High-pressure tungsten bronzes, RExWO3 with RE = La and Nd, studied by X-ray diffraction and electron microscopy

Rare earth tungsten bronzes, La0.1WO3 and NdxWO3 with x = 0.1 and 0.2, have been prepared by solid state reaction at P = 30 kbar and T = 1320-1620 K a nd structurally characterized by X-ray diffraction and electron microscopy. The hexagonal tungsten bronze (HTB) structure was found in all samples, wh ereas intergrowth tungsten bronzes of (n)-ITB and (1, n)-ITB structure type s have only been observed in samples prepared at T>1500 K. The ITB structur es can be considered as an intergrowth of slabs of WO3-type, n octahedra wi de, and slices of HTB-type. In the (II)-ITB family the slices of HTB-type a re one hexagonal tunnel row wide, whereas they are two hexagonal tunnel row s wide in the (1,n)-ITB phases. Members of the (II)-ITB family were frequen tly observed in the high-temperature samples. The LaxWO3 bronzes, (2)-ITB, (3)-ITB, (1,2)-ITB, and HTB, are new. In the Nd,WO, system a perovskite tun gsten bronze (PTB) of composition Ndapproximate to0.1WO3 and a (3)-ITB bron ze were found in addition to the previously reported high-pressure phases ( 2)-ITB and HTB. The bronze structures were well-ordered, but a few exceptio ns were observed. A couple of defects are described along with proposed def ect models. Microanalyses show that the hexagonal tunnel sites in the ITB a nd HTB structures are less than half-filled with RE atoms. The relationship s between the RExWO3 bronzes and the alkali tungsten bronzes, A(x)WO(3), ar e discussed. (C) 2000 Academic Press.