Tailoring Thermal Expansion of (LiFe)_0.5Cu_2-P₂O₇ via Codoping LiFe Diatoms in Cu₂P₂O₇ Oxide

Tailoring the thermal expansion coefficient of negative thermal expansion (NTE) materials to achieve near-zero thermal expansion has attracted great attention recently. Here, LiFe diatoms are adopted to substitute Cu in Cu₂P₂O₇ oxide to design Li-O-P and Fe-O-P linkages, with the stronger bond strength of Li-O and Fe-O compared to Cu-O and hence lowering the bond strength of P-O. With increasing the diatomic LiFe in (LiFe)_0.5Cu_2-P₂O₇, new Raman bands corresponding to LiFeP₂O₇ appear and the NTE coefficient decreases gradually to near-zero thermal expansion at x = 1 (α_v = -0.90 × 10^-6 °C^-1, -100 to 55 °C). Comparing (LiFe)_0.5CuP₂O₇ with Cu₂P₂O₇ and LiFeP₂O₇, the average bond length of P-O increases while the bond angle of P-O-P decreases, and this is verified by some weakened vibrational energies of terminal PO₃ and P-O-P, resulting in the obvious red shift of Raman bands. Ceramic (LiFe)_0.5CuP₂O₇ presents a lower difference in grain size and a higher relative density than Cu₂P₂O₇ and LiFeP₂O₇.