CRYSTAL GROWTH AND CHARACTERIZATION OF QUASI - TWO-DIMENSIONAL LANTHANIDENICKELATES
Abstract
Growth of large single crystals of quasi-two-dimensional Ln(,2)NiO(,4) (Ln = La, Pr, Nd) has been accomplished using the radio frequency technique of skull melting under controlled oxygen partial pressures. No evidence of intergrowth of other phases such as Ln(,3)Ni(,2)O(,7) or higher homologs was observed. Determination of Ni('3+) content indicates that the crystals are cation deficient. Subsolidus annealing at fixed oxygen fugacities permitted control of the extent of deviations from ideal stoichiometry. The influence of nonstoichiometry on the structures of Ln(,2)NiO(,4) compounds was investigated using X-ray powder and electron diffraction. Each of the Ln(,2)NiO(,4) compounds was found to have lower symmetry than the ideal tetragonal K(,2)NiF(,4) type and to have lattice parameters which are strongly stoichiometry dependent. Nearly stoichiometric La(,2)NiO(,4) was characterized as orthorhombic, space group Cmca, with a = 5.520(ANGSTROM), b = 5.456(ANGSTROM), and c = 12.55(ANGSTROM). Evidence for domain structures with twin and antiphase relationships was obtained. Pr(,2)NiO(,4) and Nd(,2)NiO(,4) were found to be monoclinically distorted, and to have domain structures similar to those of La(,2)NiO(,4). Pseudo-tetragonal c/a ratios were shown to decrease with increasing deviations from ideal stoichiometry as well as with decreasing lanthanide ion radius. Faraday magnetic susceptibility experiments were performed as a function of temperature, field strength, and nonstoichiometry. Evidence for long range two-dimensional antiferromagnetic order was obtained for nearly stoichiometric samples of La(,2)NiO(,4) are Pr(,2)NiO(,4) (T(,N) = 204K and 88K, respectively). Three-dimensional order sets in below 100K for La(,2)NiO(,4), which may result from interplanar exchange interactions similar to those found in K(,2)NiF(,4). Unannealed samples of La(,2)NiO(,4), Pr(,2)NiO(,4) and Nd(,2)NiO(,4) show evidence of weak ferromagnetic interactions within the basal plane at low temperaures. A transition from semiconducting to metallic behavior was observed when current flow was restricted to the basal plane, whereas insulating behavior was observed at all temperatures when current flow was along the orthogonal c direction.
Degree
Ph.D.
Subject Area
Chemistry
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