Al,Ge cation ordering in BaAl2Ge2O8-feldspar: monodomain ordering kinetics

The kinetics of monodomain order-disorder processes in monoclinic (I2/c) Ba Al2Ge2O8-feldspar have been investigated by X-ray powder diffraction, Hard Mode IR Spectroscopy, and TEM darkfield imaging on quenched samples. Compar ed to the behaviour predicted by the TDGL approximation ordering kinetics o bserved at low temperatures slow down significantly when equilibrium is app roached. Such a delay is not observed in disordering experiments starting f rom essentially ordered cation distributions. The deviation from TDGL behav iour is interpreted in terms of partial order parameter conservation in a n on-uniformly ordered phase. Modifications to the uniform TDGL rate equation are tested against the available data. An activation energy of 352 +/- 28 kJ/mol is obtained for Al,Ge ordering. The mixing coefficient zeta(c)(2)/ze ta(2), which describes the degree of order parameter conservation. is obtai ned as a function of temperature. While this coefficient vanishes in the vi cinity of the transformation temperature T-tr, it saturates towards a level of zeta(c)(2)/zeta(2) approximate to 0.4 for T --> 0. zeta(c)(2)/zeta(2) d etermines the kinetic stability of ordered clusters quenched from T approxi mate to T-tr.