Isothermal and non-isothermal kinetics study of the PbGeO3 solid-solid transition

In the last decade, studies on xPbO(1-x)GeO2 glasses received attention because of their promising optical applications when doped with rare earths [1]. The phase equilibria of the xPbO (1-x)GeO2 system have been recently investigated by our group [2], and the formation of several metastable phases was remarked. In particular, devitrification of the x=0.50 glass produced monoclinic lead metagermanate PbGeO3 via the formation of a metastable phase, which is unknown in literature. An isothermal and non-isothermal kinetic study on the formation of the metastable phase from devitrification of glassy lead metagermanate was lately reported by our group [3]. Besides an activation energy value of about 320 kJ mol-1, that is reasonably consistent with the devitrification of a number of oxide glasses, a Avrami coefficient n  1,3 was found. The attempt to relate such a coefficient with morphological evidences of secondary electron imagery collected by scanning electron microscope (SEM) did not give encouraging results. Preliminary x-ray diffraction patterns recently collected on samples heat treated at temperature just above the crystallisation peak highlighted the presence of both the unknown and the monoclinic lead germanate phases. Such an evidence indicates that pure metastable phase cannot be produced by glassy PbGeO3 devitrification. In the present paper the attention is focused on the kinetics study of the solid-solid transition between the metastable and the monoclinic PbGeO3 phases. For this purpose, single crystals of the PbGeO3 metastable phase were produced as a starting material. The kinetics parameters Ea and n, i.e. the activation energy and the Avrami coefficient, respectively, were determined by both isothermal and non-isothermal methods using differential scanning calorimetry (DSC) and x-ray powder diffraction (XRPD). A comparison between the obtained Avrami coefficient value and the morphology of thermally treated single crystals is also made. and the secondary electron imagery data collected on thermally treated single crystals is also made.