Spin Singlet Formation in [Formula presented]: Evidence of a Helical Dimerization Pattern

Abstract

The transition-metal spinel [Formula presented] undergoes a metal-insulator (M-I) transition on cooling below [Formula presented]. A sharp reduction of the magnetic susceptibility below [Formula presented] suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of [Formula presented], which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, [Formula presented] is an orbitally ordered band insulator. © 2004 The American Physical Society.