No abstract

Lead‐free double perovskites have attracted much attention as possible alternatives to lead halide based perovskites in photovoltaic applications. However, to date only few double perovskites have been successfully employed in optoelectronic device prototypes. Therefore, the search for stable and lead‐free materials is ongoing. Here, we present the successful growth of high‐quality Cs2NaFeCl6 single crystals and their temperature‐dependent structural and optical properties. By combining electron paramagnetic resonance (EPR), crystal structure analysis and density functional theory (DFT) we could determine a cubic crystal structure with a spin of 5/2 for this material, showing strongly spin polarized character. Furthermore, combining photoluminescence (PL) and optical absorption measurements we find a bandgap of approximately 2.1 eV at room temperature as well as the presence of excitonic states. Using Elliot's formula, we are able to extract the temperature‐dependent behavior of the bandgap as well as an estimated exciton binding energy of only 20 meV at 80 K.

By reacting Cs with Cs 2 O and elemental mercury the new double salt Cs 18 Hg 8 O 6 could be obtained. Its crystal structure (cubic, space group I23 with a = 13.3920(10) Å, Z = 2, R1 = 0.026/0.032 for I � 2σ(I)/all I, respectively) comprises isolated oxide anions in octahedral coordination by cesium cations next to the mercuride anion [Hg 8 ] 6À . This first isolated anion of mercury has cubic shape and is coordinated by cesium atoms capping the faces and the edges of the [Hg 8 ] 6À cube. The ionic character of the double salt is shown in DFT calculations of the electronic structure. Raman spectra show distinct features in the low energy region arising from the vibrations of the mercuride anion. Cs 18 Hg 8 O 6 has very close structural analogies to the thallide oxide Cs 18 Tl 8 O 6 .

In intermetallic chemistry, the Gd14Ag51 structure type is rather common and has many amalgam representatives. Up to today, binary amalgams of this type have been described for M = Na, Ca, Sr, Eu, Yb, and the structure family still is growing. Yb11Hg54 is the only representative with a fully ordered crystal structure, and all other representatives exhibit individual disorder phenomena or patterns. The diversity of disorder phenomena in this structural family is unique. In order to shed a light on the underlying reasons for this unexpected structural complexity, we compare the available literature structure models with three new ternary variants, Yb10.7Sr0.3Hg54, Ca4.5Eu6.5Hg54 and Ca6.9Na4.1Hg54 (all in space group type P 6 ‾ $P\overline{6}$ , a = 13.5379(12), 13.5406(8) and 13.564(5) Å, c = 9.7488(14), 9.7149 and 9.810(7) Å for Yb10.7Sr0.3Hg54, Ca4.5Eu6.5Hg54 and Ca6.9Na4.1Hg54, respectively). Their crystal structures have been examined in detail on the basis of both single crystal and powder X-ray diffraction data. Each of the three new amalgams exhibits its own set of disorder phenomena that is again different from those of the respective binary variants. The synopsis of the crystal structures and their individual disorder phenomena indicates that the reason for the disorder phenomena cannot be found only by analyzing geometric details such as atomic radii quotients or coordination polyhedral volumina, and additional electronic reasons must be assumed.