CaF2 Structure Research Articles

Isotopic effect of oxygen on the Raman mapping of a polycrystalline uranium dioxide UO2

AbstractUranium dioxide (UO2) is a widely studied material due to its use as fuel in pressurised water reactors (PWR), and Raman spectroscopy is a technique of choice to characterise the evolution of its microstructure. UO2 crystallises in a fluorite CaF2 (space group Fm‐3m) structure that gives rise to a unique Raman signature, the T2g band. However, several other bands are often detected whose attribution remains unclear. The present study gives new insights on the Raman spectrum of UO2 thanks to the combination of isotopic labelling with 18O and Raman imaging. In addition to the expected T2g, U2 (LO), 2LO and U3 bands, we have detected a doublet at 885 and 925 cm−1, a U* band at 555 cm−1 in some specific areas and two bands located at 367 and 1196 cm−1. All Raman bands shift under the effect of the replacement of 16O by 18O, excepting for the U* band that could not be detected anymore. The isotopic shift ratio could be determined for 20% and 30% 18O labelling. No discrepancy in band position is observed between grains and grain boundaries, except for the U2(LO) band. We also evidence a difference between the U defect bands and the 885 and 925 cm−1 doublet bands evolution under labelling, although the latter also seems to be connected to the presence of defects in the material.

Investigating intrinsic and extrinsic defects in an inorganic hybrid system (BaTiO3–CaF2) via vacuum ultraviolet-excited low-temperature luminescence

Investigations into the luminescence of undoped and Eu-doped BaTiO3@CaF2 hybrid compound, as well as its individual components, were performed at both room and low temperatures, and the findings were discussed in the context of intrinsic and extrinsic defects present in both matrices. The synthesis process employed an eco-friendly method, capable of producing crystalline phases in just 1 min of plateau time at temperatures as low as 140 °C. The microwave-hydrothermal method can provide free oxygen ions at the surface, allowing them to interact with the ions from the precursor solution of CaF2. For Eu-doped samples, while the isolated CaF2 showed the characteristic 4f65d1-4f7 Eu2+ transition, BaTiO3@CaF2 hybrid compound showed the association of two defects, resulting in emissions related to defects from oxygen ions as well as Eu2+. The evidence of intrinsic defects was confirmed by comparing the samples measured at 60 K, where the influence of lattice vibrations was reduced, leading to a decrease in non-radiative processes. Another difference observed was the presence of the 4f-4f transitions associated with the presence of Eu3+ in this sample. Discussions were further supported by X-ray Absorption Spectroscopy (XAS) results, performed at the Ti K-edge and Eu L3-edge. EXAFS parameters were estimated based on the incorporation of Eu into the CaF2 structure and replacing Ca sites. For the BaTiO3@CaF2:Eu system, both EuBa and EuTi had to be included in the fitting process, indicating that some of the Eu dopant is either in the lattice of CaF2 or in BaTiO3.

Experimental determination of the H–Hf phase diagram using in situ neutron diffraction

The Hf–H(D) system was studied using in situ neutron diffraction as a function of composition (up to 2 D/Hf) and temperature (up to 900 °C). This allowed to complement the previous knowledge about this system, in particular from the points of view of the phase existence and of the phase diagram. Two major findings are: the observation of a new phase (γ, PtS structure type) of composition HfH similar to the monohydride found in H–Zr system, in accordance with previous first-principles calculations, and the structure determination of the δ' phase (new structure type), an ordered superstructure of the δ phase (CaF2 structure type). Phase relations are established, and a new phase diagram is proposed.

Electron diffraction study and ionic conductivity of fluorite Ba1-xLaxF2+x and tysonite La1-yBayF3-y phases in the BaF2-LaF3 system

Crystals of fluorite phases with the CaF2 structure: Ba1−xLaxF2+x (x = 0, 0.05, 0.105, 0.25, 0.35, 0.40) and those of tysonite ones with the LaF3 structure: La1−yBayF3−y (y = 0, 0.05, 0.09) have been studied by electron diffraction and elemental analysis in a transmission electron microscope. Diffraction patterns of Ba1−xLaxF2+x crystals with x = 0.105, 0.25 exhibit diffuse scattering, which indicates the presence of clusters of structural defects. Blurred supercell reflections are seen in diffraction patterns of Ba1−xLaxF2+x crystals with a high lanthanum content (x = 0.35, 0.40) in addition to diffuse scattering. They indicate the existence of nanoscale ordered crystallites as well as clusters. It was found that LaF3 and the tysonite phase La1−yBayF3−y with a low barium content (y = 0.05) is of a low-temperature l-form (space group P3¯c1, Z = 6). A small fraction of tysonite crystallites in La1-yBayF3-y crystals with y = 0.09 is of a high-temperature h-form (space group P63/mmc, Z = 2). An intermediate crystal structure between l- and h-forms was also found.The maximum ionic conductivity σdc of tysonite crystals La1−yBayF3−y at 293 K is 800 times higher than that of fluorite ones Ba1−xLaxF2+x. A correlation was found between logarithm of conductivity and composition: lg σdc(x) = ax + b for fluorite Ba1−xLaxF2+x and tysonite La1−yBayF3−y crystals. The reason for the additive law of change in ionic conductivity in both types of nonstoichiometric phases in the BaF2-LaF3 system is apparently the similarity of defect structures of fluorite and tysonite phases.

Displacement of hydrogen position in di-hydride of V-Ti-Cr solid solution alloys

Local structure in the di-hydride (di-deuteride) phases of V-Ti-Cr solid solution alloys (V10Ti35Cr55, V50Ti20Cr30 and V80Ti8Cr12) were investigated using synchrotron X-ray and neutron total scattering experiments. Both Rietveld refinement and pair distribution function (PDF) refinement of the X-ray scattering data indicated that the crystal structure of the metal lattice was a face centered cubic (FCC) structure and no difference between their local structure and average structure was observed. Rietveld refinement of neutron scattering data showed that the crystal structure was a CaF2 structure. However, the CaF2 structure model did not reproduce the first peak corresponding to the metal-hydrogen correlation in neutron PDF patterns. When special quasi-random structure (SQS) models based on the CaF2 structure are applied for the refinements, the SQS structure model, relaxed via first-principles calculations, reproduced the whole neutron PDF pattern. Distribution of interatomic distances between hydrogen and metal atoms in the relaxed SQS models showed that interatomic distance of hydrogen with Cr was shorter than that with V and that with Ti was longer than that with V, independently of the chemical compositions. The combination of neutron PDF and first-principles calculations with the SQS models revealed that hydrogen atoms did not occupy the center of tetrahedron but moved toward Cr atoms and away from Ti.

The universal defect cluster architecture of fluorite-type nanostructured crystals

A universal cluster model applicable to the most common nonstoichiometry in inorganic fluorides with the CaF2 structure.

Luminescence of Nanocrystalline Si in (Si + CaF2)/CaF2 Structures in the Visible Range of the Spectrum

Luminescence of Nanocrystalline Si in (Si + CaF2)/CaF2 Structures in the Visible Range of the Spectrum

Displacements in the Cationic Motif of Nonstoichiometric Fluorite Phases Ba1−xRxF2+x as a Result of the Formation of {Ba8[R6F68–69]} Clusters: III. Defect Cluster Structure of the Nonstoichiometric Phase Ba0.69La0.31F2.31 and Its Dependence on Heat Treatment

The defect structure of Ba0.69La0.31F2.31 single crystals in as-grown state and after annealing at 1173 K for 336 h was studied by X-ray diffraction analysis. Both crystals belong to the CaF2 structure type (sp. gr. Fm3¯m). They have vacancies in the main anion motif and interstitial fluorine anions in Wyckoff positions 48i and 4b. Relaxation (static displacement of some main anions to Wyckoff position 32f) is observed in the annealed crystal. It was established that annealing leads to a change in the type of displacement of the main anions in Wyckoff positions 8c from dynamic to static. Displacement of La3+ cations to Wyckoff position 32f is observed in both crystals. A model of the defect structure of Ba0.69La0.31F2.31 is proposed, according to which interstitial fluorine anions and La3+ cations are aggregated into [Ba14−nLanF64+n] clusters with the cuboctahedral anionic core formed by interstitial fluorine anions in Wyckoff positions 48i. Ba2+ cations are located in the cluster in the centers of the faces, and the La3+ cations are shifted by 0.24 Å from the vertices of the cluster along the three-fold axis towards the center of the cluster. The study establishes the relationship between the defect structure of crystals and their structurally sensitive properties, and to develop approaches to their management.

Synthesis, crystal chemistry, and magnetic properties of Ce3-xRhGa10+3x (x = 0.36): A new member of the [BaAl4]m[CaF2]n[AlB2]p homologous series

Ce3-xRhGa10+3x (x = 0.36) is a new member of the linear intergrowth structure series built up from segments of the BaAl4, AlB2 and CaF2 structure types. The compound crystallizes with a disordered variant of the Pr3NiGa10 type structure. Unlike the prototype, Ce3-xRhGa10+3x exhibits notable deficiency in rare-earth content, accompanied with significant excess of Ga atoms, corresponding to additional Ga environment of one of the Ce atoms. The novel compound is a Curie-Weiss paramagnet and orders antiferromagnetically at low temperatures. The electrical transport in Ce3-xRhGa10+3x has a metallic character, and bears contributions due to crystalline electric field, Kondo and magnetic exchange interactions.

Nanostructured Crystals of Fluorite Phases Sr1–xRxF2+x (R Are Rare-Earth Elements) and Their Ordering. 16: Defect Structure of the Nonstoichiometric Phases Sr1–xRxF2+x (R = Pr, Tb–Yb) As Grown

The defect structure of as grown Sr0.72Pr0.28F2.28, Sr0.87Tb0.13F2.13, Sr0.88Dy0.12F2.12, Sr0.89Ho0.11F2.11, and Sr0.9R0.1F2.1 single crystals (R = Y, Er, Tm, Yb) prepared under identical conditions has been studied by X-ray diffraction analysis. All crystals belong to the CaF2 structure type (sp. gr. $$Fm\bar {3}m$$ ) and have vacancies in the main anion motif and interstitial fluorine anions at site 32f in the crystal with R = Pr and site 48i for the other R. Interstitial fluorine anions are found near site 4b in the crystals with R = Tb and Dy (sites 32f and 24e, respectively) and at site 4b in the crystal with R = Er. Relaxation (static displacement of some main anions to site 32f) is observed in all crystals (except for R = Tb). Displacements of some Sr2+ cations to sites 32f and 24e is simultaneously observed in crystals with R = Tb, Dy, Tm, and Yb. In crystals with R = Ho, Y, and Er, some Sr2+ cations are displaced only to site 32f; no cation displacement is observed for R = Pr. A model of the defect structure of the phases is proposed based on the structure of crystals studied, according to which interstitial fluorine anions and R3+ cations are aggregated into [Sr4–nRnF26] clusters of tetrahedral configuration for the phase with R = Pr or [Sr14–nRnF64+n] clusters of octacubic configuration for other R.

Hydrogen storage properties of Nb-based solid solution alloys with a BCC structure

Hydrogen storage properties of Nbx(Ti0.7Cr0.3)1-x (x = 0.3, 0.4, 0.5, 0.6) alloys with a body centered cubic (BCC) structure and the crystal structure of their hydrides were investigated. These alloys absorbed hydrogen up to around 1.8 H/M by forming a di-hydride phase in the absorption process, and desorbed to 0.8 H/M to transform to a mono-hydride phase during the desorption process. These behaviors are similar to those of Ti–Cr–V and Ti–Cr–Mo alloys with the BCC structure. XRD patterns indicated that the alloys transformed to the mono-hydride with a slightly distorted BCC structure (orthorhombic structure), and then to the di-hydride with a CaF2 structure. With increasing Nb content, both the lattice constants for all phases, and the equilibrium pressures for the absorption and desorption processes increased. This indicated that Nbx(Ti0.7Cr0.3)1-x does not follow the well-known relationship of equilibrium pressures decreasing with increasing lattice constants. However, the logarithmic equilibrium pressures had a linear relationship with their estimated bulk modulus.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 15. Concentration Dependence of the Defect Structure of As Grown Nonstoichiometric Phases Sr1 – xRxF2 + x (R = Sm, Gd)

The defect structure of as grown single crystals Sr1 – xSmxF2 + x (x = 0.14, 0.26) and Sr1 – xGdxF2 + x (x = 0.12, 0.19, 0.30) grown under identical conditions was studied by X-ray diffraction. All crystals belong to the CaF2 structure type, sp. gr. $$Fm\bar {3}m$$. All phases contain interstitial fluoride ions in the 48i site and vacancies in the main anionic motif. In Sr0.70Gd0.30F2.30 interstitial fluoride anions were found also in the 4b site; in Sr0.88Gd0.12F2.12, in the 32f site. Relaxation displacements of a part of F(8c) anions to the 32f site are observed in Sr0.86Sm0.14F2.14. Displacements of cations to the 24e site occur in all the phases under study; in the phases with R = Gd, there are also displacements to the 32f site. A model of the defect structure of the phases Sr1 ‒ xRxF2 + x (R = Sm, Gd) is proposed. According to this model, interstitial fluoride anions and R3+ cations are grouped into octacubic clusters [Sr14 – nRnF64 + n]. The average number of R3+ per cluster decreases from 6 to 4.5 with increasing х, the volume of the cluster core increases from 61.2(1) to 65.9(2) A3, and the volume per cluster decreases from 2419.9(5) to 783.5(2) A3.

Luminescence of Eu3+ Activated CaF2 and SrF2 Nanoparticles: Effect of the Particle Size and Codoping with Alkaline Ions

Eu3+ doped CaF2 and SrF2 nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na+ or K+ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu3+ ions enter into the fluorite CaF2 and SrF2 structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu3+ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu3+ sites are prevalent for bigger nanoparticles. Eu3+ ions in high symmetry sites present lifetimes of the 5D0 level around 27 ms, among the longest lifetimes found in the literature for Eu3+ activated materials. As a proof of concept of possible use of the Eu3+ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescenc...

Structure and reactivity of Pt–In intermetallic alloy nanoparticles: Highly selective catalysts for ethane dehydrogenation

The structure of silica supported Pt and Pt–In bimetallic catalysts with nominal In:Pt atomic ratios of 0.7 and 1.4 were determined by in situ synchrotron XAS and XRD. It was seen that the addition of In led to the formation of two different intermetallic alloy phases. At an In:Pt ratio of 0.7 the Pt3In phase with a Cu3Au structure was formed. When the ratio was increased to 1.4 a shell of PtIn2 having a CaF2 structure formed around a core of Pt3In. The catalysts were tested for ethane dehydrogenation at 600°C to determine the effect of alloying on ethylene selectivity and turnover rate (TOR). The monometallic Pt catalysts was 73% selective for ethylene and had an initial TOR of 0.7s−1. Both alloy catalysts were ≈100% selective for dehydrogenation and had higher initial TOR, 2.8s−1 and 1.6s−1 for In:Pt ratio of 0.7 and 1.4, respectively. The increase in selectivity is attributed to the elimination of large Pt ensembles resulting from geometric changes to the catalyst surface upon alloying. Electronic changes due to the formation of Pt–In bonds are thought to be responsible for the increases in TOR in the alloy catalysts.

Helium Shows New Chemistry Not Seen Anywhere Else

Work by Dong et al., recently published in Nature Chemistry , shows that the chemistry of noble gases, far from being restricted to a few instances of covalent bonding with highly electronegative elements, is more interesting than we thought—more interesting than that of most elements, even.

First principle calculation of accurate native defect levels in CaF2

We report on the first principle density functional calculation of the charge transition levels of native defects (vacancies and interstitials) in CaF2 structure. The transition level was defined as the Fermi level where two charge states of given defect have the same formation energy. The common error in the band gap inherited to semiclocal density functional has been accounted for by incorporating the hybrid density functional method, leading to correct placement of the transition levels within the band gap. The band gap size from hybrid calculation has been validated using the full potential, Linearized Augmented Planewave method with the Modified-Becke-Johnson exchange potential. Prior to level calculations, we ensured that an agreement between the formation energies from small (95–97 atoms) and large (323–325 atoms) supercells was achieved after applying the Makov-Payne correction method. Our calculated transition level for the anion vacancy was 2.97 eV below the conduction band, agreeing with the experimental optical absorption band at 3.3 eV associated with the electron transition from the ground state F-center to the conduction band in CaF2.

Hydriding of TiMo alloys at high hydrogen pressures

We studied the interaction of Ti0.40Mo0.60 and Ti0.34Mo0.66 alloys with hydrogen and obtained hydrogen desorption isotherms at pressures of up to 250 MPa. At high hydrogen pressures, we observed the formation of Ti0.40Mo0.60Н1.1 and Ti0.34Mo0.66Н0.8 hydride phases. According to X-ray diffraction data, the hydrides consisted of phases with a body-centered cubic and face-centered cubic (CaF2 structure) lattices. The structure of the deuteride based on the Ti0.40Mo0.60 alloy was studied by neutron diffraction. We identified the sites occupied by deuterium atoms and determined their occupancies.

Ionic conductivity of crystallization products of Ba1–x Yb x F2 + x melts (x = 0.1, 0.2, 0.25)

The ionic conductivity σ of the crystallization products of Ba1 - xYbxF2 + x melts with 10, 20 and 25 mol % YbF3 has been studied. A Ba0.9Yb0.1F2.1 sample is a solid solution with the CaF2 structure type, sp. gr. Fm\(\overline 3 \)m. A Ba0.8Yb0.2F2.2 sample contains two cubic forms with sp. gr. Fm\(\overline 3 \)m and Pm\(\overline 3 \)m. The σ values for Ba0.9Yb0.1F2.1 and Ba0.8Yb0.2F2.2 coincide and are equal to ~3 × 10–5 S/cm at 500 K. A Ba0.75Yb0.25F2.25 sample is heterogeneous, despite its monolithic nature and transparency. A greater part of its volume has a cubic lattice with sp. gr. Pm\(\overline 3 \)m, while the smaller part is a phase crystallizing in the orthorhombic system. A change of composition from x = 0.2 to 0.25 leads to a change in the symmetry group and type of the cluster defects in the Ba1 - xYbxF2 + x phase. The sp. gr. Fm\(\overline 3 \)m is replaced by the sp. gr. Pm\(\overline 3 \)m, and octahedral‒cubic {Ba8Yb6F69} clusters are transformed into “inverse” octahedral‒cubic {Yb8Ba6F71} clusters. These changes in the defect structure lead to an increase in conductivity by a factor of about 100. The fluorine-ionic conductivity of Ba0.75Yb0.25F2.25 is 2.5 × 10–3 S/cm at 500 K. This value exceeds the conductivity of Ba0.69La0.31F2.31 crystal by a factor of 15 (Ba0.69La0.31F2.31 has the best conducting properties among the fluorite phases of the Ba1 - xRxF2 + x family, for which σ was found to increase with a decrease in the atomic number of rare earth element (REE)).

Elasticity, ideal tensile strength and lattice dynamics of LuH2 dihydride from first-principles investigations

The structure, elasticity, tensile strength, phonon and thermodynamic properties of LuH2 with CaF2 structure have been investigated by using first-principles approach. The calculated lattice parameters and the single-crystal elastic constants of LuH2 agree well with experimental data and other theoretical results in the literature. The polycrystalline moduli of LuH2 involving volume elastic modulus, Young's modulus, and shear modulus, the brittle/ductile properties, Poisson's ratio, Debye temperature, sound velocities and hardness have been predicted. Also, the elastic anisotropy has been evaluated. The tensile strength along [100] crystallographic direction has been obtained using first principles total energy method. In addition, the phonon dispersion curves and the corresponding phonon density of states of LuH2 have been determined by virtue of a linear-response approach to density functional perturbation theory (DFPT) along with the small displacement method successfully, and the Infrared-active modes and the Raman-active modes at the Brillouin zone center have been analyzed according to the group theory. Finally, the lattice entropy and the lattice constant volume specific heat up to 1500 K have also been estimated based on quasi-harmonic approximation (QHA).

Two high-pressure phases of SiS2 as missing links between the extremes of only edge-sharing and only corner-sharing tetrahedra.

The ambient pressure phase of silicon disulfide (NP-SiS2), published in 1935, is orthorhombic and contains chains of distorted, edge-sharing SiS4 tetrahedra. The first high pressure phase, HP3-SiS2, published in 1965 and quenchable to ambient conditions, is tetragonal and contains distorted corner-sharing SiS4 tetrahedra. Here, we report on the crystal structures of two monoclinic phases, HP1-SiS2 and HP2-SiS2, which can be considered as missing links between the orthorhombic and the tetragonal phase. Both monoclinic phases contain edge- as well as corner-sharing SiS4 tetrahedra. With increasing pressure, the volume contraction (-ΔV/V) and the density, compared to the orthorhombic NP-phase, increase from only edge-sharing tetrahedra to only corner-sharing tetrahedra. The lattice and the positional parameters of NP-SiS2, HP1-SiS2, HP2-SiS2, and HP3-SiS2 were derived in good agreement with the experimental data from group-subgroup relationships with the CaF2 structure as aristotype. In addition, the Raman spectra of SiS2 show that the most intense bands of the new phases HP1-SiS2 and HP2-SiS2 (408 and 404 cm(-1), respectively) lie between those of NP-SiS2 (434 cm(-1)) and HP3-SiS2 (324 cm(-1)). Density functional theory (DFT) calculations confirm these observations.