Role of orbital selectivity on crystal structures and electronic states in ${\mathrm{BiMnO}}_3$ and ${\mathrm{LaMnO}}_3$ perovskites

Correlated oxides, such as ${\mathrm{BiMnO}}_3$ and ${\mathrm{LaMnO}}_3$, show a complex interplay of electronic correlations and crystal structure exhibiting multiple first-order phase transitions, some without a clear order parameter. The quantitative theoretical description of this temperature-dependent electronic-structural interplay in the vicinity of a Mott transition is still a challenge. Here we address this issue by simultaneously considering both structural and electronic degrees of freedom, within a self-consistent density functional theory with embedded dynamical mean field theory. Our results show the existence of novel electronic states characterized by the coexistence of insulating, semimetallic, and metallic orbitals. This state is in agreement with resonant x-ray scattering. We also show that electronic entropy plays a decisive role in both electronic and structural phase transitions. By self-consistent determination of both the electronic state and the corresponding crystal structure, we show that the temperature evolution of these phases can be quantitatively explained from first principles, thus demonstrating the predictive power of the theoretical method for both the structural and the electronic properties.

Figure 1

Room temperature (RT) structural properties for ${\mathrm{LaMnO}}_3$ and ${\mathrm{BiMnO}}_3$. (a) Schematic view of the crystal structure for ${\mathrm{LaMnO}}_3$. The longest Mn-O bond lengths are colored by magenta. (b) The octahedra distortion pattern in the (101) plane. (c) Distortions in ${\mathrm{BiMnO}}_3$ in which ${\mathrm{Mn}}_1$ ions show similar pattern to that in ${\mathrm{LaMnO}}_3$, while in ${\mathrm{Mn}}_2$ ions the long-bond distortions are confined to (101) plane. (d) Schematic view of Bi environment within a perovskite-like crystal structure; the gray arrow, pointing toward the face of the ${\mathrm{Mn}}_2{\mathrm{O}}_6$ octahedra, shows the direction of the $6s^2$ lone-pair electronic cloud within the Bi ionic environment [21]. The presence of the $6s^2$ lone pair is responsible for the appearance of two inequivalent Mn sites. Drawings were produced by VESTA [22].

Figure 2

Temperature-dependent electronic states in ${\mathrm{LaMnO}}_3$. Panels (a) to (c) show for each inequivalent Mn octahedra the following properties: (1) values of the distortion indices, $\mathbf{D}$ and $\mathbf{S}$; (2) electronic entropy per Mn site; (3) schematic view of the symmetry-inequivalent Mn-O bonds represented by red horizontal bars [the horizontal size of the bar is proportional to the bond length, short (s), medium (m), long (l); in panel (c) the three bonds are slightly distorted around the average bond length]; (4) $e_g$ and $t_{2g}$ projected density of states per formula unit versus energy in eV, for each electronic phase at a given temperature. On top of each panel we give the crystallographic symmetry at a given temperature and a name characterizing the electronic properties of the predicted electronic phases. The panels are connected by arrows suggesting the order parameter of the phase transitions.

Figure 3

Temperature-dependent electronic states in ${\mathrm{BiMnO}}_3$. Panels (a) to (e) show for each inequivalent Mn octahedra the following properties: (1) values of the distortion indices, $\mathbf{D}$ and $\mathbf{S}$; (2) electronic entropy per Mn site; (3) schematic view of the symmetry-inequivalent Mn-O bonds represented by red horizontal bars [the horizontal size of the bar is proportional to the bond length, short (s), medium (m), long (l); in panel (e) the three bonds are slightly distorted around the average bond length]; (4) $e_g$ and $t_{2g}$ projected density of states per formula unit versus energy in eV, for each electronic phase at a given temperature. On top of each panel we give the crystallographic symmetry at a given temperature and a name characterizing the electronic properties of the predicted electronic phases. The panels are connected by arrows suggesting the order parameter of the phase transitions.

Figure 4

Spectral functions for various novel phases in (Bi,$\mathrm{La}){\mathrm{MnO}}_3$. For both compounds we give the spectral functions showing the metal-to-insulator phase transitions. In panels (a) and (b) we show the spectral functions for the band-Mott and orbital-selective Mott states for ${\mathrm{LaMnO}}_3$; in panels (c) and (d) we show the spectral functions of the band-Mott and site-orbital-selective Mott states for ${\mathrm{BiMnO}}_3$.

Figure 5

Structural relaxations within $\mathrm{LDA}+\mathrm{eDMFT}$ for all the temperature-dependent novel electronic phases in (Bi,$\mathrm{La}){\mathrm{MnO}}_3$. In each panel we plot the bond distortion ($\mathbf{D}$) index versus temperature, normalized to its values at room temperature. In panel (a) we plot $\mathbf{D}$ for the Mn site in ${\mathrm{LaMnO}}_3$. In panels (b) and (c) we plot $\mathbf{D}$ for the two inequivalent Mn sites in ${\mathrm{BiMnO}}_3$. Above each panel we give the temperature-dependent crystal symmetry and the properties of that electronic state (metal or insulator). The arrows point to the type of phase transition (structural or metal-insulator phase transition).