Hexagonal $ABC$ Semiconductors as Ferroelectrics

We use a first-principles rational-design approach to identify a previously unrecognized class of ferroelectric materials in the $P{6}_{3}mc$ LiGaGe structure type. We calculate structural parameters, polarization, and ferroelectric well depths both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

Figure 1

The $P{6}_3/mmc$ ZrBeSi structure shown in (a) is the nonpolar high-symmetry reference structure for the polar $P{6}_{3}mc$ LiGaGe structure shown in (b). The two structures are related by a buckling of the planes formed by atoms at Wyckoff positions $2b$ (medium-sized dark blue balls) and $2b^{\prime }$ (small gold balls) and displacements of the planes relative to the stuffing atom at $2a$ (large green balls).

Figure 2

Difference in energy between the low ($P{6}_{3}mc$) and high ($P{6}_3/mmc$) symmetry $ABC$ structures vs polarization for all polar insulating combinations. Combinations with $A=\mathrm{Li}$ (black circles) are less likely to be switchable than those with $A=\mathrm{Na}$ (red triangles) or $A=\mathrm{K}$ (green diamonds). The two reported compounds LiBeSb and LiZnSb are labeled. The inset shows the characteristic ferroelectric double-well energy of NaMgP as a function of polar distortion obtained by linear interpolation between the polar $P{6}_{3}mc$ and nonpolar $P{6}_3/mmc$ structures.