When local deformations trigger lattice instability: Flow diagram investigations for photoinduced and quenched metastable states in a Prussian blue analog

Abstract

The structural aspects of the metastable states for K${}_{0.32}$Co[Fe(CN)${}_{6}$]${}_{0.76}\ifmmode\cdot\else\textperiodcentered\fi{}$3H${}_{2}$O have been investigated by synchrotron x-ray powder diffraction. The title compound exhibits nonequilibrium high spin (HS) states of Co${}^{\mathrm{II}}$($S=3/2$)-Fe${}^{\mathrm{III}}$($S=1/2$) configuration, induced by rapid cooling or photoexcitation from the low-temperature (LT) phase. By introducing a new local order parameter of tilting angle between cyanide-bridged Fe and Co-based octahedra, we discovered the existence of a precursor phenomenon triggering the collective instability during the thermal relaxation of the photoexcited (PX) state. Moreover, we introduced a methodology, based on the flow diagram studies, which allowed us to clearly distinguish the obtained metastable states through their strength of spin-lattice coupling, leading to various pathways in the phase space during the combined electroelastic relaxation process.