Microscopic Mechanism and Domain Formation in the Paraelectric to Ferroelectric Phase Transitions in BaTiO 3
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1034-K07-09
Microscopic Mechanism and Domain Formation in the Paraelectric to Ferroelectric Phase Transitions in BaTiO3 Marek Pasciak1, and Stefano Leoni2 1 Polish Academy of Sciences, Institute of Low Temperature and Structure Research, P.O. Box, Wroclaw 2, 1410, Poland 2 Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Strasse 40, Dresden, 01187, Germany ABSTRACT A design approach to ferroelectric materials critically depends on an accurate description of the microscopic features associated with paraelectric-to-ferroelectric phase transitions. The fine structures of domains, domain walls, and domain boundary dynamics as well as a precise understanding of local atomic displacements can be accessed using adequate potential models based on ab initio calculations and advanced molecular dynamics simulations. For BaTiO3 a complex scenario of microscopic domains in the paraelectric (cubic) phase and in the ferroelectric (tetragonal) phase is obtained. Therein, the static and dynamic role of domain/antidomain features, as well as their dependence on Ti displacements around the manifold is clearly emerging. INTRODUCTION Ferroelectric materials are well known for their extensive use as transducers, capacitors and recently as improved memory devices. Among the most used materials, also due to its robustness with respect to technical manipulations and functional stability, BaTiO3 undergoes a series of phase transitions on lowering temperature, from a paraelectric cubic phase, to a tetragonal, an orthorhombic and to a rhombohedral one. In the effort to shed light on the microscopic nature of the mechanism as a major step to understanding the nature of the ferroelectric phases, different models have been invoked over the years, among them the displacive model [1] and the order disorder model [2]. In the former different off-center displacement vectors of Ti atoms resulting from soft mode condensation are responsible for distinct symmetry lowering paths. In the latter configurational entropy is the main ingredient, owing to displacement disorder of Ti atoms along on increasing temperature. While the mentioned models do provide some pieces of truth, both do suffer from some degree of disagreement with experiments. The displacive models contradicts Raman experiments [3] and EXAFS measurements [4], which pinpoint the relevance of displacement pattern in all phases. Nor it does explain diffuse scattering effects. The latter is better accounted for in the order-disorder model, which in turn overestimates entropy effects. Experimental observations and theoretical models can be reconciled on combining the two points of view [5,6]. In the Girshberg-Yacoby theory for example off-center Ti displacements are coupling with soft modes, in a scenario of mixed displacive and order-disorder features, which can be associated with different time scales. Recent progresses from first principles calculations based on the DFT scheme provide first insights into domain/antidomain features. Calculations performed in the athermal limit
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