Phase transitions in undoped and Sc-substituted BiCrO3 perovskite


Perovskites produced by high-pressure synthesis have long been studied, allowing to obtain new multiferroics - materials that present at least two of the three phenomena: ferroelectricity, ferroelasticity and/or ferromagnetism. BiCrO3 is a notable case, subject of studies regarding its crystal and magnetic structure1. BiCrO3 is known to have a reversible phase transition upon annealing, from the monoclinic C2/c phase (resultant from high-pressure synthesis) to the non-polar orthorhombic Pnma phase above ~410 K2,3. In the BiCrO3- based solid solutions reported so far, the B-site substituting cation is very similar in size to Cr3+. A considerable size difference is suggested to induce new phases that are not observed in the end members. Entire series of the high-pressure stabilized BiFeO3-BiScO3 solid solutions where the ionic size of Sc3+ is by 24% bigger than that of Fe3+ was recently obtained4,5. In situ x-ray diffraction study of the post-synthesis thermal treatment revealed a set of annealing-stimulated irreversible phase transformations between the different metastable perovskite phases in this system5. Effect of the irreversible transformations (denominated as conversion polymorphism) is believed to be a general phenomenon that can be observed in other systems. We report on synthesis of perovskite phases of the BiCr1-xScxO3 series and detailed studies of their crystal structures and physical properties in relation to the relative amount of scandium in the materials. The compositions BiCrO3 and BiCr0.9Sc0.1O3 were synthesized under high-pressure and characterized using in-situ x-ray diffraction, scanning electron microscopy and energy dispersive x-ray spectroscopy, atomic and piezoresponse force microscopy, SQUID magnetometry, and dielectric spectroscopy. It was found from in situ x-ray diffraction experiments performed between 300 and 623 K that the reversible C2/c – Pnma transformation increases from 410-420 K for BiCrO3 to the range 470-520 K for BiCr0.9Sc0.1O3. Temperature dependences of the lattice parameters and the unit cell volume of the perovskite phases were evaluated from the Rietveld refinement data. The samples microstructure was studied ex situ using scanning electron microscopy after each stage of the step-by-step annealing at 373, 423, 448 and 473 K. The observed microstructural changes have been attributed to a jump of the unit cell volume upon C2/c – Pnma transition.


J.P. Cardoso, V. Paukšta, D. Delmonte, E. Gilioli, E.L. Fertman, A.V. Fedorchenko, V.V. Shvartsman, R. Grigalaitis, J. Banys, J.M. Vieira, A.N. Salak

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