Tetragonal Superstructure Research Articles

X-ray structural study of calcium manganese oxide by rietveld analysis at high temperatures [Ca 2MnO 4.00

The structure of the polycrystalline Ca 2MnO 4.00 has been investigated by high-temperature X-ray diffractometry and Rietveld analysis. The polycrystalline Ca 2MnO 4.00 has the K 2NiF 4-type tetragonal ( I4 1 acd ) super structure ( a=✓2a′, c=2 c′) of tetragonal ( I4 mmm ) primitive structure in the temperature range 298–977K in air. The composition remains unchanged during further conditions. This I4 1 acd super structure contains the rotated MnO 6 octahedra about their c axes, and magnitude of the rotation decreases with increasing temperature. The temperature dependence of the a parameter is governed by the rotation of the MnO 6 polyhedra, and that of the c parameter is dominated by the tetragonal distortion of the CaO arrangements in the structure.

Cation-deficient Mn, Co spinel oxides obtained by thermal decomposition of carbonate precursors

Co, Mn mixed oxides with spinel-related structure are prepared by the thermal decomposition of mixed carbonates under air atmosphere at 400 and 600°C. These oxides are cation-deficient phases with stoichiometries ranging from MO 1.333 to MO 1.6 that depend on Mn content and temperature of preparation and show tetragonal to cubic spinel structure as the Co content increases. Additionally, the X-ray diffractograms and electron microscopy data reveal the occurrence of vacancy ordering in a tetragonal superstructure, which is similar to γ-Fe 2O 3 in cubic phases and implies a′ = a and c′ = 3 c for tetragonally distorted spinels.

YCoC and Isotypic Carbides with a New, Very Simple Structure Type

Abstract Crystal Structure, Equiatomic Ternary Rare Earth Metal Cobalt Carbides The crystal structure of the new compound YCoC was determined from X-ray powder data. It is tetragonal, space group P42/mmc, with a = 0.36500(4) nm, c = 0.68636(9) nm and Z = 2 formula units per cell. The residual for a refinement of Debye-Scherrer data is R = 0.048 for 22 structure factors and 3 variable parameters. The structure is of a new type with no variable positional parameter. The arrangement of the metal atoms corresponds to that of the CsCl structure. The tetragonal superstructure with a doubled c axis arises through the ordered insertion of carbon atoms on octahedral sites formed by four Y and two Co atoms. The hydrolysis of YCoC in hydrochloric acid yields mainly methane, propane, and ethane. The compounds LnCoC (Ln = Gd -Tm, Lu) are isotypic with YCoC.

Tetragonal rare earth hydrides REH(D) ≈2.33 (RE  La, Ce, Pr, Nd, Sm) and a neutron diffraction study of NdD 2.36

The rare earth (RE) metals lanthanum, cerium, praseodymium, neodymium and samarium form hydrides REH ≈2.33 and deuterides RED ≈2.33 with a tetragonally distorted fluorite structure. X-ray investigations show a facecentred tetragonal arrangement of the rare earth cations and a lattice parameter ratio c a > 1 . Neutron diffraction studies of NdD 2.36 yield a tetragonal superstructure with c a > 2 and space group I4 1 md. The structure is related to the REH 2 (CaF 2) cell; some of the tetrahedral sites are vacant, but half or almost half of the octahedral sites are occupied by deuterium atoms in an ordered manner. These results are compared with those of other workers.

Gemischtvalente Bromoaurate(I,III) und ihre Mößbauer-Spektren / Mixed Valent Bromo Aurates(I.III) and their Mößbauer Spectra

New nonstoichiometric cesium bromo aurates(I,III) Cs1+x(AuBr4)(AuBr2, Br3,Br)x have been synthesized, in which the almost equally shaped anions AuBr3- and Br3- can interchange themselves. The compounds crystallize in a disordered cubic perovskite structure with a = 547.5 pm or in a tetragonal superstructure with a 16-fold perovskite cell (a = 1553, c = 1099 pm). The investigations show that other bromoaurates as the previously reported "Cs2AgAuBr6" can contain perceptible amounts of Br3- ions also. The IR and 197Au Mössbauer spectra of the new bromoaurates and of Cs2AuoBr6 are discussed

Mise en évidence d'une solution solide de type spinelle dans le diagramme de phase du systeme InS

Evidence is presented for normal-spinel domain of homogeneity between the compositions InS 1.50−ε and InS 1.35. Structural study of a single crystal of composition InS 1.44 indicates indium vacancies on the tetrahedral sites. The compound In 2S 3, regardless of conditions of formation, is a tetragonal superstructure of the spinel lattice ( a 0) with a = a 02 1 2 and c = 3 a 0. The spinel-type domain shows peritectic decomposition at 850°C for the composition InS 1.40.

Systeme Ga 2Se 3FeSe. Etude cristallographique diagramme de phase

This system is studied by D.T.A., metallographic and X-ray diffraction methods. Three phases are described in terms of n the atomic radio of Fe Fe+Ga . From n = 0 to n = 0.17 a solid solution of the sphalerite-type was identified; for n = 0.25 a non stoichiometric compound, CdGa 2S 4 type, tetragonal superstructure of the sphalerite-type exists and for n = 0.33 the stable phase is a FeGa 2Se 4 tetragonal pseudo-cubic compound of the CdIn 2Se 4 type. The compound FeGa 2Se 4 undergoes a peritectic decomposition at 970°C.

Optical and electrical properties of quarternary chalcogenides

Optical and electrical characterizations have been made on four compounds of the type Cu 2ZnA IVX 4 VI with A IV = Si, Ge and X VI = S, Se. These compounds belong to either a tetragonal superstructure of zincblende (stannite) or to an orthorhombic superstructure of wurtzite (wurtz-stannite) where a or = 2a w, b or = a w/ tr3, and c or = c w. The electrical and optical properties are found to parallel the size of the group IV or group VI atom present.

A neutron diffraction study of the hydrides of the early lanthanide elements at room temperature

The order structure of CeD2.29 at room temperature has been determined by profile analysis of powder neutron diffraction data. A non-stoichiometric tetragonal superstructure is adopted; space group I41md, lattice parameters a=5.538(1), c=11.046(2)AA. The ideal composition of the superstructure is CeD2.5 but the fluorite phase is not ordered at this composition because of a high level of intrinsic disorder. It is presumed that the superstructure exists over a range of composition up to approximately CeD2.4 and identical order structures have been determined in this range for LaD2.30 and PrD2.37.

Tetrahedral quaternary chalcogenides of the type Cu 2IIIVS 4(Se 4)

A survey is given on lattice data and structure types of 40 compounds of the type Cu 2IIIVS 4(Se 4) with II = Mn, Fe, Co, Ni, Zn, Cd, Hg and IV = Si, Ge, Sn. Of the 42 possible compounds, 22 had been known. The missing ones have been synthesized and characterized by x-ray powder methods. Three tetrahedral structure types, differing in symmetry and unit cell size exist: 1) The stannite structure (tetragonal superstructure of sphalerite with a tetr≈a Sph, c tetr≈2a Sph). 2) An orthorhombic superstructure of wurtzite (a or≈2a W, b or≈a W√3, c or≈c W). 3) A hitherto unknown structure type based on slightly distorted, sphalerite-sized cells of tetragonal, orthorhombic or monoclinic symmetry.

An electron microscope study of thin ferrite films prepared by the oxidation of high purity vacuum-evaporated metal thin films

The oxidation of thin films of nickel, cobalt, iron, NiFe2 and CoFe2 has been investigated between 200 and 1200° C. The oxidation products for the elemental metals differ from the oxidation products observed in previous work upon bulk material. The oxidation mechanism proposed for bulk material is in general still valid in the thin film situation. Above oxidation temperatures of approximately 850° C both NiFe2 and CoFe2 form the respective ferrites, although in the case of nickel ferrite, traces of the γ-Fe2O3 tetragonal superstructure can still be detected at oxidation temperatures of 1200° C. Films of nickel ferrite and cobalt ferrite upon single-crystal magnesium oxide substrates, produced by oxidation of vacuum-deposited NiFe2 and CoFe2 thin films, have been investigated by transmission and scanning electron microscopy. It has been found that (100) nickel ferrite prepared by this technique grows epitaxially upon (100) magnesium oxide.

On the structure of tetragonal tin-tungsten bronzes

The structures of two bronzes of composition Sn 0.11WO 3 and Sn 0.25WO 3 have been investigated by single-crystal methods. They are the well-known tetragonal structures with 5-sided and 4-sided tunnels passing through a matrix of linked W-O octahedra. Electron density maps show that the tin occupies two or three sites within the 5-sided tunnels leaving the 4-sided tunnels empty. It is suggested that the available sites are not randomly occupied, but that each crystal contains domains in each of which only one type of site is occupied. The crystal of Sn 0.11WO 3 contains two types of domain and that of Sn 0.25WO 3 contains three. Tetragonal superstructures exist with A = a✓2 C = 2 c . No investigation of the nature of the ordering has been carried out.

Electron microscopic study of the defect structure due to ordering in the alloy Ni 4Mo

Ordering of the disordered FCC alloy Ni 4Mo leads to a tetragonal superstructure with c a = 0.986 . As a result a single crystal of the alloy can present three different classes of interfaces resulting from the ordering process. The characteristic contrast features of the different types of interfaces are indicated and evidence is presented that these three classes of interfaces have been observed in Ni 4Mo.

Elektronenbeugungsuntersuchung der oxydation von UO 2 und der struktur einer U 3O 8− x-überstrukturphase

Thin films of UO 2, which were either fine-crystalline or mono-crystalline were treated in an electron diffraction set with oxygen (10 −3 torr) between 300 and 750°C for about 20 hr. Diffraction photographs were taken in transmission from time to time. The fine-crystalline specimens showed transformation to UO 2+ x and U 3O 8− x . Other phases, as U 3O 7 or U 2O 5 e.g. could not be observed. The hexagonal U 3O 8− x -phase exhibits a super structure cell with a = 6.72 A ̊ , c = 6 × 4.105 A ̊ , whose c-axis is 6 times as large as the U 3O 8 cell observed by Siegel. (3) The lattice constant of UO 2+ x decreases with increasing time of oxidation, but increases at the moment, when the U 3O 8− x -superstructure phase is formed. This shows, that part of the oxygen for the formation of U 3O 8− x must be taken from UO 2+ x . At a pressure of 10 −4 torr the mono-crystalline specimens from a superstructure with a = 3 a UO 2+ x ; at 10 −3 torr a tetragonal superstructure with a = 4 a UO 2+ x is formed. The latter phase has the composition UO 2,19, it probably forms the oxygen-rich phase border of UO 2+ x and its atomic positions were given in a former paper (1). The atomic arrangements during the oxidation of UO 2 to U 3O 8 can be described in a similar way as with the systems NbO and TaO: in the beginning additional oxygen atoms are built into the lattice in a disordered manner. At distinct concentrations they arrange in long periods, forming superstructure cells. This also is in accordance with the formation of superstructures during reduction of U 3O 8, as observed by Sato. (8,9)