Understanding the relationships between structural features and optical/magnetic properties when designing Fe(1-x)Mg(x)MoO4 as piezochromic compounds

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⁺.

Piezochromism and hydrochromism through electron transfer: new stories for viologen materials

A pyridinium-carboxylate compound undergoes reversible color change under pressure owing to the formation of radicals via electron transfer; dehydration and hydration can also trigger electron transfer.

While viologen derivatives have long been known for electrochromism and photochromism, here we demonstrated that a viologen-carboxylate zwitterionic molecule in the crystalline state exhibits piezochromic and hydrochromic behaviors. The yellow crystal undergoes a reversible color change to red under high pressure, to green after decompression, and finally back to yellow upon standing at ambient pressure. Ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance X-ray diffraction and DFT calculations suggested that the piezochromism is due to the formation of radicals via pressure-induced electron transfer from carboxylate to pyridinium, without a crystallographic phase transition. It was proposed that electron transfer is induced by pressure-forced reduction of intermolecular donor–acceptor contacts. The electron transfer can also be induced by dehydration, which gives a stable green anhydrous radical phase. The color change is reversible upon reabsorption of water, which triggers reverse electron transfer. The compound not only demonstrates new chromic phenomena for viologen compounds, but also represents the first example of organic mechanochromism and hydrochromism associated with radical formation via electron transfer.