Original Paper

Physics and Chemistry of Minerals

, Volume 40, Issue 2, pp 133-143

First online:

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

  • Mao MaoAffiliated withDepartment of Geological Sciences, University of Saskatchewan
  • , Zucheng LiAffiliated withDepartment of Geological Sciences, University of Saskatchewan
  • , Yuanming PanAffiliated withDepartment of Geological Sciences, University of Saskatchewan Email author 

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Abstract

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.

Keywords

Hemimorphite Zeolite-like mineral Single-crystal EPR DFT Phase transitions HO2 radical Proton hyperfine and superhyperfine coupling constants Proton ordering