Anharmonic properties of Prussian blue analogues MIIPtIV(CN)6 (MII = Cd, Fe, Zn)

Abstract

This work investigates the anharmonic properties of M^IIPt^IV(CN)₆ (M^II = Cd, Fe, Zn) based on three-phonon interactions. The results demonstrate that FePt(CN)₆ exhibits higher lattice thermal conductivity (LTC) with greater tunability. In all three crystals, the scattering rate decreases with increasing boundary mean free path (MFP), and within each crystal, low-frequency phonons exhibit higher scattering rates than high-frequency ones. The phonon lifetime decreases with rising temperature, with an anomalously long lifetime observed around 67 THz. It is observed that all three crystals exhibit notably negative mode-Grüneisen parameters in the low-frequency region, highlighting the dominant contribution of low-frequency phonons to negative thermal expansion (NTE). Furthermore, the recombination process prevails in the low-frequency region of the three crystals, which can be seen from the joint density of states (JDOS) plots. In the weighted joint density of states (w-JDOS) plots, CdPt(CN)₆ and ZnPt(CN)₆ exhibit an alternating pattern of recombination and decay processes at low frequencies, and FePt(CN)₆ exhibits the fewest scattering channels. The phonon branches of FePt(CN)₆ exhibit pronounced dispersion, which correlates with its higher LTC.