Crystallization of calcium carbonate, typically, progresses sequentially via metastable phases. Amorphous CaCO 3 (ACC) generally forms initially, both in vitro and in vivo, and is the precursor of the predominant anhydrous polymorphs (calcite, aragonite, and vaterite). [1][2][3][4][5][6][7][8][9][10][11][12][13] A new picture of the crystallization of calcium carbonate is emerging, which involves transformations of clusters to ACC and eventually to crystalline polymorphs. [14,15] This stepwise manner has implications for the understanding of biomineralization [16] and of crystallization. ACCs that contain additives display order over atomic length scales that are related to crystalline polymorphs; [1][2][3] ACC synthesized at high supersaturation levels without additives, [17][18][19][20] on the other hand, show no distinct short-range order. [21,22] Herein, we analyze proto-crystalline features of two amorphous intermediates, ACCI and ACCII, [15] and discuss their relevance for crystallization of CaCO 3 . We rationalize the identification of ACCI with pc-ACC (proto-calcite ACC) and ACCII with pv-ACC (proto-vaterite ACC), respectively. These ACCs were precipitated from metastable solutions of calcium carbonate at different pH values by destabilization in excess ethanol.TEM (Figure 1) reveals the ACCs as spherical particles with a diameter of approximately 20 nm. Small-angle X-ray scattering (SAXS) data support this characteristic size

Long carrier lifetime is what makes hybrid organic-inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.organic-inorganic hybrid perovskite | carrier lifetime | photoluminescence | polaron T he record efficiency of hybrid organic-inorganic perovskite (HOIP)-based solar cells has reached above 22% (1-4), which is comparable to that of silicon solar cells. The most dominant contribution to the high photovoltaic performance of HOIPs comes from their long carrier lifetimes (≥ 1 μs), which translates to large carrier diffusion lengths despite their modest charge mobilities (5). Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of ferroelectric domains (6-9), Rashba effect (10-12), photon recycling (13), and large polarons (14-16). When the HOIPs are replaced with all inorganic perovskites in the solar cell architecture, the device can still function as a solar cell. This indicates that the photons excite electrons and holes out of the inorganic metal halide atoms, which is consistent with the density functional theory (DFT) calculations that the corner interstitial cations, whether organic or inorganic, do not directly contribute to the band-edge states (17). However, the efficiency of the purely inorganic perovskites is currently at ∼11% (18-20), which is far below 22% of HOIP-based solar cells. This suggests that the presence of organic cation may be the key for achieving high solar cell efficiency. It is, however, yet to be understood how the organic cations enhance the efficiency.Among the aforementioned microscopic mechanisms, three are based on the role of organic cations. First, in the ferroelectric domain theory, nanoscale ferroelectric domains are formed due to alignment of organic cations (6-9). Such domains can spatially separate the photoexcited electron and holes and thereby reduce their recombination. Second, in the Rashba effect theory (10-12), the spin and orbit degrees of freedom of the inorganic atoms are coupled with the electric field generated by the organic cations. This results in the electronic band splitting for different spins and leads to an effectively in...

Crystallization of calcium carbonate, typically, progresses sequentially via metastable phases. Amorphous CaCO 3 (ACC) generally forms initially, both in vitro and in vivo, and is the precursor of the predominant anhydrous polymorphs (calcite, aragonite, and vaterite). [1][2][3][4][5][6][7][8][9][10][11][12][13] A new picture of the crystallization of calcium carbonate is emerging, which involves transformations of clusters to ACC and eventually to crystalline polymorphs. [14,15] This stepwise manner has implications for the understanding of biomineralization [16] and of crystallization. ACCs that contain additives display order over atomic length scales that are related to crystalline polymorphs; [1][2][3] ACC synthesized at high supersaturation levels without additives, [17][18][19][20] on the other hand, show no distinct short-range order. [21,22] Herein, we analyze proto-crystalline features of two amorphous intermediates, ACCI and ACCII, [15] and discuss their relevance for crystallization of CaCO 3 . We rationalize the identification of ACCI with pc-ACC (proto-calcite ACC) and ACCII with pv-ACC (proto-vaterite ACC), respectively. These ACCs were precipitated from metastable solutions of calcium carbonate at different pH values by destabilization in excess ethanol. TEM (Figure 1) reveals the ACCs as spherical particles with a diameter of approximately 20 nm. Small-angle X-ray scattering (SAXS) data support this characteristic size Figure 1. TEM images of pc-ACC and pv-ACC at various magnifications. Insets in (b) and (e) are selective area electron diffraction (SAED) patterns obtained from an area slightly larger than the particular image sections, SAED scale bars: 5 nm À1 . SAED patterns are shown as negatives to make weak features clear. Arrows in (f) indicate nanostructural features.

Nickel-rich layered transition metal oxides, LiNi (MnCo) O (1-x ≥ 0.5), are appealing candidates for cathodes in next-generation lithium-ion batteries (LIBs) for electric vehicles and other large-scale applications, due to their high capacity and low cost. However, synthetic control of the structural ordering in such a complex quaternary system has been a great challenge, especially in the presence of high Ni content. Herein, synthesis reactions for preparing layered LiNi Mn Co O (NMC71515) by solid-state methods are investigated through a combination of time-resolved in situ high-energy X-ray diffraction and absorption spectroscopy measurements. The real-time observation reveals a strong temperature dependence of the kinetics of cationic ordering in NMC71515 as a result of thermal-driven oxidation of transition metals and lithium/oxygen loss that concomitantly occur during heat treatment. Through synthetic control of the kinetic reaction pathway, a layered NMC71515 with low cationic disordering and a high reversible capacity is prepared in air. The findings may help to pave the way for designing high-Ni layered oxide cathodes for LIBs.

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