Electronic band structures and photovoltaic properties of MWO4 (M=Zn, Mg, Ca, Sr) compounds

Divalent metal tungstates, MWO₄, with wolframite (M=Zn and Mg) and scheelite (M=Ca and Sr) structures were prepared using a conventional solid state reaction method. Their electronic band structures were investigated by a combination of electronic band structure calculations and electrochemical measurements. From these investigations, it was found that the band structures (i.e. band positions and band gaps) of the divalent metal tungstates were significantly influenced by their crystal structural environments, such as the W-O bond length. Their photovoltaic properties were evaluated by applying to the working electrodes for dye-sensitized solar cells. The dye-sensitized solar cells employing the wolframite-structured metal tungstates (ZnWO₄ and MgWO₄) exhibited better performance than those using the scheelite-structured metal tungstates (CaWO₄ and SrWO₄), which was attributed to their enhanced electron transfer resulting from their appropriate band positions.     

Investigation of s, p, d-element Niobates and Their Solid Solution Formation Processes by Thermal Analysis

The binary and ternary systems M'O(M'CO₃)-Nb₂O₅ and M'O(M'CO₃)-MO-Nb₂O₅, where M'=Ca,Sr and Ba and M=Cu, Ni, Cd, Zn and Pb, were investigated by means of thermal analysis in the temperature range 20–1500°C. The boundaries of stability of the solid solutions Sr_2−xMe_xNb₂O₇,Sr_2−xM_xNb₂O₇, Sr_4−xM_xNb₂O₉ and Sr_6−xM_xNb₂O₁₁ were determined by means of X-ray diffraction, and IR and Raman spectroscopy. The possibility of prognostication of the phase fields of stable solid solutions by calculation from the diagrams of the ‘comparative electronegativity of atoms vs. tolerance factor’ was demonstrated. The kinetic parameters of the interactions in theSrCO₃+MO+Nb₂O₅ powder mixtures were established. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fe x Ti1−2x M x O2 (M=Nb,Ta) rutile solid solutions from gels

In this study, Fe _x Ti_1–2x M _x O₂ (M=Nb, Ta) rutile solid solutions have been synthesized from gels made from Fe(III) acetylacetonate, NbCl₅, TaCl₅, Ta(V) ethoxide, TiCl₄ and Ti(IV) isopropoxide. The results obtained are compared with those obtained by the ceramic method. The solid solutions synthesized from gels were obtained at lower temperatures than these synthesized by the ceramic method.     

Study of the Solid Solutions of LiNbO3and LiTaO3with Mn

Two unusual, extensive new solid solutions of LiNbO₃and LiTaO₃with MnO have been prepared, where 4Mn²⁺replace a combination of 3Li⁺and a pentavalent cation: Nb⁵⁺or Ta⁵⁺. The formulas are Li_1−xM_1−xMn_4xO₃, 0<x<0.13, forM=Nb and 0<x<0.23 forM=Ta. The solid solutions were characterized by X-ray powder diffraction and density measurements. The manganese oxidation states were determined by X-ray photoelectron spectroscopy.     

A correlation between the structure and some physical properties of binary molybdates (Tungstates) of uni- and bivalent metals

General principles of arrangement are discussed and analyzed for 13 known structural types (families) of binary molybdates and tungstates of uni-and bivalent metals, which were classified according to the degree of condensation of M²⁺O_n polyhedral and MoO₄ tetrahedral constructions. The structural features of some phases correlate with their ferroelectric, ion-conducting, and luminescent (laser) properties. We suggest crystallochemical criteria and mechanisms of distortion (ferroelectric) phase transitions in palmierites, langbeinites, and Rb₄Mn(MoO₄)₃ and show the paths of ionic transport in phases with NaCo₂₃₁(MoO₄)₃, alluaudite, and Na₂Mg₅(MoO₄)₆ structures. On the basis of the analysis performed, we outlined the compounds that are the most promising for the development of inorganic materials with efficient physical properties. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 6, pp. 145–157, November–December, 1994. Translated by T. Yudanova     

Phase relationships of the lithium halide spinels Li2MCl4-Li2MBr4 with M = Mn,Fe, Cd

The sections Li₂MCl_4?4xBr_4x of the quaternary systems LiCl-LiBr-MCl₂-MBr₂ with M = Mn, Cd, and Fe were studied by high-temperature X-ray diffraction patterns and DTA and DSC measurements. In the quasibinary lithium manganese halide system complete series of solid solutions exist between the inverse spinels Li₂MnCl₄ and Li₂MnBr₄. Li₂MnBr₄ and solid solutions with x > 0.54 undergo phase transitions to tetragonal spinels at lower temperatures. In the nonquasibinary system with M = Cd, only at temperatures near 400°C a complete series of mixed crystals is formed. At lower temperatures the system is mainly two-phase with rock salt-type Li_1?yCd_0.5yCl_1?xBr_x and cadmium chloride-type Cd_1?yLi_2yCl_2?2xBr_2x solid solutions in equilibrium. The lithium iron halide system is similar to that of cadmium, but spinel-type Li₂FeBr₄ does not exist at any temperature. The manganese and cadmium halide spinels and spinel solid solutions undergo phase transitions to NaCl defect structures at elevated temperatures.     

Electrical Properties of Molybdates in the Systems M2MoO4-AMoO4-Zr(MoO4)2

The temperature dependence of conductivity of molybdates in the systems M₂MoO₄-AMoO₄-Zr(MoO₄)₂ was studied. The starting molybdates and molybdates of 5 : 1 : 3 and 1 : 1 : 1 compositions, formed in the systems, exhibit mixed conduction, turning ionic at high temperatures. The sharp bends in linear portions of the temperature dependence of conductivity coincide with the temperatures of polymorphic transitions in the corresponding molybdates.     

Negative thermal expansion in rare earth molybdates

Negative thermal expansion in rare earth molybdates of A₂Mo₃O₁₂ family (A=Y, Er, Yb and Lu) is measured by high temperature X-ray diffraction and dilatometry. Rare earth molybdates which are isostructural with the corresponding rare earth tungstates, also exhibit this phenomena attributed to transverse acoustic vibrations. The rare earth molybdates of A₂M₃O₁₂ family with an orthorhombic structure (A=Y, Er, Yb and Lu) are highly hygroscopic and exhibit negative thermal expansion after the complete removal of water molecules. Axial thermal expansion co-efficient calculated from high temperature X-ray diffraction (RT-1073K) shows rare earth size effect. As the ‘A’ cation decreases in size, the thermal expansion co-efficient along ‘b’ axis and the linear thermal expansion co-efficient become less negative. The thermal expansion behaviour of the tetragonal La₂Mo₃O₁₂ is also reported to demonstrate the effect of crystal structure.     

Phase formation in Li2MoO4-Rb2MoO4-MMoO4 (M = Ca, Sr, Ba, Pb) systems and the crystal structure of α-Rb2Pb(MoO4)2

A subsolidus triangulation of Li₂MoO₄-Rb₂MoO₄-MMoO₄ (M = Ca, Sr, Pb, Ba) systems is performed. The RbLiMoO₄-Rb₂M(MoO₄)₂ (M = Pb, Ba) joins, where 11 mol.% long Rb₂M(MoO₄)₂-based solid solutions are found, are studied in most detail. Ternary molybdates do not form in the systems, which is confirmed by spontaneous flux crystallization. The α-Rb₂Pb(MoO₄)₂ crystals are obtained and their crystal structure is solved (a = 20.9724(15) ?, b = 12.1261(8) ?, c = 16.1171(10) ?, β = 115.728(13)°, C2/m space group, R = 0.0695, Z = 16), which is a monoclinic superstructure of the palmierite type and has the largest cell volume and the most complex structure among lead-containing palmierites. One of the MoO₆ tetrahedra is orientationally disordered over two sites; lead atoms are shifted from the centers of their coordination polyhedra to one of their faces and have cn = 6–8; for rubidium cations cn = 10–12.     

Zum Chemischer Transport ternärer Übergangsmetall‐ und Erdalkaliwolframate MWO4 mit Chlor

On the Chemical Vapor Transport of Ternary Transition Metal‐ and Earth Alkaline Tungstates MWO₄ with Chlorine The chemical vapor transport of transition metal tungstates MWO₄ (M=Mn, Co, Ni, Cu, Zn, Cd) was investigated in dependence on mean transport temperature (923 K to 1223 K) and amount of transport agent Cl₂. All tungstates migrate in a temperature gradient ΔT = 100 K from the region of higher temperature to the lower temperature with migration rates of 0.5 to 8 mg/h depending on experimental conditions. The transport behaviour was determined by continuous measurement of mass change during the transport experiments. The results were compared to thermo chemical calculations and the influence of moisture content discussed in detail. MgWO₄ migrates under the influence of Cl₂ in a temperature gradient 1273 K to 1173 K (migration rate 0.7 mg/h), CaWO₄ and SrWO₄ in a temperature gradient 1423 K to 1323 K (migration rate <0.1 mg/h).     

Whitlockite solid solutions Ca9?xMxR(PO4)7 (x = 1, 1.5; M = Mg, Zn, Cd; R = Ln, Y) with antiferroelectric properties

Whitlockite solid solutions Ca_9−x M _x R(PO₄)₇ (M = Mg, Zn, Cd; R = Ln, Y) were synthesized as powders and ceramics using solid-phase synthesis. Dielectric investigations and second harmonic generation (SHG) tests showed that ferroelectric (FE) phase transitions existing in samples with x = 0 change to antiferro-electric (AFE) transitions between two centrosymmetrical phases in samples with x = 1 or 1.5. The calcium-ion solid electrolyte conductivity in Ca_9−x M _x R(PO₄)₇ at high temperatures appears either as a result of an antiferroelectric-paraelectric (AFE-PE) phase transition (for x = 1) or as a result of a separate phase transition near 1173 K (for x = 1.5). The appearance of dielectric properties in whitlockites is discussed with reference to the features of their polar and centrosymmetrical structures. Original Russian Text ? A.V. Teterskii, S.Yu. Stefanovich, B.I. Lazoryak, D.A. Rusakov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp 357–363.     

Solid-phase chemical transformations in molybdate systems

The ternary salt systems M₂MoO₄−AMoO₄−Zr(MoO₄)₂ (M=K, Tl; A=Mg, Mn, Ni, Co, Cu, Zn, Cd) in a sub-solidus region were studied. New ternary molybdates with the M₅A_0.5Zr_1.5(MoO₄)₆ and MA_0.5Zr_0.5(MoO₄)₂ compositions were synthesized by solid-phase reactions in these systems. The crystallographic and thermal characteristics of the compounds were found. The electrical properties of potassium—manganese—zirconium molybdates were studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1036–1039, June, 1999.     

Contribution a l'etude des systemes Ca3(PO4)2-MSO4 (M=Sr,Ba)

In the systems Ca₃(PO₄)₂-MSO₄ (M = Sr, Ba), the series of single phase Ca_21?3xM_2xI(PO₄)_14?2x(SO₄)_2x with 0<x<0.15 forM=Sr and 0<x<0.1 forM = Ba have been prepared. These solid solutions, respectively strontium phosphosulfate and barium phosphosulfate, are isostructural with anhydrous tricalcium orthophosphate. They have been characterized by their infrared spectra and their crystallographic unit cell parameters.     

Phase equilibria,charge transport,and mass transport in Sr<Subscript>4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-“M<Subscript>4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>” (M = Cd,Cu, Ni,and Zn) systems

Homogeneous fields of Sr_{4 − x} M _x Nb₂O₉ (M = Cd, Cu, Ni, or Zn) solid solutions were determined using powder X-ray diffraction. Phase fields were plotted proceeding from the tolerance factor t and electronegativity ratio $ \bar k_A /\bar k_B $ \bar k_A /\bar k_B with a satisfactory fit of experimental results. Thermogravimetry was used to establish the major kinetic laws of solid-phase synthesis (conversion, rate-controlling stage, and effective activation energy) in (4 − x)SrCO₃ + xMO + Nb₂O₅ powdery mixtures. Direct radiometry was used to determine ⁹⁰Sr, ⁶³Ni, and ⁶⁵Zn self-diffusion coefficients in solid solutions based on the Sr₄Nb₂O₉ phase. Electrical conductivity was measured as a function of temperature for all Sr₄Nb₂O₉-“M₄Nb₂O₉” samples. The conductivity of Sr_{4 − x} M _x Nb₂O₉ (M = Cd, Cu, Ni, or Zn) solid solutions has a mixed ion-electron character.     

Thermodynamics of double oxides I. Galvanic-cell study of strontium tungstates and aluminates

Electrochemical cells employing La₂O₃ and CaO doped thoria and CaF₂ as solid electrolytes were used to determine the standard Gibbs energies of formation of some tungstates and aluminates of strontium in the range of 1100 to 1400 K. Enthalpies of formation and entropies of Sr₃WO₆, Sr₂WO₅, SrWO₄, Sr₃Al₂O₆ and SrAl₂O₄ have been calculated. Concordance of the results from cells with different combinations of electrodes confirms the reliability of the thermodynamic results.     

Electroconduction of Solid Solutions Na3 – 2x M x PO4(M = Cd, Pb)

In the sodium-orthophosphate-based solid solutions in Na_{3 – 2x} M _x PO₄ systems (M = Cd, Pb), the electroconduction is maximum near the upper concentration boundaries of the single-phase regions: x 0.4 for M = Cd and x 0.25 for M = Pb. The conductivity values at 300°C are 6.25 × 10^–3 and 2.5 × 10^–3 S/cm, respectively. The conduction of synthesized solid electrolytes has a co-cation nature. Their electric characteristics, inferior to those of the Na₃PO₄-based solid solutions obtained via heterovalent substitutions of another type, may be a manifestation of an effect similar to the polyalkali effect.     

Co-cation conduction of the (Na1-x K x )3.8M0.1P2O7 (M = Ca, Sr, Cd) solid solutions

Samples of systems (Na _1-x K _x )_3.8M_0.1P₂O₇ (M = Ca, Sr, Cd; 0 ≤x ≤ 1) are synthesized. It is established that solid solutions with the structure of γ-K₄P₂O₇ form in these systems. At room temperature, the solutions exist atx ≥ 0.7. In the calcium-containing system, they form a continuous series at temperatures exceeding ∼500°C. The co-cation character of conduction of the solutions is confirmed by measured transport numbers. The concentration dependences of the electroconductivity and its activation energy point to a polyalkali effect, which decreases with an increase in the M²⁺ ion radius. Partial conductivities of sodium and potassium cations in the (Na _1-x K _x )_3.8Ca_0.1P₂O₇ system are calculated and their dependences on the balance between alkali cations in the solid solution are discussed     

Low temperature heat capacities of mechanically alloyed La-doped PbWO4 system

Heat capacity measurements were carried out on Pb_1-xLa_xWO_4+x/2 (x=0.2) and Pb_1-xLa_2x/3WO₄ (x=0.2, 0.5) solid solutions prepared by sintering and mechanical alloying (MA) methods. For all the solid solutions, sintered samples showed slightly larger heat capacity around 100 K in comparison with MA samples, which was presumably caused by the excitation of mobile oxide ion motion. For sintered scheelite-type structured PbWO₄s, high-temperature synthesis introduced oxide ion interstitials even for the Pb_1-xLa_2x/3WO₄ system, which resulted in the excess heat capacity at low temperature for excitation. On the other hand, for the samples prepared by room-temperature MA technique, oxide ion seemed to occupy the regular sites rather than interstitial ones and excess heat capacities were not observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis of polycrystalline materials of SrWO4 and growth of its single crystal

The polycrystalline materials of SrWO₄ were synthesized by means of a solid phase reaction with analytical purity SrCO₃ and WO₃ at high temperature. The transparent SrWO₄ single crystal with dimension of ϕ 22 mm×40 mm has been successfully grown along a-axis by Czochralski method. X-ray powder diffraction results show that the as-grown SrWO₄ single crystal belongs to tetragonal system and I4₁/a space group. The measured density of SrWO₄ is 6.439 g·cm⁻³ by buoyancy method. The effective segregation coefficients of W and Sr elements in SrWO₄ single crystal are close to 1 by the X-ray fluorescence method. __________ Translated from Journal of Shandong University (Natural Science), 2005, 40(4) (in Chinese)     

Phase diagrams in the quaternary systems M1−xCuxCr2X4 with M = Cd,Mn, and X = S,Se

The phase diagrams of the spinel systems Cd_1?xCu_xCr₂S₄, Cd_1?xCu_xCr₂Se₄, and Mn_1?xCu_xCr₂S₄ have been studied on the basis of X-ray powder photographs of quenched samples and high-temperature X-ray diffraction patterns. At room temperature the mutual solid solubilities of the metallic copper and the semiconducting cadmium and manganese spinels are only small (x < 0.05 and >0.95). The interchangeability, however, increases largely with increasing temperature. Complete series of mixed crystals, as in the Zn_1?xCu_xCr₂X₄ (X = S, Se) systems, however, are not formed. The solid solutions with x > 0.07 and <0.95, x > 0.095 and <0.90, and x > 0.36 and <0.87, respectively, formed at higher temperatures cannot be quenched to room temperature without decomposition. The unit cell dimensions of the spinel solid solutions studied obviously do not obey Vegard's rule.