Negative and positive thermal expansion-like volume changes due to intermetallic charge transfer based on an ionic crystal model of transition-metal oxides

Materials showing negative thermal expansion (NTE) are technologically important and a few transition-metal oxides with the perovskite structure show large NTE-like volume changes due to intermetallic charge transfer. Here we show that the observed volume changes are primarily caused by the size effect of the constituent ions in the oxides. Not only the NTE-like behaviors but also the positive volume change due to the intermetallic charge transfer can be qualitatively reproduced with a simple ionic crystal model by using the SPuDS software program. The results demonstrate that materials showing NTE-like behavior due to intermetallic charge transfer can be strategically designed.