Comment on "mechanisms for Pressure-Induced Isostructural Phase Transitions in EuO"

Authors thank the financial support from Projects PGC2018-101464-B-I00, PGC2018-097520-A-I00, and MALTA-Consolider Team RED2018-102612-T (Ministerio de Ciencia, Innovaci´on y Universidades) is acknowledged. V. Monteseguro acknowledges the “Beatriz Galindo” fellowship (BG20/000777) and the “Juan de la Cierva” fellowship (IJC2019-041586-I)

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 diferent 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 4f 75d0 ?4f 65d1 electronic transition, no consistent model of the pressure-induced modifcations 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 feld plays a major role in triggering the observed electronic transitions from semiconductor to the half-metal and fnally to the metallic state.Authors thank the financial support from Projects PGC2018-101464-B-I00, PGC2018-097520-A-I00 and MALTA-Consolider Team RED2018-102612-T (Ministerio de Ciencia, Innovación y Universidades) is acknowledged. V. Monteseguro acknowledges the “Beatriz Galindo” fellowship (BG20/000777) and the “Juan de la Cierva” fellowship (IJC2019-041586-I). Authors are grateful to the staff of the BM23 beamline and the high-pressure laboratory at the ESRF for their support during the experiment (proposal number HC-3913), and the SERCAMAT (SCTI) of the University of Cantabria for FTIR facilities

Crystal structure solution of a high-pressure polymorph of scintillating MgMoO4 and its electronic structure

The structure of the potentially scintillating high-pressure phase of [Beta] - MgMoO 4 ( γ - MgMoO 4 ) has been solved by means of high-pressure single-crystal x-ray diffraction. The phase transition occurs above 1.5 GPa and involves an increase of the Mo coordination from fourfold to sixfold accommodated by a rotation of the polyhedra and a concommitant bond stretching resulting in an enlargement of the c axis. A previous high-pressure Raman study had proposed such changes with a symmetry change to space group P 2 / c . Here it has been found that the phase transition is isosymmetrical ( C 2 / m -> C 2 / m ). The bulk moduli and the compressibilities of the crystal axes of both the low- and the high-pressure phase, have been obtained from equation of state fits to the pressure evolution of the unit-cell parameters which were obtained from powder x-ray diffraction up to 12 GPa. The compaction of the crystal structure at the phase transition involves a doubling of the bulk modulus B 0 changing from 60.3(1) to 123.7(8) GPa and a change of the most compressible crystal axis from the (0, b , 0) direction in [Beta] - MgMoO 4 to the ( 0.9 a , 0, 0.5 a ) direction in γ - MgMoO 4 . The lattice dynamical calculations performed here on γ - MgMoO 4 served to explain the Raman spectra observed for the high-pressure phase of [Beta] - MgMoO 4 in a previous work demonstrating that the use of internal modes arguments in which the MoO n polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of γ - MgMoO 4 was also calculated and compared with the electronic structures of [Beta] - MgMoO 4 and MgWO 4 shedding some light on why MgWO 4 is a much better scintillator than any of the phases of MgMoO 4 . These calculations yielded for γ - MgMoO 4 a Y 2 Γ -> Γ indirect band gap of 3.01 eV in contrast to the direct bandgaps of [Beta] - MgMoO 4 (3.58 eV at Γ ) and MgWO 4 (3.32 eV at Z ).The authors thank I. Collings and M. Handfland from the ID15B beamline at the ESRF for their help during the experiments, and O. Gomis from the Universitat Politècnica de València for the discussions. Most of the work presented in this work benefited from the financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) under Projects No. PID2019- 106383GB-C41/43 (MCIN/AEI/10.13039/501100011033), MALTA Consolider-Team network RED2018-102612- T (MINECO/AEI/10.13039/501100003329), and from the Generalitat Valenciana under Project PROMETEO/2018/123. V.M. also thanks the MICINN for the Beatriz Galindo distinguished researcher program (BG20/00077)

Pressure-induced charge ordering transition in CaMn7O12

We use high-pressure resistivity and single crystal x-ray diffraction at ambient and low temperature to investigate the charge ordering phase transition of CaMn 7 O 12 . We have found that at ambient temperature the Jahn-Teller distortion of the Mn 3 + O 6 octahedra rapidly decreases above 20 GPa, and vanishes at 28 GPa, when two Mn octahedral sites initially occupied by Mn 3 + and Mn 4 + become regular and equivalent as the result of a charge delocalization. Such a change correlates with a two orders of magnitude drop in the resistivity and a symmetry increase from the low-pressure rhombohedral R ¯ 3 phase to the cubic Im ¯ 3 structure, the same as one found at ambient pressure above 440 K. This yields the slope of the charge ordering phase boundary of d T c / d p ? ? 6 K/GPa. This result is further supported by the lack of a structural phase transition up to the maximum measured pressure of 30 GPa when the experiment is performed at 70 K. The satellite reflections of the structural modulation of the multiferroic phase of CaMn 7 O 12 observed at 70 K were found to hold up to 25 GPa with the structure keeping a constant modulation vector k = ( 0 0 0.925 ) with pressure. The average structure at 70 K does not show other indications of further phase transition.Y. Li and X. Du from Peking University are greatly acknowledged for growing and providing the CaMn7O12 crystals. D. Spahr and J. König from Goethe University are acknowledged for help with the single-crystal diffraction experiments. M.S. would like to acknowledge the financial support under the DFG-ANR Grant No. WI1232/41-1 and DFG GACR Project No. WI3320/3-1. V.M. and J.R.-F. thank the financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) for the Beatriz Galindo Program (BG20/000777) and for the Project No. PGC2018-097520- A-I00, respectively. DESY Photon Science is gratefully acknowledged. PETRA III at DESY is a member of the Helmholtz Association (HGF)

Física II (G417). Julio 2023

Grado en Ingeniería en Tecnologías Industriale

Física II (G417). Junio 2023

Grado en Ingeniería en Tecnologías Industriale