Linear Compressibility and Thermal Expansion of KMn[Ag(CN)₂]₃ Studied by Raman Spectroscopy and First-Principles Calculations

We have investigated the linear compressibility and thermal expansion properties of KMn[Ag(CN) 2 ] 3 using Raman spectroscopy and DFT calculations. Phonon frequencies and mode assignments from polarized Raman measurements and DFT calculations agree with each other satisfactorily. Computed linear compressibilities and thermal expansion coefficients corroborate the reported measured values. Pressure variation of mean partial phonon frequencies of KMn[Ag(CN) 2 ] 3 shows that large amplitude anharmonic displacements of Ag atoms can occur with minimum enthalpy cost even with increasing pressure. This means that Ag layer can be squeezed relatively easily, which manifests as PLC in the basal plane and coupled NLC along trigonal axis due to the rigid Mn-NC-Ag-CN-Mn chain along ⟨101⟩ crystal direction. Elastic constants of KMn[Ag(CN) 2 ] 3 indicate that the crystal is highly anisotropic and becomes unstable with increasing pressure. Directional Gruneisen parameters of KMn[Ag(CN) 2 ] 3 are found to be highly anisotropic. These properties show that NLC/NTE and PLC/PTE in KMn[Ag(CN) 2 ] 3 are driven by elastic and Gruneisen anisotropies combined with anharmonic lattice vibrations of Ag atoms. Partial phonon frequencies of KMn[Ag(CN) 2 ] 3 are found to be higher than those of Ag 3 [Co(CN) 6 ]. The partial frequency of K atom increases rapidly with pressure and becomes comparable to that of Ag around 2 GPa. This shows that K inclusion stiffens the lattice and changes the dynamics, causing the pressure-induced phase transformation of KMn[Ag(CN) 2 ] 3 to occur at a higher pressure rather than at 0.2 GPa as in Ag 3 [Co(CN) 6 ]. The phase transformation can be attributed to the softening of two low-energy optic modes (A 2 and E) and the softening of C 44 shear elastic constant and hence the transverse acoustic mode.