Sub-liquidus co-crystallization in the Ln 2O 3–BaO–CoO system: growth of large LnBaCo 2O 5+x (Ln=Eu, Gd, Tb, Dy) single crystals

Abstract

An original approach to flux growth has been developed to provide for the first time large and high-quality single crystals of oxygen deficient LnBaCo 2O 5+ δ (Ln=Eu, Gd, Tb, Dy, Tb 0.9Dy 0.1). The data from DTA studies of both the BaO–CoO binary eutectic and temperature of crystal nucleation as well as analysis of more than 100 runs of sub-liquidus co-crystallization allow one to infer the tentative ( T–x) phase diagram of a quasi-binary cut in the Ln 2O 3–BaO–CoO system with the estimated primary crystallization field for the LnBaCo 2O 5+ δ (Ln-112) phase. As a result, in successful growth runs several crystals of a parallelepiped shape and up to 120 mm 3 in volume were grown. The dependence of the crystal shape on the rare earth ion is discussed. For most flux melts close to the ternary eutectic compositions of the corresponding quasi-binary cuts, co-crystallization of the layered cobaltite phase and a new one of LnBaCo 4O 7 (Ln-114) was found in a wide range of temperatures. Data on the twinned crystal structure of the optimally oxidized ( δ = 0.5 ) orthorhombic Ln-112 crystals and crystal characterization by chemical composition, magnetic susceptibility and resonance photoemission spectroscopy are discussed.