Low temperature Terahertz Spectroscopy of LaFeO3_3, PrFeO3_3, ErFeO3_3, and LuFeO3_3: Quasimagnon resonances and ground multiplet transitions

We report on zone center THz excitations of non-Jahn Teller LaFeO$_3$, PrFeO$_3$, ErFeO$_3$, and LuFeO$_3$ distorted perovskites under external magnetic fields up 7 T. Low temperature-low energy absorptions of LaFeO$_3$ show antiferromagnetic and ferromagnetic quasimagnons at $\omega$AFM ~31.4 and $\omega$FM ~26.7 cm$^{-1}$ in the $\Gamma$4 (Gx, Ay, Fz) representation. LuFeO$_3$ is characterized by zero field magnetic resonances at $\omega$AFM ~26.3 cm$^{-1}$ and $\omega$FM ~22.4 cm$^{-1}$ in addition to Fe$^{3+}$ Zeeman-split crystal field (CF) 6A$_1$ ground transitions at ~10.4 cm$^{-1}$ triggered by structural deviations induced by smaller Lu 4f$^{14}$. This local non-centrosymmetric departure is also found in ErFeO$_3$ (Kramers 4f$^{11}$ Er$^{3+}$ (4I15/2); {\Gamma}2 (Fx, Cy, Gz) <TSR ~93 K), but with the ~4 cm$^{-1}$ Fe$^{3+}$ Zeeman branching strongly biased toward higher energies. Quasimagnons at $\omega$AFM ~31.5 cm$^{-1}$ and $\omega$FM ~21.5 cm$^{-1}$ in ErFeO$_3$ do not undergo field induced band splits but a 13-fold increase in the antiferro ($\omega$AMF) /ferro($\omega$AFM) intensity ratio. There is a remarkable field-dependent CF matching population balance between Fe$^{3+}$ higher and Er$^{3+}$ lower Zeeman branches. Antiferro- and ferro- resonances in PrFeO$_3$ turn much broader as non-Kramers Pr$^3$ introduces ligand changes at the A site leading into near degeneracy the antiferromagnetic mode and the lowest Pr$^{3+}$ CF transition. We conclude that low energy excitations in RFeO$_3$ (R=rare earth) strongly depend on the lanthanide ionic size. Minute lattice displacements also underlie considering non-centrosymmetric the most distorted RFeO$_3$ (R=rare earth). Changes triggered by the smaller rare earth and the nonlinear intrinsic oxygen ion polarizability provide grounds for interplay of ionic and electronic interactions yielding ferroelectric spontaneous polarization.Comment: Full Manuscript and Supplemental Material, 73 pages, 27 figure

Short-range charge-order in RRNiO3_{3} perovskites (RR=Pr,Nd,Eu) probed by X-ray absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic $R$NiO$_{3}$ ($R$=Pr,Nd,Eu) perovskites has been studied over a wide temperature range by Ni K-edge x-ray absorption spectroscopy. Our results demonstrate that two different Ni sites, with different average Ni-O bond lengths, coexist in those orthorhombic compounds and that important modifications in the Ni nearest neighbors environment take place across the metal-insulator transition. We report evidences for the existence of short-range charge-order in the insulating state, as found in the monoclinic compounds. Moreover, our results suggest that the two different Ni sites coexists even in the metallic state. The coexistence of two different Ni sites, independently on the $R$ ion, provides a common ground to describe these compounds and shed new light in the understanding of the phonon-assisted conduction mechanism and unusual antiferromagnetism present in all $R$NiO$_{3}$ compounds.Comment: 4 pages, 3 figures, accepted PRB - Brief Report Dec.200

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29

Temperature and high-pressure dependent x-ray absorption of SmNiO3 at the Ni K and Sm L3 edges

International audienceWereport on x-ray absorption near-edge structure (XANES) and extended x-ray absorption finestructure (EXAFS) measurements of SmNiO3 from 20 Kto 600 Kand up to 38 GPa at the NiKand SmL3 edges. A multiple component pre-NiKedge tail is understood, originating from 1 s transitions to3d-4p states while a post-edge shoulder increases distinctively smoothly, at about the insulator tometal phase transition (TIM), due to the reduction of electron–phonon interactions as the Ni 3d andO2p band overlap triggers the metallic phase. This effect is concomitant with pressure-induced Ni-O-Niangle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group.Room temperature pressure-dependent Ni white line peak energies have an abrupt∼3.10±0.04 GPavalence discontinuity from non-equivalent Ni3+δ+Ni3–δ charge disproportionate net unresolvedabsorber turning at∼TIM into Ni3+ of the orthorhombic Pbnm metal oxide phase. At 20 Kthe overallwhite line response, still distinctive at TIM∼8.1±0.6 GPa is much smoother due to localization.Octahedral bond contraction up to 38 GPa and at 300 Kand 20 Kshow breaks in its monotonicincrease at the different structural changes. The SmL3 edge does not show distinctive behaviors eitherat 300 Kor 20 Kup to about 35 GPa but the perovskite Smcage, coordinated to eight oxygen atoms,undergoes strong uneven bond contractions at intermediate pressures where we found the coexistenceof octahedral and rhombohedral superexchange angle distortions.Wefound that the white linepressure-dependent anomaly may be used as an accurate alternative for delineating pressure–temperature phase diagrams

Infrared reflectivity of Co-x(SiO2)(1-x)(x similar to 0.85, 0.55, 0.38) granular films on SiO2 glass substrates

We report the infrared specular reflectivity of Co-x(SiO2)(1-x) (x similar to 0.85, 0.55, 0.38) films on SiO2 glass spanning from a metal-like to insulating behavior. While films for x similar to 0.85 show carrier metallic shielding and hopping conductivity, for x similar to 0.65 and lower concentrations, the nanoparticles' number and size promote a localization edge near the highest longitudinal optical frequency. Such an edge is associated with a reflectivity minimum and a higher frequency band connoting strong electron-phonon interactions, carrier phonon assisted hopping, and polaron formation. Optical conductivity fits with current polaron models provide grounds toward a microscopic understanding of transport properties in these as-prepared granular films. (C) 2007 Elsevier Ltd. All rights reserved

Raman and infrared spectroscopy of Sr2B′UO6 (B′ = Ni; Co) double perovskites

Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co).Fil: Moreira, A. F. L.. Universidade Estadual de Campinas; BrasilFil: García Flores, A. F.. Universidade Estadual de Campinas; BrasilFil: Granados, E.. Universidade Estadual de Campinas; BrasilFil: Massa, Nestor Emilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Química Inorgánica "Dr. Pedro J. Aymonino". Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Centro de Química Inorgánica "Dr. Pedro J. Aymonino"; ArgentinaFil: Pinacca, Ruben Miguel. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia; ArgentinaFil: Pedregosa, Jose Carmelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Investigaciones en Tecnología Química. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Instituto de Investigaciones en Tecnología Química; Argentina. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia; ArgentinaFil: Carbonio, Raul Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Muñoz, A.. Universidad Carlos III de Madrid. Instituto de Salud; EspañaFil: Martínez Lope, M. J.. Instituto de Ciencia de Materiales de Madrid; EspañaFil: Alonso, J. A.. Instituto de Ciencia de Materiales de Madrid; EspañaFil: del Campo, L.. Conditions Extrêmes et Matériaux Haute Température et Irradiation; FranciaFil: De Sousa Meneses, D.. Conditions Extrêmes et Matériaux Haute Température et Irradiation; FranciaFil: Echegut, P.. Conditions Extrêmes et Matériaux Haute Température et Irradiation; Franci

The Black Polymorph of TTF-CA: TTF Polymorphism and Solvent Effects in Mechanochemical and Vapor Digestion Syntheses, FT-IR, Crystal Packing, and Electronic Structure

Tetrathiafulvalene-chloranil (TTF-CA) was synthesized by two methods, liquid assisted grinding (LAG) and vapor digestion (VD), which largely reduce the use of reaction solvents. The effects of the small quantities of LAG solvent and solvent vapors in VD toward the formation of a particular TTF-CA product polymorph were studied from both tetrathiafulvalene forms (orange and brown) as reactants. It was concluded that a high solvent polarity index favors the formation of the ionic black polymorph of TTF-CA vs the quasi-neutral green form, whereas the crystal structure and crystal habit of the orange tetrathiafulvalene polymorph also favors the formation of the black TTF-CA. The crystal structure of the black TTF-CA was determined from synchrotron X-ray powder diffraction (XRPD), and it consists of dimerized TTF^+• and CA^–• radical ions, in agreement with room temperature magnetic susceptibility measurements indicating the material is diamagnetic. FT-IR showed that the compound is a semiconductor with a small band gap of ∼0.198 eV and it remains ionic at low temperatures. The latter was confirmed by XRPD showing the black TTF-CA does not undergo a phase transition in the range 298–20 K. Band structure calculations are in good agreement with the measured band gap

The Black Polymorph of TTF-CA: TTF Polymorphism and Solvent Effects in Mechanochemical and Vapor Digestion Syntheses, FT-IR, Crystal Packing, and Electronic Structure

Tetrathiafulvalene-chloranil (TTF-CA) was synthesized by two methods, liquid assisted grinding (LAG) and vapor digestion (VD), which largely reduce the use of reaction solvents. The effects of the small quantities of LAG solvent and solvent vapors in VD toward the formation of a particular TTF-CA product polymorph were studied from both tetrathiafulvalene forms (orange and brown) as reactants. It was concluded that a high solvent polarity index favors the formation of the ionic black polymorph of TTF-CA vs the quasi-neutral green form, whereas the crystal structure and crystal habit of the orange tetrathiafulvalene polymorph also favors the formation of the black TTF-CA. The crystal structure of the black TTF-CA was determined from synchrotron X-ray powder diffraction (XRPD), and it consists of dimerized TTF^+• and CA^–• radical ions, in agreement with room temperature magnetic susceptibility measurements indicating the material is diamagnetic. FT-IR showed that the compound is a semiconductor with a small band gap of ∼0.198 eV and it remains ionic at low temperatures. The latter was confirmed by XRPD showing the black TTF-CA does not undergo a phase transition in the range 298–20 K. Band structure calculations are in good agreement with the measured band gap