Role ofA-site cations in the metal-insulator transition inPr0.5Ca0.5CoO

Abstract

Remarkable structural changes take place during the metal to insulator first-order transition in Pr 0.50 Ca 0.50 CoO 3 : a contraction of the unit-cell volume, the bending of the Co-O-Co bond angle, and a decrease in ͑Pr,Ca͒-O bond distance. However, the Co-O bond length and the volume of the CoO 6 octahedra remain almost unaltered during the transition. In view of these data and the absence of transition in ͑Nd 2/3 La 1/3 ͒ 0.5 Ca 0.5 CoO 3 , 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.

Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites

“…This scenario was originally suggested in order to account for anomalous shortening of some Pr-O bondlengths, whereas the expected shrink of the CoO 6 octahedra due to the stabilization of lower spin states of Co has not been observed [6]. The existence of Pr→CoO 3 electron transfer was supported later theoretically by GGA+U electronic structure calculations exploiting the temperature dependence of the structural experimental data for Pr 0.5 Ca 0.5 CoO 3 [7], and experimentally, in particular by observation of Schottky peak in the low temperature specific heat, related to Zeeman splitting of the ground doublet of Kramers ions Pr 4+ [8].…”

Thermally and field-driven spin-state transitions in (Pr1−yYy)0.7Ca0.3CoO3

“…[3][4][5] Based on the electronic structure calculations and detailed analysis of the structural, electrical, magnetic, and specific heat data, it has been found that the transition is accompanied by a significant charge transfer between cobalt and praseodymium ions. In particular for the system (Pr 1Ày Y y ) 0.7 Ca 0.3 CoO 3 , about onethird of the Pr 3þ ions changes their valency to Pe 4þ during the SST on decreasing temperature.…”

Magnetostructural coupling, magnetic ordering, and cobalt spin reorientation in metallicPr0.5Sr0.5CoO

“…The appearance of the 4-fold tetragonal axis brings on degeneration and two equivalent magnetic easy-axes, which are associated to the two conjugated magnetic domains with spin orientations [110] and [1][2][3][4][5][6][7][8][9][10]. Therefore, the loss of magnetization (negative step) under low fields (H< H cr ) is produced by the presence of conjugated [110] and [1][2][3][4][5][6][7][8][9][10] ferromagnetic domains after the Imma → I4/mcm transition. A schematic view of magnetic ordering and magnetic domains above and below T S1 is shown in Figure 9.…”