Size-Dependent High-Pressure Behavior of Pure and Eu3+-Doped Y2O3 Nanoparticles: Insights from Experimental and Theoretical Investigations.

We report a joint high-pressure experimental and theoretical study of the structural, vibrational, and photoluminescent properties of pure and Eu3+-doped cubic Y2O3 nanoparticles with two very different average particle sizes. We compare the results of synchrotron X-ray diffraction, Raman scattering, and photoluminescence measurements in nanoparticles with ab initio density-functional simulations in bulk material with the aim to understand the influence of the average particle size on the properties of pure and doped Y2O3 nanoparticles under compression. We observe that the high-pressure phase behavior of Y2O3 nanoparticles depends on the average particle size, but in a different way to that previously reported. Nanoparticles with an average particle size of ~37 nm show the same pressure-induced phase transition sequence on upstroke and downstroke as the bulk sample; however, nanoparticles with an average particle size of ~6 nm undergo an irreversible pressure-induced amorphization above 16 GPa that is completed above 24 GPa. On downstroke, 6 nm nanoparticles likely consist of an amorphous phase.

Joint experimental and theoretical study of PbGa2S4 under compression.

The effect of pressure on the structural, vibrational, and optical properties of lead thiogallate, PbGa2S4, crystallizing under room conditions in the orthorhombic EuGa2S4-type structure (space group Fddd), is investigated. The results from X-ray diffraction, Raman scattering, and optical-absorption measurements at a high pressure beyond 20 GPa are reported and compared not only to ab initio calculations, but also to the related compounds α'-Ga2S3, CdGa2S4, and HgGa2S4. Evidence of a partially reversible pressure-induced decomposition of PbGa2S4 into a mixture of Pb6Ga10S21 and Ga2S3 above 15 GPa is reported. Thus, our measurements and calculations show a route for the high-pressure synthesis of Pb6Ga10S21, which is isostructural to the stable Pb6In10S21 compound at room pressure.

Unveiling the role of the lone electron pair in sesquioxides at high pressure: compressibility of ß-Sb2O3.

The structural, vibrational and electronic properties of the compressed ß-Sb2O3 polymorph, a.k.a. mineral valentinite, have been investigated in a joint experimental and theoretical study up to 23 GPa. The compressibility of the lattice parameters, unit-cell volume and polyhedral unit volume as well as the behaviour of its Raman- and IR-active modes under compression have been interpreted on the basis of ab initio theoretical simulations. Valentinite shows an unusual compressibility up to 15 GPa with four different pressure ranges, whose critical pressures are 2, 4, and 10 GPa. The pressure dependence of the main structural units, the lack of soft phonons, and the electronic density charge topology address the changes at those critical pressures to isostructural phase transitions of degree higher than 2. In particular, the transitions at 2 and 4 GPa can be ascribed to the changes in the interaction between the stereochemically-active lone electron pairs of Sb atoms under compression. The changes observed above 10 GPa, characterized by a general softening of several Raman- and IR-active modes, point to a structural instability prior to the 1st-order transition occurring above 15 GPa. Above this pressure, a tentative new high-pressure phase (s.g. Pcc2) has been assigned by single-crystal and powder X-ray diffraction measurements.

PrVO4 under High Pressure: Effects on Structural, Optical, and Electrical Properties.

In the pursuit of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO4 under high-pressure conditions, up to 24 GPa. We have found that PrVO4 undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO4-II type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. The monazite phase is thus a metastable polymorph of PrVO4. The monazite-BaWO4-II transition is found instead to be reversible and occurs with a similar volume change. Here we report and discuss the axial and bulk compressibility of all phases. We also compare our results with those for other rare-earth orthovanadates. Finally, by means of optical-absorption experiments and resistivity measurements, we determined the effect of pressure on the electronic properties of PrVO4. We found that the zircon-monazite transition produces a collapse of the band gap and an abrupt decrease in the resistivity. The physical reasons for this behavior are discussed. Density functional theory simulations support our conclusions.

High-pressure Raman investigation of high index facets bounded α-Fe2O3 pseudocubic crystals.

High index facet bounded α-Fe2O3 pseudocubic crystals has gained the attention of the scientific community due to its promising electrochemical sensing response towards aqueous ammonia. The structural stability of α-Fe2O3 pseudocubic crystals is investigated through high-pressure Raman spectroscopy up to 22.2 GPa, and those results are compared with our ab initio theoretical calculations. The symmetry of the experimental Raman-active modes has been assigned by comparison with theoretical data. In addition to the Raman-active modes, two additional Raman features are also detected, whose intensity increases with compression. The origin of these two additional peaks addressed in this study, reveals a strong dependence on the geometry and the low dimensionality as the most plausible explanation.

Structural and Lattice-Dynamical Properties of Tb2O3 under Compression: A Comparative Study with Rare Earth and Related Sesquioxides.

We report a joint experimental and theoretical investigation of the high pressure structural and vibrational properties of terbium sesquioxide (Tb2O3). Powder X-ray diffraction and Raman scattering measurements show that cubic Ia3̅ (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic C2/m (B-type) phase at ∼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3̅m1 (A-type) phase at ∼12 GPa. Thus, Tb2O3 is found to follow the well-known C → B → A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb. Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results. Moreover, Raman-active modes of the three phases have been monitored as a function of pressure, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase. Finally, we extract the bulk moduli and the Raman-active mode frequencies together with their pressure coefficients for the three phases of Tb2O3. These results are thoroughly compared and discussed in relation to those reported for rare earth and other related sesquioxides as well as with new calculations for selected sesquioxides. It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals.

Orpiment under compression: metavalent bonding at high pressure.

We report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of layered monoclinic arsenic sulfide crystals (α-As2S3), aka mineral orpiment, under compression. X-ray diffraction and Raman scattering measurements performed on orpiment samples at high pressure and combined with ab initio calculations have allowed us to determine the equation of state and the tentative assignment of the symmetry of many Raman-active modes of orpiment. From our results, we conclude that no first-order phase transition occurs up to 25 GPa at room temperature; however, compression leads to an isostructural phase transition above 20 GPa. In fact, the As coordination increases from threefold at room pressure to more than fivefold above 20 GPa. This increase in coordination can be understood as the transformation from a solid with covalent bonding to a solid with metavalent bonding at high pressure, which results in a progressive decrease of the electronic and optical bandgap, an increase of the dielectric tensor components and Born effective charges, and a considerable softening of many high-frequency optical modes with increasing pressure. Moreover, we propose that the formation of metavalent bonding at high pressures may also explain the behavior of other group-15 sesquichalcogenides under compression. In fact, our results suggest that group-15 sesquichalcogenides either show metavalent bonding at room pressure or undergo a transition from p-type covalent bonding at room pressure towards metavalent bonding at high pressure, as a precursor towards metallic bonding at very high pressure.

Experimental and Theoretical Study of SbPO4 under Compression.

SbPO4 is a complex monoclinic layered material characterized by a strong activity of the nonbonding lone electron pair (LEP) of Sb. The strong cation LEP leads to the formation of layers piled up along the a axis and linked by weak Sb-O electrostatic interactions. In fact, Sb has 4-fold coordination with O similarly to what occurs with the P-O coordination, despite the large difference in ionic radii and electronegativity between both elements. Here we report a joint experimental and theoretical study of the structural and vibrational properties of SbPO4 at high pressure. We show that SbPO4 is not only one of the most compressible phosphates but also one of the most compressible compounds of the ABO4 family. Moreover, it has a considerable anisotropic compression behavior, with the largest compression occurring along a direction close to the a axis and governed by the compression of the LEP and the weak interlayer Sb-O bonds. The strong compression along the a axis leads to a subtle modification of the monoclinic crystal structure above 3 GPa, leading from a 2D to a 3D material. Moreover, the onset of a reversible pressure-induced phase transition is observed above 9 GPa, which is completed above 20 GPa. We propose that the high-pressure phase is a triclinic distortion of the original monoclinic phase. The understanding of the compression mechanism of SbPO4 can aid to improve the ion intercalation and catalytic properties of this layered compound.

Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations.

The high-pressure behavior of technologically important visible-light photocatalytic semiconductor InNbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calculations. The experimental results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compound beyond 10.8 GPa. The large volume collapse associated with the phase transition and the coexistence of two phases observed over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination number around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The experimental pressure-volume data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low- and high-pressure phases, respectively. The pressure dependence of the Raman mode frequencies and Grüneisen parameters was determined for both phases by experimental and theoretical methods. The same information is obtained for the infrared modes from first-principles calculations. Results from theoretical calculations corroborate the experimental findings. They also provide information on the compressibility of interatomic bonds, which is correlated with the macroscopic properties of InNbO4.

Pressure-induced phase transition in hydrothermally grown ZnO nanoflowers investigated by Raman and photoluminescence spectroscopy.

This paper reports the pressure-dependent photoluminescence and Raman spectral investigation of hydrothermally synthesized ZnO nanoflowers at room temperature. Intrinsic near-band-edge UV emission from ZnO nanoflowers is monotonously blue-shifted under pressures up to 13.8 GPa with a pressure coefficient of 26 meV GPa(-1), and this pressure value is nearly 5 GPa above the transition pressure from the wurtzite to the rock salt phase for bulk ZnO. The Raman band corresponds to the wurtzite phase, the [Formula: see text] and [Formula: see text] modes were observed up to about 11 GPa from the spectra. The apparent discrepancy in the transition pressures as determined from photoluminescence and Raman studies suggests that it is a consequence of the gradual phase transition, in which the smallest nanoparticles are expected to remain in the wurtzite phase up to 13-15 GPa.

Compressibility and structural stability of nanocrystalline TiO2 anatase synthesized from freeze-dried precursors.

The high-pressure structural behavior of 30 nm nanoparticles of anatase TiO2 was studied under hydrostatic and quasi-hydrostatic conditions up to 25 GPa. We found that the structural sequence is not sensitive to the use of different pressure transmitting media. Anatase-type nanoparticles exhibit a phase transition beyond 12 GPa toward a baddeleyite-type structure. Under decompression this phase transition is irreversible, and a metastable columbite-type structure is recovered at ambient conditions. The bulk modulus of anatase-type nanoparticles was determined confirming that nanoparticles of TiO2 are more compressible than bulk TiO2. Similar conclusions were obtained after the determination of the bulk modulus of baddeleyite-type nanoparticles. Furthermore, axial compressibilities and the effect of pressure in atomic positions, bond distances, and bond angles are determined. Finally, a possible physical explanation for the destabilization of anatase under pressure is proposed based upon this information.

Status of the hard X-ray microprobe beamline ID22 of the European Synchrotron Radiation Facility.

The ESRF synchrotron beamline ID22, dedicated to hard X-ray microanalysis and consisting of the combination of X-ray fluorescence, X-ray absorption spectroscopy, diffraction and 2D/3D X-ray imaging techniques, is one of the most versatile instruments in hard X-ray microscopy science. This paper describes the present beamline characteristics, recent technical developments, as well as a few scientific examples from recent years of the beamline operation. The upgrade plans to adapt the beamline to the growing needs of the user community are briefly discussed.