AN ELECTRON-SPIN-RESONANCE STUDY OF CLO3 RADICALS IN NH4CLO4 SINGLE-CRYSTALS

An ESR study of ClO3 centres in x-irradiated ammonium perchlorate (NH4 ClO4) single crystals is carried out that is aimed at resolving the co ntroversy that has recently arisen over the possible occurrence of low -temperature phase transitions in the compound (room temperature struc ture: orthorhombic; space group Pnma). The ESR spectra are recorded as a function of the orientation of the magnetic field H in the three or thogonal crystallographic planes at 300 K, 77 K, and 4.2 K, and also a s a function of temperature for H parallel to the principal z-componen t A(zz)(Cl) of the Cl-35 hyperfine coupling tenser. The analysis of th e spectra shows that while the g-tensor remained axial over the entire temperature range (g(parallel to) = 2.0052(3), 2.0055(3), 2.0057(3) a nd g(perpendicular to) = 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 temperatur e (A(zz) = 429.77(1.0) MHz, A(yy) = 316.15(1.0) MHz, and A(xx) = 325.9 8(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(zz) = 445.55(1.0) MHz, A(yy) = 299.84(1.0) MHz , and A(xx) = 303.43(1.0) MHz at 4.2 K). No changes in the spectra att ributable to any phase transition were observed, and we conclude that NH4ClO4 does not undergo any phase transition below room temperature. Furthermore, it is found that during ClO4- --> ClO3 radiative decompos ition 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.