Polyoxoarsenotungstate in Oxalis Triangularis Arrangement Favors Slow Magnetic Relaxation for Both Prolate and Oblate Lanthanides

A series of Ln(III)-substituted polyoxometalates (POMs) having molecular formula Cs_xK_15−x[(AsW₉O₃₃)₃Ln₂(H₂O)₇W₄O₉]·yH₂O were synthesized by using dilacunary arsenotungstate [As₂W₁₉O₆₇(H₂O)]¹⁴⁻ as a precursor with Ln(III) ions [Ln = Dy (1), Tb (2), Er (3), Ho (4), Yb (5), Eu (6), x = 6−7, y = 24−40]. The Ln-POM complexes comprise a fusion of three trivacant arsenotungstates (ATs) [α-AsW₉O₃₃]⁹⁻ building units resembling an oxalis triangularis-like arrangement sealing a heterometallic [Ln₂(H₂O)₇(W₄O₉)]¹²⁺ core unit. Detailed magnetic studies disclose that complex 1 shows slow relaxation of magnetization at zero and 2000 Oe external dc fields due to its high axial anisotropy. An energy barrier of 56.48 K is obtained under the 2000 Oe applied dc field, whereas only the QTM process is favorable in the absence of dc field. On the other hand, complex 5 shows field-induced slow relaxation of magnetization with 5 and 6 K phenomenological energy barriers under 1500 and 2000 Oe applied dc fields. Furthermore, ab initio calculations confirm that high axial ground states get stabilized with minimum transverse anisotropy for complex 1 whereas easy plane anisotropy is observed for complex 5. It is also observed that the arrangement of the POM cluster provides intermediate axial symmetry (C_2v) around the metal centers, stabilizing higher m_j states along with high anisotropy for both Dy(III) and Yb(III) ions in complexes 1 and 5. Thus, both Dy(III) (oblate ion) and Yb(III) (prolate ion) Ln-POM clusters display slow magnetic relaxation, despite having different spatial distributions of 4f electronic density. The solid-state photoluminescence spectra of complexes 1−4 and 6 were investigated, which displays the characteristic emission of Ln(III) components based on 4f−4f transitions.