Magnetic ground state of NiCl2/Br2 monolayers encapsulated in UiO-67 MOF, effect of partially filled pores: Ab-initio and Monte Carlo theoretical study

Ab initio calculations are used to study the properties of two dimensional (2D) Transition Metal Halides (TMH, TM=Ni; H = Cl, Br) encapsulated inside a derivative of UiO-67 Metal-Organic Frameworks (MOF). Two monolayers of TMHs (NiCl2 and NiBr2) were isolated from the experimental Crystallographic Information File (CIF) and used in this study. We found that the structure of both monolayers within the MOF (TMH@MOF) have very similar geometries compared to their perfect 2D sheets hypothetically derived from their bulk structures (TMH@bulk) with small distortions in the Metal-Metal (M-M) and Metal-Halide (M-H) bonds. NiCl2 and NiBr2 sheets have a Ferromagnetic (FM) ground state with large intrinsic magnetic moments localized at each metal site. The predicted FM ground state is also confirmed in the magnetic exchange coupling constant (J) calculations between M centers which are found all positive, but non isotropic within each sheet. Weaker interactions with J approximate to 0.2 meV are found among NN centers in the outer ring compared to J approximate to 0.85 meV in the inner rings of both monolayers. The calculated Js are inserted in a Monte Carlo (MC) code to afford Curie temperatures of 27 K and 22 K for NiCl2/Br2 monolayers respectively. Finally, the experimentally collected structures at low metal loading were found to have distinct magnetic properties compared to the completely filled sheets.