Electronic Ferroelectricity Resulting in a Valence Bond Solid in the Triangular Lattice Organic Mott Insulator κ-(BEDT-TTF)₂Cu₂(CN)₃

A triangular lattice of dimer molecular sites (BEDT-TTF)₂ in a Mott insulator κ-(BEDT-TTF)₂Cu₂(CN)₃ (BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene) can simultaneously show magnetic and ferroelectric properties. Each site carries one electron, with S=1/2 spins interacting antiferromagnetically, while a non-equilibrium probability of the electrons to reside on individual molecules of dimer sites results in ferroelectricity. We use Raman scattering spectroscopy to identify the low temperature state of the system which is a consequence of the interactions of magnetic, charge, and lattice degrees of freedom. We identified electric dipole fluctuations within molecular dimer sites, emerging below 40 K and freezing below 20 K, through line-shape analysis of charge-sensitive molecular vibrations. We demonstrate that the charge dipoles are coupled to the lattice, so that freezing of the fluctuations is observed through anisotropic broadening of lattice phonons related to whole-molecule motion. We argue that this freezing of electric dipoles leads to spins freezing into a disordered valence bond solid state at 6 K, with ferroelectric domain walls carrying orphan spins.