Temperature Dependence of EPR Spectrum from VO2+ Ions in (NH4)2SO4

Temperature dependence of EPR spectra from polar VO 2+ probes doped in ferroelectric (NH 4 ) 2 SO 4 crystal was studied in the range between 110 K and 300 K. Impurity ion, VO 2+ , takes substitutionally the position of two NH 4 + ion vacancies so as to satisfy the local charge neutrality. Three kinds of VO 2+ sites in the crystal are identified. One is VO 2+ substituted for II-II pair of NH 4 + ions, another is I-II and the other is I-I. When the temperature is lowered, VO 2+ molecular axes are slightly displaced in a different manner, respectively. From this fact it is concluded that the local electric field is reflected in the VO 2+ displacement. The analyses of the polar angles to the crystal c -axis for VO 2+ in II-II and I-I sites show quite similar profile with anomalous temperature dependence of polarization observed by Unruh. The two-sublattice model for (NH 4 ) 2 SO 4 is supported by the present study.