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

Abstract Rubidium manganese hexacyanidoferrate, Rb 0.97 Mn[Fe(CN) 6 ] 0.99 ⋅ 0.5H 2 O, 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 Å 3 (HT phase) to 34.6 Å 3 (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.