Molecular spin qubits have received significant attention in quantum information science research due to the accessibility of electron spins that can be addressed using microwave frequencies, and the scalability and tunability of molecular systems. Lanthanide polyoxometalates (LnPOMs) are a class of materials with ideal properties for applications in molecular spin qubits with model example in Na₉[Ho(W₅O₁₈)₂].35H₂O (HoW₁₀). Our goal is 1) to improve the performance of established LnPOM systems with respect to their coherence time and 2) to extend this chemistry towards the actinide series as the AnPOM systems have been relatively underexplored compared to the LnPOM. To this end, we have conducted a full investigation into the fundamental structural and vibrational properties of the LnW₁₀ (Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Yb³⁺, and Lu³⁺) series to develop a detailed understanding of how structural changes effect vibrational characteristics, which is particularly relevant for QIS applications as decoherence events can be triggered by both unwanted spin-spin and spin-phonon coupling. From this investigation we have found that the crystal packing of LnW₁₀ structures is the primary driver of distortion of the LnW₁₀POMs away from ideal D_4d coordination symmetry, which manifests as changes in Ln³⁺ crystal fields and impacts the energy and mixing of Ln³⁺ground state wavefunctions. Based on this result, we are investigating manipulation of spin and vibrational manifolds in HoW₁₀complexes by including or fully substituting the Na⁺ charge balancing cations with other alkali metal counter cations (Li⁺, K⁺, Rb⁺, Cs⁺). This modification to the second sphere of the HoW₁₀complexes has resulted in different degrees of distortion away from ideal D_4d symmetry and distinguishable Raman and FIR spectra. Preliminary X-band and high frequency EPR measurements reveal variations in g-values that indicate the changes in structural packing are leading to changes in metal-ion electronic structure. We have also conducted similar investigation on the UW₁₀ complexes in preparation to move into the transuranic series.

Benjamin Stein

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