The effects of developmental stress on circadian rhythm development and the expression of diurnal behaviours (Thesis data)

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.