High Temperature Crystal Chemistry and Oxygen Permeation Properties of the Mixed Ionic–Electronic Conductors LnBaCo[sub 2]O[sub 5+δ] (Ln=Lanthanide)

Abstract

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo2normalO5+δ perovskite oxides [lanthanide (Ln)=Pr , Nd, and Sm] have been investigated in comparison with the cation-disordered La0.5Ba0.5CoO3−δ perovskite. The LnBaCo2normalO5+δ ( Ln=Pr , Nd, and Sm) oxides exhibit a metal-insulator transition at T<200°C , as evidenced by total conductivity measurements and high temperature X-ray diffraction data and an oxygen vacancy order–disorder transition at T350°C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln=La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln=Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L≥1.1mm .

Defect equilibrium in PrBaCo2O5 at elevated temperatures

“…When heated in He the cobaltite acquires tetragonal structure (S.G. P4/mmm) [25] with δ¼ 0.48 at 503 1С. Similar structural transition near 500 1С was observed also in other rare-earth cobaltites LnBaCo 2 O 5+δ , where Ln¼ Nd, Sm, Gd [26,27]. Therefore, the isothermal dependencies of oxygen content (5+δ) vs. pO 2 in Fig.…”

“…2 (pO 2 -Т-δ diagram) pertain to tetragonal phase modification of PrBaCo 2 O 5+δ . The smooth shape of the isotherms gives additional evidence to the absence of phase transitions in PrBaCo 2 O 5+δ within the studied range of oxygen pressure and temperature values; the stability of tetragonal structure at elevated temperatures is due to randomization of oxygen ions O 2-and vacancies V O over O3 positions [25][26][27]. Simple consideration of charge balance suggests coexistence of Co 2+ and Co 3+ cations mostly in PrBaCo 2 O 5+δ at δo0.5.…”

Crystal chemistry and properties of mixed ionic-electronic conductors

“…The PrBaCo 2 O 5.48 sample has an orthorhombic structure with the oxide-ion vacancy ordering at room temperature, but loses that ordering at T>780 K as evidenced by a transition from orthorhombic to tetragonal symmetry measured with high-temperature neutron diffraction. This thermally activated order-disorder transition has also been observed for the Ln = Nd, Sm, and Gd samples in the LnBaCo 2 O 5+δ [53,54] system. For example, the high-temperature X-ray diffraction (XRD) patterns of the NdBaCo 2 O 5+δ system in Fig.…”

Synthesis, crystal structure and properties of SmBaCo2−xFexO5+δ

“…An unique set of transport, thermomechanical and interfacial properties of perovskite-like LnBaCo 2 O 5+δ (Ln¼Pr-Ho, Y) makes it possible to use these materials for fuel cell electrodes and oxygenpermeable membranes [1][2][3][4] and attracts significant attention during last years [5][6][7][8][9]. Due to the large difference between the ionic radii of Ln 3+ and Ba 2+ , these cations are located in separate layers of the LnBaCo 2 O 5+δ structure, leading to doubling of c-parameter in the resultant tetragonal unit cell (a p  a p  2a p ).…”