morphology of the growing crystals [22]. The value of
n reflects the mechanism dominating the crystallization.
The smaller n values indicate that the crystallization is
dominated by surface crystallization mechanism rather
than by volume crystallization mechanism, and that the
crystallization dimension is low. Also, the greater n values
are expected only in the case of increasing nucleation rates,
i.e., [2.5 in diffusion-controlled reaction or [4 in polymorphic
transformation [40]. It is possible for each heating
rate to obtain a value of the Avrami constant, and their
corresponding mean value may be taken as the most
probable value of the quoted exponent [29]. For the
tetragonal Nd2Si2O7, the value of Avrami constant is 3.98,
indicating that the formation process of tetragonal Nd2-
Si2O7 could be nucleation and three-dimensional growth
that follows the diffusion-controlled mechanism when the
nucleation rate increases