Luminescence and energy transfer of co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺

Abstract

In this work the photoluminescence (PL) of co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ as well as the energy transfer (ET) from Ce³⁺ to Mn²⁺ was examined. To this end, powder samples with different Mn²⁺ concentrations were synthesised via high temperature solid state reaction. Phase purity of prepared samples was investigated by X-ray powder diffraction. To elucidate the PL properties of Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺, emission and excitation spectra along with diffuse reflectance spectra were recorded. Additionally, fluorescence lifetime measurements were performed. To study the temperature behaviour of the emission of Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺, PL and lifetime measurements were conducted from 100 to 500 K. Furthermore, external quantum efficiencies were determined and chromaticity coordinates were calculated. It was found that co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ possesses two emission bands located in the violet/blue and deep red range of the electromagnetic spectrum. Moreover, it turned out that ET from Ce³⁺ to Mn²⁺ in Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ takes place via an exchange interaction mechanism. Temperature dependent PLD measurements reveal that thermal quenching of emission is mainly caused by the Mn²⁺ ions. Finally, quantum efficiency of Ce³⁺ doped Sr₅MgLa₂(BO₃)₆:Ce³⁺ was found to be close to unity.

---

1. Introduction

In the last two decades light emitting diodes (LEDs) have more and more replaced conventional light sources like incandescent or fluorescent lamps. This is due to the benefits LEDs provide to lighting installations over traditional light sources. Commonly, LEDs provide longer lifetime, higher wall plug efficiency, as well as a higher colour rendering.¹ Nowadays most of the white emitting LEDs consist of a blue emitting (In,Ga)N-chip combined with a green-yellow emitting phosphor, e.g. Y₃Al₅O₁₂:Ce³⁺.² These light sources provide high luminous efficiency due to intense emission in the green spectral range. On the other site, due to the lack of red emission these systems suffer from a high colour temperature and low colour rendering index (CRI) which is unintended for domestic lighting.³ On that account an alternative approach to generate warm white light with appropriate CRI is the combination of a blue, green, and red phosphor excited by an ultraviolet (UV) emitting LED. In this way LEDs with excellent colour temperature as well as convenient CRI can be designed.⁴ Unfortunately, these packages incur a loss in blue emission due to reabsorption by the green and red phosphors. This is why many groups try to develop single phase white emitting phosphors which convert UV radiation into white light.⁵ One strategy to realize such a phosphor is to use the ion couple Ce³⁺ and Mn²⁺.⁶ In many host materials Ce³⁺ and Mn²⁺ possess broad emission bands in the blue and red spectral range, respectively.⁷ Due to additive colour mixing these emission bands can complement each other to white light. Further, the broad excitation band of the spin and parity allowed [Xe]4f¹–[Xe]5d¹ transition in Ce³⁺ is suitable for pumping with UV LEDs. Since all transitions in Mn²⁺ are fully forbidden, the blue emission of Ce³⁺ can also be used for sensitizing the [Ar]3d⁵–[Ar]3d⁵ excitation transition via energy transfer (ET).

To investigate the photoluminescence (PL) properties as well as the ET from Ce³⁺ to Mn²⁺ in co-activated Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ a series of powder samples with various Mn²⁺ contents was prepared. Sr₅MgLa₂(BO₃)₆ belongs to a class of ortho-borates, firstly described by Schaffers et al. in 1994.⁸ Further, Sankar and Rao reported on the PL of different compositions of said material with numerous activators.^9,10 Sr₅MgLa₂(BO₃)₆ crystalizes in a high-symmetry trigonal structure with space group R3̄. Sr₅MgLa₂(BO₃)₆ comprises a 9-fold coordinated site as well as two different octahedrally coordinated sites. The 9-fold coordinated sites are shared by Sr²⁺ and La³⁺ ions in the ratio 0.67 to 0.33. The octahedrally coordinated sites differ in average bond length of the surrounding oxygen ions. The larger sites exhibit an average bond length of 0.2298 Å and are occupied by Sr²⁺ ions whereas the smaller octahedral sites with a mean distance of 0.2092 Å are occupied by Mg²⁺ ions. The ionic radii of La³⁺ and Sr²⁺ for 9-fold coordination are 1.216 and 1.13 Å. Sr²⁺ and Mg²⁺ exhibit ionic radii of 1.18 and 0.72 Å in an octahedral coordination. The ionic radii of Ce³⁺ for 9-fold and 6-fold coordination are 1.01 and 1.196 Å. 6-fold coordinated Mn²⁺ has an ionic radius of 0.83 Å.¹¹ Considering the ionic radii as well as charge balancing, it is assumed that the Ce³⁺ ions primarily going to occupy the 9-fold coordinated La³⁺ sites whereas Mn²⁺ ions tend to occupy the octahedral Mg²⁺ sites.

In this contribution, synthesis, optical properties, and ET from Ce³⁺ to Mn²⁺ in co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ are discussed. Therefore, samples with different doping concentrations were prepared via high temperature solid state synthesis. Phase purity was validated using X-ray powder diffractometry (XRD). Diffuse reflectance (DR) spectra were recorded to examine the body colour of Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺. PL properties were investigated by recording PL spectra and photoluminescence excitation spectra (PLE). Temperature behaviour of the PL was examined by recording PL spectra from 100 to 500 K. In addition, photoluminescence decay (PLD) measurements were performed from 100 to 500 K to study the origin of thermal quenching. To monitor the ET from Ce³⁺ to Mn²⁺ PLD measurements were performed on samples with different Mn²⁺ concentrations. Furthermore, external quantum efficiencies η_ext were determined and chromaticity coordinates according to Commission International de l'Eclairage 1931 (CIE) were calculated.

2. Experimental

All investigated Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ samples were prepared by high temperature solid state synthesis. To this end, high purity educts SrCO₃ (Aldrich, 99.9%), MgO (Merck KGaA, p.a.), La₂O₃ (Treibacher Industrie AG, 99.995%), H₃BO₃ (Merck KGaA, Ph. Eur.), CeO₂ (Atomergic Chemetals Co., 99.9%), and MnC₂O₄·2H₂O (Dr Paul Lohmann, chem. pure) were weighted in stoichiometric amounts. Though, an excess of 5% of boric acid was used to compensate evaporation. The starting materials were thoroughly blended in acetone in an agate mortar. After drying at ambient temperatures, the obtained powder blends were heated at 650 °C for 1 h in air to decompose the boric acid. Afterwards, the samples were ground and calcined at 1200 °C for 6 h in a corundum crucible imbedded in active carbon. Obtained white sinter bodies were subsequently ground to a fine μ-powder.

Phase purity of the synthesized Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ samples was investigated using XRD. XRD patterns were collected on a Rigaku MiniFlex II diffractometer working in Bragg–Brentano geometry using Cu K_α radiation. Step width and integration time were set to 0.02° and 1 s, respectively.

PL spectra as well as PLE spectra were recorded on an Edinburgh Instruments FSL900 spectrometer equipped with a Xe arc lamp (450 W) and a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R2658P). Obtained PL spectra were corrected by applying a correction file obtained from a tungsten incandescent lamp certified by the National Physics Laboratory U.K.

For PLD measurements on Ce³⁺, a picosecond pulsed LED (40 μW, pulse width = 912.6 ps, λ_em = 267 nm) was attached to the spectrometer. For PLD measurements on Mn²⁺ a microsecond pulsed Xe lamp (100 W, pulse width = 1 μs) was used.

PL measurements at 2.9 K were performed using the closed cycle cryocooler OptistatAC-V12 from Oxford Instruments. Helium was used as a cooling agent. Temperature dependent PL measurements from 100 to 500 K were performed using the Oxford Instruments cryostat MicrostatN2. Liquid nitrogen was used as a cooling agent. Temperature stabilization time was 60 s and tolerance was set to ±3 K.

DR spectra were recorded on an Edinburgh Instruments FS900 spectrometer equipped with a Xe arc lamp (450 W), a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928) as well as a Teflon-coated integration sphere. BaSO₄ (99.998%, Sigma-Aldrich) was used as a reflectance standard.

External quantum efficiencies η_ext were determined using the approach of Kawamura et al.¹² Therefore, PL spectra of the samples as well as of the excitation source were recorded in a Teflon-coated integration sphere. From this, η_ext can be calculated using the following relation:

here, I_em(λ) is the emission intensity of the sample, I_ex(λ) is the intensity of the excitation lamp in absence of the sample and I'_ex(λ) is the intensity of the excitation lamp in presence of the sample.

3. Results and discussion

To investigate phase purity of the synthesised Sr₅MgLa₂(BO₃)₆ samples XRD patterns were recorded. Fig. 1 shows the collected XRD patterns of doped and undoped Sr₅MgLa₂(BO₃)₆ as well as the ICDD reference card of Sr₅MgLa₂(BO₃)₆. The diffractograms indicate the formation of the trigonal Sr₅MgLa₂(BO₃)₆ phase. No change in the host structure can be observed with increasing doping concentration. This observation confirms the suitability of the utilized synthesis route.

Fig. 1 XRD patterns of Sr₅MgLa₂(BO₃)₆, Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆, Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆, and ICDD reference card.

Fig. 2 depicts the DR spectra of undoped Sr₅MgLa₂(BO₃)₆ as well as of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ and Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆. The undoped Sr₅MgLa₂(BO₃)₆ sample shows an absorption band in the UV range below 400 nm. This band is tentatively assigned to band-to-band absorption of the host structure.⁹ The absorption in the DR spectrum of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ is assigned to the spin and parity allowed [Xe]4f¹–[Xe]5d¹ interconfigurational electric-dipole transition in Ce³⁺. The DR spectrum of Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆ shows slight absorption at about 470 nm which is ascribed to the transition from the ⁶A₁(⁶S) ground state to the ⁴T₁(⁴G) excited state in Mn²⁺.

Fig. 2 DR spectra of Sr₅MgLa₂(BO₃)₆, Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆, and Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆.

Room temperature PLE and PL spectra of singly doped Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ and Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆ as well as of co-doped Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ are depicted in Fig. 3a–c. The PLE spectrum of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ (Fig. 3a) was recorded monitoring the 400 nm-emission and shows a broad band ranging from about 240 to 370 nm. This band consists of various unresolved bands due to excitation from the ²F_7/2,5/2 ground state to the [Xe]5d¹ multiplet of Ce³⁺. The PL spectrum of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ was recorded using an excitation wavelength of λ_ex = 300 nm. The spectrum exhibits a broad band peaking at about 397 nm and a tailing towards lower energy of the electromagnetic range. This emission band occurs due to radiative relaxation from the excited ²D_j state of the [Xe]5d¹ configuration to the ²F_7/2,5/2 ground state of Ce³⁺. The tailing is caused by Ce³⁺ ions occupying different lattice sites and will be discussed later in the article in more detail. Fig. 3b illustrates the PLE and PL spectra of Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆. Monitoring the 715 nm-emission of Mn²⁺, the obtained PLE spectrum exhibits various bands at about 305, 319, 354, 378, 398, 415 and 474 nm. These bands were attributed to transitions from the ⁶A₁(⁶S) ground state to ⁴T₁(⁴F), ⁴A₂(⁴F), ⁴T₁(⁴P), ⁴E(⁴D), ⁴T₂(⁴D), [⁴E(⁴G)⁴A₁(⁴G)], and ⁴T₁(⁴G), respectively. The PL spectrum of Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆ shows a broad band with a maximum at about 714 nm and a tailing to the long wavelength range. This emission is due radiative relaxation from the ⁴T₁(⁴G) excited state to the ⁶A₁(⁶S) ground state in Mn²⁺. PLE and PL spectra of Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ are depicted in Fig. 3c. PLE spectra were recorded monitoring the emission of Mn²⁺ and Ce³⁺ at 715 and 400 nm, respectively. The obtained PLE spectra appear similar in shape indicating the occurrence of ET from Ce³⁺ ions to Mn²⁺ ions. The PL spectrum was recorded upon excitation with a wavelength of λ_ex = 300 nm and shows two bands peaking at about 403 and 713 nm. These bands originate from radiative relaxation from [Xe]5d¹ to ²F_7/2,5/2 in Ce³⁺ and from ⁴T₁(⁴G) to ⁶A₁(⁶S) in Mn²⁺.

Fig. 3 Room temperature PLE and PL spectra of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ (a), Sr₅Mg_0.97Mn_0.03La₂(BO₃)₆ (b), Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ (c).

For further investigations on the origin of the tailing of the Ce³⁺ emission band, a PL spectrum of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ was recorded at 2.9 K. This spectrum is illustrated in Fig. 4. Commonly, Ce³⁺ emission consists of a doublet band due to spin–orbit-splitting of the ²F state ([Xe]4f¹ configuration) into the ²F_7/2 and ²F_5/2 states. The energetic difference between the ²F_7/2 and ²F_5/2 states usually amounts to about 2000 cm⁻¹.¹³ Therefore, it should be possible to fit the present Ce³⁺ emission band with two Gaussian curves with an approximate distance of 2000 cm⁻¹. However, fitting the current PL spectrum with two Gaussian curves leads to an approximate distance of about 1399 cm⁻¹. This indicates the occupation of more than one distinct crystallographic site. This is thoroughly possible since Sr₅MgLa₂(BO₃)₆ provides also 6-fold coordinated Sr²⁺ sites aside from the 9-fold coordinated site. Against this, fitting the Ce³⁺ emission band with a sum of four Gaussian curves A, B, C, and D results in approximate distances of 2018 and 1979 cm⁻¹. Based on this result, it is assumed that the Ce³⁺ ions also occupy the 6-fold coordinated Sr²⁺ sites. Since crystal field splitting of the 5d states decreases with increasing coordination number, the energetic difference between d and f states increases. Therefore, it is concluded that curves A and B belong to Ce³⁺ occupying a 6-fold coordinated Sr²⁺ site (Ce2) whereas curves C and D were assigned to Ce³⁺ on a 9-fold coordinated site (Ce1). The occupation of two different crystallographic sites is also reflected in the PLE spectra depicted in the inset of Fig. 4. Monitoring the emission of Ce³⁺ at 345, 380, and 415 nm results in PLE spectra looking significantly different in shape. Shifting the monitored wavelength from 345 to 415 nm, the obtained PLE spectrum gets broader and a new band arises. This observation also indicates PL from different crystallographic sites.

Fig. 4 Low temperature PL spectra of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆ and Gaussian curves A, B, C, and D. Inset: low temperature PLE spectra of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆.

Fig. 5 depicts PL spectra of Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆ with x = 0.000, 0.005, 0.010, 0.020, 0.050, 0.080, 0.100, 0.150, and 0.200. Up to a Mn²⁺ content of x = 0.100 PL intensity of Mn²⁺ increases. At higher Mn²⁺ contents, the PL intensity of Mn²⁺ starts to decrease due to concentration quenching. Furthermore, with increasing Mn²⁺ content the Ce³⁺ PL intensity decreases implying the occurrence of ET from Ce³⁺ to Mn²⁺.

Fig. 5 PL spectra of Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆. Inset: integrated PL intensity in dependence of the Ce³⁺ and Mn²⁺ concentration.

Further, fluorescence lifetime measurements were performed monitoring the Ce³⁺ emission of co-doped Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆. The obtained PLD curves are plotted in Fig. 6 and can be best fitted with a bi-exponential function using the following equation:

here, I is the PL intensity at the time t, A₁ and A₂ are fitting parameters, and τ₁ and τ₂ are the partial fluorescence lifetimes of the exponential components. The bi-exponential behaviour of the PLD curves reflects PL of two different crystallographic sites as detailed above. The average fluorescence lifetimes τ of the Ce³⁺ emission were obtained applying eqn (3):Calculated average fluorescence lifetimes τ as well as the emission fractions frac₁ and frac₂ are summarized in Table 1. It turned out that with increasing Mn²⁺ content fluorescence lifetimes τ of the Ce³⁺ emission decreases. Since ET processes are magnitudes faster than radiative transitions, this behaviour indicates the occurrence of ET from Ce³⁺ to Mn²⁺. The ET efficiency η_T can be calculated using the following equation:¹⁴here, τ_S and τ_S0 are the fluorescence lifetimes of Ce³⁺ in presence and absence of Mn²⁺. The calculated values for η_T are summarized in Table 1 and also plotted in the inset of Fig. 6. It can be seen that the values for η_T show a continuous growth with increasing Mn²⁺ content.

Fig. 6 PLD curves of Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆. Inset: ET transfer efficiency η_T in dependence of the Mn²⁺ concentration.

Table 1 Fluorescence lifetimes τ as well as partial lifetimes τ₁ and τ₂ and the emission fractions frac₁ and frac₂ of Ce³⁺ and ET efficiency η_T in Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆

Sample (x)	frac1 (%)	τ1 (ns)	frac2 (%)	τ2 (ns)	τ(ns)	ηT
0.000	4	9.3	96	31.0	30	0
0.005	5	9.4	95	30.7	30	0.02
0.010	5	8.9	95	30.1	29	0.04
0.020	4	7.5	96	30.0	29	0.04
0.050	6	8.5	94	29.9	29	0.05
0.080	7	6.9	93	28.7	27	0.10
0.100	7	6.3	93	28.5	27	0.11
0.150	11	7.6	89	27.8	26	0.15
0.200	12	6.7	88	27.0	25	0.19

---

To identify the nature of the ET from Ce³⁺ to Mn²⁺ in co-doped Sr₅La₂Mg(BO₃)₆:Ce³⁺,Mn²⁺ Dexter's approach of exchange interactions and multipolar interactions was used.

On the basis of Dexter's ET formula and Reisfeld's approximation the following relations are obtained:¹⁵

In these equations η₀ and η are the internal luminescence quantum efficiencies of Ce³⁺ in the absence and presence of Mn²⁺. C is the concentration of Mn²⁺ in the samples. Eqn (5) corresponds to exchange interaction whereas eqn (6) with α = 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interaction, respectively. η₀ and η can approximately be estimated by the fluorescence lifetimes τ₀ and τ. Fig. 7 shows the plots of relation (5) and (6). A linear behaviour is only observed for relation (5) and suggests ET via exchange interaction.

Fig. 7 Dependence of ln(τ₀/τ) on C_Mn (a) and τ₀/τ on C^6/3 (b), C^8/3 (c), and C^10/3 (d).

To validate this result, the integrated PL intensities of the Ce³⁺ and Mn²⁺ emission were investigated using van Uitert's approach:¹⁶

here, I and I₀ are the luminescence Intensities in the absence and presence of Mn²⁺, A is a constant, and x is the concentration of Mn²⁺ ions. θ = 3, 6, 8, and 10 represents exchange interaction, dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively.¹⁷ Experimental data is plotted in Fig. 8 and best fitting is obtained when plugging in 3 for θ. This also suggests that ET from Ce³⁺ to Mn²⁺ in Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ occurs via exchange interaction. This observation is rather uncommon for the ET from Ce³⁺ and Mn²⁺. Since exchange interactions proceed only over short distances this result implies a non-statistical distribution of the Ce³⁺ and Mn²⁺ ions. To estimate the critical distance between Ce³⁺ and Mn²⁺ further investigations were carried out using the Inokuti–Hirayama equation for exchange interaction:¹⁸In this equation I(t) is the intensity at a certain time and I₀ is the intensity after excitation. t is the time and τ is the intrinsic fluorescence lifetime of Ce³⁺ in absence of Mn²⁺. C is the concentration of Mn²⁺ ions per unit volume and C₀ is the critical concentration. γ is the transfer volume and g(z) is defined ashere, z and y are defined as

Fig. 8 Dependence of I/I₀ on the Mn²⁺ concentration.

Eqn (9) can be expressed by a Taylor series as described by Inokuti and Hirayama which converges. This leads to the following formula:

For the coefficients h₁, h₂, and h₃ the values 1.73164699, 5.93433597, and 5.44487446, respectively, given by Inokuti and Hirayama were plugged in. Fitting the PLD curves of the investigated samples with eqn (12) yields the C/C₀ values. C/C₀ values are plotted against 3/(4πR_c³) in Fig. 9. Since C₀ = 3/(4πR_c³), the slope of a linear fit through the data points is equal to R_c³.^18,19 Therefore, the critical distance R_c for ET from Ce³⁺ to Mn²⁺ was calculated to be 4.6 Å.

Fig. 9 Dependence of C/C₀ on 4πC/3.

To investigate the temperature behaviour of the PL of Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ PL spectra from 100 to 500 K were recorded. The obtained PL spectra are illustrated in Fig. 10. Due to thermal quenching, the PL intensity of Ce³⁺ and Mn²⁺ decreases with increasing temperature. In addition, the maximum of the Mn²⁺ emission band shows a shift towards higher energy of the electromagnetic spectrum. Since the equilibrium distance between the Mn²⁺ ions and its oxygen ligands extends with increasing temperature, the energy difference of the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition increases.²⁰ Therefore, the maximum of the Mn²⁺ emission is shifted towards the blue range. The inset of Fig. 10 depicts the PL integrals of the Ce³⁺ and Mn²⁺ emission bands in dependence of temperature. By fitting the data points with a Fermi–Dirac distribution activation energy E_A for thermal quenching can be calculated.

here, I(T) is the integrated PL intensity at a certain temperature and I₀ is the PL intensity at zero Kelvin. B is the frequency factor for thermal quenching, T is the temperature and k is the Boltzmann constant (k = 8.617 × 10⁻⁵ eV K⁻¹). The values of the parameters I₀ and B were derived from the fitting function and are about 0.95 and 333, respectively. Fitting the data points with this equation yields an activation energy of E_A = 0.18 eV. From this the T_1/2 value can be derived by using the following relation:T_1/2 describes the temperature at which PL intensity of a luminescent centre is decreased to one-half of its maximum intensity. For the investigated Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ sample T_1/2 was calculated to be 355 K.

Fig. 10 PL spectra of Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ from 100 to 500. Inset: integrated PL intensity in dependence of temperature.

For further investigations on the origin of thermal quenching in Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ fluorescence lifetime measurements were performed. Therefore, PLD curves of the Ce³⁺ as well as of the Mn²⁺ emission were recorded from 100 to 500 K. Fig. 11 depicts the obtained PLD curves monitoring the 400 nm-emission of Ce³⁺. The PLD curves show a bi-exponential behaviour reflecting PL from two distinct crystallographic sites as mentioned above. The fluorescence lifetimes of Ce³⁺ are illustrated in the inset of Fig. 11 and are summarized in Table 2. From this finding it can be derived that the fluorescence lifetimes remain stable up to a temperature of 300 K. At 350 K the fluorescence lifetimes start to decrease indicating a decline of radiative transitions due to thermal quenching. Fig. 12 shows the PLD curves of Mn²⁺ monitoring the emission at 725 nm. It is obvious from the inset that the Mn²⁺ PL lifetimes decrease continuously over the complete temperature range. Fitting the fluorescence lifetimes of Ce³⁺ and Mn²⁺ with eqn (13) yields the T_1/2 values for the Ce³⁺ (T_1/2,Ce) and Mn²⁺ (T_1/2,Mn) emission. T_1/2,Ce and T_1/2,Mn were calculated to be 492 and 323 K, respectively. This result indicates that the overall thermal quenching of the PL in Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ is mainly caused by the presence of Mn²⁺.

Fig. 11 Obtained PLD curves of Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ at 100, 400, and 500 K monitoring the PL of Ce³⁺. Inset: Fluorescence lifetimes of the Ce³⁺ emission in dependence of temperature.

Table 2 Fluorescence lifetimes τ as well as partial lifetimes τ₁ and τ₂ and the emission fractions frac₁ and frac₂ of Ce³⁺ and Mn²⁺ in Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆ from 100 to 500 K

T (K)	Ce^3+					Mn^2+
	τ1 (ns)	frac1 (%)	τ2 (ns)	frac2 (%)	τ (ns)	τ1 (ms)	frac1 (%)	τ2 (ms)	frac2 (%)	τ (ms)
100	10.6	8	29.7	92	28	14.8	6	48.2	94	46
150	9.6	7	29.6	93	28	13.1	5	45.9	95	44
200	10.2	9	30.0	91	28	16.5	9	42.9	91	41
250	8.7	7	29.5	93	28	16.8	13	37.9	87	35
300	8.7	8	29.2	92	28	7.8	8	28.9	92	27
350	8.9	11	28.5	89	26	5.8	15	20.7	85	18
400	9.1	17	26.8	83	24	3.5	27	13.8	73	11
450	7.2	21	22.4	79	19	1.7	30	6.5	70	5
500	4.8	29	16.2	71	13	0.3	12	1.7	88	2

---

Fig. 12 Obtained PLD curves of Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆ at 100, 300, and 400 K monitoring the PL of Mn²⁺. Inset: Fluorescence lifetimes of the Mn²⁺ emission in dependence of temperature.

The efficiency of the prepared Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ samples was examined by determining external quantum efficiency η_ext as mentioned above. To our understanding, η_ext is defined as the ratio between the number of emitted photons N_emission and the number of absorbed photons N_absorption:

The calculated external quantum efficiencies are summarized in Table 3. It turned out that with increasing Mn²⁺ concentration η_ext continuously decreases. This is probably caused by concentration quenching due to ET between the Mn²⁺ ions.

Table 3 External quantum efficiencies η_ext and CIE colour coordinates of Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆

Sample (x)	ηext	CIE (x|y)
0.000	0.97	(0.160|0.046)
0.005	0.72	(0.160|0.046)
0.010	0.53	(0.161|0.046)
0.020	0.57	(0.164|0.048)
0.050	0.53	(0.169|0.050)
0.080	0.33	(0.175|0.052)
0.100	0.33	(0.178|0.052)
0.150	0.20	(0.188|0.059)
0.200	0.18	(0.194|0.062)

---

To illustrate the colour of the emission, CIE chromaticity coordinates were calculated according to the relevant PL spectra. The obtained colour points are depicted in Fig. 13. All colour points are located in the violet region of the chromaticity coordinates diagram. With increasing Mn²⁺ content the colour points show a shift towards the red region. Since the Mn²⁺ emission band is located in the deep red range of the visible spectrum where the eye sensitivity is rather low, the observed shift of the colour points is rather low too. A larger shift into the red region of the chromaticity coordinates diagram using co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ could be obtained using a lower Ce³⁺ content. In this way the Mn²⁺ emission would be more intense compared to the Ce³⁺ emission band resulting in a larger shift.

Fig. 13 CIE chromaticity diagram of Sr₅Mg_1−xMn_x(La_0.99Ce_0.01)₂(BO₃)₆ with different Mn²⁺ concentrations. Inset: Enlarged section of the relevant range.

4. Conclusions

To investigate the PL properties and the ET between Ce³⁺ and Mn²⁺ of co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ a series of powder samples with various Mn²⁺ contents was prepared. Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ shows two emission bands located at 403 and 713 nm upon excitation at λ_ex = 300 nm. Low temperature PL measurement on the Ce³⁺ emission reveals that the Ce³⁺ ions mainly occupy the 9-fold coordinated sites as well as the 6-fold coordinated Sr²⁺ sites. This statement is backed by the bi-exponential PLD curves of Sr₅Mg(La_0.99Ce_0.01)₂(BO₃)₆. The highest PL intensity was found for a Mn²⁺ content of x = 0.100. Furthermore, it was demonstrated that the ET from Ce³⁺ to Mn²⁺ occurs via an exchange interaction mechanism. Critical distance R_c was determined to be 4.6 Å. Temperature depended PL measurements prove a T_1/2 value of 355 K for Sr₅Mg_0.90Mn_0.10(La_0.99Ce_0.01)₂(BO₃)₆. Furthermore, temperature dependent fluorescence lifetimes revealed that thermal quenching is mainly caused by the Mn²⁺ ions. Finally, the external quantum efficiencies η_ext were determined to be close to unity of the Ce³⁺ doped samples. With increasing Mn²⁺ η_ext continuously decreases. Calculated chromaticity coordinates are all located in the violet region of the CIE diagram. Therefore, co-doped Sr₅MgLa₂(BO₃)₆:Ce³⁺,Mn²⁺ with the herein investigated doping levels is not suitable as a white or red emitting phosphor.

Acknowledgements

The authors are grateful to Merck KGaA Darmstadt, Germany for generous financial support.

References

1. (a) C. Feldmann, T. Jüstel, C. R. Ronda and P. J. Schmidt, Adv. Funct. Mater., 2003, 13, 511–516; (b) E. F. Schubert and J. K. Kim, Science, 2005, 308, 1274–1278.
2. S. Nakamura, P. Stephen and F. Gerhard, The Blue Laser Diode: The Complete Story, Springer-Verlag, Berlin/Heidelberg, 1997.
3. H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon and S. S. Kim, J. Lumin., 2007, 126, 371–377.
4. Y. Uchida and T. Taguchi, Opt. Eng., 2005, 44, 124003.
5. M. Shang, C. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 1372.
6. (a) M. Müller and T. Jüstel, J. Lumin., 2014, 155, 398–404; (b) T. Jia, Z. Ci, Q. Wu, G. Zhu, C. Wang and Y. Wang, ECS J. Solid State Sci. Technol., 2015, 4, R78–R82.
7. (a) R. Yu, H. Li, H. Ma, C. Wang, H. Wang and J. McKittrick, J. Am. Ceram. Soc., 2014, 97, 1151–1156; (b) Q. Liu, Y. Liu, Y. Ding, Z. Peng, Q. Yu, X. Tian and G. Dong, J. Sol-Gel Sci. Technol., 2014, 71, 276–282; (c) Y. Shi, Y. Wen, M. Que, G. Zhu and Y. Wang, Dalton Trans., 2014, 2418–2423; (d) L. Wu, B. Wang, Y. Zhang, L. Li, H. R. Wang, H. Yi, Y. F. Kong and J. J. Xu, Dalton Trans., 2014, 13845.
8. K. I. Schaffers, P. D. Thompson, T. Alekel III, J. R. Cox and D. A. Keszler, Chem. Mater., 1994, 6, 2014–2022.
9. R. Sankar and G. Subba Rao, J. Alloys Compd., 1998, 281, 126–136.
10. R. Sankar, Solid State Sci., 2008, 10, 1864–1874.
11. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
12. Y. Kawamura, H. Sasabe and C. Adachi, Jpn. J. Appl. Phys., 2004, 43, 7729–7730.
13. (a) F. A. Kröger and J. Bakker, Physica, 1941, 8, 628–646; (b) G. Blasse and A. Bril, J. Chem. Phys., 1967, 47, 5139–5145.
14. (a) H. C. Kandpal and H. B. Tripathi, Indian J. Pure Appl. Phys., 1979, 17, 587–589; (b) P. Paulose, G. Jose, V. Thomas, N. Unnikrishnan and M. Warrier, J. Phys. Chem. Solids, 2003, 64, 841–846.
15. (a) D. L. Dexter, J. Chem. Phys., 1953, 21, 836; (b) D. L. Dexter and J. H. Schulman, J. Chem. Phys., 1954, 22, 1063; (c) R. Reisfeld, E. Greenberg, R. Velapoldi and B. Barnett, J. Chem. Phys., 1972, 56, 1698.
16. L. G. van Uitert, J. Electrochem. Soc., 1967, 114, 1048.
17. (a) L. Zhang, Z. Lu, P. Han, L. Wang and Q. Zhang, J. Nanomater., 2012, 2012, 1–7; (b) F. Zhang and W. Tang, Luminescence, 2015, 30, 216–220.
18. M. Inokuti and F. Hirayama, J. Chem. Phys., 1965, 43, 1978.
19. J. S. Lee, S. Unithrattil and W. B. Im, J. Alloys Compd., 2013, 555, 297–303.
20. Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn., 1954, 9, 766–779.

---