Characteristic Phonon Spectrum of Negative Thermal Expansion Materials with Framework Structure through Calorimetric Study of Sc2M3O12(M = W and Mo)

To study the relation between the structural characteristics and the phonon property in the negative thermal expansion (NTE) compounds, the heat capacities of Sc2W3O12 and Sc2Mo3O12 were measured. Spectrum analysis of heat capacity provided their effective phonon densities of states (DOS). The DOS of Sc2W3O12 shows three features; low-energy phonon modes with negative mode-Grüneisen parameter (γi) around 5 meV, high-energy phonon modes, and separation of phonon DOS into two regions with a wide gap. The relative contribution of γiCi, where Ci is heat capacity of each vibrational mode i, reveals that the low-energy phonon modes with negative γi cause the NTE and that the latter two features are necessary to maintain the NTE in a wide temperature range. Sc2Mo3O12 has the low-energy mode with the negative γi. This fact indicates that Sc2Mo3O12 potentially has the NTE property even in its low-temperature phase showing positive thermal expansion. A comparison of the phonon DOS with other oxides shows that the phonon features are common in the NTE oxides and related to their common chemical and structural characteristics, "strong bond" and "framework structure". This finding gives us an important guide to search for new actual and/or potential NTE compounds.