Structural Phase Transition Behavior Observed for a Single Crystal of the Tetrabutylammonium Salt of a Mo18 Polyoxometalate

Ionic single crystals of the polyoxometalate [Mo^V₂Mo^VI₁₆O₅₄(SO₃)₂]^6– salted with tetrabutylammonium were found to exhibit dielectric anomalies at 235 and 302 K. The clear DSC peak at 235 K indicates the first-order nature of transition. The next transition at 302 K was concluded to be second-order in nature, demonstrating the existence of three different phases (α, β, and γ). Temperature-dependent single-crystal X-ray diffraction analysis revealed that the structural phase transitions occur owing to orientational changes in the structures of the cation alkyl chains and the crystalline solvent of acetonitrile where the structure of Mo18 polyoxometalate was comparable. The polymorphous and flexible configuration of the alkyl chains was reasoned to be the origin of the temperature dependence of the structure.