High-Pressure Synthesis of A2 NiO2 Ag2 Se2 (A=Sr, Ba) with a High-Spin Ni2+ in Square-Planar Coordination

Observation and enhancement through alkali metal doping of p-type conductivity in the layered oxyselenides Sr2ZnO2Cu2Se2 and Ba2Zn1-x O2-x Cu2Se2

The optoelectronic properties of two layered copper oxyselenide compounds, with nominal composition Sr2ZnO2Cu2Se2 and Ba2ZnO2Cu2Se2, have been investigated to determine their suitability as p-type conductors. The structure, band gaps and electrical conductivity of pristine and alkali-metal-doped samples have been determined. We find that the strontium-containing compound, Sr2ZnO2Cu2Se2, adopts the expected tetragonal Sr 2 Mn 3 SbO 2 structure with I4/mmm symmetry, and has a band gap of 2.16 eV, and a room temperature conductivity of 4.8 × 10-1 S cm-1. The conductivity of the compound could be increased to 4.2 S cm-1 when sodium doped to a nominal composition of Na0.1Sr1.9ZnO2Cu2Se2. In contrast, the barium containing material was found to have a small zinc oxide deficiency, with a sample dependent compositional range of Ba2Zn1-x O2-x Cu2Se2 where 0.01 < x < 0.06, as determined by single crystal X-ray diffraction and powder neutron diffraction. The barium-containing structure could also be modelled using the tetragonal I4/mmm structure, but significant elongation of the oxygen displacement ellipsoid along the Zn-O bonds in the average structure was observed. This indicated that the oxide ion position was better modelled as a disordered split site with a displacement to change the local zinc coordination from square planar to linear. Electron diffraction data confirmed that the oxide site in Ba2Zn1-x O2-x Cu2Se2 does not adopt a long range ordered arrangement, but also that the idealised I4/mmm structure with an unsplit oxide site was not consistent with the extra reflections observed in the electron diffractograms. The band gap and conductivity of Ba2Zn1-x O2-x Cu2Se2 were determined to be 2.22 eV and 2.0 × 10-3 S cm-1 respectively. The conductivity could be increased to 1.5 × 10-1 S cm-1 with potassium doping in K0.1Ba1.9Zn1-x O2-x Cu2Se2. Hall measurements confirmed that both materials were p-type conductors with holes as the dominant charge carriers.

High- versus Low-Spin Ni2+ in Elongated Octahedral Environments: Sr2NiO2Cu2Se2, Sr2NiO2Cu2S2, and Sr2NiO2Cu2(Se1-x S x )2

Sr₂NiO₂Cu₂Se₂, comprising alternating [Sr₂NiO₂]²⁺ and [Cu₂Se₂]^2- layers, is reported. Powder neutron diffraction shows that the Ni²⁺ ions, which are in a highly elongated NiO₄Se₂ environment with D_4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μ_B per Ni²⁺ ion. The adoption of the high-spin configuration for this d ⁸ cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr₂NiO₂Cu₂S₂, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni²⁺, and accordingly, there is no evidence for magnetic moment on the Ni²⁺ ions. Examination of the solid solution Sr₂NiO₂Cu₂(Se_1-x S _x )₂ shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr₂NiO₂Cu₂Se₂ up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni²⁺ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr₂NiO₂Cu₂Se₂ and Sr₂NiO₂Cu₂S₂, suggest that simple high-spin and low-spin models for Ni²⁺ may not be entirely appropriate and point to further complexities in these compounds.

Superconductivity in infinite-layer nickelates

The recent discovery of the superconductivity in the doped infinite layer nickelatesRNiO₂(R= La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca, Sr)CuO₂having superconducting transition temperature (T_c) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of highT_csuperconductivity in correlated electron systems. In this paper, we review experimental and theoretical works on this new superconductor and discuss the future perspectives for the 'nickel age' of superconductivity.

High-pressure Synthesis of Ba2CoO2Ag2Te2 with Extended CoO2 Planes

Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba₂CoO₂Ag₂Te₂ (space group: I4/mmm) with alternating stacks of CoO₂ and Ag₂Te₂ layers, separated by Ba atoms. The CoO₂ plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag₂Te₂ layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO₄X₂ (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of d_Co-X/d_Co-O. However, applying this relation to Ba₂CoO₂Ag₂Te₂ yields a magnetic moment of ∼4 μ_B, which is nearly twice the experimentally observed value of 1.87(17) μ_B. This result, along with a reduced Néel temperature (T_N = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.

Structure of the water-splitting photocatalyst oxysulfide α-LaOInS2 and ab initio prediction of new polymorphs

We unveil the structure and investigate the visible light water-splitting of the photocatalyst α-LaOInS₂, the second polymorph in this composition. This remarkable oxysulfide exhibits rare mixed anion InS₅O octahedra leading to both O-2p and S-3p hybridized with indium states in the vicinity of the Fermi level. Ab initio structure prediction shows the stability of such heteroleptic environments and points to other hypothetical polymorphs.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

The relationship between ligand design and spin state in base metal compounds is surveyed. Implications and applications of these principles for light-harvesting dyes, catalysis and materials chemistry are summarised.