Synthesis and characterisation of the geometric ferroelectric LaTaO₄

Abstract

LaTaO₄ is one of very few reported n=2 Carpy-Galy oxides. It is isostructural to the BaMF₄ fluorides which were subject of much prior study due to their ferroelectric and multiferroic properties. Previous studies of LaTaO₄ have been consistent in their conclusion that it can adopt either a low temperature non-polar monoclinic, or high temperature polar orthorhombic phase, with the monoclinic – orthorhombic transition temperature (Tₘ₋ₒ) being dependent on the preparation method. Recent studies, however, have reported an unidentified dielectric anomaly in the orthorhombic phase which alludes to an additional subtle structural transition above Tₘ₋ₒ. Powder neutron (PND) and electron diffraction studies have revealed that the previously unidentified dielectric anomaly denotes a transition between an incommensurately modulated and unmodulated orthorhombic (Cmc2₁) structure at ~503 K. The phase transition sequence is re-investigated in light of this new modulated phase, with resonant ultrasound (RUS) and Raman spectroscopy being utilised to elucidate the dynamics of transition and show the incommensurate – commensurate transition to be unusually 1ˢᵗ order, but nearly continuous (tri-critical). Refinement of PND data in the Cmc2₁(α00)0s0 superspace group, indicates a modulation wavevector q = (0.456 0 0) at 483 K. The modulation arises from a variation in La³⁺ positions and octahedral tilts which propagate along the polar a-axis and is speculated to arise as a mechanism to improve overall bonding environment of La³⁺. Electron microscopy reveals that the modulation arises from partially ordered sub-units, which are 2.5× and 3× expansions of the original orthorhombic cell. A model has been constructed which combines the local structural origin with the observed macroscopic modulation, where sub-units partially order with an average periodicity of ~11a (~0.456 a*). Previous studies report a relationship between the stability of each phase of LaTaO₄ and A-cation size. To explore the possibility that the modulation arises purely by an A-cation size effect, the series La₁₋ₓLnₓTaO₄ is synthesized, where Ln = Ce³⁺, Pr³⁺ and Nd³⁺ (0 ≤ x ≤ 0.3). Doping with these isovalent cations shows a general trend of stabilisation of the low temperature phases with decreasing average A-cation radii; the effect is amplified in Ce³⁺-doped samples and preliminary analysis suggests that this is due to a mix of oxidation states and the associated inter- and de-intercalation of interstitial oxygens during thermal cycling.