Synthesis and characterization of visible light absorbing nanosized (1 − x)KNbO3–x($${\textbf{BaNi}}_{\mathbf {\frac{1}{2}}} {\textbf{Nb}}_{\mathbf{\frac{1}{2}}} {\textbf{O}}_{\mathbf{3 - \delta } }$$) (x = 0, 0.05, 0.1, 0.15 and 0.2) by solution combustion method and its enhanced photocatalytic activity