Evolution Of Spin And Valence States Of (Pr0.7sm0.3)(0.7)Ca0.3coo3 At High Temperature And High Pressure

The changes of physical properties occurring at a first-order-like transition among a wide family of (Pr(1-y)Ln(y))(1-x)CaxCoO3 compounds (Ln being a lanthanide) are controversially discussed as a transition of Co3+ ions from states of low spin (LS) to intermediate spin (IS), or from LS to a mixture of LS and high spin (HS). The debate also includes the nature of the spin state of Co4+, as well as the degree of valence shift between Pr3+ and Pr4+ which accompanies this transition. In the present paper, we investigated the evolution of spin and valence states of Co ions and the valence state of Pr ions in (Pr0.7Sm0.3)(0.7)Ca0.3CoO3, at high temperature up to 750 K and under pressure up to 36.5 GPa. For this purpose, the data of three spectroscopic techniques were combined: high-resolution Co K-edge partial-fluorescence-yield x-ray absorption spectra, Pr L-2-edge x-ray absorption spectra, and Co K beta x-ray emission spectra. The spectral weight transfer of the pre-edge peaks of the Co K-edge spectra reveals a continuous redistribution of 3d electrons between the t(2g) and e(g) levels of Co ions, reflecting a gradual increase of the average Co spin state upon heating. This is further confirmed by Co K beta x-ray emission spectra, which show an increase of HS Co3+ population when increasing temperature from 300 to 750 K. Applying high pressure at 300 K, the Co K beta x-ray emission spectra indicate a relatively sharp increase of LS Co3+ population below 4 GPa. Pr L-2-edge x-ray absorption spectra of (Pr0.7Sm0.3)(0.7)Ca0.3CoO3 exhibit a valence-state transition from Pr3+ to Pr3.26+, occurring predominantly upon applying pressure in the range similar to 4 to similar to 12 GPa. Owing to the charge balance, increasing pressure in this regime induces complex valence and spin-state transitions within the Co ions. Moreover, at high enough pressure, it is found that the Co4+ can also undergo a spin-state transition (from IS to LS), in addition to that affecting the Co3+ (HS to LS).