Phase transition and negative thermal expansion in copper pyrophosphate Cu 2 P 2 O 7 studied by neutron total scattering and the reverse Monte Carlo method

The phase transition in copper pyrophosphate, Cu 2 P 2 O 7 , has been studied using neutron total scattering and the reverse Monte Carlo method. The results show the growth of disorder of the orientations of the PO 4 tetrahedra and of the positions of the copper atoms along the long O–Cu–O linkage in the CuO 6 octahedra distorted by the Jahn–Teller effect. These two types of disorder are coupled, and neither show the preference for distinct sites of partial atomic occupancy in the high-temperature phase. The origin of the negative thermal expansion identified in Cu 2 P 2 O 7 can be understood as arising from the ordering of the copper atoms leading to one Cu–O bond expanding more than the other contracts.