A new compound K2Cr8O16 with hollandite type structure

A new compound, K₂Cr₈O₁₆ with the hollandite-type structure was synthesized under high temperature-pressure conditions ranging from 1100° to 1250° C and from 55 to 65 kbars. Its crystal structure was classified as C³₂h - C2/m with the lattice parameters a=13.820, b=2.941, c=9.772 A and β=135°. Ferromagnetism was observed in this compound whose Curie temperature was 225 K and magnetic moment was approaximately 18 μ_B.     

Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba₃Cr₂O₈ and Sr₃Cr₂O₈. For Ba₃Cr₂O₈, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature T_JT = 70 K. However, no anomaly in the ε′(T) curve is observed near T_JT = 275 K for the isostructural compound Sr₃Cr₂O₈. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr⁵⁺ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.     

New compounds A2−xV8O16 (A = K,Tl) with hollandite type structure

K₂V₈O₁₆, K_1.8V₈O₁₆ and Tl_1.74V₈O₁₆ were synthesized by high pressure- high temperature treatment of KVO₃ and TlVO₃ in a modified Belt-type apparatus. The compounds crystallize tetragonal, space group I4/m. They are the first well-characterized Vanadium-hollandites and show a remarkable high occupation of the tunnel sites with univalent cations.     

Synthesis and selected properties of CrSbVO6 and phase relations in the V2O5–Cr2O3–α-Sb2O4 system in the solid state

Phase relations in the ternary oxide system V₂O₅–Cr₂O₃–α-Sb₂O₄ in the solid state in the atmosphere of air have been investigated by using the XRD, DTA/TG and IRS methods. Obtained results have shown that in the system the compound CrSbVO₆ is formed. This compound has been obtained both from oxides and from a mixture comprising CrSbO₄, CrVO₄ and SbVO₅ as well as from mixtures: CrSbO₄/V₂O₅, CrVO₄/α-Sb₂O₄ and SbVO₅/Cr₂O₃. A Solid product of incongruent melting of CrSbVO₆ at ∼1300°C is Cr₂O₃. CrSbVO₆ crystallizes in the tetragonal system and its calculated unit cell parameters amount to: a = b = 0.45719(12) nm, c = 0.30282(8) nm, Z = 2. The obtained results have allowed us also to divide the investigated system V₂O₅–Cr₂O₃–α-Sb₂O₄ into seven subsidiary subsystems and to determine temperatures and components concentration range in which CrSbVO₆ remains at equilibrium in the solid state with other phases formed in corresponding binary systems.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Crystal structure of K4[H2J2O10].8H20

Crystals of K₄ [H₂J₂O₁₀] 8H₂O belong to the triclinic system, space group P 1 with a = 7.161 (2) $\text{A}\text{?}\text{, b}\text{= 10,553}$ (5) $\text{A}\text{?}\text{, c}\text{= 7,081}$ (2) $\text{A}\text{?}\text{, α = 98°1′}$, β = 117°8′, γ = 90°6′ and Z = 1. The crystal structure has been determined on the basis of photographic data from 877 independent reflections, with the final R value of 6,6%. The iodine atoms are surrounded by a distorted octahedra consisting of five oxygen atoms and one OH group. The average J-O distance is 2.03 Å. There are 2 independent K atoms in the structure. K(1) has a coordination number of eight, while K(2) is surrounded by 6 (5 water molecules + one OH group) nearest neighbours.     

An ESR study of the heterogeneous system Cr2O3V2O5

An ESR study of the ‘products’ of the high temperature solid state reaction xCr₂O₃+yV₂O₅ ($\text{0.05≦(}\text{x}\text{y}\text{)≦2.0}$), aided by powder X-ray diffraction and IR spectra, has revealed (i) ‘unfamiliar’ paramagnetic intermediates Cr⁴⁺ and Cr⁵⁺, (ii) provided some clues to the oxidation of Cr³⁺ to Cr⁶⁺, and (iii) demonstrated a variety of exchange interactions involving V⁴⁺ and Cr³⁺.     

On the pyrochlore type Ln2V2O7 (Ln: Rare-earth elements)

Pyrochlore type rare-earth vanadites (Ln₂V₂O₇) were prepared and some physical properties were investigated. Ln₂V₂O₇ (Ln:Tm, Yb or Lu) crystallized in the cubic space group $\text{O}{}_{\mathrm{<mtext>h</mtext>}}\text{=}\text{Fd}{}_3\text{m}\text{,}\text{a}{}_0\text{= 9.9575}\text{A}\text{?}\text{, 9.9346}\text{A}\text{?}\text{and}\text{9.9231}\text{A}\text{?}$ for Tm₂V₂O₇, Yb₂V₂O₇ and Lu₂V₂O₇, respectively. These compounds were all n-type semiconductors and paramagnets in the temperature range 90–300K.     

Nouvelles phases multiples obtenues par intercroissance des reseaux “M6X4O26” et “M8O21”: Les oxydes K10(M8O21)2·M6X4O26

New oxides K₁₀(M₈O₂₁)₂·M₆X₄O₂₆ (M = Nb, Ta; X = Si, Ge) were synthesized. These compounds were investigated by electron diffraction and microscopy, the symmetry is hexagonal, space group P62m, with $\text{a}\text{? 9}\text{A}\text{?}$, $\text{c}\text{? 20}\text{A}\text{?}$. They show an intergrowth of two structural types: (M₈O₂₁) and (M₆X₄O₂₆) layers. Results of structural study by X-ray powder diffraction are discussed in terms of the stabilization of A₃M₈O₂₁ and A_6?xM₆Si₄O₂₆ structures.     

An electrochemical study of the mixed beta-vanadium bronzes LiyNaxV2O5 and LiyKxV2O5

It has been found that lithium may be rapidly inserted into β-phase sodium and potassium vanadium bronzes to yield phases of composition Li_yNa_xV₂O₅ and Li_yK_xV₂O₅. For (x + y) values less than about 2/3, a single phase region is found. For higher lithium contents two additional narrow phases are formed; the maximum alkali metal content appears to correspond to a vanadium oxidation state of 4. The reversibility of the insertion reactions and x-ray powder diffraction indicate that the structures of the higher lithium content phases are closely related to the β-phase. Thermodynamic data have been derived from the electrochemical results.     

Phase relations of the Li2O-Ta2O5-B2O3 and Li2O-WO3-B2O3 systems and promising nonlinear optical compounds in K2O-Ta2O5-B2O3 system

The subsolidus phase equilibria of the Li₂O-Ta₂O₅-B₂O₃, K₂O-Ta₂O₅-B₂O₃ and Li₂O-WO₃-B₂O₃ systems have been investigated mainly by means of the powder X-ray diffraction method. Two ternary compounds, KTaB₂O₆ and K₃Ta₃B₂O₁₂ were confirmed in the system K₂O-Ta₂O₅-B₂O₃. Crystal structure of compound KTaB₂O₆ has been refined from X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the orthorhombic, space group Pmn2₁ (No. 31), with lattice parameters a = 7.3253(4) Å, b = 3.8402(2) Å, c = 9.3040(5) Å, z = 2 and D_calc = 4.283 g/cm³. The powder second harmonic generation (SHG) coefficients of KTaB₂O₆ and K₃Ta₃B₂O₁₂ were five times and two times as large as that of KH₂PO₄ (KDP), respectively.     

TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17

TiO₂(B) a new form of titanium dioxide, the octatitanate K₂Ti₈O₁₇ and intergrowths between K₂ Ti₄ O₉ and K₂ Ti₈ O₁₇ have been prepared by hydrolysis of K₂ Ti₄ O₉ followed by heating at 500°C. TiO₂ (B) and K₂Ti₈ O₁₇ have the host covalent framework of the bronzes Na_x TiO₂ and K₃Ti₈O₁₇ respectively.     

Synthesis and characterization of Mg3Cr2Si3O12-Fe3Cr2Si3O12 garnet solid solutions

Homogeneous garnet Fe₃Cr₂Si₃O₁₂ and its solid solutions with garnet Mg₃Cr₂Si₃O₁₂ (four intermediate samples) were synthesized at high pressure. Garnet Mg₃Cr₂Si₃O₁₂ was not synthesized in pure form. The sample contained traces of the two phases, MgSiO₃ and Cr₂O₃. The products were investigated by means of X-ray powder diffraction and differential scanning calorimetry. Molar volumes of the solid solutions as a function of composition fit linear regression with an accuracy of 0.2%. No difference in heat capacity within the limits of experimental error was detected. In thermodynamic evaluations, the garnet solid solutions Fe₃Cr₂Si₃O₁₂-Mg₃Cr₂Si₃O₁₂ may be treated as ideal solid solutions.     

Ionic conductivities of Na2Ti3O7, K2Ti4O9 and their related materials

Ionic conductivities were measured on the polycrystalline samples of layered titanates, Na₂Ti₃O₇ and K₂Ti₄O₉, and their derivatives. The activation energies and the prefactors of the conductions were 0.70 eV and 7.9 × 10 (Ωcm)^?1K for Na₂Ti₃O₇ and 0.81 eB and 1.1 × 10³ (Ωcm)^?1K for K₂Ti₄O₉. A small amount of Nb₂O₅ was doped to these titanates substituting TiO₂. Remarkable enhancements of ionic conductivities were observed with the doping. A new metastable phase, Li₂Ti₃O₇, was prepared by ion-exchange of Na₂Ti₃O₇ and its ionic conductivity was measured.     

Ferromagnetic semiconductors: Magnetic properties of the pyrochlores, (YxLu1−x)2V2O7 and (ScxLu1−x)2V2O7

Some magnetic properties of eight members of the pyrochlore systems (Y_xLu_1?x2V₂O₇ and (Sc_xLu_1?x)₂V₂O₇ are reported. Critical temperatures are determined for Lu₂V₂O₇ (72.5K) and the other phases to about ± 1K. Trends in T_c, π_c, C_m, μ_eff, and to a lesser extent, μ_SAT., occur which can be correlated, qualitatively, with thermogravometric data. From (Y_.40Lu_.60) to (Sc_.20Lu_.80) the quantities mentioned are nearly constant and analysis indicates the presence of V⁴⁺ only. C_m (0.47 cm³K/mole) and μ_eff (1.94 μB) are significantly greater than the spin-only values of 0.36 and 1.73 respectively. From (Sc_.30Lu_.70) to (Sc_.50Lu_.50) a reduced form of vanadium is detected, probably V³⁺, which increases with Sc content. This is accompanied by a sharp decline in T_c and π_c and a sharp increase in C_m and μ_eff.     

Silicotantalates et siliconiobates non-stoechiometriques : Les composes K6−2xBaxTa6Si4O26 (0 ≤ x ≤ 3) et K8M14Si4O47 (M = Ta,Nb)

New oxides: K_6?2xBa_xTa₆Si₄O₂₆ (0 ≤ x ≤ 3) and K₈M₁₄Si₄O₄₇ (M = Ta, Nb) are non stoichiometric-compounds. Distorted pentagonal channels have a variable occupancy for barium and potassium of K_6?2xBa_xTa₆Si₄O₂₆ phases. The structure of K₈M₁₄Si₄O₄₇ (M = Ta, Nb) shows an intergrowth of (M₆Si₄O₂₆) and M₈O₂₁ layers. It is related to that of K₆M₆Si₄O₂₆ (M = Ta, Nb) as evidenced by the “transformation” K₆Ta₆Si₄O₂₆ → K₈Ta₁₄Si₄O₄₇ observed at 1350°C.     

Crystal structure and properties of the new complex vanadium oxide K2SrV3O9

The new complex vanadium oxide K₂SrV₃O₉ has been synthesized and investigated by means of X-ray powder diffraction (XPD), electron microscopy and magnetic susceptibility measurements. The oxide has an orthorhombic unit cell with lattice parameters a = 10.1922(2) Å, b = 5.4171(1) Å, c = 16.1425(3) Å, space group Pnma and Z = 4. The crystal structure of K₂SrV₃O₉ has been refined by Rietveld method using X-ray powder diffraction data. The structure contains infinite chains built by V⁴⁺O₅ square pyramids linked to each other via VO₄ tetrahedra. The chains form layers and potassium and strontium cations orderly occupy structural interstices between these layers. Electron diffraction as well as high resolution electron microscopy confirmed the structure solution. Magnetic susceptibility measurements revealed an antiferromagnetic interaction with J of the order of 100 K inside the chains and no long-range magnetic order above 2 K. The origin of the magnetic exchange is likely a result of super-exchange interaction through the two VO₄ tetrahedra linking the polyhedra with the magnetic V⁴⁺ cations.     

The preparation,crystal structures and properties of the potassium vanadium sulfides KXV5S8 (0.5≦x≦0.7)

The crystal structures of the potassium vanadium sulphides K_0.7V₅S₈ and K_0.5V₅S₈ (=KV₁₀S₁₆) have been determined. K_0.7V₅S₈ has a C-centered monoclinic unit cell of dimensions $\text{a}\text{=17.499(3)}\text{A}\text{?}$, $\text{b}\text{=3.2986(6)}\text{A}\text{?}$, $\text{c}\text{=8.489(1)}\text{A}\text{?}$, ß=103.98(1)°, spacegroup C2/m; isomorphous with TIV₅S₈; K_0.5V₅S₈ has essentially the same structure, but due to ordering of the K atoms, the monoclinic b-axis is doubled, thus forming a superstructure. The cell parameters are: $\text{a}\text{=17.462(4)}\text{A}\text{?}$, $\text{b}\text{=6.556(2)}\text{A}\text{?}$, $\text{c}\text{=8.4595(9)}\text{A}\text{?}$, ß=103.86(1)°, spacegroup P2. The structures are characterized by a three-dimensional framework of VS₆ octahedra with channels in which the K atoms are situated. Both compounds exhibit metallic behaviour.     

K2(Rb2,Cs2,Tl2)TeBr6(I6) and Rb3(Cs3)Sb2(Bi2)Br9(I9) perovskite compounds

We systematize available experimental data on the crystal structure of the ternary halides K₂(Rb₂,Cs₂,Tl₂)TeBr₆(I₆) and Rb₃(Cs₃)Sb₂(Bi₂)Br₉(I₉), analyze the general trends in the properties of their single crystals, and examine the key features of the physicochemical interaction in related systems.     

Investigation of the Fe2O3TiO2 system in a flux environment

System Fe₂O₃TiO₂ in a flux environment K₂O, V₂O₅, SiO₂ was studied. Single crystals of monoclinic Fe₂TiO₅ and Hollandite type phase K_1.45Fe_1.45Ti_6.55O₁₆ have been grown and some properties of the flux were discussed. Obtained phases were characterized by X-ray diffraction and Mössbauer spectra.