Niobium–aluminum oxynitride prepared by ammonolysis of oxide precursor obtained through the citrate route

Oxynitrides in the (Nb_1−xAl_x)(O,N) quaternary system were prepared by ammonolysis of oxide precursor obtained through the citrate route. The products at 1000 °C were a mixture of Nb(N,O) and NbN_0.95 at the niobium end (x = 0) and amorphous Al(O,N) at the aluminum end (x = 1). A new cubic compound (A) appeared mixed with Nb(N,O) in the compositional range 0.1 ≤ x ≤ 0.4. Its almost pure product was obtained at x = 0.5. The X-ray diffraction pattern was rock salt type (Nb_0.56Al_0.44)(O_0.38N_0.37□₀₂₅) in F_m−3m with a = 0.43481(1) nm. The product showed superconductivity with T_c = 15 K. Its crystallinity was much improved and its superconducting volume fraction increased to 32% after its thermal annealing at 1100 °C in evacuated sealed tube. A second cubic compound (B), rock salt type Nb[(O,N)_0.85□_0.15] with a = 0.434 nm, was observed mixed with amorphous Al(O,N) in the as-prepared products of the range 0.6 ≤ x ≤ 0.9.