Optical and ESR investigations of lanthanum aluminates LaMg1−xMnxO19 single crystals with magnetoplumbite-like structure

New lanthanum aluminates LaMAl₁₁O₁₉ (M²⁺ = Ni, Co, Mn, Mg_1?xMn_x, 0 ≤ x ≤ 1), with magnetoplumbite-like structure have been obtained as single crystals. This paper is particularly devoted to the Mn²⁺ and $\text{Mg}{}^{\mathrm{2+}}\text{Mn}{}^{\mathrm{2+}}$ mixed compounds, which exhibit promising luminescent properties. Several characteristics of the crystals are given. The absorption spectra of the materials, as grown, are assigned to Mn²⁺ ions in tetrahedral sites. After annealing in air new absorptions attributed to octahedral Mn³⁺ ions, appear. The ESR spectra of Mn²⁺ in all these crystals exhibit axial symmetry. For x ≤ 0.25 they arise from isolated Mn²⁺ ions in slightly distorted tetrahedral sites and reveal a strong disorder effect. For x ≥0.5 the spectra consist of a single line, attributed to clusters of magnetically interacting Mn²⁺ ions.     

Impact of metal substitutions for cobalt in YBaCo4O7

The “114” YBaCo₄O₇ cobaltite undergoes structural transition just beyond room temperature at T_S∼310 K. Correspondingly, its signature in the physical properties is detected by T-dependent measurements of electrical resistivity, magnetic susceptibility and thermoelectric power. It is found that low-level substitutions of divalent (M=Zn²⁺) or trivalent (M=Ga³⁺, Al³⁺) cations for cobalt according to the YBaCo_4−xM_xO₇ formula with x?0.4 have a strong impact upon this transition. On the one hand, Zn²⁺ substitutions preserve the transition but with T_S decreasing as x increases. On the other hand, for x=0.2 Ga³⁺ or Al³⁺, the transition is suppressed, i.e., for only 5% trivalent foreign cation substituted for cobalt. Though at first, this contrasted behaviour between divalent and trivalent substituting cations appears to be linked to the opposite evolution of hole carriers “Co³⁺” concentration with x, a possible destabilization of 3Co²⁺: 1Co³⁺ charge ordering induced by the M³⁺ cations is considered.     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.     

Weitere magnetische Untersuchungen an Ti3−xMxO5-Phasen (M = Al3+, Fe2+, Mn2+, Mg2+) mit einem Beitrag über CrTi2O5

Additional Magnetic Examinations of Ti_3?xM_xO₅-Phases (M = Al³⁺, Fe²⁺, Mn²⁺, Mg²⁺) with a Contribution about CrTi₂O₅ Ti_3?xM_xO₅ was prepared with M = Al³⁺, Fe²⁺, Mn²⁺, and Mg²⁺. Die magnetic properties of this phases were examinated by the Faraday method in respect to the temperature. The well known magnetic effect of Ti₃O₅ near 450 K is shifted to lower degrees if Ti is replaced by Al, Fe, Mn, or Mg. Compared to Ti_3?xV_xO₅ and Ti_3?xCr_xO₅ the stability of the low temperature-form of Ti₃O₅ is much more reduced in Ti_3?xM_xO₅ (M = Al, Fe, Mn, Mg). The crystal structure investigation of CrTi₂O₅ explained the anomalous behaviour of the Cr³⁺ and V³⁺ doped Ti₃O₅.     

Activities and their relation to cation distribution in NiAl2O4MgAl2O4 spinel solid solutions

The activity of NiAl₂O₄ in NiAl₂O₄MgAl₂O₄ solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl₂O₄ + Al₂O₃(α)/CaOZrO₂/Ni + Ni_xMg_1?xAl₂O₄ + Al₂O₃(α). Pt, in the temperature range 750–1150°C. The activities in the spinel solid solutions show negative deviations from Raoult's law. The cation distribution in the solid solutions has been calculated using site preference energies independent of composition for Ni²⁺, Mg²⁺, and Al³⁺ ions obtained from crystal field theory and measured cation disorder in pure NiAl₂O₄ and MgAl₂O₄, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, α, of Ni²⁺ ions on tetrahedral sites for 1>x>0.25, observed by Porta et al. [J. Solid State Chem.11, 135 (1974)] but do not support their tentative evidence for an increase in α for x < 0.25. The measured excess free energy of mixing can be completely accounted for by using either the calculated or the measured cation distributions. This suggests that the Madelung energy is approximately a linear function of composition in the solid solutions. The composition of NiOMgO solid solutions in equilibrium with NiAl₂O₄MgAl₂O₄ solid solutions has been calculated from the results and information available in literature.     

Structure and photoluminescence properties of Ce-doped novel silicon-oxynitride Ba4−zMzSi8O20−3xN2x (M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce³⁺-doped novel silicon-oxynitride phosphors of Ba_4−zM_zSi₈O_20−3xN_2x (M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba_4−zM_zSi₈O_20−3xN_2x oxynitride were synthesized by partial substitutions of 3O²⁻→2N³⁻ and Ba→M (M=Mg, Ca, Sr) in orthorhombic Ba₂Si₄O₁₀. The influences of the type of alkaline earth ions of M, the Ce³⁺ concentration on the photoluminescence properties and thermal quenching behaviors of Ba₃MSi₈O_20−3xN_2x (M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba₃MSi₈O_20−3xN_2x:Ce³⁺ (x=0.5) exhibits efficient blue emission centered at 400-450 nm in the range of 350-650 nm owing to the 5d→4f transition of Ce³⁺. The emission band of Ce³⁺ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce³⁺ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce³⁺ ions. Among the silicon-oxynitride phosphors of Ba₃MSi₈O_18.5N:Ce³⁺, M=Sr_0.6Ca_0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce³⁺.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Synthesis and crystal structure of mixed metal(III) tungstenyl(VI) ortho-pyrophosphates

The series of isotypic anhydrous ortho-pyrophosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: Sc, V, Cr, Fe, Mo, Ru, Rh, In, Ir) was obtained via vapor phase moderated solid state reactions in sealed ampoules. The crystal structure of the phosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: V, Ru, Rh) was solved from single crystal X-ray data (C2/c, Z = 16). Fairly regular MO₆ and distorted WO₆ octahedra share vertices with PO₄ and P₂O₇ units to form a 3D network. For the ortho-pyrophosphates with M: V³⁺, Cr³⁺, and Fe³⁺ the oxidation state of M is confirmed by magnetic measurements. ³¹P-MAS-NMR spectra of the diamagnetic phosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: Sc, In, Ir) show surprisingly different isotropic chemical shifts for the seven phosphorus sites. V^III(W^VIO₂)₂(P₂O₇)(PO₄) occurs as equilibrium phase in the quasi-binary system (V_1–xW_x)OPO₄ at x = 0.67 and exhibits a small homogeneity range 0.60 ≤ x ≤ 0.67. The scandium compound shows a fully inverted occupancy of the M sites according to the formulation W(Sc_1/2W_1/2O₂)₂(P₂O₇)(PO₄).     

Series of compositions Bi2(M′xM1−x)4O9 (M′, M=Al, Ga, Fe; 0≤x≤1) with mullite-type crystal structure: Synthesis, characterization and O/O exchange experiment

Series of compositions Bi₂(M′_xM_1−x)₄O₉ with x=0.0, 0.1,…, 1.0 and M′/M=Ga/Al, Fe/Al and Fe/Ga were synthesized by dissolving appropriate amounts of corresponding metal nitrate hydrates in glycerine, followed by gelation, calcination and final heating at 800 °C for 24 h. The new compositions with M′/M=Ga/Al form solid-solution series, which are isotypes to the two other series M′/M=Fe/Al and Fe/Ga. The XRD data analysis yielded in all cases a linear dependence of the lattice parameters related on x. Rietveld structure refinements of the XRD patterns of the new compounds, Bi₂(Ga_xAl_1−x)₄O₉ reveal a preferential occupation of Ga in tetrahedral site (4 h). The IR absorption spectra measured between 50 and 4000 cm⁻¹ of all systems show systematic shifts in peak positions related to the degree of substitution. Samples treated in ¹⁸O₂ atmosphere (16 h at 800 °C, 200 mbar, 95% ¹⁸O₂) for ¹⁸O/¹⁶O isotope exchange experiments show a well-separated IR absorption peak related to the M-¹⁸O_c-M vibration, where O_c denotes the common oxygen of two tetrahedral type MO₄ units. The intensity ratio of M-¹⁸O_c/M-¹⁶O_c IR absorption peaks and the average crystal sizes were used to estimate the tracer diffusion coefficients of polycrystalline Bi₂Al₄O₉ (D=2×10⁻²² m²s⁻¹), Bi₂Fe₄O₉ (D=5×10⁻²¹ m²s⁻¹), Bi₂(Ga/Al)₄O₉ (D=2×10⁻²¹ m²s⁻¹) and Bi₂Ga₄O₉ (D=2×10⁻²⁰ m²s⁻¹).     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

Stabilization of unusual electronic configurations of transition elements in elongated six-coordinated oxygen sites of a K2NiF4 structure

The A′_xA_2?xLi_0.5M_0.5O₄ matrix deriving from the K₂NiF₄ structure has an elongated MO₆ octahedron; it has been used to stabilize anisotropic electronic configurations such as high-spin Fe⁴⁺, low-spin Co⁴⁺, medium-spin Co³⁺, low-spin Ni³⁺, or low-spin Cu³⁺. The choice of ions to be stabilized was based on a simple model for predicting the electronic configuration of a dⁿ ion in a tetragonally distorted octahedral environment.     

Synthesis and crystal structure of two new cerium rhodium oxides: Ce2/3−xRh2O4 (x∼0.12) with Ce mixed valency and CeRh2O5

The new compounds Ce_2/3−xRh₂O₄ (x∼0.11-0.14) and CeRh₂O₅ have been prepared. Their structures were determined from single crystal X-ray diffraction data. Electrical and magnetic properties were also evaluated. Based on the structural analysis and physical properties, oxidation states for CeRh₂O₅ can be assigned as Ce⁴⁺Rh³⁺₂O₅. A small variation in x was detected for Ce_2/3−xRh₂O₄ indicating a formula ranging from Ce^3.64+_0.55Rh³⁺₂O₄ to Ce^3.81+_0.525Rh³⁺₂O₄.     

Structural study of copper-nickel aluminate (CuxNi1−xAl2O4) spinels

CuAl₂O₄, NiAl₂O₄, and three ternary spinels Cu_xNi_1?xAl₂O₃ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, NiO, and Al₂O₃ at 1223 K. X-Ray powder diffractometry, coupled with adequate computational methods, allowed determination of the unit-cell length, oxygen positional parameter, and cation distribution for each compound. Interdependence of these structural parameters is closely analyzed on the ternary oxide spinels. The one-electron difference between the Cu²⁺ and Ni²⁺ ions was found to be enough to render them distinguishable by X-ray powder diffraction.     

Sur de nouveaux ferrites oxyfluorés d'yttrium ou de gadolinium à structure grenat

The oxyfluoride garnets of formula Y₃Fe_5?xM_xO_12?xF_x and Gd₃Fe_5?xM_xO_12?xF_x (M = 3d transition element) result from partial substitution of O^2? by F^? in Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ oxides. The cationic charge compensation is obtained by replacing the Fe³⁺ ions by divalent ions as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ ions. The site occupied by some of these ions (Mn²⁺, Ni²⁺, Zn²⁺) is determined by magnetic or Mössbauer measurements.     

Lithium-substituted cobalt oxide spinels LixM1−xCo2O4 (M = Co2+, Zn2+; 0 ≤ x ≤ 0.4)

Substitution of Li⁺ into Co₃O₄ and ZnCo₂O₄ gives rise to the solid solution series Li_xM_1?xCo₂O₄ (M = Co²⁺ or Zn²⁺) having the spinel structure upto x = 0.4. X-Ray diffraction intensities show that the spinel solid solutions are likely to have the following cation distributions: (Co²⁺)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄ and (Zn²⁺_1?xCo²⁺_x)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄. Electrical resistivity and Seebeck coefficient data indicate that the electron transport in these systems occurs by a small-polaron hopping mechanism.     

Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

An iets study of alumina and magnesia supported —sihx and their use as substrates for inorganic vibrational spectroscopy

Tunneling spectra of Al₂O₃/—SiH_x, MgO/—SiH_x, Al₂O₃/—SiD_x, and Al₂O₃/—SiH_x + NCS^? are reported. Analysis of the vibrational spectra observed from isotopic substitution of the barriers obtained by deposition of a thin film of SiO onto alumina and magnesia indicate that the supported species is —SiH. The Al₂O₃/—SiH barrier can be used as a support for studying inorganic ions by IETS.     

The influence of Ni2+ lons on the distribution of Mg2+ and Cu2+ lons in spinel ferrites

Cation distribution in quenched and furnace-cooled samples of composition Ni_xM_1?xFe₂O₄ (where M is either Mg²⁺ or Cu²⁺) has been studied through magnetization measurements. It has been found that cation distribution in these mixed ferrites cannot be predicted by site preference energies. In magnesium-nickel ferrites, cation distribution is controlled by heat treatment up to x = 0.5, beyond which the effect of heat treatment diminishes. Addition of Ni²⁺ ions in copper ferrite reduces the diffusibility of Cu²⁺ ions and the distribution tends toward inverse spinel in the high-nickel region.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.