Magnetism and piezoelectricity in stable transition metal silicate monolayers

Two-dimensional van der Waals (2D vdW) single layered materials with ferromagnetism and piezoelectricity have been a subject of recent attention. Despite numerous reports of 2D ferromagnetic materials, developing an air stable and transferable vdW material has been challenging. To address this problem, we studied layered transition metal silicates that are derivatives of kaolinites and lizardites with transition metals substituting on ${\mathrm{Al}}^{3+}$ and ${\mathrm{Mg}}^{2+}$ sites using ab initio density functional theory calculations. This class of materials is appealing because they meet the symmetry requirements for piezoelectricity and can host a range of transition metal cations. As oxides, these materials are inherently stable in air. Following our previous experimental work, we predict that these compounds are stable under varying ${\mathrm{O}}_2$ partial pressure and can be synthesized using a surface-assisted method. We also show that the oxidation states of the substituted transition metal ions can be tuned through the level of hydrogenation.

Figure 1

Top view of the structural templates of the isolated 2D silicate layers studied in this work: (a) nontronite-like, (b) kaolinite-like, and (c) lizardite-like transition metal silicates. Respective side views are indicated in (d), (e), and (f). Blue, red, and orange indicate Si, O, and transition metal atom sites, respectively. Blue polyhedra highlight Si-O polyhedra, and gold polyhedra surround the transition metals. In side images, hydrogen atoms are indicated in white for the highest hydrogenation possible in their respective structures.

Figure 2

Phase diagrams for 2D ${\mathrm{Fe}}_3{\mathrm{Si}}_2{\mathrm{O}}_9{\mathrm{H}}_4$ with (a) hydrogen chemical potential ${\mu }_{\text{H}}$ is set to 0 eV, (b) ${\mu }_{\text{H}}=-0.133$ eV, and (c) ${\mu }_{\text{H}}=-0.267$ eV. Stability regions of ${\mathrm{Fe}}_3{\mathrm{Si}}_2{\mathrm{O}}_9{\mathrm{H}}_4$ are shaded in gray. In (c) the shaded area is given in the inset for further detail.

Figure 3

Phase diagrams for ${\mathrm{Fe}}_2{\mathrm{Si}}_2{\mathrm{O}}_9{\mathrm{H}}_4$: (a) hydrogen chemical potential ${\mu }_{\text{H}}$ is set to $-0.103$ eV, (b) ${\mu }_{\text{H}}$ is set to $-0.584$ eV, and (c) ${\mu }_{\text{H}}$ is set to $-1.27$ eV. Stability regions are shaded in gray. Insets show the shaded regions in greater detail.

Figure 4

Gibbs free energy, $\mathrm{\Delta }G$, of the hydrogenation reaction of Eq. (1) for ${\mathrm{Fe}}_3{\mathrm{Si}}_2{\mathrm{O}}_9{\mathrm{H}}_n$ as a function of the chemical potential of hydrogen, ${\mu }_{\text{H}}$.

Figure 5

Gibbs free energy, $\mathrm{\Delta }G$, for Eq. (1) and ${\mathrm{Fe}}_2{\mathrm{Si}}_2{\mathrm{O}}_9{\mathrm{H}}_n$ as a function of the chemical potential of hydrogen.

Figure 6

(a) FM and (b) G-AFM ordering considered for kaolinite structures. O and H atoms are omitted for clarity; only the transition metal (TM) atoms and Si are shown in gold and blue, respectively. Up and down spin directions are shown with green and magenta arrows, respectively. Distortions from the perfect honeycomb lattice are displayed also: longer TM bonds are shown in black, and shorter TM bonds are shown in red. The primitive cell of the structure is shown with black lines at the center of each figure.

Figure 7

Distortion of the triangular lattice in lizardites. Gold spheres show the TM atoms, blue spheres show Si atoms. O and H are omitted for clarity. Longer TM-TM bonds are shown using black, whereas the shorter TM-TM bonds are shown using red lines. The primitive cell of the structure is indicated using black lines in the center of the figure.