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[Display omitted] Theoretical assessment of Cl-excessive, mixed-halide grain boundaries in hybrid perovskite thin films demonstrates that ordered 3D/2D interfaces along perovskite (100) crystallographic planes are the most energetically favored among other possible cases. [ABSTRACT FROM AUTHOR]

TmMgB5O10 spontaneous crystals were synthesized via the flux-growth technique from a K2Mo3O10-based solvent. The crystal structure of the compound was solved and refined within the space group P21/n. The first principles calculations of the electronic structure reveal that TmMg-pentaborate with an ideal not defected crystal structure is an insulator with an indirect energy band gap of approximately 6.37 eV. Differential scanning calorimetry measurements and powder X-ray diffraction studies of heat-treated solids show that TmMgB5O10 is an incongruent melting compound. A characteristic band of the Tm3+ cation corresponding to the 3H6 → 1D2 transition is observed in the photoluminescence excitation spectra of TmMg-borate. The as-obtained crystals exhibit intense blue emission with the emission peaks centered at 455, 479, 667, and 753 nm. The most intensive band corresponds to the 1D2 → 3F4 transition. TmMgB5O10 solids demonstrate the thermal stability of photoluminescence. [ABSTRACT FROM AUTHOR]

In solar cells, hybrid halide perovskites operate under constant bias, thus their stability towards electric field-induced degradation is of key importance. Here we report on evidence of previously unidentified electric field-induced transitions and degradation path of CH3NH3PbI3 (MAPbI3) using elemental and phase mapping. Thin films of MAPbI3 were deposited onto 1–2 µm-pitch interdigitated electrodes and subjected to direct current (DC)-polarization. The MAPbI3 layer polarized with < 0.8 V/µm DC electric field undergoes pronounced ion redistribution to methylammonium-rich MAPbI3−y (y < 0.6) and iodine-rich MA1−xPbI3 (x < 0.3) regions. Polarization-induced loss of both methylammonium and iodine provokes degradation of MAPbI3. Using nanofocus grazing-incidence wide-angle X-ray scattering (GIWAXS), we unambiguously showed that the bias voltage induces the transformation of β-MAPbI3 to metastable δ-MAPbI3 polymorph via alignment of polar organic cation with the electric field. This transformation is partially reversible upon field removal. However, once formed, δ-MAPbI3 disrupts the morphology of pristine film and undergoes decomposition to β-MAPbI3 (β-MAPI) and PbI2. With the aforementioned compositional and phase changes, only MA-rich part serves as the charge separation layer, while the I-rich excitation is blocked with the PbI2 barrier serving as holes trapping layer. These observations reveal the intermediate steps in electric-field-driven degradation of halide perovskites and show the role of polar cations in the process, which is instructive for further material design with higher stability metrics. [ABSTRACT FROM AUTHOR]

Hybrid halocuprates(I) are nowadays the subject of intensive studies as promising materials for various optoelectronic applications. This class of materials is characterized by wide structural diversity enabled by a great variety in the size and shape of organic cations. Therefore, the study of composition–structure–property relationships is a key step for the rational design of new halocuprate materials with desired properties. In this paper, we comprehensively studied MABr/CuBr and FABr/CuBr systems (MA+ = methylammonium and FA+ = formamidinium) and established the existence of five novel phases (namely, MACu2Br3, FA2[Cu4Br6], MACuBr2, FACuBr2, and FA3CuBr4) related to four different structural types and three distinct A+ : Cu+ stoichiometries (A+ = MA+/FA+). The optical properties of the discovered phases are studied by absorption and low-temperature photoluminescence spectroscopy. Based on a crystal–chemical analysis, we explained a unique structural diversity of the MA- and FA-based bromocuprates, as well as revealed new structure–property relationships. [ABSTRACT FROM AUTHOR]

In this study, we systematically investigated the phase diversity and crystallization pathways of the FABr excessive regions of two ternary systems of FABr-PbBr2-DMF and FABr-PbBr2-DMSO (where FA+—formamidinium cations, DMF—dimethylformamide and DMSO—dimethyl sulfoxide solvents). In these systems, a new FA3PbBr5 phase with a structure containing chains of vertex-connected PbBr6 octahedra is discovered, and its crystal structure is refined. We experimentally assess fundamental information on differences in the mechanisms of crystallization process in FABr-PbBr2-DMF and FABr-PbBr2-DMSO systems and determine possible pathways of crystallization of hybrid perovskites. We show that intermediate solvate phases are not observed in the system with DMF solvent, while a number of crystalline solvates tend to form in the system with DMSO at various amounts of FABr excess. [ABSTRACT FROM AUTHOR]

Metal halide perovskites APbX3 (A+ = FA+ (formamidinium), MA+ (methylammonium) or Cs+, X− = I− or Br−) are considered as prominent innovative components in nowadays perovskite solar cells. Crystallization of these materials is often complicated by the formation of various phases with the same stoichiometry but structural types deviating from perovskites such as the well-known hexagonal delta FAPbI3 polytype. Such phases are rarely placed in the focus of device engineering due to their unattractive optoelectronic properties while they are, indeed, highly important because they influence the optoelectronic properties and efficiency of the final devices. However, the total number of such phases has not been yet discovered and the complete configurational space of the polytypes and their band structures have not been studied systematically. In this work, we predict and describe all possible hexagonal polytypes of hybrid lead halides with the APbI3 composition using the group theory approach, also we analyze theoretically the relationship between the configuration of close-packed layers in polytypes and their band gap using DFT calculations. Two main factors affecting the bandgap were found including the ratio of cubic (c) and hexagonal (h) close-packed layers and the thickness of blocks of cubic layers in the structures. We also show that the dependence of the band gap on the ratio of cubic (c) and hexagonal (h) layers in these structures is non-linear. We believe that the presence of such polytypes in the perovskite matrix might be a reason for the decrease in the charge carrier mobility and therefore it would be an obstacle for efficient charge transport causing negative consequences for the efficiency of solar cell devices. [ABSTRACT FROM AUTHOR]

Using the evolutionary crystal structure prediction algorithm USPEX, we showed that at pressures of the Earth's lower mantle (24-136 GPa) CaAl2O4 is the only stable calcium aluminate. At pressures above 7.0 GPa it has the CaFe2O4-type structure with space group Pnma. This phase is one of the prime candidate aluminous phases in the lower mantle of the Earth. We show that at low pressures 5CaO • 3Al2O3 (C5A3) with space group Cmc21, CaAl4O7 (with space group C2/c) and CaAl2O4 (space group P21/m) are stable at pressures of up to 2.1, 1.8 and 7.0 GPa, respectively. The previously unknown structure of the orthorhombic "CA-III" phase is also found in our calculations. This phase is metastable at 0 K and has a layered structure with space group P21212. [ABSTRACT FROM AUTHOR]

Hybrid halocuprates(I) are nowadays the subject of intensive studies as promising materials for various optoelectronic applications. This class of materials is characterized by wide structural diversity enabled by a great variety in the size and shape of organic cations. Therefore, the study of composition-structure-property relationships is a key step for the rational design of new halocuprate materials with desired properties. In this paper, we comprehensively studied MABr/CuBr and FABr/CuBr systems (MA + = methylammonium and FA + = formamidinium) and established the existence of five novel phases (namely, MACu 2 Br 3 , FA 2 [Cu 4 Br 6 ], MACuBr 2 , FACuBr 2 , and FA 3 CuBr 4 ) related to four different structural types and three distinct A +  : Cu + stoichiometries (A + = MA + /FA + ). The optical properties of the discovered phases are studied by absorption and low-temperature photoluminescence spectroscopy. Based on a crystal-chemical analysis, we explained a unique structural diversity of the MA- and FA-based bromocuprates, as well as revealed new structure-property relationships.

A new solvent system for PbI 2 based on HI solution in acetone with a low boiling point is proposed. High solubility of PbI 2 is caused by the formation of iodoplumbate complexes, and reaches a concentration of 1.6 M. Upon its crystallization metastable solvate phases PbI 2 ∙HI∙n{(CH 3 ) 2 CO} are formed. The latter allows for their easy deposition on substrates in a form of smooth and uniform thin films by spin-coating. Through a fast acid-base reaction with a gaseous amine, the films of the intermediate phase can be completely converted to single-phase perovskite films. The developed method allows one to form smooth perovskite films with high crystallinity with a thickness up to 1 μm. Due to easy and fast processing, the developed method can be promising for perovskite technology upscaling., Competing Interests: The authors declare no conflict of interest.