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1. Measurements of Density of Liquid Oxides with an Aero-Acoustic Levitator

   https://doi.org/10.3390/ma14040822  

   Ushakov, Sergey V.; Niessen, Jonas; Quirinale, Dante G.; Prieler, Robert; Navrotsky, Alexandra; Telle, Rainer (February 2021, Materials)

   null (Ed.)

   Densities of liquid oxide melts with melting temperatures above 2000 °C are required to establish mixing models in the liquid state for thermodynamic modeling and advanced additive manufacturing and laser welding of ceramics. Accurate measurements of molten rare earth oxide density were recently reported from experiments with an electrostatic levitator on board the International Space Station. In this work, we present an approach to terrestrial measurements of density and thermal expansion of liquid oxides from high-speed videography using an aero-acoustic levitator with laser heating and machine vision algorithms. The following density values for liquid oxides at melting temperature were obtained: Y2O3 4.6 ± 0.15; Yb2O3 8.4 ± 0.2; Zr0.9Y0.1O1.95 4.7 ± 0.2; Zr0.95Y0.05O1.975 4.9 ± 0.2; HfO2 8.2 ± 0.3 g/cm3. The accuracy of density and thermal expansion measurements can be improved by employing backlight illumination, spectropyrometry and a multi-emitter acoustic levitator. 

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2. Shocked silica aerogel radiance transition

   https://doi.org/10.1063/5.0105194  

   Henderson, B. J.; Rygg, J. R.; Marshall, M. C.; Ginnane, M. K.; Hansen, L. E.; Davies, E.; Celliers, P. M.; Collins, G. W. (September 2022, Journal of Applied Physics)

   Silica (SiO2) aerogel is widely used in high-energy-density shock experiments due to its low and adjustable density. Reported here are measurements of the shock velocity, optical radiance, and reflectivity of shocked SiO2 aerogel with initial densities of 0.1, 0.2, and 0.3 g/cm3. These results are compared with similar data from three solid polymorphs of SiO2, silica, quartz, and stishovite with initial densities 2.2, 2.65, and 4.3 g/cm3, respectively. Interestingly, below a brightness temperature of Tbright≈35,000 K, the slope of the radiance vs shock velocity is the same for each of the SiO2 aerogels and solid polymorphs. At Tbright≈35000 K, there is an abrupt change in the radiance vs shock velocity slope for aerogels, but not seen in the solid polymorphs over the pressures and temperatures explored here. An empirical model of shock front radiance as a function of SiO2 density and laser drive parameters is reported to aid in the design of experiments requiring maximum shock front radiance. 

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3. Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors

   https://doi.org/10.1038/s41535-022-00492-0  

   Xiao, Qian; Zhang, Wenliang; Asmara, Teguh Citra; Li, Dong; Li, Qizhi; Zhang, Shilong; Tseng, Yi; Dong, Xiaoli; Wang, Yao; Chen, Cheng-Chien; et al (December 2022, npj Quantum Materials)

   Abstract The superconducting critical temperature T c of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high- T c monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high- T c iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer in both superconducting 28 K and 42 K samples. Using atomic multiplet calculations, we assign the low-energy ~0.3 and 0.7 eV Raman peaks as local e g  −  e g and e g  −  t 2 g orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a dominating contribution of the orbital fluctuations. Our results highlight the importance of the orbital degree of freedom for high- T c iron selenides. 

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4. Raman study of layered breathing kagome lattice semiconductor Nb ₃ Cl ₈

   https://doi.org/10.1088/2053-1583/acfa10  

   Jeff, Dylan A; Gonzalez, Favian; Harrison, Kamal; Zhao, Yuzhou; Fernando, Tharindu; Regmi, Sabin; Liu, Zhaoyu; Gutierrez, Humberto R; Neupane, Madhab; Yang, Jihui; et al (September 2023, 2D Materials)

   Abstract Niobium chloride (Nb₃Cl₈) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb₃Cl₈in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which areA_1gmodes while the remaining seven areE_gmodes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperatureP $\overline{\text{3}m}$1 phase to the low-temperatureR $\overline{3m}$phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb₃Cl₈material which can be further exploited for future applications. 

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5. Optimizing Raman spectral collection for quartz and zircon crystals for elastic thermobarometry

   https://doi.org/10.2138/am-2022-8423  

   Cizina, Mayara F.; Mikesell, T. Dylan; Kohn, Matthew J. (May 2023, American Mineralogist: Journal of Earth and Planetary Materials)

   Abstract Raman spectroscopy is widely used to identify mineral and fluid inclusions in host crystals, as well as to calculate pressure-temperature (P-T) conditions with mineral inclusion elastic thermobarometry, for example quartz-in-garnet barometry (QuiG) and zircon-in-garnet thermometry (ZiG). For thermobarometric applications, P-T precision and accuracy depend crucially on the reproducibility of Raman peak position measurements. In this study, we monitored long-term instrument stability and varied analytical parameters to quantify peak position reproducibility for Raman spectra from quartz and zircon inclusions and reference crystals. Our ultimate goal was to determine the reproducibility of calculated inclusion pressures (“Pinc”) and entrapment pressures (“Ptrap”) or temperatures (“Ttrap”) by quantifying diverse analytical errors, as well as to identify optimal measurement conditions and provide a baseline for interlaboratory comparisons. Most tests emphasized 442 nm (blue) and 532 nm (green) laser sources, although repeated analysis of a quartz inclusion in garnet additionally used a 632.8 nm (red) laser. Power density was varied from <1 to >100 mW and acquisition time from 3 to 270s. A correction is proposed to suppress interference on the ~206 cm–1 peak in quartz spectra by a broad nearby (~220 cm–1) peak in garnet spectra. Rapid peak drift up to 1 cm–1/h occurred after powering the laser source, followed by minimal drift (<0.2 cm–1/h) for several hours thereafter. However, abrupt shifts in peak positions as large as 2–3 cm–1 sometimes occurred within periods of minutes, commonly either positively or negatively correlated to changes in room temperature. An external Hg-emission line (fluorescent light) can be observed in spectra collected with the green laser and shows highly correlated but attenuated directional shifts compared to quartz and zircon peaks. Varying power density and acquisition time did not affect Raman peak positions of either quartz or zircon grains, possibly because power densities at the levels of inclusions were low. However, some zircon inclusions were damaged at higher power levels of the blue laser source, likely because of laser-induced heating. Using a combination of 1, 2, or 3 peak positions for the ~128, ~206, and ~464 cm–1 peaks in quartz to calculate Pinc and Ptrap showed that use of the blue laser source results in the most reproducible Ptrap values for all methods (0.59 to 0.68 GPa at an assumed temperature of 450 °C), with precisions for a single method as small as ±0.03 GPa (2σ). Using the green and red lasers, some methods of calculating Ptrap produce nearly identical estimates as the blue laser with similarly good precision (±0.02 GPa for green laser, ±0.03 GPa for red laser). However, using 1- and 2-peak methods to calculate Ptrap can yield values that range from 0.52 ± 0.06 to 0.93 ± 0.16 GPa for the green laser, and 0.53 ± 0.08 GPa to 1.00 ± 0.45 GPa for the red laser. Semiquantitative calculations for zircon, assuming a typical error of ±0.25 cm–1 in the position of the ~1008 cm–1 peak, imply reproducibility in temperature (at an assumed pressure) of approximately ±65 °C. For optimal applications to elastic thermobarometry, analysts should: (1) delay data collection approximately one hour after laser startup, or leave lasers on; (2) collect a Hg-emission line simultaneously with Raman spectra when using a green laser to correct for externally induced shifts in peak positions; (3) correct for garnet interference on the quartz 206 cm–1 peak; and either (4a) use a short wavelength (blue) laser for quartz and zircon crystals for P-T calculations, but use very low-laser power (<12 mW) to avoid overheating and damage or (4b) use either the intermediate wavelength (green; quartz and zircon) or long wavelength (red; zircon) laser for P-T calculations, but restrict calculations to specific methods. Implementation of our recommendations should optimize reproducibility for elastic geothermobarometry, especially QuiG barometry and ZiG thermometry. 

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