Emergent physical properties of perovskite-type oxides prepared under high pressure

Investigating the potential of triclinic ABSe3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) perovskites as a new class of lead-free photovoltaic materials

In recent years, chalcogenide perovskites have emerged as promising candidates with favorable structural, electrical, and optical properties for photovoltaic applications. This paper explores the structural, electronic, and optical characteristics of ABSe3 perovskites (where A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) in their triclinic crystallographic phases using density functional theory. The stability of these materials is ensured by calculating formation energies, tolerance factors (Tf), and phonon dispersion. The Eform values of all ABSe3 are negative, suggesting favorable thermodynamic stability. The Tf values range between 0.82 and 1.1, which is consistent with stable perovskites. The phonon dispersion analysis of the chalcogenide perovskites revealed no imaginary frequencies in any of the vibrational modes, confirming their stability. The electronic band structures and corresponding density of states are computed to unveil the semiconducting nature of the studied compounds. These perovskites are promising for high-performance solar cells due to their indirect bandgaps (Eg, 1.10-2.33 eV) and a small difference between these indirect and direct gaps (0.149-0.493 eV). The Eg values increase as the ionic radii of A-site elements increase (Li < Na < K < Rb < Cs). At the B-site, Si-based chalcogenides have the largest Eg values, followed by Sn-based and then Ge-based materials. Furthermore, optical properties such as the real part and imaginary part of the dielectric function, refractive index extinction coefficient, optical conductivity, absorption coefficient, reflectivity, and energy loss are predicted within the energy range of 0-50 eV. Several ABSe3 materials, particularly LiGeSe3 and NaGeSe3, demonstrated optical properties comparable to both traditional and emerging materials, suggesting their potential for effective use in solar cells.

1 : 1 Ca2+ :Cu2+ A-site Order in a Ferrimagnetic Double Double Perovskite

Cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO6 with the tetragonal AA'BB'O6 double double perovskite structure that was previously only reported for A'=Mn compositions. CaCuFeReO6 occurs in the same phase field as CaCu3 Fe2 Re2 O12 demonstrating that different A-cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO6 has ferrimagnetic order of Fe, Re and Cu spins below TC =567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO6 displays low-field "butterfly" hysteresis with an unusual change from negative to positive values as field increases. Many more AA'BB'O6 double double perovskites may be accessible for A'=Cu and other divalent transition metals at high pressure, so the presently known phases likely represent only the "tip of the iceberg" for this family.