Structure determination of (3D) P2NbS8

Tridimensional P₂NbS₈ crystallizes in P4?n2 tetragonal space group, with $\text{a}\text{= 12.0483(4)}\text{A}\text{?}$, $\text{c}\text{= 7.2070(5)}\text{A}\text{?}$, $\text{V}\text{= 1046.2(1)}\text{A}\text{?}{}^3$ and Z = 4. The structure was anisotropically refined down to R = 2.3% from 468 reflexions and 53 variables. It is built from [Nb₂S₁₂] biprismatic bicapped units (average $\text{d}{}_{\mathrm{Nb?S}}\text{= 2.571}\text{A}\text{?}$) made of S^?II and S^?II₂ anions (dianionic distance of 2.014(3) Å). The niobium atoms are found as isolated Nb^IV ? Nb^IV pairs ($\text{d}{}_{\mathrm{Nb?Nb}}\text{= 2.859(1)}\text{A}\text{?}$) in these niobium group otherwise linked to each other through (PS₄) tetrahedral units (average $\text{d}{}_{\mathrm{P?S}}\text{= 2.051}\text{A}\text{?}$) themselves constituting interbonded [P₄S₁₂] rings. The P₂NbS₈ three-dimensional network thus obtained is compared to the (2D) P₂NbS₈, layered phase already described.     

Etude par spectrometrie d'absorption infra-rouge des facteurs influencant la transformation ordre-desordre de ferrites γ lacunaires derivant de l'oxydation de spinelles ferreux

The order-disorder phenomenon in defect γ-spinels of type $\text{(Fe}{}_{\mathrm{<mtext>(8?8y)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{<mtext>8y</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ and $\text{Fe}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{□}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ with M³⁺ = A1³⁺, Cr³⁺ and M²⁺ = Zn²⁺, Co²⁺, Mn²⁺ obtained from ferrous spinels was investigated using IR spectrometry. These compounds possess a vacancy ordering on octahedral sites for substitution extents of less than 0.30 and no ordering has been observed for substitution extent > 0.40. The ordering process is also influenced by the nature and position of the trivalent or divalent cation, particle size, heating temperature and oxidation time.     

Copper doped zinc oxide Y2BaZnO5: ERS and optical investigation

In copper doped Y₂BaZnO₅ oxides, copper exhibits a distorted square pyramidal coordination which is consistant with the values of g and A tensors obtained from O band ERS spectrum for a sample containing about 1 % Cu. Three values for g and A are observed, g₁ = 2.049₅, g₂ = 2.051₅, g₃ = 2.275, $\text{¦}\text{A}{}_1\text{¦ = 13 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$, $\text{¦}\text{A}{}_2\text{¦ = 10 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$ and $\text{¦}\text{A}{}_3\text{¦ = 147.5 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$. Since $\text{g}{}_1\text{?}\text{g}{}_2$ an approximate C_4v point symmetry can be assumed for copper. The electronic spectrum shows three bands at 11700, 14500 and 20500 cm^?1 which can be assigned to the transitions A₁ → B₁, B₂ → B₁ and E → B₁ respectively. The orbital reduction parameters are calculated and the bonding covalency is discussed.     

Refinement of the structure of K2Cr2O7

The parameters for the structure of K₂Cr₂O₇ have been refined from 7511 observed reflections; $\text{a}{}_{\mathrm{o}}\text{= 7.4200(6)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 13.399(3)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 7.3845(9)}\text{A}\text{?}$, cosα = ?0.1396(2) (α = 98°2'), cosβ = ?0.0154(2) (β = 90°53'), cosγ = ?0.1078(2) (γ = 96°11'). The discrepancy factor R(F_o²) = 0.0702 with a type 2 extinction correction. The average CrO distances are 1.609Å (unshared) and 1.783Å (shared).     

The absolute configuration of RbClO3

The positional parameters of the structure of RbClO₃ have been determined from 171 x-ray reflections; $\text{a}{}_{\mathrm{<mtext>no</mtext>}}\text{= 6.092(1)}\text{A}\text{?}$$\text{c}{}_{\mathrm{<mtext>o</mtext>}}\text{= 8.173(2)}\text{A}\text{?}\text{, ?}\text{cal}\text{= 3.203}\text{g/cc}{}^3\text{and S.G.}\text{=}\text{R}\text{3}\text{m}$. The absolute configuration of the structure was determined by a least-squares adjustment of the complex anomalous dispersion of Rb and Cl. The ClO distances are 1.478(5)Å and the RbO distances are 3.530(8)Å.     

Spectres vibrationnels,transitions electroniques et structures a courtes distances dans les verres: Sulfures de terres rares - sulfure de gallium

Absorption spectra of chalcogenide glasses of general formula Ln₂S₃-1,8 Ga₂S₃ (Ln = La⁺³, Ce⁺³, Pr⁺³, Nd⁺³, Gd⁺³ were investigated continously from far infrared to ultra-violet region. Raman spectra were recorded in the 5cm^?1–600 cm^?1 range. Vibrational spectra of glasses have been compared to those of polycrystalline compounds $\text{Ln}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{Ga}{}_6\text{S}{}_{14}$ (Ln = La⁺³, Ce⁺³) and $\text{Ln}{}_6\text{Ga}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{S}{}_{14}\text{(}\text{Ln = La}{}^{\mathrm{+3}}\text{to Gd}{}^{\mathrm{+3}}\text{)}$.From the observed spectra it could be concluded that the short distance order around gallium atom is strongly modified in going from crystalline to glassy state.Electronic transitions of Ce⁺³, Pr⁺³ and Nd⁺³ ions have been also identified in the visible region and in the mid infrared absorption spectra.     

Nature de la fluorescence de l'europium divalent dans les fluorures

The levels of the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ (4f⁷) and 4f⁶5d¹ states of europium (+II) have been determined from fluorescence spectra in a series of fluorides M_xB_yF_z (M = alkaline-earth element, B = Li, Be, Mg, Y, Si). As in BaY₂F₈, SrSiF₆ and BaSiF₆, the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ level lies even at room temperature far below the 4f⁶5d¹ band, only the f → f emission is observed. The influence of the choice of the alkaline-earth and the B cations and of the coordination of europium on the relative positions of the energy levels is discussed.     

The synthesis of the first stage graphite salt C8+AsF6− and its relationship to the first stage graphite/AsF5 intercalate

Oxidation of graphite with excess O₂⁺AsF₆^?, in suspension in SO₂C1F, produces the blue first-stage graphite salt of composition C₈AsF₆, which X-ray single crystal photographs show is hexagonal with a = 4.92(2), c = 8.10(2), $\text{V}\text{= 170}\text{A}\text{?}{}^3$. The blue first-stage material of approximate composition C₁₀AsF₅ obtained from graphite and AsF₅ has a related pseudo cell. Arsenic X-ray absorption-edge spectra show that C₈AsF₆ contains AsF₆^? alone, and that the graphite/AsF₅ intercalte contains AsF₆^? and AsF₃ in accord with the AsF₅ reduction:

$\text{3 AsF}{}_5\text{+ 2 e}{}^{\mathrm{?}}\text{→ 2 AsF}{}_6{}^{\mathrm{?}}\text{+ AsF}{}_3$

Treatment of the graphite/AsF₅ compound with F₂ gas results in conversion of all of the intercalated arsenic to AsF₆^?.     

Structure cristalline et proprietes physiques de Cu0.75 VS2

A new compound with composition Cu_0.75 VS₂ has been prepared. Its preparation, X-Ray structure, electrical and magnetic properties are reported. The structure is related to the CdI₂-type, as in the case of the previously described Cu_xTiS₂ (1); in Cu_0.75 VS₂, Cu atoms are ordered in tetrahedral sites between the CdI₂-type subunits, whereas in Cu_xTiS₂ Cu atoms are disordered in two independent sites. The vanadium atoms are shifted with respect to the titanium sites which leads a monoclinic distortion of the hexagonal cell. The relation between the CdI₂ unit cell and the true monoclinic cell of Cu_0.75 VS₂ is:

$\text{a}{}_{\mathrm{<mtext>mono</mtext>}}\text{= 2}\text{a}{}_{\mathrm{<mtext>hex</mtext>}}\text{3}\text{;}\text{b}{}_{\mathrm{<mtext>mono</mtext>}}\text{=}\text{4}\text{3}\text{a}{}^2{}_{\mathrm{<mtext>hex</mtext>}}\text{+}\text{c}{}^2{}_{\mathrm{<mtext>hex</mtext>}}\text{9}\text{;}\text{c}{}_{\mathrm{<mtext>mono</mtext>}}\text{= 2}\text{a}{}_{\mathrm{<mtext>hex</mtext>}}$

In Cu_0.75 VS₂, vanadium atoms occupy two independent sites, three vanadium atoms forming a triangular cluster (V₂—V₃ distances are 2.91 Å and V₃—V₃ are 2.92 Å) while one vanadium atom is isolated ($\text{V}{}_1\text{?}\text{V}{}_2\text{= 3.36}\text{A}\text{?}$ and $\text{V}{}_1\text{?}\text{V}{}_3\text{= 3.37}\text{A}\text{?}$. The physical properties exhibit a transition at 50°K approximately, the magnetic susceptibility being temperature-independent above and temperature dependent below the transition (Curie-Weiss behavior). Resistivity and Hall measurements confirm the metallic nature of the compound and show the existence of the low temperature transition. The observed properties could be interpreted as a result of the low temperature localisation of the 3d electron of V₁.     

Electronic and spin configurations of Co3+ and Ni3+ ions in oxides of K2NiF4 structure: A magnetic susceptibility study

In La₄LiCoO₈, Li⁺ and Co³⁺ ions are ordered in two dimensions and Co³⁺ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both Sr₄TaCoO₈ and Sr₄NbCoO₈ exhibit low to intermediate-spin state transitions of Co³⁺ ions. In the system LaSr_1?xBa_xNiO₄, the e_g electrons are essentially in extended states forming a band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin Ni³⁺ ions are formed to a small extent with increasing x, but there is no spin-state transition. In LaSrAl_1?xNi_xO₄, at small x, there is a small proportion of high-spin Ni³⁺; when x ≈ 0.6, there is an abrupt decrease in the $\text{c}$/$\text{a}$ ratio, signalling the formation of the band. In LnSrNiO₄, the $\text{c}$/$\text{a}$ ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the band.     

Rate law versus oxygen pressure in spinel oxidation to γ lacunar spinels

In the controlled oxidation of spinels (Fe²⁺Fe³⁺_2?xM³⁺_x)O^2?₄ (M³⁺=Al³⁺, Cr³⁺ ; 0 < x < 2) and (Fe²⁺Al³⁺_2?xCr³⁺_x)O^2?₄ to the metastable phases γ(Fe³⁺_1?yM³⁺_y)₂O^2?₃ and $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{O}{}^{\mathrm{2?}}{}_{\mathrm{3?}}\text{(0 <}\text{y}\text{<}\text{2}\text{3}\text{)}$, over the temperature range 200–450°C, the rate law is usually v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{2}$. This law is well interpreted if we consider a total association vacancy-positive charge. However, for low oxidation extents a (a < 0.3) the law written as is in better agreement with a partial association of defects. For a total dissociation of defects exponent n of the pressure law is always smaller.     

Crystal structures and crystal chemistry in the system Na1+xZr2SixP3−xO12

As part of a search for skeleton structures for fast alkali-ion transport, the system Na_1+xZr₂Si_xP_3?xO₁₂ has been prepared, analyzed structurally and ion exchanged reversibly with Li⁺, Ag⁺, and K⁺ ions. Single-crystal x-ray analysis was used to identify the composition NaZr₂P₃O₁₂ and to refine its structure, which has rhombohedral space group R3?c with cell parameters $\text{a}{}_{\mathrm{<mtext>r</mtext>}}\text{= 8.815(1)}\text{A}\text{?}$ and $\text{c}{}_{\mathrm{<mtext>r</mtext>}}\text{= 22.746(7)}\text{A}\text{?}$. A small distortion to monoclinic symmetry occurs in the interval 1.8 ≤x≤ 2.2. The structure for Na₃Zr₂Si₂PO₁₂, proposed from powder data, has space group $\text{C}\text{2}\text{c}$ with $\text{a}{}_{\mathrm{<mtext>m</mtext>}}\text{= 15.586(9)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.029(4)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.205(5)}\text{A}\text{?}$, and β = 123.70(5)° Both structures contain a rigid, three-dimensional network of PO₄ or (SiO₄) tetrahedra sharing corners with ZrO₆ octahedra and a three-dimensionally linked interstitial space. Of the two distinguishable alkali-ion sites in the rhombohedral structure, one is completely occupied in both end members, the occupancy of the other varies across the system from 0 to 100 percent. Several properties are compared with the fast Na⁺-ion conductor β-alumina.     

Crystal structure and fluorescence lifetime of a laser material NdNa5(WO4)4

The structure of NdNa₅(WO₄)₄, determined by single-crystal x-ray analysis, is tetragonal with space group I 4₁/a and cell parameters $\text{a}\text{= 11.559(2)}\text{A}\text{?}$, $\text{b}\text{= 11.453(2)}\text{A}\text{?}$, z = 4. A full-matrix, least-squares refinement gives R = 0.077. The W atoms and the Nd atoms occupy isolated tetrahedra and dodecahedra, respectively. These polyhedra are connected by the Na atoms, which are located at two different sites, tetrahedral and octahedral. The fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ transition of Nd⁺³ is reported for the system La_1?xNd_xNa₅(WO₄)₄. The lifetime of NdNa₅(WO₄)₄ is 85 ± 5 μsec, which is comparable to NdP₅O₁₄. Concentration quenching is considerably reduced in both host structures, which we conclude is due to the isolation of the rare-earth polyhedra.     

A series of rare earth silicates RE3+2M3+[SiO4]2 (OH)

A series of isotypic silicates of composition RE₂M[SiO₄]₂ (OH) with RE = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, and M = Al³⁺, Fe³⁺ has been synthesized under hydrothermal conditions. Lattice constants of two members as determined from single crystal X-ray diffraction data are: La₂Al[SiO₄]₂ (OH) (La₂Fe[SiO₄]₂ (OH)) $\text{a}{}_{\mathrm{o}}\text{= 7.401 (7.346)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 5.702 (5.862)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 17.072 (97.196)}\text{A}\text{?}$, gb = 112.4 (112.5°), $\text{P}\text{2}{}_1\text{c}$, Z=4.     

Synthese et caracterisation du pentafluorotellurate (IV) de sodium NaTeF5

Sodium pentafluorotellurate (IV) NaTeF₅ has been prepared by slow evaporation of a $\text{1}\text{1}$ mixture of sodium fluoride and tellurium dioxyde in 50 % aqueous hydrofluoric acid.NaTeF₅ has an orthorhombic structure, parameters of which are $\text{a}\text{= 9.10}{}_5\text{A}\text{?}$$\text{b}\text{= 11.30}{}_6\text{A}\text{?}$$\text{c}\text{= 10.12}\text{A}\text{?}$     

Sr2Mn2O5, an oxygen-defect perovskite with Mn(III) in square pyramidal coordination

An oxygen-defect perovskite Sr₂Mn₂O₅ was isolated by reduction of SrMnO_3?x perovskites in the presence of zirconium. Its structure, similar to that of Ca₂Mn₂O₅, has been determined by X-ray powder diffraction and HREM. The orthorhombic cell has the parameters : $\text{a}\text{= 5.523(1)}\text{A}\text{?}$, $\text{b}\text{= 10.761(5)}\text{A}\text{?}$, $\text{c}\text{= 3.811(1)}\text{A}\text{?}$. The possible space groups are Pbam and Pba2. The framework is built up from corner-sharing MnO₄ pyramids forming pseudo-hexagonal tunne$\text{l}$s running along 〈001〉 and perovskite tunnels running along 〈110〉 and 〈110〉. This oxide is antiferromagnetic with $\text{T}{}_{\mathrm{N}}\text{? 380 ± 10}\text{K}$ and $\text{θ}{}_{\mathrm{<mtext>P</mtext>}}\text{? 300 ± 10}\text{K}$.     

Structure de PbU2Se5

PbU₂Se₅ crystallizes in a monoclinic unit cell $\text{a}\text{= 8,605}\text{A}\text{?}$; $\text{b}\text{= 7,788}\text{A}\text{?}$; $\text{c}\text{= 12,27}\text{A}\text{?}$; β = 90° of space group $\text{P}\text{2}{}_1\text{c}$, Z = 4. The crystal structure has been determined for 1251 independant reflections and refined to a final R value of 0,068. Two kinds of uranium sites are found; the average U-Se distances are 2,97 Å for eight-coordination and 2,88 Å in seven-coordination. The average Pb-Se distance for eight-coordination is 3,20 Å. This structure explains the solid solution between PbU₂Se₅ and U₃Se₅.     

Crystal structure and fluorescence lifetime of NdAl3(BO3)4, a promising laser material

The structure of NdAl₃(BO₃)₄, determined by single-crystal x-ray analysis, is rhombohedral with space group R32 and cell parameters $\text{a}\text{= 9.3416 (6)}\text{A}\text{?}$, $\text{c}\text{= 7.3066 (8)}\text{A}\text{?}$, Z = 3. A full-matrix, least-squares refinement gives R = 0.033 and R_w = 0.037. The Nd atoms, Al atoms and B atoms occupy trigonal prisms, octahedra, and triangles of oxygen, respectively. Edge-shared Al octahedra form helices along the c-axis. These helices are connected by isolated B triangles and isolated Nd trigonal prisms. The fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ transition of Nd⁺³ is reported for the system Nd_xGd_1?xAl₃(BO₃)₄. The lifetime of NdAl₃(BO₃)₄ is 19us, and concentration quenching is reduced in the series, as happens in NdNa₅(WO₄)₄ (85μs) and NdP₅O₁₄ (115μs). The shorter lifetime is attributed to noncentrosymmetry; the reduced concentration quenching to isolation of Nd polyhedra.     

Study of the oxidation kinetics of finely-divided magnetites III — Influence of chromium and aluminum surstitution

The oxidation kinetics of aluminum and chromium-substituted magnetites (Fe²⁺Cr³⁺_xAl³⁺_2?x)O^2?₄ (0< x < 2) into the γ defect phase of type $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{)}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion under variable working conditions for samples prepared at 700°C and whose size is less than 400 Å. When particle size is caused to increase by annealing the kinetic curves are sigmoidal but only for specimens with high chromium substitution. For compounds rich in chromium observations of morphology show facetting during oxidation but no fracturing was observed.     

ESR evidence for the existence of rutile-type,nonstoichiometric vanadium antimonate

A nonstoichiometric vanadium antimonate has been synthesized from a high temperature solid state reaction of V₂O₅ and Sb₂O₃. Powder-X-ray diffraction data of this phase could be indexed on a tetragonal basis with $\text{a}\text{= 4.60}\text{A}\text{?}$, $\text{c}\text{= 3.02}\text{A}\text{?}$. The phase exhibits a well resolved hyperfine ESR spectrum at 298 K due to V(4+)(3d′) ions interacting with ${}^{51}\text{V}\text{(}\text{I}\text{=}\text{7}\text{2}\text{)}$, thus establishing its identity as a rutile type phase postulated earlier. The ESR parameters are $\text{g}{}_{\mathrm{}}\text{= 1.933±0.002}$, g_⊥ = 1.984±0.002, $\text{A}{}_{\mathrm{}}\text{= 193±3}\text{G}$, A_⊥ = 75±3G. Present results are discussed in relation to ESR of V(4+) in other rutile-type hosts.