NUCLEATION AND GROWTH OF CRYSTALS IN A ZBLYAN GLASS

Nucleation frequencies and growth rates of crystals in a 51ZrF(4)-18.5 BaF(2)-4LaF(3)-4.5A1F(3)-20NaF-2YF(3) (ZBLYAN) glass. Crystal morpholo gies were measured at surfaces and in the bulk of the samples differ. The size of crystals on the surface increased linearly as a function o f annealing time, while that of bulk crystals increased with the squar e root of time. The growth rate of surface crystals ranges from 10(-8) to 10(-7) ms(-1) in the temperature range between 603 and 643 K. The homogeneous nucleation frequency has been calculated as a function of temperature. A comparison is made between computed and experimental nu cleation frequencies for bulk crystals. Calculated values are several orders of magnitude less than those measured. Therefore, the experimen tal data suggest a heterogeneous nucleation mechanism, possibly on hom ogeneously distributed impurities.