Spin and orbital ordering in ternary transition metal oxides
View/ Open
Date
05/2008Author
Kimber, Simon A. J.
Metadata
Abstract
Spin and orbital orderings are amongst the most important phenomena in the solid state
chemistry of oxides. Physical property and powder neutron and X-ray diffraction
measurements are reported for a range of mostly low dimensional ternary transition metal
oxides which display spin or orbital order.
Extensive studies of the physical properties and crystal structure of In2VO5 are reported. The
structure of this material consists of one dimensional zig-zag chains of orbitally ordered S =
1/2 V4+. Magnetic susceptibility measurements show an unusual crossover from dominant
ferromagnetic (θ = 17 K) to antiferromagnetic (θ = -70 K) exchange at 120 K, which is
attributed to ferromagnetic dimerisation driven by magnetic frustration. The magnetic
moment also increases from 1.81 to 2.2μB at the 120 K crossover. Heat capacity
measurements confirm this scenario as the magnetic entropy tends towards 1/2 Rln3 below
120 K before approximating to Rln2 at high temperature. Synchrotron x-ray diffraction and
high resolution neutron powder diffraction show no bulk structural changes, but the b axis,
along which the VO6 chains run, shows an anomalous expansion below 120 K. At low
temperatures, a downturn in the magnetic susceptibility is seen at 2.5 K, signifying a spin
freezing transition. Heat capacity and powder neutron diffraction measurements show no
evidence for long range magnetic order down to 0.42 K.
The low dimensional brannerite materials MV2O6 (M = Mn, Co, Ni) were synthesised by a
sol-gel method. Magnetic properties were investigated by magnetisation, powder neutron
diffraction and in the case of CoV2O6, heat capacity measurements. The structure of these
materials consists of linear chains of edge sharing MO6 octahedra. Monoclinic MnV2O6 is an
isotropic antiferromagnet with TN = 20 K and a reduced magnetic coherence length due to 3
% Mn/V antisite disorder. The magnetic structure consists of ferromagnetic edge-sharing
chains with k = (0,0,1/2) and a refined Mn moment of 4.77(7) μB. The triclinic materials
CoV2O6 and NiV2O6 are also antiferromagnetic with TN = 7 and 14 K respectively and both
show metamagnetic type transitions. Unusually, M(H) isotherms recorded below 5 K for
CoV2O6 show a plateau at 1/3 of the saturation magnetisation. This feature, together with a
long period modulated magnetic structure, is attributed to strong single ion (Ising) type
anisotropy and nearest neighbour ferromagnetic exchange. Preliminary high pressure
experiments on NiV2O6 have confirmed a previously reported transition to a columbite
phase at 6 GPa and 900 °C. The high pressure polymorph is also antiferromagnetic with TN =
2.5 K.
The previously uncharacterised perovskite, PbRuO3 has been prepared using high
pressure/temperature synthesis techniques (10 GPa, 1000 °C). Synchrotron powder X-ray
diffraction measurements show that the room temperature structure is orthorhombic, Pnma.
A first order orbital ordering transition occurs at 75 K with an associated metal insulator
transition. Below 75 K, the dxz orbitals are preferentially occupied and the structure is orthorhombic Imma. The transition may be driven by an increase in antiferroelectric Pb2+
displacements, whcih reach a peak at ~ 125 K. A further structural transition to a larger
monoclinic cell is also identified at 9.7 K.
The physical properties and crystal structures of two low dimensional lead manganese
oxides have also been investigated. Acentric Pb2MnO4, which has a structure consisting of
edge sharing chains, is antiferromagnetic with TN = 18 K. Powder neutron diffraction shows
the magnetic structure consists of antiferromagnetic chains with k = (0,0,0) and a refined Mn
moment of 2.74(2) μB. The crystal point group allows piezoelectricity and the magnetic point
group symmetry allows piezomagnetism. We speculate that coupled magnetic and electric
properties may be observed in this material. The layered material, Pb3Mn7O15, with a
structure consisting of 1/2 filled Kagomé layers has also been studied. Single crystals were
prepared by a flux growth method and polycrystalline material was prepared by the ceramic
method. Powder neutron and synchrotron x-ray diffraction studies show that the single
crystals are hexagonal and that the polycrystalline material is orthorhombic. Furthermore,
heat capacity measurements show that the hexagonal single crystal material undergoes a
glassy magnetic transition. In contrast, powder neutron diffraction shows that the
orthorhombic polycrystalline material has coherent long range magnetic order. These
differences are attributed to an oxygen deficiency in the polycrystalline magnetic order. These
differences are attributed to an oxygen deficiency in the polycrystalline material