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Effect of Ga Content on the Instantaneous Structure of Al(1−x)GaxPO4 Solid Solutions at High Temperature

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journal contribution
posted on 27.01.2009, 00:00 by Olivier Cambon, Julien Haines, Martine Cambon, David A. Keen, Matthew G. Tucker, Laurent Chapon, Niels K. Hansen, Mohamed Souhassou, Florence Porcher
Al(1−x)GaxPO4 solid solutions (x= 0.14, 0.2, and 0.63) were studied at high temperature up to 1173 K by neutron total scattering. Rietveld refinements confirm the presence of the β-quartz-type structure for the lowest Ga content. On the basis of the refinement of the average structure at high temperature, the Al−O and Ga−O distances cannot be distinguished and all intratetrahedral bond distances decrease, which is not reasonable when compared to the thermal expansion behavior of this material. This apparent behavior is due to the presence of a high degree of dynamic disorder at high temperature. In contrast, reverse Monte Carlo (RMC) modeling of neutron total scattering data gives the instantaneous structure in which the two bonds (Al−O) at ∼1.75 Å and (Ga−O) at ∼1.84 Å are partially resolved and increase in length with temperature. The Ga content of the solid solutions is found to modify the structural geometry at high temperature. Increasing the Ga content reduces the Al−O−P angles compared to pure AlPO4 and lessens their temperature dependence. The results also show that the dynamic disorder in the oxygen sublattice decreases with the Ga-content. As in quartz, the displacive α−β phase transition (for x = 0.14) can be linked to cation displacements as the oxygen sublattice presents a high degree of dynamic disorder well below the transition temperature. The increased temperature stability for Ga-rich compositions can be linked to stronger covalent bonding as is found from the analysis of electron density maps of the pure AlPO4 and GaPO4 end members. This approach, based on using chemical substitution to reinforce the covalent nature of chemical bonds, may have implications for improving the thermal stability of a variety of materials used in high-temperature applications.