Rhombohedral Modification Research Articles

An Investigation of the Antimony-Containing Minerals used by the Romans to Prepare Opaque Colored Glasses

ABSTRACTRoman colored opaque vessel glasses and mosaic tesserae were examined using energy dispersive X-ray analysis, wavelength dispersive X-ray analysis, and scanning electron microscopy in order to identify the origins of the antimony-based glass opacifying agents used in the Roman period. Bindheimite and stibnite were considered as mineralogical sources of antimony, and antimonial litharge was investigated as a metallurgical source of antimony. The refining of antimonial silver ores was discussed as a source for antimonial litharge in the Roman period. The morphologies of the antimonate crystallites, their distributions, and the observed correlations of lead to antimony in the glasses indicated that roasted stibnite was the antimony source for the white and blue opaque glasses and antimonial litharge was the antimony source for the yellow and green opaque glasses. Opaque yellow Roman glasses were found to contain a mixture of clastic, subhedral, and euhedral lead pyroantimonate (Pb2Sb2O7) particles. The euhedral crystallites are a rhombohedral modification of Pb2Sb2O7that is formed above 900 °C.

Sub-solidus phase equilibria in the NdF 3-Nd 2O 3 system

The sub-solidus phase equilibria in the NdF 3-Nd 2O 3 system was investigated over the whole composition range by powder X-ray diffraction (XRD) and differential thermal analysis (DTA) between room temperature and 1573K. The intermediate compound neodymium oxyfluoride was found to exhibit tetragonal, rhombohedral and cubic modifications. The tetragonal phase exists over 20 to 40 m% Nd 2O 3 and rhombohedral phase has a fixed composition of 50 m% Nd 2O 3. The rhombohedral phase transforms to cubic at around 826K.

High-temperature X-ray diffraction study of the rhombohedral-cubic phase transition of ROF with R=Y, La, Pr, Nd, SmEr

The rhombohedral-cubic phase transition of ROF with R=Y, La, Nd, SmEr and the expansion coefficients of both modifications were studied by the high-temperature Guinier X-ray powder diffraction method. It was found that a constant formula volume increment of 0.37±0.04 Å 3, which is independent of the cationic size, is characteristic of this transition. The formula volumes depend practically linearly on temperature, and the volume expansion coefficients, which are generally larger for the cubic than for the rhombohedral modification, vary irregularly across the R 3+ series. For the case of YOF, LaOF, NdOF, SmOF, GdOF and ErOF it could be shown that experimental data for the transitional volume changes, entropies and pressure dependence of the transition temperatures obey the Clausius-Clapeyron equation within the accuracy limits of the data.

Study of the thermal behaviour of [Cr(NH3)6]MF6 (M=CR, Al, Fe, Ga and In) and [Cr(NH3)6]F3·HF·H2O

The thermal behaviour of the hexamminechromium hexafluorometallates and [Cr(NH3)6]F3. HF·H2O was investigated by non-reciprocal quasi-isobaric thermal analysis, X-ray diffraction and i.r. spectroscopy. Reduction of chromium(III) was not observed, neither during the decomposition [Cr(NH3)6]F3·HF·H2O nor during the decomposition of any of the title hexamminechromium hexafluorometallates. Obviously this reduction is not promoted by the coordinative Cr−N bonds, neither these in the starting materials nor those in the intermediately formed phases. Under non-reciprocal quasi-isobaric conditions, hexamminechromium hexafluorometallates are ideal precursors for preparing mixed cationic fluorides both in the rhombohedral modification and in the modification of the hexagonal tungsten bronze type structure.

Lithium disorder in the vicinity of the superionic phase transition in monoclinic and rhombohedral Li3In2(PO4)3

Rhombohedral and monoclinic modifications of the Li3In2(PO4)3 compound have been studied by X-ray diffraction, infrared (i.r.) analysis, Raman light scattering, differential thermal analysis (DTA), differential scanning calorimetry (DSC) and impedance spectroscopy, in a wide temperature range of 290–600 K. Analysis of the data allows the suggestion that the superionic phase transition, observed in these materials in the temperature interval 370–385 K, belongs to Faraday type phase transformations. PO4 tetrahedra, which are a partition of the (In2P3O12) 300 3− rigid skeleton, have been found deformed. This seems to be due to an ordered lithium ion-vacancy distribution in the low temperature, non-superionic, polymorphs. With increasing temperature these deformations disappear. This disappearance can be caused by a decrease of the correlation length of the lithium ion-vacancy order.

The sputtering of different modifications of boron nitride under ion and electron irradiation

The sputtering of boron nitride crystals of hexagonal (h-BN) and rhombohedral (r-BN) modifications was studied experimentally and by computer simulation. It was shown that sputtering yields are greater and change more rapidly with the energy, E, of the incident argon ions ( E = 0.3–3 keV) for r-BN than for h-BN. The energy and space distributions of N and B atoms sputtered from the (0001) face are different for the two modifications of BN. The features of the observed phenomena are discussed.

Energy-band structure of SiC polytypes by interface matching of electronic wave functions.

We interpret SiC polytypes as natural superlattices, consisting of mutually twisted cubic layers. A method is presented to calculate the electron band structure of any polytype, based on an empirical pseudopotential description of cubic SiC. Bloch and evanescent waves, belonging to cubic layers are matched at interfaces in order to make up the wave functions of the respective polytypes. Band gaps of hexagonal and rhombohedral modifications are in excellent agreement with experimental data such that the nearly linear relationship between the indirect gap and the hexagonal nature is reproduced. A simple explanation of this relationship is given in terms of a Kronig-Penney-like model.

Structural features of boron nitride dense phase formation from rhombohedral modification under high static pressure

The real structure of boron nitride (BN) polycrystals after high-pressure treatment was investigated by X-ray and transmission electron microscopy methods. Different stages of boron nitride polycrystal evolution under high quasistatic pressure and high temperature were fixed, involving rhombohedral boron nitride twinning, one-dimensionally disordered structure, 2H, 4H, 3C dense phase formation and 3C-BN recrystallization. A new structural scheme of BNr-BNw transition is proposed.

Beiträge zum thermischen Verhalten von Sulfaten

Abstract Well shaped crystals of Cr2(SO4)3 and Al2(SO4)3 could be prepared by chemical transport reactions using HCl (Cr2(SO4)3) or SOCl2 (Al2(SO4)3) as transport agent. Cr2(SO4)3 and Al2(SO4)3 are isostructural to the rhombohedral modification of Fe2(SO4)3 and crystallize in the trigonal space group R[unk] with a = 8.1245(7) (8.0246(4)) Å and c = 21.944(3) (21.357(1)) Å, Z = 6 (hexagonal setting). The crystal structures were refined to a residual R = 0.035 using 828 unique reflections with |F|≥3 σ|F| for Cr2(SO4)3 and to R = 0.026 (772 unique reflections with |F|≥3 σ|F|) for Al2(SO4)3. Main building units of the two compounds are MO6-octahedra and SO4 −-tetrahedra which are linked via common corners to a 3-dimensional network.

Shear mechanisms of phase transitions in boron nitride

Abstract The behaviour of rhombohedral modification of boron nitride has been studied under static pressures within the range from 3.5 to 7.7 GPa. At low temperatures when diffusion processes are ruled out a great amount of wurtzite boron nitride was found to form. The analysis of the texture phase relation for compacts prior to and after thermobaric treatment indicates unambiguously that a transformation mechanism is realized which is associated with the splitting of basic layers by buckling. For this mechanism of phase transformation the orientation relation of crystal lattices is as follows (100)w//(001)r; [1100]w//[0001]r which corresponds to texture relations found experimentally.

“Rigid” skeleton dynamics of Li 3In 2(PO 4) 3 superionic conductor

The effect studied is made by the thermal prehistory of the Li 3In 2(PO 4) 3 ceramics on their structural and electrophysical characteristics. Annealing at T>1290 K is shown to result in a monoclinic (HT) and at T<1230 K, rhombohedral (LT) modifications of the crystal structure of Li 3In 2(PO 4) 3. At the same temperature the ionic conductivity (σ) of the HT-modification is found to be higher than that of LT-modification. Annealing of LT-ceramics at ∼985 K leads to a decrease of σ by as much as two order of magnitude seems due to Li +-ions redistribution in the [In 2P 3O 12] 3- 3∞ skeleton cavities. In the 290–650 K temperature range phase transitions both in LT-and HT-Li 3In 2(PO 4) 3 are observed.

Defect Morphology of GeSexTe1−x Single Crystals grown by physical vapor transport

AbstractSlip lines and bands, mosaic patterns, dislocations, and twins, produced by the atom displacement type phase transformation from the cubic (high T) to the rhombohedral (room T) modification of GeSexTe1−x single crystals (x = 0.1–0.4), were observed by chemical etching, microscopic, and Laue X‐ray back reflection techniques. Structural defects increase with Se content of the crystals. Post‐growth annealing improves the crystal morphology. Vacancy clusters in Ge0.49 (SexTe1−x)0.51 (x = 0.1, 0.2) are Ge vacancies.

Molecular dynamics in perfluoro-n-eicosane. II. Components of disorder

The defect structure in the rhombohedral modification R of perfluoro-n-eicosane has been analyzed in a temperature-dependent x-ray and neutron scattering study. The analysis was based on an evaluation of Bragg-reflection intensities and of the diffuse scattering. Different components of disorder were detected and characterized. At the transition from the low temperature phase to modification R, rotation of the molecules about their long axes sets in. As shown by the azimuthal distribution of the diffuse scattering, rotations occur cooperatively with correlations between neighbors. The Debye–Waller factors measured for the layer reflections are indicative for a moderate roughening of the interfaces due to a restricted longitudinal diffusive motion. Molecules are perturbed by the formation of helix reversal defects. Evaluation of the diffuse scattering yields the associated finite azimuthal correlation length. There is also evidence for a continuous slow untwisting of the helix. With increasing temperature, it approaches the all-trans form.

Intrinsic magnetic properties of R2Fe17CyNx compounds: (R=Y, Sm, Er, and Tm)

Samples of R2Fe17CyNx (R=Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17CyNx irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17CyNx are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17CyNx compounds at room temperature because some N was lost during the heating to Tc. In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17CyNx when y&amp;lt;1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17CyNx are higher than that of Sm2Fe17Nx. Indications of a magnetic phase transition were found also in Sm2Fe17C0.7Nx and Sm2Fe17C0.9Nx.

The reversible crystal-quasicrystal transitions in icosahedral Al—Cu—Fe alloys

Abstract A perfect icosahedral phase, which shows narrow diffraction peaks, exists in Al—Cu—Fe alloys only within a restricted temperature range, between 1020 K and 1120 K. It undergoes two types of reversible transitions. Above the upper temperature limit, the icosahedral phase transforms to a mixture of crystalline phases and residual liquid by a peritectic reaction. Below the lower-temperature limit, it turns into a closely related rhombohedral modification through a solid state transition. The two types of reactions were studied in detail by powder neutron diffraction techniques the results of which are reported on. These results are complemented by electron microscopy data.

Structural phase transition in the ordered fluorides MIIZrF6 (MII=Co,Zn). I. Structural study

The mixed fluorides with formula MIIZrF6 (MII=Co,Zn) exhibit an ordered ReO3-type structure: this structure is made of a three-dimensional arrangement of corner-shared octahedra with an ordered distribution of MII and ZrIV cations in the octahedra sites. These materials undergo around room temperature a first-order ferroelastic structural phase transition from a cubic phase (Fm3m) to a rhombohedral modification (R3), as established by neutron and X-ray diffraction experiments. From a structural determination of CoZrF6 at different temperatures through the phase transition, three order parameters have been measured, which enable one to describe completely the extent of lattice distortion in the rhombohedral phase: the spontaneous strain es, the coordinate of octahedra rotation around the threefold axis R and the octahedra internal deformation coordinate Q. Important thermal variations are observed for es and R, but the coordinate Q always remains very small, showing that the Co(Zr)F6 octahedra are essentially rigid bodies. X-ray diffuse scattering experiments on CoZrF6 at room temperature (cubic phase) are consistent with the existence of structural disorder due to rotations of the Co(Zr)f6 octahedra around the cubic fourfold axes.

Structural phase transition and structure refinement of triclinic EuMo6S8 at 14 K studied by x-ray powder diffraction.

Lattice-parameter measurements of ${\mathrm{EuMo}}_{6}$${\mathrm{S}}_{8}$ as a function of temperature show an increase of the cell volume by 1.0 A${\r{}}^{3}$ as the rhombohedral high-temperature modification transforms at ${T}_{l}$\ensuremath{\sim}110 K into the triclinic low-temperature modification. The triclinic structure at 14 K was refined from film data by using the x-ray Rietveld method. The results show a distortion of the ${\mathrm{Mo}}_{6}$ octahedra by up to 0.07 A\r{} compared to those in the rhombohedral high-temperature modification.

Structure and properties of triclinic Ni 0.85Mo 6Te 8

The structure of Ni 0.85Mo 6Te 8 was refined from single-crystal X-ray diffraction data at room temperature. It is triclinic, space group P 1, a = 7.028(2) A ̊ , b = 7.100(2) A ̊ , c = 7.102(2) A ̊ , α = 91.23(2)°, β = 95.73(2)°, γ = 90.82(2)°, V = 352.48 A ̊ 3, Z = 1 ; 1619 reflections, 75 refined parameters, R = 0.031. The Mo atoms form distorted octahedral clusters (2.69 Å ≤ d intra[MoMo] ≤ 2.81 Å; 3.58 Å < d inter[MoMo]). The Ni atoms are disordered (site occupancy: 0.423(7); d[ NiNi] = 2.586(6) A ̊ ) , and interact strongly with one Mo 6 cluster ( d[ NiMo] = 2.603(3) and 2.958(3) A ̊ ), and weakly with another ( d[ NiMo] = 2.985(3) A ̊ ). The structure transforms at 1057(5) K into a rhombohedral modification ( a hex = 10.457(2) A ̊ , c hex = 11.866(3) A ̊ at 1073 K). Measurements on powders suggest metallic conductivity (5.1 × 10 −4 Ω-cm at 293 K) and weakly temperature-dependent paramagnetism (110 × 10 −6 emu/g at 100 K).

Structure and some properties of fine-grained graphite-like boron nitride

The structure of boron nitride synthesized by carbothermal reduction and simultaneous nitridation in the temperature interval 1000–1450 °C was studied by transmission electron microscopy and X-ray powder methods. A starting mixture based on the homogeneously distributed fine-grained amorphous boron oxide and carbon phases was employed for synthesis. It was found that the graphite-like BN produced consisted of a mixture of hexagonal and rhombohedral modifications; the content of the latter amounts to up to 30%. It was concluded that the formation of the polycrystalline particles from BN with various extents of three-dimensional ordering is due to contact reduction of B 2O 3; the prismatic crystals of the rhombohedral phase are formed via gas-phase reactions. The chemical stability in acids of BN produced at 1400 °C is comparable with the stability of coarse hexagonal BN powders.

The deposition characteristics and the structural nature of the deposit in the chemical vapour deposition of boron

Boron was deposited onto graphite by a chemical vapour deposition (CVD) technique using gaseous mixtures of BCl 3 and hydrogen. The CVD of boron is a thermally activated process with an apparent activation energy of 18.8±0.7 kcal mol -1 at low temperatures and high gas flow rates. At high deposition temperatures and low gas flow rates the deposition rate depends on the gas phase mass transfer. Boron is deposited as an amorphous material below 1250 K, and mainly as the β rhombohedral modification above 1250 K. However, a small amount of β tetragonal boron is deposited together with the β rhombohedral boron. The β rhombohedral boron has a strong (104) preferred orientation at 1500 K. However, the preference for the (104) orientation is reduced as the BCl 3 fraction or the gas flow rate increases.