An attempt to improve photochromic coloration of LaNbO4, GdNbO4 and YbNbO4 has been made by doping with several kinds of impurities. These include praseodymium in the rare earth group, calcium, strontium or barium in the IIa group, and germanium or tin in the IVb group. The divalent IIa group ions and the quadrivalent IVb group ions are considered to reduce partially the host trivalent rare earth ions into the divalent ions and incidentally to produce a corresponding amount of oxygen vacancies. In fact, doping of each dopant has been shown to heighten the photochromic characteristics of the three compounds mentioned above. Furthermore, multiple doping of praseodymium and the other impurities has been found to be markedly effective also in regard to these compounds. Codoped impurities of certain optimum amounts lead to realization of very high level photochromic characteristics.
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Yasutaka Suemune (1984) studied this question.
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