Rhombohedral Modification Research Articles

New MnO–Nb(Ta)2O5 Phases Produced at High Pressures and Temperatures

Phase formation at high pressures and temperatures were studied in the MnO–Nb(Ta)2O5 system. New rhombohedral modifications of Mn4Nb2O9 and Mn4Ta2O9, two new modifications of MnTa2O6, and two modifications of Mn2Ta2O7 were obtained. They were structurally identified.

High‐Pressure High‐Temperature Synthesis of Mn4Nb2O9. A XRD and TEM Study.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Dynamics of anions and cations in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A proton nuclear magnetic resonance study

Second moments (M2) and spin-lattice relaxation times (T1) of proton nuclear magnetic resonance (NMR) measurements were analyzed for the hydrogensulfides of sodium, potassium, and rubidium. Three modifications are known at ambient pressure for each of MHS with M=Na, K,Rb. They are isotypic for the different cations, with an ordered monoclinic low temperature modification, a dynamically disordered rhombohedral middle temperature modification (MTM), and a cubic high temperature modification (HTM). The number of proton sites for one proton increases from two in the MTM to at least eight in the HTM. The title compounds were investigated by proton NMR in the temperature range 180 K⩽T⩽560 K with a resonance frequency of ν0=400 MHz. In addition, KHS was measured from T=90 K up to T=297 K, using proton resonance frequencies ν0=45 MHz and ν0=96 MHz. The M2 calculated from the absorption signal of the individual modification for the various cations do not differ from those calculated for the known crystal structures. The minima of T1 are in good agreement with results of calculations based on the crystal structure of the MTM of these compounds and two-site 180° reorientations of the anions. Activation enthalpies and attempt frequencies (NaHS:26.2(4) kJ mol−1, 6.1(2)×1014 s−1, KHS:19.0(4) kJ mol−1, 4.0(7)×1013 s−1, RbHS:16.3(4) kJ mol−1, 2.0(2)×1013 s−1) agree with those determined by quasielastic neutron scattering [Haarmann et al., J. Chem. Phys. 113, 8161 (2000)]. For the cubic HTM, translational motion of the cations is probable. Only in the case of KHS and RbHS, was the onset of this translational motion already observed in the MTM at temperatures close to the phase transition into the HTM. This seems to be a difference in the mechanism of the MTM⇄HTM phase transition for NaHS, on the one hand, and KHS, RbHS, on the other hand.

High-pressure high-temperature synthesis of Mn4Nb2O9. A XRD and TEM study

Abstract A new high-pressure, high-temperature modification of Mn 4 Nb 2 O 9 , has been prepared at T =1700 °C, P =9 GPa. The XRD study based on Rietveld technique suggests the structure to be a rhombohedral (R3c) modification of Mn 4 Nb 2 O 9 , (LiNbO 3 type structure) with the chemical formula Mn(Mn 1/3 Nb 2/3 )O 3 and unit cell parameters a =5.34139(2), c =14.15807(7) A. However, electron diffraction (ED) and high resolution electron microscopy (HREM) images of crystallites found in an XRD single phase sample revealed frequent disorder, but also the existence of a new monoclinic modification of Mn 4 Nb 2 O 9 often intergrown with the corundum related P 3 c1-Mn 4 Nb 2 O 9 modification stable at ambient pressure and temperature. This new monoclinic modification has the unit cell parameters: a =9.93, b =5.34, c =13.20 A, β =92.9° and space group C2/c or Cc. It is closely related to the corundum lattice since its c ∗ axis is parallel to 〈2 0 2〉 ∗ R 3 c in the rhombohedral sublattice.

ANALYSIS OF THE EFFECT OF NONHYDROSTATIC COMPRESSION CONDITIONS ON DIRECT PHASE TRANSFORMATIONS IN CARBON

We have analyzed the effect of deviations from hydrostatic compression conditions on the driving force for transformations of graphite to dense phases. We consider transformations of the hexagonal (2H) and rhombohedral (3R) modifications of graphite to lonsdaleite and diamond by martensitic mechanisms, and also a direct diffusional transformation. Comparative estimates of the driving forces were made for hydrostatic and uniaxial compression schemes, and also for shear loading. We show that a loading scheme combining hydrostatic compression with uniaxial compression is more favorable for the martensitic transformation of 2H-graphite to lonsdaleite, while shear loading and uniaxial loading are more favorable for the transformations of 3R-graphite; and lonsdaleite is formed in the first case, while diamond is formed in the second case.

A rhombohedral polymorph of aqua(malonato)cadmium(II) hydrate

The Cd atom in the rhombohedral modification of aqua(malonato)cadmium(II) hydrate, [Cd(C3H2O4)(H2O)].H2O, shows pentagonal bipyramidal coordination. The malonate group chelates to the water-coordinated Cd atom; its two carboxyl groups also chelate adjacent Cd atoms.

Reorientational motion of the HS− ions in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A quasielastic neutron scattering study

The isostructural hydrogensulfides of the alkali metals sodium, potassium, and rubidium were investigated by quasielastic neutron scattering in order to gain insight into the dynamics of the molecular anion HS− as a function of temperature. A phase transition between the rhombohedral middle temperature modification (MTM) and the cubic high temperature modification (HTM) of the title compounds was of special interest. A two site jump model is clearly supported by the data of the MTM and an eight site jump model is consistent with the data of the HTM. The jump distances are in reasonable agreement with distances of the hydrogen positions determined by neutron powder diffraction on the fully deuterated samples. At the MTM→HTM phase transition the correlation times of the reorientational motion decrease by about one order of magnitude. Within the investigated temperature range (T=170 K to T=450 K) they change from 10−8 s to 10−12 s. They show Arrhenius behavior. The activation enthalpies of the MTM decrease with increasing radius of the cation from 26(1) kJ mol−1, 22(1) kJ mol−1 to 18(2) kJ mol−1 for NaHS, KHS and RbHS respectively. In the HTM they are equal within the estimated error 6(1) kJ mol−1 and 7(1) kJ mol−1 for NaHS and KHS.

New high-pressure phases of lithium.

Lithium is considered a 'simple' metal because, under ordinary conditions of pressure and temperature, the motion of conduction electrons is only weakly perturbed by interactions with the cubic lattice of atomic cores. It was recently predicted that at pressures below 100 GPa, dense Li may undergo several structural transitions, possibly leading to a 'paired-atom' phase with low symmetry and near-insulating properties. Here we report synchrotron X-ray diffraction measurements that confirm that Li undergoes pronounced structural changes under pressure. Near 39 GPa, the element transforms from a high-pressure face-centred-cubic phase, through an intermediate rhombohedral modification, to a cubic polymorph with 16 atoms per unit cell. This cubic phase has not been observed previously in any element; unusually, its calculated electronic density of states exhibits a pronounced semimetal-like minimum near the Fermi energy. We present total-energy calculations that provide theoretical support for the observed phase transition sequence. Our calculations indicate a large stability range of the 16-atom cubic phase relative to various other crystal structures tested here.

Epitaxial CaGe 2 films on germanium

The epitaxial growth of thin CaGe 2 films with reactive deposition epitaxy (RDE) on Ge(1 1 1) substrates is described. The films consist in general of a mixture of the known trigonal rhombohedral tr6 modification and a hexagonal h2 modification of CaGe 2 containing two Ca and two buckled Ge layers per unit cell in a twofold stacking sequence whose formation appears to be favored by strain. Epitaxial layers of both polytypes show remarkably higher crystalline quality compared to epitaxial CaSi 2 films grown on silicon substrates. The tr6 modification is found to be unstable in air in contrast to the h2 modification.

Wurtzitic boron nitride thermal stability and transformation into the rhombohedral boron nitride modification when heated

Samples of wurtzitic boron nitride (wBN) were obtained from the rhombohedral BN (rBN) modification as a result of phase transformation under high pressures. Structure features and kinetics of wBN→graphite-like BN transformation in heating have been investigated. The nature of the low thermal stability of wBN is discussed.

Phase transformations of the rhombohedral modification of boron nitride with shock compression

Features of crystal-orientated transformations of the rhombohedral modification of boron nitride (rBN) into dense modifications of the wurtzite (wBN) and sphalerite (spBN) types with shock compression in the range from 15 to 50 GPa are studied. Three compression schemes are used: in annular ampules (∼15 GPa); in the ampules of K. M. Rabinin (25–35 GPa); in a plane shock wave (∼50 GPa). With an increase in pressure from 15 to 50 GPa and a corresponding change in the loading scheme the degree of rBN transformation into dense modifications increases from 0 to 40%. There is a change in the rBN transformation mechanism: from longitudinal bending of layers (as a result of which wBN forms, and spBN is formed from it) to corrugation leading to spBN formation directly from rBN.

On the rhombohedral-cubic phase transition of solid solutions ROFCaF2 with RGd, Tb and Dy

The rhombohedral-cubic phase transition typical of stoichiometric rare earth oxide fluorides of the formula type ROF was investigated for the ROF-based solid solution systems R1−xCaxO1+x F1+x, with RGd, Tb, Dy, by differential scanning calorimetry and by X-ray powder diffraction. It was found that the solubility limit of CaF2 in these ROF is almost identical with x⪷ 0.09 and that the transition temperatures decrease with increasing CaF2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb temperatures decrease with increasing CaF2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb and Dy, respectively, as compared to the transition temperatures of pure ROF. The formula volumes of the rhombohedral as well as the cubic solid solutions were determined as a function of x, and the structural implications of the observed trend is discussed for the rhombohedral modification. A partial tentative phase diagram is exemplarily proposed for Gd1 −xCaxO1 −x F1 −x.

Pressure-Induced Fm-3m R-3 Phase Transition in NaSbF6

High-pressure X-ray diffraction measurements on NaSbF6 powder were performed up to 5.63 (7) GPa. At ∼0.1 GPa the cubic low-pressure phase with ordered ReO3-type crystal structure undergoes a phase transition into a rhombohedral LiSbF6-type modification. The high-pressure behaviour of this phase is characterized by rotations and distortions of the coordination octahedra, but it also can be described with a sphere-packing deformation. The octahedral distortions are probably caused by cation–cation repulsions.

Chemical bond and electronic structure anisotropies in the graphitic and rhombohedral modifications of boron nitride

The pseudopotential method is used to calculate the band spectra, valence charge densities, and electroneutrality levels in the graphitic (h-BN) and rhombohedral (r-BN) modifications of boron nitride. It is shown that these compounds are indirect-band insulators with the minimum energies of the indirect (4.65 and 4.8 eV) and direct (5.27 and 5.5 eV) forbidden zones in h- and r-BN, respectively. The band energies of h-BN correlate well with the experimental data. Both crystals are characterized by ionic-covalent chemical bonds. In the plane of hexagonal layers, the bonds are more covalent in h-BN. In h- and r-BN, the electroneutrality levels correspond to the energy of about 1.9 eV over the top of the valence band, which is responsible for the hole conductivity of the substances. In superlattices based on these compounds, the rhombohedral phase is a quantum well for holes and a barrier for electrons.

Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds

The framework compounds M 2(SO 4) 3 with M = ( Ti Fe), ( V Fe), Fe and Li x M 2( PO 4) 3 with M = Ti, ( V Fe), Fe, were synthesized and electrochemically characterized by the coin-cell method. Use of larger (XO 4) n− polyanions not only allows fast Li +-ion conduction in an open three-dimensional framework that is selective for the working alkali ion on discharge; it also stabilizes operative redox potentials Fe 3+ Fe 2+ , Ti 4+ Ti 3+ and V 3+ V 2+ that give open-circuit voltages V oc > 2.5 V as well as access to V 4+ V 3+ , Ti 3+ Ti 2+ and Fe 2+ Fe + couples. Separation of the V 4+ V 3+ and V 3+ V 2+ couples were found to be 2.0 V. Fe 2(SO 4) 3 has both monoclinic and rhombohedral modifications that give a flat open-circuit voltage V oc = 3.6 V versus Li and a reversible capacity for ~ 1.8 lithium atoms per formula unit. Li x Fe 2(SO 4) 3 shows an abrupt voltage drop occurring for x > 2 that can be held in check by the addition of buffers such as Li 3Fe 2(PO 4) 3, FeV(SO 4) 3 and LiTi 2(PO 4) 3. Changing the polyanion group from (SO 4) 2− to (PO 4) 3− in these framework compounds decreases the redox potentials from 3.2 to 2.5 V for the Ti 4+ Ti 3+ couple, 2.5 to 1.7 V for the V 3+ V 2+ couple and 3.6 to 2.8 V for the Fe 3+ Fe 2+ couple. Comparative advantages and disadvantages of framework cathodes for Li rechargeable battery applications are discussed.

Structural mechanisms of rhombohedral BN transformations into diamond-like phases

Structural features and mechanisms of transformation of graphite-like rhombohedral boron nitride modification (rBN) into the diamond-like wurtzite (wBN) and zinc blende (zBN) types have been studied. The puckering mechanism of the rBN→zBN transformation in shock waves was first proved structurally. It has been found that in static compression of CVD rBN (8 GPa, 20–2600 °C), transformations occur in the following sequence: rBN→(S,T)→I g→wBN→I d→cBN, where (S,T) are structures deformed via basal shears or twinning, and I g and I d are intermediate graphite- and diamond-like structures, respectively. The structure of wBN formed from rBN has been shown to be much more disordered than that of wBN obtained from the BN hexagonal modification (hBN).

Band energy structreu and optical spectra

Abstract The article deals with the simulation of band energy structure of rhombohedral and monoclinic modifications of Cs2Cdl4 crystals.

Intercalation reactions and carbide formation in graphite-lithium system

Abstract In the present work we have investigated a lithium-graphite system by DTA, DSC, heat flow calorimetric, and volumetric methods in a “piston-cylinder apparatus” under high pressures (23 kbar). For the first time, we have determined the enthalpies (ΔHform) of LiC6 (1 stage) and LiC12 (2 stage) formation by the direct calorimetric method at 455 K. It was stated that the values are −13.9 ± 1.2 kJ/mol Li and −24.8 ± 1 kJ/mol Li, respectively. We have defined the enthalpies and temperatures of (3D) → (2D) transitions in LiC6 (T = 711 ± 5 K, ΔHph.tr. = 1.1 ± 0.3 kJ/mol Li) and in LiC12 (T = 472 ± 5K, ΔHph.tr = 1.45 ± 0.2 kJ/mol Li). The rhombohedral modification of Li2C2 was explored at T = 293–1000 K. Only one phase transition at T = 716 ± 6 K (ΔHph.tr. = 5.56 ± 0.09 kJ/mol Li2C2) was detected. The compressibility of Li2C2 was investigated at pressures up to 23 kbar and at room temperature. The change of molar volume ( ΔV V 0 , % ) was −2.07 ± 0.07 (10 kbar) and −3.46 ± 0.14 (22 kbar). We studied the carbide formation in Li-GICs at ambient and high pressure (P = 60 kbar) as well.

Kronig-Penney-like description for band gap variation in SiC polytypes

A one-dimensional Kronig-Penney-like model for envelope wave functions is presented to explain the band gap variation of SiC polytypes. In this model the envelope functions obey discontinuous boundary conditions. The electronic band gaps of cubic and several hexagonal and rhombohedral SiC polytypes are calculated. The polytypic superlattices are assumed to be stackings of differently sized and orientated cubic SiC segments. The empirical Choyke-Hamilton-Patrick relation is understood and deviating trends for small hexagonalities and rhombohedral modifications are predicted.

Structural, Magnetic, and Thermal Properties of La1−tCatCrO3−δ

A broad study on the properties of Ca-substituted LaCrO3is reported. Problems in the synthesis of Ca-rich samples of La1−tCatCrO3−δare addressed. For the La-rich region, synthesis in air provides La1−tCatCrO3with partial formation of CrIV. At reducing conditions, CrIVcan be converted to CrIII. The properties of oxidized and reduced samples are compared since differences are of importance for applications. The maximum reduction, without decomposition, corresponds ideally to δ =t/2, but this could not be fully achieved in 10% H2+ 90% N2at 1263 K. Unit cell data, atomic coordinates (from powder neutron diffraction data), thermal expansion, and magnetic property data are provided. The crystal structure is orthorhombic (GdFeO3-type) at 298 K; the rhomobohedral LaAlO3-type variant exists at high temperatures. Sample reduction influences slightly the orthorhombic distortion, increases the unit cell volume quite considerably, enhances the temperature domain of the orthorhombic versus the rhombohedral modification, and increases the magnetic moment and the antiferromagnetic ordering temperature. The microscopic thermal expansion is independent of the Ca content (0.00 ≤t≤ 0.30) and is slightly enhanced for the rhombohedral phase. The antiferromagneticGx-type ordering in LaCrO3is retained upon substitution. However, for oxidized samples, a smallerAzcomponent develops upon substitution, whereas for field cooled samples parasitic ferromagnetism appears. For fully reduced samples, the aliovalent Ca substitution implies unit cell compression and oxygen vacancy formation with modest effect on magnetic properties. The additional partial exchange of CrIII(d3) by CrIV(d2) for oxidized samples has significant impact on the magnetic properties.