Mechanochromic materials with aggregation-induced enhanced emission (AIEE) characteristic have been intensively expanded in the past few years. In general, intermolecular interactions invariably alter photophysical processes, while their role in the luminescence properties of these AIEE-active molecules is difficult to fully recognize because the pressurized samples possess amorphous nature in many cases. We now report the high-pressure studies on a prototype AIEE-active molecule, tetraphenylethene, using diamond anvil cell technique with associated spectroscopic measurements. An unusual pressure-dependent color, intensity, and lifetime change in tetraphenylethene has been detected by steady-state photoluminescence and time-resolved emission decay measurements. The flexible role of the aromatic C-H···π and C-H···C contacts in structural recovery, conformational modification, and emission efficiency modulation upon compression is demonstrated through structure and infrared analysis.
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.
We report the high-pressure response of three forms (α, δ, and γ) of pyrazinamide (C(5)H(5)N(3)O, PZA) by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 14 GPa. These different forms are characterized by various intermolecular bonding schemes. High-pressure experimental results show that the γ phase undergoes phase transition to the β phase at a pressure of about 4 GPa, whereas the other two forms retain their original structures at a high pressure. We propose that the stabilities of the α and δ forms upon compression are due to the special dimer connection that these forms possess. On the other hand, the γ form, which does not have this connection, prefers to transform to the closely related β form when pressure is applied. The detailed mechanism of the phase transition together with the stability of the three polymorphs is discussed by taking molecular stacking into account.
A combination of synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy has been used to study high-pressure behavior of the zircon-type LaVO4 nanorods. In situ high-pressure XRD results identified an irreversible zircon-to-monazite phase transition at ∼5 GPa and a reversible transition to an undetermined second high-pressure phase (phase III) at ∼12.9 GPa. Through Le Bail refinements of the XRD patterns with zircon-type structure, we show that the zircon-type LaVO4 nanorods possess the smallest bulk modulus among zircon-type rare-earth orthovanadates. Furthermore, negative pressure coefficients of external translational T(Eg) and internal υ2(B2g) bending modes have been observed in Raman measurements. The Raman spectra of phase III with distinctive features have been fully recorded for the first time, and a related structure associated with a coordination increase for V is suggested in terms of the postmonazite phase in LaVO4 nanorods. Finally, analysis of the transmission electron microscopy both before and after compression indicates that a large number of nanorods can be recovered in the quenched samples, allowing us to verify the orientation relationship for zircon-to-monazite phase transformation.
The high-pressure behavior of zircon-structured YPO 4 (with/without Eu 3+ doping) nanoparticles was examined at room temperature using in situ synchrotron X-ray diffraction (XRD) and photoluminescence (PL) measurements. In contrast with the reported XRD results of bulk YPO 4 upon compression, the nanoparticles showed a distinct transition sequence: zircon phase → scheelite phase (∼18 GPa) without the metastable monazite phase. By the return to ambient pressure, both XRD and PL results revealed that the scheelite phase could be reserved. Further Raman experiments helped us to identify the valuable mode ν 1 (A g ) of the scheelite structure in the quenched samples. The dopants effect, quasi-hydrostatic stress, and nanoscale-induced surface energy difference are considered to explain the high-pressure behavior of the nanoparticles. It is proposed that the nanoscale-induced higher surface energy contribution plays a crucial role in the distinctive high-pressure behavior of the nanoparticles.
We probed the high-pressure response of the YV 1−x P x O 4 :Eu 3+ (x = 0, 0.5, 0.7, 1.0) solid-solution nanoparticles using angular dispersive synchrotron X-ray diffraction (XRD) and Raman techniques at room temperature. In situ diffraction results showed that the overall nanoparticles underwent an irreversible zircon-to-scheelite structural transformation. The transition pressures were ∼9.3, ∼12.1, ∼14, and ∼18.4 GPa for the YV 1−x P x O 4 :Eu 3+ (x = 0, 0.5, 0.7, 1.0) samples, respectively. Coupled with the zircon-toscheelite transition features, it was proposed that the transition pressure was probably governed by the stiffness of VO 4 /PO 4 units in the solid solutions. This claim was verified by further Raman measurements, which revealed that the stiffness of VO 4 /PO 4 units was enhanced with increasing P contents. The structural refinements showed that the samples with comparable particle size (20−90 nm) became less compressible with increasing P content (x = 0 → 0.7 → 1.0). However, the compressibility of the YV 0.5 P 0.5 O 4 :Eu 3+ sample with smaller particle size (10−30 nm) was similar to that of the YV 0.3 P 0.7 O 4 :Eu 3+ sample. The general compressibility behavior as a function of P content was ascribed to the special packing style related to the stiffness of VO 4 /PO 4 tetrahedra in zircon structure, and the higher surface energy contribution was responsible for the exceptional compressibility in the smaller nanoparticles. ■ INTRODUCTIONZircon-type ABO 4 ternary oxides are common accessory minerals, and they share many physical properties, as well as displaying variable degrees of solid solutions among end members existing in a wide variety of sedimentary, igneous, and metamorphic rocks. 1 The high-pressure research on zircon-type ABO 4 compounds has drawn considerable interest in the past several decades because it could provide a wide range of geochemical and geophysical investigations, including studies on the evolution of Earth's crust and mantle. 1,2 Apart from the geophysical importance, the rare-earth orthovanadates RVO 4 and orthophosphates RPO 4 currently attract considerable interest by virtue of their wide potential applications and interesting optical/luminescent properties. 3−6 They generally crystallize, depending on the ionic radii of the R cation, in two different structural types: zircon [space group (SG): I4 1 /amd, Z = 4] and monazite [SG: P2 1 /n, Z = 4]. Those with a small R size (r R < r La for RVO 4 and r R < r Gd for RPO 4 ) adopt the zircon structure under ambient conditions, whereas the others have the lower-symmetry monoclinic monazite structure. 7−9 Specifically, depending on the growth conditions, GdPO 4 , TbPO 4 , DyPO 4 , and HoPO 4 can adopt either zircon or monazite structure. 10 The high-pressure behavior of zircon-type RVO 4 and RPO 4 compounds was addressed recently. Upon compression, a direct transition to scheelite structure [SG: I4 1 /a, Z = 4] occurs in almost all the zircon-type RVO 4 compounds except CeVO 4 , which displays the complete zircon-to-monazite...
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