Felix Eder

Recent transport experiments reported a quantum spin Hall insulator phase in monolayer 1T-TaIrTe4 gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative Van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe4 using microfocus laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which linear transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of Van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe4.

Layered delafossite-type compounds and related transition metal dichalcogenides, characterized by their triangular net structures, serve as prototypical systems for exploring the intricate interplay between crystal structure and magnetic behavior. Herein, we report on the discovery of the compound K_1-xCrSe₂ (x ≈ 0.13), an incommensurately modulated phase. Single crystals of this compound were grown for the first time using a K/Se self-flux. We find a monoclinic crystal structure with incommensurate modulation that can be rationalized by a 3 + 1-dimensional model. This modulation compensates for the under-stoichiometry of K cations, creating pronounced undulations in the CrSe₂ layers. Our anisotropic magnetization measurements reveal that K_1-xCrSe₂ undergoes a transition to a long-range magnetically ordered state below T_N = 133 K, a temperature 1.6 to 3.3 times higher than in earlier reported KCrSe₂ compounds. Our findings open new avenues for tuning the magnetic properties of these layered materials through structural modulation.

Monoclinic single crystals of Cd₅(TeO₃)₄(NO₃)₂ (space group P2₁/c), pentacadmium tetrakis[oxidotellurate(IV)] dinitrate, and of Cd₄Te₅O₁₄ (space group C2/c), tetracadmium pentaoxidotellurate(IV), were obtained under the same hydrothermal conditions. Whereas the crystal structure of Cd₅(TeO₃)₄(NO₃)₂ is distinctively layered, that of Cd₄Te₅O₁₄ exhibits a tri-periodic framework. In Cd₅(TeO₃)₄(NO₃)₂, the three Cd^II atoms have coordination numbers (CN) of 7, 6 and 6. The two types of [CdO₆] and the [CdO₇] polyhedra [bond lengths range from 2.179 (3) to 2.658 (2) Å] share corners and edges, resulting in layers extending parallel to (100). Both Te^IV atoms are coordinated by three oxygen atoms in a trigonal–pyramidal shape. The oxygen atoms of the isolated [TeO₃] groups [bond lengths range from 1.847 (3) to 1.886 (3) Å] all are part of the cadmium–oxygen layer. The electron lone pairs ψ of the Te^IV atoms are directed away from the layer on both sides. The available interlayer space is co-occupied by the nitrate group, which is directly connected with two of its O atoms to the layer whereas the third O atom is solely bonded to the N atom and points towards the adjacent layer. In Cd₄Te₅O₁₄, all three unique Cd^II atoms are coordinated by six oxygen atoms, considering Cd—O distances from 2.235 (2) to 2.539 (2) Å. By edge- and corner-sharing, the distorted [CdO₆] polyhedra form an open framework that is partially filled with three different stereochemically active Te^IV atoms. All of them exhibit a CN of 4, with Te—O bonds in a range from 1.859 (2) to 2.476 (2) Å. The corresponding [TeO₄] units are linked to each other by corner- and edge-sharing, forming infinite helical ¹ _∞[Te₁₀O₂₈] chains extending parallel to [203]. The connectivity in the chains can be described as (...–⋄–⋄=⋄–⋄–⋄–⋄–⋄=⋄–⋄–⋄–...) _n where ‘⋄’ denotes a [TeO₄] unit, ‘–’ a linkage via corners and ‘=’ a linkage via edges. Such a structural motif is unprecedented in the crystal chemistry of oxidotellurate(IV) compounds.

Na₂(Fe_2/3Te_4/3)O₆ (Z = 3) or Na₃(FeTe₂)O₉ (Z = 2), trisodium iron(III) ditellurium(VI) nonaoxide, adopts the ilmenite (FeTiO₃, Z = 6) structure type with the Ti site (site symmetry 3.) replaced by Na and the Fe site (site symmetry 3.) replaced by a mixed-occupied (Fe^III,Te^VI) site in a Fe:Te ratio of 1:2. Whereas the [(Fe,Te)O₆] octahedron is only slightly distorted, the [NaO₆] octahedron shows much stronger distortions, as revealed by a larger spread of the bond lengths and some distortion parameters.

The title compound, ammonium trinickel(II) hydrogen arsenate arsenate dihydroxide, was synthesized under hydrothermal conditions. Its crystal structure is isotypic with that of K[Cu₃(HAsO₄)(AsO₄)(OH)₂] and is characterized by pseudo-hexagonal (001) ² _∞[Ni₃As₂O_18/3(OH)_6/3O_1/1(OH)_1/1]⁻ layers formed from vertex- and edge-sharing [NiO₄(OH)₂] octahedra and [AsO_3.5(OH)_0.5] tetrahedra as the building units. The hydrogen atom of the OH group shows occupational disorder and was refined with a site occupation factor of 1/2, indicating the equal presence of [HAsO₄]^2– and [AsO₄]^3– groups. Strong asymmetric hydrogen bonds between symmetry-related (O,OH) groups of the arsenate units [O...O = 2.588 (18) Å] as well as hydrogen bonds accepted by these (O,OH) groups from OH groups bonded to the Ni^II atoms [O...O = 2.848 (12) Å] link adjacent layers. Additional consolidation of the packing is achieved through N—H...O hydrogen bonds from the ammonium ion, which is sandwiched between adjacent layers [N...O = 2.930 (7) Å] although the H atoms could not be located in the present study. The presence of the pseudo-hexagonal ² _∞[Ni₃As₂O_18/3(OH)_6/3O_1/1(OH)_1/1]⁻ layers may be the reason for the systematic threefold twinning of (NH₄)[Ni₃(HAsO₄)(AsO₄)(OH)₂] crystals. Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.

Single crystals of two basic cadmium phosphates, dicadmium orthophosphate hydroxide, Cd₂(PO₄)OH, and pentacadmium bis(orthophosphate) tetrakis(hydroxide), Cd₅(PO₄)₂(OH)₄, were obtained under hydrothermal conditions. Cd₂(PO₄)OH adopts the triplite [(Mn,Fe)₂(PO₄)F] structure type. Its asymmetric unit comprises two Cd, one P and five O sites, all situated at the general Wyckoff position 8 f of space group I2/a; two of the O atoms are positionally disordered over two sites, and the H atom could not be localized. Disregarding the disorder, distorted [CdO₆] polyhedra form a tri-periodic network by edge-sharing with neighbouring [CdO₆] units and by vertex-sharing with [PO₄] units. The site associated with the OH group is coordinated by four Cd atoms in a distorted tetrahedral manner forming ¹ _∞[(OH)Cd_4/2] chains parallel to [001]. The oxygen environment around the OH site suggests multiple acceptor atoms for possible O—H...O hydrogen-bonding interactions and is the putative reason for the disorder. Cd₅(PO₄)₂(OH)₄ adopts the arsenoclasite [Mn₅(AsO₄)₂(OH)₄] structure type. Its asymmetric unit comprises five Cd, two P, and twelve O sites all located at the general Wyckoff position 4 a of space group P2₁2₁2₁; the H atoms could not be localized. The crystal structure of Cd₅(PO₄)₂(OH)₄ can be subdivided into two main sub-units. One consists of three edge-sharing [CdO₆] octahedra, and the other of two edge- and vertex-sharing [CdO₆] octahedra. Each sub-unit forms corrugated ribbons extending parallel to [100]. The two types of ribbons are linked into the tri-periodic arrangement through vertex-sharing and through common [PO₄] tetrahedra. Quantitative structure comparisons are made with isotypic M ₅(XO₄)₂(OH)₄ crystal structures (M = Cd, Mn, Co; X = P, As, V).

The crystal structure of K₆[Zn(CO₃)₄], hexapotassium tetracarbonatozincate(II), comprises four unique potassium cations (two located on a general position, and two on the twofold rotation axis of the space groupC2/c) and a [Zn(CO₃)₄]⁶⁻anion. The Zn^IIatom of the latter is located on the twofold rotation axis and is surrounded in a slightly distorted tetrahedral manner by two pairs of monodentately binding carbonate groups, with Zn—O distances of 1.9554 (18) and 1.9839 (18) Å. Both carbonate groups exhibit a slight deviation from planarity, with the C atom being shifted by 0.008 (2) and 0.006 (3) Å, respectively, from the plane of the three O atoms. The coordination numbers of the potassium cations range from 6 to 8, using a threshold of 3.0 Å for K—O bonding interactions being significant. In the crystal structure, [KO_x] polyhedra and [Zn(CO₃)₄]⁶⁻groups share O atoms to build up the framework structure.