A novel perovskite-like Ta-bronze KTa1+zO3: preparation, stoichiometry, conductivity and crystal structure studies.

A new cubic Ta-bronze (1) KTa(1+z)(+(5-delta))O(3) [z approximately 0.107 (3)] was obtained on a cathode by molten salt electrolysis of the system K(2)TaOF(5)-K(3)TaO(2)F(4)-(KF + NaF + LiF)(eutectic). Black, metallic cubic crystals of (1) are formed together with tetragonal beta-Ta. The perovskite-like crystal structure of (1) [a = 4.005 (1) A, space group Pm3m] was refined with anharmonic displacement parameters for Ta and K atoms and anisotropic displacement parameters for a split O-atom position [KM4CCD diffractometer; lambda(Mo Kalpha); 3320 measured reflections with I > 3sigma(I); R = 0.0095, wR = 0.0065, Deltarho(min) = -0.91 e A(-3), Deltarho(max) = 0.65 e A(-3)]. Defects in the O and K atomic positions were found. (1) is a semiconductor in the temperature range 4-300 K, whereas the well studied and closely related colourless transparent crystals KTa(+5)O(3) (2) are dielectric. Differences in the properties of (1) and (2) are assumed to be connected with the existence of Ta dumb-bells statistically distributed into the KTaO(3) matrix.