A NEUTRON POWDER DIFFRACTION STUDY OF THE BETA TO GAMMA PHASE-TRANSFORMATION IN NAFEO2

The beta- (low temperature, Pn2(1)a) to gamma- (high temperature I, P4(1)2(1)2) phase transformation in NaFeO2 has been studied by Rietveld analysis of neutron powder diffraction data collected over the temperature range 298 to 1373 K. The results are consistent with a mechanism that involves cooperative rotation of FeO4 tetrahedra in the cristobalite-related tetrahedral framework. The sodium atoms occupy tetrahedral sites in the framework cavities in both structures. The transformation involves a relocation of the framework oxygen atoms around the sodium atoms to change the articulation between NaO4 and FeO4 from corner to edge sharing. The driving force for the transformation appears to be strain, arising from incompatibility between an expanding cavity volume and the bond valence requirements of the sodium atoms in the beta phase. The transformation to the gamma form allows an increase in the Fe-O-Fe bridging angle to occur while at the same time permitting the volume of the NaO4 tetrahedron to decrease. The number of nearest neighbour Na-Fe distances decreases from 8 in the beta phase to 7 in the gamma phase. Diffraction data were collected for two successive traverses of the transition temperature to study a "memory effect", which is manifested as a lower recorded transition temperature for the second traverse. The origin of the effect was interpreted from the structure analysis results as a retention of uncorrelated domains of the gamma phase within the beta phase on cooling.