Spin Singlet Formation inMgTi2O4: Evidence of a Helical Dimerization Pattern

Abstract

The transition-metal spinel ${\mathrm{M}\mathrm{g}\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_4$ undergoes a metal-insulator (M-I) transition on cooling below $T_{\mathrm{M}\mathrm{\text{−}}\mathrm{I}}=260\mathrm{K}$. A sharp reduction of the magnetic susceptibility below $T_{\mathrm{M}\mathrm{\text{−}}\mathrm{I}}$ 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 ${\mathrm{M}\mathrm{g}\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_4$, 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, ${\mathrm{M}\mathrm{g}\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_4$ is an orbitally ordered band insulator.