Probing Co- and Fe-doped LaMO (M = Ga, Al) perovskites as thermal sensors.

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O perovskite via the Pechini process aimed to achieve a color change induced by temperature and associated with spin crossover (SCO). In Fe-doped samples, iron was shown to be in the high-spin state, whereas SCO from the low-spin to the high-spin configuration was detected in Co-doped compounds when the temperature increased. Fe-doped compounds clearly adopted the high-spin configuration even down to 4 K on the basis of Mössbauer spectroscopic analysis. The original SCO phenomenon in the Co-doped compounds LaGaCoO (0 < x < 0.1) was evidenced and discussed on the basis of in situ X-ray diffraction analysis and UV-vis spectroscopy. This SCO is progressive as a function of temperature and occurs over a broad range of temperatures between roughly 300 °C and 600 °C. The determination of a crystal field strength of about 2 eV and a Racah parameter B of about 500 cm for Co (3d) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGaCoO compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO in which Co is stabilized in the LS state shows that electronic effects with the creation of M-O covalent bonds are predominant and contribute to the stabilization of a high crystal field around Co (LS) although its ionic radius is smaller in comparison with that of Ga.