Monoclinic Monazite Research Articles

Systematic synthesis and characterization of single-crystal lanthanide orthophosphate nanowires.

A simple hydrothermal method has been developed for the systematic synthesis of lanthanide orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure LnPO(4) compounds change structure with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured lanthanide orthophosphate LnPO(4) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike morphology. In contrast, tetragonal LnPO(4) (Ln = Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same experimental conditions consist of nanoparticles. The obtained hexagonal LnPO(4) (Ln = La --> Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 degrees C in air (Hexagonal DyPO(4) is an exceptional case, it transformed to tetragonal DyPO(4) by calcination), while the tetragonal structure for (Ho--> Lu, Y)PO(4) remains unchanged by calcination. The resulting LnPO(4) (Ln = La --> Dy) products consist almost entirely of nanowires/nanorods with diameters of 5-120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium doped LaPO(4) nanowires were also prepared, and their photoluminescent properties were reported. The optical absorption spectrum of CePO(4) nanowires was measured and showed some differences from that of bulk CePO(4) materials. The possible growth mechanism of lanthanide phosphate nanowires was explored in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra, and photoluminescence spectra have been employed to characterize these materials.

Synthesis, Growth, and Er3+ Luminescence of Lanthanide Phosphate Nanoparticles

The complete series of all nonradioactive lanthanide phosphates has been prepared in organic solution as ligand-capped nanoparticles. In all cases, well-dispersed particles with mean sizes below 10 nm are obtained. Despite the similar chemical properties of the lanthanides, the growth, the crystal structure, and the mean size of the nanocrystals are found to be strongly affected by the lanthanide ion employed. Very small nanoparticles are obtained for lanthanides for which the lattice energies of the bulk tetragonal xenotime phase and the bulk monoclinic monazite phase are similar. These smaller nanoparticles do not show the expected phase transition from monoclinic to tetragonal and seem to have their own unique crystal structure. Possible explanations for these observations are discussed. Finally, we present the first results on the IR emission of Er3+-doped YbPO4 and (Lu, Yb)PO4 nanoparticles in solution.

Synthesis, Structure, and Properties of Monazite, Pretulite, and Xenotime

### General characteristics Orthophosphate compounds of the type A(PO4) include the minerals monazite [RE(PO4) with RE = the light rare-earth ions, e.g., La, Ce, Nd…], xenotime [Y(PO4), and also incorporating the heavy RE ions], and pretulite [Sc(PO4)]. The name of the mineral monazite is derived from the Greek word monazein , “to be solitary,” whereas that of xenotime is derived from the Greek words xenos , meaning “foreign” and time , meaning “honor.” The mineral pretulite is named for Pretul Mountain, located in the Fischbacher Alps in Styria, Austria where the mineral was discovered (Bernhard et al. 1998). Pretulite is only the sixth mineral to have been found in which scandium is a major constituent. The anhydrous rare-earth (RE) orthophosphates can be structurally divided between the light RE-element compounds with the monoclinic monazite structure (space group P 21/ n , Z = 4) and the heavier RE compounds with the tetragonal (zircon-type) xenotime structure (space group I 41/ amd , Z = 4). In terms of the crystal chemistry of these two groups of compounds, a primary distinguishing structural feature is the coordination of the RE, Y, or Sc ions. In the monoclinic monazite structure, the RE ion is located in a polyhedron in which it is coordinated with nine oxygen ions. In the tetragonal xenotime structure, the heavier REs, Y, or Sc are located in a polyhedron in which they are coordinated with eight oxygen ions. Scandium orthophosphate is the basic constituent of the recently identified mineral, pretulite, which belongs to the orthophosphate group with the tetragonal xenotime structure (Bernhard et al. 1998). Both monazite and xenotime are relatively widely distributed as microcrystalline (or small crystalline) accessory inclusions in granitic rocks, rhyolites, pegmatites, and gneisses. Monazite also occurs in carbonatites, charnockites, migmatites, and …

Studies on triple orthovanadates: VIII. Synthesis and spectrostructural characterization of triple orthovanadates BaLnTh(VO 4) 3 (Ln=La or Pr) and BaLnCe(VO 4) 3 (Ln=La, Pr, Nd or Sm)

To study the effect of substitution at the tetravalent ion, some barium lanthanide containing triple orthovanadates have been synthesised and structurally examined. The X-ray diffraction and the infrared spectroscopic data are analysed and interpretations are reported. It is observed that thorium containing compounds crystallise with a monoclinic monazite structure while those containing cerium are tetragonal superstructured zircon.

Studies on thorium containing triple orthoarsenates: M RE Th (AsO4)3 (M=Mg, Sr, Ba, Cd and Pb, RE=La, Pr, Nd, Sm, Gd, Tb, Dy and Y)

In order to extend systematically the available information on the triple orthoarsenates, some new divalent and lanthanide ion containing thorium compounds have been synthesised and structurally examined. Their X-ray diffraction patterns were scanned and found to be characteristic of either monoclinic monazite or tetragonal zircon structure. The unit cell parameters are reported, which indicate a regular decrease when the mass of lanthanide increases. Infrared spectroscopic studies were also carried out and the major assignments of the vibrational bands have been made as a supplementary evidence.

Experimental determination of REE distributions between monazite and xenotime: potential for temperature-calibrated geochronology

Solid solutions of the REE+Y orthophosphates monazite and xenotime were synthesized hydrothermally between 300°C and 800°C at 2 and 5 kbar using REE+Y-oxides and H 3PO 4 as starting materials. The REE bulk composition was similar to those found in metapelitic rocks. All experiments produced two solid phases, monoclinic monazite and tetragonal xenotime, in volumetric ratios similar to relative abundances in metapelites. REE concentrations of coexisting monazite–xenotime pairs were determined by EMP analysis, and REE distribution coefficients D REE= X REE monazite/ X REE xenotime were derived for the entire REE+Y suite. Values of D REE depend strongly on ionic radii of the REEs and on temperature. The log D REE vs. ionic radius curve shows a nearly linear decrease from La to Lu and comprises almost five orders of magnitude from D La≈10 2.5 to D Lu≈10 −2. Between 450° and 800°C, most D REE values vary continuously by a factor of about 2 to 3. D Nd decreases from 48 to 16 and D Sm from 9 to 5; D Tb increases from about 0.2 to about 0.4, D Dy from 0.1 to 0.2 and D Y from 0.06 to 0.12 along this temperature range. Experimentally determined values of D REE are in close agreement with those derived from natural monazite–xenotime pairs equilibrated between 400° and 700°C. REE partitioning between monazite and xenotime is appropriate for geothermometry. The most useful application is that equilibrium and disequilibrium partitioning of REE in metamorphic rocks are easily recognized. In the case of equilibrium distributions, U–Pb chronometry of single monazites or xenotimes provides a direct link to their growth conditions and information on temperature–time paths. Equilibrium distributions of Sm and Nd, expressed in molar fractions, result in K D=( X Sm monazite X Nd xenotime/( X Sm xenotime X Nd monazite)=0.2 to 0.3 from 600° to 800°C, and Sm–Nd chronometry should provide reasonable two-mineral isochrons for such monazite–xenotime pairs.

Synthesis and Optical Spectroscopy of MnO42--Doped Crystals of Cs2CrO4, SrCrO4, CsBr, and CsI

The first low-temperature absorption and luminescence spectra of MnO42--doped crystals of Cs2CrO4, SrCrO4, CsBr, and CsI are reported. Single crystals of Cs2CrO4:MnO42-, which crystallize in the orthorhombic β-K2SO4 structure, were grown from basic aqueous solutions. An alkali−chloride flux composition was used to obtain single crystals of SrCrO4:MnO42- with the monoclinic monazite structure. Crystals of CsBr:MnO42- and CsI:MnO42- were prepared by slowly cooling the melt. The site symmetry in SrCrO4 is C1, but the deviations from D2 symmetry are small, and the spectra are analyzed in the approximate D2 symmetry. The 2E ↔ 2T2 ligand-field transitions are strongly c polarized, and in absorption only the transition to the lowest-energy component of the split 2T2 state is observed with its origin at 10 303 cm-1. Luminescence is exclusively observed to the lower-energy component of the 2E state. By a combination of absorption and luminescence data, it is shown that, in the cubic CsBr and CsI lattices, MnO42- i...

Monazite–xenotime miscibility gap thermometry. I. An empirical calibration

Rare earth element (REE) and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the Variscan fold belt in NE Bavaria, Germany. The metapelites include a continuous prograde, mainly low‐P (3–5 kbar) metamorphic profile from greenschist (c. 400 °C) to lower granulite facies conditions (c. 700 °C). The LREE (La–Sm) are incorporated preferentially in monoclinic monazite (REO9 polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO8 polyhedron). The major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La, Ce and Nd (La0.20–0.23, Ce0.41–0.45, Nd0.15–0.18)PO4, all other elements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 with some Dy and Gd solid solutions (Y0.76–0.80, Dy0.05–0.07, Gd0.04–0.06). In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3–5 from greenschist to granulite facies rocks. Monazite crystals often show zonation with cores low in HREE and rims high in HREE that is interpreted as growth zonation attained during prograde metamorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3–4 with increasing metamorphic grade. Prograde zonation in single crystals of xenotime was not observed. The XHREE+Y in monazite and XLREE in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 °C to 700 °C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U–Pb age determination. The combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature–time paths.

Computer simulation of collision cascades in monazite

Collision cascades in the monoclinic mineral monazite have been studied with a modified version of the computer-simulation program MARLOWE. Most of the calculations were made for the irradiation of polycrystalline CeP${\mathrm{O}}_{4}$ by normally incident 50-eV to 100-keV Ar atoms. A few calculations were made for 1-keV Cu atoms recoiling from lattice sites in a hypothetical form of Cu with the monazite structure. Although there are many fewer replacements and substantially more channeling in the mineral, collision cascades in monazite are generally similar to those in metals in size and shape. The dimensions of the calculated cascades suggest that the amorphization of monazite by Ar-ion irradiation probably commences when collision cascades begin to overlap. The calculations give a detailed view of the structure of the cascades in monazite that should be useful in interpreting radiation effects in proposed nuclear-waste isolation media.

EPR investigations of Gd3+ in single crystals and powders of the zircon–structure orthophosphates YPO4, ScPO4, and LuPO4

The tetragonal xenotime (YPO4) and monoclinic monazite (CePO4) orthophosphates are known to be chemically and physically stable in geological environments and are also radiation resistant. For these reasons, materials of this type are currently candidates for the primary containment of radioactive actinide solid wastes. Single crystals of the tetragonal orthophosphates YPO4, ScPO4, and LuPO4 doped with Gd3+ were grown by slowly cooling a Pb2P2O7 flux. Finely divided (<0.1 μ), rare-earth-doped powders of these materials were also prepared using a technique of precipitation in molten urea. Electron paramagnetic resonance investigations have shown that the Gd3+ impurities occupy identical substitutional sites in both single crystal and powder specimens of the tetragonal-symmetry orthophosphates. These results provide a basis for future investigations of the effects of heavy-particle radiation damage in the lanthanide orthophosphates.

Epr Investigations of Impurities in the Lanthanide Orthophosphates

ABSTRACTLanthanide orthophosphates formed from elements in the first half of the 4f transition series are analogs of the monoclinic mineral monazite. The known geologic properties of this mineral make the general class of lanthanide orthophosphate compounds attractive substances for long-term containment and disposal of α-active actinide nuclear wastes. EPR spectroscopy has been used to investigate the structural properties and solid state chemical properties of impurities in these materials and to compare the characteristics of single crystals and polycrystalline bodies.

A high pressure polymorph of CaSO4

CaSO4 has been investigated at room temperature over the pressure range of 0 to 160 kbar. A reversible, sluggish transition that occurs at about 20 kbar with a volume change of about 4% has been confirmed. The x-ray powder data collected at pressure can be indexed on the monoclinic monazite structure cell (P21/n) which is considered to be the most likely model structure for the new HP CaSO4 phase. The transition is largely distortional and involves rotation and displacement, but essentially preserves the chains of edge shared CaO8(9) and SO4 polyhedra found in the orthorhombic anhydrite (Bbmm) structure.