Structural Study of La₂Ti₂O₇ by Convergent-Beam Electron Diffraction and Electron Microscopy

Abstract

A convergent-beam electron diffraction study has revealed that the space group of the room temperature phase of La2Ti2O7 is P21. A dark-field electron microscopy has made clear a twinned domain structure in this phase. C BED patterns obtained at above 1053 K were compatible with the space group Cmc21. An incommensurate phase exists between 993 K and 1053 K. The incommensurate reflections are characterized with the vector q̄=((1/2)-δ)ā 0 * from the fundamental reflections of the high temperature phase, a 0 * being the reciprocal lattice parameter of the high temperature phase. The value of δ increases from 0.015 to 0.017 as the temperature is raised. The transition between the room temperature phase and the incommensurate phase is of the first order, and that between the latter phase and the high temperature phase is of the second order.

The twin structure of La2Ti2O7: X-ray and transmission electron microscopy studies

“…3b). The twin planes are only readily visible in the thicker parts of the crystal away from the edge, as an abrupt change forms at 993 K there is a transition to one or other maximally tilted orientation resulting in the appearance of twin bands noted earlier (Tanaka et al, 1985) and here. Alternatively, both twin types of the monoclinic modification may be represented by slightly different structural forms in the incommensurate phase region, with the transition at 1053 K, not 993 K, determining the final twin distribution at room temperature.…”

“…Our simulations confirmed that, with use of the identical choice of unit cell and orientation, the two twins (or their enantiomorphs) cannot be distinguished by their image contrast alone. However, the change of cell orientation across the twin plane in the experimental image clearly indicates its location.There appears to be no strong structural reason favouring either of the two octahedral layer confor- forms at 993 K there is a transition to one or other maximally tilted orientation resulting in the appearance of twin bands noted earlier (Tanaka et al, 1985) and here. Alternatively, both twin types of the monoclinic modification may be represented by slightly different structural forms in the incommensurate phase region, with the transition at 1053 K, not 993 K, determining the final twin distribution at room temperature.…”

“…The study by Ishizawa et al (1977) also noted the presence of twinning in this material. It was pointed out by Tanaka et al (1985) that the earlier incorrect choice of orthorhombic symmetry was probably due to twinning. However, the correct structure determination by Gasperin (1975) was apparently performed for an untwinned crystal, prepared by growth in a PbO flux cooled from 1373 to 1073 K at 2 K h -1 and then presumably quenched rapidly through the orthorhombic-to-incommensurate and incommensurate-to-monoclinic transitions (Nanot et al, 1973).…”

Phase relations, crystal chemistry, and dielectric properties in sections of the La2O3–CaO–MgO–TiO2 system

“…These homologs exhibit orthorhombic subcells with similar a-(B5.5 Å ) and c-(B3.9 Å ) parameters, and variable b-parameters that reflect slab thickness and translational symmetry-they are readily distinguished by characteristic diffraction lines below 15 2y: Many A n B n O 3n+2 structures feature sequences of reversible displacive phase transitions associated with cooperative rotations of the [BO 6 ] octahedra [49]. In particular, La 2 Ti 2 O 7 (=La 4 Ti 4 O 14 , n ¼ 4) undergoes the following sequence of reversible tilting phase transitions on cooling [50]: Amam-A2 1 amðincommensurate phaseÞ-P2 1 : For this compound, the Amam-A2 1 am transition is associated with in-phase rotation of octahedra about the c-axis, while the subsequent A2 1 am-P2 1 symmetry reduction (through the incommensurate phase) is caused by octahedral tilting about the long b-axis; the latter transition doubles the c-lattice parameter. A classification scheme that relates the stacking sequences of the {110} perovskitelike layers and octahedral tilt systems to the symmetry of the A n B n O 3n+2 structures has been reported previously [49].…”

Phase Composition and Disorder in La2(Sn,Ti)2O7 Ceramics: New Insights from NMR Crystallography

“…Below 1.46 a transformation to a defect fluorite structure (with disorder on both cation and anion lattices) is expected while, above 1.78, materials are predicted to adopt a layered perovskite-related (La 2 Ti 2 O 7 ) structure, as shown in Figure 1b. 8−11 There are four crystallographically distinct types of La and Ti sites in this structure. Materials that exhibit this structural type have been investigated for uses in photocatalysis and electrical applications that exploit their piezoelectric and ferroelectric properties.…”