posted on 2019-12-12, 14:37authored byMarie
S. Norre, Chen Gao, Sourav Dey, Sandeep K. Gupta, Aditya Borah, Ramaswamy Murugavel, Gopalan Rajaraman, Jacob Overgaard
Single-ion magnets based on lanthanide ions in pseudo-D5h symmetry have gained much
attention in recent years as they are reported to possess a large
blocking temperature and a large barrier for magnetization reversal.
Magneto-structural correlations reveal that the axial O–Ln–O
angle is an important parameter to control the barrier, and while
it can be fine-tuned by chemical modification, an alternative would
be to utilize hydrostatic pressure. Herein, we report the crystal
structures and static magnetic properties of two air-stable isostructural
lanthanide SIMs under applied pressures. The complexes exhibit pseudo-D5h symmetry around the Ln(III)-ion
(Ln = Dy or Ho), which coordinates to five equatorial water molecules
and two large neutral phosphonic diamide ligands along the axial direction.
High-pressure single-crystal X-ray diffraction experiments revealed
two phase-transitions and an increasing deviation from D5h-symmetry between ambient pressure
and 3.6 GPa. High-pressure direct-current magnetic measurements of
the Dy(III) compound showed large steps in the hysteresis loops near
zero field, indicative of quantum tunneling of magnetization (QTM).
These steps grow in size with increasing pressure, suggesting that
QTM becomes progressively more active, which correlates well with
the pressure-induced increased overall deviation from pseudo-D5h symmetry and decreasing
axial O–Dy–O angle. A strong temperature dependence
of the step size is seen at 0.3 GPa, which shows that the SMM character
persists even at this pressure. To understand the origin of significant
variation in the tunneling probability upon pressure, we performed
a range of ab initio calculations based on the CASSCF/RASSI-SO/SINGLE_ANISO
method on both Dy and Ho complexes. From the energies and magnetic
anisotropy of the mJ sublevels, we find
a complex variation of the energy barrier with pressure, and using
a constructed geometrical parameter, R, taking into
account changes in both bond angles and distances, we link the magnetic
properties to the first coordination sphere of the molecules.