Monoclinic superstructure V<Subscript>14</Subscript>O<Subscript>6</Subscript> of the tetragonal solid solution of oxygen in vanadium

The monoclinic (space group C2/m) superstructure of V₁₄O₆, which is formed in the atom-vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is studied by the methods of x-ray diffraction and symmetry analysis. It has been found that the channel of the order-disorder phase transition attributed to the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars {k ₁₋₁}, {k ₁₋₂}, and {k _1–3} of one type {k ₁}. The distribution function of the O atoms in the V₁₄O₆ monoclinic superstructure has been calculated. It has been shown that the displacements of V atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the fcc sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

Ordered monoclinic vanadium suboxide V<Subscript>14</Subscript>O<Subscript>6</Subscript>

The monoclinic (space group C2/m) superstructure of the suboxide V₁₄O₆, which is formed as a result of the atomic and vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is investigated using X-ray diffraction and symmetry analysis. The monoclinic suboxide V₁₄O₆ is observed in the vanadium oxide samples VO_0.57, VO_0.81, and VO_0.86 synthesized at 1770 K and the samples VO _y (0.87 ≤ y ≤ 0.98) additionally annealed at 1470 K after the synthesis. It is established that the channel of the disorder-order phase transition associated with the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars of one type {k ₁}. The distribution function of the oxygen atoms in the monoclinic superstructure of the suboxide V₁₄O₆ is calculated. It is demonstrated that the displacements of vanadium atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the facecentered cubic sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

V<Subscript>52</Subscript>O<Subscript>64</Subscript> tetragonal superstructure of cubic vanadium monoxide with vacancies in the metal sublattice

The atom-vacancy ordering of cubic vanadium monoxide VO_1.29, which has basis cubic structure B1 and structural vacancies in the metal sublattice, has been studied using the x-ray diffraction method. It has been shown that the formation of the tetragonal (space group I4₁/amd) ordered phase V₅₂O₆₄ of cubic vanadium monoxide VO_y proceeds as a first-order phase transition through the disorder-order channel including 22 nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₂}. The distribution function of the vanadium atoms in the V₅₂O₆₄ tetragonal superstructure has been calculated.     

The disorder-order transition in cubic vanadium monoxide with vacancies in the metal sublattice

The disorder-order transition in cubic vanadium monoxide VO_y (y = 1.29, 1.30) possessing a B1 type structure and containing vacancies only in the metal sublattice has been studied by X-ray diffraction and symmetry analyses. It is established that the formation of a tetragonal (space group I4₁/amd) ordered V₅₂O₆₄ phase in cubic vanadium monoxide VO_y proceeds in the form of the first-order phase transition via a channel involving 22 nonequivalent superstructural vectors of four stars ({k ₁₀}, {k ₄}, {k ₃}, and {k ₂}). The distribution function of V atoms in the tetragonal V₅₂O₆₄ superstructure is calculated, and it is found that the real ordered V_51.6O₆₄ phase exhibits significant atomic displacements. The boundaries of the domain of existence of the V₅₂O₆₄ phase at 54–60% O are determined in the phase diagram of the V-O system.     

Long- and short-range order in the Pd<Subscript>6</Subscript>B monoclinic superstructure and M<Subscript>6</Subscript>X<Subscript>5</Subscript> and M<Subscript>6</Subscript>X allied superstructures

Symmetry analysis of the Pd₆B monoclinic superstructure (space group C2/c) formed in the cubic (with the B1 structure) solid solution of boron in palladium (PdB _y ) has been carried out. The formation of this superstructure proceeds as a first-order phase transition via the disorder-order channel including nine nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₀}. For the Pd₆B monoclinic super-structure (space group C2/c), the distribution function for boron atoms is calculated and the interval of admissible values of the long-range order parameters is defined. It is shown that the transition channel determined in this way coincides with the channel in which the M₆X monoclinic superstructure (space group C2) is formed; therefore, the Pd₆B superstructure can also be described in space group C2 to the same degree of accuracy. The higher symmetry of the monoclinic model (space group C2/c) suggests that it describes the structure of the Pd₆B phase (Pd₆B□₅), as well as of mutually inverse phases M₆X□₅ and M₆X₅□, more adequately than the model with space group C2. It is shown that superstructures of the M₆X□₅ type (space groups C2/c, C2, C2/m, and P3₁) and inverse superstructures of the M₆X₅□ type with the same space groups have the positions of the nearest surrounding of metal atoms by two types of nonmetallic sublattice sites located in the first and second coordination spheres.     

Symmetry analysis of the monoclinic Pd<Subscript>6</Subscript>B superstructure: Long- and short-range orders

A symmetry analysis of the monoclinic (space group C2/c) Pd₆B superstructure formed in the cubic (with structure B1) boron solid solution PdB _y in palladium has been performed. The formation of this superstructure occurs as a first-order phase transition via the disorder-order transition channel including nine nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₀}. For the monoclinic (space group C2) Pd₆B superstructure, the distribution function of boron atoms has been calculated and the interval of admissible values of long-range order parameters has been determined. It has been shown that the found transition channel is identical to the channel of the formation of the monoclinic (space group C2/c) M ₆ X superstructure; therefore, the Pd₆B superstructure can be described with the same accuracy in space group C2. The higher symmetry of the monoclinic (space group C2/c) model suggests that it more accurately describes the structure of the phase Pd₆B (Pd₆B□₅) and mutually inverse phases M ₆ X□₅ and M ₆ X ₅□ than the model with space group C2. It has been demonstrated that there are two types of the nearest environment of metal atoms with non-metal sublattice sites arranged in the first and second coordination spheres in M ₆ X□₅-type superstructures (space groups C2/c, C2, C2/m, and P3₁) and in inverse M ₆ X ₅□-type superstructures with the same space groups.     

Atomic displacements in the V<Subscript>52</Subscript>O<Subscript>64</Subscript> superstructure and the short-range order in superstoichiometric cubic VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium monoxide with metal vacancies

Atomic displacements in the lattice of the tetragonal V₅₂O₆₄ superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V₅₂O₆₄ superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the superstoichiometric VO_>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MX _y (X = C, N, O) of transition metals.     

Neutron diffraction analysis of a defect vanadium monoxide close to the equiatomic vanadium monoxide

Neutron and X-ray diffraction analyses are applied to studying the defect structure of synthesis-temperature quenched and low-temperature annealed vanadium monoxides VO _y (0.90 ≤ y ≤ 0.97) close to the equiatomic monoxide VO_1.0. It is found that the monoxides VO_0.90 and VO_0.97 contain structural vacancies not only in the oxygen sublattice, but also in the metal sublattice. In addition to the cubic disordered phase VO _y with the structure B1, the monoclinic superstructure V₁₄O₆ with space group C2/m is present in the synthesized VO_0.90 sample and in the annealed VO_0.90 and VO_0.97 samples. The formation of the V₁₄O₆ superstructure is attributed to the ordering of oxygen atoms and nonmetal vacancies in the lattice of the tetragonal solid solution of oxygen in vanadium. No simultaneous ordering of metal and oxygen vacancies in two sublattices of the cubic vanadium is observed.     

Ti5O5 superstructures of cubic titanium monoxide

A cubic model is proposed for the Ti₅O₅ (Ti₅?O₅□ ≡ Ti₉₀?₁₈O₉₀□₁₈) superstructure of nonstoichiometric titanium monoxide Ti _x O _z with double imperfection. The unit cell of the cubic Ti₅O₅ superstructure has the threefold lattice parameter of the unit cell of the basis disordered B1 structure of Ti _x O _z monoxide and belongs to space group $Pm\bar 3m$ . The channel of the disorder-order transition, i.e., Ti _x O _z (space group $Fm\bar 3m$ )-Ti₅O₅ (space group $Pm\bar 3m$ ), includes 75 superstructure vectors of seven stars {k ₁₀}, {k ₇}, {k ₆₍₁₎}, {k ₆₍₂₎}, {k ₄₍₁₎}, {k ₄₍₂₎}, and {k ₁}. The distribution functions of Ti and O atoms over the sites of the cubic Ti₅O₅ superstructure are calculated. A comparison of the X-ray and electron diffraction data obtained for ordered TiO_1.087 monoxide with the theoretical simulation results supports the existence of the cubic Ti₅O₅ superstructure. The cubic (space group $Pm\bar 3m$ ) Ti₅O₅ superstructure is shown to be a high-temperature structure relative to the well-known monoclinic (space group C2/m) superstructure of the same type.     

Diffraction of electrons in the CubicTi5O5 superstructure of titanium monoxide

Annealed titanium monoxide TiO_1.087 has been studied by the electron diffraction method. A cubic model of the Ti₅O₅ superstructure (Ti₅O₅ (Ti₉₀?₁₈O₉₀??₁₈)) of nonstoichiometric titanium monoxide Ti _x O _z has been proposed on the basis of experimental data and representations about the disorder-order transition channel. It has been shown that reflections observed on the electron diffraction pattern are identified in the space group $Pm\bar 3m$ . The period of the unit cell of the cubic Ti₅O₅ superstructure is larger than that for the B1 basic disordered structure of Ti _x O _z monoxide by a factor of 3. The disorder-order transition channel Ti _x O _z (space group $Fm\bar 3m$ )-Ti₅O₅ (space group $Pm\bar 3m$ ) includes 75 superstructure vectors of seven stars {k ₁₀}, {k ₇}, {k ₆₍₁₎}, {k ₆₍₂₎}, {k ₄₍₁₎}, {k ₄₍₂₎}, and {k ₁}. The distribution functions of Ti and O atoms over the sites of the cubic Ti₅O₅ (space group $Pm\bar 3m$ ) superstructure have been calculated.     

Argentite-Acanthite Transformation in Silver Sulfide as a Disorder-Order Transition

An alternative model has been proposed for the phase transition from cubic argentite ß-Ag₂S to monoclinic acanthite α-Ag₂S in silver sulfide as a disorder–order transition. It has been shown that, as the temperature decreases below the transition temperature T_trans, S atoms equiprobably occupying the sites of the body centered cubic (bcc) nonmetal sublattice of argentite are concentrated at four sites of the monoclinic nonmetal sublattice, whereas the other sites remain vacant. A disorder-order transition channel including three superstructure vectors of k₉ and k₄ stars has been determined. The distribution function of sulfur atoms in monoclinic acanthite α-Ag₂S has been calculated. It has been shown that displacements of sulfur atoms distort the bcc nonmetal sublattice of argentite, forming a monoclinic lattice, where silver atoms are spaced by quite large distances and occupy their crystallographic positions with a probability of 1. The region of allowed values of the long-range order parameters η₉ and η₄ for the model monoclinic ordered phase α-Ag₂S has been determined.

     

Charge ordering in crystals of Pr1? x Sr x MnO3 ( x = 0.22, 0.24) ferromagnetic manganites

The structural and magnetic states of Pr_1−x Sr _x MnO₃ (x = 0.22, 0.24) manganite crystals were studied over a wide temperature range. Measurements of the magnetic susceptibility and electrical resistivity demonstrated that these manganites belong to the class of ferromagnetic semiconductors. Thermal neutron elastic scattering patterns revealed that, in the temperature range 4.2–350 K, the manganites have an orthorhombic structure (space group Pnma) with a well-pronounced cooperative Jahn-Teller effect. Major emphasis was placed on the nuclear magnetic superstructure with a wave vector q = (2π/2a, 0, 2π/2c). It was shown that this superstructure suggests 1/4-type charge ordering in the manganites under investigation. Original Russian Text ? S.F. Dubinin, S.G. Teploukhov, V.E. Arkhipov, V.D. Parkhomenko, é.A. Neĭfel’d, A.V. Korolev, N.A. Ugryumova, Ya.M. Mukovskiĭ, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 4, pp. 704–710.     

Neutron scattering by anharmonic crystals and the effect of sublattice displacements

A theory has been given for the scattering of neutrons by anharmonic crystals, for which terms of the typeV ⁽³⁾ (k _1j1; —k _1j1;o _j) which contribute to the sublattice displacements are not neglected. Using the standard perturbation theory in the interaction picture or Green’s function method, an expression has been derived for the differential scattering cross-section which brings in the shift and the width of the phonons in one-phonon energy exchange processes. It is shown that the sublattice displacements will modify the phase factor arising from the scattering by any atom in the unit cell, and the Debye-Waller factor also gets altered both by the sublattice displacements as well as by higher order terms arising from anharmonicity. It is shown that the differential scattering cross-section contains a term linearly depending on the third order anharmonicity coefficientV ⁽³⁾ (k _1j1;k _2j2;k _3j3) and neutron scattering by crystals should provide a useful method for evaluating the third order anharmonicity coefficients.     

Propene-V2O2 interactions studied by infrared emission spectroscopy

Infrared emission spectroscopy was used to investigate the interactions between propene and vanadium hemipentoxide. The spectrum obtained for V₂O₅ alone was very similar to the spectrum given by KBr disc transmission method. Reaction of propene on V₂O₅ was performed between 110 and 250°C; IR spectra were recorded in situ. Reduction of the oxide occurred and its bands were strongly altered. The bands attributed to the terminal oxygen (1018 cm^?1) and to the doubly bridged oxygen (820 cm^?1) were mainly affected by the reaction with propene. For a reaction temperature of 250°C, the recorded spectrum was close to that given by hydrogen reduction at 230°C. It can be concluded that V₂O₅ was reduced by propene with the formation of a superficial suboxide. By oxygen treatment, the reduced form was restored to the initial V₂O₅ sample. Infrared emission spectroscopy appears as a very suitable method for studying the interactions of the reactants with the catalysts.     

The cluster blocking distribution function in the spin glass transition range of FexV2O5

The cluster blocking distribution function for the iron sublattice of the iron vanadium oxide bronzes Fe_xV₂O₅ /x = 0.33?0.37/ was experimentally established in a range 4 – 36 K by means of Wohlfarth method. It shows that the blocking process starts already in the paramagnetic phase and is most intense just below T_g.     

Luminescence properties of Eu2+ in M2MgSi2O7 (M=Ca, Sr, and Ba) phosphors

The photoluminescence properties of alkali-earth magnesium silicates (M₂MgSi₂O₇, M=Ca, Sr, and Ba) doped with Eu²⁺ were investigated. Solid solutions of Ba _x Sr_2−x Si₂O₇, Ca₂MgSi₂O₇, and Sr₂MgSi₂O₇ were prepared. Ba _x Sr_2−x Si₂O₇ retained a tetragonal crystal structure similar to the structure of the other compounds up to a stoichiometry of x=1.6, which enabled a systematic study of the common structure. Monoclinic Ba₂MgSi₂O₇ was prepared, and the luminescence properties were compared with those of other samples. The emission and excitation spectra of tetragonal M₂MgSi₂O₇ (M=Ca, Sr, and Ba) changed as a function of the covalency, site symmetry, and crystal field strength. The luminescence properties showed excellent agreement with theoretical predictions based on these factors. The Stokes shift differentiated the emission behaviors of the tetragonal and monoclinic structures.     

The electric field gradient at111Cd in vanadium oxides

The electric field gradient (efg) of¹¹¹Cd in polycrystalline V₂O₅ was studied using perturbed angular correlation (PAC) spectroscopy, with the¹¹¹In activity ion-implanted at 400 keV. Between the individual steps of an isochronal annealing program, a distinct efg (v _Q 1=88.1(3) MHz, ₁=0.62(2)) was recorded the contribution of which increased with annealing temperature up to 74% at 870 K. Corresponding X-ray analysis of inactive V₂O₅ samples, which underwent the same annealing treatment, proved that the sample always stayed as V₂O₅. Since V₂O₅ has only one equivalent cation site, it is concluded that this efg belongs to¹¹¹Cd at this site. Oxidation of a vanadium foil atT=675 and 800 K at =200 mbar also yielded this efg. From PAC measurements in VO₂, two well-defined efg's were found above and below the metal-semiconductor transition at 340 K, which are tentatively attributed to the monoclinic and the tetragonal phase.Supported by Deutsche Forschungsgemeinschaft and DAAD.On leave from the University of Durban-Westville, South Africa.     

DO<Subscript>3</Subscript>-type cubic structure of nonstoichiometric vanadium monoxide

An X-ray diffraction study indicates that nonstoichiometric vanadium monoxide VO _y ≡ V _x O _z (y = z/x) has a cubic structure of the DO₃ type (space group Fm $ \bar 3 $ \bar 3 m), where vanadium atoms are not only at the 4(a) sites of the metal fcc sublattice, but also at the tetrahedral 8(c) sites. This circumstance fundamentally distinguishes monoxide VO _y from strongly nonstoichiometric MX _y compounds with the B1 structure and the same space group, where atoms M and X and structural vacancies ▪ and ▭ of the metal and nonmetal sublattices, respectively, are distributed over the 4(a) and 4(b) sites. The dependence of the filling factor q of the tetrahedral interstices by vanadium atoms on the relative content y of oxygen in VO _y has been obtained. It has been shown that the composition of cubic vanadium monoxide should be represented as VO _y ≡ V _x O _z ≡ V _{x − 2q} V_2q ^(t)▪_{1 − x + 2q} O _z ▭_{1 − z} , taking into account the structure.     

Activation energy and conductivity of glasses in the P2O5–V2O5–Li2O system

The conductivity of glasses in the 50\textP_\text2 \textO_\text5 - x\textV_\text2 \textO_\text5 - ( 50 - x )\textLi_\text2 \textO50{\text{P}}_{\text{2}} {\text{O}}_{\text{5}} - x{\text{V}}_{\text{2}} {\text{O}}_{\text{5}} - \left( {50 - x} \right){\text{Li}}_{\text{2}} {\text{O}} system was studied as a function of temperature and composition. For all compositions, the conductivity variation as a function of temperature followed an Arrhenius type relationship. Isothermal variation of conductivity as a function of composition showed a minimum for a molar ratio x near 20. Probable mechanisms for decrease of conductivity with decrease of vanadium oxide concentration were explained. The minimum in room temperature was attributed to increase of V⁴⁺/V⁵⁺ with decrease of vanadium oxide in specific concentrations of vanadium oxide. Activation energy increased with decrease of V₂O₅ content. This behavior was attributed to increase of average spacing between vanadium ions.     

Electronic conductivity and thermopower of Ca2NaMg2V3O12−x garnet

Results are reported from conductivity and thermoelectric power measurements on partially reduced Ca₂NaMg₂V₃O_12?x, with x < 5.10^?2, at temperatures of 300–1100 K. The conductivity is thermally activated with activation energies 0.26 ? E_a ? 1.28 eV for differently reduced samples. The thermopower is temperature independent in the 300–800 K region. These results are shown to be consistent with the adiabatic hopping of small polarons localised on the vanadium sublattice, where defect interactions result in the formation of multiple conduction pathways.