Topochemical Synthesis of Superconducting Li_1-xNbO₂ Crystals and Thin Films

We are investigating the synthesis of superconducting infinite layer lithium niobate. Our Li_1-xNbO₂ samples are synthesized through the topochemical CaH₂ reaction of single crystals and epitaxial thin films of the parent compound LiNbO₃. This approach yields interconnected 2D sheets of NbO₂, where a Li_1-x content yields a Nb^(3+x)+ valence, with an electron configuration of [Kr] 4d_2-x. Reduction of the parent phase LiNbO₃ requires processing at 650 C, and longer duration, on the order of tens of days. Our single crystals reduced for 20 days achieve a superconducting transition with an onset at 14.2 K.

To synthesize our thin films, the parent compound LiNbO₃ is deposited epitaxially on sapphire substrates. Then the film is reduced through the topochemical CaH₂ reaction to achieve Li_1-xNbO₂ thin films. Our thin films, grown to approximately 50 nm, that are reduced for 24 hours show the transition at 5 K. Time of flight secondary ion mass spectrometry reveals a complex microstructure including lithium gradients and the presence of lithium hydrides near the surface. We will discuss the composition-structure-superconducting properties relationship of the materials characterized through various means.