(Invited) Ferroelectric Materials Study Using First-Principles Calculations and Materials Informatics
Authors: Moriwake Hiroki 
Published: May 2020

In the past a couples of decades, first-principles calculation has become powerful tool for materials study. In our group, ferroelectric material is one of important research target.
Softmodes and the structural phase transitions associated with them are is one of most important research topic in the field of ferroelectric materials. In the 2000s, a technique for calculating phonon using first-principles calculations was developed. Since then, softmode in ferroelectric materials have been extensively studied by many groups. More reentry, an automated method for comprehensive softmode analysis using simple algorithm for automated searching of phase-transition pathways based upon first-principles phonon calculations has been developed by Togo and Tanaka.[1] Figuer1(a) shows how the procedure is composed of multiple calculation “units.” Each calculation takes a crystal structure as its input. The initial structure is relaxed under the constraints of its crystal symmetry, i.e., during relaxation no lowering of crystal symmetry is allowed. However, increase in symmetry is allowed and often occurs. In such higher symmetry cases, the space group of the crystal structure may not be a subgroup of the space group of the initial structure, but it is a subgroup of the space group of the relaxed (final) structure. The dynamical stability of the final structure is then examined by calculate supercell force constant. If any instabilities are found, the structure is deformed along the relevant direction. These calculations are repeated until all of softmodes are eliminated.
Using this procedure, we can examine the mechanisms of softmode related structural phase transitions. Another advantage of this method is that it allows phase transition pathway via metastable structures to be proved, which may enable. Possible phase transition pathways are difficult to determine to be discovered experimentally. Fluorite structured hafnia (HfO2) is used as a high-k gate dielectric material in Si semiconductor devices. Recently, the ferroelectric behavior of thin film HfO2 has been reported. However, the origin and mechanism of this ferroelectric phase transition are still not well understood. Identifying the origin of this ferroelectricity would allow hafnia-based materials to be developed for application in nonvolatile memories and ferroelectric field effect transistors. Hence, in this study, we have attempted to elucidate the mechanism of the ferroelectric phase transition in thin-film HfO2 by performing comprehensive softmode analysis.
Our results are summarized in Fig.1(b). Under ambient pressure, no softmode is observed for the tetragonal P42/nmc phase. However, under slight negative (tensile) pressure (-3 GPa), a softmode leading resulting in a transition to the experimentally observed ferroelectric Pca21 phase was found. This softmode path is sensitive to pressure, as it vanished we applying positive (compressive) pressure, or zero pressure. In thin film form, the volume change HfO2 need to undergo during the first-order phase transition from tetragonal to monoclinic is suppressed by the constraining substrate. Under these condition, a ferroelectric phase transition through this softmode path can occur.[2]

References

[1] A. Togo and I. Tanaka, Phys. Rev. B, 87, 184104 (2013).
[2] H. Moriwake, A. Taguchi, C.A.J. Fisher, A. Togo, T. Shimizu, H. Funakubo, in preparation.

Figure 1