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6 páginas, 5 figuras, 1 tabla.-- PACS number(s): 71.30.+h, 75.25.Dk, 75.30.Wx, 78.70.Dm.-- et al. ; X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is a homogeneous Co3.5-δ state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3++Co3.5→1/2Pr3+2δ+Co3.5-δ, with 2δ ≈ 0.26e-. ; Financial support from MICINN (Spanish government) under Projects No. MAT2006-11080-C02-02 and No. MAT2009-09308, and NANOSELECT under Project No. CSD2007-00041 is acknowledged. The ALICE diffractometer is funded through the BMBF under Contract No. 05KS7PC1. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 226716 ; Peer reviewed

6 páginas, 4 figuras, 1 tabla.-- PACS number(s): 71.30.+h, 61.05.fm, 75.30.Kz, 75.47.De ; Remarkable structural changes take place during the metal to insulator first-order transition in Pr0.50Ca0.50CoO3: a contraction of the unit-cell volume, the bending of the Co-O-Co bond angle, and a decrease in (Pr,Ca)-O bond distance. However, the Co-O bond length and the volume of the CoO6 octahedra remain almost unaltered during the transition. In view of these data and the absence of transition in (Nd2/3La1/3)0.5Ca0.5CoO3, we review the current spin-state transition picture, emphasizing that an active role is played by the A-site Pr cations that could be related with the effect of Pr in 123 superconducting cuprates. ; Financial support from MICINN (Spanish government) under Projects No. MAT2006-11080-C02-02 and No. NANOSELECT CSD2007-00041, Generalitat de Catalunya (Grant No. 2005-GRQ-00509) and FAME European Network of Excellence is acknowledged. ; Peer reviewed

Pr0.5Ca0.5CoO3 exhibits a non-conventional metal-insulator transition (MIT) in which the insulating state is due toa volume contraction (∼2%) and electron transfer from Pr to Co sites. Pr1-xCaxCoO3cobaltites with Ca doping around x ∼ 0.5 (0.45 ≤ x ≤ 0.55) have been investigated. Different ground states and phase transitions are reported in samples at both sides of the half-doped composition. The relevance of the Pr/Ca ratio around half-doping and the amplitude of the changes observed at the MIT are analyzed. Besides the expected variation in the transition temperature, we report an enhanced Pr3+/Pr4+ transformation at the MIT in Pr0.45Y0.05Ca0.50CoO3[0.25(5) e-/Pr ion and T MI = 120 K] with respect to Pr0.5Ca0.5CoO3[0.15(5) e-/ Pr ion and T MI = 75 K]. © 2013 The Korean Physical Society. ; We thank financial support from MICINN (Spanish government) under projects No. MAT2009-09308 and No.CSD2007-00041 (NANOSELECT)and CRG-D1Band the ILL and HZB for granting beamtime. J.H.-M. thanks CSIC for JAEDoc contract, cofinanced by the European Social Fund. ; Peer Reviewed

6 páginas, 4 figuras.-- PACS number(s): 71.30.+h, 78.70.Dm, 75.30.Wx, 75.25.Dk.-- et al. ; X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(±5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomiclike 4f1 states (Pr4+) and f-symmetry states on the O 2p valence band (Pr3+L̲ states) indicative of a strong 4f-2p interaction. ; We acknowledge financial support from MICINN (Spanish government) under Projects No. MAT2009-09308 and Nanoselect Project No. CSD2007-00041. We acknowledge ILL (and the CRG-D1B), and HZB for the provision of beam time. The ALICE diffractometer is funded through the BMBF under Contract No. 05KS7PC1. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 226716. J.H.M. thanks CSIC for JAEdoc contract. ; Peer reviewed

We present a study of the Co spin-state changes in Pr 0.5Ca 0.5CoO 3 across the metal-insulator transition by means of the analysis of two complementary synchrotron x-ray spectroscopic techniques. Recorded experimental and charge transfer multiplet-based calculated Co L 2,3 absorption spectra, in combination with Co Kβ main emission lines, indicate that Co3 + ion spin state evolves from low in the insulating phase to a low:high mixed state in the metallic regime, close to a 1:1 ratio at ambient temperature. A pure high spin state can be discarded, while the occurrence of an intermediate state is also questioned. © 2012 American Physical Society. ; We are thankful for financial support from MICINN (Spanish government) under projects MAT2009-09308, NANOSELECT under Project No. CSD2007-00041, and the ESRF and BESSY for the provision of beamtime. The research leading to these results has received funding from the European Community's Seventh Framework Program (FP/2007-2013) under Grant No. 226716. J.H.-M. also thanks CSIC for the JAEdoc contract, cofinanced by the European Social Fund. ; Peer Reviewed

The low temperature properties of Pr0.50Sr0.50CoO3 are studied in comparison to Pr0.50Ca0.50CoO3, which is known to exhibit a Pr3+/Pr4+ valence shift below TMI∼80 K. The evolution of the Pr-O network strongly differs in the Sr and Ca compounds, but both have in common a prominent and sudden contraction of some Pr-O bonds. The structural phase transition that accompanies the so-called magnetocrystalline anisotropy transition at TS∼120 K in Pr0.50Sr0.50CoO3 brings about a notable contraction of some Pr-O2 bonds. The observations give support to the active participation of Pr-4f electrons at the intriguing magnetostructural transition (TS) in Pr0.50Sr0.50CoO3. In addition, we have confirmed that (in absence of praseodymium) the magnetostructural transition is suppressed in (Nd2/3La1/3)0.50Sr0.50CoO3, a perovskite with the same average cationic size at the A-site than Pr0.50Sr0.50CoO3. ; We thank MINECO (Spanish Government) for financial support under Projects MAT2009-09308, MAT2012-38213-C02-02 and CSD2007-00041 (NANOSELECT), and the ILL for granting beamtime. J. P-P thanks CSIC for JAE‐Pre contract. ; Peer Reviewed