Low to High Spin-State Transition Induced by Charge Ordering in AntiferromagneticYBaCo2O5

Abstract

The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering. We report here on structurally related LBaCo 2 O 51x materials, whose structures are derived from perovskites via ordering of the rare earth ͑L͒ and Ba cations into layers along c and removing oxygen exclusively from the L layer [6,7]. This creates an apically connected double layer of corner-sharing CoO 5 pyramids. For x . 0 the extra oxygen ions are incorporated into the L layer of LBaCo 2 O 51x to form disordered octahedra along the c axis. We have synthesized LBaCo 2 O 5.00 and studied the thermal evolution of its structure and properties using synchrotron x-ray [8] (Fig. 1) and the appearance of magnetic superstructure reflections in neutron powder diffraction data. Synchrotron x-ray powder diffraction measurements show that this magnetic transition occurs simultaneously with a tetragonal-to-orthorhombic ͑T -O͒ structural phase transition. Furthermore, as mentioned above, the minority tetragonal phase persists below the T -O transition. The two-phase coexistence was confirmed by ultra-highresolution diffraction measurements using a crystal analyzer. We stress that the observation of two phases in synchrotron x-ray powder diffraction experiments, with an order of magnitude better resolution than available with a standard laboratory x-ray diffractometer, can be traced to minute oxygen-content variations in the order of magnitude around x 0.01 [11]. In the presence of strong lattice coupling these small compositional variations can lead to phase coexistence, which can be observed in high-resolution experiments. This behavior beautifully illustrates how sensitive phase transitions within these systems are to minute variations in the oxygen stoichiometry.At room temperature there is no evidence for any long-range charge ordering. However, upon cooling long-range charge ordering is detected below 200 K, by the appearance of the ͑

Defect equilibrium in PrBaCo2O5 at elevated temperatures

“…The using of the negative enthalpy ΔH 3 D ¼ À 24.4 kJ mol -1 obtained from the defect model in [28] would lead to a strong decrease in the amount of Co 3+ cations with temperature so that their concentration would become negligently small at near room temperatures. Such a conclusion occurs in a strong controversy with the available literature data [1][2][3][4].…”

“…The air synthesized cobaltite PrBaCo 2 O 5+δ was confirmed to be a single phase (Fig. 1) having an orthorhombic structure (space group Pmmm) with the unit cell parameters a ¼3.8981(1), b¼ 7.8069(2) and c ¼7.6345(2) Å characteristic of a double perovskite [1][2][3][4]. A part of the obtained PrBaCo 2 O 5+δ was crashed into a thin powder and used for determination of absolute amount of oxygen in the as-prepared material.…”

“…It must be noted also that the obtained ΔH 3 D is approximately equal, as expected, to the energy gap between t 2g and e g levels in the rare-earth double perovskite cobaltites [31]. The using of the negative enthalpy ΔH 3 D ¼ À 24.4 kJ mol -1 obtained from the defect model in [28] would lead to a strong decrease in the amount of Co 3+ cations with temperature so that their concentration would become negligently small at near room temperatures.…”

“…In difference with Ca or Sr substituted cobaltites where doping cations randomly replace lanthanide cations within the entire solubility range, the larger barium cations tend to ordering so that composition Ln 0.5 Ba 0.5 CoO 3 À δ is characterized by alternation of Ln and Ba cations along c-axis [1][2][3][4]. Consequently, the elementary unit parameters a, b and c for Ln 0.5 Ba 0.5 CoO 3 À δ approximately relate with the elementary unit parameter a p for the parent perovskite-like cobaltite as a p , 2a p and 2a p , respectively, while the obtained oxides are called "double perovskite" cobaltites and their chemical formula is usually represented as LnBaCo 2 O 5+δ .…”

Local Lattice Distortion Caused by Short-Range Charge Ordering in Transition Metal Oxides

“…The inequivalent Co sites with the valences of +2 and +3 are alternately arranged in the unit cell. Although the cubic phase of LiMn 2 O 4 , and the tetragonal and intermediate orthorhombic phases of YBaCo 2 O 5 have the transition metal ions with the half-integer valences, their electrical conductivities are non-metallic, [8,[12][13][14][15][16][17] suggesting the short-range charge ordering. Then we have performed the local structural analyses by using PDFs obtained in the cubic phase of LiMn 2 O 4 and the tetragonal phase of YBaCo 2 O 5 to reveal the local lattice distortion caused by the short-range charge ordering.…”