Fabrication and Characterization of NiO Bicrystals

Abstract

AbstractFabrication of NiO bicrystals having Σ5 (310) and Σ13 (510) coincidence-site tilt boundaries was successfully carried out by a CVT (Chemical Vapor Transport) method. The CVT method was a very advantageous way to grow ultra pure crystals since it preferentially transported NiO from the source pellet to the substrate by reaction with a HCl carrying gas. Single crystal MgO was used as a substrate for epitaxial growth of NiO as the readily available MgO crystals have only a 1% lattice mismatch with NiO. Moreover, MgO is soluble in acids while NiO is not. This permitted removal of the substrate crystal after growth to provide a free-standing NiO crystal. Using two single crystals of MgO with the desired tilt orientation as a substrate, NiO bicrystals were fabricated at growth rates greater than 100 μm/hour at 1,400K using 250 torr of HCI(g) as a carrying agent. The purity of the epitaxial NiO crystals was determined by mass spectrometry and neutron activation analysis. The grain boundary in the bicrystals is exactly perpendicular to the (100) growth surface. Highly-reflective facets along the growth direction suggest high mechanical quality. High-resolution transmission electron microscopy of the Σ13 boundary revealed structure at the atomic scale that provided no evidence for segregated phases.