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A team of chemists and physicists at the universities of Liverpool and Oxford is the first to show that hydrogen transmits magnetism. They created a new magnetic-oxide material in which, for the first time, the dominant magnetic interaction is produced by a negatively charged hydrogen atom, known as a hydride ion. In the past, many types of magnetic oxides have been prepared that have important magnetic conducting and even superconducting properties, but the new material - LaSrCoO3H0.7 -is the first in which oxide and hydride ions coexist. The team - headed by Professor Matthew Rosseinsky of the Department of Chemistry, University of Liverpool, and Dr Stephen Blundell of the Physics Department, University of Oxford - confirmed the magnetic properties involved. This was achieved by measuring the new material using particles known as muons. "Muons behave as tiny gyroscopes and spin round when they experience a magnetic field," said Dr Blundell. "When implanted in the new material, we found that they carried on spinning round as we warmed the sample from a degree above absolute zero to room temperature, demonstrating that the sample was magnetic over the whole region. That was a surprise because without the hydrogen in there, we would have expected the oxide chains to lose their magnetism at all but the lowest temperatures." LaSrCoO3H0.7 adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network. The metal centres are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering up to at least 350 Kelvin (K). The synthetic route is sufficiently general to allow the prediction of a new class of transition metal-containing electronic and magnetic materials. Professor Rosseinsky said: "The chemistry leading to this compound was totally unexpected. Before this work, most chemists would not have believed that anyone could synthesise a material with this composition." In this approach the team used soft chemistry or what is known as chimie douce. This is carried out under moderate conditions, typically fewer than 500 degrees Celsius, and thus preserves the structural elements of the solid-state reactants but changes their composition." This low-temperature synthetic route shows that ternary transition metal oxides can form oxide-hydrides, despite the expectation that in a high-temperature solid-state synthesis with a hydride source, the transition metal cation would be reduced to the metal. The synthesis also shows that reduction by hydride ions can yield unexpected results as with the Liverpool/Oxford project. Their discovery of this useful new material is example of how this may be achieved by the detailed understanding and control over the coupling. This approach has been greatly accelerated in recent years because a range of techniques is now available to help researchers achieve this goal. Among the most significant of these has been the synthesis and in-depth characterisation of an unusual mixed-metal materials that contain both oxide and hydride ions. For example, cobalt-hydride-cobalt bonding provides a new pathway for electronic and magnetic interactions. Essentially, the interest in hydrides mainly stems from their strong reducing properties. The present wide range of characterisation techniques that can be brought to bear on this activity reflects how far solid-state chemistry has advanced in recent years. It also demonstrates how a series of linked techniques from powder diffraction for indexing purposes, to neutron and synchotron X-ray diffraction, electron microscopy, and muon spin rotation can be combined to carry out a complete characterisation.
With oxygen linking two metal centres, the interaction can either be along or perpendicular to the bond axis. On the other hand, hydrides allow only the interaction along the axis. Also, the strength of interactions are different. The promising result, yet to be fully explored, is that this new material and other future family members can thus be expected to have interesting new magnetic and electronic properties. Source: London Press Service, web site at: http://www.london.press.net Published on 29th July 2002
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Not only a new class of magnetic materials but the exciting prospect of the first step towards a whole new field of chemistry is the likely result of a discovery by scientists in the United Kingdom.
Muon spin rotation in particular has established that extra diffraction peaks in the neutron diffraction pattern were caused by magnetic ordering. The magnetic and electronic properties of oxides are controlled through the exchange pathway from the metal through oxygen back to the metal.
