Spin State Control of the Perovskite Rh/Co Oxides

We show why and how the spin state of transition-metal ions affects the thermoelectric properties of transition-metal oxides by investigating two perovskite-related oxides. In the A-site ordered cobalt oxide Sr [Formula: see text] YCo [Formula: see text] O [Formula: see text] , partial substitution of Ca for Sr acts as chemical pressure, which compresses the unit cell volume to drive the spin state crossover, and concomitantly changes the magnetization and thermopower. In the perovskite rhodium oxide LaRhO [Formula: see text] , partial substitution of Sr for La acts as hole-doping, and the resistivity and thermopower decrease systematically with the Sr concentration. The thermopower remains large values at high temperatures (>150 μV/K at 800 K), which makes a remarkable contrast to La [Formula: see text] Sr [Formula: see text] CoO [Formula: see text]. We associate this with the stability of the low spin state of the Rh [Formula: see text] ions.