New high-resolution K-edge XAS spectra have been obtained for Cu(II) ion in liquid and frozen 1 M HClO₄ solution.

EXAFS to k=18 Å^-1 was achieved using low-zinc copper. Rising K-edge XANES spectra of [Cu(aq)]²⁺ imply structural variation between frozen and liquid phases. EXAFS and MXAN fits to XAS spectra consistently yielded non-centrosymmetric models for dissolved [Cu(aq)]²⁺. The equatorial plane of four water oxygen ligands at ~1.96 Å is stably present in both phases and all models. First shell disparity was restricted to the variable presence and fluctuating distances of axial water molecules. Both EXAFS and MXAN analysis further indicated that the organization of more distant second shell water molecules also varies between liquid and frozen phases. Additionally, both the first and second shells of water about Cu(aq)²⁺ were found to vary structurally within each phase. That is, in each phase [Cu(aq)]²⁺ is at least structurally dimorphous.

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