Cation distribution of pseudobrookite-type titanates and their phase stability

Abstract

Pseudobrookite-type titanate has two types of oxide octahedra for different cation types. The cation distributions in the octahedra of three pseudobrookite type oxides, Al 2 TiO 5 , MgTi 2 O 5 and Fe 2 TiO 5 , and in a solid solution of 0.5Al 2 TiO 5 0.5MgTi 2 O 5 were studied by the Rietveld method. The analysis revealed that the cation distributions of Al 2 TiO 5 and Fe 2 TiO 5 did not depend on the annealing temperature, whereas that of MgTi 2 O 5 depended on the annealing temperature. The estimated cation distributions determined by the Rietveld method agreed with those determined by bond valence sum estimation. The configuration entropies of Al 2 TiO 5 and Fe 2 TiO 5 were almost the same as that of the maximum value of the random distribution of cations. Although the configuration entropy stabilized Al 2 TiO 5 thermodynamically, the decomposition of Al 2 TiO 5 should be attributed to the stable nature of corundum, and the rather "stretched" structure of Al 2 TiO 5 would also cause its instability at low temperature.

Coupled Experimental Study and Thermodynamic Modeling of the Al₂O₃–Ti₂O₃–TiO₂ System

“…They reported about 0.4 occupancy of Ti 4 + in 4c site. It should be noted that the experimental data from Ohya et al 42) and Skala et al 45) are consistent each other, and they are reasonably well reproduced in the present study. As discussed above in section 2, the end member Gibbs energy of G TA was determined to reproduce this cation distribution data.…”

“…Calculated cation distribution in Al2TiO5 pseudobrookite solution along with the experimental data. 42,45) mized melting and decomposition temperatures of Al 2 TiO 5 are 1 860 and 1 279°C, respectively, in the present study. Apart from Al 2 TiO 5 , Lejus et al 51) found two new compounds with compositions between 33 and 40 mol% TiO 2 which are stable in the approximate temperature range between 1 800 and 1 900°C.…”

“…5. Recently, Ohya et al 42) determined the cation distribution in Al 2 TiO 5 pseudobrookite compound at 600-1 500°C using XRD Rietveld It is well-known from Jung et al 43,44) that the cation distribution can be frozen at low temperature due to the slow kinetic of cation-exchange reactions. Therefore, the experimental data collected at 600°C can most probably be in non-equilibrium state.…”

Circumventing Thermodynamic Constraints in Nucleation-Controlled Crystallization of Al2TiO5-Based Chemical Vapor Deposition Coatings

“…The presence of structural relaxations within our amorphous samples can be linked to our previous Raman spectroscopic evaluations, showing that extensive structural rearrangements and coordination changes occur during annealing. 31 In addition, because there are few constraints on the individual placements of cations in Al 2 TiO 5 , owing to cationic mixing, 33,42,91,92 these reorganizations may also induce secondary nucleation events where structurally related phases are formed besides the main Al 2 TiO 5 phase. Such phases include Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 , whose crystallographic differences compared to Al 2 TiO 5 are very subtle, and can primarily be traced to different stacking sequences along the phases' respective c-axes, leading to elongations of ditto.…”

Selective kinetic growth and role of local coordination in forming Al2TiO5-based coatings at lower temperatures

“…The cationic mixing on both sites cause local distortions to the oxygen octahedrons 81 and, conceivably, contributes to this compound's sizeable configurational entropy term. 78 Few studies have so far investigated the formation of Al 2 TiO 5 as coatings, which would extend the material's applicability in harsh thermal environments as a potential thermal barrier-and corrosion-resistant coating. Principal studies using CVD as a method includes the work made by D. Leinen et al 82 In their study, a combined ion-beam and CVD set-up was used to deposit amorphous coatings of Al x Ti y O z , which reportedly transformed into a sole crystalline TiO 2 phase upon subsequent annealing.…”