Etude des transformations allotropiques de Mg2NiH4

Three allotropic varieties of Mg₂NiH₄ with monoclinic, orthorhombic and cubic symmetry have been identified. The unitcell dimensions are : $\text{a}\text{= 12.99}\text{A}\text{?}$, $\text{b}\text{= 6.390}\text{A}\text{?}$, $\text{c}\text{= 6.598}\text{A}\text{?}$ and $\text{β = 93.22}\text{A}\text{?}$ for the monoclinic form. The orthorhombic structure whose parameters are : $\text{a}\text{= 6.499}\text{A}\text{?}$, $\text{b}\text{= 6.415}\text{A}\text{?}$ et $\text{c}\text{= 6.589}\text{A}\text{?}$, represents a slight distortion from the cubic one. Under pressure, an irreversible transformation from the monoclinic to the cubic variety has been observed. The transition temperature is 245°C at 1 bar. On the other hand, the orthorhombic form transforms reversibly into the cubic one about 235°C under 1 bar pressure. The orthorhombic-monoclinic transition occurs only at very high pressure.     

Etude structurale du system Mg2NiH2: II — La variete haute temperature de Mg2NiH4 de symetrie cubique

The structure of the cubic variety of Mg₂NiD₄ has been studied by neutron diffraction at 240°C. The metal atoms form a fluorite-type structure and most of deuterium is distributed among the (48 h) sites (x,x,0) with $\text{x}\text{? 0.2}$, of space groupe Fm3m. The ideal positions form $\text{2}\text{3}$ filled octahedra around the magnesium atoms with $\text{MgD}\text{? 1.66}\text{A}\text{?}$. Such a distance suggests a preferential MgD bond and explains that the thermal stability of Mg₂NiD₄ is close to that of MgD₂. At rising temperature the deuterium atoms tend to be redistributed in the two 48 h (x,x,0) and 24 h (x,0,0) sites. This repartition corresponds to preferential diffusion ways in the Mg₂NiH₄ lattice.     

(NbSe4)3I low temperature structure

Low temperature (128 K) structural determination of (NbSe₄)₃I is reported. (NbSe₄)₃I is found to undergo a second-order displacive phase transition at 274 K. Symmetry is tetragonal, space group P4?2₁c with $\text{a}\text{= 9.450(7)}\text{A}\text{?}$$\text{c}\text{= 19.08(6)}\text{A}\text{?}$. (NbSe₄)₃I is no more centrosymmetric : NbSe₄ chains are shifted in respect of each other. Some experimental results are discussed in connection with the transition.     

温度对Mg4Nb2O9合成的影响

Mg4Nb2O9具有与α-Al2O3相同的刚玉型晶体结构,可望成为取代氧化铝陶瓷的新一代高Q值基板陶瓷.在900～1400℃温度范围内,合成了Mg4Nb2O9化合物,采用X射线粉末衍射法进行了相结构分析.结果表明,生成物中含有Mg4Nb2O9、Mg4Nb2O6和MgO三种物相,主晶相是Mg4Nb2O9;在900～1300℃温度范围内,随着温度的升高,MgNb2O6和MgO反应生成Mg4Nb2O9相,主晶相含量线性增加,但在1300℃以上,主晶相含量随温度的升高而减小;Mg4Nb2O9相的最佳合成温度为1300℃.这些结果对研究开发Mg4Nb2O9微波基板材料有着重要的意义.     

Elastic tensor of the forsterite (Mg2SiO4) under pressure

A complete elastic tensor of the low-pressure structure of the magnesium orthosilicate (Mg₂SiO₄, forsterite) is determined by an ab initio technique for the pressure range P=0–240 kB. The geologically important quantities: density, sound velocity, Young's modulus, Poisson's ratio, crystal anisotropy, are derived from the calculated data. A systematic increase of crystal's anisotropy with pressure has been noticed. The results agree well with the available experimental data.     

Low-temperature sintering of (Zn0.8 Mg0.2)2SiO4-TiO2 ceramics

Effect of Li₂O-B₂O₃-SiO₂ (LBS) glass on the sintering behavior and the microwave dielectric properties of (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ (ZMST) ceramics were investigated. The Li₂O-B₂O₃-SiO₂ glass lowered the sintering temperature of ZMST ceramics effectively from 1250 to 870 °C. The unknown second phase, which was formed in the ZMST ceramics increased with the addition of LBS glass. With increasing the LBS glass content, the bulk density, dielectric constant (ε_r) and the maximum Q × f value decreased, and the temperature coefficient of resonant frequency (τ_f) shifted to a negative value. (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ ceramics with 3 wt.% Li₂O-B₂O₃-SiO₂ glass sintered at 870 °C for 2 h shows excellent dielectric properties: ε_r = 8.48, Q × f = 11500 GHz, and τ_f = 0 ppm/°C.     

Oxidative dehydrogenation of cyclohexane with Mg3(VO4)2 synthesized by the citrate process

Citric acid complexation under mild condition was proposed to prepare monophasic and well crystallized Mg₃(VO₄)₂ particle to be used as an active catalyst for the oxidative dehydrogenation of cyclohexane to cyclohexene. The catalyst prepared above was characterized by N₂-physisorption, X-ray diffraction, scanning electron microscopy, and thermal gravimetric analysis. The characterization results displayed that the Mg₃(VO₄)₂ particle was typically 100-160 nm and the specific surface area was 12.0-26.7 m²/g. Moreover, it showed that the purity and the structure of the catalyst were principally subjected to the calcination temperature and the amount of citric acid used in the sol-gel procedure. The Mg₃(VO₄)₂ catalyst calcined at 823 K for 6 h with a molar ratio of (Mg + V):citric acid = 1:1.2 exhibited the best catalytic performance with an excellent thermal stability.     

The synthesis and selected properties of new compounds: Mg3Fe4(VO4)6 and Zn3Fe4(VO4)6

New compounds: Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ were obtained from a solid state reaction. The temperatures of melting of Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ amount to 950±5 and 850±5°C, respectively. The indexing results and the calculated unit cell parameters for both compounds are given and suggest that both phases are isotypic with Mn₃Fe₄(VO₄)₆. The IR spectra of the above-mentioned compounds are presented.     

Dielectric and electric studies of the [N(CH3)4][N(C2H5)4]ZnCl4 compound at low temperature

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound was prepared and characterized by electrical technique. The temperature dependence of the dielectric permittivity shows that this compound is ferroelectric below T = 268 K. The two semi-circles observed in the complex impedance identify the presence of the grain interior and grain boundary contributions to the electrical response in the material. The equivalent circuit is modeled by a combination series of two parallel R_P–CPE circuits. The frequency dependent conductivity is interpreted in term of Jonscher's law. The modulus plots can be characterized by the empirical Kohlrausch–Williams–Watts (K.W.W.) function: ?(t) = exp [(−t/τ)^β]. The temperature dependence of the alternative current conductivity (σ_p), direct current conductivity (σ_dc) and the relaxation frequency (f_p) confirm the presence of the ferroelectric–paraelectric phase transition.     

The crystal structure of Pb8Bi2(PO4)6O2

Single crystals of Pb₈Bi₂(PO₄)₆O₂ were grown by the Czochralski technique, and the structure of this compound was determined. It was found to crystallize in Pnma orthorhombic symmetry with a = 13.313, b = 10.284 and $\text{c}\text{= 9.219}\text{A}\text{?}$. The structure was refined to an R value of 7%. Powder diffraction data are also reported.     

Preparation of (NH4)Zr2(PO4)3 and HZr2(PO4)3

(NH₄)Zr₂(PO₄)₃ has been prepared, hydrothermally, from α-zirconium phosphate in three different ways; (1) from amine intercalates at 300°C, (2) from mixtures of ZrOCl₂·8H₂O in excess (NH₄)H₂PO₄ and (3) reaction of NH₄Cl with Zr(NaPO₄)₂. Ammonium dizirconium triphosphate is rhombohedral with a = 8.676(1) and $\text{c}\text{= 24.288(5)}\text{A}\text{?}$. It decomposed on heating to HZr₂(PO₄)₃. Below 600°C a complex, as yet unindexed, X-ray pattern was obtained. A very similar X-ray pattern was obtained by washing LiTi_0.1Zr_1.9(PO₄)₃ with 0.3N HCl. Heating this phase or NH₄Zr₂(PO₄)₃, above 600°C resulted in the appearance of a rhombohedral phase of HZr₂(PO₄)₃ with cell dimensions a = 8.803(5) and $\text{c}\text{= 23.23(1)}\text{A}\text{?}$. The protons were not completely removed until about 1150°C. Decomposition of (NH₄)Zr₂(PO₄)₃ at 450°C yielded an acidic gas whereas at 700°C NH₃ was evolved. A possible explanation for this behavior is presented.     

Syntheses and structures of Ta2(WO2)0.87H0.26(PO4)4 and Ta2(MoO2)(PO4)4

Tantalum hydrogen phosphate, β-TaH(PO₄)₂, has a three-dimensional structure that is stable to remarkably high temperature (∼600 °C) presumably due to the presence of strong hydrogen bonds. Impedance measurements indicate a low conductivity, 2.0 × 10⁻⁶ S/cm at 200 °C in 5% H₂. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO₄)₂-W₂P₂O₁₁ system at 380 °C. As a result, a new phase, Ta₂(WO₂)_0.87H_0.26(PO₄)₄, was identified and subsequently the molybdenum analog Ta₂(MoO₂)(PO₄)₄ was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta₂(WO₂)_0.87H_0.26(PO₄)₄ and Ta₂(MoO₂)(PO₄)₄ can be formally derived from the structure of β-TaH(PO₄)₂ by the replacement of two P-OH protons with an MO₂²⁺ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra.     

Temperature dependence of critical stress and phase diagram of ferroelastic Pb3 (PO4)2 Pb3 (VO4)2

The temperature dependence of the critical stress in ferroelastic Pb₃ (PO₄)₂ reveals a Curie-Weiß law ($\text{ß =}\text{1}\text{2}$) up to 145°C. Between 160°C and the transition point at 180°C a crossover to a $\text{ß =}\text{1}\text{3}$ regime was found. For mixed crystals Pb₃ (PO₄)₂ ? Pb₃ (VO₄)₂ a phase diagram is suggested from optical, dielectric and Raman spectroscopical experiments.     

Synthesis, structure refinement and characterisation of a new oxyphosphate Mg0.50TiO(PO4)

A new titanium oxyphosphate Mg_0.50TiO(PO₄) has been synthesized and characterized by several physical techniques: X-ray diffraction, ³¹P MAS-NMR, Raman diffusion, infrared absorption and diffuse reflectance spectroscopy. It crystallizes in the monoclinic system with unit cell parameters: a = 7.367(9), b = 7.385(8), c = 7.373(9) Å, β = 120.23(1), with the space group P2₁/c (no. 14), Z = 4. The crystal structure has been refined by the Rietveld method using X-ray powder diffraction. The conventional R indices obtained are R_wp = 0.138, R_p = 0.096 and R_B = 0.0459. The structure of Mg_0.50TiO(PO₄) consists of infinite chains of corner-shared [TiO₆] octahedra parallel to the c-axis, crosslinked by corner-shared [PO₄] tetrahedra. These infinite chains have alternating short (1.74 Å) and long (2.26 Å) TiO bonds and are similar to those found in titanium oxyphosphate M^II_0.50TiO(PO₄) (M²⁺ = Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺). The magnesium atom is located in an antiprism between two [TiO₆] octahedra. ³¹P MAS NMR showed only a single ³¹P resonance line, in a good agreement with the crystal structure. Raman and IR spectra show strong bands respectively at 765 and 815 cm⁻¹, attributed to the vibration of TiOTiO bonds in the infinite chains. The gap due to the Oxygen-Titanium(IV) charge transfer is 3.37 eV.     

Die strukturen des MgAl2S4 und Mg0.830In2.113S4

MgAl₂S₄ crystallizes in the orthorhombic system, space group Pnma (No 62) with the lattice constants: a=1251.1 ± 0.6 pm, b =722.0 ± 0.4 pm, c=592.1 ± 0.4 pm The compound is isotypic to BeAl₂O₄.Mg_0.830In_2.113S₄ forms the spinel structure, space group Fd3m (No 227) with a=1071.9 ± 0.6 pm.     

On the luminescence of Ti4+ In Mg5SnB2O10 and Mg3ZrB2O8

The following new compounds are reported: Mg₅TiB₂O₁₀ (ludwigite structure), Mg₅SnB₂O₁₀ (orthopinakiolite structure) and Mg₃ZrB₂O₈ (warwickite structure). The luminescences of the systems Mg₅(Sn,Tl)B₂O₁₀ and Mg₃(Zr,Ti)B₂O₈ were measured. The results are discussed in terms of delocalization in clusters of titanate octahedra. A general scheme for the luminescence of Ti⁴⁺-activated compositions is given. The composition Mg₅SnB₂O₁₀: Ti is a very efficient luminescent material for titanium concentrations below some 20 mole percent.     

Synthesis of Nanotubular Mg3Si2O5(OH)4-Ni3Si2O5(OH)4 Silicates at Elevated Temperatures and Pressures

Mg₃Si₂O₅(OH)₄-Ni₃Si₂O₅(OH)₄ nanotubes with the chrysotile structure and MgO : NiO molar ratios of 1 : 2 and 2 : 1 are synthesized by hydrothermal reactions at temperatures from 250 to 450°C and pressures from 30 to 100 MPa. The reaction path and kinetics, as well as the dimensions and morphology of the resulting nanotubes, are shown to depend on the nature of the starting reagents, chemical composition of the reaction system, and hydrothermal synthesis conditions. At higher nickel concentrations in the hydrous silicates, nanotube formation requires higher temperatures, longer hydrothermal treatment times, and higher NaOH concentrations in the reaction system.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 849–855.Original Russian Text Copyright © 2005 by Korytkova, Maslov, Pivovarova, Polegotchenkova, Povinich, Gusarov.     

Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.     

Dielectric properties of Pb3 (PO4)2 | Pb3 (AsO4)2

The dielectric constants of Pb₃ (PO₄)₂ | Pb₃ (AsO₄)₂ at room temperature are intrinsic and fulfill the Lyddane - Teller - Sachs relation. At higher temperatures the specific conductivity increases with an activation energy of 0.56 eV leading to Maxwell - Wagner polarization effects thereby increasing the effective dielectric constant. Corresponding peaks in ∈' (T) are extrinsic and not attributed to structural phase transformations.     

Etude structurale du systeme Mg2NiH2 I - La solution solide Mg2NiHx (x=0, 30)

The structure of Mg₂NiD_{0, 30} has been studied by neutron diffraction after preparation at 300°C under 2.5 bar deuterium pressure, by “in situ” decomposition of cubic Mg₂NiD₄. Mg₂Ni and Mg₂NiD_0.30 show similar neutron diffraction patterns (space-group P6₂22), the metal atoms being in the same crystallographic positions. Three hypotheses have been considered to determine the coordinates of the deuterium atoms. The final R factors obtained for each are very close ($\text{R}\text{? 0.05}$). The approximative size of deuterium inserted in the metallic sublattice has been calculated. It was shown that in Mg₂NiD_0.30 hydrogen diffusion occurs by preferential ways.