Dodecahedral Sites Research Articles

Yb3+ doping effects on thermal conductivity and thermal expansion of yttrium aluminium garnet

Yttrium Aluminium Garnet (YAG) is an attractive candidate as thermal barrier material used for turbine blade in aero engines, due to its relatively low thermal conductivity, low oxygen diffusivity and good phase stability at high temperature. YAG has a complex crystal structure, in which Y3+ ions locate in dodecahedron and Al3+ ions in octahedron and tetrahedron. Replacing the host cations with rare earth elements can cause the structure change which influences the thermal properties of YAG. Because the space inside the octahedron is relatively small, Yb3+ ions which have the smallest ionic radial size in the lanthanide series, have been selected and attempted to be doped on dodecahedral and octahedral sites to investigate the effects on thermal conductivity and thermal expansion. The variation of lattice constant indicates that Yb3+ ions are located on the dodecahedron or octahedron. In addition, when Yb3+ ions replace Al3+ ions on octahedral sites, the thermal conductivity at room temperature is dramatically reduced and the coefficient of thermal expansion is over 10×10−6K−1 at high temperature, which results from the expansion of octahedron due to the much larger radius of Yb3+ ion compared with the host cation (Al3+ ion). On the contrary, replacing Y3+ ions with Yb3+ ions in dodecahedron, the thermal conductivity also gradually reduces to the similar value but the coefficient of thermal expansion is getting smaller, due to the relatively small ionic radius of Yb3+ causing the contraction of the dodecahedron. Therefore, a dopant with much larger radius would be preferred in both dodecahedron and octahedron to significant reduce thermal conductivity as well as increase coefficient of thermal expansion of YAG, by introducing large radial difference between the dopant and the host cations.

Multicomponent diffusion in aluminosilicate garnet: coupling effects due to charge compensation

ABSTRACTThree kinds of multicomponent diffusion effects, arising from three distinct physical mechanisms, are evident in stranded diffusion profiles at the rims of partially resorbed garnets from the contact aureole of the Makhavinekh Lake Pluton, northern Labrador. Profiles that display a subtle maximum in Ca concentration are explained by the prevailing ideal mean-field theory of multicomponent diffusion, but models implementing that theory cannot replicate inverted profiles for Li and internal maxima for Nd, Sm, and Eu. The anomalous profiles are quantitatively reproduced, however, by numerical simulations employing a model based on coupled movement of charge-compensating groups during diffusional transport of yttrium and the rare-earth elements (Y+REEs). An alkali-type charge-compensation mechanism for the heterovalent substitution of Y+REEs on dodecahedral sites in garnet produces direct charge coupling between Li+ and (Y+REE)3+ and leads to co-diffusion of Li+-(Y+REE)3+ pairs, with the result that Li profiles closely mimic those for Y+REEs. A menzerite-type charge-compensation mechanism produces indirect charge coupling among all Y+REE components, with the result that the fluxes of low-abundance REEs become partly dependent on the fluxes of Y+REEs present in higher abundance. These findings have implications for the robustness of Li profiles in garnet as monitors of fluid–rock interaction, for geochronology based on the Sm–Nd and Lu–Hf systems, and for future experimental attempts to quantify rates of diffusion in garnet.

Correlation factors for impurity diffusion on the sublattice of dodecahedral sites in garnet

The method of Koiwa and Ishioka (Philos Mag A 47:927–938, 1983) is used, with slight modification, to evaluate the correlation factor for vacancy-mediated diffusion of impurity atoms on the sublattice of dodecahedral sites in garnet, as a function of the relevant vacancy-jump frequencies. The required values of the lattice Green’s function were obtained from multiple Monte Carlo simulations in lattices of progressively larger size, extrapolated to an infinite lattice using a model that linearizes the dependence of the functional value on lattice size. As Online Resources, codes are provided that permit evaluation of the correlation factor for any chosen set of vacancy-jump frequencies, for implementation in either Mathematica® or Matlab®.

Na Modification of Lanthanide Doped Ca3Nb1.5Ga3.5O12-Type Laser Garnets: Czochralski Crystal Growth and Characterization

The Na+-Yb3+ (or Er3+) co-substitution of Ca2+ in Ca3Nb1.5Ga3.5O12 (CNGG) laser crystal is studied. In contrast to other garnets whose structural disorder is exclusively based on the presence of differently sized cations on the same crystal sites, Na+ incorporated in the dodecahedral site (a site also shared by Ca2+ and trivalent lanthanides) creates diverse electric charge distributions over the dodecahedral sublattice, which adds to the disorder associated with Nb5+ and Ga3+ simultaneous occupation of the octahedral and tetrahedral sites. The currently determined cationic compositions of Czochralski grown congruent CNGG and Na-modified CNGG crystals show that Na+ incorporation reduces the cationic vacancy concentration on dodecahedral and octahedral sites but does not affect that in tetrahedral sites. Physical properties of interest for laser design (optical transmission, elastic constants, hardness, specific heat, thermal conductivity, thermal expansion, refractive index dispersion, group velocity disp...

Influence of initial charge composition and growth/annealing atmospheres on the structural parameters of Czochralski-grown (NaxGd1−x)MoO4crystals

The composition of microparts of sodium-gadolinium molybdate crystals, grown by the Czochralski method in different atmospheres from stoichiometric ((Na0.5Gd0.5)MoO4; group I) and nonstoichiometric ((Na2/7Gd4/71/7)MoO4 (II.1) and (Na6/15Gd8/151/15)MoO4 (II.2); group II) initial charge compositions (where = vacancies), was refined by X-ray diffraction analysis. All the crystals under investigation were found to crystallize in the space group I41/a without any signs of structural modulation and the refined crystal compositions were different from the initial charge compositions. Crystals from group I had a gadolinium excess and sodium deficiency without vacancies in the dodecahedral site. Vacancies were found in the Mo and O sites of the structure of the dark-gray micropart of the crystal grown in an Ar atmosphere, with the composition of the near-colorless micropart of the same crystal being close to stoichiometry. Microparts of the crystals from group II contain vacancies in both cation sites in (Na0.348(8)Gd0.5280.124)(Mo6+0.996(3)0.004)O4 (II.1) and in the dodecahedral and oxygen sites in (Na0.300(8)Gd0.5760.124)Mo6+(O3.880(10)0.120) (II.2). It was revealed that an additional annealing in air of the crystals grown in a slightly oxidizing atmosphere reduces the oxygen vacancies content. The possibility of growing crystals with an ordered structure depending on the ratio of the formula coefficients of components in the dodecahedral site of scheelite structure is thus discussed herein.

Clarifying the preferential occupation of Ga3+ ions in YAG:Ce,Ga nanocrystals with various Ga3+-doping concentrations by nuclear magnetic resonance spectroscopy

Preferential substitution of Ga3+ ions for octahedral Al3+ ions in YAG:Ce,Ga nanophosphors enhances the luminescence properties of Ce3+ activators.

High Li ion conductivity in a garnet-type solid electrolyte via unusual site occupation of the doping Ca ions

The search for solid electrolytes with high stability and ionic conductivity is a long sought-after goal in the development of safe and high energy density Li-ion batteries. Garnet-type lithium conductors form a most promising family of materials due to their good chemical stability. However, the low conductivity at room temperature prevents wider application of these materials. It has long been recognized that alkaline earth metals are confined exclusively to dodecahedral 8-coordinated sites within the garnet framework. In contrast to this dominant viewpoint, we show that Ca2+ cations can occupy the octahedral 6-coordinated sites, leading to an enhanced room temperature conductivity of 5.2×10−4Scm−1 and reduced activation energy of 0.27eV, together with a 0 to 9V electrochemical stability window. This finding opens up a new opportunity for the design of ceramic electrolytes with higher conductivities, providing added impetus for further exploration of oxide electrolyte chemistry.

Bi‐YIG system: The effect of cation relocation on structural, hyperfine, and magnetic properties

BixY3–xFe5O12 (x = 0.0, 0.9, 1, 1.1, 1.2, and 1.3) were synthesized by the sol–gel method to investigate the effect of Bi substitution on the structural and magnetic properties of YIG. The XRD results indicate that all the diffraction peaks belong to the pure phase. The Mössbauer analysis shows an increase in the ratio and also a line broadening at tetrahedral sites, which suggests a relocation of yttrium ions at dodecahedral sites with iron ions at octahedral sites. The saturation magnetization fluctuates with increasing bismuth content, which has been explained based on Mössbauer results. The change in vibrational band broadening and the observation of a new band in the far‐FTIR spectra confirm the proposed cation relocation.

High-pressure synthesis of skiagite-majorite garnet and investigation of its crystal structure

Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia 3 d , a = 11.7511(2) A, V = 1622.69(5) A3, D calc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mossbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X-ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X-ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

Fabrication of cerium-doped yttrium aluminum garnet thin films by a mist CVD method

We synthesized thin films, consisting of yttrium aluminum garnet doped with Ce3+ (YAG:Ce), using the mist chemical vapor deposition (CVD) method, which allows the fabrication of high-quality thin films under atmospheric conditions without the use of vacuum equipment. Under a deposition rate of approximately 1μm/h, the obtained thin films had a typical thickness of 2μm. The XRD analysis indicated that the thin films consisted of single-phase YAG:Ce. The Rutherford backscattering confirmed the stoichiometry; the composition of the film was determined to be (Y, Ce)3Al5O12, with a Ce content of Ce/(Y+Ce)=2.5%. The YAG:Ce thin films exhibited fluorescence due to the 5d–4f electronic transitions characteristic of the Ce ions occupying the eight-coordinated dodecahedral sites in the YAG lattice.

Thermodynamics and phonon dispersion of pyrope and grossular silicate garnets from ab initio simulations

The phonon dispersion and thermodynamic properties of pyrope ( $$\hbox {Mg}_3\hbox {Al}_2\hbox {Si}_3\hbox {O}_{12}$$ ) and grossular ( $$\hbox {Ca}_3\hbox {Al}_2\hbox {Si}_3\hbox {O}_{12}$$ ) have been computed by using an ab initio quantum mechanical approach, an all-electron variational Gaussian-type basis set and the B3LYP hybrid functional, as implemented in the Crystal program. Dispersion effects in the phonon bands have been simulated by using supercells of increasing size, containing 80, 160, 320, 640, 1280 and 2160 atoms, corresponding to 1, 2, 4, 8, 16 and 27 $$\mathbf {k}$$ points in the first Brillouin zone. Phonon band structures, density of states and corresponding inelastic neutron scattering spectra are reported. Full convergence of the various thermodynamic properties, in particular entropy (S) and specific heat at constant volume ( $$C_\mathrm{{V}}$$ ), with the number of $$\mathbf {k}$$ points is achieved with 27 $$\mathbf {k}$$ points. The very regular behavior of the S(T) and $$C_\mathrm{{V}}(T)$$ curves as a function of the number of $$\mathbf {k}$$ points, determined by high numerical stability of the code, permits extrapolation to an infinite number of $$\mathbf {k}$$ points. The limiting value differs from the 27- $$\mathbf {k}$$ case by only 0.40 % at 100 K for S (the difference decreasing to 0.11 % at 1000 K) and by 0.29 % (0.05 % at 1000 K) for $$C_\mathrm{{V}}$$ . The agreement with the experimental data is rather satisfactory. We also address the problem of the relative entropy of pyrope and grossular, a still debated question. Our lattice dynamical calculations correctly describe the larger entropy of pyrope than grossular by taking into account merely vibrational contributions and without invoking “static disorder” of the Mg ions in dodecahedral sites. However, as the computed entropy difference is found to be larger than the experimental one by a factor of 2–3, present calculations cannot exclude possible thermally induced structural changes, which could lead to further conformational contributions to the entropy.

Tuning the composition and magnetostructure of dysprosium iron garnets by Co-substitution: An XRD, FT-IR, XPS and VSM study

We report the Co-substituting on the synthesis and properties of garnet type dysprosium ferrite nanoparticles by basic composition Dy3Fe5−xCoxO12 (x=0–0.06) synthesized through glycine assisted combustion method. A possible formation mechanism of synthesized Dy3Fe5−xCoxO12 samples by controlling the synthesis process has been proposed. XRD, FT-IR, XPS and VSM studies were used to investigate the compositional and magnetostructural properties of the prepared nanoparticles. XRD results confirm that all the samples are single-phase cubic garnet structure with mean crystallite size of 97–105nm obtained from Scherrer method and 95–102nm from W–H method. FT-IR analysis shows the presence of three expected bands in the frequency limit of 450–600cm−1 attributed to metal–O stretching vibration in tetrahedral site of garnet structure. A typical survey spectrum from XPS results confirmed the presence of Dy, Fe, Co and O elements in the samples. This study also to characterize the different oxidation states of the samples by fitting the parameters of high resolution Dy 4d, Fe 2p, Co 2p and O 1s XPS spectra. The XPS data of Dy 4d spectrum show that Dy3+ ion occupy in dodecahedral (D) site. The XPS analysis of Fe 2p and Co 2p data suggests that (Fe3+ and Fe2+), (Co3+ and Co2+) are distributed in tetrahedral and octahedral sites. Magnetic study reveals that these nanoparticles exhibit ferromagnetism. The saturation magnetization, coercivity, anisotropy constant and remanent magnetization changes appreciably with increase in Co content in dysprosium ferrite.

Photoluminescence of Eu<sup>3+</sup> Ions in Lu<sub>3</sub>(Ga,In)<sub>5</sub>O<sub>12</sub>:Eu<sup>3+</sup> Films Grown by LPE Method

High quality thin epitaxial films of Lu3(Ga,In)5O12:Eu3+ were grown by liquid phase epitaxy (LPE) method from Bi2O3-based flux. Excitation and emission spectra as well as luminescence decay kinetics of Lu3(Ga,In)5O12:Eu3+ epitaxial films were studied under UV excitation. The photoluminescence spectra of Eu3+ ions in Lu3(Ga,In)5O12:Eu3+ are characteristic to the f→f transition in the Eu3+ ions that occupied dodecahedral sites in the garnet structure. The efficient energy transfer between Bi3+→Eu3+ is observed at the excitation of Lu3(Ga,In)5O12:Eu3+ films in Bi3+ excitation band with maximum at 288 nm. The Eu3+ decay curves of all investigated films show non-single exponential behavior at room temperature.

On the dodecahedral site substituted Lu3Fe5O12 ceramics for room temperature magneto-dielectric applications

Materials exhibiting low-field magneto-electric and magneto-dielectric (MD) response at room temperature are essential for utilization in multifunctional devices. Magneto-dielectric properties of Lu3Fe5O12 substituted with a magnetic ion (Terbium) at its dodecahedral (non-magnetic) site are investigated. The measured dielectric permittivity at various magnetic fields exhibits a giant MD effect (~65%) at remarkably low fields (H~1kOe) at room temperature. The MD response of Lu3Fe5O12 is enhanced due to the coupling effect of substituted magnetic Terbium ions in the non-magnetic Lu-site interacting with the neighboring Fe-sites. Studies suggest a route towards future applications of the MD effect in advanced devices.

Structural perfection of (Na0.5Gd0.5)MoO4:Yb laser crystals

Czochralski-grown (Na0.5Gd0.5)MoO4 single crystals doped with different concentrations of Yb3+ ions have been investigated by X-ray diffraction. The type and concentration of point defects (different Na:Gd ratios, vacancies in Mo and O sites), and, consequently, the real compositions of the investigated crystals have been determined. The presence of additional reflections (50%) for the (Na0.5Gd0.5)MoO4:10%Yb crystal was revealed, indicating the symmetry reduction from space group I41/a to P4_ without any change in the unit cell parameters. Crystal refinement within the framework of space group I41/a revealed an increase in the unit cell parameters for both (Na0.5Gd0.5)MoO4:3%Yb and (Na0.5Gd0.5)MoO4:10%Yb crystals; it can be explained by the kinetic phase transition of order–disorder type caused by the cation ordering at the dodecahedral sites in the local regions of the crystals. The reasons for the possible symmetry reduction in scheelite family crystals are discussed.

Luminescence and magnetic properties of novel nanoparticle-sheathed 3D Micro-Architectures of Fe0.5R0.5(MoO4)1.5:Ln3+ (R = Gd3+, La3+), (Ln = Eu, Tb, Dy) for bifunctional application

For the first time, we report the successful synthesis of novel nanoparticle-sheathed bipyramid-like and almond-like Fe0.5R0.5(MoO4)1.5:Ln3+ (R = Gd3+, La3+), (Ln = Eu, Tb, Dy) 3D hierarchical microstructures through a simple disodium ethylenediaminetetraacetic acid (Na2EDTA) facilitated hydrothermal method. Interestingly, time-dependent experiments confirm that the assembly-disassembly process is responsible for the formation of self-aggregated 3D architectures via Ostwald ripening phenomena. The resultant products are characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and magnetic measurements. The growth and formation mechanisms of the self-assembled 3D micro structures are discussed in detail. To confirm the presence of all the elements in the microstructure, the energy loss induced by the K, L shell electron ionization is observed in order to map the Fe, Gd, Mo, O, and Eu components. The photo luminescence properties of Fe0.5R0.5(MoO4)1.5 doped with Eu3+, Tb3+, Dy3+ are investigated. The room temperature and low temperature magnetic properties suggest that the interaction between the local-fields introduced by the magnetic Fe3+ ions and the R3+ (La, Gd) ions in the dodecahedral sites determine the magnetism in Fe0.5R0.5(MoO4)1.5:Eu3+. This work provides a new approach to synthesizing the novel Fe0.5R0.5(MoO4)1.5:Ln3+ for bi-functional magnetic and luminescence applications.

Intercalation Route to Complex Perovskites AM0.2Ta0.8O2.8N0.2 (A = Sr, Ba; M = Li, Na): Neutron Diffraction and Nuclear Magnetic Resonance Study

Oxynitride-type complex perovskites, AM0.2Ta0.8O2.8N0.2 (A = Sr, Ba; M = Li, Na), were synthesized by the ammonolytic heating of a layered perovskite, A5Ta4O15, with 0.5M2CO3. A Rietveld refinement of the synchrotron X-ray and neutron powder diffraction patterns confirmed the complete structural transformation from a hexagonal layered-perovskite to a three-dimensional perovskite type, as well as the stabilization of alkali cations on the octahedral sites rather than on the dodecahedral sites in the latter. In all four compounds, M+ and Ta5+ were disordered completely despite a charge difference as much as 4. The crystal symmetry of the average structure depended on the size of the dodecahedral cation: simple cubic for BaM0.2Ta0.8O2.8N0.2 and body-centered tetragonal for SrM0.2Ta0.8O2.8N0.2. This trend coincides with the symmetry transition from BaTaO2N (Pm3m) to SrTaO2N (I4/mcm). In both SrM0.2Ta0.8O2.8N0.2, nitrogen atoms preferentially occupied the c-axial 4a site of the tetragonal cell. Solid state ma...

Syntheses, crystal structures, and magnetic properties of Mn–Nb and Co–Nb cyanido-bridged bimetallic assemblies

We report three-dimensional (3D) cyanido-bridged Mn–Nb and Co–Nb bimetal assemblies, MnII2[NbIV(CN)8]·(L)2·nH2O (1: L=5-aminopyrimidine, n=5; 2: L=5-methylpyrimidine, n=4), and CoII2[NbIV(CN)8] (4-pyridinealdoxime)8·2H2O (3). The single-crystal X-ray structural analyses show that 1 and 2 consist of isomorphic 3D cyanido-bridged network (monoclinic, P21/n) composed of square antiprism Nb sites and distorted octahedral Mn sites. Both 1 and 2 show ferrimagnetism with Curie temperatures (TC) of 32 and 34K, respectively, which is caused by the antiferromagnetic superexchange interaction between MnII (S=5/2) and NbIV (S=1/2) through the CN groups. 3 has a 3D cyanido-bridged network (tetragonal, I41/a) composed of a dodecahedral Nb site and axially distorted pseudo-octahedral Co site. This compound shows ferromagnetic ordering with a TC of 18K. Magnetization versus rotation angle of single crystal reveals that 3 has a large coercive field of 15000Oe as crystal is oriented to a-axis, which is due to the strong magnetic anisotropy on distorted CoII.

Magnetic and fluorescence properties of cerium-doped yttrium gadolinium aluminum iron garnet crystals

Magnetic and fluorescence properties of chemically synthesized Ce:Gd-YAIG (Ce0.05GdxY2.95−xAl5−yFeyO12) nanocrystals have been investigated. The structural characterization by X-ray diffraction (XRD) shows that a garnet phase has been identified in samples with 0 ≤ x ≤ 2.95 and 0 ≤ y ≤ 3.0. When y = 0, only garnet peaks are observed for 0 ≤ x ≤ 2.5, while both garnet and perovskite phases are present for x > 2.5. It is found from XRD Rietveld analyses that the site occupancy of Fe3+ at the tetrahedral and octahedral sites in the garnet is independent of the amount of Y3+ substituted by Ce3+ and Gd3+ at the dodecahedral sites. The saturation magnetization for the sample with x = 0 and y = 3.0 is 4.35 emu/g, while that with x = 2.5 and y = 3.0 is 87.5 emu/g. When the Fe3+ composition y is varied from 0 to 3.0 at x = 2.5, the intensity of fluorescence at the emission wavelength ∼570 nm significantly decreases presumably due to absorption by Fe3+ that is increased in the crystal.

Local lattice distortion induced by substitution of Si for Ge in Li2CaGe1−xSixO4 solid solutions

Possible reasons for essential differences in values of the total crystal field splitting and centroid shift of Ce3+ 5d configuration for isostructural Li2CaGeO4 and Li2CaSiO4 were analyzed by means of X-ray diffraction method and luminescent spectroscopy. It was found that the increase in Si concentration in Ce3+-doped Li2CaGe1−xSixO4 solid solutions leads to increasing of noncubic crystal field splitting parameter (∆) of the Ce3+ 5d configuration, indicating to an enhancement of distortion from the cubic symmetry of the calcium dodecahedral site upon going from Li2CaGeO4 to Li2CaSiO4. The results of X-ray diffraction analysis of Li2CaGe1−xSixO4 show that the linear decrease of the unit-cell parameters with increasing Si-content is followed by complex changes in the geometry of the Ca polyhedra. Changes in the electronic structure with the composition of the solid solutions are also discussed.