RESUMEN
Tungstates and molybdates possessing the scheelite- and wolframite-type (if present) structures hold a significant functional value. Their high-pressure phase diagrams are very complicated and controversial, and even some parts have not been characterized yet. In this study, we investigate the sequence of pressure driven structural phase transitions up to 100 GPa in these tungstate and molybdate families via first-principles structure predictions. Based on our structural predictions, it is possible for isostructural tungstates and molybdates to exhibit a phase transition sequence that is either similar or identical. Examples of these compounds are CaWO4, CaMoO4, and CdMoO4, in addition to EuWO4 and EuMoO4. However, the phase transition sequences of some tungstates and molybdates, especially those with different divalent cations, display noteworthy variations, revealing the intricate influence of ionic radii and electronic properties on crystal configurations. To obtain a deeper understanding of the high-pressure phase transition behavior of tungstates and molybdates, we analyze the high-pressure phase diagrams of MgWO4, SrWO4, and CaMoO4, representative examples of wolframite-type tungstate, scheelite-type tungstate, and scheelite-type molybdate, respectively, using x-ray powder diffraction. Our x-ray diffraction experiments and structure predictions consistently verify that the orthorhombic Cmca phase is a high-pressure phase of SrWO4. Structural configurations and mechanical properties of these predicted structures are discussed, and electronic properties are given. This study could have important implications for the fields of seismology and geophysics, as well as the utilization of these materials in various capacities, such as photocatalysts, photoanodes, and phosphors.
RESUMEN
Pressure-induced emission enhancement (PIEE), a novel phenomenon in the enhancement of the solid-state emission efficiency of fluorophores, has been arousing wide attention in recent years. However, research on PIEE is still in the early stage. To further pursue more enhanced efficiency, discovering and designing more PIEE systems would be urgently desirable and of great importance. In this Letter, we found that carbazole presented a conspicuous emission enhancement under high pressure up to 1.0 GPa. In situ high-pressure infrared spectroscopy and angle-dispersive X-ray diffraction analysis combined with Hirshfeld surface theory calculation indicated that the PIEE of carbazole was attributed to the decrease of the nonradiation vibration process. This phenomenon mainly resulted from restriction of the N-H stretching vibration by increased N-H···π interactions under high pressure. Our study puts forward a mechanism of PIEE related to the restriction of intramolecular vibration, which provided deep insight into the essential role of intermolecular interaction in fluorescence emission properties.
RESUMEN
Negative linear compressibility (NLC) is a rare and counterintuitive phenomenon because materials with this property would expand along one specific direction when uniformly compressed. NLC materials have a broad range of potential applications in designing pressure sensors, artificial muscles, and so on. Designing and searching for systems with NLC is desired and crucial for material and compression science. Herein, with the help of high-pressure X-ray diffraction measurements and density functional theory calculations, we find that the 2D layered Co(SCN)2(pyrazine)2 exhibits NLC with a new mechanism: layer sliding. When compressed, the ab planes slide along the a axis, leading to the decrease of lattice parameter ß, which results in the NLC effect along principal axis X3 (≈ -0.84a - 0.55c). The layer sliding mechanism opens exciting opportunities for seeking, designing, and synthesizing new classes of materials with anomalous mechanical properties in monoclinic layered or other related systems.
RESUMEN
Negative linear compressibility (NLC) is a relatively uncommon phenomenon and rarely studied in organic systems. Here we provide the direct evidence of the persistent NLC in organic mineral ammonium oxalate monohydrate under high pressure using synchrotron X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculation. Synchrotron X-ray powder diffraction measurement reveals that ammonium oxalate monohydrate shows both positive and negative linear compressibility along b-axis before 11.5 GPa. The red shift of the external Raman modes and abnormal changes of several selected internal modes in high-pressure Raman spectra further confirmed the NLC. DFT calculations demonstrate that the N-H···O hydrogen bonding "wine-rack" motifs result in the NLC along b-axis in ammonium oxalate monohydrate. We anticipate the high-pressure study of ammonium oxalate monohydrate may represent a promising strategy for accelerating the pace of exploitation and improvement of NLC materials especially in organic systems.