Effects of chemical and hydrostatic pressures on structural, magnetic, and electronic properties of $R_2\mathrm{NiMn}{\mathrm{O}}_6$ $\left(R=\mathrm{rare}\text{−}\mathrm{earth}\mathrm{ion}\right)$ double perovskites

The effects of chemical and hydrostatic pressures on structural, magnetic, and electronic properties of $R_2\mathrm{NiMn}{\mathrm{O}}_6$ double perovskites, with $R$ being a rare-earth ion, have been systematically studied by using specific first-principles calculations. These latter reproduce well the correlation between several properties (e.g., lattice parameters, Ni-O-Mn bond angles, magnetic Curie temperature, and electronic band gap) and the rare-earth ionic radius (i.e., the chemical pressure). They also provide novel predictions awaiting experimental confirmation, such as (i) that many physical quantities respond in dramatically different manners to chemical versus hydrostatic pressure, unlike as commonly thought for perovskites containing rare-earth ions, and (ii) a dependence of antipolar displacements on chemical and hydrostatic pressures, which would further explain why the recently predicted electrical polarization of $\mathrm{L}{\mathrm{a}}_2\mathrm{NiMn}{\mathrm{O}}_6\text{/}{R}_2\mathrm{NiMn}{\mathrm{O}}_6$ superlattices [H. J. Zhao, W. Ren, Y. Yang, J. Íñiguez, X. M. Chen, and L. Bellaiche, Nat. Commun. 5, 4021 (2014)] can be created and controlled by playing with the rare-earth element.

Figure 1

Structural properties of $R_2\mathrm{NiMn}{\mathrm{O}}_6$ double perovskite oxides as a function of chemical pressure (i.e., the rare-earth ionic radius, $r$_R). (a) Lattice parameters $a,b,c$ and monoclinic angle β; (b) Ni-O and Mn-O bond lengths; (c) the average antiphase tilting ${\omega }_R$ (about the pseudocubic [100] or [010] direction), in-phase tilting ${\omega }_M$ (about the pseudocubic [001] direction), and in-plane and out-of-plane Ni-O-Mn bond angles; (d) antipolar displacement of the $R$ ions. Panels (e) and (f) schematize the patterns associated with the antipolar displacements discussed in the text, with the pseudocubic [100], [010], and [001] directions also indicated there. The light blue and purple colors are used to represent $\mathrm{Ni}{\mathrm{O}}_6$ and $\mathrm{Mn}{\mathrm{O}}_6$ octahedrons, respectively. The AFE displacements associated with the $R$ point (e) and $X$ point (f) of the cubic first Brillouin zone are along the pseudocubic $\left[1\overline{1}0\right]$ direction (which is the $a$ axis) and pseudocubic [110] direction (which is the $b$ axis), respectively.

Figure 2

Magnetic Curie temperature, $T$_C, of $R_2\mathrm{NiMn}{\mathrm{O}}_6$ double perovskite oxides as a function of chemical pressure calculated via the PBEsol and PBE functionals for different effective Hubbard $U$ values for Ni and Mn ions $(U_{\mathrm{Ni}},U_{\mathrm{Mn}})$. The inset shows $T$_C as a function of the mean value of $\mathrm{co}{\mathrm{s}}^2\theta$, with θ being the Ni-O-Mn bond angle, for the PBEsol functional with $\left(U_{\mathrm{Ni}}=3\mathrm{eV},U_{\mathrm{Mn}}=3\mathrm{eV}\right)$.

Figure 3

Similar to Fig. 1, but for $\mathrm{S}{\mathrm{m}}_2\mathrm{NiMn}{\mathrm{O}}_6$ double perovskite as a function of hydrostatic pressure.

Figure 4

Similar to Fig. 2, but for $\mathrm{S}{\mathrm{m}}_2\mathrm{NiMn}{\mathrm{O}}_6$ double perovskite as a function of hydrostatic pressure.

Figure 5

Calculated DOS of $\mathrm{S}{\mathrm{m}}_2\mathrm{NiMn}{\mathrm{O}}_6$ (0 kbar, top panel), $\mathrm{E}{\mathrm{r}}_2\mathrm{NiMn}{\mathrm{O}}_6$ (0 kbar, middle panel), and $\mathrm{S}{\mathrm{m}}_2\mathrm{NiMn}{\mathrm{O}}_6$ (110 kbar, bottom panel), respectively. The total DOS are shown by means of black lines, while the contribution of Ni $3d$ and Mn $3d$ bands are represented via green and purple lines, respectively. For each panel, the upper part shows the spin-up channel, and the bottom part depicts the results for the spin-down channel.

Figure 6

Band gap of (a) $R_2\mathrm{NiMn}{\mathrm{O}}_6$ as a function of chemical pressure and of (b) $\mathrm{S}{\mathrm{m}}_2\mathrm{NiMn}{\mathrm{O}}_6$ as a function of hydrostatic pressure.