Morphology-Tuned Phase Transitions of Horseshoe Shaped BaTiO₃ Nanomaterials under High Pressure

Exploring new physical properties of nanomaterials with special morphology have been important topics in nanoscience and nanotechnology. Here we report a morphology-tuned structural phase transition under high pressure in the horseshoe shaped BaTiO₃ nanomaterials with an average diameter of 26 ± 4 nm. A direct structural phase transition from the tetragonal to the cubic phase without local rhombohedral distortion was observed at about 7.7 GPa by in situ high-pressure X-ray diffraction and Raman spectroscopy, which is clearly different from the phase transition processes of the BaTiO₃ bulks and nanoparticles. Additionally, bulk modulus of the tetragonal and cubic phases were determined to be 125.0 and 211.7 GPa, respectively, obviously smaller than the estimated values for BaTiO₃ nanoparticles with the same grain size. Further analysis shows that the unique phase transition process and the enhanced structural stability of the tetragonal horseshoe shaped BaTiO₃ nanomaterials, may be attributed to the similar axes compressibility. Comparing with the high-pressure study on BaTiO₃ nanoparticles, this study suggests that the morphology plays an important role in the pressure-induced phase transition of BaTiO₃ nanomaterials

Pressure-induced enhancement and retainability of optoelectronic properties of NiPS₃

Here, we report significant pressure-modulate optoelectronic properties of NiPS3. Upon compression, NiPS3 exhibited a photocurrent increase of five orders of magnitude over the initial value. Interestingly, when NiPS3 was finally decompressed to ambient conditions, the photocurrent could maintain a two-order-of-magnitude enhancement. In addition, the spectral response range was extended to the near-infrared spectral range (up to 1650 nm) under high pressure. Raman and XRD measurements and theoretical calculations revealed significant enhancement in both interlayer and intralayer interactions during compression, leading to a remarkable modulation of the optoelectronic properties. By applying pressure, giant enhancement of photocurrent and tunable spectral response range were achieved in NiPS3, which provides a potential way to modify the optoelectronic properties for materials.</p

Significant Enhancement of Optoelectronic Properties in CuInP₂S₆ via Pressure-Induced Structural Phase Transition

Quaternary layered transition metal thiophosphate CuInP2S6 (CIPS) has attracted extensive research interest because of its outstanding optical and ferroelectric properties. Pressure-tuned phase transition is an efficient method to regulate the properties of functional materials in situ, yet there is still much to explore. Herein, we studied the pressure-regulated optoelectronic properties of CIPS and found a four-stage evolution of photoresponsivity under compression. The photoresponse of CIPS barely changes with pressure initially but increases dramatically above 4.2 GPa. Under further compression, the photoresponse first shows a decrease above 7.5 GPa and then a significant increase up to 23.5 GPa. Remarkably, the photoresponse at the highest pressure enhances by two orders of magnitude compared with the starting value. To investigate the origin of these abnormal variations in CIPS, high-pressure UV–vis absorption, Raman, and XRD measurements were conducted and a phase transition from Cc to P3̅1m symmetry was found at approximately 4.0 GPa. We suggest that the pressure-modulated optoelectronic properties in CIPS are closely related to the conductivity change of CIPS caused by its structural phase transition. Our study spotlights the outstanding pressure regulation of optoelectronic properties in CIPS, which paves the way for modifying the behavior of other optoelectronic materials

The Multifunctionality of Lanthanum–Strontium Cobaltite Nanopowder: High-Pressure Magnetic Studies and Excellent Electrocatalytic Properties for OER

Simultaneous study of magnetic and electrocatalytic properties of cobaltites under extreme conditions expands the understanding of physical and chemical processes proceeding in them with the possibility of their further practical application. Therefore, La0.6Sr0.4CoO3 (LSCO) nanopowders were synthesized at different annealing temperatures tann = 850–900 °C, and their multifunctional properties were studied comprehensively. As tann increases, the rhombohedral perovskite structure of the LSCO becomes more single-phase, whereas the average particle size and dispersion grow. Co3+ and Co4+ are the major components. It has been found that LSCO-900 shows two main Curie temperatures, TC1 and TC2, associated with a particle size distribution. As pressure P increases, average ⟨TC1⟩ and ⟨TC2⟩ increase from 253 and 175 K under ambient pressure to 268 and 180 K under P = 0.8 GPa, respectively. The increment of ⟨dTC/dP⟩ for the smaller and bigger particles is sufficiently high and equals 10 and 13 K/GPa, respectively. The magnetocaloric effect in the LSCO-900 nanopowder demonstrates an extremely wide peak δTfwhm > 50 K that can be used as one of the composite components, expanding its working temperature window. Moreover, all LSCO samples showed excellent electrocatalytic performance for the oxygen evolution reaction (OER) process (overpotentials of only 265–285 mV at a current density of 10 mA cm–2) with minimal η10 for LSCO-900. Based on the experimental data, it was concluded that the formation of a dense amorphous layer on the surface of the particles ensures high stability as a catalyst (at least 24 h) during electrolysis in 1 M KOH electrolyte

Tuning the Flat Band in Bi₂O₂Se by Pressure to Induce Superconductivity

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron–phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity

Linear Tunability of the Band Gap and Two-Dimensional (2D) to Three-Dimensional (3D) Isostructural Transition in WSe₂ under High Pressure

Transition metal dichalcogenides (TMDs) have recently gained tremendous interest for use in electronic and optoelectronic applications. Unfortunately, the electronic structure or band gap of most TMDs shows noncontinuously tunable characteristics, which limits their application to energy-variable optoelectronics. Thus, layered materials with better tunability in their electronic structures and band gaps are desired. Herein, we experimentally demonstrated that layered WSe₂ possessed highly tunable transport properties under various pressures, with a linearly decreasing band gap that culminates in metallization. Pressure tuned the band gap of WSe₂ linearly, at a rate of 25 meV/GPa. The high tunability of WSe₂ was attributed to the larger electron orbitals of W²⁺ and Se^2– in WSe₂ compared to the Mo²⁺ and S^2– in MoS₂. WSe₂ underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like bonding was observed in the valence electron localization function (ELF). Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering

Structural Phase Transition and Photoluminescence Properties of YF₃:Eu³⁺ Nanocrystals under High Pressure

High-pressure behaviors of YF₃:Eu³⁺ nanocrystals with an average grain size of 40 nm were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 31.1 GPa at ambient temperature. The pressure-induced structural phase transition starts at 11.8 GPa and completes at 23.3 GPa. YF₃:Eu³⁺ nanocrystals with a starting phase of orthorhombic structure transform into a high-pressure phase, which is inferred to be hexagonal structure. The high-pressure structure returned to the orthorhombic phase after release of pressure. The transition pressure is enhanced in nanosized YF₃:Eu³⁺ as compared to submicrometer size samples, which is due to the surface energy differences between submicrometer size and nanosized materials. The nanosized samples of high-pressure phase were easier to compress with smaller bulk modulus than the submicrometer size samples. The photoluminescence properties of YF₃:Eu³⁺ have also been studied in the pressure range from ambient pressure to 25.0 GPa at room temperature. Accompanied by the structure transformation, the Eu³⁺ ion luminescence from ⁵D₀ → ⁷F_1,2,3,4 transition in YF₃:Eu³⁺ nanocrystals emerges obvious changes, which indicate the variation of the local symmetry of Eu³⁺ ions