Showing papers on "Ionic radius published in 2020"

Machine learning modeling of lattice constants for half-Heusler alloys

Abstract: The Gaussian process regression model is developed as a machine learning tool to find statistical correlations among lattice constants, a0, of half-Heusler compounds, ionic radii, and Pauling electronegativity of their alloying elements. Nearly 140 half-Heusler samples, containing alloying elements of Cr, Mn, Fe, Co, Ni, Rh, Ti, V, Al, Ga, In, Si, Ge, Sn, P, As, and Sb, are explored for this purpose. The modeling approach demonstrates a high degree of accuracy and stability, contributing to efficient and low-cost estimations of lattice constants of half-Heusler compounds.

Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites

Abstract: Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure-composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX3 (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic, and monoclinic perovskite phases to obtain phonon-stable structures for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide computer simulations and experimental synthesis in material screening.

XRD structural studies on cobalt doped zinc oxide nanoparticles synthesized by coprecipitation method: Williamson-Hall and size-strain plot approaches

Abstract: Cobalt doped Zinc Oxide (Zn1-xCoxO) (x = 0.03) nanoparticles have been synthesized by chemical co-precipitation method at room temperature and characterized by X-ray diffraction (XRD) study. The XRD pattern indicates that Co doped ZnO NPs are with hexagonal wurtzite geometry and diffraction peaks get shifted to higher angles which is the characteristic influence of dopant Co that has an ionic radius smaller than the host cation. The true values of lattice constants have been calculated using Nelson–Riley Function. Crystallite size calculated using Scherrer formula has been compared with that estimated by uniform deformation (UDM), uniform stress deformation (USDM) and uniform deformation energy density (UDEDM) models of Williamson – Hall method, and also by size-strain plot (SSP) method. The lattice strain has also been calculated. The surface morphology and elemental analysis of the product have been characterized by field emission scanning electron microscopy (FESEM) and energy dispersive (EDAX) spectra.

Machine learning lattice constants for cubic perovskite A22+BB′O6 compounds

Abstract: Double perovskite oxides have attracted great attention in the past decade due to their unique and versatile material properties. The lattice constant, a, as the only variable parameter among the six parameters in the cubic structure, has a significant impact on the structural stability, electronic structure, magnetic ordering, and thus material performance. In this work, a Gaussian process regression (GPR) model is developed to elucidate the statistical relationship among ionic radii, electronegativities, oxidation states, and lattice constants for cubic perovskite A22+BB′O6 compounds. A total of 147 samples with lattice constants ranging from 7.700 A to 8.890 A are explored. The modeling approach demonstrates a high degree of accuracy and stability, contributing to efficient and low-cost estimations of lattice constants.

Impact of rare earth Dy +3 cations on the various parameters of nanocrystalline nickel spinel ferrite

Abstract: In this work, nanocrystalline NiDyxFe2-xO4 ferrites (0.0 ≤ x ≤ 0.1) were synthesized via co-precipitation route. The impact of Dy+3 substitution on the various properties has been investigated. Structural, electrical, spectral and magnetic parameters were measured by XRD (X-ray diffraction), I–V (current-voltage), FTIR (Fourier transform infrared spectroscopy) and VSM (vibrating sample magnetometer) respectively. Morphological analysis was done by FE-SEM (Field emission scanning electron microscope). The formation of a single-phase FCC cubic spinel structure was confirmed by XRD patterns. With the help of XRD data, bond lengths and ionic radii were also calculated. FTIR spectra showed the formation of two typical frequency bands (υ1 and υ2) which represent metal-oxygen (M–O) vibrations at octahedral (B) and tetrahedral (A) sites. EDX spectrographs confirmed elemental composition. The M H curves demonstrated a decrease in saturation magnetization (Ms) and Bohr magneton (nB). The coercivity (Hc) showed a non –linear behaviour while anisotropy constant first increased and then decreased with Dy-substitution. NiDy0.025Fe1.975O4 showed maximum coercivity (102.06 Oe).

Lattice constant prediction of A2XY6 cubic crystals (A = K, Cs, Rb, TI; X = tetravalent cation; Y = F, Cl, Br, I) using computational intelligence approach

Abstract: Lattice constant mismatch between materials affects the quality of thin film fabrication. For this reason, lattice constants information is vital in the design of materials for technological applications. The determination of lattice constants via experimental analysis is relatively expensive and laborious. As a result, several linear empirical models have been proposed to predict the lattice constant of crystal structures. However, the accuracies of these models are limited partly due to their failure to account for nonlinearity in the atomic parameters-lattice constant relationship. Machine learning techniques have shown excellent ability to deal with nonlinear problems in many areas of materials science; hence, they are considered suitable computation tools to study the crystal structure of materials. In this contribution, we developed a support vector regression (SVR) model to predict the lattice constant of cubic crystals of the form A2XY6 (A = K, Cs, Rb, TI; X = tetravalent cation; and Y = F, Cl, Br, I). The SVR algorithm uses the ionic radii and electronegativities data of the constituent elements of A2XY6 cubic crystals as model inputs. The robustness of the proposed model was demonstrated by comparing our result with an existing linear model based on 26 cubic crystal samples. The result revealed a total relative deviation of 1.757 and 2.704 for the SVR model and the existing linear equation, respectively. This result proves that the SVR model has a huge potential in the search for new materials for different applications.

High-throughput investigation of the formation of double spinels

Abstract: Spinel compounds, with the general chemical formula AB2O4, are a wide class of materials, where A and B can be a variety of cations, providing this structure with a great deal of functional flexibility and giving rise to its considerable scientific interest. Recently, a spinel with the general formula ABB′O4 has been predicted, increasing the possible usability of the spinels due to the higher cation diversity in the so-called double spinel structure. Here, we use density functional theory calculations to predict if double spinels can be formed between experimentally synthesized normal and inverse single spinels. Our computations reveal that 49 double spinels have negative mixing enthalpies and are thus thermodynamically stable, with most of the stable compounds being formed from one of two distinct cation orderings. We show that the 17 different cations that form the different double spinels have a preferred site, tetrahedral or octahedral, except for Mn, Fe and Co which can occupy both sites interchangeably. We also study the relation between mixing enthalpies and cation-specific properties, as well as ways to classify the double spinels into distinct types and spinel groups depending on the cation ordering and cation oxidation states, respectively. By applying the Sure Independence Screening and Sparsifying Operator (SISSO) approach on the coordination-dependent ionic radii of the elemental constituents, we show that an interplay of local strain and electrostatic dominated terms can be used to separate the double spinels into distinct structural types depending on the cation order and their oxidation states.

Machine learning lattice constants from ionic radii and electronegativities for cubic perovskite $$A_{2}XY_{6}$$ A 2 X Y 6 compounds

Abstract: Metal halide perovskites have attracted great attention in the past decade due to unique and tunable optical and electrical properties, which are promising candidates for various applications such as solar cells, light-emitting diodes, and laser cooling devices. For cubic perovskites, the lattice constant, a, representing the size of the unit cell, has a significant impact on the structural stability, bandgap structure, and thus materials performance. In this study, we develop the Gaussian process regression (GPR) model to shed light on the relationship among ionic radii, electronegativities, and lattice constants for cubic perovskite $$A_{2}XY_{6}$$ compounds. A total of 79 samples with lattice constants ranging from 8.109 to 11.790 $$\mathring{\rm A}$$ are examined. The model has a high degree of accuracy and stability, contributing to fast, robust, and low-cost estimations of lattice constants.

Improved Lithium Ion Diffusion and Stability of a LiNi0.8Co0.1Mn0.1O2 Cathode via the Synergistic Effect of Na and Mg Dual-Metal Cations for Lithium Ion Battery

Abstract: Nickel-rich ternary material LiNi0.8Co0.1Mn0.1O2 is the promising cathode for lithium ion battery, but it has the disadvantage of structural instability. The common method for modifying its electrochemical performance is element doping, such as sodium, aluminum, magnesium and other elements, because the elements with different types and valences have varied ionic radii and binding energies to oxygen. They are incorporated into the layered nickel-rich ternary material to play the distinct roles. In this work, we investigate the effect of Na and Mg co-doping on Li(Ni0.8Co0.1Mn0.1)O2. Our results suggest that the mixed doping has no effect on the crystal structure and morphology of the material. The Rietveld refinement results demonstrated that cation mixing reduced. The electrochemical test results show that the proper amount of Na and Mg mixed doping improves the cyclic reversibility and reduces the resistance of the material. Na and Mg elements can promote each other to improve the electrochemical performance of the material. At the same time we note that high doping concentrations might mitigate the Li diffusion rates.

Structural, thermomagnetic, and dielectric properties of Mn 0.5 Zn 0.5 Gd x Fe 2– x O 4 ( x = 0, 0.025, 0.050, 0.075, and 0.1)

Abstract: In this paper, effect of Gd3+ was investigated on structural, magnetic, and dielectric properties of Mn0.5Zn0.5GdxFe2−xO4(x = 0, 0.025, 0.050, 0.075, and 0.1) nanoparticles prepared by facile coprecipitation method. X-ray diffraction (XRD) studies confirmed the single cubic spinel phase for all the samples and showed that lattice parameter (aexp) was found to increase from 8.414 to 8.446 A with the substitution of Gd3+ ions due to their larger ionic radii than the replaced Fe3+ ions. Shape and size of developed nanoparticles were studied using transmission electron microscopy (TEM) and found that particle size decreased from 31.06 to 21.12 nm for x = 0–0.1. Magnetic properties showed that maximum magnetization decreased from 39.21 to 23.59 emu/g, and Curie temperature decreased from 192 to 176 °C with increase in x from 0 to 0.1 due to weakening of superexchange interaction. Dielectric parameters like dielectric constant (e′ and e″), dielectric loss (tanδ), AC conductivity (σac), and impedance (Z′ and Z″) as a function of frequency and composition were analyzed and discussed. It was found that e′, e″, σac, and tanδ values decreased with Gd substitution, which has been explained based on Maxwell-Wagner theory and hopping mechanism of electrons between Fe3+ and Fe2+ ions at octahedral sites. Nyquist plots for all the developed compositions showed single semi-circular arc which indicate the dominant effect of grain boundaries.

Influence of Oxide Glass Modifiers on the Structural and Spectroscopic Properties of Phosphate Glasses for Visible and Near-Infrared Photonic Applications.

Abstract: The effect of oxide modifiers on multiple properties (structural and spectroscopic) of phosphate glasses with molar composition 60P2O5-(10-x)Ga2O3-30MO-xEu2O3 and 60P2O5-(10-y)Ga2O3-30MO-yEr2O3 (where M = Ca, Sr, Ba; x = 0, 0.5; y = 0, 1) were systematically examined and discussed. The local structure of systems was evidenced by the infrared (IR-ATR) and Raman spectroscopic techniques. The spectroscopic behaviors of the studied glass systems were determined based on analysis of recorded spectra (excitation and emission) as well as luminescence decay curves. Intense red and near-infrared emissions (1.5 μm) were observed for samples doped with Eu3+ and Er3+ ions, respectively. It was found that the value of fluorescence intensity ratio R/O related to 5D0→7F2 (red) and 5D0→7F1 (orange) transition of Eu3+ ions depends on the oxide modifiers MO in the glass host. However, no clear influence of glass modifiers on the luminescence linewidth (FWHM) was observed for phosphate systems doped with Er3+ ions. Moreover, the 5D0 and 4I13/2 luminescence lifetimes of Eu3+ and Er3+ ions increase with the increasing ionic radius of M2+ (M = Ca, Sr, Ba) in the host matrix. The obtained results suggest the applicability of the phosphate glasses with oxide modifiers as potential red and near-infrared photoluminescent materials in photonic devices.

A-Site and B-Site Cation Ordering Induces Polar and Multiferroic Behavior in the Perovskite NaLnNiWO6 (Ln = Y, Dy, Ho, and Yb)

Abstract: Octahedral distortion in ABO3 perovskite materials is ubiquitous because of the ionic size mismatch between A and B cations, leading to various kinds of crystal symmetry. However, such a distortion always results in centrosymmetric structures except for the covalent bond formation because of the second-order Jahn–Teller (SOJT) effect that occurs with d0 or lone-pair cations. Here, we report that an unusual combination of the layered A-site cation ordering and B-site rock salt ordering in NaYNiWO6 prepared under high-pressure and high-temperature conditions results not only in a polar (P21) structure, as revealed by the neutron diffraction analysis, but exhibits multiferroic properties below the magnetic ordering of Ni2+ ions (TN = 21 K). Analysis of the neutron diffraction data at 20 K reveals an incommensurate sinusoidal spin ordering with the propagation vector, ki = (0.471(2), 0, 0.491(4)), and a commensurate collinear spin structure with kc = (0.5, 0, 0.5) below 18 K. X-ray diffraction data confirm the polar structure in Dy, Ho, and Yb compounds. All four compounds exhibit a switchable change in electric polarization (ΔP) at the magnetic ordering temperatures, demonstrating coupling between ferroelectricity and magnetism.

The relationship between enhanced dielectric property and structural distortion in Ca doped Ba2NaNb5O15 tungsten bronze ceramics

Abstract: The relationship between enhanced dielectric property and structural distortion in tungsten bronze structure ceramics was discussed in this work. The ceramics with the composition of (Ba1-xCax)2NaNb5O15 (x = 0, 0.1, 0.2, 0.3, 0.4) were fabricated via conventional solid-state method. All ceramics were pure without secondary phase and the distinct lattice distortion in structure was testified by Rietveld XRD refinement. Compared with the un-doped composition, the maximum polarization and energy storage density were strongly enhanced according to the ferroelectric property measurements, which were contributed to the distortion of NbO6 octahedron induced by the variation of ionic radius. The actually pulsed charge-discharge property of x = 0.3 ceramic was tested, whist excellent power density (PD = 35.106 MV/cm3) and discharge energy density (Wd = 0.29 J/cm3) were obtained at 100 °C under 120 kV/cm, revealing the potential for application of Ba2NaNb5O15 system-based ceramics in harsh environment.

Poly (1-vinylpyrrolidone-co-vinyl acetate) (PVP-co-VAc) based gel polymer electrolytes for electric double layer capacitors (EDLC)

Abstract: The present work describes the electrochemical performance of gel polymer electrolytes containing two different sodium salts such as sodium trifluoromethanesulfonimide (NaTFSI) and sodium trifluoromethanesulfonate (NaOTF)). Poly (1-vinylpyrrolidone-co-vinyl acetate) P(VP-co-VAc) and the mixture of ethylene carbonate and propylene carbonate (EC: PC) have been employed as the host polymer and plasticizer, respectively. Through electrochemical impedance spectroscopy study, the GPE system containing 50% of NaTFSI (System 1: A5) has achieved higher ionic conductivity (1.79 × 10−3 S cm−1) compared to the GPE system containing 50% of NaOTF (1.21 × 10−3 S cm−1) (System 2: B5). The higher ionic conductivity of System 1: A5 was owing to the larger ionic radius of (TFSI−) anion. Temperature dependent studies affirmed that the ionic conductivity of all samples obeyed Arrhenius behavior. Fourier transform infrared spectroscopy revealed the formation of complex within the GPE systems which indicates the good interaction between the host polymer and the salts. X-ray diffraction analysis demonstrated the reduction in crystallinity of System 1: A5 is greater than that of System 2: B5. The maximum specific capacitance achieved by the EDLC employing System 1: A5 and System 2: B5 was 13.44 F/g (67% capacitance retention) and 13.33 F/g (4.2% capacitance retention), respectively.

Structural and Lattice-Dynamical Properties of Tb2O3 under Compression: A Comparative Study with Rare Earth and Related Sesquioxides

Abstract: We report a joint experimental and theoretical investigation of the high pressure structural and vibrational properties of terbium sesquioxide (Tb2O3) Powder X-ray diffraction and Raman scattering measurements show that cubic Ia3 (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa We observe a first irreversible reconstructive transition to the monoclinic C2/m (B-type) phase at ∼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3m1 (A-type) phase at ∼12 GPa Thus, Tb2O3 is found to follow the well-known C → B → A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results Moreover, Raman-active modes of the three phases have been monitored as a function of pressure, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase Finally, we extract the bulk moduli and the Raman-active mode frequencies together with their pressure coefficients for the three phases of Tb2O3 These results are thoroughly compared and discussed in relation to those reported for rare earth and other related sesquioxides as well as with new calculations for selected sesquioxides It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals