Preferential occupancy of Eu³⁺ and energy transfer in Eu³⁺ doped Sr₂V₂O₇, Sr₉Gd(VO₄)₇ and Sr₂V₂O₇/Sr₉Gd(VO₄)₇ phosphors

Abstract

The vanadate-based phosphors Sr₂V₂O₇:Eu³⁺ (SV:Eu³⁺), Sr₉Gd(VO₄)₇:Eu³⁺ (SGV:Eu³⁺) and Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu³⁺ (SGV/SV:Eu³⁺) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu³⁺ based on the bond valence model. By comparing the change of bond energy when the Eu³⁺ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu³⁺ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu³⁺, SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu³⁺ can be obtained through changing the concentration of Eu³⁺ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O²⁻ → V⁵⁺ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f–f transitions of Eu³⁺. The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O²⁻ → V⁵⁺ CT of SV, and a sharp characteristic spectrum (⁵D₀–⁷F₂) at about 615 and 621 nm.

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1. Introduction

Recently, vanadate-based phosphors have drawn increasing attention due to the self-activated emitting properties of [VO₄]³⁻ group, the sensitization from [VO₄]³⁻ to rare earth ions as well as their long wavelength excitation and the excellent chemical stabilities.^1–8 The vanadate group, namely, [VO₄]³⁻, in which the central metal ion V is coordinated by four oxygen ligands in a tetragonal symmetry, exhibits broad and intense charge transfer (CT) absorption bands in the UV region and some of them can produce intense broadband CT emission spectra from 400 to more than 700 nm related to the local structure.^9–11 When excited by UV light, these vanadates or rare earth ions-doped materials have the capability to convert the ultraviolet emission into white light.^4,12,13

In general, the first essential factor that determines the luminescence quantum efficiency of vanadate-based phosphors originating from O²⁻ → V⁵⁺ CT transition is the distortion of the VO₄ tetrahedron. The excitation process of O²⁻ → V⁵⁺ CT is always allowed; thus, most of the vanadates show self-activated properties, while the intersystem crossing (¹T₁, ¹T₂ → ³T₁, ³T₂) and luminescence process (³T₁, ³T₂ → ¹A₁) are forbidden in the ideal T_d symmetry due to the spin selection rule.¹ For example, in the crystal YVO₄, O²⁻ → V⁵⁺ CT luminescence process is forbidden and thus, the luminescence of O–V CT cannot be observed at room temperature because in this crystal, V atom is coordinated with four equal oxygens and shows ideal T_d symmetry (all four Y–O bond lengths are 1.7 Å).¹⁴ However, in Eu³⁺-doped YVO₄, the O²⁻ → V⁵⁺ CT energy can effectively be transferred to Eu³⁺ and shows intense red photoluminescence corresponding to the electric dipole transition, ⁵D₀ → ⁷F₂, of Eu³⁺ ions.¹⁵ However, the structure of the VO₄ tetrahedron has, to some extent, a distorted T_d symmetry as compared to that of an ideal tetrahedron; thus, these forbidden processes are partially allowed due to the spin–orbit interaction.^1,10 The vanadates with this type of structure can show intense O²⁻ → V⁵⁺ CT emission. For example, AVO₃ (A: Rb and Cs) exhibits intense broadband emission from 400 nm to more than 700 nm under UV excitation.^5,9,16 M₂V₂O₇ (M = Ca, Sr, Ba) with distorted T_d symmetry around V atoms can emit strong O²⁻ → V⁵⁺ CT luminescence.¹⁷ Although only a few Eu³⁺ doped with this type of vanadates, such as Ba₃LiMgV₃O₁₂:Eu³⁺,¹⁸ showed white light, while most rare earth ions-doped with this type of vanadates only produced red, yellow or green or blue-green light; other examples include M₂V₂O₇ (M = Ca, Sr, Ba):Eu³⁺,¹⁹ Li₂Ca₂ScV₃O₁₂:Eu³⁺,⁶ and Ba₂Y_2/3V₂O₈:Eu³⁺.¹³

The vanadate phosphors exhibit two types of important advantages according to the symmetrical characteristic of VO₄: first, self-activated emission arising from O²⁻ → V⁵⁺ CT with distorted T_d symmetry and second, the efficient energy transfer from self-activated O²⁻ → V⁵⁺ CT to Eu³⁺ with T_d symmetry.¹ It is very interesting and important to investigate the preferential occupancy of Eu³⁺ and PL properties in the mixed phosphor. This is because the Sr₉Gd(VO₄)₇ and Sr₂V₂O₇ vanadates show two different types of important advantages according to the symmetrical characteristic of VO₄. Sr₂V₂O₇ shows self-activated emission arising from O²⁻ → V⁵⁺ CT with distorted T_d symmetry, but the energy transfer from O²⁻ → V⁵⁺ CT to the Eu ions is not effective. However, Eu³⁺ doped Sr₉Gd(VO₄)₇ shows efficient energy transfer from self-activated O²⁻ → V⁵⁺ CT to Eu³⁺, but Sr₉Gd(VO₄)₇ could not produce self-activated emission due to the symmetrical characteristic of VO₄ with T_d symmetry. Eu³⁺ doped Sr₂V₂O₇/Sr₂Gd(VO₄)₇ possesses the two advantages of self-activated emission and efficient energy transfer from the host lattice to Eu³⁺. In order to investigate the structure and the photoluminescence of vanadate phosphors, we synthesized Sr₂V₂O₇ (SV), Sr₉Gd(VO₄)₇:Eu³⁺ (SGV:Eu³⁺) and Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu³⁺ (SGV/SV:Eu³⁺); in particular, Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu³⁺ can possess both the above advantages of vanadates. The occupying sites of Eu³⁺, photoluminescence properties and the relationship between O²⁻ → V⁵⁺ CT energy and crystal structure are discussed. The photoluminescence and excitation mechanism as well as energy transfer phenomenon between the host lattice and Eu³⁺ are investigated.

2. Experimental section

2.1. Materials and synthesis

The phosphor with nominal composition Sr₉Gd(VO₄)₇:5% Eu³⁺ was synthesized using a high-temperature solid-state reaction method from a stoichiometric mixture of SrCO₃ (99.9%), V₂O₅ (99.9%), Gd₂O₃ (99.99%), and Eu₂O₃ (99.99%). The mixture was ground in alumina crucibles, homogeneously mixed; finally the mixtures were heated at 1023 K for 12 h and at 1223 K for 12 h and then cooled down to room temperature to obtain a white powder. Sr₂V₂O₇/Sr₉Gd(VO₄)₇:5% Eu³⁺ can be obtained when heated at 1123 K for 12 h.

Pure Sr₂V₂O₇:Eu³⁺ was prepared using traditional solid-state method from a stoichiometric mixture of SrCO₃ (99.9%), V₂O₅ (99.9%) and Eu₂O₃ (99.99%). After grinding in an alumina crucible and mixing homogeneously, the mixture was heated at 1323 K for 24 h and then cooled down to room temperature to obtain a white powder.

2.2. Characterizations

Powder X-ray diffraction (XRD) measurements were recorded on a D/MAX 2500 instrument (Rigaku) with a Rint 2000 wide angle goniometer and Cu Kα1 radiation (λ = 1.54056 Å) at 40 kV and 100 mA. The diffraction patterns were scanned over an angular (2θ) range of 20−80° at intervals of 0.02° with a counting time of 0.6 s per step. Photoluminescence (PL) studies were conducted on a fluorescence spectrophotometer (Photon Technology International) equipped with a 60 W Xe-arc lamp as the excitation light source. All the measurements were recorded at room temperature.

2.3. Theoretical method

Based on the chemical bond viewpoint, the dopants preferentially occupy the sites with smaller alterations of bond energy.²⁰ The variation of bond energy can be measured by the following expressionwhere N_M is the dopant content in the unit of mol. For the same host lattice, the N_M is the same, so when we discuss the occupancy of any anion, we only analyse the ΔE^Sr/V/Gd_M–O value do not consider the value. Ē_M–O and Ē_Sr/V/Gd–O are the mean bond energies of M–O bond and Sr–O or V–O or Gd–O, which can be expressed as

The total bond energy, ∑E_Sr/V/Gd–O, in kcal of the crystals can be regarded as a bond-energy sum of all constituent chemical bonds. For example, in Sr₂V₂O₇, there are four different sites,²¹ that is, Sr1, Sr2, Sr3 and Sr4. Taking Sr1 site as an example, there are eight types of constituent Sr1–O, that is, Sr1–O1, Sr1–O2, Sr1–O6 (there are two Sr1–O6 bonds with different distance), Sr1–O8, Sr1–O9, Sr1–O11 and Sr1–O14. Thus, the bond energy, ∑E_Sr1–O, can be calculated using the following formula:

The ∑E_M–O is the sum of bond energies of different dopants in the crystallographic frame. When the dopant ion is Eu³⁺, E_M–O can be estimated through the following equation²⁰

where V_{Sr²⁺/V⁵⁺/Gd³⁺} presents the valence state of Sr or V or Gd and V_Eu³⁺ is the dopant valence of Eu³⁺. This indicates that the valence state has influence on the crystal bond energy if the valence state of the dopant is not equal to that of the original ions. The coefficient J is equal to the standard atomization energy, which can be estimated using the following formula:²²where m is the number of cations in the formal molecule, z is the cation valence and E⁰_a is molar atomization energy (kcal mol⁻¹) of an oxide crystal M_mO_n at the standard state (normal pressure, 298 K). E⁰_a may be expressed aswhere ΔH^Θ_f is the standard heat of formation of M_mO_n (−592, −1550, −1816 and −1652 kJ mol⁻¹ for SrO, V₂O₅, Gd₂O₃ and Eu₂O₃, respectively), S is the heat of metal sublimation or, more generally, heat of atomization of M (164, 515, 397.5 and 177.4 kJ mol⁻¹ for Sr, V, Gd and Eu, respectively) and D is the heat of dissociation of O₂ molecule (493.804 kJ mol⁻¹). According to the equation above, we can determine that the J values of Sr²⁺–O, V⁵⁺–O, Gd³⁺–O and Eu³⁺–O are 119.80, 91.13, 133.45 and 109.40 kcal mol⁻¹, respectively.

In eqn (5), d₀ is an empirically determined parameter, which is measured by processing all available crystallographic data; it is constant for a given atom pair.²³ The d₀ of Sr²⁺–O²⁻, V⁵⁺–O²⁻ and Eu³⁺–O²⁻ are 2.118, 1.917, and 2.074 Å, respectively. Furthermore, the d₀ value of Gd³⁺–O²⁻ can be estimated by the formula below:

d₀ = r_c + A × r_a + P − D − F (8)

where r_c and r_a are contributions to d₀ from the cation and anion, respectively. The multiplier A is set to 0.8 for transition metal ions with d electrons or else it is set to 1.0. P, D and F are corrections required when cation contains non-bonding p, d and f electrons, respectively. The r_c, r_a and D values can be obtained in the ref. 23. P and F can be calculated using

For lanthanide ions, the calculated d₀ values using (8) are larger 0.079 than those obtained in the ref. 23. Hence, we corrected the formula (8) to be as below:

Hence, the d₀ of Gd³⁺–O²⁻ is 2.049 Å.

3. Results and discussion

3.1. Phase identification of Sr₂V₂O₇:Eu³⁺, Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu³⁺ and Sr₉Gd(VO₄)₇:Eu³⁺

Fig. 1 shows the representative X-ray diffraction patterns for Sr₂V₂O₇:Eu, Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu and Sr₉Gd(VO₄)₇:Eu samples. Fig. 1(a) shows the XRD patterns for Sr₂V₂O₇:Eu. All the diffraction peaks of Sr₂V₂O₇:Eu can be indexed to the reported JCPDS 48-0148 card (60° > 2θ > 20°). It can be demonstrated that the obtained Sr₂V₂O₇:Eu sample is pure. Fig. 1(e) shows that the XRD patterns of Sr₉Gd(VO₄)₇:Eu are similar to those of Sr₃(VO₄)₂ (Fig. 1(d), JCPDS file number 29-1318);²⁴ no peaks from other phases such as Gd₂O₃, SrO, or V₂O₅ appear. The structure of Sr₉Gd(VO₄)₇:Eu is similar to that of Sr₉Lu(VO₄)₇, which is found to be isotypic with Ca₃(VO₄)₂ (ref. 25) or Sr₃(VO₄)₂.²⁴ The above results indicate the formation of a pure Sr₉Gd(VO₄)₇:Eu. The XRD patterns of Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu component is clearly shown in Fig. 1(c). The detailed crystal plane diffraction peaks ascribed to Sr₂V₂O₇ and Sr₉Gd(VO₄)₇:Eu are labeled and distinguished using blue and red colors, respectively. Except the diffraction peaks arising from Sr₂V₂O₇ and Sr₉Gd(VO₄)₇ crystals, no other peaks can be found, which indicates that we have obtained the component of “pure” Sr₉Gd(VO₄)₇/Sr₂V₂O₇. From Fig. 1(c) and (e), we can judge that the mechanism of producing Sr₂V₂O₇, Sr₉Gd(VO₄)₇/Sr₂V₂O₇, and Sr₉Gd(VO₄)₇ is as below:

2SrCO₃ + V₂O₅ ⇔ Sr₂V₂O₇ + 2CO₂↑ (11)

18SrCO₃ + Gd₂O₃ + 7V₂O₅ ⇔ 3.5xSr₂V₂O₇ + 0.5xGd₂O₃ + 2xSrO + (2 − x)Sr₉Gd(VO₄)₇ + 18CO₂ ⇔ 2Sr₉Gd(VO₄)₇ + 18CO₂↑ (12)

Fig. 1 (a) XRD patterns of Sr₂V₂O₇, and (b) the JCPDS card 48-0145 of Sr₂V₂O₇ (blue color), (c) XRD patterns of Sr₉Gd(VO₄)₇/Sr₂V₂O₇:5% Eu³⁺ (red color); (d) the JCPDS card 29-1318 of Sr₃(VO₄)₂ (pure Sr₉Gd(VO₄)₇:5% Eu³⁺ phase is isotypic with Sr₃(VO₄)₂) (black color) as well as (e) XRD patterns of Sr₉Gd(VO₄)₇.

From mechanism (12), it can be shown that there are some Gd₂O₃ and SrO except for the main products of Sr₉Gd(VO₄)₇/Sr₂V₂O₇; however, it cannot be observed in Fig. 1(c) because the relative quantities of Gd₂O₃ and SrO are so small that XRD patterns cannot be indexed. In our investigation, we ignored the existence of the samples Gd₂O₃ and SrO because they do not influence the photoluminescence of Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu.

3.2. Photoluminescence properties

3.2.1. Site occupancy and PL properties of Sr₂V₂O₇:Eu. The PL properties of pure Sr₂V₂O₇ were reinvestigated for the occupancy of Eu³⁺ in SV sample. Fig. 2 shows the PL excitation and emission spectra of pure Sr₂V₂O₇ at room temperature. Upon monitoring the wavelength at 526 nm, Sr₂V₂O₇ shows a broad excitation band with peak at 355 nm spanning from 200 to 400 nm, which can be matched well with the excitation band of UV-LED chips. The excitation spectrum originates from O²⁻ → V⁵⁺ CT transition, which can be deconvoluted into two peaks at 330 (3.76 eV) and 356 nm (3.48 eV) corresponding to the ¹A₁ → ¹T₂ and ¹A₁ → ¹T₁ transition of VO₄³⁻ group, respectively, as shown in Fig. 2(a) and (c). The values are listed in Table 2. In Fig. 2(a), red dotted lines indicate excitation spectra fitted with two Gaussian curves (green dotted lines) corresponding to two excitation bands: Ex₁ and Ex₂. The energy difference between the peaks Ex₁ and Ex₂ is 0.28 eV, which is ascribed to the energy difference between ¹T₂ and ¹T₁ as reported in many ref. 6, 18 and 26. The corresponding excitation and emission mechanisms are shown in Fig. 2(c). Under the excitation of 355 nm UV-light irradiation, Sr₂V₂O₇ shows a strong green emission (Fig. 2(b)) due to the typical V–O charge transfer (CT) emission of VO₄³⁻ ions, consisting of a strong broad band (400–650 nm) with a maximum at 526 nm. Fig. 2(d) shows a graphic of the Commission Internationale de L'Eclairage (CIE) 1931 chromaticity coordinate of pure Sr₂V₂O₇ phosphors excited at 325 nm. The (x, y) chromaticity coordinate of Sr₂V₂O₇ is (0.308, 0.427) in the green region. The emission spectrum can be further fitted by two Gaussian components with two peaks at Em₁ (490 nm, i.e. 2.53 eV) and Em₂ (550 nm, i.e. 2.25 eV), which are labeled using green color of Fig. 2(b). The energy gap between the two peaks is 0.28 eV, which is ascribed to the energy difference between ³T₁ and ³T₂ as reported in many ref. 6, 18 and 26.

Fig. 2 PL excitation spectra (a) and emission spectra (b) of Sr₂V₂O₇. Red dotted lines indicate excitation spectra or emission spectra fitted with two Gaussian curves (green dotted lines) corresponding to excitation bands Ex₁, Ex₂ and emission bands Em₁, Em₂, respectively. (c) Schematic model of absorption and emission processes of [VO₄]³⁻ tetrahedron in Sr₂V₂O₇. Ex₁ and Ex₂ represent excitation processes ¹A₁ → ¹T₁ and ¹A₁ → ¹T₂, respectively. Em₁ and Em₂ represent emission processes ³T₂ → ¹A₁ and ³T₁ → ¹A₁, respectively. (d) CIE chromaticity diagram shows colour coordinates of the luminescence of Sr₂V₂O₇.

Fig. 3 shows the PL properties of Eu³⁺ in SV, which are different in the ref. 2 and 19. The emission spectrum (excited at 355 nm) is shown in Fig. 3(b). The emission spectrum contains a broadband emission in the 400–590 nm wavelength region with a maximum at about 530 nm and a sharp peak at 614 nm. The broad peak has a 30 nm redshift compared with the reported value of 500 nm, which is very large, but the breadth and peak of the broad band emission is similar to that of SV. This corresponds to the charge transfer from the O²⁻ to V⁵⁺ localized within the tetrahedrally coordinated VO₄³⁻ group. The sharp peak at 614 nm originates from the ⁵D₀ → ⁷F₂ transition in Eu³⁺ dopant.

Fig. 3 PL excitation spectra (a) and emission spectra (b) of Sr₂V₂O₇:5% Eu (SV:Eu).

In Fig. 3(a), the excitation spectrum monitored at 614 nm has a broadband in the 250–400 nm wavelength region with the peak at 355 nm, which is close to the excitation spectrum monitored at 530 nm. This indicates that the broad excitation band arises due to the charge transfer from oxygen 2p orbital of O²⁻ to an empty d orbital of V⁵⁺, and not to the orbital of Eu³⁺.

Coordination atoms and their bond lengths of four sites of Sr and four sites of V are listed in columns 2 and 3 of Table 1, respectively. All calculated , and ΔE^Sr/V_M–O values of various possible substituted ions including Sr²⁺ and V⁵⁺ on both Sr²⁺ and V⁵⁺ sites are listed in Table 1. These results indicate that the value of bond energy variation is in the order Sr4 < Sr2 < Sr3 < Sr1 < V1 < V2 < V3 < V4. According to the bond energy method, Eu³⁺ ions should preferentially occupy the sites with smaller alterations of bond energy values, ΔE^Sr/V_Eu–O. Within this energy formation argument, the priority site for the Eu³⁺ incorporation of the luminescent centers is Sr4.

Table 1 Single and sum bond energies of Sr–O or V–O bonds in Sr₂V₂O₇ (and ), single and sum bond energies of Eu–O bonds in Sr₂V₂O₇:Eu ( and ) and variation of bond energy (ΔE^Sr/V_M–O) when the Eu³⁺ ion is located at different Sr²⁺ and V⁵⁺ sites. All units of bond energy are kcal mol⁻¹

Central atom	Coordination atom	d (Å)					ΔE^Sr/VEu–O
Sr1	O14	2.4848	44.45	254.1	24.03	137.3	15.84
	O1	2.5205	40.37		21.82
	O6	2.5283	39.52		21.36
	O6	2.5613	36.15		19.54
	O11	2.5807	34.30		18.54
	O2	2.7065	24.42		13.20
	O9	2.7374	22.46		12.14
	O8	2.9578	12.38		6.692
Sr2	O10	2.5407	38.22	229.8	20.66	124.2	11.7
	O4	2.5415	38.14		20.62
	O5	2.6112	31.59		17.08
	O2	2.6231	30.59		16.54
	O12	2.6275	30.23		16.34
	O9	2.6771	26.44		14.29
	O13	2.8247	17.74		9.589
	O11	2.9948	11.20		6.055
	O7	3.2494	5.629		3.043
Sr3	O4	2.5307	39.27	234.9	21.23	127.0	12.0
	O2	2.6431	28.98		15.67
	O10	2.6439	28.92		15.63
	O1	2.6481	28.59		15.45
	O7	2.6617	27.56		14.90
	O13	2.6949	25.19		13.62
	O10	2.7036	24.61		13.30
	O5	2.8155	18.19		9.831
	O8	2.9238	13.57		7.336
Sr4	O5	2.5484	37.43	238.5	20.23	128.9	10.96
	O9	2.5537	36.90		19.95
	O6	2.5653	35.76		19.33
	O14	2.6598	27.70		14.97
	O3	2.662	27.54		14.89
	O8	2.7251	23.22		12.55
	O13	2.7647	20.86		11.28
	O11	2.8376	17.13		9.261
	O7	3.1886	6.635		3.586
	O1	3.2685	5.346		2.890
V1	O14	1.6626	133.2	465.4	554.3	1937	368
	O13	1.6828	126.1		524.9
	O9	1.7047	118.9		494.7
	O3	1.8192	87.23		363.0
V2	O4	1.6677	131.4	467.1	546.7	1944	369
	O11	1.6693	130.8	—	544.4
	O2	1.7114	116.7	—	485.8
	O12	1.8151	88.20	—	367.1
V3	O7	1.6572	135.1	468.1	562.5	1948	370
	O10	1.6868	124.8		519.2
	O5	1.6959	121.7		506.6
	O12	1.8222	86.52		360.1
V4	O8	1.6597	134.2	474.2	558.7	1974	375
	O1	1.6758	128.5		534.9
	O6	1.695	122.0		507.8
	O3	1.81	89.42		372.2

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3.2.2. Site occupancy and PL properties of SGV and SGV:Eu. PL emission of pure SGV cannot be observed at room temperature, but Eu³⁺-doped SGV can emit strong red light. Fig. 4(a) and (b) show the typical excitation and emission spectra of SGV:Eu. The excitation spectrum of SGV:Eu obtained by monitoring the ⁵D₀ → ⁷F₂ at 617 nm is shown in Fig. 4(a). It consists of a broad excitation band with peak at 327 nm spanning from 200 to 350 nm, which can be matched well with the UV-LED chips, along with some dominated sharp lines in the wavelength region of 350 to 500 nm, which arise due to the characteristic f–f transition of Eu³⁺ at about 398 and 469 nm. The excitation is the typical O²⁻ → V⁵⁺ CT transition spectrum of [VO₄]³⁻, which can be separated into two peaks at 300 and 327 nm corresponding to the ¹A₁ → ¹T₂ and ¹A₁ → ¹T₁ transition of [VO₄]³⁻ group, respectively. This also indicates that the energy transfer can be taken place efficiently from VO₄³⁻ to Eu³⁺ ions in SGV:Eu. The mechanism of energy transfer and luminescence of Eu³⁺ are shown in Fig. 4(c), which is similar to the energy transfer from [VO₄]³⁻ to Eu³⁺ ions in YVO₄:Eu.^15,27

Fig. 4 PL excitation spectra (a) and emission spectra (b) of SGV:5% Eu³⁺. Red dotted lines indicate excitation spectra fitted with two Gaussian curves (green dotted lines) corresponding to excitation bands Ex₁, Ex₂. (c) Schematic model of excitation, energy transfer and emission processes of VO₄ tetrahedron in SGV:Eu³⁺. Ex₁ and Ex₂ represent excitation processes ¹A₁–¹T₁ and ¹A₁–¹T₂, respectively. (d) CIE chromaticity diagram showing color coordinates of the luminescence of SGV:Eu³⁺.

Comparing the O–V CT energy in pure sample SV (355 nm), the CT energy of [VO₄]³⁻ in pure SGV (327 nm) is much higher. This is because the crystal field or environmental factor (h_e) around V atoms in the two samples is different.²⁸

Fig. 4(b) shows the emission spectra of SGV:Eu excited at 327 nm. The SGV:Eu phosphor shows bright red color. The (x, y) chromaticity coordinate of SGV is (0.547, 0.333) in the red region. Fig. 4(d) shows a graphic of the CIE 1931 chromaticity coordinate of pure SGV phosphors excited at 327 nm. As shown in Fig. 4(b), the dominant red emission bands of 615 and 620 nm are attributed to the electric dipole transition ⁵D₀ → ⁷F₂, indicating that Eu³⁺ ions are located at the sites of non-inversion symmetry. The emission peaks at about 573, 595, 650, and 700–705 nm are derived from the transition of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄, respectively, which are much weaker than the intensity of ⁵D₀ → ⁷F₂.

Consequently, ⁵D₀ → ⁷F₂ red emission (615 and 620 nm) presents the most prominent intensity in the emission spectrum. In SGV:Eu, the structure of SGV is isotypical with that of Sr₉Lu(VO₄)₇, so Sr₉Gd(VO₄)₇ can be analyzed according to the structure in the reference. In Sr₉Gd(VO₄)₇, there are three different Sr or V sites and one Gd site; their coordination atoms are summarized in Table 3.

All calculated , and ΔE^Sr/V_Eu–O values of various possible substituted ions including Sr²⁺, Gd³⁺and V⁵⁺ on both Sr²⁺, Gd³⁺and V⁵⁺ sites are listed in Table 3. The result indicates that the value of bond energy variation is in the order Gd1 < Sr3 < Sr1 < Sr2 < V2 < V3 < V1. According to the bond energy method, Eu³⁺ ions should preferentially occupy the sites with smaller alterations of bond energy values ΔE^Sr/Gd/V_Eu–O. Within this energy formation argument, the priority site for the Eu³⁺ incorporation of the luminescent centers is Gd.

3.2.3. Site occupancy and PL properties of SGV/SV and SGV/SV:Eu³⁺. On the basis of the above PLE spectra of SGV:Eu³⁺ and SV, it can be observed that both of them have a broad absorption range in the UV region. Eu³⁺-doped SGV exhibits efficient energy transfer from [VO₄]³⁻ to Eu³⁺ and emits strong red light and SV can emit strong green light. According to the calculated ΔE^Sr/Gd/V_Eu–O, in the “pure” SGV/SV system, the result indicates that the smallest value of bond energy variation would be at the Gd site. Consequently, we designed Eu³⁺-doped “pure” SGV/SV system expecting that Eu³⁺ can enter the site of SGV and emit strong red light, while SV can retain its strong green light and thus, the composition of red and green will produce white light or exhibit excellent luminescence properties.

The excitation and emission spectra of “pure” SGV/SV are shown in Fig. 5. The peaks, full wave at half maximum (FWHM), Ex₁ and Ex₂ as well as Em₁ and Em₂ of excitation and emission spectra for “pure” SGV/SV are listed in Table 2.

Fig. 5 PL excitation spectra (a) and emission spectra (b) of Sr₉Gd(VO₄)₇/Sr₂V₂O₇. Red dotted lines indicate excitation spectra or emission spectra fitted with two Gaussian curves (green dotted lines) corresponding to excitation bands Ex₁, Ex₂ and emission bands Em₁, Em₂, respectively.

Table 2 Comparing peaks and full wave at half maximum (FWHM) of the excitation and emission spectra SV, SGV and SGV:Eu with those of the mixed-compound SGV/SV

Compound	Excitation spectrum (nm)			Emission spectrum (nm)
	Peak position		FWHM	Peak position		FWHM
SV	355	330, 356	50	526	550, 490	140
SGV	327	300, 327	50	No	No	No
SGV/SV	355	323, 356	52	525	562, 502	137

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Table 3 Single and sum bond energies of Sr–O or V–O bonds in Sr₉Gd(VO₄)₇ ( and ), single and sum bond energies of Eu–O bonds in Sr₉Gd(VO₄)₇:Eu ( and ) and variation of bond energy (|ΔE|) when the Eu³⁺ ion is located at different Sr²⁺ and V⁵⁺ sites. All units of bond energy are kcal mol⁻¹

Central atom	Coordination atom	d (Å)					|ΔE|
Sr1	O8	2.4607	47.45	248.0	25.65	134.0	15.49
	O10	2.496	43.13		23.31
	O7	2.5922	33.25		17.98
	O2	2.6146	31.30		16.92
	O5	2.6353	29.60		16.00
	O6	2.6453	28.81		15.57
	O6	2.7103	24.17		13.06
	O4	3.0266	10.28		5.556
Sr2	O5	2.5086	41.69	240.2	22.53	129.8	15.76
	O2	2.5139	41.09		22.21
	O4	2.5867	33.75		18.24
	O3	2.5871	33.72		18.22
	O9	2.6318	29.88		16.15
	O9	2.6856	25.84		13.97
	O8	2.8354	17.23		9.316
	O7	2.8411	16.97		9.173
Sr3	O5	2.5899	33.46	216.5	18.09	117.0	11.05
	O4	2.5902	33.44		18.07
	O7	2.6004	32.53		17.58
	O1	2.7138	23.94		12.94
	O10	2.7203	23.52		12.72
	O10	2.7381	22.42		12.12
	O8	2.7733	20.38		11.02
	O3	2.8996	14.49		7.832
	O2	2.9599	12.31		6.654
Lu1/Gd1	O6	2.1762	72.32	401.3	83.00	460.5	9.89
	O6	2.1762	72.32		83.00
	O6	2.1762	72.32		83.00
	O9	2.2365	61.44		70.51
	O9	2.2365	61.44		70.51
	O9	2.2365	61.44		70.51
V1	O1	1.6822	126.3	489.1	525.7	2036	386.8
	O2	1.6983	120.9		503.3
	O2	1.6983	120.9		503.3
	O2	1.6983	120.9		503.3
V2	O3	1.6505	137.6	477.5	572.7	1988	377.5
	O5	1.7026	119.5		497.5
	O4	1.7078	117.9		490.6
	O6	1.7594	102.5		426.7
V3	O7	1.6726	129.6	483.4	539.5	2012	382.3
	O10	1.687	124.7		518.9
	O8	1.7157	115.4		480.2
	O9	1.721	113.7		473.4

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In contrast with the values of pure SV, all the parameters are almost the same, which demonstrate that the emission with the peak at 525 nm comes from the SV in SGV/SV.

The typical excitation and emission of SGV/SV:5% Eu³⁺, represented by SGV/SV:5% Eu³⁺, monitored with different wavelengths and excited at different wavelengths are shown in Fig. 6(a) and (b). Similar excitation spectra with two strong broad bands (at about 320 and 350 nm) can be observed in SGV/SV:Ln³⁺ (Ln³⁺ = Sm³⁺, Dy³⁺, or Tm³⁺) as shown in Fig. S1.† It therefore can be demonstrated that the two broad excitation spectra of SGV/SV:Eu³⁺ arise from the O → V charge transfer of the host lattice, i.e., SGV/SV and not from the CT transition of O²⁻ → Eu³⁺. Under 315 nm UV light irradiation, SGV/SV:5% Eu³⁺ shows warm white light emission, consisting of a strong broad band (400–650 nm) with a maximum at 530 nm and some sharp f–f transitions of Eu³⁺ ions with the peaks at 595, 615 and 621, 700 and 705 nm attributed to, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄, respectively. The f–f transition peak position of Eu³⁺ in SGV/SV:5% Eu³⁺ are similar to those in SGV:5% Eu³⁺, which demonstrates that the Eu³⁺ ions enter the crystal of SGV. However, the relative strengths at 595, 621 and 700 nm of SGV/SV:5% Eu³⁺ are changed, which can be expressed using I(⁵D₀ → ⁷F₁) : I(⁵D₀ → ⁷F₂) : I(⁵D₀ → ⁷F₄), that is, 2.1 : 6.57 : 1. Moreover, comparing with the f–f transition of Eu³⁺ in pure SGV (Fig. 4(b)), whose relative value is 1.74 : 4.48 : 1, the red light was demonstrated to be enhanced in SGV/SV:5% Eu³⁺ compared with the pure SGV:5% Eu³⁺.

Fig. 6 (a), (b) and (c) are excitation spectra of SGV/SV:Eu³⁺ and (d), (e) and (f) are emission spectra of SGV/SV:Eu³⁺ under different excitation spectra.

The excitation spectrum (Fig. 6(b), blue line) recorded by monitoring the emission of 621 nm (the strongest emission line in the PL spectrum shown in Fig. 6(e)) contains two visible broad absorption bands at 315 and 350 nm as well as some dominated sharp lines in the wavelength region of 350 to 500 nm due to the characteristic f–f transition of Eu³⁺ at about 398 and 469 nm. Under 350 nm UV radiation excitation, the SGV/SV:Eu exhibits green emission, and the obtained emission spectrum consists of a broad band with the peak at 530 nm due to the CT transition of O²⁻ → V⁵⁺ of SV and very weak f–f transition lines at 614 nm within the Eu³⁺ electron configuration. Thus, SV in SGV/SV can retain its excitation energy for strong green emission and its energy transfer efficiency to Eu³⁺ is lower. Monitored at 530 nm, the excitation spectrum of SGV/SV:Eu³⁺ sample displays a broad absorption band with the peak at 350 nm, as shown in Fig. 6(c), which is similar to the peak of pure SV. This asserts that the broad excitation band in SGV/SV:Eu³⁺ originates from the CT transition of O²⁻ → V⁵⁺ of SV. Another broad band at 315 nm in SGV/SV:Eu³⁺ must arise from the CT transition of O²⁻ → V⁵⁺, which has larger blue-shift compared with the O²⁻ → V⁵⁺ CT transition of pure SGV at 327 nm (Fig. 4(a)). This is due to the change of crystal environment around V atoms and the existence of oxygen deficiency.²⁹ The SGV:Eu³⁺ was obtained at 950 °C, and the SGV/SV:Eu was obtained at 750 °C. With increasing calcination temperature, the crystallization of SGV increases, which changes the lattice constant and the oxygen deficiency around V atoms. The change of the lattice constant is small, which generally makes a 1–5 nm shift in CT band, while 5–15 nm blue-shift primarily arises due to of oxygen deficiency.²⁹

3.2.4. The dependence of photoluminescence of SGV/SV:Eu³⁺ on the concentration of Eu³⁺. Changing the concentration of activator doped in host lattice is a feasible route to realize color-tunable emission; a white emission can be obtained through mixing the green and red light sources at a suitable ratio.^30–32 In our case, the effects of concentration of Eu³⁺ on the PL excitation and emission spectra of SGV/SV:Eu³⁺ were investigated. Fig. 7(a) and (b) show the variation of PL spectra and emission intensity of Eu³⁺ in SGV/SV:Eu³⁺ samples with the increase of Eu³⁺-doping concentrations from 0 to 20 mol% (λ_ex = 320 nm), respectively. The changing curve of relative intensity of f–f transition emission of Eu³⁺ at 621 nm (labelled using red circles) and the V–O CT green emission at 530 nm (labelled using green circles) followed the change in concentration of Eu³⁺ as shown in Fig. 7(c). Although the concentration of Eu³⁺ is changed, the emission intensity of V–O CT changed slightly, which indicates that the intensity of V–O CT emission originating from SV in SGV/SV is independent of the concentration of Eu³⁺. This further confirmed that the Eu ions enter the sites of SGV in SGV/SV, while no (or a little) Eu ions can be doped into the sites of SV in SGV/SV. This is because the radius and properties of Eu³⁺ ions are similar to those of Gd³⁺ in SGV; thus, they will occupy the sites of Gd³⁺ ions on priority. The experimental analysis is consistent with the theoretical result. The excitation spectrum monitoring at Eu^{3+ 5}D₀ → ⁷F₂ at 621 nm clearly shows two broad bands: 200–345 nm (centered at 315 nm) and 345–400 nm (centred at 350 nm), which occur due to the O²⁻ → V⁵⁺ CT transition of SGV and SV in SGV/SV, respectively. Furthermore, the excitation intensity at 320 nm is stronger than that at 350 nm, which demonstrates that the O²⁻ → V⁵⁺ CT transition energy can be efficiently transferred to the Eu³⁺ ion. The energy transfer and luminescence mechanism of SGV/SV:Eu is shown in Fig. 7(e). Moreover, the emission intensity of Eu³⁺ first increases with increasing Eu³⁺ from 0 to 15 mol%, and then decreases at 20 mol%. Thus, the emitting color of SGV/SV:Eu³⁺ samples can be tuned through changing the concentration of Eu³⁺. The results can also be confirmed by their CIE chromaticity coordinates shown in Fig. 7(d). The CIE chromaticity coordinates (x, y) for SGV/SV:xEu³⁺ phosphors excited at 320 nm are listed in Table 4.

Fig. 7 (a) and (b) are PLE (λ_em = 621) and PL of SGV/SV:Eu³⁺, respectively. (c) The PL intensity of the O²⁻ → V⁵⁺ charge transfer emission and Eu³⁺-emission. (d) CIE chromaticity diagram showing color coordinates of the luminescence of SGV/SV:Eu³⁺. (e) Schematic model of excitation, energy transfer and emission processes of SGV/SV:Eu³⁺.

Table 4 A comparison of the CIE chromaticity coordinates (x, y) for SGV/SV:xEu³⁺ phosphors excited at 320 nm

Sample no	Sample composition (x)	CIE coordinates (x, y)
A	0	(0.303, 0.436)
B	0.005	(0.349, 0.401)
C	0.01	(0.370, 0.395)
D	0.05	(0.376, 0.391)
E	0.06	(0.406, 0.384)
F	0.1	(0.418, 0.379)
G	0.15	(0.431, 0.382)

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Except that the emission of “pure” SGV/SV is at green region, all other SGV/SV:xEu³⁺ phosphors (x = 0.005, 0.01, 0.05, 0.1, and 0.15) exhibit warm-white-light emissions. Therefore, we can realize white light in a single component. This component consists of two types of host lattices and one type of rare earth ion activator (Eu³⁺), which can achieve the perfect union between the red f–f transition emission of Eu³⁺ and the O²⁻ → V⁵⁺ CT transition emission from the two different vanadate host lattices with different [VO₄]³⁻ symmetries under the UV light excitation. This is a very promising novel method with a wide range of adaptability to obtain white or other color lights.

4. Conclusions

1. The vanadate-based phosphors Sr₂V₂O₇:Eu³⁺ (SV:Eu³⁺), and Sr₉Gd(VO₄)₇:Eu³⁺ (SGV:Eu³⁺) and their products Sr₉Gd(VO₄)₇/Sr₂V₂O₇:xEu³⁺ (SGV/SV:xEu³⁺) were prepared by solid-state reaction at low temperature.

2. The bond-energy method is used to investigate the site occupancy preference of Eu³⁺ based on the bond valence model. By comparing the change in bond energy when the Eu³⁺ ions are incorporated into different Sr or V or Gd sites, we observed that Eu³⁺ in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites Sr4, Gd and Gd sites, respectively.

3. PL properties of Sr₂V₂O₇:Eu³⁺ (SV:Eu³⁺) and Sr₉Gd(VO₄)₇:Eu³⁺ (SGV:Eu³⁺) and their products Sr₉Gd(VO₄)₇/Sr₂V₂O₇:xEu³⁺ (SGV/SV:xEu³⁺) were investigated, which shows their excitation and emission characteristics. Their excitation wavelengths ranging from 220 to 400 nm fit well with the characteristic emission of UV light-emitting diode (LED) chips. Two broad charge transfer bands arising from O²⁻ → V⁵⁺ of [VO₄]³⁻ with the peaks at about 325 and 350 nm coexist in SGV/SV:Eu³⁺, which indicates that the phosphors can be efficiently excited in the UV region.

4. Eu³⁺ ions primarily enter the sites of Gd³⁺ of SGV. The photoluminescence properties of SGV/SV:Eu³⁺ component shows two prominent properties: first is that V–O CT energy of SGV can be efficiently transferred to Eu³⁺ and strong red photoluminescence can occur coming from the ⁵D₀ → ⁷F₂ transition of Eu³⁺; second is that the intensity of V–O CT transition emission originating from SV is not influenced by the concentration of Eu³⁺ and retains green light emission.

5. With the increase in concentration of Eu³⁺ from 0.5 to 15 mol%, the intensity of red f–f transition of Eu³⁺ at 621 nm increases, and concentration quenching was observed 20 mol%. All the SGV/SV:xEu³⁺ (x = 0.005, 0.01, 0.05, 0.1, and 0.15) phosphors exhibit warm-white-light emission.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2013012655). “Preferential Occupancy of Eu³⁺ and energy transfer in Eu³⁺ doped Sr₂V₂O₇, Sr₉Gd(VO₄)₇ and Sr₂V₂O₇/Sr₉Gd(VO₄)₇ Phosphors” was supplied by the Display and Lighting Phosphor Bank at Pukyong National University. This work is also supported by the National Natural Science Foundations of China (Grant No. 21301053).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra08089a

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