Phase transitions and proton ordering in hemimorphite: new insights from single-crystal EPR experiments and DFT calculations

Single-crystal electron paramagnetic resonance spectra of gamma-ray-irradiated hemimorphite (Mapimi, Durango, Mexico) after storage at room temperature for 3 months, measured from 4 to 275 K, reveal a hydroperoxy radical HO2 derived from the water molecule in the channel. The EPR spectra of the HO2 radical confirm that hemimorphite undergoes two reversible phase transitions at similar to 98 and similar to 21 K and allow determinations of its spin Hamiltonian parameters, including superhyperfine coupling constants of two more-distant protons from the neighboring hydroxyl groups, at 110, 85, 40 and 7 K. These EPR results show that the HO2 radical changes in site symmetry from monoclinic to triclinic related to the ordering and rotation of its precursor water molecule in the channel at < 98 K. The monoclinic structure of hemimorphite with completely ordered O-H systems at low temperature has been evaluated by both the EPR spectra of the HO2 radical at < 21 K and periodic density functional theory calculations.