Transition Metal Ditellurides Research Articles

What drives superconductivity in Pt-doped IrTe2?

The emergency of superconductivity at the edge of charge density wave (CDW) state by pressure or chemical doping in 1T transition metal dichalcogenides (TMDs) has attracted much attention in past decade [1, 2]. The interplay of CDW and superconductivity has triggered the hot debate: why does superconductivity arise after suppressing the CDW and what’s the driving force of the charge modulation? For the 5d transition metal ditelluride IrTe2, a layered compound with trigonal symmetry, previous studies have shown an ordered superstructure below *280 K, and a superconducting state when doped or intercalated with Pd or Pt [3, 4]. In IrTe2, the Fermi surface nesting has been excluded as the impetus of superstructure; instead, splitting of Te orbitals by the crystal field [5] or destabilization of polymeric Te–Te networks was proposed [6], evident from the visualized periodic dimerization of Te atoms by scanning tunneling microscopy (STM) [7]. Meanwhile, a charge disproportionated CDW by covalent bonding Ir dimers was also presented [8]. However, though the origin of the superstructure formation becomes more and more clear, the underlying mechanism of the appearance of superconductivity by doping Pd or Pt is still obscure. Recently, Ruan et al. [9] have studied the structural phase transition and electronic structure evolution in pure and Pt-doped IrTe2 single crystals using low-temperature STM combined with first-principles calculations. The authors presented a clear evidence for a commensurate 1/6 charge modulation as the ground state of high-quality IrTe2 single crystals, consistent with previous reports [7, 10]. By substituting Pt for Ir, the system exhibits longrange ordered 1/5 phase and then goes to a new threefold quasiperiodic phase. In addition to a full picture of phase evolution, the most important finding is that the Pt dopants actually act as chemical strain, generating local impurity states rather than injecting itinerant charge carriers [9]. The weakened intralayer Ir dimerization effectively leads to the recovery of polymeric Te bonds across layers, lifting the density of states at Fermi level and making the system more three-dimensional [9]. Therefore, it turns out that the higher dimensionality tips the balance in favor of superconductivity in this system. This work presents a coherent story and also stimulates us to think more about the role of strain in modulating new order.

Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1−xNbxTe2

MoTe2 is a rare transition-metal ditelluride having two kinds of layered polytypes, hexagonal structure with trigonal prismatic Mo coordination and monoclinic structure with octahedral Mo coordination. The monoclinic distortion in the latter is caused by anisotropic metal-metal bonding. In this work, we have examined the Nb doping effect on both polytypes of MoTe2 and clarified a structural phase diagram for Mo1−xNbxTe2 containing four kinds of polytypes. A rhombohedral polytype crystallizing in polar space group has been newly identified as a high-temperature metastable phase at slightly Nb-rich composition. Considering the results of thermoelectric measurements and the first-principles calculations, the Nb ion seemingly acts as a hole dopant in the rigid band scheme. On the other hand, the significant interlayer contraction upon the Nb doping, associated with the Te p-p hybridization, is confirmed especially for the monoclinic phase, which implies a shift of the p-band energy level. The origin of the metal-metal bonding in the monoclinic structure is discussed in terms of the d electron counting and the Te p-p hybridization.

Nucleation and growth of noble metals on transition-metal di-tellurides

Transition-metal di-tellurides (α- and β-MoTe 2 and WTe 2) were used as substrates for nucleation and growth studies of noble metals. They represent a group of chemically closely related compounds with different surface topographies. Nucleation and growth of Ag and Au at room temperature were studied by means of UHV-STM, AFM and TEM. The results revealed that the growth and orientation of these metals are influenced by the topography of the substrate surfaces. Contrary to the growth on atomically flat α-MoTe 2, there is an enhanced diffusion and nucleation along the periodic surface troughs on β-MoTe 2 and WTe 2. The topography of their (001) surfaces is responsible for the orientation of metal (112) planes being parallel to the substrate surface.)

Solid-state tellurium-125 nuclear magnetic resonance studies of transition-metal ditellurides †

Solid-state 125Te NMR studies of inorganic compounds containing tellurium in oxidations states ranging from –II to VI have been made. The chemical shift tensors of Te(OH)6, TeCl4, TeO2 and elemental tellurium have been determined by simulation of magic angle spinning (MAS) NMR spectra. Binary transition-metal ditellurides have been studied by MAS and static 125Te NMR techniques. The observed MTe2 NMR shifts range from ca. 500 to 8000 ppm [from aqueous Te(OH)6] and are shown to be correlated with the expected tellurium oxidation state. Separation of the chemical shift and Knight shift contributions has been attempted.

Dynamics of125Te in transition metal ditellurides

The125Te Mossbauer effect in the transition metal ditellurides NiTe2, FeTe2, and MnTe2 was studied at atmospheric pressure between 78 and 297 K and a pressure of 7 GPa (78 or 297 K). The determined Debye temperatures are related to the crystal structure and electronic state of Te. The pressure-induced changes in the hyperfine interaction parameters are discussed in terms of charge transfer and structure type.

Importance of short interlayer Te···Te contacts for the structural distortions and physical properties of CdI 2-type layered transition-metal ditellurides

We examined how the short intra- and interlayer Te···Te contacts of layered transition-metal tellurides affect their structures and physical properties by carrying out tight-binding band electronic structure calculations for the CdI 2-type layered transition-metal dichalcogenides Ti X 2 (X =S, Se, Te) and MTe 2 (M =V, Nb, Ta) on the basis of the extended Huöckel method. In the CdI 2-type tellurides, the top portion of the Te p-block bands overlaps significantly with the bottom portion of the metal d-block bands, thereby causing a substantial electron transfer from the p- to the d-block bands. For this p → d electron transfer, the interlayer Te···Te contacts are found to be essential because the overlap between the Te p z -orbitals (perpendicular to the layer) associated with the interlayer Te···Te contacts is most effective in raising the top portion of the Te p-block bands. As a consequence, layered transition-metal tellurides are likely to possess a three-dimensional metallic character, and a slight change in their interlayer Te···Te contacts significantly affects their electrical and other physical properties.

Electronic structure and properties of anionic mixed valence and layered CrTe 3: The question of extended tellurium bonding in transition metal tellurides

CrTe 3 is a layered material whose tellurium sublattice contains Te, Te 2, and Te 3 entities. Our magnetic susceptibility measurements are consistent with a + 3 oxidation state for Cr. This indicates three different oxidation states for the tellurium atoms, i.e., Te 2−, Te 2− 2, and Te 2− 3. Conductivity measurements show this material to be a semiconductor with an activation energy of 0.35 eV. Our tight-binding band structure calculations indicate that it is a Mott-Hubbard semiconductor rather than a regular one. A study of the Te···Te overlap populations suggests that CrTe 3 is a new transition metal telluride with a polymerized tellurium network. Analysis of this additional bonding within the tellurium sublattice in this phase and in related tellurides leads to a classification of the transition metal ditellurides taking into account both dimensionality and degree of polymerization.

Raman spectroscopic studies of MX2-type layered compounds

The Raman spectra of transition metal ditellurides (TiTe2, ZrTe2, HfTe2, NbTe2 and TaTe2) have been measured. The spectra of TiTe2, ZrTe2 and HfTe2 show two Raman lines, respectively, in accordance with the group theoretical prediction, whereas those of NbTe2 and TaTe2 show complicated structures due to the distorted octahedral crystal structure. These complicated sectra are explained by the folding of the phonon dispersion curves which is brought about by the enlarged unit cell compared with that of the normal octahedral (1T) structure. The Raman spectra of transition metal dichalcogenides (1T-TaS2, 2H-TaS2, -TaSe2, -NbS2 and -NbSe2) intercalated with organic molecules have also been measured to investigate the effect of the intercalation on the lattice dynamics and the phase transitions in these crystals.