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

Abstract Rare earth tungsten bronzes, La 0.1 WO 3 and Nd x WO 3 with x =0.1 and 0.2, have been prepared by solid state reaction at P =30 kbar and T =1320–1620 K and structurally characterized by X-ray diffraction and electron microscopy. The hexagonal tungsten bronze (HTB) structure was found in all samples, whereas intergrowth tungsten bronzes of ( n )-ITB and (1, n )-ITB structure types have only been observed in samples prepared at T >1500 K. The ITB structures can be considered as an intergrowth of slabs of WO 3 -type, n octahedra wide, and slices of HTB-type. In the ( n )-ITB family the slices of HTB-type are one hexagonal tunnel row wide, whereas they are two hexagonal tunnel rows wide in the (1, n )-ITB phases. Members of the ( n )-ITB family were frequently observed in the high-temperature samples. The La x WO 3 bronzes, (2)-ITB, (3)-ITB, (1, 2)-ITB, and HTB, are new. In the Nd x WO 3 system a perovskite tungsten bronze (PTB) of composition Nd ≈0.1 WO 3 and a (3)-ITB bronze were found in addition to the previously reported high-pressure phases (2)-ITB and HTB. The bronze structures were well-ordered, but a few exceptions were observed. A couple of defects are described along with proposed defect models. Microanalyses show that the hexagonal tunnel sites in the ITB and HTB structures are less than half-filled with RE atoms. The relationships between the RE x WO 3 bronzes and the alkali tungsten bronzes, A x WO 3 , are discussed.