Oxygen content annealing, effect

Color." The most obvious effect produced in silica gel by radiation is a grayish-purple color (64), which can be almost surely attributed to the same type of center as that responsible for the similar color in irradiated quartz, namely, a positive hole trapped at an oxygen ion adjacent to a substitutional Al + impurity ion (65-67). The attribution rests on the similarity in optical absorption between irradiated gel and irradiated quartz (66), on the dependence of the intensity of the color on the aluminum content (69), and on the observation of a hyperfine interaction characteristic of the spin of the 2 a1 nucleus (I = 5/2) in the ESR spectrum of the irradiated gel (70). Furthermore, the ESR sextet and the color are annealed at comparable rates above 200° (70) and are both destroyed by adsorption of H2 at room temperature (64, 70). Their intensities increase in parallel as the aluminum content, the severity of preirradiation heat treatment, or the length of irradiation is increased (70). The concentration of the center does not increase indefinitely. After some lO i ev/gm, it approaches a limiting value which depends on the impurity content, for typical gels around lO H2/gm (69). 

These results show that it is difficult to compare EPR data of high-Tc cuprates from different laboratories. The number of parameters (i.e. annealing procedure, Sr content, oxygen stoichiometry, 3d-doping level, preparation procedure of the samples etc.) is too big, and hence the quality of the crystals, including effects like electronic phase separation, is too different in order to allow reliable comparisons. One way to avoid these difficulties is to carry out the investigation over a broad range of all these parameters and direct the attention to a well-defined preparation procedure for all samples used. 

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