Spin-crossover-induced Mott transition and the other scenarios of metallization in $3d^n$ metal compounds

A different “Hubbard energy control” mechanism of the insulator-metal transition (IMT) in Mott-Hubbard insulators is discussed. This mechanism can be initiated by the lattice compression and it is driven by a spin crossover of $3d^5$ ions from the high-spin state to the low-spin state. The spin crossover suppresses the effective Hubbard parameter $U_{\text{eff}}$ down to the value enabling the insulator-metal transition according to the Mott mechanism $U_{\text{eff}}/W\approx 1$. The classification of possible scenarios of metallization in the other $3d^n$ metal compounds is also performed.

Figure 1

(Color online) The pressure dependences of the optical gap in selected $3d^5$ systems. At the HS-LS spin transitions of ${\text{Fe}}^{3+}$ ions around 50 GPa, the optical gap in ${\text{FeBO}}_3$, ${\text{GdFe}}_3{\left({\text{BO}}_3\right)}_4$, ${\text{LuFeO}}_3$, and ${\text{NdFeO}}_3$ decreases dramatically to below 1 eV range, typical of semiconductors. In the same pressure range, the optical gaps in ${\text{BiFeO}}_3$ and ${\text{Y}}_3{\text{Fe}}_5{\text{O}}_{12}$ reveal a drop to nearly zero value indicating possible metallization.

Figure 2

(Color online) The pressure effect on resistance is shown in (a) ${\text{FeBO}}_3$ (Ref. 19), (b) ${\text{LaFeO}}_3$ (Ref. 13), and (c) ${\text{BiFeO}}_3$ (Ref. 1) near the HS-LS spin-crossover transitions in ${\text{Fe}}^{3+}$ ions. The insets show the temperature dependence of resistance at several pressures. At the spin transition, ${\text{FeBO}}_3$ and ${\text{LaFeO}}_3$ reveal dramatic drop of resistance and transform to a semiconducting state, while in ${\text{BiFeO}}_3$ the insulator-metal transition is observed.

Figure 3

(Color online) The spin-crossover transition in ${\text{Fe}}^{3+}$ ions is evidenced by the pressure behavior of magnetic hyperfine fields at ${}^{57}\text{F}\text{e}$ nuclei obtained from conventional and synchrotron Mössbauer technique in several materials: (a) ${\text{FeBO}}_3$ (Ref. 6), $\alpha {\text{-Fe}}_2{\text{O}}_3$, (b) ${\text{LaFeO}}_3$ (Ref. 10), ${\text{PrFeO}}_3$ (Ref. 13), (c) ${\text{Y}}_3{\text{Fe}}_5{\text{O}}_{12}$ (Ref. 16), ${\text{BiFeO}}_3$ (Refs. 1, 18). The inset in (c) shows the HS-LS spin crossover probed by x-ray ${\text{Fe-K}}_{\beta }$ emission spectroscopy in ${\text{BiFeO}}_3$ (Ref. 1).

Figure 4

(Color online) (a) A diagram elucidating the Hubbard–energy control mechanism of the Mott-Hubbard transition. The dependencies of $U_{\text{eff}}$ (solid red lines) and $W$ (dashed blue lines labeled 1, 2, and 3) on pressure are drawn for the $d^5$ configuration taking into account the spin crossover effects. Dashed green horizontal line corresponds to the constant level $U_0$ when $U_{\text{eff}}$ is independent of $P$. On the $U_{\text{eff}}\left(P\right)$ curve, the points $P_1^{\text{cross}}$ and $P_2^{\text{cross}}$ correspond to the spin crossover for $d^6$ and for $d^4$, $d^5$ configurations, respectively, while the crosses of the $U_{\text{eff}}\left(P\right)$ and $W\left(P\right)$ lines at the $P_1$, $P_2$, and $P_4$ points correspond to IMT. Three different $W\left(P\right)$ lines 1, 2, and 3 correspond to different dependence of $W$ on pressure $\left({\alpha }_{W1}>{\alpha }_{W2}>{\alpha }_{W3}\right)$. Points $P_1$, $P_2$, and $P_4$ are the pressures of the insulator-metal Mott-Hubbard transitions in the Hubbard–energy control mechanism. (b) The types of pressure dependencies of the band gap $E_g$ as a result of different scenarios from the theoretical model in part (a). The case of ${\text{BiFeO}}_3$ is indicated by an arrow. The inset shows the experimental examples of pressure dependence of the gap for ${\text{BiFeO}}_3$ and ${\text{FeBO}}_3$ [filled points correspond to the thermoactivating gap from resistivity measurements (Ref. 19); open symbols correspond to the optical gap from optical absorption]. The case of ${\text{FeBO}}_3$ is a good example of scenario 3 from part (a).