterized by high-resolution transmission electron microscopy (HRTEM) and high-resolution x-ray diffraction. The dependence of the cation-anion bond length to Mg content has been studied. A virtual crystal model of MgZnO has been applied to interpret the bond-length variation. HRTEM results indicate that the initial stage of the MgZnO growth on a MgO buffer layer starts with a cubic structure even in the case of a wurtzite structure at the end of growth.

Nonideal anion displacement, band gap variation, and valence band splitting in Cu–In–Se compounds

Polycrystalline thin films of ternary chalcopyrite CuInSe 2 and defect compounds CuIn 3Se 5 and CuIn 5Se 8 are prepared in vacuum by three-source coevaporation method. Structural and optical characterizations of the films are done using X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and optical absorbance spectra measurements. With variation in the composition of CuInSe 2, a change over from p-type to n-type conductivity is observed (as noted by the hot probe method). The deformation parameters and the anion displacements are calculated from the X-ray diffraction data, and the cation–anion bond lengths are deduced. The dependence of band gap variation on nonideal anion displacement in the ternary compounds and the effect of Se- p–Cu- d repulsion on band gap are studied. The threefold optical structure observed in the fundamental absorption region of the absorption spectra is analysed to extract the valence band splitting parameters. Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation is used to determine the crystal field splittings and spin orbit splittings, and the linear hybridization model is used to calculate the percentage of d-orbital and p-orbital contribution to hybridization in the compounds under consideration.

Bond Valence Analysis of Tetragonal Zirconias

In tetragonal zirconia, the cation is coordinated by two interpenetrating tetrahedra of oxygen ions, implying two different cation–oxygen bond lengths. On substituting the different tetravalent ions Ge, Ti, Sn, and Ce into tetragonal ZrO2–2 mol% Y2O3, the mean value of the shorter cation–anion bond length varies linearly with the concentration of the substituent ion where the bond length increases or decreases depending on whether the substituted ion is larger or smaller than the zirconium ion it replaces. It is argued in this paper that the length of the longer bond is determined by the requirement that the bond valence sum remains constant. In each case the length of the longer bond calculated on this basis is in good agreement with the measured bond length (from neutron diffraction), and following small adjustments of the bond valence constants, excellent agreement is obtained. The requirement for the bond valence sum evidently accounts for the physics of the situation, and at the same time the available bond length data allow very precise determination of the bond valence constants of the different ions in the tetragonal zirconia environment. It is shown how these bond length considerations provide an explanation for the variation with composition of oxygen position and lattice parameters in all of the materials considered. Among the interesting features accounted for by this analysis are the increase in cell volume occurring when Zr is replaced by the smaller Sn ion, and slight departures from Vegard's law observed in the substitution of Zr by Ti.