Trigonal Prismatic Layers Research Articles

Dz2 orbital character of polyhedra in complex solid-state transition-metal compounds.

In transition-metal compounds, the character of the d orbitals often plays an important role in the development and enhancement of novel physical and chemical properties. Density functional theory calculations of the electronic structures of various d0- and d1-complex transition-metal compounds consisting of either face-sharing octahedra, edge-sharing octahedra, or edge-sharing trigonal prismatic layers were performed to investigate the nature of their d orbitals. The dz2 orbital of the transition metal was shown to make a significant contribution to the electronic structure near the Fermi level in nine different complex transition-metal compounds (oxides, nitrides, and sulfides), regardless of the type of polyhedral geometry and connectivity. The importance of controlling and designing the dz2 orbital character of transition metals near the Fermi level was shown to be important in developing novel physical and chemical properties in complex transition-metal compounds.

Octahedral and trigonal-prismatic coordination preferences in Nb-, Mo-, Ta-, and W-based ABX2 layered oxides, oxynitrides, and nitrides

Crystallographic and electronic structures of Nb-, Mo-, Ta-, and W-based layered oxides, oxynitrides, and nitrides were analyzed to elucidate the structural relationship between layered oxides and nitrides consisting of octahedral and trigonal-prismatic layers. The electron density, as derived by synchrotron X-ray analysis of LiNbO2 and Ta5−x(O,N)6, showed orbital overlaps between Nb–Nb and Ta–Ta metals in the trigonal layers. Computational calculations based on DFT exhibited that these overlaps stabilized these structures by lowering the hybridization states composed of the dxy, dx2−y2, and dz2 orbitals below the Fermi level. Crystal structures and formation energies suggest that tuning the Fermi level through the substitutions and vacancies of the cation/anion sites determines the structural preferences of the coordination. The properties and syntheses of these compounds are briefly described. This study enhances the understanding of layered oxides, oxynitrides, and nitrides to further the development of new synthetic approaches, compounds, and applications.

Crystal Structures of Complex Lanthanide Sulfides With Built-in Non-Commensurability

Non-commensurability on the boundaries of adjacent structural layers (rods, blocks) represents an important structure-building principle for several structural families of complex lanthanide sulfides. In the layer-misfit sulfides 'ABS3', pseudotetragonal layers with lanthanide ions alternate with octahedral layers or trigonal prismatic layers. Among more complex, fragment-recombination structures based on similar layer strips, (A) a family of derivatives of the structure of Yb3S4, (B) structures that represent anti-(out-of-)phase modulated non-commensurate layer sequences, and (c) lanthanide sulfosalts (with bismuth and antimony) are described in some detail. Layer-match modes, their relationships to the ratios of cation radii, and relations to simpler archetypal structures are analysed.

Fermi surface measurements in layer chalcogenide crystals

Quantum oscillations in the frequency range 0.04–8 MG have been observed in the magnetoresistance of single crystals of 4Hb-TaS 2. These have been studied as a function of applied pressure up to 7 kbar. Pressures in this range depress the charge density wave in the trigonal prismatic layers and the effects on the amplitudes and frequencies of the magnetoquantum oscillations have been studied. Application of pressure produces a dramatic reduction of amplitude for most oscillations, and several undergo definite frequency shifts. These effects are interpreted as resulting from a sensitive pressure dependence of the charge density wave gaps which dominate the Fermi surface topology. Magnetic breakdown and interference orbits may also play a role.

The crystal structure of 3- R Nb 1.06S 2

The X-ray single crystal structure of 3- R Nb 1.06S 2 has been determined. The material crystallizes in the space group R 3 m with a = 3.3285(4) and c = 17.910(4) Å when indexed on a hexagonal lattice. The structure was refined by full matrix least squares procedures to a final residual of R = 0.026 based on 79 observed ( I > 3 σ I) reflections. The sulfurs form closest-packed layers with the majority of the metal in sheets of trigonal prismatic sites. A small portion of niobium was found to occupy octahedral sites, between the van der Waals gaps of the sulfur lattice. Niobium in the van der Waals region is trigonally distorted from octahedral symmetry, with niobium-sulfur distances of 2.234(8) and 2.577(11) Å, because of repulsion from niobium in adjacent trigonal prismatic layers.

Optical properties of mixed coordination polytypes of TaS2in the photon energy range 0.5-6.0 eV

It is shown that the transmission and reflectivity spectra obtained by averaging the dielectric function of trigonal prismatic coordination 2H-TaS2 and octahedral coordination 1T-TaS2 are very similar to the measured spectra of 4Hb and 6R-TaS2, two of the mixed coordination polytypes of TaS2. This indicates that the individual layers of TaS2 macromolecules contribute additively to the physical properties of this layer material. The main difference between the calculated and measured spectra indicates charge transfer of up to 0.2 of an electron per TaS2 formula unit from the octahedral to the trigonal prismatic layers in both 4Hb and 6R-TaS2.

Quantum oscillations and the Fermi surface of2H-TaS2

Quantum oscillations in the magnetoresistance and susceptibility of $2H$-Ta${\mathrm{S}}_{2}$ have been used to determine Fermi-surface cross sections. Eleven frequencies in the range from 0.04 to 6.44 MG have been observed corresponding to sections arising from the charge-density-wave transition at 75 K. Comparison to the frequencies observed in $4Hb$-Ta${\mathrm{S}}_{2}$ shows that the Fermi-surface cross sections are similar in both the $2H$ and $4Hb$ phases indicating relatively little charge transfer between octahedral and trigonal prismatic layers in $4Hb$-Ta${\mathrm{S}}_{2}$. The angular dependence observed for a number of sections in $2H$-Ta${\mathrm{S}}_{2}$ shows a $sec\ensuremath{\theta}$ dependence characteristic of cylinders while others show a more three-dimensional behavior in contrast to $4Hb$-Ta${\mathrm{S}}_{2}$ where all of the lowest frequencies show a $sec\ensuremath{\theta}$ dependence. Comparison to results on $2H$-Ta${\mathrm{Se}}_{2}$ and to band-structure calculations and band-folding models will also be made.

Electrical conductivity and charge density wave formation in 4HbTaS2under pressure

The two charge density wave (CDW) transitions in 4Hb TaS2 have been followed under pressure. The two transition temperatures were lowered when pressure was applied; the 315K atmospheric pressure transition associated with CDW formation in the octahedral layers was suppressed by 35 kbar, the 22K transition associated with CDW formation in the trigonal prismatic layers was suppressed at 8.5 kbar. The fall in the 22K transition correlates with the rise in superconducting transition temperature. The behaviour of the resistivity at the CDW transitions is discussed and charge transfer from the octahedral layers to the trigonal prismatic layers is proposed. The effect of pressure on CDW formation in trigonal prismatic material is related to its superconducting behaviour under pressure through the microscopic CDW theory of Chan and Heine (1973).

The effect of pressure on the charge density wave transitions in 4H b TaS2

The two charge density wave (CDW) transitions in 4Hb TaS2 have been followed under pressure. They both fell in temperature; the 315 K atmospheric-pressure transition would be suppressed at 35 kbar, the 22 K transition was suppressed at 8.5 kbar. The fall in the 22 K transition, associated with CDW formation in the trigonal prismatic layers, correlates with the rise in superconducting transition temperature.

LAYER COMPOUNDS. CHARGE DENSITY WAVES IN TRANSITIONS METAL COMPOUNDS.ELECTRONIC PROPERTIES OF TRANSITION METAL DICHALCOGENIDES : CONNECTION BETWEEN STRUCTURAL INSTABILITIES AND SUPERCONDUCTIVITY

We discuss the electronic properties of the transition metal dichalcogenide layer compounds which have been investigated in detail because they undergo phase transitions accompanied by the occurence of charge density waves (CDW). The CDW state is favoured by the cylindrical nature of the Fermi surface and also by the possible existence of Van-Hove singularities in the band structure close to the Fermi level of the compounds TX2(T = Nb, Ta, X = S, Se). The anisotropy of the layer compounds is related to physical properties such as : compressibility, electrical conductivity and the superconducting properties. In the trigonal prismatic polytypes 2H-NbSe2, TaSe2, TaS2 the onset of a CDW is characterized by a weak anomaly in the resistivity ; whereas in the octahedral coordinated polytypes (1T-TaS2), the formation of a CDW induces a metal-insulator transition. A detailed study of the CDW state by NMR on 2H-NbSe2 will be presented. Below the onset temperature, a redistribution of the conduction electrons causes an inhomogeneous'broadening of the 93Nb quadrupolar satellites. From the interpretation of the line shape of the NMR lines we have deduced the existence of 3 incommensurate CDW in 2H-NbSe2 below 33 K. The amplitude of the CDW measured by NMR is proportional to the onset temperature as expected in a mean field theory. The pretransitional effects observed above the transition are related to static perturbations of the electron distribution induced by the charged impurities. A contribution of dynamical critical fluctuations above 33 K to the 93Nb spin-lattice relaxation rate has also been observed. The CDW states have been studied by resistivity measurements in other systems such as 2H-TaSe2 where a lock-in of the ICDW occurs through a 1st order phase transition between 87 and 93 K, or in the mixed polytype 4 HbTaS2 with a lock-in in 1T layers at 320 K and an onset in the 2H layers at 22 K. A pressure investigation shows a connection between the enhancement of Tc and the existence of a structural phase transition at T0 > Tc.The CDW phase transition will be discussed in terms of the phenomenological theory of McMillan. In this respect 4 HbTaS2 is of particular interest since the onset of CDW's occurs through a 1st order transition and therefore suggests that the coupling between the trigonal prismatic layers is relatively weaker in the mixed polytypes than in the pure 2H polytypes.