Effect of H₂O Molecules on Thermal Expansion of TiCo(CN)₆

Understanding the role of guest molecules in the lattice void of open-framework structures is vital for tailoring thermal expansion. Here, we take a new negative thermal expansion (NTE) compound, TiCo(CN)₆, as a case study from the local structure perspective to investigate the effect of H₂O molecules on thermal expansion. The in situ synchrotron X-ray diffraction results showed that the as-prepared TiCo(CN)₆·2H₂O has near-zero thermal expansion behavior (100-300 K), while TiCo(CN)₆ without water in the lattice void exhibits a linear NTE (α_l = -4.05 × 10^-6 K^-1, 100-475 K). Combined with the results of extended X-ray absorption fine structure, it was found that the intercalation of H₂O molecules has the clear effect of inhibiting transverse thermal vibrations of Ti-N bonds, while the effect on the Co-C bonds is negligible. The present work displays the inhibition mechanism of H₂O molecules on thermal expansion of TiCo(CN)₆, which also provides insight into the thermal expansion control of other NTE compounds with open-framework structures.