Paolo G. Radaelli

Structural phenomena associated with the spin-state transition in LaCoO 3

The structural properties of

Structure and superconductivity of HgBa2CuO4+δ

{\mathrm{LaCoO}}_{3}

The Preparation and Structures of Hydrogen Ordered Phases of Ice

were studied by means of high-resolution neutron powder diffraction in the temperature range

Formation of isomorphic Ir3+ and Ir4+ octamers and spin dimerization in the spinel CuIr2S4.

5l~Tl~1000\mathrm{K}.

Long Range Charge Ordering in Magnetite Below the Verwey Transition

Changes of the

Ice XV: a new thermodynamically stable phase of ice.

{\mathrm{Co}}^{+3}

Structure, doping and superconductivity in HgBa2CaCu2O6+δ (Tc ⩽ 128 K)

spin states in this temperature interval are shown to affect not only the unit-cell volume, as previously known, but also internal parameters such as the metal-oxygen bond lengths. These data, as well as the temperature-dependent magnetic susceptibility, can be qualitatively modeled based on a three-state (low-spin, intermediate-spin, and high-spin) activated behavior, but correction terms are required for quantitative agreement. Our fits consistently indicate that the ionic radius of the intermediate-spin state (\ensuremath{\sim}0.56 \AA{}) is smaller than the low-spin/high-spin average (\ensuremath{\sim}0.58 \AA{}). We also present evidence of a third lattice anomaly, occurring around 800 K, which we attribute to the formation of oxygen vacancies.

Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices

Abstract We have used neutron powder diffraction to investigate the defect structure of HgBa 2 CuO 4+δ . An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T c onset , of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

Two hydrogen ordered phases of ice were prepared by cooling the hydrogen disordered ices V and XII under pressure. Previous attempts to unlock the geometrical frustration in hydrogen-bonded structures have focused on doping with potassium hydroxide and have had success in partially increasing the hydrogen ordering in hexagonal ice I (ice Ih). By doping ices V and XII with hydrochloric acid, we have prepared ice XIII and ice XIV, and we analyzed their structures by powder neutron diffraction. The use of hydrogen chloride to release geometrical frustration opens up the possibility of completing the phase diagram of ice.

The polymorphism of ice: five unresolved questions

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network. The CuIr2S4 thiospinel is another example, well known for the presence of a metal–insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal–insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition—a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.