Pressure-induced suppression of charge order and nanosecond valence dynamics in Fe₂OBO₃

Valence order and fluctuations in the mixed-valence warwickite Fe ₂OBO₃ have been explored by 57Fe Mössbauer effect spectroscopy at pressures up to 30 GPa in diamond anvil cell experiments. At room temperature a drastic disruption of charge order is evident at ∼11 GPa. There is coexistence of charge order and a progressively increasing abundance of fluctuating valence states in the range extending to ∼16 GPa. At P16 GPa only signatures of electron exchange relaxation, Fe2⁺ Fe3 ⁺, where represents the resonating mobile carrier, are discerned. Spectral signatures indicate that electron hopping is on a timescale of ∼50 ns, that is, in a time window to which the nuclear resonance technique is particularly sensitive. Low-temperature quenching (∼110 K) at these high pressures (i) is not sufficient to inhibit electron exchange for charge order to reemerge and (ii) reveals that magnetic ordering typical of the charge-ordered phase at low pressure is completely altered to entail new spin dynamics. This evidences the strong interplay between charge order and magnetism and establishes P∼16 GPa as a new electronic phase transition boundary for this system. Nanosecond valence fluctuation signatures persist upon further pressurization to ∼30 GPa at 300 K, suggestive of continued confinement of the mobile carrier to the Fe2⁺ Fe3⁺ pair at these extremes.