Orbital ordering in Mott-insulators La2O3Fe2Se2 and La2O3Co2Se2

The electronic structure and magnetic properties of new layered oxyselenide compounds La₂O₃Fe₂Se₂ and La₂O₃Co₂Se₂ are studied by first-principles calculations. Our results indicate that both compounds are Mott-insulators with orbital ordering. The ground states of both compounds are the checkerboard antiferromagnetic states, which are different from the iron pnictide superconductors, although their structures are similar to those of the Fe-As-based superconductors.     

EPR and magnetic properties of MgO-CaO-SiO2-P2O5-CaF2-Fe2O3 glass-ceramics

Glass-ceramics have been derived from 4.5MgO(45−x)CaO34SiO₂16P₂O₅0.5CaF₂xFe₂O₃ (x=5, 10, 15, 20 wt%) glasses by heat treatment. Room temperature electron paramagnetic resonance (EPR) spectra and temperature-dependent magnetic susceptibility (χ) of the glass-ceramics have been obtained. The EPR absorption line centered at g≈4.3 disappeared at higher concentrations of iron oxide. The intensity and line width of the EPR absorption line centered at g≈2.1 increased as the iron oxide concentration was increased. Temperature-dependent magnetization of samples with low iron oxide content revealed ferrimagnetic as well as paramagnetic contributions. Information about the structural changes involving iron ions, their valence state and the type of magnetic interactions between the Fe ions as a function of composition was obtained using EPR and χ studies.     

Structural study of the Lu2Si2O7-Sc2Si2O7 system

Different compositions in the Lu₂Si₂O₇-Sc₂Si₂O₇ system have been synthesized following the ceramic method. All XRD patterns are compatible with the thortveitite structure (β-RE₂Si₂O₇ polymorph). Unit cell parameters change linearly with composition, which indicates a complete solid solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇. ²⁹Si MAS NMR spectra show a decrease of the ²⁹Si chemical shift with increasing Sc content. A correlation reported in the literature to predict ²⁹Si chemical shifts in silicates is applied here to obtain the theoretical variation in ²⁹Si chemical shift values in the system Lu₂Si₂O₇-Sc₂Si₂O₇ and the results compare favourably with the values obtained experimentally. The FWHM values of the ²⁹Si MAS NMR curves indicate a random distribution of Lu and Sc in the structure of the intermediate members. Finally, the IR study of the system confirms the solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇, showing the splitting of several modes in the intermediate members and a linear shift of the frequency on going from one end-member to the other.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Synthesis and crystallographic studies of garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

High-purity powder specimens of AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ have been successfully synthesized by solid-state chemical reaction. The Rietveld refinements from X-ray powder diffraction data verified that these compounds have the garnet-type structure (space group , No. 230) with the lattice constant of a=12.596(2) Å for AgCa₂Mn₂V₃O₁₂ and a=12.876(2) Å for NaPb₂Mn₂V₃O₁₂. Calculation of the bond valence sum supported that Mn is divalent and V is pentavalent in these garnets. Estimation of the quadratic elongation and the bond angle variance showed that the distortions of the MnO₆ octahedra and the VO₄ tetrahedra are significantly suppressed. Our new results of AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ are compared to those of AgCa₂M₂V₃O₁₂ and NaPb₂M₂V₃O₁₂ (M=Mg, Co, Ni, Zn).     

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.     

Synthesis and characterization of thermoluminescent glass-ceramics Li2O-Al2O3-SiO2:CeO2

Vitroceramic powders of Li₂O-Al₂O₃-SiO₂ systems (LAS), doped with 1% (LAS:1Ce) and 10% (LAS:10Ce) molar of cerianite (CeO₂) were synthesized by means of the gelification technique of metal formates of aluminum and lithium, in the presence of tetraethoxy silane and CeO₂. The gels obtained were dried (120 °C, 2.5 h), calcined (480 °C, 5 h) and sinterized (1250 °C, 30 min). The sinterized samples were characterized by X-ray difraction (XRD), scanning electron microscopy (SEM) and microchemical analysis (EDS). There is evidence for a mixture of two phases of 64% β-spodumene (Li₂O-Al₂O₃-4SiO₂) and 36% β-eucryptite (Li₂O-Al₂O₃-2SiO₂). The LAS:1Ce system was enriched in aluminum, the LAS:10Ce system showed areas of heterogeneous composition; some regions with a shortage of CeO₂, while others zones with cerium cumulus. From the microscopy images it was found that CeO₂ acts as a densificant agent in LAS system, favoring the sintering in the host. The chemical route and the sintering processes utilized allow the production of samples exhibiting an acceptable linear correlation between total thermoluminescent emission intensity and the irradiation dose when the CeO₂ concentration is low (less than 1%), opening the possibility of using this kind of glass-ceramic in dosimetry.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.     

Incommensurate modulations in Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8

Incommensurate modulations in Ba₂TiSi₂O₈, Sr₂TiSi₂O₈, and Ba₂TiGe₂O₈ are compared based on their corresponding electron diffraction patterns. The dependence of the modulations on chemical composition provides a suitable model system for the investigation of incommensurations in framework structures using high-resolution transmission electron microscopy (HRTEM). A widening of quantitative HRTEM utilising the concept of rigid units is proposed allowing for a determination of atomistic displacements responsible for the formation of incommensurately modulated structures.     

Emission analysis of Dy and Pr:Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses

In this paper, we present the spectral results of Dy³⁺ and Pr³⁺ (1.0 mol%) ions doped Bi₂O₃-ZnF₂-B₂O₃-Li₂O-Na₂O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy³⁺:glass has shown two emission transitions ⁴F_7/2→⁶H_15/2 (482 nm) and ⁴F_7/2→⁶H_13/2 (576 nm) with an excitation at 390 nm wavelength and Pr³⁺:glass has shown a strong emission transition ¹D₂→³H₄ (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy³⁺ and Pr³⁺ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.     

Structure and surface characterization of ZrO2-Y2O3-Cr2O3 system

3-mol% Y₂O₃ and 0.3 to 3-mol % Cr₂O₃ co-doped ZrO₂ nanopowders were synthesized using co-precipitation technique and investigated by terms of X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Structural analysis shows no significant impact of chromium on powders structure except of presence of small amount of m-phase. Surface analysis reveals segregation of yttrium and chromium atoms to the surface along with surface enrichment by oxygen that can be attributed to residual water. Chromium surface atoms present in three oxidation states with catalytically active Cr²⁺ sites possibly controlling m-phase appearance through lattice distortion.     

In vitro evaluation of bioactivity of CaO-SiO2-P2O5-Na2O-Fe2O3 glasses

Glasses with compositions 41CaO(52 − x)SiO₂4P₂O₅·xFe₂O₃3Na₂O (2 ≤ x ≤ 10 mol.%) were prepared by melt quenching method. Bioactivity of the different glass compositions was studied in vitro by treating them with simulated body fluid (SBF). The glasses treated for various time periods in SBF were evaluated by examining apatite formation on their surface using grazing incidence X-ray diffraction, Fourier transform infrared reflection spectroscopy, scanning electron microscopy and energy dispersive spectroscopy techniques. Increase in bioactivity with increasing iron oxide content was observed. The results have been used to understand the evolution of the apatite surface layer as a function of immersion time in SBF and glass composition.     

Glass formation in the As2Se3-As2Te3-Sb2Te3 system

Chalcogenide glasses from the As₂Se₃-As₂Te₃-Sb₂Te₃ system were synthesized for the first time. The glass-forming region was determined by X-ray diffraction and electron microscopic analyses.The basic physicochemical parameters such as density (d), microhardness (HV) and temperatures of phase transformations (glass transition T_g, crystallization T_cr and melting T_m) were measured. Compactness and some thermomechanical characteristics such as volume (V_h) and formation energy (E_h) of micro-voids in the glassy network as well as the elasticity module (E) were calculated. The glass-forming ability was evaluated according to Hruby's criteria (K_G). The correlation between composition and properties of the (As₂Se₃)_x(As₂Te₃)_y(Sb₂Te₃)_z glasses was established and comprehensively discussed.     

Magneto-structural correlations in Pr0.15Gd0.85Mn2Ge2

Magneto-structural correlations in Pr_0.15Gd_0.85Mn₂Ge₂ have been studied by synchrotron diffraction in the temperature range between 11 and 300 K. This compound crystallizes in the ThCr₂Si₂-type structure (space group ). The unit cell parameters a and c were determined by Rietveld refinements as a function of temperature. Anomalies in the temperature dependence of the unit cell parameters a and c, the c/a ratio and the unit cell volume V at about 240 and 140 K, which is close to the magnetic phase transition temperatures, indicate a pronounced magneto-structural correlation. Spontaneous volume change and linear magnetostrictions are derived as a function of temperature.     

Fluoride glasses in the InF3-GaF3-YF3-PbF2-CaF2-ZnF2 system

New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35−x) InF₃-xGaF₃-10YF₃-25PbF₂-15CaF₂-15ZnF₂. T_g lies between 260 and 296 °C while melting starts around 480 °C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density.     

Dielectric properties of P2O5-Na2O-Li2O glasses containing WO3, CoO or Fe2O3

The aim of this work was to study some polarization mechanisms in phosphate glasses containing different transition metal oxides, performed at radio frequency (100 kHz) and in the microwave region (9 GHz). Cobalt, iron and tungsten oxides were chosen for the present investigation. The results show that the dielectric constant increases linearly with increasing Co concentration, whereas for iron and tungsten ions, the dielectric constant values were found to decrease up to a given concentration and further, increase for higher concentration of W and Fe. This anomaly was observed both at 100 kHz and 9.00 GHz and was attributed to the valence state of the transition metal ions in the glass structure. The dielectric constant is lower in the microwave region, due to structural relaxation mechanisms.     

Multicomponent GeSe2-CdI2-TeO2 (Bi2O3) systems: glass formation and properties

The glass-forming regions of the GeSe₂-CdI₂-TeO₂ (I), GeSe₂-CdI₂-Bi₂O₃ (II) and GeSe₂-TeO₂-Bi₂O₃ (III) systems have been determined. The obtained glassy phases have been characterized by their basic physicochemical parameters such as temperatures of glass transition, crystallization, and melting, density and microhardness. The phase T-X diagram of the GeSe₂-CdI₂ system, which is the basic joint line for systems I and II, has been specified. Three non-variant equilibria (two eutectic and one syntectic) have been observed at temperatures 350, 280 and 375 °C for compositions containing 15, 95 and 33.3 mol% GeSe₂, respectively. A new intermediate phase with probable composition of 2CdI₂·GeSe₂ has been formed.     

Thermoluminescence glow curves analysis of pure and CeO2-doped Li2O-Al2O3-SiO2 glass ceramics

A comparative study of the thermoluminescence (TL) emission between beta-irradiated lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂ (LAS), and beta-irradiated CeO₂-doped lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂-CeO₂ (LAS:Ce), grown by sol-gel technique and preannealed at 1250 °C is presented. It is found that doping reinforces the result of preannealing the sample; the incorporation of CeO₂ at low concentrations shifts the TL curves towards higher temperatures and increases the total intensity with respect to samples without the dopant. This behavior, together with the fact that CeO₂ is a good densification agent, suggests the possibility of using CeO₂ to improve the technological properties of LAS. Deconvolution analysis of the thermoluminescence glow curves of doped materials under general order kinetics shows that the relevant temperatures at which the component signals appear are virtually the same as for pure samples. This suggests that CeO₂ does not introduce new types of traps, but only changes the population and distribution of impurities responsible for the TL in the original matrix of LAS. Finally, the kinetic parameters for the deconvoluted curves are reported. They corroborate that changes in CeO₂ concentration only vary the depth and distribution of the same kind of original traps.