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structure, which further revealed that these systems are essentially single-band systems with a remarkably two- dimensional Fermi surface derived from the Ni-dx2-y2 states (Figures 97b and c). In addition, the corresponding charge- transfer energy is pushed towards the charge-insulator regime compared to the infinite-layer case.

Beyond these findings, the successful synthesis of single- layer T nickelates under different synthesis conditions

PRINCIPAL PUBLICATION AND AUTHORS

Single-Layer T Nickelates: Synthesis of the La and Pr Members and Electronic Properties across the Rare-Earth Series, K. Wissel (a), F. Bernardini (b), H. Oh (a), S. Vasala (c), R. Schoch (d), B. Blaschkowski (a), P. Glatzel (b), M. Bauer (d), O. Clemens (a), A. Cano (e), Chem. Mater. 34 (16), 7201-7209 (2022); https:/doi.org/10.1021/acs.chemmater.2c00726 (a) University of Stuttgart, Stuttgart (Germany) (b) University of Cagliari, Cagliari (Italy) (c) ESRF (d) Paderborn University, Paderborn (Germany) (e) Institut NEEL-CNRS, Grenoble (France)

REFERENCES

[1] A.S. Botana et al., JETP 159, 711 (2021). [2] D. Li et al., Nature 572, 624 (2019). [3] K. Wissel et al., Chem. Mater. 32 (7), 3160-3179 (2020). [4] F. Bernardini et al., Phys. Rev. Mater. 5, L061801 (2021).

Anisotropic crystal lattice dynamics of black phosphorus

The phonon dispersion curves of black phosphorus were experimentally determined by inelastic X-ray scattering, supported by ab-initio simulations. The retrieved dispersions made it possible to extract the full elastic tensor, sound velocities and elastic moduli, of great relevance for the development of technological applications for black phosphorus and phosphorene-based nanomaterials.

Layered crystals have recently emerged as a starting material for the manufacture of two-dimensional (2D) crystals with novel and largely tunable physical properties, useful for integration into miniaturised devices. Black phosphorus is the most stable allotrope of phosphorus at ambient conditions and has a layered crystal structure consisting of vertically stacked 2D single-layers of phosphorene, forming an A17 face centered orthorhombic lattice. It was first synthesised by Bridgman in 1914, and interest in it has recently seen a renaissance [1] due to its promising semiconducting properties, featuring a finite near-infrared bandgap and high hole mobility, which have enabled the manufacture of high-performance electronic and optoelectronic devices [2]. The interest in black phosphorus is also motivated by its strong in- plane anisotropy, due to the puckered structure of the phosphorene layers, which is periodic along the armchair (AC) and zigzag (ZZ) directions (Figure 98a). The in- plane anisotropy results in anisotropic electronic, optical, electrical and thermal properties, and large tunability of

the electrical and thermal properties is expected upon mechanical deformation based on the instability and anisotropy of the microscopic structure [3].

Accurate characterisation of the elastic properties of black phosphorus has been challenged by the limited availability of large, high-quality crystal samples, such that the elastic tensor was only partially characterised by inelastic neutron scattering and ultrasounds, providing estimations inconsistent with atomistic model calculations.

A complete characterisation of the elasticity of black phosphorus is here provided by a combination of diffuse scattering and inelastic X-ray scattering (IXS) measurements performed at beamline ID28, which made it possible to probe the entire Brillouin zone with a high- quality micrometre-size sample, prepared by the Bridgman method and laser-cut to a cylinder shape (height 50 microns, diameter 150 microns). Diffuse scattering measurements were applied to map the reciprocal space of black phosphorus, unveiling thermal diffuse scattering dominated by the soft transverse acoustic modes propagating along the stacking direction. The dispersion curves of the 12 phonon branches were then determined by IXS (Figure 98b), along the principal high-symmetry crystallographic directions.

The experimental phonon dispersion curves are in good agreement with the reported ab-initio calculations, which were used to evaluate the phonon dispersions along unexplored crystallographic directions and to evaluate the vibrational density of states. This latter allowed the

combined with first-principles calculations revealed a comparatively high degree of thermodynamic stability. In addition, the observed changes in the lattice parameters suggest the possibility of tuning the F content and thereby the electronic configuration of these systems. Thus, these unprecedented nickelates offer additional possibilities to investigate cuprate analogues and thereby to tackle the challenge of unconventional high-temperature superconductivity.