Structural, magnetic, transport, and thermoelectric properties of the pseudobrookite AlTi2O5−Ti3O5 system

We investigated the structural, magnetic, transport, and high-temperature thermoelectric properties of single crystals of the pseudobrookite ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_{5}$ for $0\ensuremath{\le}x\ensuremath{\le}1$ grown using a floating zone. We found a correlation of spin-singlet ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ dimers coupled with the lattice even in the conductive $\ensuremath{\alpha}$ and $\ensuremath{\lambda}$ phases which develops with increasing $x$. This developing dimer correlation reduces the number of unpaired ${\mathrm{Ti}}^{3+}$ ions, which makes the compound more conductive owing to the suppression of disorder for $x$ up to about 0.75. The dimer fluctuation causes a critical enhancement of the magnetic susceptibility at around 150 K in the $\ensuremath{\lambda}$ phase near the boundary ($x\ensuremath{\sim}0.9$) between the $\ensuremath{\lambda}$ and the $\ensuremath{\beta}$ phases. Such a correlation of the spin-singlet ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ dimers may produce a high Seebeck coefficient in the conductive $\ensuremath{\alpha}$ and $\ensuremath{\lambda}$ phases leading to a large thermoelectric power factor at high temperatures.