EPR investigation of superparamagnetism in nanometer-size LaFeO(3+d) and La(2)NiO(4+d) perovskites

Two perovskite-like catalysts (LaFeO3 and La2NiO4) have been prepared, characterised by high thermal stability and phase purity. SEM analysis showed that they consisted of 20-60 nm spheroids, generally clustered into one order of magnitude larger particles. A temperature-dependent EPR pattern was obtained with LaFeO3. In particular, at 110 K the spectrum was formed of an asymmetric pattern attributable to an axially symmetric paramagnetic system with gx = gy = 2, gz = 3.2. This feature became progressively more symmetric with increasing temperature, the apparent gz value decreasing down to 2.3 at 480 K. The EPR spectrum of La2NiO4 was even more temperature-dependent than that of LaFeO3. However, the EPR line of the former became more symmetric than that of the latter at high temperature. Indeed, gx = gy = 2.13 at any temperature and gz decreased from 5.5 down to 2.9 when increasing temperature from 110 K up to 380 K. The results show that both samples possess superparamagnetic properties, connected with particles ca. 20 nm in size. Furthermore, the EPR patterns were broader with La2NiO4, than with LaFeO3, indicating a greater amount of local disorder in the former. This was attributed to the presence of more oxygen vacancies in the Ni-based with respect to the Fe-based sample, in line with what elsewhere hypothesised on the base of catalytic properties of the two samples for the flameless combustion of methane.