Lead-free magnetic double perovskites for photovoltaic and photocatalysis applications

The magnetic spin degrees of freedom in magnetic materials serve as an additional way to tune materials properties, thereby invoking a magneto-optical response. Herein, we report the magneto-optoelectronic properties of a family of lead-free magnetic double perovskites of the form ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ ($T=\mathrm{Sc},\mathrm{Ti},\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni},\mathrm{Cu};X=\mathrm{Cl},\mathrm{Br},\mathrm{I}$). These provide an extremely fertile series, giving rise to potential candidate materials for photovoltaic applications. In conjunction with a high absorption coefficient and a high simulated power-conversion efficiency for photovoltaic applications, a few compounds in this series exhibit magnetic character useful for spintronic applications. The interaction between magnetism and light can have far-reaching effects on the photovoltaic properties as a consequence of the shift in the defect energy levels due to the Zeeman effect. This subsequently affects the recombination rate of minority carriers, and hence the photoconversion efficiency. Moreover, the distinct ferromagnetic and antiferromagnetic ordering driven by hybridization and the superexchange mechanism can play a significant role in breaking the time-reversal and/or inversion symmetry. Such a coalescence of magnetism and efficient optoelectronic response has the potential to trigger a magnetic/spin anomalous photovoltaic (nonlinear optical) effect in this ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ family. These insights can thus channelize the advancement of lead-free double perovskites in the magnetic/spin anomalous-photovoltaic-effect field as well.

Figure 1

Crystal structure of ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ (where $T$ is a transition element) in (a) the cubic and (b) hexagonal phases. FM and AFM indicate ferromagnetic and antiferromagnetic ordering, respectively.

Figure 2

Formation energies ($\mathrm{\Delta }{E}_F$) of ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ ($T=\mathrm{Sc},\mathrm{Ti},\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni},\mathrm{Cu}$; $X=\mathrm{Cl},\mathrm{Br},\mathrm{I}$) in two different structures [cubic and hexagonal (hex)] and three different magnetic phases (NM, FM, and AFM). An asterisk indicates the energetically stable phase for each compound.

Figure 3

Chemical phase diagrams of (a) ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Fe}\mathrm{Cl}}_6$ and (b) ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Cr}\mathrm{Cl}}_6$. The gray area indicates the extent of the stability region for the target systems, whereas the cyan area represents secondary phases.

Figure 4

Spin-resolved band structure and PDOS for (a) cubic AFM ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Fe}\mathrm{Cl}}_6$ and (b) hexagonal FM ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Cr}\mathrm{Cl}}_6$. The former is an AFM semiconductor with a band gap of 1.17 eV, while the latter is a FM semiconductor with band gaps of 3.51 and 1.69 eV for spin-down and spin-up channels, respectively.

Figure 5

Absorption coefficient (${\alpha }_{ii}$) in the in-plane direction and the out-of-plane direction for selected (a),(b) nonmagnetic, (c),(d) antiferromagnetic, and (e),(f) ferromagnetic systems. SLME of the same set of (g) nonmagnetic, (h) antiferromagnetic, and (i) ferromagnetic systems.