Terahertz Wave Absorption of a Rubidium Manganese-Iron Prussian Blue Phase Transition Material

Rubidium manganese hexacyanidoferrate, Rb_0.97Mn[Fe(CN)₆]_0.99 ⋅ 0.5H₂O, which has a three-dimensional cyanido-bridged Mn−Fe framework encapsulating Rb⁺ ions, shows a terahertz (THz) wave absorption property. This compound undergoes a charge-transfer phase transition between Fe^III-CN-Mn^II [high temperature (HT) phase] and Fe^II-CN-Mn^III [low temperature (LT) phase] near room temperature. The HT phase exhibits a THz wave absorption at 1.06 THz due to the slow vibration of the heavy Rb⁺ ions encapsulated in the three-dimensional framework. By contrast, the LT phase displays a higher resonance frequency at 1.13 THz. This shift to a higher frequency is attributed to the framework shrinkage, which decreases the space available for the trapped Rb⁺ ion. Indeed, the void volume accompanying the phase transition decreases by 16.4 % from 41.4 ų (HT phase) to 34.6 ų (LT phase). Tuning the THz wave absorption frequency is necessary for THz devices such as absorbers, switches, filters, and modulators, especially for THz technology with THz waves in the sub-THz region or near 1 THz.