Phase relationships, transparency, and conductivity in the cadmium indate-cadmium stannate system

A novel transparent conducting spinel solid solution Cd(1+x)In(2-2x)Sn(x)O₄ (0 ≤ x ≤ 0.75 at 1175 °C) between cubic CdIn₂O₄ and orthorhombic Cd₂SnO₄ has been discovered. The extent of this solid solution is noteworthy given that bulk phase relations determined in air over the temperature range 800-1175 °C demonstrate that cubic (spinel) Cd₂SnO₄ is metastable. Reduced samples exhibit exceptional four-point dc conductivities exceeding 3500 S/cm (x = 0.70). The maximum optical gap estimated from diffuse reflectance spectra is ~3.0 eV for 0 ≤ x ≤ 0.15. The optical gap shows negligible change with reduction, possibly because of combined Moss-Burstein and band-narrowing effects. Abrupt conductivity increases and optical gap decreases at specific compositions, i.e., x = 0.4 and 0.2 respectively, may be indicative of a change in cation distribution between tetrahedral and octahedral sites. Room temperature thermopower measurements indicate the change in conductivity across the solution is a consequence of an increase in carrier density and that mobility remains relatively constant.