Composition evolution of crystal structure and negative thermal expansion in pyro-vanadate–phosphate Cu1.8Zn0.2V2–<i>x</i>P<i>x</i>O7

We constructed a composition–temperature phase diagram for Cu1.8Zn0.2V2–xPxO7 based on structural analysis conducted using synchrotron radiation over the entire compositional range and a wide temperature (110–760 K) range. As x increases, the structure changes from the β-Cu2V2O7 structure (C2/c) to the α-Cu2P2O7 structure (C2/c), and then to the β-Cu2P2O7 structure (C2/m). Accordingly, the crystallographic negative thermal expansion (NTE) weakens, especially in the β-Cu2P2O7 structure, but it never disappears completely. We also discovered that for x = 0.75–1.0, a phase transition occurs from the α-Cu2P2O7 structure to the β-Cu2P2O7 structure with increasing temperature, accompanied by large volume contraction of at most 1.5%. The anisotropic thermal deformation of the unit cell, which indicates the effectiveness of the microstructural effects enhancing the NTE, also weakens as x increases. Elucidation of NTE requires discussion beyond the crystal structure alone, such as local structure related to the soft linkage between polyhedra.