Coexisting nanoscale inverse spinel and rock salt crystallographic phases in NiCo_2O_4 epitaxial thin films grown by pulsed laser deposition

摘要

The origin of alternating wavy dark-bright stripe-like contrast in strain contrast transmission electron microscopy images of NiCo_2O_4 (NCO) epitaxial thin films grown by pulsed laser deposition has been investigated. The nanoscale stripe-like pattern is determined to be associated with coexisting rock salt (RS) and inverse spinel crystal phases. The presence of two different phases, not addressed in previous reports, is experimentally confirmed by both electron diffraction and high resolution transmission electron microscopy imaging. First principles based calculations, together with compressive strain present in the films, support the formation of such coexisting crystallographic phases in NCO. Similar microstructural patterns and RS structure are not observed in epitaxial films of two other oxides of the spinel family, namely, NiFe_2O_4 and CoFe_2O_4. A correlation between the coexisting structures and the macroscopic physical properties of NCO is discussed.