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

Abstract

Lanthanide-based single-ion magnets (Ln-SIMs) have garnered significant interest for their potential application in molecular-level information storage devices. Among various strategies to enhance the magnetization blocking barrier 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 of lanthanide mononitrides [LnN] (where Ln = Dy(III) and Tb(III)) and their encapsulation in zigzag boron nitride nanotubes (BNNTs) with diameters of (8,0) and (9,0) to explore novel hybrid assemblies. Using periodic density functional theory calculations and energy decomposition analysis, we have thoroughly analyzed the structural and energetic perspectives towards encapsulation of [LnN] molecules in parallel and perpendicular modes in BNNT(8,0) (8Ln_|| and 8Ln_⊥) and BNNT(9,0) (9Ln_|| and 9Ln_⊥) tubes. Binding energy calculations suggest that the parallel arrangement of [LnN] is energetically more favourable (>30 kJ mol⁻¹) than the perpendicular arrangement, with the BNNT(8,0) tube being energetically more preferred over the BNNT(9,0) tube for encapsulation. Non-covalent interaction plots clearly show dominant van der Waals interactions in 8Dy_||/8Tb_||, stabilizing it compared to other assemblies. CASSCF calculations suggest that both [TbN] and [DyN] show a pure Ising-type ground state with a giant barrier height of >1800 cm⁻¹ and strictly no ground-state quantum tunnelling 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 unfavourable 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 materials for information storage applications.