The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

Abstract

A-Site doping with alkali ions, and/or metal substitution at the B and B ′-sites, are among the key strategies in the innovative development of A 2 BB ′ X 6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2 AgRhCl 6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2 AgRhCl 6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs 2 AgBiX 6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2 AgRhCl 6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2 AgRhCl 6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs 2 AgRhCl 6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable. Keywords: A 2 AgRhCl 6 halide double perovskites, first-principles studies, optoelectronic properties, geometrical, dynamical and mechanical stabilities, DOS and band structures Varadwaj and Marques Stable Halide Double Perovskites

Hybrid Organic–Inorganic Halide Derivatives of the 2H Hexagonal Perovskite Structure

“…Their results predict direct band gap transitions with band gap energies ranging from 0.58 eV for Cs 2 AgRhCl 6 to 0.66 eV for Li 2 AgRhCl 6 . Band structure and density of states (DOS) calculations suggest the valence band maximum and conduction band minimum arise from a mixture of Rh 4d and Cl 3p orbitals . Of these compositions, Cs 2 AgRhCl 6 is the only one that is calculated to be thermodynamically stable.…”

“…Their calculations show significantly greater dispersion in the Rh e g σ* conduction bands, suggesting a greater degree of hybridization between the two metal sites. 7 Increased bandwidth is generally attractive for photovoltaic applications, and it would be interesting to experimentally compare the properties of a cubic double perovskite like Cs 2 AgRhCl 6 if this composition can be synthetically realized. This raises an interesting question.…”

“…Band structure and density of states (DOS) calculations suggest the valence band maximum and conduction band minimum arise from a mixture of Rh 4d and Cl 3p orbitals. 7 Of these compositions, Cs 2 AgRhCl 6 is the only one that is calculated to be thermodynamically stable. A similar study of the analogous bromide compositions yielded similar conclusions, with Cs 2 AgRhBr 6 predicted to be stable.…”

Structural stabilities, mechanical and thermodynamic properties of chalcogenide perovskite ABS3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) from first-principles study

“…The valance state of cation and anion are used to categorize perovskite. There are three categories of simple compounds: A +2 B +4 X 3 (2,4), A +3 B +3 X 3 (3,3), and A +1 B +5 X 3 (1,5). The cubic perovskite structure belongs to Pm-3 m crystallography space group.…”

“…Based on the analysis of tolerance factor value, in the range of 0.813 < t < 1.107 indicates ideal cubic structures or a distorted perovskite structure with tilted octahedra. Structures having t values outside the range cannot be considered stable perovskites [3][4][5]. The mechanism of the distortion from ideal cubic perovskite can be classified into five versions [6]: (i) distortion of the BX 6 octahedron, e.g., by the Jahn-Teller effect, (ii) off-center displacement of the B cations in the BX 6 octahedron, this effect is the basis for possible ferroelectricity, (iii) so-called tilting of the octahedron framework, usually occurring as a result of a too-small A cation at the cuboctahedral site, (iv) ordering of more than one kind of cations A or B, or vacancies, and (v) formation of oxygen vacancies.…”