Spectroscopic investigation of pressure-induced phase transitions in TCNQ complex salts

In most of the TCNQ complex salts, conduction electrons are localized on specific TCNQ sites, so that these salts show nonmetalic behavior. The caesium salt, Cs2(TCNQ)3, is one of the 2:3 complex salts. In the crystal, TCNQ molecules form trimeric units, which consist of two TCNQ radical anion sandwiching a neutral TCNQ along the column. The rubidium salt, Rb2(TCNQ)3, also has a similar crystal structure to Cs2(TCNQ)3. We measured infrared absorption (IR) and Raman spectra for these salts under high pressure by using a diamond anvil cell. In the case of IR spectra, Cs2(TCNQ)3 showed a spectral change probably due to a pressure-induced phase transition. Similar feature was not clearly observed in the Rb2(TCNQ)3. On the other hand, the Raman spectra, Cs2(TCNQ)3 showed two phase transition at 2.5 and 4.1 GPa in the compression stage. The change from localization phase to delocalization phase occurred at latter transition with large hysteresis. Similar phase transition occurred at 3.2 GPa in the Rb2(TCNQ)3. The reason for the difference in transition pressure is that the ion radius of Rb+ is smaller than that of Cs+, because a small ion radius of the counter ion probably favors the charge localization-delocalization transition of the TCNQ column.