Monoclinic Monazite Research Articles

A comprehensive study on crystal structure, phase compositions of the BiVO4-LaVO4 binary dielectric ceramic system and a typical design of dielectric resonator antenna for C-band applications

Novel multi-component (Bi1-xLax)VO4 (BLVOx) (0.05 ≤ x ≤ 0.9) solid-solution/composite ceramics crystal structures, phase compositions as well as its applications in the C-band dielectric resonator antenna have been designed and investigated. With the increased substitution of La3+, phase transitions among the monoclinic scheelite (mBiVO4), tetragonal zircon (zt(La,Bi)VO4) and monoclinic monazite (mLaVO4), driven by the compositional changes, were detected. La3+ played an essential role in the optimization of the dielectric properties of BLVOx. The measured Q × f values at the microwave band are considerably larger around the range of compositions forming the zircon phase and reached the maximum value (12,800 GHz) at pure zircon phase (x = 0.5), demonstrating that the impact of zircon structure on the intrinsic loss should not be disregarded and is beneficial for low dielectric loss. Meanwhile, the εr and TCF values could be tuned by regulating the concentration ratio of Bi3+/La3+ to obtain composites or solid-solutions with appropriate εr and near-zero TCF values. Additionally, a cylindrical dielectric resonator antenna (CDRA) of BVLO0.5 ceramic was designed with high simulated radiation efficiency (99.8%) and gain (4.58 dBi) at the center frequency (6.73 GHz), indicating its potential application in C-band because of its tunable permittivity and low-loss.

Structural, vibrational and electronic properties of Nb substituted orthovanadates LaV1−xNbxO4

We investigate the structural, vibrational, morphological, and electronic properties of Nb substituted orthovanadate LaV1−xNbxO4 samples prepared by the solid-state reaction method. The x-ray diffraction (XRD) analysis reveals the presence of three crystal structures [monoclinic monazite (m − m) type for the x = 0, two-phase equilibrium of monoclinic monazite (m − m) and tetragonal scheelite (t − s) type for the 0.2 ≤ x ≤ 0.8, and monoclinic fergusonite (m − f) type for the x = 1 samples] with an increase in Nb5+ concentration. The Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were employed to study the vibrational and electronic properties of all the samples, respectively. In order to choose an excitation wavelength that does not cause undesirable fluorescence and has observable intensities of all the vibrational modes, the Raman spectra are collected using 532 nm, 633 nm, and 785 nm laser lines. With increasing the Nb5+ concentration, new Raman modes associated with Nb-bonds are clearly visible and the intensity of V-bonds assigned modes is decreasing. The XPS analysis shows the unchanged 3+ oxidation state of La ion where the intensity of the V 2p core-level decreases while the Nb 3d core-level increases with x. The equal spin-orbit energy splitting of the states is confirmed by the average energy difference (across La core-level spectra for all the samples) for state I as well as bonding and anti-bonding of state II. Interesting, the relative intensity of La 3d state I and state II show systematic change with Nb doping altering the metal ligand overlap. We discuss and provide insight into the evolution of the structural, morphological, and chemical features with Nb substitution in LaV1−xNbxO4 samples.

Spectroscopic study of the 4f n−15d transitions of LaPO4 doped with Pr3+ or co-doped with Pr3+ and Gd3+ in the vacuum ultra violet region

The energy of f-d transitions depends on the crystalline field in which the lanthanide ion is inserted. Depending on the experimental setup, these transitions could occur at high energy, so several studies regarding theoretical data have been conducted. Here, we present the experimental determination of the energy of interconfigurational 4f n → 4f n−15d (f-d) transitions from Pr3+ ions to the lanthanum orthophosphate LaPO4 matrix; we have also determined the bandgap value for this host. The experiments were carried out at the Synchrotron setup of the Brazilian LNLS laboratory. Specifically, we synthesized LaPO4:Pr3+ and LaPO4:Pr3+/Gd3+ by the hydrothermal method under different pH conditions or by spray pyrolysis. The particles resulting from hydrothermal synthesis had different morphologies and the influence of pH value was showed: the reaction medium was controlled along the process, which changed the surface potential. On the basis of Raman spectroscopy and x-ray diffraction analyses, we found that the crystalline phase was monoclinic monazite for all the samples. We studied the 4f5d level and bandgap transitions at high energy by absorption analysis in the VUV range. The experimental results were 7.5 eV (LaPO4 bandgap) and 5 eV (4f n → 4f n−15d transition of the Pr3+ ion), which were close to the theoretical values reported in the literature for this ion and this matrix.

Structural phase-transition in CeVO4 nanobelts by P-doping enables better levofloxacin photocatalysis

The visible-light-induced photocatalytic degradation of levofloxacin was investigated based on belt-like CeVO4 homojunctions via P-doping-induced partial phase transition from tetragonal zircon (t-CeVO4) to monoclinic monazite (m-CeVO4). Such a homojunction allowed for the complete interface optimization of t-CeVO4/m-CeVO4, to boost visible-light harvesting and enhance electron-hole transferring and separating, making a contribution to an effective visible-light-induced photocatalysis. P-doped CeVO4 homostructured nanobelts achieved outstanding photocatalytic activity, high long-term stability, and superior reusability in consecutive levofloxacin photocatalysis experiments. The striking photocatalytic performance of t-CeVO4/m-CeVO4 homojunction can be attributed to a favorable band structure, interface electronic properties, surface oxygen vacancy, and unique cumulative intrinsic properties of belt-like t-CeVO4 and m-CeVO4, synergistically created by this doping-induced structural phase transition strategy. A reaction mechanism for levofloxacin degradation is proposed in terms of band-gap discussion, and trapping experiments. This work might provide a deep insight into designing novel homojunction photocatalysts to facilitate the wastewater decontamination, and environmental remediation using photocatalytic technology.

Surface Plasmon Enhancement of Eu3+ Emission Intensity in LaPO4/Ag Nanoparticles.

A promising way to improve the performance of luminescent materials is to combine them with noble metal nanoparticles. Herein, a set of silver/europium-doped lanthanum orthophosphate (Ag/La0.95Eu0.05PO4) nanostructures with different concentrations of silver nanoparticles were prepared and investigated. The presented overlap between the strongest europium (Eu3+) excitation line and the broad silver nanoparticle surface plasmon resonance makes the combination prospective for coupling. X-ray powder diffraction confirmed the monoclinic monazite structure. The transmission electron microscopy revealed particles with a rod-like shape and ~4 aspect ratio. Photoluminescence spectra show characteristic Eu3+ ion red emission. One of the requirements for an enhanced luminescence effect is the precise control of the distance between the noble metal nanoparticles and the emitter ion. The distance is indirectly varied throughout the change of Ag nanoparticle concentration in the La0.95Eu0.05PO4 host. The emission intensity increases with the increase in Ag nanoparticles up to 0.6 mol %, after which the luminescence decreases due to the nanoparticles’ close packing and aggregation leading to the displacement of La0.95Eu0.05PO4 from the vicinity of the metal particles and reabsorption of the emitted light. The emission intensity of La0.95Eu0.05PO4 increases more than three times when the Eu3+ excitation is supported by the localized surface plasmon resonance in the Ag/La0.95Eu0.05PO4 nanostructures.

Preferred orientations and microstructures of lanthanum phosphate films prepared via laser chemical vapor deposition

In this study, the vapor-phase growth of lanthanum phosphate (LaPO4 monazite) films having preferred orientations on polycrystalline alumina substrates was demonstrated via laser chemical vapor deposition using metal-organic precursors. The effects of the laser power densities and oxygen (O2) partial pressure on the crystal orientations, microstructures, and textures of the LaPO4 films were investigated. Monoclinic LaPO4 monazite was grown at laser power densities of 62–96 W cm−2 (deposition temperatures of 1075–1120 K) and an O2 partial pressure of 257 Pa. The (111¯)-oriented LaPO4 films with small pyramidal facets of less than 1 μm in sizes and large platelets were grown at laser power densities lower than 85 W cm−2. The preferred orientation of the monoclinic LaPO4 changed from (111¯) to (101¯) as the laser power density was increased to 95 W cm−2, at which large platelets inclined from the substrate surface were formed. Deposition with a reduced O2 partial pressure of 200 Pa induced the growth of highly (111¯)-oriented LaPO4 films with small pyramidal facets, which comprised (001), (100), and (010) faceted planes. At a low O2 partial pressure of 133 Pa, (100)-oriented LaPO4 films with platelet grains vertically grown from the substrate were formed. The deposition rates of the LaPO4 films were as high as 58.6 μm h−1.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties.

We have investigated the high-pressure behavior of PbCrO4. In particular, we have probed the existence of structural transitions under high pressure (at 4.5 GPa) by single-crystal X-ray diffraction and density functional theory calculations. The structural sequence of PbCrO4 is different than previously determined. Specifically, we have established that PbCrO4, under pressure, displays a monoclinic-tetragonal phase transition, with no intermediate phases between the low-pressure monoclinic monazite structure (space group P21/ n) and the high-pressure tetragonal structure. The crystal structure of the high-pressure polymorph is, for the first time, undoubtedly determined to a tetragonal scheelite-type structure (space group I41/ a) with unit-cell parameters a = 5.1102(3) Å and c = 12.213(3) Å. These findings have been used for a reinterpretation of previously published Raman and optical-absorption results. Information of calculated infrared-active phonons will be also provided. In addition, the pressure dependence of the unit-cell parameters, atomic positions, bond distances, and polyhedral coordination are discussed. The softest and stiffest direction of compression for monazite-type PbCrO4 are also reported. Finally, the theoretical pressure dependence of infrared-active modes is given, for the first time, for both polymorphs.

Detection of Cu2+ ions in aqueous solution via emission quenching of colloidal EuPO4 ultrasmall nanoparticles

EuPO4 and Eu3+-activated REPO4 (RE = La, Dy, Er) ultrasmall fluorescent nanoparticles (NPs) with an average diameter of 2.1 nm were prepared via colloidal synthesis and tested for sensing Cu2+ and other heavy-metal ions. The pure monoclinic monazite crystal structure (space group P121/n1) of the synthesized particles was confirmed using X-ray diffraction measurements, and transmission electron microscopy images showed round particles with a narrow particle-size distribution. The NPs exhibited intense red emission, which is characteristic of the f-f electronic transition of Eu3+, and the quenching of their emission in the presence of heavy-metal ions was revealed by dispersing colloidal particles in a TRIS buffer and then performing photoluminescence measurements. Strong quenching of the emission (at a rate of 0.195 μM−1) was observed upon the addition of Cu2+ ions over the concentration range of 0–10 μM with a limit of detection of 60 nM for Cu2+. The recovery of nearly 90% of the original emission intensity of the probe was achieved via the addition of ethylendiaminetetraacetic acid and was possible in five quenching/recovery cycles.

High resolution luminescence spectroscopy and thermoluminescence of different size LaPO4:Eu3+ nanoparticles

Nanoparticles (5 nm) and nanorods (2 nm × 15 nm and 4 nm × 20 nm) of monoclinic monazite LaPO4:Eu3+ were prepared by reverse micelle and co-precipitation techniques. Effects of the particle size and surface defects on the intensity of luminescence and the emission spectrum shapes were analyzed by high resolution spectroscopy under laser (266 nm) and X-rays excitation. All synthesized LaPO4:Eu3+ samples showed similar spectral features with characteristic Eu3+ ions emission bands: 5D0→7F0 centered at 578.4 nm, magnetic-dipole transition 5D0→7F1 at 588–595 nm, electric-dipole transition 5D0→7F2 at 611.5–620.5 nm, 5D0→7F3 at (648–652 nm) and 5D0→7F4 at (684–702.5 nm), with the most dominant electric-dipole 5D0→7F2 transition. Additionally, the thermally stimulated luminescence was studied for the most dominant peak at 611.5 nm. It was shown that the Eu3+ doping creates traps in all samples. Two prominent and well resolved glow peaks at 58.7 K and 172.3 K were detected for 5 nm nanoparticles, while low-intensity glow-peaks at 212.1 K and 212.2 K were observed in the X-rays irradiated nanorods. Displayed glows could be attributed to free and bound electrons and holes or to the recombination of electrons of ionized oxygen vacancies with photogenerated holes. To obtain information about the processes and specific defect type it is necessary to carry out additional analysis for all synthesized samples. The glow curves were analyzed and trap parameters were estimated and discussed throughout the paper.

Sol-gel synthesis and up-conversion luminescence of GdPO4–Gd3PO7:Yb3+, Ln3+ (Ln = Er, Ho, Tm) phosphor

This work reports the structural, morphological and luminescent properties of GdPO4–Gd3PO7:Yb3+, Ln3+ (Ln = Er, Ho, Tm) phosphors which have been synthesized by Pechini sol-gel process. The XRD patterns indicated that all the samples are crystalline and have a mixture of monoclinic monazite and monoclinic phases. Some samples were synthesized with ammonium hydroxide (NH4OH) and had better crystallinity than these synthesized without NH4OH. The up-conversion emissions were obtained under excitation at 967 nm. According to our results, the color emission was tuned from red to green by increasing the Yb3+ concentration from 5 mol% to 20 mol%. Moreover, the use of NH4OH to synthesize the samples doped with 20 mol% of Yb3+ enhanced their green emission band ≈3 times. The results reveal that doping the GdPO4–Gd3PO7 particles with rare earths produce an up-conversion emission which can be useful for the fabrication of luminescent coatings or lighting devices.

Structural, spectroscopic and optical properties of pure and gadolinium doped neodymium phosphate nanoparticles prepared using wet chemical route

ABSTRACTNanoparticles of neodymium phosphate and 5% gadolinium (Gd) doped neodymium phosphate prepared by co-precipitation technique belong to the monoclinic monazite phase with space group P21/n. The average crystallite size calculated using Debye-Scherrer and Williamson Hall method matches very well with grain size observed from scanning electron microscope images and Image J software. FTIR analysis signifies the presence of vibration modes along with the phosphate group and a slight shift in wave number was observed with the substitution of gadolinium into neodymium phosphate. Ultraviolet absorption spectrophotometer show transitions from ground state to various excited state within the 4f shell and the absorption edge shifts towards the lower wavelength side (blue shift) with the addition of dopant. Raman spectrum further confirms the formation of phosphate group with relevant peaks.

Facile wet chemical route synthesis, characterization and spectroscopic analysis of yttrium-doped lanthanum phosphate nanoparticles

Pure and yttrium-doped lanthanum phosphate nanoparticles were successfully prepared through a simple one-step co-precipitation method. The phase, morphology and composition of Y x La1 − x PO4 powders with varying dopant concentration (x = 0.00, 0.01, 0.03, 0.05) were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) supplemented with energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM). XRD analysis reveals the nanocrystalline nature with monoclinic monazite crystal system. SEM and TEM micrographs indicate the formation of ultrafine particles depicting spherical morphology with slight agglomeration and cluster formation. Fourier transform infrared spectroscopy (FTIR) signifies the presence of water vibration modes in a particular wave number along with phosphate group and a slight shift in wave numbers when yttrium is induced into lanthanum phosphate. Thermogravimetric analysis (TGA) reveals that the structural phase transition takes place above 800 °C. Raman spectra gives insight into the order–disorder in the system and shows relevant peaks for symmetric and anti-symmetric of PO4 3−, O–P–O bending mode, rare earth—oxygen vibrations. The fluorescence spectra of the grown compositions were investigated. The results show strong emission peaks at 270 nm excitation, whose intensity increases along with the increasing dopant concentration. Ultraviolet (UV) spectroscopy reveals that the absorption bands lie in the ultraviolet range and the bands are particularly sensitive to the incorporation of dopant ion, i.e., with effect of doping bands shift towards the lower wavelength side.

Cathodoluminescence response of natural and synthetic lanthanide-rich phosphates (Ln3+: Ce, Nd)

This paper reports on the cathodoluminescence (CL) emission of both natural and synthetic lanthanide-rich phosphates (Ln3+: Ce, Nd) previously characterized by X-ray Diffraction (XRD), Environmental Scanning Electronic Microscopy (ESEM) and Energy Dispersive Spectroscopy. The thermal treatment at 700°C performed on the synthetic sample obtained by chemical precipitation, promotes increasing of the crystallinity degree giving rise to a phase transition from the hexagonal (comprising monazite and rabdophane) into the monoclinic (cerianite and monazite) structures detected by XRD. Despite the size and the morphology of the grains are similar under ESEM, it could be appreciated significant differences among CL signals attending to the shape (with well-defined peaks for the annealed sample) and intensity (with lower emission for the non-thermally pretreated synthetic phosphate). The main wavebands centered at (i) 360, 380 and 490nm are associated respectively with 5D3/2 → 2F5/2 and 5D3/2 → 2F7/2 transitions as well as a redox reaction assigned to the presence of Ce3+, (ii) 276, 424, 516 and 531nm are linked respectively to 2G9/2→4I9/2, 2P1/2→4I9/2, 4G9/2→4I9/2 and 4G7/2→4I9/2 Nd3+ transitions and (iii) 400–490nm is due to non-bridging oxygen hole centers related to the tetrahedral PO43- groups or structural defects for the heated synthetic samples. The natural sample from Madagascar, with a very complex CL spectrum, displays a characteristic band emission in the green-yellow and red regions corresponding to [UO2]2+ groups and Sm3+ respectively.

Transcription of Nanofibrous Cerium Phosphate Using a pH-Sensitive Lipodipeptide Hydrogel Template

A novel and simple transcription strategy has been designed for the template-synthesis of CePO4·xH2O nanofibers having an improved nanofibrous morphology using a pH-sensitive nanofibrous hydrogel (glycine-alanine lipodipeptide) as structure-directing scaffold. The phosphorylated hydrogel was employed as a template to direct the mineralization of high aspect ratio nanofibrous cerium phosphate, which in-situ formed by diffusion of aqueous CeCl3 and subsequent drying (60 °C) and annealing treatments (250, 600 and 900 °C). Dried xerogels and annealed CePO4 powders were characterized by conventional thermal and thermogravimetric analysis (DTA/TG), and Wide-Angle X-ray powder diffraction (WAXD) and X-ray powder diffraction (XRD) techniques. A molecular packing model for the formation of the fibrous xerogel template was proposed, in accordance with results from Fourier-Transformed Infrarred (FTIR) and WAXD measurements. The morphology, crystalline structure and composition of CePO4 nanofibers were characterized by electron microscopy techniques (Field-Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy/High-Resolution Transmission Electron Microscopy (TEM/HRTEM), and Scanning Transmission Electron Microscopy working in High Angle Annular Dark-Field (STEM-HAADF)) with associated X-ray energy-dispersive detector (EDS) and Scanning Transmission Electron Microscopy-Electron Energy Loss (STEM-EELS) spectroscopies. Noteworthy, this templating approach successfully led to the formation of CePO4·H2O nanofibrous bundles of rather co-aligned and elongated nanofibers (10–20 nm thick and up to ca. 1 μm long). The formed nanofibers consisted of hexagonal (P6222) CePO4 nanocrystals (at 60 and 250 °C), with a better-grown and more homogeneous fibrous morphology with respect to a reference CePO4 prepared under similar (non-templated) conditions, and transformed into nanofibrous monoclinic monazite (P21/n) around 600 °C. The nanofibrous morphology was highly preserved after annealing at 900 °C under N2, although collapsed under air conditions. The nanofibrous CePO4 (as-prepared hexagonal and 900 °C-annealed monoclinic) exhibited an enhanced UV photo-luminescent emission with respect to non-fibrous homologues.

Structural and NIR to visible upconversion properties of Er3+-doped LaPO4 phosphors

Various concentrations of Er3+-doped LaPO4 powder phosphors have been synthesized by the conventional solid state reaction method and are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and upconversion emission measurements. The XRD results show that all the concentration of Er3+-doped samples are assigned to the monoclinic monazite type structure of the LaPO4 phase. An intense green emission band at 547nm (4S3/2→4I15/2) along with a weak red emission at 657nm (4F9/2→4I15/2) are observed under NIR excitation with 975nm laser diode. Energy transfer and upconversion fluorescence properties of Er3+-doped phosphors have also been studied. The possible mechanisms involved in the upconversion process have been discussed and compared with other reported systems.

Preparation and luminescence characteristics of monazite Eu3+:LaPO4 nanocrystals in NH4NO3 molten salt

Molten-salt method and NH4NO3 flux were developed to fabricate monoclinic monazite Eu3+:LaPO4 nanocrystals for the first time. The products were characterized by X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, excitation spectra, emission spectra and luminescence decay curves. The as-obtained products were quasihexagonal Eu3+:LaPO4 nanocrystals with the mean size of 30nm. The room temperature charge transfer bands (CTB) exhibited red-shift and spectral broadening in comparison with 10K CTB. The optimal Eu3+ concentration was determined to be 8mol% by a comparative study of the relative emission intensities for different Eu3+ doping concentrations. The higher concentration quenching could be caused by the possible nonradiative energy transfer (electric multipole–multipole interaction). The relationship between the FL lifetime of 5D0 energy level and Eu3+ concentration was investigated based on Auzel’s model. This work is important not only to understand the unique physical properties of Eu3+:LaPO4 nanocrystals but also to bring an opportunity for the development of the other nanocrystals via the molten salt synthesis in NH4NO3 flux.

Preparation by chemical co-precipitation, spectral and electrical characteristics of neodymium orthophosphate nanoparticles

Nanoparticles of neodymium orthophosphate (NdPO4) were prepared through a facile wet co-precipitation technique. The X-ray diffraction (XRD) studies revealed the material belonging to monoclinic monazite phase with crystallite size of 30.8 nm. Scanning electron microscopic studies showed that the material was composed of a large number of agglomerated spherical particles having grain size of 92 nm. Thermogravimetric analysis suggested that the structural phase transition was seen above 800 °C. The spectroscopic investigations were carried out using Fourier transform infrared spectroscopy (FTIR). UV-VIS-NIR absorption spectrophotometer showed various transitions from the ground state 4I9/2 to various excited states within the 4f shell and the optical band gap came out to be 3.3 eV. The dielectric constant (ε′), dielectric loss (tan d) and ac conductivity ln(sac) were measured in the frequency range of 5 kHz to 1 MHz and in the temperature range of 40 to 500 °C. The dielectric measurement suggested that there was a shift in the value of transition temperature (Tc), thereby indicating relaxor behaviour of the material. The activation energy value decreased with increase in frequency, thereby suggesting the role of mixed ionic-polaronic conductivity.

Polymorphs of CaSeO4 under pressure: a first-principles study of structural, electronic, and vibrational properties.

In this paper we report a theoretical study of the CaSeO4 compound at ambient pressure and under pressure. Here we made a structural analysis of its three known polymorphs--orthorhombic (Cmca), monoclinic monazite, and tetragonal scheelite--where direct comparison with experimental measurements is done. Besides, the electronic and vibrational structures are reported for the first time for those structures. In addition, the behavior of CaSeO4 as a function of pressure is studied, where phase transitions are investigated by considering a quasiharmonic approximation at 300 K. After a total energy study of 14 possible high-pressure phases of CaSeO4, the following sequence of pressure-driven structural transitions has been found: orthorhombic (Cmca) → tetragonal scheelite → monoclinic AgMnO4-type structure. It was observed that monazite is less stable as temperature increases, while the opposite occurs for the AgMnO4-type structure, this being a novel polymorph. This high-pressure structure is a distortion of the monazite structure and resembles the distorted barite-type structure (P2(1)/n) of CaSO4. The equation of state and the pressure evolution of the structural, electronic, and vibrational properties are also reported.

Enhanced upconversion luminescence induced by structrual evolution of lanthanum niobate phosphor

A series of Yb3+-Er3+ coactivated La(Nb1-xVx)O4 (0≤x≤1) upconversion (UC) emission phosphors are prepared by high-temperature solid-state reaction method. Experimental results demonstrate that the introduction of V5+ expands and distorts the unit cell volume of LaNbO4, and induces structural transformation of LaNbO4 from Phase 1 with monoclinic fergusonite structure (0<x≤0.2) to Phase 2 with tetragonal scheelite structure (0.25≤x≤0.5) to Phase 3 with monoclinic monazite structure (x=1). Under 976nm excitation, continuously enhanced green UC emission is observed in Phase 1 with gradual replacement of Nb5+ with the V5+ (0<x≤0.2). At x=0.2, the luminescence intensity is enhanced 1.3 times than that of the pristine sample. In Phase 2 and Phase 3, the intensity of green UC emission is decreased gradually with increasing the V5+ concentration. Based on the XRD results and crystal structural analysis, the enhanced luminescence is attributed to the lowered symmetry around La3+ ions (Er3+ ions), which relaxes the parity-forbidden selection rules of Er3+ luminescent centers. This study provides a useful approach in developing UC luminescence materials with controllable optical properties based on variations in local coordination environments through composition substitutions.

31P Magic Angle Spinning NMR Study of Flux-Grown Rare-Earth Element Orthophosphate (Monazite/Xenotime) Solid Solutions: Evidence of Random Cation Distribution from Paramagnetically Shifted NMR Resonances

We present (31)P magic angle spinning nuclear magnetic resonance spectra of flux-grown solid solutions of La(1-x)Ce(x)PO4 (x between 0.027 and 0.32) having the monoclinic monazite structure, and of Y(1-x)M(x)PO4 (M = V(n+), Ce(3+), Nd(3+), x between 0.001 and 0.014) having the tetragonal zircon structure. Paramagnetically shifted NMR resonances are observed in all samples due to the presence of paramagnetic V(n+), Ce(3+), and Nd(3+) in the diamagnetic LaPO4 or YPO4. As a first-order observation, the number and relative intensities of these peaks are related to the symmetry and structure of the diamagnetic host phase. The presence of paramagnetic shifts allows for increased resolution between NMR resonances for distinct atomic species which leads to the observation of low intensity peaks related to PO4 species having more than one paramagnetic neighbor two or four atomic bonds away. Through careful analysis of peak areas and comparison with predictions for simple models, it was determined that solid solutions in the systems examined here are characterized by complete disorder (random distribution) of diamagnetic La(3+) or Y(3+) with the paramagnetic substitutional species Ce(3+) and Nd(3+). The increased resolution given by the paramagnetic interactions also leads to the observation of splitting of specific resonances in the (31)P NMR spectra that may be caused by local, small-scale distortions from the substitution of ions having dissimilar ionic radii.