Single-ion magnet behaviour in highly axial lanthanide mononitrides encapsulated in boron nitride nanotubes: a quantum chemical investigation

Lanthanide-based single-ion magnets (Ln-SIMs) have garnered significant interest for their potential application in designing molecular-level information storage devices. Among various strategies to enhance magnetization blocking barriers in SIMs, synthesizing highly axially symmetric compounds is the most promising approach. In the present work, using state-of-the-art computational tools, we have thoroughly examined the electronic structure, bonding, and magnetic anisotropy in lanthanide mononitrides [LnN](where Ln = Dy(III) and Tb(III)) and their encapsulation in zigzag boron nitride nanotubes (BNNT) of diameters (8,0) and (9,0) to design novel hybrid assemblies. Using periodic density functional theory calculations, we have thoroughly analyzed the structural and energetic perspective towards encapsulation of [LnN] molecules in parallel and perpendicular modes in BNNT(8,0) (8Ln_∥ and 8Ln_⊥) and BNNT(9,0) tubes (9Ln_∥ and 9Ln_⊥). Binding energy calculations suggest that the parallel arrangement of [LnN] is energetically more favorable (>30kJ/mol) than the perpendicular arrangement, with the BNNT(8,0) tube being energetically more preferred over the BNNT(9,0) tube. Non-covalent interaction plots clearly show dominant van der Waals stabilizing interaction in 8Dy_∥ and 8Tb_∥ compared to other assemblies. CASSCF calculations suggest that both the [DyN] and [TbN] show pure Ising type ground state with a giant barrier height of >1800 cm^-1 and strictly no ground state quantum tunneling of magnetization. CASSCF calculations predict that the 8Dy_∥ and 8Tb_∥ assemblies show record high ab initio blockade barrier (U_cal) values of ~1707 and 1015 cm^-1, respectively. Although 9Dy_⊥ is an energetically unfavorable mode, this orientation benefits from the tube’s crystal field, which leads to a U_cal value of ~1939 cm^-1, suggesting that encapsulation could further enhance the U_cal values. Contrarily, the [TbN] molecules show a dramatic increase in the tunnel splitting values upon encapsulation in BNNT tubes, leading to a drastic decrease in U_cal values. Our in-silico strategy offers insights into the magnetic anisotropy of simple [DyN] and [TbN] molecules and possible ways to integrate these molecules into BNNTs to generate hybrid magnetic material for information storage applications.