Hierarchical microstructures lead to high thermoelectric performance in Cum+nPb100SbmTe100Se2m (CLAST) through synergistically optimizing carrier and phonon transport.
Perovskite-related materials with various dimensionalities have attracted sustained attention owing to their extraordinary electronic and optoelectronic properties, but it is still challenging in the synthesis of compounds with desired compositions and structures. Herein, a two-dimensional (2D) CsPb 2 I 5 perovskite has been synthesized by the conversion of CsPbI 3 at high-pressure and high-temperature (high P−T) conditions, which is quenchable at ambient conditions. In situ synchrotron X-ray diffraction shows that high-pressure monoclinic CsPbI 3 converts into tetragonal CsPb 2 I 5 and cubic CsI at 8.7 GPa upon heating from 644 to 666 K. Keeping the tetragonal structure stable, CsPb 2 I 5 exhibits tunable optical properties with the bandgap changing from ∼2.4 eV at ambient pressure to ∼1.4 eV at 36.9 GPa. Further experiments demonstrate similar structural evolution in the typical three-dimensional CsPbBr 3 perovskite into 2D CsPb 2 Br 5 at high P−T conditions, indicating that the conversion of CsPbX 3 (X = Br and I) into CsPb 2 X 5 is ubiquitous.
Determination of the structures of materials involving
more light
elements such as boron-rich compounds is challenging and technically
important in understanding their varied compositions and superior
functionalities. Here we resolve the long-standing uncertainties in
structure and composition about the highest boride (termed MoB4, Mo1–x
B3, or
MoB3) through the rapid formation of large-sized boron-rich
molybdenum boride under pressure. Using high-quality single-crystal
X-ray diffraction analysis and aberration-corrected scanning transmission
electron microscopy, we reveal that boron-rich molybdenum boride with
a composition of Mo0.757B3 exhibits P63/mmc symmetry with a partial
occupancy of 0.514 in 2b Mo sites (Mo1), and direct
observations reveal the short-range ordering of cation vacancies in
(010) crystal planes. Large anisotropic Young’s moduli and
Vickers hardness are seen for Mo0.757B3, which
may be attributed by its two-dimensional boron distributions. Mo0.757B3 is also found to be superconducting with
a transition temperature (T
c) of ∼2.4
K, which was confirmed by measurements of resistivity and magnetic
susceptibility. Theoretical calculations suggest that the partial
occupancy of Mo atoms plays a crucial role in the emergence of superconductivity.
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