Cobalt spin state above the valence and spin-state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3

Abstract

Pr 0.7 Sm 0.3 ) 0.7 Ca 0.3 CoO 3 belongs to a class of cobalt oxides undergoing a first-order transition (T*  90 K) associated to a coupled change in the valence and spin-state degrees of freedom. The Curie-Weiss regime present around room temperature (T >> T*) was analysed in detail to address the controversial issue of the cobalt spin states above the transition. This magnetic investigation indicates that the Co 4+ are in an intermediate spin-state, while the Co 3+ are in a mixed state combining low-spin and high-spin states. These results are discussed with respect to the literature on related compounds and recent results of x-ray absorption spectroscopy.

Simultaneous First-Order Valence and Oxygen Vacancy Order/Disorder Transitions in (Pr0.85Y0.15)0.7Ca0.3CoO3−δ via Analytical Transmission Electron Microscopy

“…We have thus directly measured, using EELS, and in combination with high spatial resolution imaging, a formal Co valence change from 3.3+ to 3.0+ on cooling, along with a corresponding upward shift in Pr valence. This is qualitatively consistent with prior work using XAS and XANES, ,− although it should be noted that this is the first report (to our knowledge) using EELS as opposed to X-ray spectroscopy, the first to quantify the valence change from the O K -edge (via calibration with PCCO samples), the first to combine spectroscopy with imaging, and one of the few to simultaneously probe both the Co and Pr valence states. In terms of quantitative comparison with other results, the extracted (or deduced) Co valence shifts vary somewhat with precise composition, measurement technique, and so forth, but lie in the approximate range 0.13–0.30, ,− consistent with our result.…”

“…A flurry of investigations ensued, leading to the remarkable conclusion, backed up by numerous observations, that the phase transition is actually a valence transition , initiated by a shift from the standard 3+ valence for Pr toward the unexpected Pr 4+ . − This upward shift in Pr valence necessarily results in a reduction in the effective Co valence on cooling, hence driving the first-order metal–insulator and magnetic transitions. The latter has been shown to be a spin-state transition .…”

“…We note, as an aside, that a valence shift on Tb in a Tb-substituted system has also been claimed, but this is of a much smaller size than for the Pr ion. While spectroscopic studies using X-ray absorption spectroscopy (XAS) have now been performed and have directly verified the valence shifts, − much remains to be understood about this remarkable phenomenon. As specific examples, we currently have little understanding of critical factors for, or precursors to, the first-order valence transition, and little understanding of the homogeneity of the starting and final states.…”

“…As specific examples, we currently have little understanding of critical factors for, or precursors to, the first-order valence transition, and little understanding of the homogeneity of the starting and final states. It is known that the shift from Pr 3+ valence is only partial (not completely to 4+) ,− but it is unclear whether this is a homogeneous partial shift, or whether the Pr valence change varies from site to site. This is important as the magnetic properties of systems such as (Pr 0.85 Y 0.15 ) 0.7 Ca 0.3 CoO 3‑δ (hereafter denoted PYCCO), have been found to be highly inhomogeneous at low temperatures. , In addition, while spectroscopic studies at different elemental edges have been performed, it is beneficial to study not only multiple edges simultaneously (ideally with full quantification), but also with high spatial resolution.…”

Magnetism and transport in transparent high-mobility BaSnO3 films doped with La, Pr, Nd, and Gd

“…While most of the rare-earth ions are trivalent, both Pr 4+ with an electron configuration of 4 f 1 5d 0 6s 0 and Pr 3+ with 4 f 2 5d 0 6s 0 are stable depending on the surrounding environment. For example, PrCoO 3 is primarily in a Pr 3+ valence state, but below its spin state transition there is a charge transfer between the cations resulting in a detectable amount of Pr 4+ in the x-ray absorption spectroscopy spectra [57][58][59][60]. Pr can also be found on the B-site in an octahedral environment.…”