Lithium Intercalation into the Excitonic Insulator Candidate Ta2NiSe5

P A Hyde; J Cen; S J Cassidy; N H Rees; P Holdship; R I Smith; B Zhu; D O Scanlon; S J Clarke

doi:10.1021/acs.inorgchem.3c01510

Lithium Intercalation into the Excitonic Insulator Candidate Ta₂NiSe₅

Inorg Chem. 2023 Jul 31;62(30):12027-12037. doi: 10.1021/acs.inorgchem.3c01510. Epub 2023 Jul 19.

Authors

P A Hyde¹, J Cen^{2

3}, S J Cassidy¹, N H Rees¹, P Holdship⁴, R I Smith⁵, B Zhu^{2

3}, D O Scanlon^{2

3}, S J Clarke¹

Affiliations

¹ Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
² Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
³ Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.
⁴ Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, U.K.
⁵ Rutherford Appleton Laboratory, ISIS Facility, Harwell Campus, Didcot, Oxon OX11 0QX, U.K.

Abstract

A new reduced phase derived from the excitonic insulator candidate Ta₂NiSe₅ has been synthesized via the intercalation of lithium. LiTa₂NiSe₅ crystallizes in the orthorhombic space group Pmnb (no. 62) with lattice parameters a = 3.50247(3) Å, b = 13.4053(4) Å, c = 15.7396(2) Å, and Z = 4, with an increase of the unit cell volume by 5.44(1)% compared with Ta₂NiSe₅. Significant rearrangement of the Ta-Ni-Se layers is observed, in particular a very significant relative displacement of the layers compared to the parent phase, similar to that which occurs under hydrostatic pressure. Neutron powder diffraction experiments and computational analysis confirm that Li occupies a distorted triangular prismatic site formed by Se atoms of adjacent Ta₂NiSe₅ layers with an average Li-Se bond length of 2.724(2) Å. Li-NMR experiments show a single Li environment at ambient temperature. Intercalation suppresses the distortion to monoclinic symmetry that occurs in Ta₂NiSe₅ at 328 K and that is believed to be driven by the formation of an excitonic insulating state. Magnetometry data show that the reduced phase has a smaller net diamagnetic susceptibility than Ta₂NiSe₅ due to the enhancement of the temperature-independent Pauli paramagnetism caused by the increased density of states at the Fermi level evident also from the calculations, consistent with the injection of electrons during intercalation and formation of a metallic phase.