Gadolinium Molybdate Research Articles

Atomistic simulation of sodium–gadolinium molybdate of stoichiometric (Na1/2Gd1/2MoO4) and cation-deficient (Na2/7Gd4/7MoO4) compositions

Crystals of sodium–gadolinium molybdates of two compositions: stoichiometric (Na1/2Gd1/2MoO4) and cation-deficient (Na2/7Gd4/7MoO4) composition in which 1/7 of the corresponding cation positions are not occupied are simulated by the method of interatomic potentials. For cation-deficient crystals, two kinds of cation position distribution are considered: the statistical distribution of sodium, gadolinium, and unoccupied cation positions in the I41/a structure and their partial ordering in the I space group. As a result of the simulation, structural characteristics of sodium–gadolinium molybdates agreeing well with the known experimental data are obtained. In addition, a number of important elastic and thermodynamic properties of these compounds are predicted. The results obtained in the partial-occupancy approximation and by constructing a 7 × 2 × 2 supercell are compared. The local structure of sodium–gadolinium molybdates are analyzed in detail. The influence of the deviation from the stoichiometry as well as cation ordering on the properties of these crystals is discussed.

Magnetic properties of iron-doped gadolinium molybdate

ABSTRACTIn search of new multiferroic materials we introduced iron into the lattice of improper ferroelectric gadolinium molybdate. Magnetic measurements show weak ferromagnetic-like hysteresis in addition to the primary paramagnetic behavior. The likely reason of this contribution is clustering of magnetic ions into separate iron-rich particles.

Role of ytterbium-erbium co-doped gadolinium molybdate (Gd2(MoO4)3:Yb/Er) nanophosphors in solar cells.

Insufficient harvest of solar light energy is one of the obstacles for current photovoltaic devices to achieve high performance. Especially, conventional organic/inorganic hybrid solar cells (HSCs) based on PTB7 as p-type semiconductor can only utilize 400-800 nm solar spectrum. One effective strategy to overcome this obstacle is the introduction of up-conversion nanophosphors (NPs), in the virtue of utilizing the near infrared region (NIR) of solar radiation. Up-conversion can convert low-energy photons to high-energy ones through multi-photon processes, by which the solar spectrum is tailored to well match the absorptive domain of the absorber. Herein we incorporate erbium-ytterbium co-doped gadolinium molybdate (Gd2(MoO4)3, GMO), denoted as GMO:Yb/Er, into TiO2 acceptor film in HSCs to enhance the light harvest. Here Er3+ acts as activator while Yb-MoO4 2- is the joint sensitizer. Facts proved that the GMO:Yb/Er single crystal NPs are capable of turning NIR photons to visible photons that can be easily captured by PTB7. Studies on time-resolved photoluminescence demonstrate that electron transfer rate at the interface increases sharply from 0.65 to 1.42 × 109 s-1. As a result, the photoelectric conversion efficiency of the GMO:Yb/Er doped TiO2/PTB7 HSCs reach 3.67%, which is increased by around 25% compared to their neat PTB7/TiO2 counterparts (2.94%). This work may open a hopeful way to take the advantage of those conversional rare-earth ion doped oxides that function in tailoring solar light spectrum for optoelectronic applications.

NaGd(MoO4)2 nanocrystals with diverse morphologies: controlled synthesis, growth mechanism, photoluminescence and thermometric properties.

Pure tetragonal phase, uniform and well-crystallized sodium gadolinium molybdate (NaGd(MoO4)2) nanocrystals with diverse morphologies, e.g. nanocylinders, nanocubes and square nanoplates have been selectively synthesized via oleic acid-mediated hydrothermal method. The phase, structure, morphology and composition of the as-synthesized products are studied. Contents of both sodium molybdate and oleic acid of the precursor solutions are found to affect the morphologies of the products significantly, and oleic acid plays a key role in the morphology-controlled synthesis of NaGd(MoO4)2 nanocrystals with diverse morphologies. Growth mechanism of NaGd(MoO4)2 nanocrystals is proposed based on time-dependent morphology evolution and X-ray diffraction analysis. Morphology-dependent down-shifting photoluminescence properties of NaGd(MoO4)2: Eu3+ nanocrystals, and upconversion photoluminescence properties of NaGd(MoO4)2: Yb3+/Er3+ and Yb3+/Tm3+ nanoplates are investigated in detail. Charge transfer band in the down-shifting excitation spectra shows a slight blue-shift, and the luminescence intensities and lifetimes of Eu3+ are decreased gradually with the morphology of the nanocrystals varying from nanocubes to thin square nanoplates. Upconversion energy transfer mechanisms of NaGd(MoO4)2: Yb3+/Er3+, Yb3+/Tm3+ nanoplates are proposed based on the energy level scheme and power dependence of upconversion emissions. Thermometric properties of NaGd(MoO4)2: Yb3+/Er3+ nanoplates are investigated, and the maximum sensitivity is determined to be 0.01333 K−1 at 285 K.

Tunable properties of spin waves in magnetoelastic heterostructure

Full ferroelastic and simultaneously ferroelectric materials are interesting candidates for applications in devices based on multiferroic heterostructures. They should allow for non-volatile and low-power writing of data bits in magnetoelectric random access memories. Moreover, ferroelasticity, in contrast to piezoelectric material, make magnetic information in ferromagnetic film resistant to external fields. As an example for such a system, we have studied the magnetoelastic interaction between a thin ferromagnetic layer of with a full ferroelastic–ferroelectric gadolinium molybdate crystal. We have investigated the influence of spontaneous strain onto magnetic properties of thin ferromagnetic film. Particularly, we have shown by Brillouin spectroscopy, that it is possible to modulate surface spin wave frequency of by spontaneous strain of gadolinium molybdate substrate.

Heat capacity, entropy of Ln2(MoO4)3 (Ln = La, Sm, and Gd), and the high-temperature enthalpy of Ln2(MoO4)3 (Ln = Eu, Dy, and Ho)

The low-temperature heat capacity of Ln2(MoO4)3 (Ln = La, Sm, and Gd) is investigated by means of adiabatic calorimetry within the range of 60–300 K. The temperature dependences of the heat capacity are found and the values of the standard entropy are calculated, based on extrapolations to 0 K. Characteristic temperatures for molybdates are determined from the results of IR spectroscopic studies. The high-temperature enthalpy of Ln2(MoO4)3 (Ln = Eu, Dy, and Ho) is measured via high-temperature microcalorimetry, and the temperature dependence of heat capacity is calculated in the range of 298–1000 K. Since samarium and gadolinium molybdates are of the same structural type as terbium molybdate, we can estimate the anomaly of the heat capacity in the low-temperature region using the data for terbium molybdate and find the entropy of samarium and gadolinium molybdates.

Electronic structure and inner-shell excited luminescence in gadolinium molybdate single crystals

An electronic structure of Gd2(MoO4)3 was studied using methods of luminescence and X-ray photoelectron spectroscopy. Luminescent properties under excitation were investigated in energy ranges 125–165 and 526–553 eV which correspond to Gd4d and O1s shells. Luminescence excitation spectra were antibate to X-ray absorption spectra. The X-ray photoelectron spectroscopy appended these data by density of states in energy ranges where no resonant structures in excitation spectra were found. A low-temperature emission of Gd3+ ions was registered in gadolinium molybdate for the first time. Possible models of energy transfer in cation sublattice are discussed.

Efficient charge transfer and utilization of near-infrared solar spectrum by ytterbium and thulium codoped gadolinium molybdate (Gd2(MoO4)3:Yb/Tm) nanophosphor in hybrid solar cells.

In this work, thulium and ytterbium codoped gadolinium molybdate (Gd2(MoO4)3:Yb/Tm) nanophosphors (NPs) have been synthesized, followed by being incorporated into a photo-catalytic titania (TiO2) nanoparticle layer. In detail, morphology and phase identification of the prepared NPs are first characterized and then the up-conversion of the Gd2(MoO4)3:Yb/Tm NPs is studied. Electron transfer dynamics after interfacing with bare or NP-doped electron donor TiO2 and the corresponding photovoltaic performance of solar cells are explored. The results show that Gd2(MoO4)3:Yb/Tm NPs excited at 976 nm exhibit intense blue (460-498 nm) and weak red (627-669 nm) emissions. The lifetime of electron transfer is shortened from 817 to 316 ps after incorporating NPs and correspondingly the electron transfer rate outstrips by 3 times that of the bare TiO2. Consequently, a notable power conversion efficiency of 4.15% is achieved as compared to 3.17% of pure TiO2/PTB7. This work demonstrates that the co-doping of robust rare earth ions with different unique functions can widen the harvesting range of the solar spectrum, boost electron transfer rate and eventually strengthen device performance, without complicated interfacial and structural engineering.

Up/down-converted green luminescence of Er3+–Yb3+ doped paramagnetic gadolinium molybdate: a highly sensitive thermographic phosphor for multifunctional applications

Optical temperature sensing and nano-heating behavior of Er3+–Yb3+ doped multifunctional gadolinium molybdate phosphor.

New insight into rare-earth doped gadolinium molybdate nanophosphor assisted broad spectral converters from UV to NIR for silicon solar cells

Demonstration of novel rare-earth doped gadolinium molybdate nanophosphor assisted broad spectral converters from UV to NIR for Si-solar cell application.

Concentration quenching of praseodymium ions Pr3+ in BaGd2(MoO4)4 crystals

The concentration effect on the photoluminescence (PL) of the praseodymium Pr(3+) ion is studied at 298-12 K for barium gadolinium molybdate (BaGd2(MoO4)4, called BGM) crystals with a wide Pr(3+) concentration range of 0.05-25.0 mol%. Three types of concentration dependences are observed for the emissions although all types show PL quenching at high concentrations. The first type (Type A) has the maximum PL intensity at about 10 mol% with a non-zero intensity at high concentrations, which is observed for the (3)P0 emissions except for emission at 621 nm. The second and third types (Type B-1 and B-2) have the maximum at about 1 mol% with a finite residual intensity and nearly zero intensity at high concentrations, respectively, which are observed for the 621 nm emission and all the (1)D2 emissions. It is suggested that the energy migration mechanism is responsible for Type A, while the non-resonant cross-relaxation is responsible for Type B-1 and the resonant cross-relaxation for Type B-2.

ChemInform Abstract: Magnetic and Magnetoelectric Properties of Rare Earth Molybdates

AbstractReview: 68 refs.

Intrinsic and radiation-induced defect luminescence of gadolinium molybdate under UV- and VUV-excitation

Abstract A study of intrinsic luminescent properties of two polymorphic modifications of gadolinium molybdate was conducted using synchrotron radiation at low temperature. Excitation spectra in the UV- and VUV-ranges (Eexc = 3.7–21 eV) were measured. Energy transfer from Gd3+ to ( MoO 4 ) 2 − centers was observed. Change of crystal structure influenced luminescent properties, with less symmetric phase having higher quenching temperature and lower emission. After irradiation of the crystals with fast electrons, the creation of a new luminescent center was registered with a separate excitation and emission bands. Temperature dependences of luminescence yield of all bands were measured.

Band gap, CIE and trap depth parameters of rare earth molybdate phosphors for optoelectronic applications

This paper reports on the comparative investigation of optical, luminescence and dielectric properties of Tb doped lanthanum and gadolinium molybdate phosphors. The crystal structures of the prepared phosphors were identified from XRD studies. The emission spectra of both the phosphors show the most intense peak at 545 nm due to 5D4 → 7F5 transition. The optical absorption and diffuse reflectance spectra of both the phosphors show the main peak at 210 nm due to the band gap. Thermoluminescence studies were carried out after irradiating the phosphors by γ-rays. Trap depth associated with different trap levels was calculated from thermoluminescence data using various glow curve analysis techniques. The dielectric properties such as permittivity and dielectric loss were studied as a function of frequency.

Magnetic and Magnetoelectric Properties of Rare Earth Molybdates

We present results on ferroelectric, magnetic, magneto-optical properties and magnetoelectric effect of rare earth molybdates (gadolinium molybdate, GMO, and terbium molybdate, TMO, and samarium molybdate, SMO), belonging to a new type of ferroelectrics predicted by Levanyuk and Sannikov. While cooling the tetragonal β-phase becomes unstable with respect to two degenerate modes of lattice vibrations. The β-β′ transition is induced by this instability. The spontaneous polarization appears as a by-product of the lattice transformation. The electric order in TMO is of antiferroelectric type. Ferroelectric and ferroelastic GMO and TMO at room temperature are paramagnets. At low temperatures GMO and TMO are antiferromagnetic with the Neel temperatures TN=0.3 K (GMO) and TN=0.45 K (TMO). TMO shows the spontaneous destruction at 40 kOe magnetic field. Temperature and field dependences of the magnetization in TMO are well described by the magnetism theory of singlets at 4.2 K ≤ T ≤ 30 K. The magnetoelectric effect in SMO, GMO and TMO, the anisotropy of magnetoelectric effect in TMO at T = (1.8–4.2) K, the Zeeman effect in TMO, the inversion of the electric polarization induced by the laser beam are discussed. The correlation between the magnetic moment of rare earth ion and the magnetoelectric effect value is predicted. The giant fluctuations of the acoustic resonance peak intensity near the Curie point are observed.

QUADRUPOLE ARRANGEMENTS IN SOLID GADOLINIUM MOLYBDATE

Quadrupole arrangements in Gadolinium Molybdate Gd2 (MoO4)3 crystal are carried out by using group theoretical method of Opechowski and Guccione for the classification of magnetic spin arrangements in the various phase transitions. It turns out that the quadrupole arrangement of Gadolinium Molybdate in the tetragonal phase is �� 2�� 3 with non-axial symmetric quadrupoles. In orthorhombic phase there are two possible quadrupole arrangements with low energy in which one is C 8 2v and the other is D 1 2 both are axially symmetric quadrupoles.

Upconversion luminescence of KGd(MoO 4) 2:Er 3+,Yb 3+ powder prepared by Pechini method

Er 3+ doped potassium gadolinium molybdate (KGM) phosphor with sensitizer Yb 3+ ion was synthesized by the Pechini method using citric acid and ethylene glycol. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential scanning calorimetry (TG-DSC), which indicated that ultrafine uniform crystallites of KGM:Er,Yb were obtained by sintering the precursors at above 650 °C for 5 h. Upconversion luminescence (UL) spectra of the samples were studied by a 976 nm semiconductor laser diode (LD) excitation. The UL spectra exhibited the green and red emission bands that were attributed to the 2H 11/2, 4S 3/2→ 4I 15/2 and 4F 9/2→ 4I 15/2 transitions of Er 3+, respectively. The possible UL mechanisms of Er 3+ were explained by means of an energy level diagram. The maximum luminescent intensity was achieved when the concentration of Er 3+, Yb 3+ arrived at 1 mol.% and 20 mol.%, respectively.

High‐pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization

AbstractPolarized Raman spectra of a single crystal of gadolinium molybdate [Gd2(MoO4)3] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure‐transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure‐transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO4 stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short‐range and long‐range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure‐transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A1 symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm−1, soften continuously through the first three phases when increasing pressure. The strong A2 mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm−1 at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the MoO stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.

Study of Multiferroic Materials at Nano-Scale

With the growing interest in nanotechnology, the multifunctional materials are being imaged with much advanced precision microscopes. Difference in properties at bulk and nano-level has provided new insight and direction to the researchers to meet the ever-increasing demand of smaller and more efficient devices. This paper reports a study of the polar regions, ferroelectric domains and characterization on different sample surfaces of interest. The Scanning Probe Microscope has been used in its contact mode to obtain the nano-scale surface images of some important multifunctional ferroics such as Lead Potassium Niobate, Bismuth Titanate and Gadolinium Molybdate. The piezoresponse and ferroelectric properties have been the primary area of concentration. The study shows the interesting ferroelectric domain structure on various pristine sample surfaces.

Tunable quasi-cw two-micron lasing in diode-pumped crystals of mixed Tm3+-doped sodium — lanthanum — gadolinium molybdates and tungstates

Two-micron lasing is obtained for the first time on the 3F4 → 3H6 transition of Tm3+ ions in diode-pumped crystals of mixed sodium — lanthanum — gadolinium tungstate Tm:NaLa1/2Gd1/2(WO4)2 (CTm = 3.6 at %) (3.6Tm : NLGW) and molybdate Tm:NaLa1/3Gd2/3(MoO4)2 (CTm = 4.8 at %) (4.8Tm : NLGM). For the 3.6Tm : NLGW crystal, the quasi-cw laser output power exceeded 200 mW and the slope efficiency (with respect to absorbed pump power) for the π- and σ-polarisations at wavelengths of 1908 and 1918 nm was 34% and 30%, respectively. The laser wavelength of this crystal was continuously tuned within the spectral range of 1860 — 1935 nm. For the 4.8Tm : NLGM crystal, the slope efficiency for the π- and σ-polarisations at wavelengths of 1910 and 1918 nm was 27% and 23%, respectively, and the laser wavelength was tunable within the spectral range of 1870 — 1950 nm.