We prepare colloidal nanoplatelets of methylammonium lead bromide (MAPbBr3) perovskite and compare the optical signatures of excitons in these two-dimensional systems to spherical perovskite nanocrystals and the corresponding bulk phase. We find that excitonic features that had previously been attributed to quantum confinement in MAPbBr3 nanocrystals are in fact a property of the bulk perovskite phase. Furthermore, we find that higher-energy absorption features originate from two-dimensional nanoplatelets, which are present in the nanocrystal reaction product. Upon further purification, we obtain colloidal nanoplatelets with predominantly single unit cell thickness and submicron lateral dimensions, which are stable in solution and exhibit a sharp excitonic absorption feature 0.5 eV blue-shifted from that of the three-dimensional bulk MAPbBr3 phase, representing a new addition to the growing family of colloidal two-dimensional nanostructures.

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SignificanceEarth's outer core is composed of a liquid iron alloy with up to 10% of unknown light elements, likely silicon, oxygen, sulfur, carbon, or hydrogen. The release of these light elements upon freezing of the solid iron inner core plays an important role in sustaining Earth's magnetic field, but the exact chemical makeup of the core is widely debated. In this paper, we perform high-pressure, high-temperature melting experiments and first-principles simulations on iron alloys containing silicon and oxygen and find that two distinct liquids form at high pressures. The presence of immiscible liquids may explain a seismically observed stratified layer atop the outer core and suggests that an Fe-Si-O composition can explain multiple observations of the outer core.

Laser-heated diamond-anvil cell (LHDAC) experiments reveal electronic changes in KBr at pressures between ~13-81 GPa when heated to high temperatures that cause runaway heating to temperatures in excess of ~5000 K. The drastic changes in absorption behavior of KBr are interpreted as rapid formation of high-pressure F-center defects. The defects are localized to the heated region and thus do not change the long-range crystalline order of KBr. The results have significant consequences for temperature measurements in LHDAC experiments and extend the persistence of F-centers in alkali halides to at least 81 GPa.

The melting curve of elemental sulfur was measured to pressures of 65 GPa in a laser-heated diamond-anvil cell using ex-situ textural analyses combined with spectroradiometry and benchmarked with laser-power-temperature functions. The melting curve reaches temperatures of ∼1800 K by 65 GPa and is smooth in the range of 23-65 GPa with a Clapeyron slope of ∼14 K/GPa at 23 GPa. This is consistent with melting of a single tetragonal sulfur structure in this range, which is confirmed by in-situ x-ray diffraction. An updated equation of state for tetragonal sulfur is determined, and the high-pressure, high-temperature stability region of tetragonal sulfur is reassessed.