Phase Relationships, Transparency, and Conductivity in the Cadmium Indate−Cadmium Stannate System

A novel transparent conducting spinel solid solution Cd 1+ x In 2 - 2 x Sn x O 4 (0 ≤ x ≤ 0.75 at 1175 °C) between cubic CdIn 2 O 4 and orthorhombic Cd 2 SnO 4 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) Cd 2 SnO 4 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.