Raman relaxation in Yb(iii) molecular qubits: non-trivial correlations between spin-phonon coupling and molecular structure.

The coordination complexes of Yb(iii) exhibit some of the longest spin coherence times among 4f compounds, making them a promising platform for molecular quantum technologies. While spin-phonon relaxation remains a limiting factor for coherence times even at low temperature, its control through chemical design has the potential to push these spin qubit prototypes beyond current limits. With the aim of providing insights into how to chemically control spin-phonon relaxation, we here present a full ab initio study of spin-phonon dynamics for three Yb(iii) molecules exhibiting minimal chemical differences, yet quantitatively different spin relaxation times. The results show that low-temperature relaxation is governed by Raman processes triggered by a small group of largely delocalized low-energy phonons. The analysis of these contributions highlights that the modulation of spin-phonon coupling by molecular structure modifications beyond the first coordination shell is highly non-trivial in nature and hard to rationalize in simple chemical terms. These findings call for a conceptual step change from the attempt to use simple magneto-structural correlations to interpret the effect of molecular structural modifications on spin-phonon relaxation and present predictive first-principles frameworks as a potential driving force of future chemical design strategies.