Showing papers by "Shigeo Mori published in 2006"

Spontaneous formation of vanadium "molecules" in a geometrically frustrated crystal: AlV2O4.

Abstract: We investigated the charge-ordered (CO) state in spinel AlV2O4 by electron diffraction, synchrotron x-ray diffraction, and magnetic measurements. It was found that the CO structure appearing below 700 K is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. Theoretical consideration indicates that this unique molecularlike V heptamer is stabilized by a strong bonding of vanadium t(2g) orbitals.

Solid state self-assembly of nanocheckerboards

Abstract: The authors introduce an emergent method to fabricate a few-nanometer-size columnar superlattice with a checkerboard pattern in inorganic spinels by harnessing the Jahn-Teller structural distortion. Transmission electron microscope images reveal that the fundamental building blocks are two types of long nanorods with the ∼4×4×70nm3 size, which are alternatively stacked in a way that the cross sectional and side views show checkerboard and herringbone patterns, respectively. The authors discuss that the strain induced by the Jahn-Teller distortion causes this peculiar self-assembled nanostructure in the coherent mixture of two spinel phases. This pure solid state self-assembly can be implemented to fabricate heterogeneous nanostructures with practical functionalities.

Doping Effect on the Charge Ordering in LuFe2O4

Abstract: Change of the charge ordered (CO) structure by substituting Cu 2+ for Fe 2+ in LuFe 2 O 4 was investigated by means of the transmission electron microscopy. The CO structure in LuFe 2 O 4 is characterized by the modulated structure with the wave vector of q=1/3[1-13/2] and the average size of the CO domains can be estimated to be about 10-20nm. On the contrary, the Cu 2+ substitution in LuFe 2 O 4 destroyed the CO structure drastically and induced characteristic local lattice distortion, which gives rise to characteristic diffuse scattering in the reciprocal space. High-resolution lattice images revealed that there exist nano-scale clusters, which are characterized as the short-range ordering of the Fe 3+ and Cu 2+ ions on the triangular lattice. In addition, the magnetic measurement revealed that LuFeCuO 4 exhibits an antiferromagnetic transition around 50K, which is lower than the Neel temperature of 250K in LuFe 2 O 4 .

Conduction Characteristics of Charge Ordering Type Ferroelectrics,YFe2O4

Abstract: Electrical conduction properties of charge ordering type ferroelectrics YFe2O4 were investigated. YFe2O4 was synthesized in reduced atmosphere at 1200 °C. Oxygen partial pressure of the reduced atmosphere was controlled by the equilibrium state of CO and CO2. YFe2O4 is paramagnetic at room temperature and has Neel temperature around 250K. The Neel temperature was decreased with increasing the amount of oxygen deficiency. Moreover, YFe2O4 showed ohmic conduction from 260 to 100 K. The temperature dependence of the DC conductivity showed an inflection point at the Neel temperature, which indicated the development of charge ordering of Fe2+ and Fe3+ ions. From the complex impedance measurements, the equivalent circuits of YFe2O4 with different oxygen deficiency were determined at various temperatures.

Ferroelectricity from Electron Ordering

Abstract: We report ferroelectricity in triangular mixed valence material RFe2O4 (R=Y, Dy, Ho, Er, Tm, Yb, Lu). This ferroelectricity originates from the polar ordering of Fe3+ and Fe2+ or the polar ordering of electrons on Fe3+.