Rare Earth Apatite Research Articles

Diffuse reflectance and photoluminescence spectroscopic investigations of Sr2RE8(SiO4)6O2 (RE = rare earth) apatites

Apatite-type compounds have attracted attention due to their flexible crystalline structure besides high chemical stability, which increases the interest in understanding and modifying their properties to achieve the requirements for applications in catalysis and energy devices. In order to attend these demands, this paper investigates the optical properties of apatite-type Sr2 RE 8(SiO4)6O2 ceramics (RE = rare earth) by using diffuse reflectance and photoluminescence spectroscopies. Intraconfigurational f-f transitions, charge transfer bands, and energy gaps were identified and detailed discussed in terms of their relationships with the structural features of the rare earth apatites. Although all materials have the same hexagonal space group (P6 3 /m, # 176), the band gaps varied between 3.51 and 3.95 eV, for Eu- and Yb-containing apatites, respectively, showing the influence of their ionic radii and their electronic configurations on this property. In this sense, diffuse reflectance spectroscopy emerges as a powerful tool to study the optical properties of rare earth-containing materials as a complementary technique to photoluminescence spectroscopy. In this work, these techniques were discussed in association, which allowed us to provide a complete set of optical information that improves the understanding of apatite-type materials, as required in advanced technological demands.

Crystallization and phase evolution in novel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 environmental barrier coating

AbstractDesign of multiple rare‐earth (RE) principal elements is a practical strategy to optimize key property of RE disilicate environmental barrier coating (EBC). However, multi‐RE component in the feedstocks may also introduce critical fabrication problems due to the different evaporating and crystallizing behaviors of the EBC containing various RE composition. We herein report the preparation of a novel multi‐RE disilicate (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 EBC by atmospheric plasma spraying (APS). The crystallization behavior of the as‐deposited coating annealed at 1100°C–1300°C with different durations (5 and 20 h) in air and argon atmospheres were investigated through comprehensive characterization techniques. RE disilicate, RE mono‐silicate, and RE apatite were detected in the as‐sprayed coatings, and mono‐silicate and apatite transform to RE2Si2O7 with the increment of temperature and extension of annealing duration. Furthermore, the contribution of multiple RE to the formation of apatite phase is discussed. The results provide a fundamental understanding and may guide the controllable preparation of multi‐RE disilicate EBC via APS method.

Chemical compatibility of rare earth apatite with yttria-stabilized zirconia

The chemical compatibility of a series of rare earth apatite (RE-apatite), with Y2O3-stabilized ZrO2 (YSZ) has been investigated. Three types of RE-apatite powders with different ionic radius (RE = Gd, Nd and La) were prepared, and bulks prepared from the powder mixtures of RE-apatite and YSZ were heat-treated at 1300 °C up to 100 h in this study. It was found that Gd-apatite reacted with YSZ and formed a reaction layer (Gd2Si2O7) at the Gd-apatite/YSZ interface. Meanwhile, the intense Gd3+ diffusion resulted in the formation of Gd solid solutions in YSZ and much YSZ phase transformation. In contrary, as for Nd- or La-apatite/YSZ composite, which has larger ionic radius, no reaction product was observed at interface and there was less RE diffusion into YSZ as well as YSZ transformation. These results clearly indicated that large ionic radius of RE3+ could enhance the chemical compatibility of RE-apatite with YSZ.

АКЦЕСОРНІ МІНЕРАЛИ РІДКІСНИХ МЕТАЛІВ У ГРОРУДИТАХ СХІДНОГО ПРИАЗОВ'Я (УКРАЇНА)

Unlike other Precambrian platforms and shields, alkaline granites and their hypabyssal and effusive variaties in Ukraine have limited distribution. In Eastern Azov region dike analogs of alkaline granites (grorudites) are known. They are different in chemical and mineral composition and considered as analogs of pantellerites (aegirine hihg-Ti) and comendites (amphibole low-Ti). The high-Ti aegirine grorudites are more intensively enriched with incompatible rare elements (REE, Zr, Nb) compared with their low-Ti varieties. Despite the high or elevated concentration of HFSE in grorudites, there are few of their own minerals in these rocks. Thus, in high-Ti grorudites there have been identified such minerals of rare elements as monazite-(Ce), bastnäsite-(Ce), britholite-like mineral and rare earth apatite, zircon and undiagnosed zirconium mineral, whereas only zircon and niobium-containing rutile are diagnosed in amphibole one. These minerals are very small in size, the largest of them can reach 15-20 μm (sometimes up to 50 μm), although most of them do not exceed 10 μm (usually 5-6 μm). It is assumed that a significant part of rare metals are isomorphically included in rock-forming minerals (alkaline pyroxenes and amphiboles), and zirconium minerals, at least part of them, are secondary and formed as result of changing of primary sodium (eudialyte, catapleite, ilerite) or calcium (gittingsite) zirconosilicates which are more typical for peralkaline (agpaitic) rocks. Taking into account the peculiarities of the mineral composition, geochemical features and rare-earth mineralization of the Azov region, there is reason to believe that the HFSE mineralization of these rocks is a consequence of the differentiation of the primary igneous silica unsaturated melt(s). Secondary hydrothermal processes are weakly manifested in the studied rocks and probably presented by replacement of primary accessory minerals. Elevated or high concentrations of Nb in high-Ti grorudites and absence of Nb-minerals may indicate that the PTcondition of differentiation of these rocks (low F concentration, high fO2, and hypabyssal conditions of crystallization) did not contribute to their crystallization. The presence of grorudites in this region increase the prospects of discovering small alkaline granite massifs (holocrystalline analogues of grorudites) to which deposits and/or occurrences of Nb, REE, Zr, Sn, Be can be related

A simple and general route for the preparation of pure and high crystalline nanosized lanthanide silicates with the structure of apatite at low temperature

Rare earth silicates with the structure of apatite are attracting considerable interest since they show oxygen ion conductivities higher than that of yttria-stabilized zirconia (YSZ) at moderate temperature. Based on the hydrothermal synthesis we presented a simple one step process for the direct preparation of the pure and the high crystalline nanosized rare earth silicates with the structure of apatite under a mild condition (230 °C). Since the preparation of the high crystalline silicon based rare earth apatites is performed at high temperature previously and accompanied by subsequent process of grinding, results of this work provide a promising alternative of the existing methodology. Furthermore, due to the relatively low temperature of the preparation of these materials, high doping of monovalent cation can be done, which was not achieved before.

Influence of anionic vacancies on the ionic conductivity of silicated rare earth apatites

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for Solid Oxide Fuel Cells (SOFC). Doped silicated rare earth apatites with formula La 9.33− x Ca x (SiO 4) 6O 2− x/2 (0 ≤ x ≤ 1) have been prepared by solid state reaction at high temperature in order to determine the influence of anionic vacancies on the electrical properties of the material. The incorporation of calcium in the structure has been checked by characterizations of the powders (X-ray diffraction, helium pycnometry). The cell parameters of the hexagonal apatite were refined. Samples were sintered at 1550 °C. Electrical properties of each composition have been studied between 280 and 620 °C by the complex impedance method. The evolution of the bulk conductivity and of the activation energy with the substituting ratio gives information on the conductivity mechanism in these materials. An improvement of ionic conductivity about one order of magnitude has been observed for low calcium substitution ratios.

Synthesis and characterization of oxide ions conductors with the apatite structure for intermediate temperature SOFC

Silicated rare earth apatite with formula La 9.33(SiO 4) 6O 2 has been prepared by solid state reaction at high temperature. The reagents (La 2O 3 and SiO 2) have first been characterized. Then the synthesis process was studied and optimized. Samples with various densities (from 67 to 92% of the theoretical value) have been obtained by hot pressing at 1400 °C or by sintering at 1500 and 1550 °C. The presence of secondary phases leads to the formation of a liquid phase at temperatures above 1600 °C. A microstructural study has been performed on these samples. Electrical properties of all the samples have been characterized between 400 and 900 °C by the complex impedance method. Conductivity values of about 2 × 10 −4 S cm −1 have been measured at 700 °C. The samples present activation energies of less than 1 eV. The influences of the densification ratio and of the microstructure on the electrical properties of the material have been underscored.

Apatite-type Pr9K(SiO4)6O2—a potential oxide ion conductor

Pr9K(SiO4)6O2 (hexagonal, P63/m (No. 176), a=b=9.6466(8), c=7.1136(6) Å, Z=1) was synthesised in a potassium fluoride flux. The chemical composition was proven by EPMA/EDX and the structure refined from X-ray single crystal diffraction data (R1(F2>2σ(F2))=0.0204). The compound crystallises in the regular apatite structure. Potassium fills one (4f) out of two metal positions present in the structure with an occupancy factor of 25%. The remaining places of this site (Pr2/K2) are occupied by praseodymium. Oxygens of the silicate groups form a coordination polyhedron (CN=9) in the shape of a distorted threefold capped trigonal prism. These face sharing [(Pr2/K2)O9]-polyhedra build up chains, which are inter-connected via SiO4-groups. The resulting channel framework accommodates sevenfold oxygen-coordinated praseodymium (Pr1), attached to the inside of the tubes that are aligned parallel to the c-axis. The oxide ions O4, located on the longitudinal axis of the channels, exhibit anomalously high atomic displacement parameters along the c-direction. It can be shown that this property is in agreement with literature structural data as well as oxide ion conductivity of similar rare earth apatites.

Cratonic tectonic controls of Late Precambrian iron deposits and Mississippi Valley-type deposits in the Central United States

Middle Proterozoic age iron, light rare-earth apatite, copper deposits are exposed in the St. Francois Mountains in a larger alkalic granite and rhyolite terrane that is favorable for the occurrence of world-class deposits of the Olympic Dam type. The Precambrian age structural network of lineaments cut the Proterozoic rocks above where movement has been revived in post-Precambrian time including both Paleozoic and Mesozoic age rocks. The Mississippi Valley-type of deposits in Paleozoic-age rocks are located at the crests of domes (Central Kentucky and Central Tennessee districts), or faulted collapsed domes (Illinois-Kentucky) on half-domes (Upper Mississippi Valley and Tri-State districts), or they surround Precambrian islands in an open oval in fault zones and facies changes where shales, carbonate rocks and sandstones pinch out (Southeast Missouri). Some ores were deposited as veins in faults, as replacements in adjacent permeable beds, in cryptovolcanic breccia (part of Illinois-Kentucky), or in fault zones adjacent to and above Precambrian rift valleys (eastern northern Arkansas and parts of the Illinois-Kentucky districts). In the Central States major structures of repeated uplift and subsidence are crossed by a complex system of intersecting structural lineaments that form a cratonic network across the large domes, basins and rift valleys. The lineaments trend northwest, northeast, east and north. Some are curved and many have a strike-slip component of movement

Flux growth of rare earth silicates and aluminosilicates

Crystals of R2SiO5, R2Si2O7, rare earth apatites, and square-faceted rare earth aluminium garnets containing silicon have been grown from fluxed melts. Silicon was provided in two ways: by vapour transport of a siliceous vapour species which diffused into the melts (“vapour-flux” method), and by including SiO2 in the initial mixtures. The results of these two methods are compared.