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

A new cubic Ta-bronze (1) KTa1+z+(5-delta)O3 [z similar or equal to 0.107 (3)] was obtained on a cathode by molten salt electrolysis of the system K2TaOF5-K3TaO2F4-(KF + NaF + LiF)(cutectic). Black, metallic cubic crystals of (1) are formed together with tetragonal B-Ta. The perovskite-like crystal structure of (1) [a = 4.005 (1) Angstrom, space group Pm (3) over barm] 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 K alpha): 3320 measured reflections with I > 3 sigma (I), R = 0.0095, wR = 0.0065, Delta rho (min) = -0.91 e Angstrom (-3), Delta rho (max) = 0.65 e Angstrom (-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+5O3 (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 KTaO3 matrix. [References: 12] 12