Antiphase inversion domains in lithium cobaltite thin films deposited on single-crystal sapphire substrates

Abstract

Antiphase inversion domains in LiCoO2 thin films prepared by pulsed laser deposition on sapphire single-crystal substrates are analyzed using a combination of (scanning) transmission electron microscopy and first-principles calculations. Domains form epitaxially on the substrates with orientation relationships of [112¯0]LiCoO2(0001)LiCoO2//[11¯00]α-Al2O3(0001)α-Al2O3 and [1¯1¯20]LiCoO2(0001)LiCoO2//[11¯00]α-Al2O3(0001)α-Al2O3. In addition, substrate/film interfaces with the above orientation relationships always have the same stacking sequence of Al–O–Co–O–Li–O. This is confirmed to be the most energetically stable stacking arrangement according to first-principles calculations. Individual domains form as a result of steps one (0001) O–Al–O spacing in height on the otherwise flat substrate surface. Because the orientation of adjacent (0001) AlO6 octahedra in Al2O3 are rotated by 180°, while LiO6 and CoO6 octahedra in LiCoO2 are all aligned in the same direction, substrate steps produce LiCoO2 domains rotated 180° relative to their neighbors. The similar size of oxygen octahedra in the two materials also means that the step height is close to the layer spacing in LiCoO2, so that (0001) Li and Co layers of adjacent domains are shifted by one layer relative to each other at each domain boundary, aligning Li layers with Co layers across the boundary. The combination of these two effects generates antiphase inversion domains. The domain boundaries effectively sever Li-ion diffusion pathways in the (0001) planes between domains and thus are expected to have a detrimental effect on Li-ion conductivity.