Hexagonal Description Research Articles

A model structure for interfacial phase change memories: Epitaxial trigonal Ge1Sb2Te4

The structural characteristics of epitaxial Ge1Sb2Te4 deposited by MOVPE are reported. X-ray diffraction, electron backscatter diffraction as well as high resolution transmission electron microscopy were carried out. The Ge1Sb2Te4 layers crystallize in the trigonal space group R3¯m with lattice constants a=4.27Å and c=41.0Å (in hexagonal description). Seven alternating anion and cation layers forming blocks separated by (van der Waals) gaps oriented parallel to the Si (111) substrate can be used to describe the structure. Except for the formation of crystallographic twins (rotation by 60° around the [0001] direction), no other defects are found. The results indicate the existence of mixed cation layers and a stacking sequence (Ge0.5Sb0.5)-Te-(Ge0.25Sb0.75)-Te-Te-(Ge0.25Sb0.75)-Te. The structural relation to interfacial phase change memories based on superlattices is discussed.

Exploring structural similarities between crystal phases using EBSD pattern comparison

We discuss the application of EBSD for the analysis of structural features of magnetite and hematite relevant in the topotactic growth of both phases. The orientation relationships of both phases are investigated using characteristically related sets of Kikuchi patterns, which were collected from topotactically intergrown hematite variants and their parent magnetite crystal grain. We address the hexagonal description of trigonal and cubic phases with respect to crystallographic relationships. The combination of locally resolved EBSD measurements and microstructural investigations thus offers a promising opportunity to evaluate orientation relationships and intergrowth between magnetite and hematite.

Crystal growth and characterization of cuprous ferrite (CuFeO2)

The growth of bulk cuprous ferrite (CuFeO2) single crystals by the travelling-solvent, floating-zone technique is reported. Crystals 5 to 8 mm in diameter by 10 to 30 mm have been grown from two sets of starting materials: Cu2O + Fe2O3 and 2CuO + Fe2O3, respectively. The trigonal unit-cell parameters determined by X-ray diffraction are ah = 3.035(1) Å and ch = 17.161(4) Å in the hexagonal description. Mössbauer studies reveal an isomer shift for 57Fe at room temperature of 0.40 mm/s with respect to metallic iron, confirming that iron is in the trivalent state and that CuFeO2 is cuprous metaferrite (Cu+Fe3+O2). Magnetic susceptibility indicates that CuFeO2 undergoes two successive magnetic transition temperatures around TN1 = 13.5 K and TN2 = 9.5 K, respectively. Thermal analysis reveals that, under an argon atmosphere, CuFeO2 partially decomposes at 1180°C to form Fe2O3 (or Fe3O4) and Cu2O.

Phase equilibria in the ternary Ho [sbnd]Fe [sbnd]Nd system at 500°C

The solid state phase equilibria and phase relationship in the ternary Ho Fe Nd system at 500°C were investigated mainly by X-ray powder diffraction and by differential thermal microanalysis. The isothermal section at 500°C of this system has been established. The existence of the binary compounds Fe 1, Nd 2, in the Fe Nd system and the δ phase (Sm type) in the Ho Nd system were confirmed. The δ phase belongs to the rhombohedral system with a = 0.3636 nm and c = 26146 nm in the hexagonal description. The composition range of the δ phase is from 45 to 65 at.% Nd. The isothermal section consists of nine single-phase regions, 15 two-phase regions and six three-phase regions. The compounds Fe 2, Nd 2, and Fe 1, Ho, form a continuous series of solid solutions. At 500°C. the solid solubilities of Fe in α-Ho, α-Nd or the δ phase are small and Fe 1, Nd 2, Fe 3, Ho 6, Fe 3Ho and Fe 3Ho are non-variable compounds.

A New Diamond-Related Compound in the System Al 2O 3-Al 4C 3-AlN

A new compound has been identified in the system Al 2O 3 Al 4C 3-AlN, which crystallizes in a diamond-related structure. This compound, for which the acronym "ALCON" is used, has the composition Al 28O 21C 6N 6. ALCON crystallizes in a rhombohedral space group ( R - 3 m, no. 166): a = 5.459(2) Å and c = 14,952(6) Å (hexagonal description) or a = 5.897(2) Å and α = 55.15(4)° (rhombohedral description). ALCON is a compound with a very limited phase width with respect to the molar C/O ratio and the aluminium content. Electron diffraction results are in agreement with the structure as determined by X-ray powder diffraction. High-resolution transmission electron microscopy revealed the existence of twin defects in the (100) plane (hexagonal description).

The crystal structure of AuNi 2Sn 4

A hitherto unknown, virtually stoichiometric phase with composition AuNi 2Sn 4 has been identified in the AuNiSn system, and its structure has been determined by direct methods. The unit cell is rhombohedral (space group: R 3 m) with dimensions a = 422.6(1) pm and c = 2656.6(4) pm in the hexagonal setting and a = 918.2(2) pm and α = 26.60(1)° in the rhombohedral setting. According to the hexagonal description Au is in 3 b and Ni, Sn(I), and Sn(II) are in 6 c with z = 0.40091(6), 029965(3), and 0.11204(3), respectively. AuNi 2Sn 4 proves to be isostructural with the mineral smythite (Fe 3S 4- s). The Fe 3S 4- s-type structure belongs to the NiAs-type family and certain aspects of the structural relationships are briefly discussed.

Growth and characterization of CuFeO 2 single crystals

In this paper, the first successful growth of CuFeO 2 single crystals by applying the traveling solvent floating-zone technique is reported. Typical crystals of 5–8 mm in diameter and 10–30 mm in length have been obtained from the starting materials Cu 2O and Fe 2O 3 with the molar ratio 1:1. The as-grown crystal is homogeneous and of the trigonal system with cell parameters a h = 3.04Å and c h = 17.17Å in the hexagonal description. In Mössbauer effect (ME) measurements, the isomer shift for 57Fe in CuFeO 2 at room temperature is 0.40 mm/s with respect to metallic iron, which offers experimental confirmation that the iron is present in the trivalent state. Magnetic susceptibility measurements indicate that CuFeO 2 has two successive magnetic transition temperatures at T N = 13.5 K and T N = 9.5 K, respectively. Thermal analysis reveals that CuFeO 2 partially decomposes into Fe 2O 3 and Cu 2O at about 1180°C.

Powder neutron diffraction studies of the crystallographic and magnetic structures of Ho2Fe17-xAlxC(x=4 and 7)

The crystallographic and magnetic structures of Ho2Fe17-xAlxC(x=4 and 7) were investigated by powder neutron diffraction at room temperature. The results indicate that both compositions are isostructural with Th2Zn17 (space group, R3m). Aluminium substitutes for iron in three of the four Fe sites (18h, 18f and 6c in the hexagonal description of the rhombohedral cell) but not in 9d, and the fractional occupancies have been determined for both compositions. The carbon atoms occupy 9e sites. The saturation magnetization and Curie temperatures of samples with x in the range 0-7 are given; in the room-temperature magnetic structure of the phase with x=4 the magnetic moments of all the atoms lie in the plane perpendicular to the 3 axis, with the Fe moments being ferrimagnetically coupled to those of Ho.

Crystal structure of the ordered Nb 10Ge 7 phase

The structure of a new ordered phase, Nb 10Ge 7 (or Nb 5Ge 3Ge 0.5; trigonal), is described on the basis of electron microscopic and electron diffraction data. The new phase was found in a Nb Ge tape prepared by the coevaporation technique. The new structure can be derived from the hexagonal D8 8-structure of Nb 5Ge 3 by periodically inserting Ge-atoms at octahedral sites. The lattice parameters of this structure can be described as follows, using the lattice parameters of the D8 8-structure a= √3 ·a Nb 5Ge 3(D 88 and c=C Nb 5Ge 3(D 88) in hexagonal description. The grains of this phase are usually elongated along the c-direction. Structural data of the new phase: Nb 10Ge 7,trigonal, hP51, P 3¯2/ml,a= 1.337nm,c= 0.537nm,Z= 3,V Nb 10Ge 7 ≈ 3V Nb 5Ge 3(D 88) . The structure represents a new structure type with an ordered atom arrangement, intermediate between the Mn 5Si 3 type and the Ti 5Ga 4 type, in which the octahedral sites are only partly occupied.

Crystal structure of cadmium cyclohexaphosphate hexahydrate: Cd3P6O18 · 6 H2O

AbstractChemical preparation and crystal structure of Cd3P6O18 · 6 H2O are described. This compound is trigonal (rhomboedral) with the following unit‐cell dimensions: or The space group is R3 with Z = 6 for the hexagonal description. The crystal structure has been solved by using 1355 unique reflexions with a final R value 0.013.The P6O18 ring anions have a threefold symmetry. The cadmium coordination polyhedron is a slightly distorted octahedron built up by four oxygen atoms and two water molecules. The water molecules link themselves as to build six‐member rings having a 3 symmetry alterning with the P6O18 phosphoric rings along the 3 axes.

Crystal chemistry of some addition compounds of alkali iodates with telluric acid

Five addition compounds of telluric acid with alkali iodates are described. Three of these compounds are orthorhombic and isotypic: RbIO3·Te(OH)6, KIO3·Te(OH)6, and NH4IO3·Te(OH)6. The crystal structure of this class of compounds has been solved with the potassium salt whose unit-cell dimensions are a = 14.30(1) Å, b = 6.734(5) Å, c = 8.711(5) Å, Pc21n (C92v). Z = 4. Crystal data are given for the two isotypic salts. The crystal structures of two other compounds have been determined. 2KIO3·Te(OH)6 is trigonal, R3̄, with the unit cell dimensions a = 6.500(5) Å, c = 25.717(8) Å, Z = 3 for the hexagonal description. NH4IO3·Te(OH)6·H2O is triclinic (P1̄) with a = 10.99(1) Å, b = 6.932(4) Å, c = 6.559(4) Å, α = 88.89(5), β = 90.88(5), γ = 104.48(5)°, and Z = 2. For these three different crystal structure determinations the final R values are, respectively, 0.047, 0.030, and 0.036. The first class of compounds described in this paper, MIO3·Te(OH)6 (M = K, Rb, NH4) is a new class of piezoelectric materials.