Structure, phase transition, and tailoring thermal expansion in Cu2-xMnxP2O7

Among the A2P2O7 (A = divalent metal) series compounds, Cu2P2O7 has attracted a lot of attention because of its unique negative thermal expansion properties. In this work, we have synthesized a series of Cu2-xMnxP2O7 (0 ≤ x ≤ 2) compounds by solid state reaction method. The crystal structure of the Cu2-xMnxP2O7 (0 ≤ x ≤ 2) experiences three changes from α-Cu2P2O7 to β-Cu2P2O7 and then to Mn2P2O7 structure. The thermal expansion in Cu2-xMnxP2O7 is gradually regulated from strong negative (αl = −12.5 × 10−6 K−1, 173–373 K) to near zero (αl = 1.17 × 10−6 K−1, 173–273 K), and then to positive with increasing Mn content. The first-principles calculation has revealed that the Mn substitution diminishes structural flexibility and depress the NTE of Cu2-xMnxP2O7 (0 ≤ x ≤ 2). The present work not only achieves controlled thermal expansion, but also deepens the understanding of the correlation between structure and properties of frame-structured negative thermal expansion compounds.