Phase transitions and proton ordering in hemimorphite: new insights from single-crystal EPR experiments and DFT calculations
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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 ~98 and ~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.
KeywordsHemimorphite Zeolite-like mineral Single-crystal EPR DFT Phase transitions HO2 radical Proton hyperfine and superhyperfine coupling constants Proton ordering
We thank Dr. Milan Rieder and two reviewers for incisive criticisms and helpful suggestions, Dr. Mark J. Nilges of the Illinois EPR Research Center for attempts of pulsed ENDOR and ESEEM measurements, Prof. Jin-Xiao Mi for assistance with matrix transformation, and the Natural Science and Engineering Research Council (NSERC) of Canada for financial support of this study. DFT calculations in this research have been enabled by the use of Westgrid computing resources, which are funded in part by the Canadian Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions. Westgrid equipment is provided by IBM, Hewlett Packard and SGI.
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