An investigation on the structural evolution of Pb2YbTaO6

The structure of the complex perovskite Pb2YbTaO6, reported to undergo two phase transitions at 285 and 180°C, respectively, was analysed within the temperature range 30–320°C by DSC measurements and X-ray diffraction. The occurrence of just one thermal effect was revealed in DSC curves, corresponding to the first-order paraelectric–antiferroelectric transition, while no signal was detected at the reported antiferroelectric–ferroelectric transition. The structure of the high-temperature phase was confirmed to be elpasolite-type, with complete ordering of Yb and Ta on octahedral sites. The structure at 220°C was refined in space group Pbnm (n. 62), with lattice parameters derived from the cubic one as ao=(ac+bc)/2, bo=2(−ac+bc) and co=cc. The shifts of all atoms with respect to the cubic positions occur along the (100) orthorhombic direction, which corresponds to an original 〈110〉 cubic one. The two independent Pb atoms are shifted in opposite directions. A mode analysis shows that the shifts of heavy atoms can be described by the modulation associated with the condensation of the lattice mode Σ3 of wavevector q:(3/4,3/4,0).