Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba0.5–x­TaO3–x is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba0.5–x­TaO3–x (x = 0.25–0.325) compositions are stabilized via the interpolation of Ba2+ cations and (TaO)3+ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the c axis. The slightly nonstoichiometric Ba0.5–x­TaO3–x (x = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< x < 0.25 nonstoichiometric range, resulting in two closely related (TaO)3+-containing and (TaO)3+-free TTB phases. The highly nonstoichiometric (TaO)3+-containing phase exhibits Ba2+ cationic migration. The incorporation of (TaO)3+ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)3+ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions.