Effects of Pressure and Temperature on the Formation of Tungsten-Bronze-Related Phases, RExWO3+y, with RE=La and Nd

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 RExWO3, 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 PTB bronze RExWO3 with x~ 0.10 was formed at por=to20 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 RExWO3+3x/2 with x