Negative thermal expansion in Co2-Mn P2O7

Negative thermal expansion (NTE) is an abnormal phenomenon that only exists in few materials but is of importance in tailoring the coefficients of thermal expansion (CTE) in some advanced devices and instruments. Herein, we find that a large volume shrinkage occurs in Co2P2O7 crossing its structural transformation from α (P21/C) to β phase (C2/m) around 584 K which can be tuned even to below room temperature (RT) by the substitution of Mn2+ for Co2+. It is also found that the sintering temperature and the content of Mn2+ are crucial factors in controlling the proportions of α- and β-phases crystallized at RT and the rates of phase transition. A zero thermal expansion within a wide temperature window (173–573 K) is realized for Co1.4Mn0.6P2O7 sintered at 1173 K. The NTE arises from the lattice volume difference between the low temperature α-phase and high temperature β-phase while a higher content of Mn2+ and lower sintering temperature favorites for the coexistence of both phases at RT and lowering the phase transition rate. This work not only adds new members for the NTE family but also presents a method for regulating the onset, width, and rate of NTE by the interplay of substitution content and sintering temperature.