Like their chemical cousins, the high-temperaturesuperconductors (see p.14), they possess complicated electronic structures.Yet unravelling their behaviour – over a range of temperatures or in magnetic fields – is essential to understanding CMR and finding suitable CMR materials for commercial application.
Our contribution to this active research area has been to investigate, in these materials, an attribute of electronic structure that has hitherto been difficult to probe experimentally – the electron orbitals. Electrons involved in chemical bonding have three defining properties laid down by quantum mechanics, their charge, spin and orbital.The orbital refers to the shape of the volume that the electron is likely to occupy (the electron cloud). In transition metals such as manganese, it is the outer 'd' orbitals that are involved in bonding and give heavy metal compounds their electronic and structural characteristics.
In the CMR manganites, the manganese atom is triply charged (Mn3+) and is bound to six neighbouring oxygen atoms in an octahedral shape as shown in Figure 1.Mn3+ has one d electron involved in bonding. This 'valence' electron occupies one of two possible d orbitals, as shown in Figure 2.These orbitals are thought to be significant in CMR. Each can mix, or 'hybridise' with the oxygen orbitals to form bonding orbitals; each is geometrically different and is associated with differing bond lengths.
The structure of 50 per-cent hole-doped
lanthanum strontium manganite
(LaSr2Mn2O7), at a temperature of 165K,
looking down through the manganese-oxygen
octahedra. Neutron diffraction allowed us to
determine the position of both manganese
and oxygen atoms and thus the Mn–O bond
distances.This revealed the ordering of the
Mn3+ and Mn4+ ions (A) and the ordering of
the hybridised orbitals (B)
Abel A. Colmenares E.