The mechanism of the intracrystalline cation exchange reaction in a natural columbite sample from Kragero, with composition (Mn0.85Fe0.15)(Nb1.8Ta0.2)O6, has been characterised by means of TEM and HR synchrotron-radiation powder diffraction. In columbite, cations may occupy two different octahedral sites, with divalent cations preferring the A site and pentavalent cations preferring the B site. When both the octahedral sites are randomly occupied, columbite adopts a α-PbO2-type crystal structure; ordering of divalent cations on A sites and pentavalent cations on B sites gives rise to an ...ABBABB… sequence along the a direction resulting in a tri-α-PbO2-type structure in which the Pbcn symmetry is maintained. The completely ordered scheme represents the only thermodynamically stable state. Nevertheless, partially disordered distributions of cations among the sites are often found in natural samples. High temperature treatment of these samples leads to the completely ordered state [1, 2]. Two single crystals with different degrees of order obtained by quenching experiments were prepared for TEM study. They revealed a pervasive presence of nano-sized domains with rhombic shape and sides parallel to (310) planes. Diffraction contrast images and high resolution images clearly show that rhombic domains are ordered and dispersed in a disordered matrix. In situ high temperature powder diffraction data collected at ESRF ID31 starting from untreated powders (degree of cation order Q ~ 0.15) and until completion of the ordering process shows that the sluggish cation ordering process in columbite is more complicated than the simple picture which is deducible from laboratory X-ray diffraction studies. The diffraction patterns show split reflections and require analysis using two distinct columbite phases with different degrees of order. The phase characterised by a higher Q value has slightly broader peak profiles. This is consistent with TEM observations of some small ordered domains in a largely disordered material. Furthermore the size of these ordered domains in the starting material, as derived by peak-profile analysis, is also consistent with TEM observations. Preliminary Rietveld analyses exclude a nucleation and growth model followed by coarsening of the domains, and indicate two stages for the cation ordering mechanism: a first stage in which the degree of order increases and the domains size remains nearly constant and a second one in which the growth of the domains accompanies the achievement of the complete cation order. PDF (Pair Distribution Function) analysis of patterns collected at different Q will allow to give further insights and a better understanding of the factors controlling the columbite ordering process.
Tarantino, S., Zema, M., Capitani, G., Scavini, M., Brunelli, M., Ghigna, P. (2008). CATION ORDERING AND MICROSTRUCTURES IN COLUMBITE. Intervento presentato a: 1st SIMP-AIC joint meeting, Sestri Levante (Italy).
CATION ORDERING AND MICROSTRUCTURES IN COLUMBITE
CAPITANI, GIANCARLO;
2008
Abstract
The mechanism of the intracrystalline cation exchange reaction in a natural columbite sample from Kragero, with composition (Mn0.85Fe0.15)(Nb1.8Ta0.2)O6, has been characterised by means of TEM and HR synchrotron-radiation powder diffraction. In columbite, cations may occupy two different octahedral sites, with divalent cations preferring the A site and pentavalent cations preferring the B site. When both the octahedral sites are randomly occupied, columbite adopts a α-PbO2-type crystal structure; ordering of divalent cations on A sites and pentavalent cations on B sites gives rise to an ...ABBABB… sequence along the a direction resulting in a tri-α-PbO2-type structure in which the Pbcn symmetry is maintained. The completely ordered scheme represents the only thermodynamically stable state. Nevertheless, partially disordered distributions of cations among the sites are often found in natural samples. High temperature treatment of these samples leads to the completely ordered state [1, 2]. Two single crystals with different degrees of order obtained by quenching experiments were prepared for TEM study. They revealed a pervasive presence of nano-sized domains with rhombic shape and sides parallel to (310) planes. Diffraction contrast images and high resolution images clearly show that rhombic domains are ordered and dispersed in a disordered matrix. In situ high temperature powder diffraction data collected at ESRF ID31 starting from untreated powders (degree of cation order Q ~ 0.15) and until completion of the ordering process shows that the sluggish cation ordering process in columbite is more complicated than the simple picture which is deducible from laboratory X-ray diffraction studies. The diffraction patterns show split reflections and require analysis using two distinct columbite phases with different degrees of order. The phase characterised by a higher Q value has slightly broader peak profiles. This is consistent with TEM observations of some small ordered domains in a largely disordered material. Furthermore the size of these ordered domains in the starting material, as derived by peak-profile analysis, is also consistent with TEM observations. Preliminary Rietveld analyses exclude a nucleation and growth model followed by coarsening of the domains, and indicate two stages for the cation ordering mechanism: a first stage in which the degree of order increases and the domains size remains nearly constant and a second one in which the growth of the domains accompanies the achievement of the complete cation order. PDF (Pair Distribution Function) analysis of patterns collected at different Q will allow to give further insights and a better understanding of the factors controlling the columbite ordering process.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
