Perovskite lanthanum tantalate La1/3TaO3 (LT) precursors were prepared by a new polymeric complex sol–gel method using Ta-citrate or -tartrate complexes in different solvents (ethanol or methanol) and calcination between 750 and 1050 °C. The structural properties of precursors were characterised by X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetric analysis (DSC/TG), Fourier transform infrared (FTIR) and Raman spectroscopy, scanning and transmission electron microscopy (SEM, TEM). The fluorite orthorhombic tungsten bronze (TB) of La4.67Ta22O62 and perovskite La1/3TaO3 phases with a small amount of pyrochlore monoclinic LaTaO4 phase were formed after calcination at 900–1050 °C respectively, irregardless of solvent. The particle morphology changed from larger agglomerates composed of spherical 5–20 nm amorphous clusters without visible crystallization at 750 °C to plate-like shaped of well-ordered LT nanoparticles (ethanol) or individual spherical clusters with the fine inner structure (methanol) at 1050 °C. The same effect of alcohol solvent on phase transformation during annealing was shown, whereas the effect of chelating agent was insignificant. Selected area electron diffraction (SAED) patterns of La1/3TaO3 precursors after calcination at 900 °C (a) metastable fluorite La4.67Ta22O62, (b) perovskite La1/3TaO3 and (c) LaTaO4 phases and (d) multidomain structure of super cell of La2/3Ta2O9
 .

Structural properties and phase transformation of sol–gel prepared lanthanum tantalates

Nb K-edge x-ray absorption spectroscopy is utilized to investigate the changes in the local structure of the A-site deficient double perovskite La1/3NbO3 which undergoes a pressure induced irreversible amorphization. EXAFS results show that with increasing pressure up to 7.5 GPa, the average Nb-O bond distance decreases in agreement with the expected compression and tilting of the NbO6 octahedra. On the contrary, above 7.5 GPa, the average Nb-O bond distance show a tendency to increase. Significant changes in the Nb K-edge XANES spectrum with evident low energy shift of the pre-peak and the absorption edge is found to happen in La1/3NbO3 above 6.3 GPa. These changes evidence a gradual reduction of the Nb cations from Nb(5+) towards Nb(4+) above 6.3 GPa. Such a valence change accompanied by the elongation of the average Nb-O bond distances in the octahedra, introduces repulsion forces between non-bonding adjacent oxygen anions in the unoccupied A-sites. Above a critical pressure, the Nb reduction mechanism can no longer be sustained by the changing local structure and amorphization occurs, apparently due to the build-up of local strain. EXAFS and XANES results indicate two distinct pressure regimes having different local and electronic response in the La1/3NbO3 system before the occurence of the pressure induced amorphization at  ∼14.5 GPa.

https://iopscience.iop.org/article/10.1088/0953-8984/28/4/045401/pdf

Nb K-edge x-ray absorption investigation of the pressure induced amorphization in A-site deficient double perovskite La1/3NbO3.

High-pressure X-ray diffraction measurements have demonstrated that the cation-deficient perovskites Pr1/3NbO3, Pr1/3TaO3, Nd1/3NbO3, and Nd1/3TaO3 undergo irreversible pressure-induced amorphization (PIA). This occurs near 14.5 GPa for the niobates and 18.5 GPa for the tantalates. The unit cell volumes of the four oxides show an almost linear decrease as the pressure is increased. It is concluded that the PIA transition occurs at higher pressures in the tantalates due to the lower MO6 initial tilting at ambient conditions, which is associated with the larger atomic mass of the tantalum. The behavior of these oxides is compared to that of CaTiO3, and the role of both the weakening of the M–O–M π-bonding and the cation vacancies on the observed structural changes is discussed.

Pressure-induced amorphization of A-site-deficient double perovskite Ln1/3MO3 (Ln = Pr, Nd, M = Nb, Ta)

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Pressure-induced amorphization of La1/3TaO3

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Pressure-induced amorphization of La1/3TaO3

Pressure-induced amorphization of A-site-deficient double perovskite Ln1/3MO3 (Ln = Pr, Nd, M = Nb, Ta)