Rare-earth valence and doping in T-, T'-, and T*-phase R2CuO4 (R=rare earths)

Using applied pressure to guide materials design: a neutron diffraction study of La2NiO4+δ and Pr2NiO4+δ

The compression behaviours of La2NiO4+δ and Pr2NiO4+δ have been studied up to a pressure of 2.8 and 2.2 GPa respectively. Using neutron diffraction, the mechanism of compression, and the behaviour of the NiO6 and La/PrO9 polyhedra in these layered perovskite materials have been determined. Their compression mechanisms have then been compared to related materials (La2-xPrxNiO4, Pr2-xNdxNiO4, La2-xSrxNiO4 and Pr2-xCaxNiO4) where the unit-cell volume has been reduced by controlling the composition (x), which acts as an 'effective chemical pressure'. Understanding the effects of both has implications for materials design; pressure can be used to finely tune a property, which theoretically may then be emulated using chemical doping.

Local structural disorder in REFeAsO oxypnictides by RE L(3) edge XANES

The REFeAsO (RE = La, Pr, Nd and Sm) system has been studied by RE L(3) x-ray absorption near edge structure (XANES) spectroscopy to explore the contribution of the REO spacers between the electronically active FeAs slabs in these materials. The XANES spectra have been simulated by full multiple scattering calculations to describe the different experimental features and their evolution with the RE size. The near edge feature just above the L(3) white line is found to be sensitive to the ordering/disordering of oxygen atoms in the REO layers. In addition, shape resonance peaks due to As and O scattering change systematically, indicating local structural changes in the FeAs slabs and the REO spacers due to RE size. The results suggest that interlayer coupling and oxygen order/disorder in the REO spacers may have an important role in the superconductivity and itinerant magnetism of the oxypnictides.

RE L(3) x-ray absorption study of REO(1-x)F(x)FeAs (RE = La, Pr, Nd, Sm) oxypnictides

Rare earth L(3)-edge x-ray absorption near-edge structure (XANES) spectroscopy has been used to study REOFeAs (RE = La, Pr, Nd, Sm) oxypnictides. The Nd L(3) XANES due to the [Formula: see text] transition shows a substantial change in both white line (WL) spectral weight and the higher energy multiple scattering resonances with the partial substitution of O by F. A systematic change in the XANES features is seen due to varying lattice parameters with ionic radius of the rare earth. On the other hand, we hardly see any change across the structural phase transition. The results provide timely information on the local atomic correlations showing the importance of the local structural chemistry of the REO spacer layer and interlayer coupling in the competing superconductivity and itinerant striped magnetic phase of the oxypnictides.