Crystal Structures and Phase Transitions in A-Site Deficient Perovskites Ln_1/3TaO₃

Abstract

The synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/m m m/La → C m m m/Ce−Gd → P m m a/Tb, Dy → P m c21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating.

Synthesis and thermophysical properties of RETa3O9 (RE = Ce, Nd, Sm, Eu, Gd, Dy, Er) as promising thermal barrier coatings

“…The vibrational models around 100 cm À1 and 200 cm À1 stem from the bending vibration of TaO 6 and lattice vibration of RETa 3 O 9 , as we can see that RETa 3 O 9 (RE = Ce, Nd, Sm, Eu) almost have the same Raman vibration in this region because of the same distortion TaO 6 units according to Ref. [20]. But due to the decrease in RE 3+ ionic radius the TaO 6 units distort further and lead to the appearance of different vibrational models in RETa 3 O 9 (RE = Gd, Dy, Er) compounds.…”

“…The thermal conductivity in this region increases rapidly (~T 3 ) with temperature. 20 Someone may argue that the strange thermal conductivity trend in this work may link to phase transition, but one can see from Figure 6A the differential scanning calorimetry (DSC) curves of RETa 3 O 9 specimens that there is no endothermal or exothermal peak, indicating phase transformation did not happen under the current test temperature. At temperature higher the peak (≧RT), the thermal conductivity decreases as 1/T largely due to anharmonic phonon scattering, the Umklapp phonon-phonon process.…”

“…[35], the changes of thermal conductivity from being characteristic of a well ordered solid to that of a glass can be achieved in a highly controllable fashion by introducing vacancies on the perovskite A-site. 20 Someone may argue that the strange thermal conductivity trend in this work may link to phase transition, but one can see from Figure 6A the differential scanning calorimetry (DSC) curves of RETa 3 O 9 specimens that there is no endothermal or exothermal peak, indicating phase transformation did not happen under the current test temperature. Figure 6B shows the thermogravimetric analysis (TG) curves of all species, when the temperature was increasing there was little loss of quality but less than 0.2%, it should be caused by the moisture in the specimens.…”

“…One can see from Figure 2B the main diffraction peaks shift forward higher angle as the ionic radius of rare earth element decreases. [20]. A larger radius of the RE 3+ ion leads to the increase in crystalline volume and a decrease in bulk density, which contribute to reducing the thermal conductivity of RETa 3 O 9 ceramics.…”

Structural and Electronic Parameters of Complex Niobates and Tantalates

“…Presently only a few photocatalytic niobates and tantalates have been accurately observed with XPS, and further accumulation of experimental results is beneficial in order to establish reliable correlations between the structural and electronic parameters, which will be helpful for the design of new effective catalytic materials. A set of chemically stable rare-earth-bearing perovskites Ln 1/3 MO 3 would be considered for this purpose [49,50].…”