Spin-Lattice Coupling in the Spin-1 Kagome Compound Na₂Ti₃Cl₈

The coupling between the lattice vibrations or static distortions and the magnetic Hamiltonians (the spin-phonon and spin-lattice couplings) are ubiquitous in ionic transition metal compounds. These effects give rise to phenomena such as spin-splitting of phonon modes, or breaking of the degeneracy of elastic constants below magnetic ordering temperatures. Historically, first principles computational approaches have been extremely successful in predicting and explaining the coupling between the crystal structure, its excitations, and the magnetic properties. In this talk, after giving a brief introduction to the idea of spin-lattice coupling, I am going to discuss a case study from a first principles theory point of view: Na2Ti3Cl8 is a 2 dimensional frustrated antiferromagnet, which hosts spin-1 Ti cations on a Kagome lattice. The crystal structure of Na2Ti3Cl8 undergoes a phase transition at 200 K, below which the system develops a polarization, which is in principle switchable, and the magnetic moments are suppressed. Our combination of first principles density functional theory and exact diagonalization calculations show that this compound not only has a magnetic Hamiltonian with unexpected higher order `ring exchange’ terms, but also that the coincident structural and magnetic phase transition is driven by the strong spin-lattice coupling. I will conclude by a discussion on how the lower-T electronic structure of this compound is better explained from a molecular point of view, rather than the usual approach of well defined atomic moments in a solid.