The phase transition in copper pyrophosphate, Cu2P2O7: Insights and implications for the interpretation of negative thermal expansion

Abstract

The origin of negative thermal expansion (NTE) in ${\text{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$${\text{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ has been examined using diffraction data at ambient and raised pressure. From an analysis of spontaneous strains and order parameters associated with the structural phase transition, we show that the NTE can be explained wholly on the basis of the structural distortions that accompany the phase transition. We demonstrate that the NTE diverges at the phase transition, because the phase transition shows tricritical behavior. The unusual aspect of the phase transition is that it gives rise to an expansion on cooling, whereas normally, a structural phase transition leads to an overall volume reduction. This behavior, which we identify as a uniaxial spontaneous strain, can be understood in terms of ordering of pairs of long Cu–O bonds associated with buckling of the atomic layers in the crystal structure. We demonstrate that the NTE is enhanced under pressure.