Potassium- and Lithium-Ammonia Intercalation into Excitonic Insulator Candidate Ta₂NiSe₅

Two new reduced phases derived from the topical excitonic insulator candidate Ta₂NiSe₅ have been synthesized via the intercalation of lithium and potassium from solutions of the metals in liquid ammonia. Li(NH₃)Ta₂NiSe₅ and KTa₂NiSe₅ both crystallize in orthorhombic space group Pmnb with the following lattice parameters: a = 3.5175(1) Å, b = 18.7828(7) Å, and c = 15.7520(3) Å and a = 3.50247(3) Å, b = 13.4053(4) Å, and c = 15.7396(2) Å, respectively. They have increased unit cell volumes of 48% and 31%, respectively, relative to that of Ta₂NiSe₅. Significant rearrangement of the transition metal selenide layers is observed in both intercalates compared to the parent phase. In Li(NH₃)Ta₂NiSe₅, neutron diffraction experiments confirm the location of the light Li, N, and H atoms, and solid-state nuclear magnetic resonance (NMR) experiments show that H, N, and Li each occupy a single environment at ambient temperature on the NMR time scale. Magnetometry data show that both intercalates have increased magnetic susceptibilities relative to that of Ta₂NiSe₅, consistent with the injection of electrons during intercalation and an enhancement of the Pauli paramagnetism.