Probing Co- and Fe-doped LaMO₃ (M = Ga, Al) perovskites as thermal sensors

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O₃ 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 LaGa_1-xCo_xO₃ (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^-1 for Co³⁺ (3d⁶) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa_1-xCo_xO₃ compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO₃ in which Co³⁺ 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 Co³⁺ (LS) although its ionic radius is smaller in comparison with that of Ga³⁺.