Iridium valence variation and carrier sign tuning in (Ca,Ba)xLa2−xCuIrO6 double perovskites

We report the structure and properties of ${(\mathrm{Ca},\mathrm{Ba})}_x{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{CuIrO}}₆$. The rock-salt double perovskite structure goes through a $P\overline{1}$ to $P{2}₁/n$ phase transition with increasing Ba content but not with Ca content. In both cases, the Ir ion is oxidized from $4+$ to $5+$ with increasing substitution. Transport and magnetic properties measurements reveal that all compositions are insulating with hysteretic spin-freezing magnetic behavior. Seebeck coefficient measurements reveal a large thermopower change from $+176\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}$/K in ${\mathrm{La}}₂{\mathrm{CuIrO}}₆$ to $\ensuremath{-}223\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}$/K with increasing Ba content in ${\mathrm{Ba}}_x{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{CuIrO}}₆$. This behavior is modeled well by the Heikes formula for correlated hopping conduction. ${(\mathrm{Ca},\mathrm{Ba})}_x{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{CuIrO}}₆$ provides a model system for investigations of other systems with similar carrier sign tuning, with applications toward semiconductors and thermoelectric materials.