Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn). From the experimental results their band-gap energy has been determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV (CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The results are discussed in terms of the electronic structure of the studied tungstates. It has been found that those compounds where only the s electron states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4) have larger band-gap energies than those where also p, d, and f states of the A2+ cation contribute to the top of the valence band and the bottom of the conduction band (e.g. PbWO4). The results are of importance in view of the large discrepancies existent in prevoiusly published data.Comment: 16 pages, 3 figures, 1 tabl

Compressibility and structural stability of ultra-incompressible bimetallic interstitial carbides and nitrides

We have investigated by means of high-pressure x-ray diffraction the structural stability of Pd2Mo3N, Ni2Mo3C0.52N0.48, Co3Mo3C0.62N0.38, and Fe3Mo3C. We have found that they remain stable in their ambient-pressure cubic phase at least up to 48 GPa. All of them have a bulk modulus larger than 330 GPa, being the least compressible material Fe3Mo3C, B0 = 374(3) GPa. In addition, apparently a reduction of compressibility is detected as the carbon content increased. The equation of state for each material is determined. A comparison with other refractory materials indicates that interstitial nitrides and carbides behave as ultra-incompressible materials.Comment: 14 pages, 3 figures, 1 tabl

High-pressure study of substrate material ScAlMgO4

We report on the structural properties of ScAlMgO4 studied under quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at 300 K and 100 K. We found that the low-pressure phase remains stable up to 24 GPa. At 28 GPa, we detected a reversible phase transformation. The high-pressure phase is assigned to a monoclinic distortion of the low-pressure phase. No additional phase transition is observed up to 40 GPa. In addition, the equation of state, compressibility tensor, and thermal expansion coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found to be 143(8) GPa, with a strong compressibility anisotropy. For the trigonal low-pressure phase, the compressibility along the c-axis is twice than perpendicular one. A perfect lattice match with ZnO is retained under pressure in the pressure range of stability of wurtzite ZnO.Comment: 22 pages, 5 figures, 4 tables, 24 reference

Reversible Tuning of Ca Nanoparticles Embedded in a Superionic CaF2 Matrix

ABSTRACT: Controlling the size and shape of metallic colloids is crucial for a number of nanotechnological applications ranging from medical diagnosis to electronics. Yet, achieving tunability of morphological changes at the nanoscale is technically difficult and the structural modifications made on nanoparticles generally are irreversible. Here, we present a simple non-chemical method for controlling the size of metallic colloids in a reversible manner. Our strategy consists on applying hydrostatic pressure on a Ca cationic sublattice embedded in the irradiated matrix of CaF2 containing a large concentration of defects. Application of our method to CaF2 along with in situ optical absorption of the Ca plasmon shows that the radii of the Ca nanoparticles can be reduced with an almost constant rate of −1.2 nm/GPa up to a threshold pressure of ∼ 9.4 GPa. We demonstrate recovery of the original nanoparticles upon decompression of the irradiated matrix. The mechanisms for reversible nanocolloid-size variation are analyzed with first-principles simulations. We show that a pressure-driven increase in the binding energy between fluorine centers is responsible for the observed nanoparticle shrinkage. We argue that the same method can be used to generate other metallic colloids (Li, K, Sr, and Cs) with tailored dimensions by simply selecting an appropriate matrix

High-pressure structural investigation of several zircon-type orthovanadates

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low- and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO4 is less compressible than EuVO4 and LuVO4, being the most incompressible orthovanadate studied to date. The sequence of structural transitions and compressibilities are discussed in comparison with other zircon-type oxides.Comment: 34 pages, 2 Tables, 11 Figure

Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectric A(x)Ba(1-x)Nb(2)O(6) (A: Sr,Ca)

[EN] In this letter, we have investigated the electronic structure of A(x)Ba(1-x)Nb(2)O(6) relaxor ferroelectrics on the basis of optical absorption spectroscopy in unpoled single crystals with A = Sr and Ca under high pressure. The direct character of the fundamental transition could be established by fitting Urbach's rule to the photon energy dependence of the absorption edge yielding bandgaps of 3.44(1) eV and 3.57(1) eV for A = Sr and Ca, respectively. The light scattering by ferroelectric domains in the pre-edge spectral range has been studied as a function of composition and pressure. After confirming with x-ray diffraction the occurrence of the previously observed ferroelectric to paraelelectric phase transition at 4 GPa, the light scattering produced by micro-and nano-ferroelectric domains at 3.3 eV in Ca0.28Ba0.72Nb2O6 has been probed. The direct bandgap remains virtually constant under compression with a drop of only 0.01 eV around the phase transition. Interestingly, we have also found that light scattering by the polar nanoregions in the paraelectric phase is comparable to the dispersion due to ferroelectric microdomains in the ferroelectric state. Finally, we have obtained that the bulk modulus of the ferroelectric phase of Ca0.28Ba0.72Nb2O6 is B-0 = 222(9) GPa. Published by AIP Publishing.J.R.-F. acknowledges the Spanish MINECO for the Juan de la Cierva (IJCI-2014-20513) Program and Dr. Bayarjargal from the Goethe-Universitat Frankfurt for providing the CBN28 samples. This work was supported by Spanish MINECO under Grant No. MAT2016-75586-C4-1-P/2-P. The high pressure x-ray diffraction experiments were performed at MSPD beamline at ALBA Synchrotron (Project 2016021588) with the collaboration of ALBA staff.Ruiz-Fuertes, J.; Gomis, O.; Segura, A.; Bettinelli, M.; Burianek, M.; Muehlberg, M. (2018). Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectric A(x)Ba(1-x)Nb(2)O(6) (A: Sr,Ca). Applied Physics Letters. 112(4). https://doi.org/10.1063/1.5012111S112

Crystal-field mediated electronic transitions of EuS up to 35 GPa

An advanced experimental and theoretical model to explain the correlation between the electronic and local structure of Eu2+ in two different environments within a same compound, EuS, is presented. EuX monochalcogenides (X: O, S, Se, Te) exhibit anomalies in all their properties around 14 GPa with a semiconductor to metal transition. Although it is known that these changes are related to the 4f75d0 → 4f65d1 electronic transition, no consistent model of the pressure-induced modifications of the electronic structure currently exists. We show, by optical and x-ray absorption spectroscopy, and by ab initio calculations up to 35 GPa, that the pressure evolution of the crystal field plays a major role in triggering the observed electronic transitions from semiconductor to the half-metal and finally to the metallic state

Tracking Turbulent Coherent Structures by Means of Neural Networks

[EN] The behaviours of individual flow structures have become a relevant matter of study in turbulent flows as the computational power to allow their study feasible has become available. Especially, high instantaneous Reynolds Stress events have been found to dominate the behaviour of the logarithmic layer. In this work, we present a viability study where two machine learning solutions are proposed to reduce the computational cost of tracking such structures in large domains. The first one is a Multi-Layer Perceptron. The second one uses Long Short-Term Memory (LSTM). Both of the methods are developed with the objective of taking the the structures' geometrical features as inputs from which to predict the structures' geometrical features in future time steps. Some of the tested Multi-Layer Perceptron architectures proved to perform better and achieve higher accuracy than the LSTM architectures tested, providing lower errors on the predictions and achieving higher accuracy in relating the structures in the consecutive time steps.This work was supported by RTI2018-102256-B-I00 of MINECO/FEDER. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, reference AECT-2020-2-0005.Aguilar-Fuertes, JJ.; Noguero-Rodríguez, F.; Jaen Ruiz, JC.; García-Raffi, LM.; Hoyas, S. (2021). Tracking Turbulent Coherent Structures by Means of Neural Networks. Energies. 14(4):1-15. https://doi.org/10.3390/en1404098411514

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate

The double carbonate BaCa(CO3)2 holds potential as host compound for carbon in the Earth?s crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO3)2 phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO3)2 polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO3 and BaCO3 end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.Authors thank the fnancial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) and the Agencia Estatal de Investigación under projects MALTA Consolider Ingenio 2010 network (RED2018-102612-T), PID2019-106383GB-C44, FIS2017-83295-P and PGC2018-097520-A-I00 (cofnanced by EU FEDER funds), and from the Generalitat Valenciana under project PROMETEO/2018/123. A.O.R. acknowledges the fnancial support of the Spanish MINECO RyC-2016-20301 Ramon y Cajal Grant. Authors also thank Dr. Nicolescu and the Mineralogy and Meteoritic Department of the Yale Peabody Museum of Natural History for providing the mineral samples, the MALTA Consolider supercomputing centre and Compute Canada for computational resources, the General Services of Research Support (SEGAI) at La Laguna University and ALBA-CELLS synchrotron for providing beamtime under experiments 2020084419 and 2021024988. Tese experiments were performed at the MSPD beamline with the collaboration of ALBA staf