Effects of pressure and temperature on the formation of tungsten-bronze-related phases, RExWO3+y, with RE = La and Nd

Abstract The influence of pressure ( P ) and temperature ( T ) on the formation of tungsten-bronze-related phases containing lanthanum and neodymium was investigated. A large number of samples with bulk compositions RE x WO 3 , prepared by solid-state reaction in the pressure and temperature regions P = 10–80 kbar and T = 1170–1620 K were examined by X-ray powder diffraction and electron microscopy, and a ( P – T ) diagram showing the phase relations was drawn. Three tungsten-bronze-related phases with perovskite (PTB)-, hexagonal (HTB)- and intergrowth (ITB)-type structures were identified. The P TB bronze RE x WO 3 with x ≈ 0.10 was formed at p ≤50 kbar. The HTB-related phase with x ≈ 0.10 was observed in samples prepared at P ≥20 kbar, whereas phases of ( n )-ITB-type were observed only in the 25–50 kbar region. In the latter pressure region, the PTB and ITB phases were only seen in samples prepared at T > 1520 K, while the HTB-related phase was found in almost all samples. The HTB- and ITB-related compounds are metastable, probably fully oxidized, high-pressure phases of composition RE x WO 3+3 x /2 with x ≤0.13. They transform to a cubic PTB bronze during annealing in inert atmosphere under ambient pressure conditions. According to microanalysis studies of individual crystals, less than 40% of the hexagonal tunnel sites in the HTB and ITB structures are occupied by RE 3+ ions. A superstructure of HTB-type with ≈60% occupancy of the hexagonal tunnel sites ( x ≈0.20) was observed in a few crystals from the samples prepared at P = 80 kbar. Ordered, defect and intergrowth structures are presented.