Detection of copper precipitates in Cu2O by NMR

Abstract

Weak 63Cu NMR signals characteristic of metallic copper have been found in three type-C Cu2O crystals, but not in types A and B. This is in accordance with presently accepted models of these materials. The resonances are asymmetric in two of the three crystals. One shows skin-depth effects, implying particle sizes [Formula: see text], the other shows some second-order quadrupole broadening, presumably associated with strains or surface effects. From the areas under the absorption curves, mass concentrations of copper precipitates ranging from 0.023 to 0.034% are deduced. However, the smallest value is found from an asymmetric resonance where significant loss of signal must have occurred. The data are not inconsistent with a concentration of about 0.034% for all three crystals. For type-A and -B material, an upper limit of 3–7 ppm can be placed on the concentration, depending on the assumed contribution of the satellites to the resonance. 63Cu resonances associated with the transitions between Zeeman-split ± 1/2 quadrupole states in Cu2O were troublesome, since they overlapped the metal resonance at many sample orientations. Strong Cu2O signals, observed at one orientation, indicated that part of the crystal must have grown with {111} faces.