Crystal structure and phase transition in the defect pyrochlore NH4NbWO6

The crystal structure of ammonium tungstoniobate is revised as some feature s observed in its X-ray diffraction pattern are not consistent with the ext inction conditions of space group Fd (3) over bar m, commonly assigned to t his compound. A better agreement between the observed and calculated X-ray powder diffraction patterns was obtained by performing a Rietveld refinemen t with a structural model using the noncentrosymmetric space group F (4) ov er bar 3m. The temperature dependence of the Fd (3) over bar m-forbidden 42 0 Bragg peak intensity indicates a continuous transition at 455 K from the space group F (4) over bar 3m to a more symmetrical, high-temperature phase , with space group Fd (3) over bar m. (C) 1998 Academic Press.