Phase diagram and isotope effect in(Pr1−yEuy)0.7

Abstract

Phase diagram and isotope effect in (PrWe present the study of magnetization, thermal expansion, specific heat, resistivity, and a.c. susceptibility of (Pr1−yEuy)0.7Ca0.3CoO3 cobaltites. The measurements were performed on ceramic samples with y = 0.12 − 0.26 and y = 1. Based on these results, we construct the phase diagram, including magnetic and spin-state transitions. The transition from the low-to intermediate-spin state is observed for the samples with y > 0.18, whereas for a lower Eu-doping level, there are no spinstate transitions, but a crossover between the ferromagnetic and paramagnetic states occurs. The effect of oxygen isotope substitution along with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope substitution (16 O by 18 O) is found to shift both the magnetic and spin-state phase boundaries to lower Eu concentrations. The isotope effect on the spin-state transition temperature (y > 0.18) is rather strong, but it is much weaker for the transition to a ferromagnetic state (y < 0.18). The ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by the Co 4+ ions, which favor the formation of the intermediate-spin state of neighboring Co 3+ ions.

Electronic phase separation: Recent progress in the old problem

“…(83) and the energy of the "coated" magnetic polaron is given by Eqs. (81) and (82), where θ n = π/2 and ϕ n are the solutions of Eqs. (87).…”

“…Usually the electron polaron effect and polaron effect of the phonon origin play in concert leading to the multiplicative effect for the bandwidth narrowing. In this connection, it is important to mention the giant oxygen isotope effects in manganites [77,78,79] (see also [80] and references therein) and in cobaltites [81], which are closely related both to the polaron effect and phase separation. Moreover, the oxygen isotope exchange can even induce the metal-insulator transition in manganites [82,83,84].…”

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

“…Recently, another type of SST was revealed in orthorhombic perovskites of compositions (Pr 1−y Ln y ) 1−x Ca x CoO 3 , where Ln = Sm, Eu, Gd, Tb or Y , while x and y are approximately within the ranges 0.2-0.5 and 0-0.3, respectively [3][4][5][6][7][8][9]. Contrary to the crossover-like behavior observed in LaCoO 3 , these compounds exhibit a sharp transition (at a temperature hereafter referred to as T*) which can be regarded as being first-order in that it corresponds to abrupt jumps in magnetization, unit-cell volume and entropy [3,4,10].…”

Valence change of praseodymium in Pr0.5Ca0.5CoO3investigated by

“…With Ln = Nd or smaller lanthanides in Ln 1−x Ca x CoO 3 (x 0.5) the conductivity decreases, but there are no signs of abrupt spin-state changes. [17][18][19][20] suggesting only smooth thermally assisted variations of the Co 3+ spin state. In addition, a first-order MIT is only found as a function of temperature or applied pressure in (Pr 1−y Ln y ) 1−x Ca x CoO 3 compositions having Pr atoms.…”