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

• Published in: Journal of physical chemistry. C 2013-12, Vol.117 (48), p.25704-25713
• Main Authors: Kamali, K, Ravi, C, Ravindran, T. R, Sarguna, R. M, Sairam, T. N, Kaur, Gurpreet
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Deformation and plasticity (including yield, ductility, and superplasticity); Elasticity. Plasticity; Exact sciences and technology; Materials science; Mechanical and acoustical properties of condensed matter; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Mechanical properties of solids; Metals. Metallurgy; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Thermal expansion; thermomechanical effects and density; Thermal properties of condensed matter; Thermal properties of crystalline solids
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp410214y

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 Ag3[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 Ag3[Co(CN)6]. The phase transformation can be attributed to the softening of two low-energy optic modes (A2 and E) and the softening of C 44 shear elastic constant and hence the transverse acoustic mode.