Solid Solution Range Research Articles

Phase stability of solid solution La1-xRxRh3B (R = Gd, Lu and Sc) with cubic anti-perovskite type structurecompounds

We have investigated the solid solution range of single phase La1-xRxRh3B (R = Gd, Lu and Sc) compounds with a cubic anti-perovskite type structure. The behaviors of lattice parameters, hardness, and thermogravimetry–differential thermal analysis (TG-DTA) were also investigated. The cubic anti-perovskite phase exists over the entire composition range x from 0.0 to 1.0 for all La-Gd, La-Lu and La-Sc systems. Both the lattice parameters and the hardness in all La-Gd, La-Lu and La-Sc systems show a linear dependence on the degree of the substitution x of La atom. The results of TG-DTA measurements indicate that oxidation of the compounds in air begins at about 500-600 K. The mixed phases of RBO3, R2O3 and Rh are identified as oxidized products around x = 0.4 to 0.6 composition. The oxidation onset temperature, and weight gains due to the oxidation depend on the degree of substitution x. The behavior of crystallographic and physical/chemical properties in the present compounds La1-xRxRh3B strongly depend on the atomic size of the rare-earth atoms that form the cubic framework.

Co‐immobilization of a PuO2 surrogate and contaminated stainless steel within a zirconolite matrix

AbstractThe aqueous reprocessing of spent nuclear fuel generates a considerable amount of plutonium alongside contaminated stainless steel, necessitating meticulous handling for safe decommission and long‐term management. The present work investigated the co‐immobilization of CeO2 (as an inactive surrogate for PuO2) with metallic Fe and Cr (representing a simplified stainless steel) within a zirconolite ceramic wasteform, nominally targeting Ca1−xCexZrTi2−2xFexCrxO7. After sintering at 1400°C under an air atmosphere, the zirconolite phase constituted 90.8–95.1 wt.% of the product across the solid solution range of 0.05 ≤ x ≤ 0.25, alongside perovskite and baddeleyite secondary phases in varying proportion; no evidence of the unincorporated CeO2 or metallic and oxidized Fe or Cr were identified. Above x = 0.30 CeO2 was detected inferring, the solubility limit was reached. A polytype transformation from zirconolite 2M to 3T was confirmed by X‐ray diffraction and selected area electron diffraction results, with the relative fraction of the 3T phase gradually increasing to 49.5 wt.% at x = 0.30. Deconvolution of X‐ray photoelectron spectroscopy data revealed the partial reduction of Ce4+–Ce3+, whereas Fe and Cr species maintained trivalent, in agreement with the targeted substitution scheme. Benefitting from the excellent chemical flexibility of zirconolite structured compounds, the co‐immobilization approach may be an effective disposal pathway for Pu‐containing wastes and contaminated stainless steel residues.

How alloying and processing effects can influence the microstructure and mechanical properties of directly extruded thin zinc wires

Zinc (Zn) in particular has gained attention as biodegradable metal due to its advantageous corrosion rates compared to magnesium (Mg) or iron (Fe). Still, strength and ductility of zinc are found to be unfavorable for many medical applications. Strategies to overcome such issues base on a distinct grain refinement of the respective product. One important condition of the metal is assumed to be in the form of wires, which in the present work stem from a direct extrusion setup and high degrees of deformation, therefore a hot forming procedure as the underlying thermomechanical treatment. A basic binary alloying approach with Mg, manganese (Mn) and copper (Cu) is applied, limiting the content to a solid solution range of the alloys. The processability and the processing ranges are examined as well as their impact on the microstructure development and the resulting mechanical behavior. Higher extrusion speed leads to inhomogeneous material flow during extrusion. Alloying Zn can reduce the influence of process parameters and decrease the average grain sizes of wires which experienced lower temperature impact. The forming ability of pure Zn and ZnMg-alloy remain limited whereas they appear more beneficial for the alloys with Mn and especially Cu.

Li-diffusion in lithium niobate - tantalate solid solutions

Among the class of ferroelectric crystals, the lithium-niobate-tantalate (LiNb1-xTaxO3) system attracts growing attention for applications in the field of nonlinear optics, functional electronics and piezoelectrics. The self-diffusion of the ionic constituents, especially Li, is important for the overall electric conductivity. The aim of this investigation is to figure out how Li diffusion in lithium niobate-tantalate solid solution crystals behaves as a function of the Ta content. Specially grown LiNb1-xTaxO3 single crystals with different niobium to tantalum ratios are used for this purpose. Li tracer self-diffusion is analysed using secondary ion mass spectrometry after applying an isotope-enriched 6LiNbO3 tracer layer and subsequent annealing. All investigations are carried out at 250 and 600 °C, respectively. Despite of the theoretically predicted non-linear variation of the volume of the unit cell between the values of the end compounds, the results indicate no significant modification of Li diffusivities within error limits in the whole solid solution range.

Structure and superconducting properties of Ru1–x Mo x (x = 0.1–0.9) alloys

We report the detailed crystal structures and physical properties of Ru1−x Mo x alloys in the solid solution range of x = 0.1–0.9. Structure characterizations indicate that the crystal structure changes from the hcp-Mg-type, to β-CrFe-type, and then bcc-W-type. The measurements of physical properties show that the Ru1−x Mo x samples with x ≥ 0.2 are superconductors and the superconducting transition temperature T c as a function of Mo content exhibits a dome-like behavior.

Challenging thermodynamics: combining immiscible elements in a single-phase nano-ceramic

The Hume-Rothery rules governing solid-state miscibility limit the compositional space for new inorganic material discovery. Here, we report a non-equilibrium, one-step, and scalable flame synthesis method to overcome thermodynamic limits and incorporate immiscible elements into single phase ceramic nanoshells. Starting from prototype examples including (NiMg)O, (NiAl)Ox, and (NiZr)Ox, we then extend this method to a broad range of Ni-containing ceramic solid solutions, and finally to general binary combinations of elements. Furthermore, we report an “encapsulated exsolution” phenomenon observed upon reducing the metastable porous (Ni0.07Al0.93)Ox to create ultra-stable Ni nanoparticles embedded within the walls of porous Al2O3 nanoshells. This nanoconfined structure demonstrated high sintering resistance during 640 h of catalysis of CO2 reforming of methane, maintaining constant 96% CH4 and CO2 conversion at 800 °C and dramatically outperforming conventional catalysts. Our findings could greatly expand opportunities to develop novel inorganic energy, structural, and functional materials.

Extending the solid solution range of sodium ferric pyrophosphate: Off‐stoichiometric Na3Fe2.5(P2O7)2 as a novel cathode for sodium‐ion batteries

AbstractIron‐based pyrophosphates are attractive cathodes for sodium‐ion batteries due to their large framework, cost‐effectiveness, and high energy density. However, the understanding of the crystal structure is scarce and only a limited candidates have been reported so far. In this work, we found for the first time that a continuous solid solution, Na4−αFe2+α/2(P2O7)2 (0 ≤ α ≤ 1, could be obtained by mutual substitution of cations at center‐symmetric Na3 and Na4 sites while keeping the crystal building blocks of anionic P2O7 unchanged. In particular, a novel off‐stoichiometric Na3Fe2.5(P2O7)2 is thus proposed, and its structure, energy storage mechanism, and electrochemical performance are extensively investigated to unveil the structure–function relationship. The as‐prepared off‐stoichiometric electrode delivers appealing performance with a reversible discharge capacity of 83 mAh g−1, a working voltage of 2.9 V (vs. Na+/Na), the retention of 89.2% of the initial capacity after 500 cycles, and enhanced rate capability of 51 mAh g−1 at a current density of 1600 mA g−1. This research shows that sodium ferric pyrophosphate could form extended solid solution composition and promising phase is concealed in the range of Na4−αFe2+α/2(P2O7)2, offering more chances for exploration of new cathode materials for the construction of high‐performance SIBs.

The Magnetism of Nonstoichiometric Sr2Cr1+xRe1-xO6 (0 < x < 0.5) Double Perovskites.

The effect of nonstoichiometry on the cation distribution, crystal structure, and magnetic properties of a series of Cr-rich Sr2Cr1+xRe1-xO6 samples has been investigated. The double perovskite structure is maintained over a wide solid solution range that extends from x = 0 to approximately x = 0.5. For most of the solid solution range, the Cr-rich octahedral site maintains a nearly constant occupancy, 87% Cr and 13% Re, that is comparable to prior studies of Sr2CrReO6, while Cr steadily replaces Re on the other octahedral site. As x approaches 0.5, long-range Cr/Re ordering drops precipitously. Analysis of X-ray powder diffraction peak shapes reveals antiphase boundaries, associated with Cr/Re ordering, the concentration of which increases steadily with increasing x. Neutron powder diffraction studies confirm antiferromagnetic coupling between antisite Cr3+ ions and Cr3+ ions that occupy the normal sites, leading to site-dependent ferrimagnetic ordering. Density functional theory calculations indicate that chromium maintains a +3 oxidation state across the series, while the oxidation state of rhenium increases with increasing x. Calculations are also used to explore the energies of competing magnetic ground states. Except for the most chromium-rich compositions (x ≈ 0.5), site-dependent ferrimagnetism is retained with only a modest reduction in TC. The saturation magnetization steadily decreases as the chromium content increases due to a combination of Cr/Re antisite disorder and antiphase boundaries.

Alloying and Doping Control in the Layered Metal Phosphide Thermoelectric CaCuP.

We recently identified CaCuP as a potential low cost, low density thermoelectric material, achieving zT = 0.5 at 792 K. Its performance is limited by a large lattice thermal conductivity, κL, and by intrinsically large p-type doping levels. In this paper, we address the thermal and electronic tunability of CaCuP. Isovalent alloying with As is possible over the full solid solution range in the CaCuP1-xAsx series. This leads to a reduction in κL due to mass fluctuations but also to a detrimental increase in p-type doping due to increasing Cu vacancies, which prevents zT improvement. Phase boundary mapping, exploiting small deviations from 1:1:1 stoichiometry, was used to explore doping tunability, finding increasing p-type doping to be much easier than decreasing the doping level. Calculation of the Lorenz number within the single parabolic band approximation leads to an unrealistic low κL for highly doped samples consistent with the multiband behavior in these materials. Overall, CaCuP and slightly Cu-enriched CaCu1.02P yield the best performance, with zT approaching 0.6 at 873 K.

Synthesis and Electrical Behavior of Sodium Doped Monoclinic SrSiO3

The operating temperature of solid oxide fuel cells, oxygen division membranes, and oxygen sensors is determined by oxide-ion electrolytes. There is a strong incentive to reduce the operating temperature in solid oxide fuel cells, from 800°C to 500°C. The use of low-cost Na+ instead of K+ as dopant in monoclinic SrSiO3 offers a wider solid solution range (0.1&lt;x&lt; 0.5) in Sr1-xNaxSiO3-δ and obtains an oxide ion conductivity of 10-2 Scm-1 at 600°C, reducing the temperature of a smooth transition to full impairment of mobile oxide ions. For electrochemical characterization, the flat surfaces of the pellets were pasted with silver (Ag) paste and then sintered at 1200°C for 24 hours. The production of the Na2Si2O5 phase was observed for most compositions due to thermal treatment. Crystallization of Na2Si2O5 from glass was obtained in single-step calcination at 850°C after synthesis in an acetone medium, resulting in the highest conductivity. Although double calcination reduced conductivity, it improved thermal stability. Due to its low activation energy and lack of crystallization of other silicates, this material showed maximum conductivity after long-standing maturity at 600°C. Ethanol was used in place of acetone for powder assimilation and double calcination was also performed.

Discovery of Guinier-Preston zone hardening in refractory nitride ceramics

Traditional age hardening mechanisms in refractory ceramics consist of precipitation of fine particles. These processes are vital for widespread wear-resistant coating applications. Here, we report novel Guinier-Preston zone hardening, previously only known to operate in soft light-metal alloys, taking place in refractory ceramics like multicomponent nitrides. The added superhardening, discovered in thin films of Ti-Al-W-N upon high temperature annealing, comes from the formation of atomic-plane-thick W disks populating {111} planes of the cubic matrix, as observed by atomically resolved high resolution scanning transmission electron microscopy and corroborated by ab initio calculations and molecular dynamics simulations. Guinier-Preston zone hardening concurrent with spinodal decomposition is projected to exist in a range of other ceramic solid solutions and thus provides a new approach for the development of advanced materials with outstanding mechanical properties and higher operational temperature range for the future demanding applications.

Synergistic photocatalytic degradation mechanism of BiOClxI1-x-OVs based on oxygen vacancies and internal electric field-mediated solid solution

The easy recombination of photoexcited electron-hole pairs is a serious constraint for the application of photocatalysts. In this work, a range of BiOClxI1-x solid solutions with abundant oxygen vacancies (BiOClxI1-x-OVs) were synthesized. In particular, the optimal BiOCl0.5I0.5-OVs sample exhibited almost 100% removal of bisphenol A (BPA) within 45 min visible light exposure, which was 22.4, 3.1 and 4.5 times greater than BiOCl, BiOCl-OVs and BiOCl0.5I0.5, respectively. Besides, the apparent quantum yield of BPA degradation reaches 0.24%, better than some other photocatalysts. Benefiting from the synergism of oxygen vacancies and solid solution, BiOCl0.5I0.5-OVs gained an enhanced photocatalytic capacity. Oxygen vacancies induced an intermediate defective energy level in BiOClxI1-x-OVs materials, promoting the generation of photogenerated electrons and the molecular oxygen adsorption to produce more active oxygen radicals. Meanwhile, the fabricated solid solution structure enhanced the internal electric field between BiOCl layers, achieving rapid migration of photoexcited electrons and effective segregation of photoinduced charge carriers. Thus, this study provides a viable idea to solve the problems of poor visible light absorption of BiOCl-based photocatalysts and easy reorganization of electrons and holes in the photocatalysts.

Evidence of Yb3+ band broadening and continuous wave laser operation in bismuth modified KY1-x-yBixYby(WO4)2 mixed single crystals

To increase the Yb3+ optical bandwidths in highly anisotropic β:KY(WO4)2 (KYW) type laser single crystals, the simultaneous contributions of the Bi3+ 6 s2 lone electron pair and Y3+/Bi3+ ionic radii mismatch is proposed. The top seeded solution growth method with K2W2O7 as solvent was used to grow Yb doped single crystals of the KY(WO4)2/KBi(WO4)2 (KYBiW) solid solution. Crystals obtained across the used KY1xBixW solid solution range show always the low temperature β phase, with the monoclinic C2/c crystalline structure, but important differences between the Y/Bi composition of the starting solute and that of the crystal grown are found. The incorporation of Y and Yb into the crystal is preferential, resulting in a low Bi segregation coefficient, SBi= 13%, on the KYW side but near SBi= 90% in the KBiW side (Yb is still strongly incorporated filling the remaining 10%). The saturation temperature of the melt decreases as the Bi concentration in the precursor KY1-x-yBixYby(WO4)2/K2W2O7 mixture increases. Evidence of optical absorption and photoluminescence broadening of the Yb3+ bands with Bi3+ incorporation is derived from 6 K and 300 K measurements. Laser operation of KY1-x-yBixYby(WO4)2 crystals is shown to be comparable to that of KY1−yYby(WO4)2 ones. Up to ≈ 0.5 W of output power is obtained pumping in continuous wave regime with a Ti sapphire laser with slope efficiency η ≈ 65% and pump threshold< 200 mW, which is within the usual performance of KYW:Yb crystals, but the spectral distribution of the laser output in KYBiW:Yb crystals is larger than that for KYW:Yb. This spectral broadening may help in the reduction of ultrashort (fs) laser pulse duration generated by modelocking.

Evaluating the pressure dependence of PZT structures using a virtual reality environment

Pb–Zr–Ti–O (PZT) perovskites span a large solid-solution range and have found widespread use due to their piezoelectric and ferroelectric properties that also span a large range. Crystal structure analysis via Rietveld refinement facilitates materials analysis via the extraction of the structural parameters. These parameters, often obtained as a function of an additional dimension (e.g., pressure), can help to diagnose materials response within a use environment. Often referred to as “in-situ” studies, these experiments provide an abundance of data. Viewing structural changes due to applied pressure conditions can give much-needed insight into materials performance. However, challenges exist for viewing/presenting results when the details are inherently three-dimensional (3D) in nature. For PZT perovskites, the use of polyhedra (e.g., Zr/Ti–O6 octahedra) to view bonding/connectivity is beneficial; however, the visualization of the octahedra behavior with pressure dependence is less easily demonstrated due to the complexity of the added pressure dimension. We present a more intuitive visualization by projecting structural data into virtual reality (VR). We employ previously published structural data for Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 as an exemplar for VR visualization of the PZT R3c crystal structure between ambient and 0.62 GPa pressure. This is accomplished via our in-house CAD2VR™ software platform and the new CrystalVR plugin. The use of the VR environment enables a more intuitive viewing experience, while enabling on-the-fly evaluation of crystal data, to form a detailed and comprehensive understanding of in-situ datasets. Discussion of methodology and tools for viewing are given, along with how recording results in video form can enable the viewing experience.

Characterization of Ruddlesden-Popper La2−xBaxNiO4±δ Nickelates as Potential Electrocatalysts for Solid Oxide Cells

Ruddlesden-Popper La2-xBaxNiO4±δ (x = 0-1.1) nickelates were prepared by a glycine-nitrate combustion route combined with high-temperature processing and evaluated for potential application as electrocatalysts for solid oxide cells and electrochemical NOx elimination. The characterization included structural, microstructural and dilatometric studies, determination of oxygen nonstoichiometry, measurements of electrical conductivity and oxygen permeability, and assessment of chemical compatibility with other materials. The formation range of phase-pure solid solutions was found to be limited to x = 0.5. Exceeding this limit leads to the co-existence of the main nickelate phase with low-melting Ba- and Ni-based secondary phases responsible for a strong reactivity with Pt components in experimental cells. Acceptor-type substitution of lanthanum by barium in La2-xBaxNiO4+δ is charge-compensated by decreasing oxygen excess, from δ ≈ 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 °C in air, and generation of electron-holes (formation of Ni3+). This leads to an increase in p-type electronic conductivity (up to ~80 S/cm for highly porous La1.5Ba0.5NiO4+δ ceramics at 450-900 °C) and a decline of oxygen-ionic transport. La2-xBaxNiO4+δ (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 °C in air. These ceramic materials react with yttria-stabilized zirconia at 700 °C with the formation of an insulating La2Zr2O7 phase but show good chemical compatibility with BaZr0.85Y0.15O3-δ solid electrolyte.

Investigating the Li+ substructure and ionic transport in Li10GeP2-xSbxS12 (0 ≤ x ≤ 0.25).

Understanding the correlation between ionic motion and crystal structure is crucial for improving solid electrolyte conductivities. Several substitution series in the Li10GeP2S12 structure have shown a favorable impact on the ionic conductivity, e.g. the replacement of P(+V) by Sb(+V) in Li10GeP2S12. However, here the interplay between the structure and ionic motion remains elusive. X-Ray diffraction, high-resolution neutron diffraction, Raman spectroscopy and potentionstatic impedance spectroscopy are employed to explore the impact of Sb(+V) on the Li10GeP2S12 structure. The introduction of antimony elongates the unit cell in the c-direction and increases the M(1)/P(1) and Li(2) polyhedral volume. Over the solid solution range, the Li+ distribution remains similar, an inductive effect seems to be absent and the ionic conductivity is comparable for all compositions. The effect of introducing Sb(+V) in Li10GeP2S12 cannot be corroborated.

Effect of Ca2+ site substitution on structural, optical, electrical and magnetic properties in Nd3+ and Mn2+-co-doped calcium molybdato-tungstates

Ca 1-3x-y Mn y [] x Nd 2x (MoO 4 ) 1-3x (WO 4 ) 3x molybdato-tungstates (▯ denotes vacant sites) were successfully synthesized by high-temperature solid-state reaction. New materials crystallize in scheelite-type structure within whole homogeneity range of solid solution ( x ≤ 0.2000 and y = 0.0200). Morphological features and particle size distribution were investigated by SEM and laser diffraction methods, respectively. Spectroscopic measurements in the UV–vis range was carried out to determine optical direct band gap ( E g ), Urbach energy ( E U ) and confirmation of structural disorder. Refractive index ( n ) was calculated using four different models. Magnetic studies revealed paramagnetic behavior with long-range ferrimagnetic and short-range antiferromagnetic interactions. New materials showed weak n -type electrical conductivity and thermoelectric power factor (S 2 σ) that strongly depends on Nd 3+ ions content. Dielectric parameters, i.e. relative permittivity (ε r ) and energy loss ( tanδ ) are insignificantly dependent on Nd 3+ ions concentration. These effects were considered in terms of structural defects, thermal activation of charge carriers, and the Maxwell–Wagner polarization.

Exploring the limits of Fe3+ solid solution in calcium hexaluminate

Calcium hexaluminate (CA6) forms a broad range of solid solutions by substitution of Al3+ with other cations such as, for example, Fe, Co, Mn, etc. Solid solutions of CA6 with magnetic ions have attracted interest because they allow designing the magnetic behavior depending on the composition of each solid solution. In this paper, we have studied materials pertaining to an area of the CaO–Al2O3–Fe2O3 ternary phase diagram that is potentially interesting, but which has not been too much explored so far. We report on various synthesis methods of calcium hexaluminate with iron in solid solution. The percentages of iron introduced have been varied in order to determine the maximum percentage of iron that can enter into solid solution in the calcium hexaluminate such that the materials obtained exhibit magnetic behavior.Graphical Exploring the limits of Fe3+ solid solution in calcium hexaluminate.

Correlations of Composition, Structure, and Hardness in the High-Entropy Alloy System Nb–Mo–Ta–W

Refractory high-entropy alloys are of interest due to the potential of compositionally complex alloys to achieve combinations of mechanical properties such as room-temperature ductility and high-temperature strength rarely found in simpler alloys. To study a large compositional range of the system Nb–Mo–Ta–W, thin-film materials libraries were fabricated by combinatorial sputtering. High-throughput characterization methods were used to systematically determine composition-dependent properties: (I) the extent and stability of the complex solid solution range and (II) the mechanical properties (Young’s modulus, hardness). The whole investigated composition range of Nb20–59Mo9–31Ta10–42W12–32 crystallized in a bcc phase, independent of annealing temperatures ranging from 300 to 900 °C. Mechanical strength values of the Nb–Mo–Ta–W compositions were calculated using the Maresca–Curtin analytical model parameterized with experimental data. A strong positive correlation with measured hardness was observed that allows using this analytical model for optimization of the mechanical strength. We predict that compositions with high Mo contents provide the highest hardness values.

K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+ → Eu3+ Energy Transfer.

This paper describes the influence of sintering conditions and Eu3+/Tb3+ content on the structure and luminescent properties of K5Eu1-xTbx(MoO4)4 (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K5Tb(MoO4)4 (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the β ↔ α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-K5Eu0.6Tb0.4(MoO4)4 structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group C2/m(0β0)00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to 5D4 → 7FJ (J = 3-6) Tb3+ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the 5D4 → 7F5 Tb3+ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λex = 377 nm exhibited an intense red emission at ∼615 nm due to the 5D0 → 7F2 Eu3+ transition, thus indicating an efficient energy transfer from Tb3+ to Eu3+.