Defect density measurements in shocked single crystal ammonium perchlorate by x-ray photoelectron spectroscopy

Abstract

The linewidths of x-ray photoelectron spectra have been correlated with dislocation densities in a shock-damaged crystal of ammonium perchlorate (AP). A centimeter-size AP crystal was loaded at several sites with a diamond pyramid (Vickers) indenter, creating localized strain centers. The crystal was nonuniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact, and cleaved through the indentations. The cleaved planes permitted interior analysis of the crystal by x-ray photoelectron spectroscopy (XPS) over a pattern of 1 mm by 1 mm areas. The linewidth of the Cl(2p3/2) spectra ranged from 1.70 eV for the region of greatest visible damage to 1.22 eV for the region of no visible damage, the same linewidth as that obtained for unshocked AP (control). The observed damage was compared to photographs in the literature of gamma-ray irradiated AP crystals, for which dislocation densities were reported. This provided an approximate correlation of dislocation density versus XPS linewidth. The correlation was refined by chemically etching and determining densities on another cleaved plane in the recovered crystal. By this technique, a ∼100X increase in dislocation density was determined for the region of greatest shock damage relative to an unshocked crystal. The strain fields associated with the impressions were found to enhance perchlorate decomposition when driven by shock. Distortion of the molecular lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl(2p) energy level in the perchlorate anion predicted variations of 0.1 to 0.46 eV. Variations of this magnitude are sufficient to produce the observed linewidth broadening.