An electron spin-resonance study of CIO 3 radicals in NH 4 CIO 4 single crystals

An ESR study ofCIO 3 centres in x-irradiated ammonium perchlorate(NH 4 CIO 4 ) single crystals is carried out that is aimed at resolving the controversy that has recently arisen over the possible occurrence of low-temperature phase transitions in the compound (room temperature structure: orthorhombic; space group Pnma). The ESR spectra are recorded as a function of the orientation of the magnetic field H in the three orthogonal crystallographic planes at 300 K, 77 K, and 4.2 K, and also as a function of temperature for H parallel to the principal z-component A ZZ (Cl) of the 35 Cl hyperfine coupling tensor. The analysis of the spectra shows that while the g-tensor remained axial over the entire temperature range(g=2.0052(3), 2.0055(3), 2.0057(3) and g =2.0090(3), 2.0096(3), 2.0105(3) at 300 K, 77 K, and 4.2 K respectively), A (Cl), which is rhombic at room temperature (A ZZ =429.77 (1.0) MHz,A yy = 316.15 (1.0) MHz and A xx = 325.98 (1.0) MHz, at 300 K) becomes more axial as the temperature is lowered A zz = 441.90 (1.0) MHz A yy =299.09 (1.0) MHz and A xx = 301.61(1.0) MHz at 77 K, and A yy = 303.43 (1.0) MHz at 4.2K). No changes in the spectra attributable to any phase transition were observed, and we conclude that NH 4 CIO 4 does not undergo any phase transition below room temperature. Furthermore, it is found that duringCl0 4 ' CIO 3 radiative decomposition the mirror-related Cl - O bonds, even though weaker than the other Cl - O bonds, do not break, thus confirming our earlier conclusion that mirror symmetry is preserved during radiation damage.