Lithium Intercalation into the Excitonic Insulator Candidate Ta₂NiSe₅

A new reduced phase derived from the excitonic insulatorcandidateTa(2)NiSe(5) has been synthesized via the intercalationof lithium. LiTa2NiSe5 crystallizes in the orthorhombicspace group Pmnb (no. 62) with lattice parameters a = 3.50247(3) & ANGS;, b = 13.4053(4) & ANGS;, c = 15.7396(2) & ANGS;, and Z = 4, withan increase of the unit cell volume by 5.44(1)% compared with Ta2NiSe5. Significant rearrangement of the Ta-Ni-Selayers is observed, in particular a very significant relative displacementof the layers compared to the parent phase, similar to that whichoccurs under hydrostatic pressure. Neutron powder diffraction experimentsand computational analysis confirm that Li occupies a distorted triangularprismatic site formed by Se atoms of adjacent Ta2NiSe5 layers with an average Li-Se bond length of 2.724(2)& ANGS;. Li-NMR experiments show a single Li environment at ambienttemperature. Intercalation suppresses the distortion to monoclinicsymmetry that occurs in Ta2NiSe5 at 328 K andthat is believed to be driven by the formation of an excitonic insulatingstate. Magnetometry data show that the reduced phase has a smallernet diamagnetic susceptibility than Ta2NiSe5 due to the enhancement of the temperature-independent Pauli paramagnetismcaused by the increased density of states at the Fermi level evidentalso from the calculations, consistent with the injection of electronsduring intercalation and formation of a metallic phase. LiTa2NiSe5 is anew reduced metallicphase synthesized from the layered ternary chalcogenide and excitonicinsulator candidate Ta2NiSe5. Intercalationof lithium causes unit cell expansion, significant rearrangement ofthe Ta2NiSe5 layers, and suppression of themonoclinic distortion observed in Ta2NiSe5.Li occupies a single trigonal prism site, and the net diamagnetismis reduced relative to Ta2NiSe5.