Study on Soft Phonons and Mode Grüneisen Parameters in a Framework Material H3[Co(CN)6]

Last several years, the author has been involved in studies of phonons and their structure-properties correlation on several advanced functional materials that included not only flexible framework materials, Te-based oxides, and perovskites but also recently established low dimensional 2D materials, namely graphene, TiS3 nanosheet and nanofiber, VSe2 nanosheet, SnO2 nanoparticles. In this book chapter, the phonons and mode Gruneisen parameters of a flexible framework compound H3[Co(CN)6] is presented in an elaborate manner. H3[Co(CN)6] belongs to a class of flexible framework structured cyanides that exhibit negative thermal expansion  along c-axis. In-situ high-pressure Raman spectroscopic studies on H3[Co(CN)6] in a diamond-anvil cell were carried out under hydrostatic pressure from ambient to 11 GPa. From the evolution of Raman spectra, splitting of bands and discontinuous changes in the pressure dependence of band frequencies, a phase transition from trigonal-monoclinic is identified at 2.3 GPa. In contrast to other bands, a lattice band at 140 cm-1 and the Co-CN deformation band at 348 cm-1 were soften with increase in pressure. Mode Gruneisen parameters (yi) were obtained for the ambient trigonal phase using the pressure dependencies of Raman mode frequencies (d\(\Omega\)/dP). Absence of Raman band with flat spectral feature was found as the compound amorphized around 11 GPa.