The oxygen deficient cubic perovskite SrFe1−xScxO3−δ (x ≤ 0.5; 0.1 ≤ δ ≤ 0.5): Structural features and physical properties

Solid state synthesis method has been used to stabilize oxygen deficient perovskite phases SrFe_1?xSc_xO_3?δ (0 ≤ x ≤ 0.5). The good homogeneity of samples is confirmed by energy dispersive spectroscopy (EDS) analysis performed with a transmission electronic microscope (TEM). By combining X-ray and electronic diffraction (ED), it is demonstrated that the cationic substitution on the B site of the perovskite induces a decrease of the oxygen content but without inducing long range ordering phenomenon. On this basis, X-ray patterns of compounds were indexed in the cubic Pm3m space group. The oxidation states of iron evidenced by Mössbauer spectroscopy, are in good agreement with the oxygen stoichiometries determined by cerimetric titration. In the SrFe_1?xSc_xO_3?δ series, the Fe³⁺/Fe⁴⁺ origin of the electronic conductivity is clearly evidenced. The limit compound SrFe_0.5Sc_0.5O_2.5 is highly resistive and characterized by a cluster glass-like behaviour. Finally, negative magnetoresistivity properties are revealed for the x = 0.1 and x = 0.2 samples, reaching ?10% around the magnetic transition temperature in a 7T magnetic field.     

Investigation of the LiCo1−xMgxPO4 (0 ≤ x ≤ 0.1)–graphitic carbon foam composites

LiCo_1−xMg_xPO₄–graphitic carbon foam (LCMP–GCF with 0 ≤ x ≤ 0.1) composites are prepared by Pechini-assisted sol-gel method and annealed with the 2-steps annealing process (T = 300 °C for 5 min in flowing air, then at T = 730 °C for t = 12 h in flowing nitrogen). The XRD analysis, performed on powders reveals LiCoPO₄ as major crystalline phase, Co₂P and Co₂P₂O₇ as secondary phases. The morphological investigation revealed the formation and growth of microcrystalline “islands” which consist of acicular crystallites with different dimensions (typically 5–50 μm). By addition of Mg-ions, CV-curves of LCMP–GCF composites show a decrease of the surface between anodic and cathodic sweeps by cycling and a stark contribution of faradaic processes due to the graphitic structured foam. The electrochemical measurements, at a discharge rate of C/10 at room temperature, show the decrease of the discharge specific capacity from 100 mAh g⁻¹ for x = 0.0 to ∼35 mAh g⁻¹ for 0.025 ≤ x ≤ 0.05, then an increase to 69 mAh g⁻¹ for x = 0.1. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity at high Mg-ions content into the LiCoPO₄ phase (x ≥ 0.025).     

On the physical properties of Sr1−xNaxRuO3 (x = 0–0.19)

The metallic ferromagnetic perovskite-type SrRuO₃ (T_C ～ 160 K) belongs to the “class” of materials with strongly correlated electrons. Nonetheless a simple ferromagnetism associated with isotropic interactions of low spin Ru⁴⁺ ions local moments is far too simple to explain the complex interplay between charge carriers and magnetic interactions. In that sense the suppression of ferromagnetism in isoelectronic Sr_1?xCa_xRuO₃ was tentatively associated to the increased lattice distortion influencing primarily the 4d Ru bandwidths and, hence, the itinerancy and respective populations of the spin-up and spin-down electrons.In order to probe the robustness of the metallic ferromagnetism against electron occupation of 4d Ru orbital we prepared and characterized polycrystalline Sr_1?xNa_xRuO₃ (x = 0.0–0.19) ceramics. The substitution of Sr²⁺ by Na¹⁺, leading to formally mixed valence Ru⁴⁺/Ru⁵⁺, induces the decrease of the Curie temperature and spin-wave stiffness, which was determined independently from magnetic and specific heat data. On the other hand the effective paramagnetic moment remains essentially unchanged. All compounds are metallic in a sense of electrical resistivity and thermopower temperature dependence; the low temperature upturn of the electrical resistivity was explained on a base of the weak localization. The metallic nature of the samples is corroborated by Pauli paramagnetism and high Sommerfeld coefficient γ, extracted from the low temperature specific heat, which increases from 30.9 mJ mol^?1 K^?2 (x = 0.0) to 43.0 mJ mol^?1 K^?2 (x = 0.19).     

Electronic transport and magnetic properties of the perovskites La0.8Sr0.2Co1−xFexO3; x ≤ 0.3

Transport and magnetic properties of LaCoO₃-based compounds, doped with 20% Sr and 2.5, 5, 10, 15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by ⁵⁷Fe Mössbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co → Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed.     

Thermal behaviour,surface properties and vibrational spectroscopic studies of the synthesized Co3xNi3−3x(PO4)2·8H2O (0 ≤ x ≤ 1)

Co_3xNi_3−3x(PO₄)₂·8H₂O (x = 1, 0.8, 0.6, 0.4, 0.2, and 0) were synthesized via simple wet chemical reaction and energy saving route method. The final decomposition products of hydrates are corresponding anhydrous tri(cobalt nickel) diphosphates. The metal and water contents of the synthesized hydrates were confirmed by AAS and TG/DTG/DTA techniques, respectively. The observed metal and water contents agree well with the formula of the title compounds. The crystal structures and lattice parameters as well as crystallite sizes of the studied compounds were determined using XRD data. The results from XRD and TG/DTG/DTA techniques confirmed that Co_3xNi_3−3x(PO₄)₂·8H₂O at all ratios were the single phase. The FTIR spectra of studied compounds were recorded and assigned. The thermal behaviours of single and binary tri(cobalt nickel) diphosphate octahydrates were studied for the first time. The morphologies of the studied compounds were investigated by using the SEM technique. The micrographs of all studied compounds exhibited the thin plated morphology. The surface area and the pore size data of anhydrous forms were measured by N₂ adsorption at −190 °C according to the BET method. The anhydrous forms of binary metal phosphate at x = 0.8, Co_2.4Ni_0.6(PO₄)₂, exhibits the highest surface area and expects to improve the catalytic activity.     

Structural and magnetic properties of TbxY3−xFe5O12 (0 ≤ x ≤ 0.8) thin film prepared via sol–gel method

Terbium-doped yttrium iron garnet (Tb_xY_3−x Fe₅O₁₂; x = 0.0, 0.2, 0.4, 0.6 and 0.8) nanoparticles thin films have been prepared onto quartz substrate by sol–gel method followed by spin coating process. Annealing of the films was processed at 900 °C in air for 2 h. The structures were investigated by using an X-ray diffractometer (XRD) and a field emission scanning electron microscope (FE-SEM). The magnetic properties were studied by a vibrating sample magnetometer (VSM). The XRD patterns of the films were consistent with a single phase garnet structure. The lattice parameter was initially increased with Tb³⁺ concentration due to the larger size of the Tb³⁺ ion compared to Y³⁺ ion, but a decrease in lattice parameter was observed at higher Tb³⁺ concentration due to the effect of film’s thickness. FE-SEM micrographs reveal that the particles were highly agglomerated. The grain’s sizes for all films were in the range of 40–59 nm. The magnetic measurements at room temperature (25 °C) show that the saturation magnetization (M_s) of the films was reduced with the increase in Tb³⁺ ions, which due to the antiparallel alignment between Tb³⁺ ions and Fe³⁺ ions. The films illustrate normal shapes of hysteresis loops except Tb_0.2Y_2.8Fe₅O₁₂ and Tb_0.4Y_2.6Fe₅O₁₂ films exhibiting two steps increments before being saturated. The coercivity values (H_c) demonstrate non linear dependency with the terbium concentration (x).     

Structural and magnetic properties of the Zn0.8−4xHoxOy (0.05 ≤ x ≤ 0.10) compounds prepared by solid state reactions

Zn_0.8−4xHo_xO_y (0.05 ≤ x ≤ 0.10) diluted magnetic semiconductors were prepared by the solid state reaction method. We have studied the structural properties of the samples by using the XRD, SEM, and EDX techniques. The SEM results clearly demonstrate that Ho³⁺ ions are quite well substituted for Zn²⁺ in the ZnO lattice, and the grains of the samples are very well connected to each other and tightly packed. From the XRD and EDX spectra of the samples, it has been concluded that the substitution of Ho causes no change in the hexagonal wurtzite structure of ZnO. According to our M–H and M–T measurements paramagnetism has been observed for all the samples from our attainable lowest temperature of 10 K to 300 K. Furthermore, the trend of the AC-susceptibility (χ) versus temperature curves, measured under an AC-magnetic field of 10 Oe, also support our conclusion about the paramagnetic contribution in the Zn_0.8−4xHo_xO_y compounds explored in this study. In order to clearly see the paramagnetic contribution, and whether there is also a ferromagnetic or antiferromagnetic contribution or not the inverse susceptibility (1/χ) against temperature curves are also plotted. Those curves indicate that, the substitution of Ho into the ZnO compound causes, in addition to the paramagnetism, a weaker antiferromagnetic (AFM) interaction.     

First-principles calculations of the structural,elastic and electronic properties of MNxC1−x (M = Ti,Zr, Hf; 0 ≤ x ≤ 1) carbonitrides at ambient and elevated hydrostatic pressure

The structural, electronic, and elastic properties of three mixed transition metal carbonitrides TiN_xC_1−x, ZrN_xC_1−x, and HfN_xC_1−x (0 ≤ x ≤ 1) with the rock-salt structure were calculated at ambient and elevated up to 50 GPa hydrostatic pressures in the framework of the density functional theory methods. The lattice constants, densities, and bulk moduli of the considered compounds were shown to behave as linear functions of the nitrogen concentration x. The obtained linear dependencies of all these parameters allow for getting their estimates at any value of x in the range from 0 to 1. Gradual enhancement of the ionicity of the chemical bonds with gradual replacement of carbon by nitrogen was demonstrated by calculating the bond orders and electron density difference distributions.     

Structural variations and cation distributions in Mn3−xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn_3?xCo_xO₄ were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn²⁺ and/or Co²⁺ occupy the tetrahedral site while Mn³⁺, Co²⁺, Co^III (cobalt in low-spin state) and Mn⁴⁺ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn³⁺/Mn⁴⁺ and Co²⁺/Co^III on the octahedral sites.     

Inhomogeneous magnetism studied by ESR in La1-xSrxMnO3 (0.45 ≤ x ≤ 0.62)

The influence of strontium substitution on lanthanum site in La_1-xSr_xMnO₃ manganites has been investigated with x ranging from 0.55 to 0.62 in the 130–400 K temperature range. Powder X-ray diffraction reveals structural changes from rhomboedral to tetragonal and to orthorhombic structures upon increasing Sr substitution. Magnetic properties also show a rich variety of phases and behaviors, including coexistence of phases above Curie temperature. The electron spin resonance measurements allow quantifying paramagnetic phases and properties with randomly distributed spins and ferromagnetic phases or inclusions with oriented spins giving rise to a local magnetic field. When x = 0.55, 0.57, the ferromagnetic state is the minor phase embedded in a paramagnetic matrix. Conversely, when x increases, the ferromagnetic phase grows and becomes the only phase observed for x = 0.62.     

Thermal and electrical relaxation studies in Li(4+x)TixNb1−xP3O12 (0.0 ≤ x ≤ 1.0) phosphate glasses

A new superionic conducting transparent phosphate glasses with composition Li_(4+x)Ti_xNb_1?xP₃O₁₂ (0 ≤ x ≤ 1.0) were prepared by rapid melt quenching. As quenched samples were characterized by X-ray powder diffraction, differential scanning calorimetric and Fourier transform infrared spectroscopy studies. These glasses were found to have high thermal stability parameter and Li₄NbP₃O₁₂ has been found to have high glass forming ability. Electrical properties of the present glasses were studied by impedance and dielectric spectroscopy in the frequency range 10 Hz–3 MHz in the temperature range 323–523 K. Arrhenius behavior has been observed for all the glass in conductivity, dielectric loss and conductivity relaxation and their activation energies are explained and reported.     

Thermal and kinetic parameters of 30Li2O–55B2O3–5ZnO–xTiO2–(10−x)V2O5 (0 ≤ x ≤ 10) glasses

Journal of Thermal Analysis and Calorimetry - In the present investigation, the effects of titanium and vanadium concentration on the thermal properties have been studied for...     

A single crystal X-ray and powder neutron diffraction study on NASICON-type Li1+xAlxTi2−x(PO4)3 (0 ≤ x ≤ 0.5) crystals: Implications on ionic conductivity

Single crystals of NASICON-type material Li_1+xTi_2−xAl_x(PO₄)₃ (LATP) with 0 ≤ x ≤ 0.5 were successfully grown using long-term sintering techniques. Sample material was studied by chemical analysis, single crystal X-ray and neutron diffraction. The Ti⁴⁺ replacement scales very well with the Al³⁺ and Li⁺ incorporation. The additional Li⁺ thereby enters the M3 cavity of the NASICON framework at x, y, z ∼ (0.07, 0.34, 0.09) and is regarded to be responsible for the enhanced Li⁺ conduction of LATP as compared to Al-free LTP. Variations in structural parameters, associated with the Ti⁴⁺ substitution with Al³⁺ + Li⁺ will be discussed in detail in this paper.     

First-principles study on the geometries,stabilities and electronic properties of yttrium–silicon clusters (Y2Si n ; 1 ≤ n ≤ 12)

Structural Chemistry - The geometries, growth patterns, relative stabilities and electronic properties of yttrium-doped silicon clusters Y2Si n (n&nbsp;=&nbsp;1–12) are systematically...     

Calculated structural properties of TbCu7-type disordered intermetallic compounds NdCo7 and Nd1−xYxCo6.8Zr0.2 (x = 0, 0.2, 0.4, 0.6)

An improved evolution from ordered CaCu₅-type structure to disordered TbCu₇-type structure has been implemented in this work based on the atomic simulation regarding intermetallic compounds NdCo₇ and Nd_1?xY_xCo_6.8Zr_0.2 (x = 0, 0.2, 0.4, 0.6). The calculated powder patterns, phase stabilities, site preferences and structural properties, including lattice parameters, the shrinkages from 2c to 6l site in 2c layers, the 3g layer deformation, the dumbbell bond length distribution and inclination angle distribution as well as the effects of element Zr and Y in the compounds, are found to be in agreement with physical analysis and experiment.     

Crystal structures and magnetism of DyAlxGa3−x (where x = 0.33 and x = 0.85)

New ternary compounds have been obtained from arc-melting of the elements. The crystal structures have been investigated by powder X-ray diffraction analysis. DyAl_0.33Ga_2.67 crystallizes in space group P6₃/mmc (a = 6.1649(1) Å, c = 23.0792(1) Å) with Ta(Rh_0.33Pd_0.67)₃-type structure. The structure of DyAl_0.85Ga_2.15 has been refined with rhombohedral symmetry (space group R-3m, a = 6.1702(1) Å, c = 20.7797(1) Å), BaPb₃-type structure. The structures of the compounds have been analyzed crystallographically and the structural relationship has been established. Heat capacity measurements prove antiferromagnetic ordering at Néel temperatures of 12.6(1) K for DyAl_0.33Ga_2.67 and 10.0(1) K for DyAl_0.85Ga_2.15.     

Approaching compositional limits of perovskite – type oxides and oxynitrides by synthesis of Mg0.25Ca0.65Y0.1Ti(O,N)3, Ca1–xYxZr(O,N)3 (0.1 ≤ x ≤ 0.4), and Sr1–xLaxZr(O,N)3 (0.1 ≤ x ≤ 0.4)

Partial substitution of cations and anions in perovskite-type materials is a powerful way to tune the desired properties. The systematic variation of the cations size, the partial exchange of O²⁻ for N³⁻ and their effect on the size of the optical band gap and the thermal stability was investigated here. The anionic substitution resulted in the formation of the orthorhombic perovskite-type oxynitrides Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃, Ca_1-xY_xZr(O,N)₃, and Sr_1–xLa_xZr(O,N)₃. A two-step synthesis protocol was applied: i) (nano-crystalline) oxide precursors were synthesized by a Pechini method followed by ii) ammonolysis in flowing NH₃ at T = 773 K (Ti) and T = 1273 K (Zr), respectively. High-temperature synthesis of such oxide precursors by solid–state reaction generally resulted in phase separation of the different A-site cations. Changes of the crystal structures were investigated by Rietveld refinements of the powder XRD data, thermal stability by DSC/TG measurements in oxygen atmosphere, oxygen and nitrogen contents by O/N analysis using hot gas extraction technique, and optical band gaps by photoluminescence spectroscopy. By moving from Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃ via Ca_1–xY_xZr(O,N)₃ to Sr_1–xLa_xZr(O,N)₃, the degree of tilting of the octahedral network is reduced, as observed by an increase in the B–X–B angles caused by the simultaneously increasing effective ionic radius of the A-site cation(s). In general, increasing substitution levels on the A-site (Y³⁺ and La³⁺) are accompanied by an enhanced replacement of O²⁻ by N³⁻. In all three systems, this anionic substitution resulted in a reduction of the optical band gap by approximately 1 eV (Ti) and up to 2.1 eV (Zr) compared to the respective oxides. For Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃ an optical band gap of 2.2 eV was observed, appropriate for a solar water splitting photocatalyst. The Zr-based oxynitrides required a by a factor of 2 higher nitrogen contents to significantly reduce the optical band gap and the measured values of 2.9 eV–3.2 eV are larger compared to the Ti-based oxynitride. Bulk thermal stability was revealed up to T = 881 K. In general, the thermal stability decreased with increasing substitution levels due to an increasing deviation from the ideal anionic composition as demonstrated by O/N analysis.