The structural properties of EuCo₂As₂ have been studied up to 35 GPa, through the use of x-ray diffraction in a diamond anvil cell at a synchrotron source. At ambient conditions, EuCo₂As₂ ) (I4/mmm) has a tetragonal lattice structure with a bulk modulus of 48 ± 4 GPa. With the application of pressure, the a axis exhibits negative compressibility with a concurrent sharp decrease in c-axis length. The anomalous compressibility of the a axis continues until 4.7 GPa, at which point the structure undergoes a second-order phase transition to a collapsed tetragonal (CT) state with a bulk modulus of 111 ± 2 GPa. We found a strong correlation between the ambient pressure volume of 122 parents of superconductors and the corresponding tetragonal to collapsed tetragonal phase transition pressures.
The pressure-temperature (P-T) phase diagram of 1,1-diamino-2,2-dinitroethylene (FOX-7) was determined by in situ synchrotron infrared radiation spectroscopy with the resistively heated diamond anvil cell (DAC) technique. The stability of high-P-T FOX-7 polymorphs is established from ambient pressure up to 10 GPa and temperatures until decomposition. The phase diagram indicates two near isobaric phase boundaries at ∼2 GPa (α → I) and ∼5 GPa (I → II) that persists from 25 °C until the onset of decomposition at ∼300 °C. In addition, the ambient pressure, high-temperature α → β phase transition (∼111 °C) lies along a steep boundary (∼100 °C/GPa) with a α-β-δ triple point at ∼1 GPa and 300 °C. A 0.9 GPa isobaric temperature ramping measurement indicates a limited stability range for the γ-phase between 0.5 and 0.9 GPa and 180 and 260 °C, terminating in a β-γ-δ triple point. With increasing pressure, the δ-phase exhibited a small negative dT/dP slope (up to ∼0.2 GPa) before turning over to a positive 70 °C/GPa slope, at higher pressures. The decomposition boundary (∼55 °C/GPa) was identified through the emergence of spectroscopic signatures of the characteristic decomposition products as well as trapped inclusions within the solid KBr pressure media.
The structural phase stability of 1,1-diamino-2,2-dinitroethylene (FOX-7) has been studied up to 10 GPa through isothermal compression at 100 °C and 200 °C using synchrotron mid- and far-infrared spectroscopy. During isothermal compression at 100 °C changes are observed in vibrational spectra with increase in pressure that are indicative of significant distortion to monoclinic α phase or a possible structural transformation to a high pressure α(') phase at 2.2 GPa and α(") phase at 6.1 GPa. At 200 °C, for the far- and mid-IR regimes, the similar changes were observed at 2.1 (2.0) GPa and 5.3 (5.5) GPa, respectively. The observed change is nearly isobaric, consistent with previously reported high pressure and room temperature values, up to the highest temperature of 200 °C reached in our experiments. Over the total P-T range investigated, up to ∼10 GPa and 200 °C, we observed no evidence of sample decomposition. The observed changes are partially reversible with only slight evidence of the high pressure distortion remaining upon complete decompression. Additional isobaric heating at 1.07 GPa was performed in the mid-IR regime, which clearly revealed an onset of decomposition at 360 °C. Further x-ray or neutron diffraction, which are needed to fully resolve the cause of observed changes above 2 and 5 GPa, are ongoing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.