Anomalies in thermal conductivity and phonon mean free path in alkali and alkaline earth metal silicate melts: Quasi-classical and classical molecular dynamics study

A comparison between the thermal conductivities (κ) determined using quasi-classical molecular dynamics and classical molecular dynamics for 33.3Na₂O–66.7SiO₂ (mol%) shows that we can neglect the quantum effect when the temperature exceeds 1100 K. At 1200 K, we identified a linear relationship between κ and sound velocity(v) in R₂O–SiO₂ (R: Li, Na, K), despite that the density (ρ), specific heat capacity at constant pressure (C_p), and mean free path (MFP, l) depend on the composition and contribute to thermal conductivity through κ = 1/3ρC_pvl. Furthermore, for the R'O–SiO₂(R’: Ca, Sr) system at 1200 K, κ as a function of v does not conform to the line of R₂O–SiO₂, which is attributed to the shorter MFP (l) of R'O–SiO₂ than that of R₂O–SiO₂. This is counterintuitive because the O–R’–O bond would preferably act as a bridge between the two non-bridging oxygens for phonon transport.