Novel ferroelectric phase in bulk BaO obtained by application of anisotropic strain

VB Nascimento and BV da Costa and JP Rino, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 126, 744 (2020).

DOI: 10.1007/s00339-020-03930-7

Ferroelectricity is expected to be observed in alkaline earth metal oxides (BaO, MgO, CaO, and SrO) with the application of adequate strain, according to theoretical predictions by Bousquet et al. The resultant polarization, dielectric constant, and piezoelectric response would be comparable to those of typical ferroelectric materials, like perovskites. In yet another recent work, Nascimento et al. explored these theoretical predictions by simulating the behaviour of ultra-thin films of BaO under applied in-plane strain to mimic the effect of epitaxial strain induced by an adequate substrate. Effects of both in- plane compressive strain as well as temperature, varied in the ranges of 0 to - 10% and 100-1500 K, respectively, were explored in the simulations. An antipolar phase was observed for in-plane compressive strain values from 3 to 10% and a temperature range of 100 to 1500 K. However, only a compressive strain value around 9%, typically too high to be obtained by epitaxial strain, was able to produce an ordered single domain. Polarization and ferroelectricity are well known as hard to obtain in ultra-thin films due to their limited thickness. The observed existence of an antipolar phase in ultra-thin films would in principle avoid the problems associated with uncompensated surface charges created by the discontinuity of the normal polarization component at the surface. However, can ferroelectricity be obtained in thicker films or even in bulk-like BaO systems? To answer this question, we will here explore the effects of in-plane compressive strain in BaO bulk systems. Compressive in-plane strain, along both x and y directions, will be varied in 0-10% range as well as the c/a (out-of- plane/in-plane) lattice ratio for values of 1.0-1.3. The observed structural phases and associated physical properties will be presented and discussed.

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