Alkali metal suboxides: Intermediates between salts and metals

Rubidium and cesium form stable metal-rich oxides (suboxides) which have been investigated by thermal and structural analysis. The compounds Rb₉O₂, Rb₆O (? Rb₉O₂Rb₃), Cs₁₁O₃, Cs₄O (? Cs₁₁O₃Cs₁₀), Cs₁₁O₃Rb, and Cs₁₁O₃Rb₇ contain the characteristic ionic clusters Rb₉O₂ and Cs₁₁O₃. A simple model for the chemical bonding in alkali metal suboxides is described and proved by measuring the electrical properties and optical reflectivities as well as photoelectron spectra of these compounds. It is found that cesium suboxides play an important role in widely used photocathodes and image converters of type S1. The IR sensitivity of these devices is explained on the basis of surface plasmon-enhanced photoemission.     

Synthesis, V K β5β″ spectra, and magnetic properties of M4 ± x V6O16 ± x · n H2O (M = K, Rb, Cs) polyvanadates

Tetragonal polyvanadates M_{4 ± x} V₆O_{16 ± x} · nH₂O (M = K, Rb, Cs) have been synthesized under hydrothermal conditions. According to the VKβ₅β″ spectra of the hydrates, vanadium atoms have an average valence state (the V⁵⁺ ⇄ V⁴⁺ charge-fluctuation frequency is higher than 10⁻¹⁵ s). After dehydration, the phases do not change their crystal system. The thermal properties of the compounds have been studied in air and under an inert atmosphere. K_4.2V₆O_16.2 and Rb_4,1V₆O_16.1 melt congruently at 720 and 690°C, respectively. Cs_3.8V₆O_15.8 melts incongruently at 675°C. The magnetic susceptibility of all phases obeys the Curie-Weiss law in the range from 77 to 673 K. Original Russian Text ? V.L. Volkov, N.V. Podval’naya, V.M. Cherkashenko, S.N. Nemnonov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 8, pp. 1272–1276.     

Crystal chemistry of the (Ta6Si4O26)6− and (Ta14Si4O47)8− frameworks

The range of chemical flexibilities of the hexagonal frameworks (Ta₆Si₄O₂₆)^6? and (Ta₁₄Si₄O₄₇)^8? have been partially explored. This has been done with high-temperature preparations as in general ionic mobilities in these frameworks are too low to permit low-temperature ion exchange. Ionic site potential calculations indicate that preferential site-occupancy factors as well as geometric constraints are responsible for the absence of ionic motion. New phases K_6?xNa_xTa₆Si₄O₂₆ (x ? 4), K_8?xNa_xTa₁₄Si₄O₄₇ (x ? 5), and impure Ba_3?xNa_2xTa₆Si₄O₂₆ have been prepared. Introduction of up to 2 moles of Li⁺ and 1 mole of Mg²⁺ ions per formula unit into sites of the framework not normally occupied has been demonstrated as well as the possibility of partially substituting Zr⁴⁺ for Ta⁵⁺ ions. Substitutions designed to introduce large tunnel vacancies in the presence of only monovalent K⁺ or Na⁺ ions (P for Si, W for Ta and F for O) generally proved unsuccessful. Competitive phases also frustrated attempts to substitute either the larger Rb⁺ or the smaller Li⁺ ions into the large-tunnel sites. A large area of solid solution was discovered in the BaONa₂OTa₂O₅ phase diagram; it has a (TaO₃)-framework with the structure of tetragonal potassium tungsten bronze.     

Magnetic and spectroscopic investigation of partially reduced vanadium pentoxides. II. β′-CuxV2O5

The magnetic, electron paramagentic resonance (EPR), infrared (ir), and optical properties of β′-Cu_xV₂O₅ have been measured and interpreted. Magentic susceptibility studies indicate that β′-Cu_xV₂O₅ undergoes a semiconductor → metal transition near x = 0.60. For x ? 0.40, the magnetic data interpreted in terms of a ligand-field model in which the octahedral ²T_2g term of V⁴⁺ is split by the combined perturbations of axial distortion and spin-oribt coupling, with the result that the ²T_2g term is split into a magnetic ground level, a weakly magentic intermediate level, and a magnetic highest level. The results of the magnetic analysis further support the transition to metallic behavior with increasing x. The EPR spectra are motionally narrowed by electronic hopping at low temperatures, and the g-tensor and linewidth data are in good agreement with the magnetic results. The ir spectra are independent of x and exhibit narrow bands at 1020 and 995 cm^?1, which are attributed to the stretching vibration of multiple VO bonds. The optical spectra consist of two main bands whose peak positions shift to higher frequencies with increasing x, implying that the V⁴⁺O bond distances decrease with increasing x. The results of this study are in excellent agreement with Goodenough's interpretation of β-M_xV₂O₅.     

New cesium titanate layer structures

A phase study of the Cs₂OTiO₂ system in the composition range 75–100 mole% TiO₂ and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs₂Ti₅O₁₁ and Cs₂Ti₆O₁₃. The former compound undergoes a reversible hydration reaction below 200°C to form Cs₂Ti₅O₁₁ · (1 + x)H₂O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H₂O) packing between the layers. In Cs₂Ti₆O₁₃, the layers are continuous in two dimensions, whereas in Cs₂Ti₅O₁₁ and Cs₂Ti₅O₁₁ · (1 + x)H₂O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.     

Neues über Zweikernige Oxoferrate(II)

News about Binuclear Oxoferrates(II) [1] . By “reaction with the wall” of the Fe-cylinders used here we synthesized the new oxoferrates(II) Cs₆[Fe₂O₅], Cs_3.5Rb_2.5[Fe₂O₅] and Rb₄K₂[Fe₂O₅] in the form of red single crystals. The structure elucidation via four-circle-diffractometer data shows that the new oxoferrates(II) are isotypic with Cs₂(Cs_0.35K_1.65)K₂ [Fe₂O₅]. In the structure we have isolated binuclear groups [(O1)₂Fe—O(2)—Fe(O1)₂]^6?. Structure refinements is possible in the centrosymmetrial space group C2/m as well as in the space groups C2 and Cm without centre of symmetry. The existence of two further oxoferrates(II) Cs_6?xRb_x[Fe₂O₅] and Cs_6?xK_x[Fe₂O₅] which can be described as solid solutions was confirmed by power-data.     

Electrical and magnetic properties and homogeneity ranges of mixed-valence cesium-vanadium oxides

Single crystals of CsV₂O₅, Cs₂V₅O₁₃, Cs_xV₂O₅, and CsV₃O₇, grown from melts, have been subjected to electrical conductivity and magnetic susceptibility measurements. CsV₂O₅ and Cs₂V₅O₁₃ are insulators exhibiting Curie-Weiss' law behavior with p_eff close to $\text{1.73}\text{V}{}^{\mathrm{4+}}\text{ion}$. They are stoichiometric according to the X-ray studies. CsV₂O₅, which has a very limited composition range (x ≈ 0.30?;0.33), is a semiconductor with temperature-dependent paramagnetism ($\text{p}{}_{\mathrm{<mtext>eff</mtext>}}\text{=}\text{1.83}\text{V}{}^{\mathrm{4+}}\text{ion}$). Cs_xV₃O₇ (0.30 ? x ? 0.40) is antiferromagnetic with a Néel temperature of ≈230 K. The temperature variation of the lattice parameters of Cs_0.33V₃O₇ has been determined with a Bond-type diffractometer. The same crystal was used for the conductivity measurements, and together these studies indicate that the transition from the semiconducting antiferromagnetic state to the semiconducting paramagnetic state takes place stepwise. The observed properties are discussed with reference to the crystal structures of the compounds.     

Magnetic interactions in ternary cobalt oxides with cubic perovskite structure

Magnetic properties were measured on the cubic perovskite systems SrCoO_3?δ, (La_1?xSr_x)CoO₃ (0.5 ≦ x ≦ 1.0), and Sr(Co_1?xMn_x)O₃ (0 ≦ x ≦ 1.0). It is found that S²⁺ and La³⁺ ions strongly affect the spin state of the Co²⁺ ion and that the Mn⁴⁺ ion located at the octahedral site affects the spin state of Co⁴⁺ ion. The magnetic properties (T_c, T_θ, and σ) are explained by the magnetic interaction Co³⁺OCo³⁺, Co³⁺OCo⁴⁺, Co⁴⁺OCo⁴⁺, Mn⁴⁺OMn⁴⁺, and Mn⁴⁺OCo⁴⁺ in these systems.     

Thermal reactivity and1H NMR spectroscopy of Sr1−x H2x V6O16 · aq

The results of study of the thermal reactivity and¹H NMR spectroscopy of solid polyvanadates of general formula Sr_1?x H_2x V₆O₁₆ · aq are presented. Compounds withx=0 (a),x ∈ (0.3–0.6) (b) andx=1 (c) were studied. The protons are bonded in V - OH (b, c) and V - O ... H (a, b, c) groups, in H₂O molecules (a, b, c) and in H₂O ... H₂O systems (a, b, c). Dehydration of the studied compounds proceeds stepwise. Total dehydration causes decomposition of the original structures and Sr(VO₃)₂, SrV₁₂O₃₀ and V₂O₅ are formed. The results confirm the role of crystal water in stabilizing the structures of the studied compounds.     

Phase formation in the system Zn(VO<Subscript>3</Subscript>)<Subscript>2</Subscript>–HCl–VOCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

The phase and chemical compositions of precipitates formed in the system Zn(VO₃)₂–HCl–VOCl₂–H₂O at pH 1?3, molar ratio V⁴⁺: V⁵⁺ = 0.1?9, and 80°C were studied. It was shown that, within the range 0.4 ≤ V⁴⁺: V⁵⁺ ≤ 9, zinc vanadate with vanadium in a mixed oxidation state forms with the general formula Zn_xV⁴⁺ _yV⁵⁺ _2-yO₅ ? nH₂O (0.005 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.3, n = 0.5?1.2). Vanadate Zn_xV₂O₅ ? nH₂O with the maximum tetravalent vanadium content (y = 0.30) was produced within the ratio range V⁴⁺: V⁵⁺ = 1.5?9.0. Investigation of the kinetics of the formation of Zn_xV₂O₅ ? nH₂O at pH 3 determined that tetravalent vanadium ions VO2+ activate the formation of zinc vanadate, and its precipitation is described by a second-order reaction. It was demonstrated that, under hydrothermal conditions at pH 3 and 180°C, zinc decavanadate in the presence of VOCl₂ can be used as a precursor for producing V₃O₇ ? H₂O nanorods 50–100 nm in diameter.     

Selectivity for Cs and Sr in Nb-substituted titanosilicate with sitinakite topology

The 25% niobium substituted crystalline titanosilicate with the composition Na_1.5Nb_0.5Ti_1.5O₃SiO₄·2H₂O (Nb-TS) was synthesized under hydrothermal conditions. Its selectivity for radioactive ¹³⁷Cs and ⁸⁹Sr was compared with the TS, Na₂Ti₂O₃SiO₄·2H₂O, having sitinakite topology. The Nb-TS shows significantly higher uptake value for ¹³⁷Cs but lower for ⁸⁹Sr than the TS. To investigate the origin of selectivity, the ion exchanged Cs⁺ and Sr²⁺ forms with the composition, Cs_xNaH_yNb_0.5Ti_1.5O₃SiO₄·zH₂O (x=0.1, 0.2 and 0.3, x+y=0.5 and z=1-2) and Sr_0.2Na_0.6H_0.5Nb_0.5Ti_1.5O₃SiO₄·H₂O, respectively, were structurally characterized from the X-ray powder diffraction data using the Rietveld refinement technique. Simultaneously the kinetics of ¹³⁷Cs and ⁸⁹Sr uptake was investigated for the Nb^V free and doped samples. While the Cs⁺ and Sr²⁺ exchanged form of Nb-TS and the Cs⁺ exchanged form of TS retain the symmetry of the parent compound, the Sr²⁺ exchanged form of TS undergoes a symmetry change. The differences in the uptake of Cs⁺ and Sr²⁺ result from the different coordination environments of cesium and strontium in the eight-ring channel, that result from various hydration sites in the tunnel. The origin of selectivity appears to arise from the higher coordination number of cesium or strontium. Other effects due to Nb^V substitution are reflected in the increase of both, the a- and c-dimensions and thus the unit cell volume, and the population of water vs. Na⁺ in the channel to charge-balance the Nb⁵⁺↔Ti⁴⁺ substitution.     

Composition and formation kinetics of sodium polyvanadates in vanadium(IV, V) solutions

We study how VO²⁺ ions affect the composition and formation kinetics of polyvanadate precipitates in solutions with 1 ≤ pH ≤ 3 at 80–90°C. The compounds have the general formula Na_2.1?x H_xV _y ⁴⁺ V _12?y ⁵⁺ O_31?δ. nH₂O (0 ≤ x ≤ 1.1, 0.2 ≤ y ≤ 2.3, 0.1 ≤ δ ≤ 1.4). The maximal vanadium(IV) concentration in the precipitates y = 2.2 and 2.3) is achieved for the V⁴⁺/V⁵⁺ ratio in the solution equal to 0.5 and 0.3 and pH of 1.7 and 3.0, respectively. The polyvanadate precipitation at pH 1.7 has a long induction period, which is not observed when V⁴⁺/V⁵⁺ > 0.02. In the solutions with pH 3.0, the precipitation occurs only when VO²⁺ are added. The processes are controlled by second-order reactions on the polyvanadate surface.     

Solid reactions in Rubidium Carbonate-Vanadium Pentoxide system

The reactions between rubidium carbonate and vanadium pentoxide were performed at different high temperatures. Four reaction products of the compositions: I. Rb₂O · V₂O₅; II. 2 Rb₂O · V₂O₅; III. 3 Rb₂O · V₂O₅; and IV. Rb₂O · 4 V₂O₅ were obtained. According to the determination of Rb and the X-ray powder photographs of the products the existence of rubidium metavanadate (RbVO₃) and rubidium pyrovanadate (Rb₄V₂O₇) was confirmed. On the other hand, the preparation of a pure rubidium orthovanadate (Rb₃VO₄) and rubidium tetravanadate (Rb₂O · 4 V₂O₅) was not successful. The diffraction pattern of Rb₂O · V₂O₅ obeys hexagonal indexing with lattice dimensions a = 7.347 and c = 13.608 Å.     

Isomorphism in the KTaWO<Subscript>6</Subscript>-RbTaWO<Subscript>6</Subscript>-CsTaWO<Subscript>6</Subscript> system

Synthetic procedures have been developed and compounds of composition K _x Rb _y Cs _z TaWO₆ (x + y + z = 1) have been obtained. Their structure has been investigated by X-ray diffractometry. It has been shown that a continuous series of solid solutions is formed in the ternary system under study. Thermal decomposition of A^ITaWO₆ compounds (A^I = K, Rb, Cs) has been investigated by high-temperature X-ray diffractometry.     

Über „Lithovanadate”︁: Zur Kenntnis von Rb2[LiVO4] und Cs2[LiVO4]

On ?Lithovanadates”?: Rb₂[LiVO₄] and Cs₂[LiVO₄] By heating of well ground mixtures of the binary oxides [A₂O, Li₂O, V₂O₅, A : Li: V = 2.2 : 1.1 : 1.0 (A = Rb, Cs); Ni-tube, 750° 25 d] we obtained Rb₂[LiVO₄] and Cs₂[LiVO₄] colourless, orthorhombic single crystals. We found a new type of ?Lithovanadate”?-structure: space group Cmc2₁; a = 587.9(1), b = 1170.1(1), c = 793.3(1) pm, Z = 4 (A = Rb) bzw. a = 610.5(1), b = 1222.6(3), c = 815.5(2) pm, Z = 4 (A = Cs). The structure was determined by four-circle diffractometer data [MoKα -radiation; 997 from 1157 I₀(hkl), R = 7.75%, R_w = 5.54% (A = Rb); 686 from 686 I₀(hkl), R = 6.97%, R_w = 4.20% (A = Cs)] parameters see text. The Madelung part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, have been calculated.     

Electronics structures of Rb,Cs and some of their metallic oxides studied by photoelectron spectroscopy

Photoemission measurements with He and Ne resonance lines and Al Kα radiation are reported on bulk samples of the alkali metals Rb, Cs, their suboxides Cs₇O, Cs₁₁O₃ and (Cs₁₁O₃)Rb₇. For comparison, the Hel spectrum of the “normal” oxide Cs₂O is added. The occurrence of ionic clusters in a metallic matrix is typical for the suboxides. Binding energies, Auger transitions, and electron concentrations are discussed. The spectra of the suboxides show a narrow non-bonding oxygen 2p band at 2.7 eV. Different binding energies are found for Cs atoms in the clusters and for the atoms in the metallic regions of (Cs₁₁O₃)Cs₁₀. The compound Cs₁₁O₃ consists of ionic [Cs₁₁O₃]^5? clusters, which are bound by 5 free electrons in accordance with the chemical bond model.     

Substructure of a rubidium-ion-exchanged form of the silicate nepheline hydrate I

X-Ray structure analysis of a nepheline hydrate I crystal, Rb⁺-exchanged at 80°C, was performed making use only of the main diffractions. The resulting substructure was found to be orthorhombic with a = 8.0802(8), b = 15.259(2), c = 5.1584(5) Å, V = 636.0ų, space group Pnm2₁. Fourier and least-squares techniques gave the residuals R = 0.048 and R_w = 0.058, and a tentative formula of RbNa₂Al₃Si₃O₁₂ · H₂O (Z = 2, D_c = 2.65 g cm^?3). Tetrahedral distances were consistent with Al,Si alternation in the framework. Of the channel species, Na(1) and Na(2) were found not to be exchangeable at the current temperature. These sodium atoms are located in the small cages, formed by 6-rings of O atoms which connect the 8-ring channels parallel to c into two-dimensional pore systems. In the larger tunnels the replacement was complete and these contain a Rb⁺ ion and probably a water molecule in symmetry-related positions. According to this model, Rb⁺ coordinates four oxygens of an 8-ring and two water molecules, with RbO distances in the range 2.81–3.36 Å. Additional O atoms are found at greater distances.     

Metall-Koordinationsverbindungen aus Essigsäure. I Chlorometallate(III) des Eisens,Chroms und Vanadiums

Metal Coordination Compounds Prepared in Acetic Acid. I. Chlorometalates(III) of Iron, Chromium, and Vanadium Ternary chloride-hydrates A₂MCl₅ · H₂O (A = Cs, Rb, (K)) can be precipitated with HCl from solutions of MCl₃ · 6 H₂O, (M = Fe, Cr, V) and alkali metal acetates in acetic acid. Under special conditions also compounds of the composition Cs₃MCl₆ · H₂O can be obtained. After dehydration of the solutions with acetyl chloride, anhydrous compounds are formed: Cs₃Fe₂Cl₉; A₃CrCl₆ and A₃Cr₂Cl₉ with A = Cs, Rb; Cs₃VCl₆ and Cs₃V₂Cl₉. V^III is partially oxidized to V^IV by an excess of acetyl chloride. Compounds A₂VCl₆ with A = Cs, Rb can be obtained more conveniently by the reaction of VOCl₂ · H₂O in acetic acid with acetyl chloride. The lattice parameters of some compounds were determined from powder patterns in analogy to known structure families.     

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe_1−xCo_xO_3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO_3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe³⁺OCo⁴⁺↔Fe⁴⁺OCo³⁺. This model is strongly supported by ⁵⁷Fe Mössbauer measurements which show the existence of two sites at room temperature, high spin localized Fe⁴⁺ and delocalized Fe^3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO₃ structure.