Eight-coordinated Sites Research Articles

Interrupting the long-range energy migration among Eu3+ by the introduction of unequivalent [NaO8] units to achieve both high quenching concentration and quantum yield in NaY2Ga2InGe2O12

In the pursuit of optimizing white light-emitting diodes (WLEDs), Eu3+-doped phosphors have emerged as a key component in strategies that combine ultraviolet (UV) LED chips with red, green, and blue (RGB) tricolor phosphors. However, their application has been notably constrained by concentration quenching and thermal quenching. To address this challenge, we have developed a series of novel red phosphors, NaY2-xGa2InGe2O12:xEu3+. The powder X-ray diffraction (XRD) indicates that the NaY2Ga2InGe2O12(NYGIG) belongs to the garnet family. The [NaO8] unit replaces one-third of the eight-coordinated sites of garnet and leaves the other two-thirds for the [YO8], which helps to prevent long-range energy migration among Eu3+ especially when the eight-coordinated sites were highly substituted. Such a structure feature allowed the NaY2-xGa2InGe2O12:xEu3+ phosphor to achieve an optimal quantum yield of 95.6% at an extremely high Eu3+ doping x value up to 1.2. Moreover, further studies on the temperature-dependent luminescence spectra confirm that an anomalous thermal quenching phenomenon will occur when the NaY2-xGa2InGe2O12:1.2Eu3+ phosphor is properly excited. In application, the combination of the red-emitting NaY2-xGa2InGe2O12:1.2Eu3+, blue-emitting BaMgAl10O17:Eu2+, and green-emitting Zn2SiO4:Mn2+ phosphors can realize high color rendering (CRI∼93) white light with the with CIE coordinates of (0.353,0.350), indicating that the NYGIG:Eu3+ phosphor own a promising potential for WLEDs.

Understanding the β–K2CO3-Type Na(Na0.5Sc0.5)BO3:Ce3 + Phosphor

A Na(Na0.5Sc0.5)BO3:Ce3+ phosphor in the β–K2CO3-type crystal structure was developed by identifying orthoborate substitutional derivatives with distinct crystal and electronic structure and high structural rigidity. Crystal structure and compositional analysis after the high-temperature synthesis of the target phase confirmed the product’s phase purity. Photoluminescence spectroscopy revealed a bright blue emission centered at 420 nm under 340 nm excitation. Computational modeling indicated that the optical properties stem from Ce3+ having a significant site preference for the eight-coordinated Na+ crystallographic site rather than the smaller six-coordinated Na+/Sc3+ site. Analyzing the local crystal structure through a combined computational and spectroscopic analysis showed that the substitution of Ce3+ on the normally eight-coordinated site causes an unexpected evolution of the crystal structure resulting in a nine-coordinated rare-earth position leading to the blue emission. These results highlight the importance of using a combined computational and experimental evaluation to develop new phosphors and understand their ensuing complex optical properties.

A further investigation of the complex M3 murataite structure using Hf substitution and STEM-EELS techniques.

Many members of the complex crystalline fluorite supercell structures (e.g. zirconolite, pyrochlore and murataite polytypes) have been considered/studied for possible long-term radioactive-waste immobilization. The eight-coordinated sites in these crystals are of particular importance because they are preferred for the accommodation of trivalent rare earths and actinides present in radioactive waste from fuel element processing. The fluorite-type supercell structures include the murataites, M3, M5, M7, M8, having those numbers of repeating fluorite sub-cell units. One simple technique, as shown here, namely the substitution of Hf into the Zr site, is very helpful for structural analysis in these very complex cases in order to further illuminate the site preference of the Zr ion. Three M3 murataite samples, Ca-Mn-Ti-Zr-Al-Fe-O (regular M3), Ca-Ti-Zr-Al-Fe-O (Mn-free M3) and Ca-Mn-Ti-Hf-Al-Fe-O (Hf-substituted M3) are investigated and, through techniques described for larger cells, show that the Zr is very likely not to be hosted in the [6]Ti site in the M3 murataite structure, as suggested by Pakhomova et al. [(2013), Z. Kristallogr. Cryst. Mater. 228, 151-156], but more likely replaces the [8]Ca1 site and less likely the [8]Ca2 site. This adjusted site preference for each cation from the powder X-ray diffraction (PXRD) and scanning transmission electron microscopy electron energy-loss spectroscopy (STEM-EELS) methods, agrees well with the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image.

Incorporation of Sb5+into CeO2: local structural distortion of the fluorite structure from a pentavalent substituent

Hydrothermal crystallisation of CeO2 from aqueous sodium hydroxide solution at 240 °C using CeCl3·7H2O in the presence of hydrogen peroxide with addition of either SbCl3 or SbCl5 yields polycrystalline samples of antimony-containing ceria directly from solution. Powder X-ray diffraction shows a contraction of the cubic lattice parameter with increasing Sb content, and also a broadening of Bragg peaks, from which Scherrer analysis yields crystallite domain sizes of 5-20 nm. Scanning transmission electron microscopy provides consistent results with observation of highly crystalline particles of a few nm in diameter. X-ray absorption near edge structure spectroscopy at the Ce LIII- and Sb K-edges reveals the presence of Ce4+ and Sb5+ in the solids. To balance charge the presence of co-included Na is proposed, corroborated by elemental analysis. The general chemical formula of the materials can thus be written as (Ce1-xSbx)1-yNayO2-δ (where x < 0.4 and y ≥ x/3). Sb K-edge extended X-ray absorption fine structure spectroscopy of the substituted ceria samples shows that the local structure of Sb resembles that in NaSbO3, where six-coordinate metal sites are found, but with evidence of a longer interatomic correlation due to surrounding Ce/Sb atoms in the fluorite structure; this implies that the Sb is displaced from the ideal eight-coordinate site of the fluorite structure. This structural distortion gives materials that are unstable under reducing conditions, coupled by the ease of reduction to elemental antimony, which is extruded leading to phase separation.

Synthesis of new fluorite-related rare earth oxides Ln $${\text{Ln}}_{2}^{\prime }$$ Ln 2 ′ MO7 (Ln, Ln′ = rare earths; M = Nb, Sb, Ta), their structures and magnetic properties by calorimetry measurements

New fluorite-related quaternary rare earth oxides Ln $${\text{Ln}}_{2}^{\prime }$$ MO7 (Ln = Nd, Gd, Tb; Ln′ = Lu and Y; M = Nb, Sb, Ta) were prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space group C2221. The results of the Rietveld analysis indicate that there exists some disordering of the Ln and Ln′ atoms at the eight-coordinate site and the seven-coordinate site. These compounds are paramagnetic down to 1.8 K except for TbLu2SbO7. Magnetic susceptibility measurements show that TbLu2SbO7 antiferromagnetically orders at 3.0 K. Specific heat measurements also show a λ-type anomaly at the same temperature. The analysis of the magnetic specific heat for TbLu2SbO7 indicates that the magnetic transitions are due to the ground doublet state of Tb3+ ions.

Perettiite-(Y), Y3+ 2Mn2+ 4Fe2+ [Si2 B8 O24 ], a new mineral from Momeik, Myanmar

The new mineral perettiite-(Y), end-member formula Y3+2Mn2+4Fe2+[Si2B8O24], from pegmatites in the region of Momeik, north of Mogok, Myanmar, occurs as yellow needles enclosed in phenakite crystals. The mineral was named after the mineralogist and gemologist Dr. Adolf Peretti. The empirical formula using LA-ICP-MS data (based on 24 O pfu ) is Y2.06Ln0.53Zr0.02Th0.01Mn3.24Ca0.38Fe0.71Mg0.07Al0.11Li0.22Si1.95B7.44Be0.31O24. Calculation of the H2O content using IR absorption data yields a maximum hydroxyl/water content equivalent to 0.1 wt.% H2O. The strongest lines in the powder pattern [ d in A, (I meas. ), ( hkl )] are 4.63(52) (010), 4.08(28) (301, 103), 3.74(20) (210), 3.05(100) (113, 311, 303), 2.64(67) (410, 014), 2.54(60) (313), 2.12(23) (600, 006), 1.87(33) (420, 024), 1.84(52) (415, 323), 1.57(20) (026, 620), 144(25) (133, 331). The perettiite-(Y) crystals show a tetragonal X-ray diffraction pattern but the structure could only be solved as a 50/50 pseudo-merohedral orthorhombic twin with the a and c axes having the same length. The structure, refined to R 1 = 0.017 (space group Pmna with a = 12.8252(5), b = 4.6187(2), c =12.8252(5) A, V =759.71(5)A3, Z =2), is characterized by two eight-coordinated sites: one dominated by Y and lanthanoids (Ln) and the other by Mn2+ (with additional Ca2+ and Y3+). An octahedral site is occupied by (Fe2+, Mg) with additional Li+. These cation sites form an interlayer between two tetrahedral sheets of Si2B8O24 composition. Boron shows minor replacement by beryllium. Arrangement of 4-, 5- and 8-membered rings within (010) form the borosilicate tetrahedral layers. Perettiite-(Y) is brittle, with irregular fracture and (010) cleavage; the estimated Mohs hardness is ~ 7. Using the empirical formula, the density was estimated to be 4.533 g cm−3. Perettiite-(Y) is biaxial, α = 1.82(1), γ = 1.84(1) (589 nm).

Structural Phase Transformation and Luminescent Properties of Ca(2-x)SrxSiO4:Ce3+ Orthosilicate Phosphors.

The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure-property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca(2-x)SrxSiO4:Ce(3+) phosphor by Sr(2+) substituting for Ca(2+) within 0 ≤ x < 2. The crystal structures of Ca(2-x)SrxSiO4:Ce(3+) are divided into two groups, namely, β phase (0 ≤ x < 0.15) and α' phase (0.18 ≤ x < 2), and the phase transition (β → α') mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition Pnma (α'-phase) ↔ P21/n (β-phase) can be ascribed to the second-order type, and Sr(2+) ions in Ca(2-x)SrxSiO4 preferentially occupy the seven-coordinated Ca(2+) sites rather than the eight-coordinated sites with increasing Sr(2+) content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca-O bond length in the two polymorphs of Ca2SiO4. The emission peaks of Ce(3+) shift from 417 to 433 nm in the composition range of 0 ≤ x ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca(2-x)SrxSiO4:Ce(3+) samples were evaluated, and the results show that the integral emission intensities at 200 °C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors.

Structures and magnetic properties of new fluorite-related quaternary rare earth oxides LnY2TaO7 and LaLn2RuO7 (Ln=rare earths)

New fluorite-related quaternary rare earth oxides LnY2TaO7 (Ln=La–Dy) and LaLn2RuO7 (Ln=Eu–Tb) have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space groups C2221 and Cmcm, respectively. The results of the Rietveld analysis for LnY2TaO7 (Ln=La–Dy) indicate that there exists no structural ordering of the Ln and Y ions at the eight-coordinate 4b site and the seven-coordinate 8c site, but the larger Ln ions occupy the eight-coordinate 4b site rather than the seven-coordinate 8c site, with increasing the Ln ionic radius. On the other hand, the results of the Rietveld analysis for the X-ray diffraction profiles of LaTb2RuO7 showed that La and Tb atoms are almost situated at the eight-coordinated site (4a site) and seven-coordinated site (8g site), respectively, i.e., cation ordering occurs on the 4a and 8g sites.All compounds LnY2TaO7 (Ln=La–Dy) are paramagnetic down to 1.8K. On the other hand, LaTb2RuO7 shows an antiferromagnetic transition at 17K. In addition, another magnetic anomaly has been found at 10K. Analysis of the magnetic specific heat for LaTb2RuO7 indicates that the magnetic transitions at 10 and 17K are due to the magnetic ordering of Tb3+ and Ru5+ ions, respectively.

Structural, Electrical, and Optical Properties of the Tetragonal, Fluorite-Related Zn0.456In1.084Ge0.460O3

We report the discovery of Zn0.456In1.084Ge0.460O3, a material closely related to bixbyite. In contrast, however, the oxygen atoms in this new phase occupy 4 Wyckoff positions, which result in 4 four-coordinate, 24 six-coordinate (2 different Wyckoff positions), and 4 eight-coordinate sites as compared to the 32 six-coordinate (also 2 different Wyckoff positions) sites of bixbyite. This highly ordered material is related to fluorite, Ag6GeSO8, and γ-UO3 and is n-type with a bulk carrier concentration of 4.772 × 1014 cm–3. The reduced form displays an average room temperature conductivity of 99(11) S·cm–1 and an average optical band gap of 2.88(1) eV. These properties are comparable to those of In2O3, which is the host material for the current leading transparent conducting oxides. The structure of Zn0.456In1.084Ge0.460O3 is solved from a combined refinement of synchrotron X-ray powder diffraction and time-of-flight neutron powder diffraction and confirmed with electron diffraction. The solution is a new, ...

Structural chemistry and magnetic properties of Y2CoGe4O12

Polycrystalline Y2CoGe4O12 has been prepared by standard ceramic methods. The crystal structure (space group P4/nbm; a=9.8465(2), c=4.92986(9)Å) consists of metal-rich layers separated from each other by Ge4O12 groups comprised of four corner-sharing GeO4 tetrahedra. Two cation sites lie within the layers; an eight-coordinate site occupied by yttrium and a six-coordinate site occupied by a 1:1 disordered distribution of yttrium and cobalt. Neutron diffraction revealed two-fold disorder on the oxide sublattice; this has been elucidated using Co K-edge EXAFS spectroscopy. The availability of two sites allows each oxide ion to accommodate the coordination preferences of its single Co/Y neighbour; the GeO4 tetrahedra distort to absorb any consequent strain. The octahedron of anions around each Co2+ cation shows a pseudo-tetragonal distortion with a strain (Co–O)eq–(Co–O)ax/(Co–O)eq=−0.173. This results in an unusually large effective magnetic moment of 6.05µB per Co2+ cation.

First-Principles Study on Site Preference and 4f → 5d Transitions of Ce3+ in Sr3AlO4F

The local structures and 4f → 5d transition energies of Ce(3+) located on the two crystallographic strontium sites of Sr3AlO4F, with charge compensation by means of nearby sodium substitutions for strontium (NaSr') or oxygen substitutions for coordinating fluorine (OF'), have been studied using the density functional theory (DFT) within the supercell model and the wave function-based embedded cluster calculations, respectively. The DFT total energy calculations show that Ce(3+) prefers strongly to occupy the eight-coordinated (Sr2) site over the ten-coordinate (Sr1) site. On the basis of the results from embedded cluster calculations at the CASPT2 level with the spin-orbit effect, the experimentally observed excitation bands are identified in association with the charge-compensated cerium centers. Especially, the two bands observed at ∼404 and ∼440 nm have been both assigned to the Ce(3+) located at the Sr2 sites but with compensation by one and two nearest-neighbor OF' substitutions, respectively, rather than to the Ce(3+) on the Sr1 and the Sr2 sites, respectively, as proposed earlier. Furthermore, the structural and electronic reasons for the red shift of the lowest 4f → 5d transition caused by coordinating OF' substitutions are analyzed in terms of the variations in centroid energy and crystal-field splitting of the 5d(1) configuration with the local environment. Finally, the thermal quenching of 5d luminescence at relatively high Ce(3+) concentrations is discussed on the basis of the electronic properties calculated with the hybrid DFT method.

CRYSTAL STRUCTURE DETERMINATION OF A UNIQUE Nb-Ti-RICH THORITE FROM MONT SAINT-HILAIRE, QUEBEC, WITH COMMENTS ON ‘THOROGUMMITE’

The crystal chemistry of a unique Nb-Ti-rich thorite from Mont Saint-Hilaire (Quebec) has been examined by a combination of single crystal and powder X-ray diffraction, electron microprobe analyses, and Fourier-transform infrared spectroscopy. The average of 9 compositions gave (Th 0.21 Nb 0.20 Ti 0.18 Ca 0.13 Y 0.10 REE 0.09 Fe 0.03 Zr 0.01 Sr 0.01 Mn 0.01 K 0.01 Na 0 . 01 ) Σ 1.00 [(Si 0.49 □ 0.41 Al 0.08 P 0.01 S 0.01 ) Σ 1.00 (O 2.33 F 0.02 )](OH) 1.70 . This is the first example in the literature of a zircon-group mineral containing elevated concentrations of Nb (0.20 apfu , 13.33 wt.% Nb 2 O 5 ) and Ti (0.19 apfu Ti, 7.41 wt.% TiO 2 ), and evidence for the (SiO 4 ) 4− ↔ (OH) 4 4− “hydrogarnet” substitution. The crystal structure was solved and refined to R = 3.40% and wR 2 = 9.73% for 68 reflections with F o > 4σ( F o ). The studied thorite is slightly metamict, tetragonal, space group I 4 1 / amd , with a 7.058(1) A, c 6.2260(12) A, V 310.15(11) A 3 , and Z = 4. It is isostructural with other zircon-group minerals and has a unit cell which is 4% smaller than that of thorite sensu stricto , a result of the incorporation of high field-strength elements of smaller radii. The structure consists of one eight-coordinated metal site ( A = Th, Zr, U, REE, Y, Nb, Ti, etc. ), one tetrahedral site ( T ), one O site, and one variably-occupied H site. The A site is coordinated by four axial O atoms [ A –O axial = 2.428(5) A] and four equatorial O atoms [ A –O eq = 2.322(6) A], which define a bisdisphenoid with A –O> = 2.374 A. The T site in MSH thorite is only partially occupied by Si (33% vacant) and coordinated by four O with T –O = 1.641(5) A. A partially occupied H site (31%) is located 0.980 A away from the O atom, forming (O 4 H 4 ) 4− groups when the T site is vacant. Removal of the center of symmetry in the structure allows for the possibility of the presence of bimodal T –O and A –O bond lengths, leading to both short Si–O bonds and longer □–OH bonds, as well as the shorter A –O bonds required for Nb and Ti. Accommodation of Nb and Ti into the thorite structure may be facilitated by increased distortion of the A O 8 bisdisphenoid, relaxation and shortening of A –O bonds as a result of the (SiO 4 ) 4− ↔ (OH) 4 4− substitution, and the likely presence of defects (O vacancies) in regions which have undergone slight metamictization, resulting in short-range ordering of Nb, Ti, and Th. Although it is possible that a metastable, limited solid solution exists between thorite and (OH) 4 4− -dominant “thorogummite” with intermediate compositions defined by Th(SiO 4 ) 1 −x (OH) 4 x , reported compositions indicate otherwise and it is suggested that the name “thorogummite” be abandoned.

Nioboaeschynite-(Ce), Ce(NbTi)O6

Nioboaeschynite-(Ce), ideally Ce(NbTi)O6 [cerium(III) niobium(V) titanium(IV) hexa­oxide; refined formula of the natural sample is Ca0.25Ce0.79(Nb1.14Ti0.86)O6], belongs to the aeschynite mineral group which is characterized by the general formula AB 2(O,OH)6, where eight-coordinated A is a rare earth element, Ca, Th or Fe, and six-coordinated B is Ti, Nb, Ta or W. The general structural feature of nioboaeschynite-(Ce) resembles that of the other members of the aeschynite group. It is characterized by edge-sharing dimers of [(Nb,Ti)O6] octa­hedra which share corners to form a three-dimensional framework, with the A sites located in channels parallel to the b axis. The average A—O and B—O bond lengths in nioboaeschynite-(Ce) are 2.471 and 1.993 Å, respectively. Moreover, another eight-coordinated site, designated as the C site, is also located in the channels and is partially occupied by A-type cations. Additionally, the refinement revealed a splitting of the A site, with Ca displaced slightly from Ce (0.266 Å apart), presumably resulting from the crystal-chemical differences between the Ce3+ and Ca2+ cations.

Synthesis and crystal structure of Ln2M2+Ge 4O 12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln 2 M 2+Ge 4O 12, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y 2− x Er x CaGe 4O 12 ( x=0–2), Y 2−2 x Ce x Ca 1+ x Ge 4O 12 ( x=0–1), Y 2Ca 1− x Mn x Ge 4O 12 ( x=0–1) and Y 2− x Pr x MnGe 4O 12 ( x=0–0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu 2MnGe 4O 12 has been found.

THE CRYSTAL STRUCTURE OF CERVANDONITE-(Ce), AN INTERESTING EXAMPLE OF As3+ -&gt; Si DIADOCHY

Cervandonite-(Ce), a unique arsenosilicate from Alpine fissures at Pizzo Cervandone (Scherbadung) in the Central Alps between the Binn Valley (CH) and Ossola (I), has proven to be a problem in view of the extreme scarcity of single crystals suitable for the determination of its complex structure. The monoclinic supercell (Z = 6) originally described by the discoverers is confirmed; we have established the crystal structure of a trigonal subcell, with space group R 3 m and a 6.508(1), c 18.520(3) A, V 679.4(2) A 3 and Z = 3, to a final R(F) = 0.0380 for 441 observed independent reflections [ I > 2σ ( I )]. The mechanism of the very unusual As 3+ → Si (not As 5+ → Si) diadochy is accounted for by considering the presence in two different sites of sorosilicate anions Si 2 O 7 6− , which are statistically replaced by arsenite [AsO 3 3− ] anions and minor amounts of OH − ; the positions of the oxygen atoms common to these anions are the same. The asymmetric unit also contains one eight-coordinated atomic site occupied by Ce and other REE lying on the three-fold axis; a bipyramidal six-coordinated site (of symmetry .m ) occupied by Fe, Ti, or Al also is present. The originally proposed chemical formula (Ce,Nd,La)(Fe 3+ ,Fe 2+ ,Ti 4+ ,Al) 3 SiAs(Si,As)O 13 is accordingly revised to (Ce,Nd,La)(Fe 3+ ,Fe 2+ ,Ti 4+ ,Al) 3 O 2 (Si 2 O 7 ) 1− x + y (AsO 3 ) 1+ x − y (OH) 3 x −3 y , where x and y are 0.47 and 0.31, respectively, in the sample here examined.

Melilite‐Type Borates Bi2ZnB2O7 and CaBiGaB2O7.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Melilite-Type Borates Bi2ZnB2O7 and CaBiGaB2O7

The bismuth borates Bi2ZnB2O7 and CaBiGaB2O7 have been synthesized by solid-state reactions at temperatures in the 650−825 °C range at 1 atm pressure. These compounds are the only synthetic diborate members of the melilite family, A2XZ2O7, in which layers of A cations alternate with XZ2O7 tetrahedral layers. Except for CdBiGaB2O7, the synthesis of other substituted bismuth borate melilites has been unsuccessful. The crystal structures of Bi2ZnB2O7 and CaBiGaB2O7 have been determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data. CaBiGaB2O7 adopts the regular tetragonal melilite structure (P421m space group, Z = 2) containing B2O7 tetrahedral dimers. The refinement of split eight-coordinated sites for the Ca2+ and Bi3+ interlayer cations suggests the presence of additional disorder. Bi2ZnB2O7 adopts a unique orthorhombic melilite superstructure (Pba2 space group, Z = 4) containing both tetrahedral B2O7 and triangular B2O5 dimers. The Bi3+ cations occu...

Lanthanide-sensitized lanthanide luminescence: terbium-sensitized ytterbium luminescence in a trinuclear complex.

A heterotrinuclear lanthanide complex has been prepared which contains two terbium ions in DO3A-derived binding sites and a single ytterbium ion in a DTPA-like site. The luminescence properties of the system have been investigated, showing that the terbium remains in a seven-coordinate binding site throughout the synthesis, while the ytterbium occupies the eight-coordinate site. Pumping the 488 nm absorption band of the terbium ion results in energy transfer to ytterbium with emission at 980 nm.

Crystalline Host Phases for Actinides, Obtained by Self-Propagating High-Temperature Synthesis

Host matrices for actinides prepared by self-propagating high-temperature synthesis are studied. The matrices consist of a pyrochlore or fluorite phase and metallic molybdenum. The factor determining the structural type of the crystal lattice of the target phase is the ionic radius ratio. When the difference in the ionic radii is insignificant, as in the case of Y3+ (r 0.102 nm) and Zr4+ (r 0.084 nm), the oxide Zr1-xYxO2-0.5x with a fluorite structure is formed, in which the cations occupy the eight-coordinate sites. This structure permits incorporation of heavy lanthanides and tetravalent actinides: U4+ (r 0.10 nm), Np4+ (r 0.098 nm), and Pu4+ (r 0.096 nm). When the difference in the ionic radii is more considerable, as in the case of Y3+ and Ti4+ (r 0.061 nm), a pyrochlore-related structure is realized. In this case the cations occupy different (eight- or six-coordinate) sites. The pyrochlore structure is preserved if the radii of ions occupying different structural sites change in parallel. This structure is typical of zirconates of trivalent actinides and light REEs. The decision on the major host phase for actinides is determined by the waste composition. At low content of light REEs and americium an oxide with a fluorite-related structure shows promise. At high content of these elements, zirconates and titanates with the pyrochlore structure are more stable.

The structure of the Pd(111)-(√7 × √7)R19.1°-S surface: comparison with the corresponding P/Rh(111) surface

The structure of the Pd(111)-(√7 × √7)R19.1°-S surface has been determined by low-energy electron diffraction and has been shown to involve a reconstructed topmost metal layer based on a packed arrangement of Pd pentagons and triangles, with S atoms at 3/7 monolayer coverage occupying eight-coordinate sites at the centre of each Pd pentagon. Comparisons are made with the structure recently determined for the corresponding P/Rh(111) system. In both cases, the atomic density in the reconstructed layer increases to five metal atoms and three chemisorbed atoms for every seven atoms in the regular metal surface. Some details are closely similar between the two systems, but an intriguing difference is also indicated. In S/Pd(111), four metal atoms in each pentagonal ring appear close to hcp-type three-fold sites, while the other is close to a fcc-type three-fold site. This contrasts with the situation for the topmost layer of the P/Rh(111) system, for which the reversed lateral registry has been reported (i.e. the atoms of each pentagonal ring occupy three-fold sites where four are of the fcc type and one of the hcp type).