Design of Heteroanionic MoON Exhibiting a Peierls Metal-Insulator Transition

Using a first-principles approach, we design the heteroanionic oxynitride MoON to exhibit a first-order isosymmetric thermally activated Peierls-type metal-insulator transition (MIT). We identify a ground state insulating phase ($\alpha$-MoON) with monoclinic $Pc$ symmetry and a metastable high temperature metallic phase ($\beta$-MoON) of equivalent symmetry. We find that ordered fac-$\mathrm{Mo}{\mathrm{O}}_3{\mathrm{N}}_3$ octahedra with edge and corner connectivity stabilize the twisted Mo-Mo dimers present in the $\alpha$ phase, which activate the MIT through electron localization within the $4d$ $a_{1g}$ manifold. By analyzing the temperature dependence of the soft zone-boundary instability driving the MIT, we estimate an ordering temperature $T_{\mathrm{MIT}}\sim 900\mathrm{K}$. Our work shows that electronic transitions can be designed by exploiting multiple anions, and heteroanionic materials could offer new insights into the microscopic electron-lattice interactions governing unresolved transitions in homoanionic oxides.

Figure 1

(a) Total densities-of-states (DOS) for homoanionic ${\mathrm{MoO}}_2$ and ${\mathrm{MoN}}_2$ in the (inset) rutile and monoclinic structures. Orbital resolved DOS for (b) monoclinic ${\mathrm{MoO}}_2$, indicating metallicity persists despite Mo-Mo dimerization and splitting of the $4d$ orbital symmetries, and (c) rutile ${\mathrm{MoN}}_2$.

Figure 2

(a) Energies for competing polymorphs of MoON relative to the ground state $\alpha$-MoON (starred). $\beta$-MoON is the next lowest-energy polymorph (magenta bar). In all cases, the lowest energy anion order is represented. (inset) Box and whisker plot showing the distribution of energies with anion configuration. The fac-$\mathrm{Mo}{\mathrm{O}}_3{\mathrm{N}}_3$ octahedra consist of O (red) and N (yellow) anions on opposite faces. Mo-O and Mo-N bond lengths are given (in units of Å). (b) A slice of the $\alpha$- and $\beta$-MoON structures, illustrating the edge and corner connectivity of the $\mathrm{Mo}{\mathrm{O}}_3{\mathrm{N}}_3$ octahedra, with the Mo-Mo distances and dimer twist angle ($\phi$) defined. Orange arrows indicate the direction of the Mo displacements across the $\beta \to \alpha$ transition. (Anion displacements omitted for clarity). (c) Total (gray) and atom-projected DOS of $\alpha$-MoON.

Figure 3

(a) Phonon dispersion curves of $\beta$-MoON at 0 K. (b) The filled (open) symbols indicate the normalized energy gain with $Y$-mode eigendisplacement amplitude (combined with the instability near $\mathrm{\Gamma }$). (c) Electronic band structure of the $\beta$-MoON phase. (d)–(f) Evolution in the $\beta$-MoON band dispersions as a function of the Mo-Mo distances (Å) and twisting distortions ($\phi$) exhibited by the $\alpha$-MoON phase. (g) Electronic band structure for the $\alpha$-MoON phase. Note that the structure used in (e) corresponds to the energy minimum of ${\nu }_1$ (filled symbols) in (c). The Fermi level is set to 0 eV.

Figure 4

(a) Finite temperature phonon dispersion curves of $\beta$-MoON. (b) Radial distribution function $g(r)$ obtained from our finite temperature AIMD calculations. The short Mo-Mo distances in the $\alpha$ and $\beta$ phases are indicated by vertical lines.