Saeid Esmaeilzadeh

Preparation and crystal structure of a new Sr containing sialon phase Sr2AlxSi12–xN16–xO2+x (x≈2)

A nitrogen-rich sialon phase containing Sr or Eu, named the S-phase, has been reported to form in the M′-Si-Al-O-N systems with M′=Sr and Eu. A sample with overall composition Sr 2 Al 2.5 Si 10 O 4 N 14.5 hot-pressed at 1800°C for 2 h contained approximately 85 vol% of the S-phase, and in addition the α- and β-sialon phases and an amorphous phase. Its structure was solved from X-ray synchrotron powder data (λ=1.1608 A), using direct methods, and was refined by the Rietveld method from 131 reflections in the 2θ range 10-59° to R F =2.7%, with the assumed composition Sr 2 Al 2 Si 10 O 4 N 14 , space group Imm2, a=8.2788(9), b=9.5757(9), c=4.9158(4) A, V=389.7 A 3 . The structure model was confirmed by its electron diffraction pattern and by high-resolution electron microscopy studies. The structure exhibits a tetrahedral network with high connectivity, each tetrahedron sharing corners with seven surrounding tetrahedra, and the Sr atoms, irregularly coordinated by eight O/N atoms, are found in tunnels extending along [001].

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi10-xAl18+xN32-xOx (x approximate to l) has been found in the Sr-Si-Al-O-N system. The phase was characterised by ...

(3 + 1)-Dimensional structure refinement of the fresnoite framework-structure type compound Ba2TiGe2O8

The incommensurately modulated structure of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8) has been solved using a (3 + 1)-dimensional superspace approach. The structure is orthorhombic and adopts the superspace group Cmm2(0,beta,1/2)s00 with beta approximately 0.635 at room temperature. The refinement was based on neutron powder diffraction data obtained from a powdered single crystal grown by Czochralski pulling. The modulation parameters that were obtained support the idea that frozen-in rigid-unit modes cause the modulation. The modulation is mainly manifested by positional displacements of O atoms. Barium ions are either eightfold, ninefold or tenfold coordinated in the one-dimensional modulated structure. A significant improvement of the bond-valence sum for both barium positions is achieved by the introduction of the positional modulation. This finding strongly suggests that underbonded barium positions are critically involved in provoking the incommensurate modulation in Ba(2)TiGe(2)O(8).

Synthesis and phase evolution of Mg–Si–C–N ceramics prepared by pyrolysis of magnesium-filled poly(ureamethylvinyl)silazane precursor

Mg–Si–C–N ceramics were synthesized by reactive pyrolysis of poly(ureamethylvinyl)silazane (PUMVS) polymeric precursor filled with Mg metal powder. Pyrolysis of crosslinked polymeric composites with Mg loadings between 10 and 50 wt.% was carried out in argon atmosphere at temperatures up to 1600 °C. The metal–precursor reactions, ceramization and phase evolution were studied using thermogravimetry, infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. Predominantly X-ray amorphous Mg–Si–C–N materials, containing up to 15 mol% Mg, were obtained at 1000 °C, with a homogeneous distribution of magnesium at a length scale above 100 nm. The incorporation of Mg in the materials proceeds in two steps: (i) conversion of the Mg metal to magnesium compounds and (ii) solid state diffusion, assisted by vapour phase transport, of Mg into the amorphous Si–C–N–H bulk. Mg retention in the materials at temperatures above 900 °C is realized due to the formation of Si–Mg–N bonds. Primary crystallization of SiC is initiated by magnesium induced partitioning of nitrogen in the Mg–Si–C–N materials. Stability of the amorphous Mg–Si–C–N phase and crystallization of MgSiN2 at higher pyrolysis temperatures is governed by the free carbon content in the materials and the nitrogen partial pressure in the thermolysis atmosphere. The results indicate the feasibility of synthesizing monolithic MgSiN2-based ceramics through precursor pyrolysis.

Structural studies on the Fresnoite type compound Ba2VSi2O8

Abstract Single crystals of Ba2VSi2O8 have successfully been grown from a high-temperature solution and single crystal X-ray diffraction used to refine its fresnoite type average crystal structure. In contrast to other recently investigated representatives of the fresnoite family of compounds, Ba2VSi2O8 did not at first appear to be incommensurately modulated. Neither single crystal X-ray diffraction nor powder neutron diffraction gave any indication for the existence of satellite reflections in addition to the strong average structure Bragg reflections. A careful electron diffraction study, however, has revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave-vectors q1 = 0.297 [110]* ½c* and q2 = 0.297 [11̅0]* + ½c*. The reciprocal space positioning of these incommensurate primary modulation wave-vectors, the overall (3 + 2)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from the single crystal XRD refinement are all consistent with their arising from condensed Rigid Unit Mode (RUM) modes of distortion of the Ba2VSi2O8 parent structure. The weakness of the satellite reflections in Ba2VSi2O8 relative to, for example, Ba2TiGe2O8 or Sr2TiSi2O8 arises from the significantly smaller amplitude of the condensed RUM mode as is clear from the magnitudes of the refined average structure anistropic displacement ellipsoids.

Meta-stable phases in the MnTaO system

Abstract Studies of compound formation in the system MnTaO showed the existence of six new meta-stable phases. The compound MnTa 2 O 6 was found to exhibit, in addition to the stable tantalite structure, modifications related to the tri-rutile, wolframite, and wodginite structures. These were characterized by powder X-ray diffraction and, for the wodginite modification, also single crystal X-ray diffraction. Three new phases, RI -Mn 4 Ta 2 O 9 , RII -Mn 4 Ta 2 O 9 and R -Mn 11 Ta 4 O 21 were found with rhombohedral symmetries and corundum related structures. The structures of RI -Mn 4 Ta 2 O 9 and RII -Mn 4 Ta 2 O 9 were determined and refined, respectively, from X-ray powder and single-crystal diffraction data. Electron diffraction studies indicate the existence of a chemical inter-growth between corundum and rock-salt type structures.

Single Crystal Refinement of the Incommensurately Modulated Mn0.55Ta0.45O1.7, an Oxygen Deficient Fluorite Type Compound

A structural investigation of the incommensurately ordered compound Mn0.55Ta0.45O1.7 has been carried out, using single-crystal X-ray diffraction data. The basic structure is related to the fluorite type with a monoclinic distortion. Electron diffraction (ED) patterns for various crystal orientations were used for determination of unit cell, incommensurate modulation vector and superspace group. The unit cell parameters and the length and direction of the incommensurate vector were refined using X-ray powder diffraction (XRPD) data. P2/m was chosen as the three-dimensional space group for a subcell with parameters a = 3.5005(2) A (√/2/2 · af), b = 3.5730(2) (√2/2/ · af), c = 5.0015(2) A (af) and β = 91.677(7)°. From the systematic absences hklm: k + m ≠ 2n, the four-dimensional superspace group was determined to be B:P2/m(α0γ) with determined modulation vector components α = -0.1833(1) and γ = 0.3582(2). A total of 511 unique reflections (79 basic fluorite type, 268 first-order and 164 second-order satellites) were used in the structure refinement, which resulted in weighted R-values of 5.6% for the fluorite type sublattice reflections, 5.5% for the first-order satellites and 7.4% for the second-order satellites. The Mn and Ta atoms were found to be both positionally and occupationally modulated and the oxygen atoms to be highly disordered. The structure is the first fluorite-related modulated structure of an oxide that has been determined from single-crystal data. The structural relation to the previously characterised disordered cubic phase Mn0.6Ta0.4O1.65 and the diffuse scattering exhibited by it are discussed in the article. (Less)

A cubic calcium oxynitrido-silicate, Ca2.89Si2N1.76O4.24

The title compound, tricalcium oxynitride silicate, with composition Ca3-xSi2N2-2xO4+2x (x ≃ 0.12), is a perovskite-related calcium oxynitrido silicate containing isolated oxynitrido silicate 12-rings. The N atoms are statistically disordered with O atoms (occupancy ratio N:O = 0.88:0.12) and occupy the bridging positions in the 12 ring oxynitrido silicate anion, while the remaining O atoms are located at the terminal positions of the Si(O,N)4 tetrahedra. The majority of the Ca2+ cations fill the channels along [100] in the packing of the 12-ring anions. The rest of these cations are located at several positions, with partial occupancy, in channels along the body diagonals.

Determination of the magnetic structure at 10 K of Mn3Ta2O8, and refinement of the crystal structure at 295 K, using neutron powder diffraction data

The Mn 2+ -containing oxide Mn 3 Ta 2 O 8 crystallises in the space group I4 1 /a with a=11.2728(2) and c=9.8030(3) A, and with a structure related to the cubic fluorite type. The compound is antiferromagnetic with a Neel temperature of 23 K. The magnetic structure at 10 K was determined from constant-wavelength (λ=1.47 A) neutron powder diffraction (NPD) data and refined by the Rietveld method to R F =2.1% and S=1.3 for 80 reflections with magnetic contributions at 2θ≤60°. The magnetic moments, of magnitude 4.01(2) µB, lie in the ab plane and are antiparallel with the (001) planes. The spin ordering is similar to that in the cubic fluorite structure of UO 2 but with 40% of the magnetic atoms replaced with non-magnetic Ta atoms. The non-magnetic crystal structure at 295 K was refined to R F =0.6% using time-of-flight (TOF) NPD data, anisotropic thermal parameters and 1943 reflections with d≥0.55 A.

Determination of the fluorite related structure of Mn3Ta2O8, using synchrotron X-ray powder and electron diffraction data

The Mn 2+ -containing oxide Mn 3 Ta 2 O 8 has been synthesised at 1200°C in Ar atmosphere, and its structure has been solved from X-ray synchrotron powder data (λ0.65 A) by direct methods. The structure was refined by the Rietveld method to R F =6.3%, with space group I4 1 /a, a=11.2728(2), c=9.8030(3) A, V=1243.47 A 3 , from 1190 reflections with d≥0.65 A. It is related to the fluorite structure with a ≊√5a f and c≊2a f . The Ta atoms are octahedrally coordinated by oxygen atoms and the three crystallographically different Mn atoms by 7, 4 + 4 and 4 oxygen atoms. Electron diffraction patterns show the presence of weak superstructure reflections corresponding to a primitive unit cell with a′=a and c′=6c. The melting point of Mn 3 Ta 2 O 8 is 1470°C in Ar atmosphere. It is a semiconductor with an activation energy of 1.2 eV and a conductivity σ=3.7×10 –5 S cm –1 at 600°C. The magnetic susceptibility shows a maximum at 23 K and a Curie-Weiss behaviour at higher temperatures, with µ eff =5.7(1) µ B per Mn atom. When Mn 3 Ta 2 O 8 is oxidised at 1100°C in air an Mn-Ta oxide forms, which has a wolframite type structure with unit cell a=4.7574(5), b=5.7296(6), c=5.1133(4) A and β=91.202(9)°.