Phase transformation in AlPO4 and quartz studied by electron paramagnetic resonance of Fe3+

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Abstract
Electron paramagnetic resonance (EPR) of Fe3+ in AlPO4 and quartz were studied as a function of temperature using a microwave spectrometer operated at 35 GHz. Electron paramagnetic resonance of Fe3+ in AlPO4 changed with temperature and gave direct evidence of the α–β phase transition. The EPR signal from Fe3+ in quartz was greatly reduced above 670 K. The change of the EPR parameters below 670 K was similar in both materials. There is a continuous rotation of 86° of the magnetic axes between room temperature and the phase transition in AlPO4. This is compared with the rotation of the PO4 and AlO4 tetrahedra measured by X-ray diffraction. The temperature dependence, but not the magnitude, of EPR parameters calculated using a point-charge model and the Blume–Orbach mechanism with the temperature dependence of quartz crystal parameters is similar to what is observed. Increased EPR line width near the phase transition was observed and was attributed to fluctuations at the phase transition.

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