Luminescence of coesite

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Coesite is a polymorph modification of crystalline silicon dioxide with a tetrahedral structure. The luminescence of a single crystal of synthetic coesite was studied under excitation using x-rays, an electron beam, and excimer lasers KrF (248 nm), ArF (193 nm) and F2 (157 nm). Luminescence bands in the regions of 2.5 eV and 4.4 eV appear. The blue band is dependent on temperature and is composed of decay kinetics. Three main decay times are revealed, exhibiting luminescence of a different nature in the same range of the spectrum. One is in the ns range of time with a time constant of about 2 ns. The two other decay times are in the regions of 5 μs and 700 μs. The 5 μs component is also seen under KrF excitation, whereas both the 5 μs and 700 μs components are seen under ArF excitation. The time resolved spectra are mutually similar and they correspond to those under x-ray and e-beam excitation. The UV band is fast with a time constant of less than 1 ns, independent of temperature. Only the 2 ns and 5 μs components are revealed for the blue band under the KrF laser excitation. Blue luminescence thermal quenching takes place for temperatures above 50 K, with good correspondence between the intensities of the thermal dependences under different excitation and that of the decay time constant. The quenching parameters used are 0.05 eV of energy and a frequency factor of 6 · 105 s−1. The UV band is practically independent of temperature in the range 10–290 K. The nature of luminescence is ascribed to the coexistence of a host defect and a self-trapped exciton. The defect is similar to the known oxygen-deficient luminescence center in pure silica glass. The blue luminescence at 700 μs is ascribed to the self-trapped exciton being characteristic of silicon dioxide with a tetrahedral structure.