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

✍ Scribed by C. Grenthe; M. Sundberg; V.P. Filonenko; I.P. Zibrov

• Publisher: Elsevier Science
• Year: 2002
• Tongue: English
• Weight: 552 KB
• Volume: 168
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2002.9723

✦ Synopsis

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 solidstate 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-bronzerelated phases with perovskite (PTB)-, hexagonal (HTB)-and intergrowth (ITB)-type structures were identified. The PTB bronze RE x WO 3 with x E 0.10 was formed at pr50 kbar. The HTB-related phase with x E 0.10 was observed in samples prepared at PZ20 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+3x/2 with xr0.13. 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 E60% occupancy of the hexagonal tunnel sites (xE0.20) was observed in a few crystals from the samples prepared at P = 80 kbar. Ordered, defect and intergrowth structures are presented.