Ultrasmall pure hexagonal phase NaYF 4 :Yb,Er is successfully prepared via the solvothermal method. The upconversion (UC) luminescence of hexagonal phase NaYF 4 :Yb,Er nanocrystals is ten times stronger than that of cubic phase nanocrystals with the same size of 6 nm. XRD results reveal that heating above 673 K leads to conversion of the hexagonal phase to the high-temperature cubic phase, whereas the cubic nanocrystals undergo phase transformation from normal cubic at room temperature to hexagonal at 673 K and further to high-temperature cubic above 773 K. The hexagonal nanocrystals exhibit emission enhancement after heat treatment up to 573 K. Further heating above 773 K induces a decreasing trend in emission due to the phase transition to the high-temperature cubic phase. The cubic phase exhibits decreasing luminescence with temperature due to strong cross relaxation and then increasing luminescence above the temperature of 673 K due to the hexagonal phase transformation. The luminescent properties of both the normal cubic phase and high-temperature cubic phase indicate that different crystal fields exist in these two phases due to the rearrangement of ligands around Er 3+ at high temperature.
Phase transitions of the β-HMX crystal have been systematically studied via Raman, mid-infrared, and X-ray diffraction techniques under high pressures of up to 40 GPa. When using a neon pressure transmitting medium, we observed four phase transitions, including β-phase to ζ-phase at 5.4 GPa, ε-phase at 9.6 GPa, φ-phase at 21.6 GPa, and further to η-phase at 35.0 GPa. According to the high-pressure Raman spectra, the first two phase transitions of HMX are induced by the changes of the NO 2 groups and ring, whereas the ε to φ phase transition is attributed to the mutation of the CH 2 groups and ring. Under higher pressures, another newly discovered phase transition from φ to η occurred at 35.0 GPa. Based on the analysis of highpressure X-ray diffraction results, all currently observed high-pressure phases are isostructures of monoclinic HMX, and no abrupt change in the volume has been found in the volume−pressure curve below 38 GPa. These are consistent with the isentropic compression results and the conclusion of the first-principles calculation. The present work clarifies the controversial pressure-induced phase transitions for β-HMX and the newly discovered phase transitions under high pressure.
Herein, pressure-induced phase transitions of RDX up to 50 GPa were systematically studied under different compression conditions. Precise phase transition points were obtained based on high-quality Raman spectra with small pressure intervals. This favors the correctness of the theoretical formula for detonation and the design of a precision weapon. The experimental results indicated that α-RDX immediately transformed to γ-RDX at 3.5 GPa due to hydrostatic conditions and possible interaction between the penetrating helium and RDX, with helium gas as the pressure-transmitting medium (PTM). Mapping of pressure distribution in samples demonstrates that the pressure gradient is generated in the chamber and independent of other PTMs. The gradient induced the first phase transition starts at 2.3 GPa and completed at 4.1 GPa. The larger pressure gradient promoted phase transition in advance under higher pressures. Experimental results supported that there existed two conformers of AAI and AAE for γ-RDX, as proposed by another group. δ-RDX was considered to only occur in a hydrostatic environment around 18 GPa using helium as the PTM. This study confirms that δ-RDX is independent of PTM and exists under non-hydrostatic conditions. Evidence for a new phase (ζ) was found at about 28 GPa. These 4 phases have also been verified via XRD under high pressures. In addition to this, another new phase (η) may exist above 38 GPa, and it needs to be further confirmed in the future. Moreover, all the phase transitions were reversible after the pressure was released, and original α-RDX was always obtained at ambient pressure.
Laser irradiation transforms Sb2O3 from the tetragonal phase into an HD-amorphous phase under high pressure and back to cubic phase from LD-amorphous phase at ambient conditions.
To
probe the behavior of structural evolution and optical properties
in solid energetic material TATB, X-ray diffraction (XRD) and Raman
and absorption spectroscopy were performed under high pressure up
to 20 GPa. The absorption edge shifts to red, and the color significantly
varies with increasing pressure for TATB. The XRD patterns under high
pressure indicate that TATB maintains the triclinic structure within
this pressure range. An electronic structural change is observed at
∼5 GPa, resulting from the modification of conformers of TATB,
which is associated with the rotation of nitro and amino groups under
high pressure. The current experimental results clarified the absence
of phase transition below 20 GPa and confirmed that the pressure-induced
color change originates from the enhancing conjugation of π
orbital due to the shorting C–NO2 bonds and the
rotation of nitro groups with increasing pressure. The third-order
Birch–Murnaghan equation of state is obtained up to 16.5 GPa,
which is helpful for calculating researchers to verify the correctness
of their models.
The pressure-induced
structural phase transitions of ε- and γ-CL-20 were studied
by Raman and mid-infrared spectroscopy up to 60 GPa. In this work,
the phase transition of CL-20 from the ε-phase to the γ′-phase
starts at 0.9 GPa and ends at 4.4 GPa. The γ′-phase in
this work is distinctly different from the γ-phase recognized
by the energetic community in terms of the structure and properties.
Subsequently, the η phase starts at 6.9 GPa and ends at 10.6
GPa because of the slight cage distortion. With
further increase of the loading pressure, two new phases, φ
and ι, were observed at 28 and 50 GPa, respectively. The infrared
results are consistent with Raman results and show that similar phase
regions are observed for CL-20 under high pressures. The behavior
of the γ-phase under pressure indicates that the ζ-phase
appears at 1.3 GPa and sustains its stability up to 47.4 GPa. The
current results prove that the newly discovered γ′-phase
is evidently distinct from the γ-phase and they undergo different
phase transition routes under loading compression.
The high-pressure structural properties of PCP were investigated by using Raman scattering techniques in a diamond anvil cell up to 10 GPa at room temperature. Compression caused a relative flat xylyl fragment and twisted ethylene bridges in PCP. Raman analysis showed that more peaks related to vibrational modes of aliphatic CH 2 emerged due to the transition to a low symmetric phase after compression pressure reached 3.9 GPa. Other blue-shifting peaks showed discontinuity in the frequency− pressure curves at pressure 3.9 GPa because of molecular relaxation by phase transition. Compression strengthened the conjugation of xylyl fragments. When releasing pressure, the normal vibrational modes could be recovered with a hysteresis of ∼1.3 GPa. The normal phase of PCP could be fully recovered under ambient conditions.
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