Soft mode-driven reduction of switching energy barriers in fluorite ferroelectric

The low coercive field (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>E</a:mi><a:mi>c</a:mi></a:msub></a:math>) is highly desirable for developing efficient and durable ferroelectric-based devices. However, fluorite ferroelectrics such as <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>HfO</b:mi><b:mn>2</b:mn></b:msub></b:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>ZrO</c:mi><c:mn>2</c:mn></c:msub></c:math>, which are attracting significant attention as next-generation semiconductor materials due to their compatibility with complementary metal-oxide-semiconductor technology and robust ferroelectric properties, exhibit a significantly high <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>E</d:mi><d:mi>c</d:mi></d:msub></d:math>, severely limiting their application in memory devices. Here, using group theory and first-principles simulations, we reveal that phonon engineering can induce a new soft mode in the polar <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>P</e:mi><e:mi>c</e:mi><e:mi>a</e:mi><e:msub><e:mn>2</e:mn><e:mn>1</e:mn></e:msub></e:mrow></e:math> phase of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>HfO</f:mi><f:mn>2</f:mn></f:msub></f:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>ZrO</g:mi><g:mn>2</g:mn></g:msub></g:math>. This mode dominates the intermediate transition state during polarization switching and drastically reduces the energy barriers for both homogeneous switching and the uniquely observed local switching, which enables ultrahigh memory density. Furthermore, the discovery of a ternary state during local switching points to the possibility of implementing ternary logic at the material level. Our findings introduce a new pathway to lowering <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>E</h:mi><h:mi>c</h:mi></h:msub></h:math> in fluorite ferroelectrics by exploiting intrinsic lattice dynamics, offering an alternative to conventional strategies such as doping or defect engineering, and open a promising direction for future material design.