Pressure-dependent phase transitions in hybrid improper ferroelectric Ruddlesden-Popper oxides

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a combined high-pressure powder synchrotron x-ray diffraction experiment and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>a</a:mi><a:mi>b</a:mi><a:mspace width="0.16em"/><a:mi>i</a:mi><a:mi>n</a:mi><a:mi>i</a:mi><a:mi>t</a:mi><a:mi>i</a:mi><a:mi>o</a:mi></a:mrow></a:math> study on the bilayered Ruddlesden-Popper phases <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi>Ca</c:mi><c:mn>3</c:mn></c:msub><c:msub><c:mi>Mn</c:mi><c:mn>2</c:mn></c:msub><c:msub><c:mi mathvariant="normal">O</c:mi><c:mn>7</c:mn></c:msub></c:mrow></c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>Ca</e:mi><e:mn>3</e:mn></e:msub><e:msub><e:mi>Ti</e:mi><e:mn>2</e:mn></e:msub><e:msub><e:mi mathvariant="normal">O</e:mi><e:mn>7</e:mn></e:msub></e:mrow></e:math>. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mi>A</g:mi><g:msub><g:mn>2</g:mn><g:mn>1</g:mn></g:msub><g:mi>a</g:mi><g:mi>m</g:mi></g:mrow></g:math> and nonpolar <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mi>A</h:mi><h:mi>c</h:mi><h:mi>a</h:mi><h:mi>a</h:mi></h:mrow></h:math> structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar phase, as is commonly observed for proper ferroelectrics, regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability. Published by the American Physical Society 2024