Abstract
LaNiO3−δ single crystals have been obtained via high pressure floating zone growth under 149 bar of oxygen pressure. We find a radial gradient in the magnetic properties of specimens extracted from the as-grown boule, which we correlate with the appearance of ordered oxygen vacancy structures. This radial oxygen inhomogeneity has been characterized systematically using a combination of magnetization and X-ray scattering measurements. We establish the presence of rhombohedral ( R 3 ¯ c ), oxygen stoichiometric specimens at the periphery of the boule and the presence of a dilute concentration of ordered oxygen-deficient orthorhombic La2Ni2O5 in the center. Furthermore, we demonstrate that the as-grown, oxygen-deficient central regions of the crystal can be annealed under high oxygen pressure, without loss of crystallinity, into fully oxygenated LaNiO3, recovering magnetic properties that are characteristic of stoichiometric specimens from the exterior region of the crystal. Thus, single crystals of LaNiO3−δ possess oxygen content that can be reversibly modified under oxidizing and reducing conditions.
Highlights
The subject of the insulator-metal transition in rare-earth perovskite nickelates and its relationship to charge order, antiferromagnetic order, crystal structure, and chemical pressure has been a focal point of research in strongly correlated oxides for decades [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
This pO2 is near the maximum of previously reported growths, which have been carried out over a broad range in pO2, from 30 bar up to 150 bar
The oxygen inhomogeneity within a cross-sectional slice of the crystal was characterized by its effects on χ, Cp, and the crystal structure
Summary
The subject of the insulator-metal transition in rare-earth perovskite nickelates and its relationship to charge order, antiferromagnetic order, crystal structure, and chemical pressure has been a focal point of research in strongly correlated oxides for decades [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The Heidelberg group likewise concluded that long-range AFM order is not intrinsic to stoichiometric LaNiO3 by examining various pieces of crystals along the axis of crystal growth, observing a correlation between oxygen non-stoichiometry and AFM signatures in the bulk properties [22]. The AFM behavior can be reversibly cycled by consecutive annealing treatments under reducing/oxidizing atmospheres These findings lead to the conclusions that: (1) oxygen stoichiometry is inhomogeneous in both axial and radial growth directions; and (2) this inhomogeneity has a profound influence on the magnetic behavior observed in as-grown specimens and its interpretation
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