We investigated the structural, magnetic, transport, and high-temperature thermoelectric properties of single crystals of the pseudobrookite Al1-xTi2+xO5 for 0≤x≤1 grown using a floating zone. We found a correlation of spin-singlet Ti3+-Ti3+ dimers coupled with the lattice even in the conductive α and λ phases which develops with increasing x. This developing dimer correlation reduces the number of unpaired Ti3+ 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 λ phase near the boundary (x∼0.9) between the λ and the β phases. Such a correlation of the spin-singlet Ti3+-Ti3+ dimers may produce a high Seebeck coefficient in the conductive α and λ phases leading to a large thermoelectric power factor at high temperatures.
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