Structure and thermal stability of La0.10WO3+y; A hexagonal tungsten bronze related phase formed at high pressure

Abstract The structure and thermal stability of a hexagonal tungsten bronze (HTB) related compound, La x WO 3+ y with x ≈0.10 and y ≈0.15, has been studied by X-ray diffraction, thermal analysis, and electron microscopy. The structure was refined by the Rietveld method from X-ray powder diffractometer data of a La 0.10 WO 3 sample prepared at T =1250°C and P =25 kbar, which consisted of two tungsten bronze related phases in 1:1 proportion. The unit cell dimensions are as follows: La 0.108 WO 3+ y ( y ≈0.16), a =7.40890(5), and c =3.79329(4) A (HTB-related structure); La 0.091 WO 3 , a =3.82458(6) A (cubic perovskite tungsten bronze (PTB) structure). The lanthanum atoms in La 0.108 WO 3+ y are located on the hexagonal axis and statistically distributed on two sites close to the tungsten atom plane. Thermal stability studies of the La 0.10 WO 3 sample in an argon atmosphere under ambient pressure conditions revealed that the HTB-related compound is metastable, decomposing to the stable PTB-type structure and WO 3 . It was also found from the TG experiments in argon and oxygen that additional oxygen atoms ( y ) are present in the structure, thus forming a lanthanum tungsten oxide of the above composition. The electron diffraction and microanalysis studies confirmed that crystals of the HTB- and PTB-type structures were formed, with a lanthanum content of x ≈0.1.