Chemical Control of Spin‐Orbit Coupling and Charge Transfer in Vacancy‐Ordered Ruthenium(IV) Halide Perovskites

Abstract

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru IV halides are presented; A 2 RuCl 6 and A 2 RuBr 6 , where A is K, NH 4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with J eff = 0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion. Remarkable developments in perovskite-based photovoltaics over the last decade have driven the discovery of a wide range of new halide perovskites and related solids. [1-4] These include 3D perovskites with different divalent metals (i.e., Pb II and Sn II), [5, 6] 3D double perovskites with a combination of univalent and trivalent metals (i.e., K I /Bi III and Ag I / Bi III), [7, 8] as well as low dimensional 2D, [9] and 1D perovskites. [10] Progress in this area has also rekindled interest in K 2 Pt IV Cl 6-type vacancy-ordered double perovskites, comprising isolated metal halide octahedra interspersed with mono-valent cation. For example, the vacancy-ordered halides of Sn IV , [11] Se IV , [12] Te IV , [13, 14] and Ti IV , [15] have shown to be promising photovoltaic materials. The analogous variants of

Vacancy‐Ordered Double Perovskite Rb2ZrCl6−xBrx: Facile Synthesis and Insight into Efficient Intrinsic Self‐Trapped Emission

“…The crystal unit of vacancy-ordered perovskites is very similar to that of the double perovskites, formed by doubling the conventional perovskite ABX 3 unit cell along all three crystallographic axes, subsequently removing every other B site cations. [41][42][43][44] Some efforts have been made to fabricate vacancy-ordered double perovskites in the past few years. For example, solvent-thermal method was adopted to fabricate Cs 2 ZrCl 6 (need 180 °C for 10 h), [31] and Cs 2 ZrX 6 (X = Cl, Br) nanocrystals were also got by using strict air-free Schlenk line technique.…”

A review on the sensing mechanisms and recent developments on metal halide-based perovskite gas sensors

“…The magnetic moments of A 2 RuCl 6 and A 2 RuBr 6 (where A is K, NH 4 , Rb or Cs) depend on the temperature owing to their non-magnetic ground state with J eff = 0 caused by spin–orbit coupling. 76 The A 2 RuX 6 compounds follow d 4 Kotani behavior, with a drastic drop in their effective magnetic moments at low temperatures. In iron-alloyed double perovskites, Cs 2 Ag(Bi:Fe)Br 6 shows a strongly temperature-dependent magnetic response at temperatures below 30 K, 77 which is ascribed to the weak ferromagnetic or antiferromagnetic response arising from their confined regions.…”

Acid‐ and Base‐Stable Cs2Pt(Cl,Br)6 Vacancy‐Ordered Double Perovskites and Their Core–Shell Heterostructures for Solar Water Oxidation

“…þ ; B-site by tetravalent cation like Sn 4þ , [26] Pt 4þ , [27] Pd 4þ , [28] Ti 4þ , [29] and Ru 4þ , [30] and X-site by halides (Cl À , Br À , I À ). As the name implies, this class of perovskite has a vacant position between octahedra, allowing rotational freedom for the isolated [BX 6 ] 2À octahedra; enabling a wider range of elements for perovskite formation.…”

“…Vacancy‐ordered double perovskites with the formula A 2 BX 6 have recently received a lot of attention in the perovskite community due to their excellent stability, good absorption coefficient, panchromatic absorption, and compositional tunability making them a promising alternative for the conventional ABX 3 perovskite. Here, A site is occupied by a monovalent cation predominantly Cs + , Rb + , K + or organic cations like CH 3 NH 3 + or CH(NH 2 ) 2 + ; B‐site by tetravalent cation like Sn 4+ , [ 26 ] Pt 4+ , [ 27 ] Pd 4+ , [ 28 ] Ti 4+ , [ 29 ] and Ru 4+ , [ 30 ] and X‐site by halides (Cl − , Br − , I − ). As the name implies, this class of perovskite has a vacant position between octahedra, allowing rotational freedom for the isolated [BX 6 ] 2− octahedra; enabling a wider range of elements for perovskite formation.…”