Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Controlled peak wavelength shift of Ca1−xSrx(SySe1−y):Eu phosphor for LED application

The highly efficient red-orange-yellow-emitting phosphor (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺ in combination with commercial green phosphor SrGa₂S₄:Eu²⁺ and blue LED are proposed for a three-band white LED. The luminescence mechanism and optimization parameters are discussed on the basis of proposed peak wavelength diagram.     

Selective substitution of vanadium for molybdenum in Sr2(Fe1−xVx)MoO6 double perovskites

The crystal and magnetic structures of Sr₂(Fe_1−xV_x)MoO₆ (0.03?x?0.1) compounds are refined by alternately using X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) data collected at room temperature. The refinement results reveal that the V atoms selectively occupy the Mo sites instead of the Fe sites for x?0.1. The 3d/4d cation ordering decreases with the increase of the V content. Slight distortions in the lattice and metal octahedra are shown at 300 K, and the distortions increase at 4 K. The magnetic structure at 4 K can be modeled equally well with the moments aligning along [001], [110] or [111] directions. The total moments derived from the NPD data for the [110] and [111] direction models agree well with the magnetic measurements, whereas the [001] model leads to a smaller total moment. Bond valence analysis indicates that Sr ions are properly located in the structure and Mo ions are compatible with both the Fe sites and the Mo sites. The electronic effects are suggested to be responsible for the selective occupation of the V on the Mo sites due to the different distortions of the FeO₆ and MoO₆ octahedra.     

Optical properties of (Y1−xTmx)3GaO6 and subsolidus phase relation of Y2O3-Ga2O3-Tm2O3

A serial of samples in Y₂O₃-Ga₂O₃-Tm₂O₃ pseudo-ternary system are prepared by solid-state chemical reaction method. The range of solid solution in (Y_1−xTm_x)₃GaO₆ is 0<x<0.384. Powder X-ray diffraction shows that the compounds crystallize in Gd₃GaO₆ (Cmc2₁)-type structure. The solid solubilities of Y_3+xGa_5−xO₁₂ (x=0-0.77) and Tm_3+xGa_5−xO₁₂ (x=0-0.62) are 37.5-47.11 at% Y₂O₃, and 37.5-45.26 at% Tm₂O₃, respectively. PL spectra of Tm-doped Y₃GaO₆ show that there is a sharp blue emission at ∼456 nm from the ¹D₂→³F₄ transition at room temperatures with two lifetimes (∼5 and ∼15 μs) and a narrow saturation range of PL intensity for the Tm³⁺ content from x=0.005 to 0.03. The sharp emission and long lifetime of (Y_1−xTm_x)₃GaO₆ indicate that Y₃GaO₆ is a potential phosphor and laser crystal host material.     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

Solution precursor synthesis and magnetic properties of Eu1−xCaxTiO3

Europium titanate, EuTiO₃, is a paraelectric/antiferromagnetic cubic perovskite with T_N=5.5 K. It is predicted that compressive strain could induce simultaneous ferroelectricity and ferromagnetism in this material, leading to multiferroic behavior. As an alternative to epitaxial strain, we explored lattice contraction via chemical substitution of Eu²⁺ with the smaller Ca²⁺ cation as a mechanism to tune the magnetic properties of EuTiO₃. A modified sol-gel process was used to form homogeneously mixed precursors containing Eu³⁺, Ca²⁺, and Ti⁴⁺, and reductive annealing was used to transform these precursors into crystalline powders of Eu_1−xCa_xTiO₃ with x=0.00, 0.05, 0.10, 0.15, 0.25, 0.35, 0.50, 0.55, 0.60, 0.65, 0.80, and 1.00. Powder XRD data indicated that a continuous Eu_1−xCa_xTiO₃ solid solution was readily accessible, and the lattice constants agreed well with those predicted by Vegard's law. SEM imaging and EDS element mapping indicated a homogeneous distribution of Eu, Ca, and Ti throughout the polycrystalline sample, and the actual Eu:Ca ratio agreed well with the nominal stoichiometry. Measurements of magnetic susceptibility vs. temperature indicated antiferromagnetic ordering in samples with x≤0.60, with T_N decreasing from 5.4 K in EuTiO₃ to 2.6 K in Eu_0.40Ca_0.60TiO₃. No antiferromagnetic ordering above 1.8 K was detected in samples with x>0.60.     

Preparation, crystal structure, and photoluminescence of Ca2SnO4:Eu, Y

Eu³⁺-doped Ca₂SnO₄ (solid solutions of Ca_2−xEu_2xSn_1−xO₄, 0?x?0.3) and Eu³⁺ and Y³⁺-codoped Ca₂SnO₄ (Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄) were prepared by solid-state reaction at 1400 °C in air. Rietveld analysis of the X-ray powder diffraction patterns revealed that Eu³⁺ replaced Ca²⁺ and Sn⁴⁺ in Eu³⁺-doped Ca₂SnO₄, and that Eu³⁺ replaced Ca²⁺ and Y³⁺ replaced Sn⁴⁺ in Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄. Red luminescence at 616 nm due to the electric dipole transition ⁵D_o→⁷F₂ was observed in the photoluminescence (PL) spectra of Ca_2−xEu_2xSn_1−xO₄ and Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ at room temperature. The maximum PL intensity in the solid solutions of Ca_2−xEu_2xSn_1−xO₄ was obtained for x=0.1. The PL intensity of Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ was 1.26 times greater than that of Ca_2−xEu_2xSn_1−xO₄ with x=0.1.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Structural and magnetic properties of pyrochlore solid solutions (Y,Lu)2Ti2−x(Nb,Ta)xO7±y

The synthesis and characterization of the pyrochlore solid solutions, Y₂Ti_2−xNb_xO_7−y, Lu₂Ti_2−xNb_xO_7−y, Y₂Ti_2−xTa_xO_7−y and Lu₂TiTaO_7−y (−0.4<y<0.5), is described. Synthesis at 1600 °C, and 10⁻⁵ Torr yields oxygen deficiency in all systems. All compounds are found to be paramagnetic and semiconducting, with the size of the local moments being less, in some cases substantially less, than the expected value for the number of nominally unpaired electrons present. Thermogravimetric analysis (TGA) shows that all compounds can be fully oxidized while retaining the pyrochlore structure, yielding oxygen rich pyrochlores as white powders. Powder neutron diffraction of Y₂TiNbO₇-based samples was done. Refinement of the data for oxygen deficient Y₂TiNbO_6.76 indicates the presence of a distribution of oxygen over the 8b and 48f sites. Refinement of the data for oxygen rich Y₂TiNbO_7.5 shows these sites to be completely filled, with an additional half filling of the 8a site. The magnetic and TGA data strongly suggest a preference for a Ti³⁺/(Nb,Ta)⁵⁺ combination, as opposed to Ti⁴⁺/(Nb,Ta)⁴⁺, in this pyrochlore family. In addition, the evidence clearly points to Ti³⁺ as the source of the localized moments, with no evidence for localized Nb⁴⁺ moments.     

Percolative small-polarons conduction regime in Ce1−xGdxO2−x/2, probed by the EPR spectral intensity of Gd

EPR analysis is carried out with Ce_1−xGd_xO_(4−x)/2 (x=0.1; 0.2) nanopowders aiming at obtaining information about their oxidation and reduction properties. The EPR spectrum of these systems is composed of a single feature. The first derivative peak-to-peak spectral intensity decreases at higher temperatures, but this trend deviates from that of Curie's law with the x=0.1 sample, at difference with the x=0.2 sample. This unexpected result is related to oxygen deficiency, due to gas-solid equilibrium, present in the former sample only. As a consequence, some Ce³⁺ ions would form providing it with conduction electrons propagating as small polarons in a percolative way. This would result in a thinner skin depth at higher temperatures, able to explain the deviation of the spectral intensity from its expected value. Indeed, this deviation from Curie's law is reduced or disappears at all after thermal treatment of the x=0.1 sample with O₂.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Revised phase diagram of Li2MoO4-ZnMoO4 system, crystal structure and crystal growth of lithium zinc molybdate

Orthorhombic lithium zinc molybdate was first chosen and explored as a candidate for double beta decay experiments with ¹⁰⁰Mo. The phase equilibria in the system Li₂MoO₄-ZnMoO₄ were reinvestigated, the intermediate compound Li₂Zn₂(MoO₄)₃ of the α-Cu₃Fe₄(VO₄)₆ (lyonsite) type was found to be nonstoichiometric: Li_2−2xZn_2+x(MoO₄)₃ (0≤x≤0.28) at 600 °C. The eutectic point corresponds to 650 °C and 23 mol% ZnMoO₄, the peritectic point is at 885 °C and 67 mol% ZnMoO₄. Single crystals of the compound were prepared by spontaneous crystallization from the melts and fluxes. In the structures of four Li_2−2xZn_2+x(MoO₄)₃ crystals (x=0; 0.03; 0.21; 0.23), the cationic sites in the face-shared octahedral columns were found to be partially filled and responsible for the compound nonstoichiometry. It was first showed that with increasing the x value and the number of vacancies in M3 site, the average M3-O distance grows and the lithium content in this site decreases almost linearly. Using the low-thermal-gradient Czochralski technique, optically homogeneous large crystals of lithium zinc molybdate were grown and their optical, luminescent and scintillating properties were explored.     

The crystal structure, vibrational and luminescence properties of the nanocrystalline KEu(WO4)2 and KGd(WO4)2:Eu obtained by the Pechini method

The luminescent nanocrystalline KEu(WO₄)₂ and KGd_0.98Eu_0.02(WO₄)₂ have been prepared by the Pechini method. X-ray diffraction, infrared and Raman spectroscopy as well as optical spectroscopy were used to characterise the obtained materials. The crystal structure of KEu(WO₄)₂ was refined in I2/c space group indicating the isostructurality to KGd(WO₄)₂. The size of the crystalline grains depended on the annealing temperature, increasing with the increase of the temperature. The average size of crystallites of both crystals formed at 540 °C was about 50 nm. Vibrational spectra showed noticeable changes as a function of size due to, among others, phonon confinement effect. Luminescence studies did not reveal significant changes for the nanocrystallites with the lowest grain size in comparison with the bulk material. The differences observed in luminescence spectra in form of slight inhomogeneous broadening of the spectral lines and increase of the hypersensitive I_0-2/I_0-1 ratio point to very low symmetry of Eu³⁺ ions and change of the polarisation of the local vicinities of Eu³⁺. X-ray diffraction, vibrational and optical studies showed that the structure of the synthesised nanocrystalline KEu(WO₄)₂ and KGd(WO₄)₂:Eu is nearly the same as that found for the bulk material. The size-driven phase transitions were established for both compounds.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Phase segregation in the Gd1−xSrxFeO3−δ series

Gd_1−xSr_xFeO_3−δ ferrites have been studied by means of X-ray powder diffraction in the whole composition range. Single-phase solid solution is found for x<0.09 and for x>0.63. At intermediate Sr content, phase segregation takes place. Compounds with x?0.05 crystallize in the orthorhombic structure, space group Pbnm. Oxygen-deficient Gd_1−xSr_xFeO_3−δ with x?2/3 are cubic or nearly cubic. The oxygen vacancies stabilize the cubic phase for x=2/3 whereas highly oxidized samples show an orthorhombic distortion, which has not been observed earlier. Magnetic and electrical properties have been measured for the single-phase solid solutions. Gd_1−xSr_xFeO_3−δ compounds with x?2/3 order antiferromagnetically below ∼100 K. In the paramagnetic region, their susceptibility follows the Curie-Weiss law in all but SrFeO_2.96 compound. These ferrites show semiconducting behavior in the electrical transport likely related to atomic disorder. We find that the conductivity activation energy becomes larger by increasing either the Gd content or the oxygen vacancies.     

Novel powellite-based red-emitting phosphors: CaLa1−xNbMoO8:xEu for white light emitting diodes

We report the photoluminescence properties of a novel powellite-based red-emitting phosphor material: CaLa_1−xNbMoO₈:xEu³⁺ (0.01, 0.03, 0.05, 0.1) for the first time. The photoluminescence investigations indicated that CaLa_1−xNbMoO₈:xEu³⁺ emits strong red light at 615 nm originating from ⁵D₀→⁷F₂ (electric dipole transition) under excitation either into the ⁵L₀ state with 394 nm or the ⁵D₂ state with 464 nm, that correspond to the two popular emission lines from near-UV and blue LED chips, respectively. When compared with emission intensity from a CaMoO₄:Eu³⁺, the emission from CaLaNbMoO₈:Eu³⁺ showed greater intensity values under the same excitation wavelength (394 nm). The enhanced red emission is attributed to the enhanced f-f absorption of Eu³⁺. These materials could be promising red phosphors for use in generating white light in phosphor-converted white light emitting diodes (WLEDs).     

Eu luminescence in La5Si2BO13 with apatite related structure and magnetic studies in Ln5Si2BO13 (Ln=Gd, Dy)

Eu³⁺ photoluminescence is studied in La₅Si₂BO₁₃ with apatite related structure. La_5−xEu_xSi₂BO₁₃ [x=0.05, 0.1, 0.3, 0.5, 0.7, 1.0, 2.0] compositions are synthesized. The emission results shows that Eu³⁺ ions occupy two different cationic sites viz., La(1) and La(2). The increase in the intensity of ⁵D₀-⁷F₀ line with increasing Eu³⁺ content shows the preferential occupancy of Eu³⁺ in La(2) site due to the existence of short La(2)-O(4) (free oxide ion) bond. The observation of antiferromagnetic interactions in Gd and Dy analogues supports the structural features elucidates from photoluminescence studies.     

Eu luminescence—a structural probe in BiCa4(PO4)3O, an apatite related phosphate

Eu³⁺ luminescence is studied in apatite-related phosphate BiCa₄(PO₄)₃O. Compositions of the formula Bi_1−xEu_xCa₄(PO₄)₃O [x=0.05, 0.1, 0.3, 0.5, 0.8 and 1.0] are synthesized and they are isostructural with parent BiCa₄(PO₄)₃O. Room temperature photoluminescence shows the various transitions ⁵D₀→⁷F_J(=0,1,2) of Eu³⁺. The emission results of compositions with different Eu³⁺ content show the difference in site occupancy of Eu³⁺ in Bi_1−xEu_xCa₄(PO₄)₃O. The intense ⁵D₀-⁷F₀ line at 574 nm for higher Eu³⁺ content is attributed to the presence of strongly covalent Eu-O bond that is possible by substituting Bi³⁺ in the Ca(2) site. This shows the preferential occupancy of Bi³⁺ in Ca(2) site and this has been attributed to the 6s² lone pair electrons of Bi³⁺. This is further confirmed by comparing the emission results with La_0.95Eu_0.05Ca₄(PO₄)₃O.