DFT Study of the Magnetic and Structural Properties of the DTN Molecular Crystal

DTN or dichloro-tetrakis-thiourea-nickel (NiCl₂ – [SC(NH₂)₂]₄) is a molecular crystal whose soft lattice structure allows it to more easily deform, allowing stronger magneto-electric coupling (ME) manifestations, compared to typical ME materials. At low temperatures and in moderate magnetic fields DTN exhibits exotic behavior including a quantum phase transition into a Bose-Einstein condensate/XY antiferromagnetic state. Moreover, the presence of disorder introduced by Br doping experimentally shows evidence for the existence of a Bose glass state. This system poses a challenge when trying to accurately model structural and mechanical properties due to its weak intermolecular interactions. Our work attempts to understand the nature of DTN and Br-DTN using density functional theory (DFT). By including the effects of Van der Waals interactions using the DFT-D3 correction we show that we can get accurate lattice constants, bond lengths and bond angles, which allows us to further probe the mechanism responsible for the magneto-electric phenomena present in DTN. We also report our calculation of the exchange coupling constants and zero-field splitting constants responsible for the behavior of DTN at low temperatures.