Seven-Coordinated High-Pressure Phase of CrSb2 and Experimental Equation of State of MSb2 (M = Cr, Fe, Ru, Os).

The MSb2 compounds with M = Cr, Fe, Ru, and Os have been investigated under high pressures by synchrotron powder X-ray diffraction. All compounds, except CrSb2, were found to retain the marcasite structure up to the highest pressures (more than 50 GPa). In contrast, we found that CrSb2 has a structural phase transition around 10 GPa to a metastable, MoP2-type structure with Cr coordinated to seven Sb atoms. In addition, we compared ambient temperature compression with laser-heating experiments and found that laser-heating at pressures below and above this phase transition results in the known CuAl2-type structure. Density functional theory calculations show that this tetragonal structure is the most stable in the whole pressure interval. However, a crossing of the marcasite's and MoP2-like structure's enthalpies occurs between 5 and 7.5 GPa, which is in good agreement with the experimental data. The phase transition to the MoP2-type structure observed in this work opens up for discovering other compounds with this new transition pathway from the marcasite structure.

High-Pressure, High-Temperature Studies of Phase Transitions in SrOsO3─Discovery of a Post-Perovskite.

Using a recently developed method for in situ high-pressure, laser heating experiments in diamond anvil cells, we obtained a novel post-perovskite phase of SrOsO3. The phase transition from perovskite SrOsO3 was induced at 44 GPa and 1350 K in a diamond anvil cell and characterized with synchrotron powder X-ray diffraction. The newly obtained post-perovskite is quenchable and Le Bail refinements under ambient conditions yielded the unit cell parameters: a = 3.152(9) Å, b = 10.82(2) Å, c = 7.27(1) Å, V = 248.1(1) Å3. In addition, the compression of perovskite SrOsO3 at ambient temperature was investigated up to 66 GPa in a diamond anvil cell using synchrotron powder X-ray diffraction. The compression at ambient temperature showed that pressure alone does not induce the first-order phase transition to the post-perovskite structure. However, at 36 GPa, a continuous phase transition to monoclinic (P21/n) symmetry was detected, persistent up to 58 GPa, where the perovskite transitioned back to orthorhombic (Pbnm) symmetry. Fitting a third-order Birch-Murnaghan equation of state to the obtained P-V data for perovskite SrOsO3 yielded a bulk modulus of K0 = 187.4(15) GPa. Density functional theory calculations were performed to support the experimental findings in the compression study at ambient temperature. This work shows that transformations to the post-perovskite structure can be obtained for a wider range of perovskites than simple empirical rules otherwise suggest.

Discovery of Rhombohedral NaIrO3 Polymorph by In Situ High-Pressure Synthesis of High-Oxidation-State Materials Using Laser Heating in Diamond Anvil Cells.

We report a new in situ synthesis method effective for discovery of high-oxidation-state materials using laser-heated diamond anvil cells. The issue of chemical reduction during thermally induced phase transitions that occur spontaneously in a noble gas pressure transmitting media (PTM) can be overcome by thermal decomposition of an oxygen-rich solid PTM (NaCl + NaClO3). To illustrate the technical challenges the method overcomes, we applied this new method for two known pentavalent A(I)B(V)O3 postperovskite compounds. We successfully synthesized the two postperovskites, NaOsO3 and NaIrO3, and quenched to ambient conditions. Furthermore, we report the discovery of a new low-pressure polymorph of NaIrO3, illustrating the high potential for new materials discovery. This new method will enable realization of new high-oxidation-state postperovskites and can be applied for many other structure families in a P, T parameter space that is not easily accessible using conventional high-pressure synthesis methods.

Deciphering the Role of Water in Promoting the Optoelectronic Performance of Surface-Engineered Lead Halide Perovskite Nanocrystals.

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield. Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr3) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH4Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH4+, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

Tuning the Structural and Optoelectronic Properties of Cs2 AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution.

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs2 AgBiBr6 and alkali-metal-substituted (Cs1- x Yx )2 AgBiBr6 (Y: Rb+ , K+ , Na+ ; x = 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.

Tracking Structural Phase Transitions in Lead-Halide Perovskites by Means of Thermal Expansion.

The extraordinary properties of lead-halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX3 (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high-resolution capacitive dilatometer to determine the phase transition temperatures. For δ-FAPbI3 , two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr3 that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X-ray diffraction. Moreover, it is demonstrated that transport under steady-state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead-halide perovskite materials.