Van der Waals phase transition investigation toward high-voltage layered cathodes

High-voltage phase transition constitutes the major barrier to accessing high energy density in layered cathodes. However, questions remain regarding the origin of phase transition, because the interlayer weak bonding features cannot get an accurate description by experiments. Here, we determined van der Waals (vdW) interaction (vdWi) in LixCoO2 via visualizing its electron density, elucidating the origin of O3─O1 phase transition. The charge around oxygen is distorted by the increasing Co─O covalency. The charge distortion causes the difference of vdW gap between O3 and O1 phases, verified by a gap corrected vdW equation. In a high charging state, excessive covalency breaks the vdW gap balance, driving the O3 phase toward a stable O1 one. This interpretation of vdWi-dominated phase transition can be applied to other layered materials, as shown by a map regarding degree of covalence. Last, we introduce the cationic potential to provide a solution for designing high-voltage layered cathodes.

Formation of In-Plane Semiconductor-Metal Contacts in 2D Platinum Telluride by Converting PtTe2 to Pt2Te2

Monolayer PtTe₂ is a narrow gap semiconductor while Pt₂Te₂ is a metal. Here we show that the former can be transformed into the latter by reaction with vapor-deposited Pt atoms. The transformation occurs by nucleating the Pt₂Te₂ phase within PtTe₂ islands, so that a metal-semiconductor junction is formed. A flat band structure is found with the Fermi level of the metal aligning with that of the intrinsically p-doped PtTe₂. This is achieved by an interface dipole that accommodates the ∼0.2 eV shift in the work functions of the two materials. First-principles calculations indicate that the origin of the interface dipole is the atomic scale charge redistributions at the heterojunction. The demonstrated compositional phase transformation of a 2D semiconductor into a 2D metal is a promising approach for making in-plane metal contacts that are required for efficient charge injection and is of particular interest for semiconductors with large spin-orbit coupling, like PtTe₂.

Giant Thermoelectric Power Factor Anisotropy in PtSb1.4Sn0.6

We report on the giant anisotropy found in the thermoelectric power factor (S²σ) of marcasite structure-type PtSb_1.4Sn_0.6 single crystal. PtSb_1.4Sn_0.6, synthesized using an ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb₂ and PtSn₂, which crystallize in the cubic Pa3̅ pyrite and Fm3̅m fluorite unit cell symmetry, respectively. The large difference in S²σ for heat flow applied along different principal directions of the orthorhombic unit cell stems mostly from anisotropic Seebeck coefficients.

Controlling Stoichiometry in Ultrathin van der Waals Films: PtTe2, Pt2Te3, Pt3Te4, and Pt2Te2

The platinum-tellurium phase diagram exhibits various (meta)stable van der Waals (vdW) materials that can be constructed by stacking PtTe₂ and Pt₂Te₂ layers. Monophase PtTe₂, being the thermodynamically most stable compound, can readily be grown as thin films. Obtaining the other phases (Pt₂Te₃, Pt₃Te₄, Pt₂Te₂), especially in their ultimate thin form, is significantly more challenging. We show that PtTe₂ thin films can be transformed by vacuum annealing-induced Te-loss into Pt₃Te₄- and Pt₂Te₂-bilayers. These transformations are characterized by scanning tunneling microscopy and X-ray and angle resolved photoemission spectroscopy. Once Pt₃Te₄ is formed, it is thermally stable up to 350°C. To transform Pt₃Te₄ into Pt₂Te₂, a higher annealing temperature of 400°C is required. The experiments combined with density functional theory calculations provide insights into these transformation mechanisms and show that a combination of the thermodynamic preference of Pt₃Te₄ over a phase segregation into PtTe₂ and Pt₂Te₂ and an increase in the Te-vacancy formation energy for Pt₃Te₄ compared to the starting PtTe₂ material is critical to stabilize the Pt₃Te₄ bilayer. To desorb more tellurium from Pt₃Te₄ and transform the material into Pt₂Te₂, a higher Te-vacancy formation energy has to be overcome by raising the temperature. Interestingly, bilayer Pt₂Te₂ can be retellurized by exposure to Te-vapor. This causes the selective transformation of the topmost Pt₂Te₂ layer into two layers of PtTe₂, and consequently the synthesis of e Pt₂Te₃. Thus, all known Pt-telluride vdW compounds can be obtained in their ultrathin form by carefully controlling the stoichiometry of the material.

Superconductivity under pressure in the Dirac semimetal PdTe2

The Dirac semimetal PdTe₂ was recently reported to be a type-I superconductor (T _c   =  1.64 K, [Formula: see text] mT) with unusual superconductivity of the surface sheath. We here report a high-pressure study, [Formula: see text] GPa, of the superconducting phase diagram extracted from ac-susceptibility and transport measurements on single crystalline samples. T _c (p ) shows a pronounced non-monotonous variation with a maximum T _c   =  1.91 K around 0.91 GPa, followed by a gradual decrease to 1.27 K at 2.5 GPa. Surface superconductivity is robust under pressure as demonstrated by the large superconducting screening signal that persists for applied dc-fields [Formula: see text]. Surprisingly, for [Formula: see text] GPa the superconducting transition temperature at the surface [Formula: see text] is larger than T _c of the bulk. Therefore surface superconductivity may possibly have a non-trivial topological nature. We compare the measured pressure variation of T _c with recent results from band structure calculations and discuss the importance of a Van Hove singularity.

Mixed type I and type II superconductivity due to intrinsic electronic inhomogeneities in the type II Dirac semimetal PdTe2

The type II Dirac semimetal PdTe[Formula: see text] is unique in the family of topological parent materials because it displays a superconducting ground state below 1.7 K. Despite wide speculation on the possibility of an unconventional topological superconducting phase, tunneling and heat capacity measurements revealed that the superconducting phase of PdTe[Formula: see text] follows predictions of the microscopic theory of Bardeen, Cooper and Schrieffer for conventional superconductors. The superconducting phase in PdTe[Formula: see text] is further interesting because it also displays properties that are characteristic of type-I superconductors and are generally unexpected for binary compounds. Here, from scanning tunneling spectroscopic measurements we show that the surface of PdTe[Formula: see text] displays intrinsic electronic inhomogeneities in the normal state which leads to a mixed type I and type II superconducting behaviour along with a spatial distribution of critical fields in the superconducting state. Understanding of the origin of such inhomogeneities may be important for understanding the topological properties of PdTe[Formula: see text] in the normal state.

Penetration depth study of the type-I superconductor PdTe2

Superconductivity in the topological non-trivial Dirac semimetal PdTe₂ was recently shown to be type-I. We hereby report measurements of the relative magnetic penetration depth, [Formula: see text], on several single crystals using a high precision tunnel diode oscillator technique. The temperature variation [Formula: see text] follows an exponential function for [Formula: see text], consistent with a fully-gapped superconducting state and weak or moderately coupling superconductivity. By fitting the data we extract a [Formula: see text]-value of  ∼500 nm. The normalized superfluid density is in good agreement with the computed curve for a type-I superconductor with nonlocal electrodynamics. Small steps are observed in [Formula: see text], which possibly relates to a locally lower [Formula: see text] due to defects in the single crystalline sample.

Determination of minerals in platinum concentrates from the Transvaal by X-ray methods

Concentrates from the platiniferous norites of the Bushveld, Transvaal, are not completely soluble in aqua regia. The insoluble portion consists of steel-grey fragments first analysed chemically by R. A. Cooper and considered by him to be a new platinum mineral represented by the formula Pt(As,S)2. The name cooperite was proposed for the new mineral by F. Wartenweiller, and after further work Cooper decided that the arsenic found in the early analysis was due to the presence of sperrylite, and he changed the formula to PtS2. H. Schneiderhöhn observed simple twinning and, less frequently, polysynthetie lamellae on polished sections of mineral grains from the same deposits, and he suggested that cooperite is probably orthorhombic and isomorphous with marcasite. The latest account of the new mineral has been published by H. R. Adam who gave several analyses of cooperite from the Rustenburg and Potgietersrust districts and concluded that the ‘mineral is PtS2with a small amount of excess metal (platinum, palladium, and nickel) present in solid solution’.

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca₁₀(Ir₄As₈)(Fe₂As₂)₅. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir₄As₈ layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe₂As₂, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.