  A tunable conductivity has been achieved by controllable substitution of a
novel UV light activated electronic conductor. The transparent conducting oxide
system H-doped Ca12-xMgxAl14O33 (x = 0; 0.1; 0.3; 0.5; 0.8; 1.0) presents a
conductivity that is strongly dependent on the substitution level and
temperature. Four-point dc-conductivity decreases with x from 0.26 S/cm (x = 0)
to 0.106 S/cm (x = 1) at room temperature. At each composition the conductivity
increases (reversibly with temperature) until a decomposition temperature is
reached; above this value, the conductivity drops dramatically due to hydrogen
recombination and loss. The observed conductivity behavior is consistent with
the predictions of our first principles density functional calculations for the
Mg-substituted system with x=0, 1 and 2. The Seebeck coefficient is essentially
composition- and temperature-independent, the later suggesting the existence of
an activated mobility associated with small polaron conduction. The optical gap
measured remains constant near 2.6 eV while transparency increases with the
substitution level, concomitant with a decrease in carrier content.
