An electron spin-resonance study of CIO₃ radicals in NH₄ CIO₄ single crystals

Abstract

An ESR study ofCIO₃ centres in x-irradiated ammonium perchlorate(NH₄CIO₄) 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 NH₄CIO₄does not undergo any phase transition below room temperature. Furthermore, it is found that duringCl0₄' CIO₃ 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.