Energy transfer and unusual decay behaviour of BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ phosphor

Abstract

In this work the photoluminescence (PL) of BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ and the energy transfer (ET) between Eu²⁺ and Mn²⁺ were examined. A series of powder samples with various Mn²⁺ concentrations were prepared by high temperature solid state route. Phase purity was investigated using X-ray powder diffractometry. Emission and excitation spectra as well as diffuse reflectance spectra were recorded to elucidate the PL properties of co-doped BaCa₂Si₃O₉:Eu²⁺,Mn²⁺. Furthermore, fluorescence lifetime measurements were performed. PL and lifetime measurements were carried out from 100 to 800 K and 100 to 500 K, respectively. Moreover, external quantum efficiencies were determined and colour points were calculated. It turned out that ET from Eu²⁺ to Mn²⁺ is of a resonant type and occurs via dipole–quadrupole interactions. Temperature dependent PL measurements indicate high temperature stability of emission intensity in BaCa₂Si₃O₉:Eu²⁺,Mn²⁺. Finally, it was found that fluorescence lifetimes of Eu²⁺ BaCa₂Si₃O₉ show an unusual increase with increasing temperature.

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1. Introduction

Even though the invention of bright blue light emitting diodes (LEDs) by Nakamura et al. nearly dates back a quarter of a century, LEDs still have a vast impact on advances in lighting technology.¹ Meanwhile, LEDs have replaced more and more conventional light sources such as incandescent and discharge lamps. This is because of the fact that LEDs provide many benefits compared to thermal and gas discharge light sources, such as longer lifetimes, higher wall plug efficiency and higher colour rendering.² Nowadays, most of the commercially available white light emitting LEDs comprise a blue emitting (In,Ga)N chip pumping a green-yellow emitting phosphor, e.g. (Y,Gd)₃Al₅O₁₂:Ce³⁺. This setup provides a high luminous efficacy due to emission in the green spectral range. Unfortunately, these lamps suffer from high colour temperature and low colour rendering index due to the lack of emission in the red spectral range.³ Therefore, these light sources are unpopular for domestic lighting. To overcome these drawbacks, one approach is to use a phosphor blend comprising a blue, green, and red phosphor, which is excited by an ultraviolet emitting LED. Using this concept, cold and warm white emitting LEDs with excellent colour points can be realized.⁴ However, these packages undergo a loss in blue emission due to re-absorption by the green and red phosphors. For this reason, many research groups are developing single phase white emitting phosphors to convert UV radiation into white light.⁵

One approach to realize a white emitting phosphor is to use the ion couple Eu²⁺ and Mn²⁺.⁶ The broad emission bands in the blue and red spectral range of Eu²⁺ and Mn²⁺ in many host structures complement each other to white light due to additive colour mixing. Furthermore, Eu²⁺ usually exhibits a broad excitation band in the UV range due to the spin and parity allowed [Xe]4f⁷–[Xe]4f⁶5d¹ interconfigurational electric-dipole transition. Therefore, Eu²⁺ is well appropriated for pumping by UV LEDs. In addition the blue emission band of Eu²⁺ is also suitable for sensitizing the spin and parity forbidden [Ar]3d⁵–[Ar]3d⁵ excitation transitions of Mn²⁺via energy transfer (ET). To investigate the photoluminescence (PL) properties as well as the ET from Eu²⁺ to Mn²⁺, a series of co-doped BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ samples with various Mn²⁺ concentrations were synthesized. BaCa₂Si₃O₉ was described for the first time in the work of Eskola in 1922.⁷ Later on, in 1965 Alfors et al. investigated the mineral walstromite.⁸ Based on their results on optical properties and X-ray powder diffraction data they stated that BaCa₂Si₃O₉ is identical to walstromite. Glasser et al. reported on the crystal structure of synthetic BaCa₂Si₃O₉ using photographic X-ray diffraction data in 1968.⁹ The crystal structure derived from a single crystal analysis of BaCa₂Si₃O₉ was firstly published by Barkley et al. in 2011.¹⁰ Shortly afterwards, Yao et al. presented the first report on the blue emission of Eu²⁺ doped BaCa₂Si₃O₉.¹¹ Moreover, Gaft et al. published an article concerning the luminescence of natural walstromite in 2013.¹²

BaCa₂Si₃O₉ crystallizes triclinically in the space group P1̄. The structure of triclinic BaCa₂Si₃O₉ is built up of layers consisting of alternating Si₃O₉ rings. Between these layers Ba and Ca atoms are incorporated. BaCa₂Si₃O₉ comprises two nonequivalent Ca sites, namely Ca1 and Ca2. The Ca1 site is coordinated by 8 oxygen atoms with a mean distance of 2.545 Å. The Ca2 site is surrounded by 6 oxygen atoms with an average bond length of 2.370 Å. The Ba atoms are 8-fold coordinated with an average distance of 2.841 Å. The ionic radii of Ca for 8 and 6-fold coordination are 1.12 and 0.96 Å. Ba²⁺ in 8-fold coordination has an ionic radius of 1.42 Å. The ionic radii of Mn²⁺ and Eu²⁺ for 8 and 6-fold coordination are 0.96 and 0.83 as well as 1.25 and 1.17 Å, respectively.¹³ Based on the ionic radii, it can be assumed that the Mn²⁺ ions tend to occupy the Ca sites whereas the Eu²⁺ ions tend to occupy the Ba²⁺ sites as well as the 8-fold coordinated Ca²⁺ sites.

2. Experimental

Preparation

Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ samples were synthesized via conventional high temperature solid state reaction. High purity starting materials BaCO₃ (Alfa Aesar, 99.8%), CaCO₃ (Merck KGaA, Reag. Ph. Eur.), SiO₂ (Merck KGaA, Ph. Eur.), Eu₂O₃ (Treibacher Industrie AG, 99.99%) and MnC₂O₄·2H₂O (Dr Paul Lohmann, chem. pure) were weighed in stoichiometric amounts. 1 wt% H₃BO₃ (Merck KGaA, Ph. Eur.) was added as a flux. Subsequently, the educts were ground in acetone in an agate mortar. After drying at ambient temperature, the obtained powder blends were heated at 1000 °C for 2 h in air to decompose the carbonates. Afterwards the samples were ground and finally calcined in alumina boats at 1200 °C for 12 h in reducing forming gas flow (Westfalen, 5% H₂, 95% N₂). After calcination, hard sinter bodies with a slightly yellow body colour were obtained which were subsequently ground to a fine powder.

Characterization

Phase purity was investigated using X-ray powder diffractometry (XRD). XRD patterns were recorded on a Rigaku MiniFlex II diffractometer working in Bragg–Brentano geometry using Cu K_α radiation. Step width and integration time were 0.02 and 1 s, respectively.

PL spectra and photoluminescence excitation (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). PL spectra were corrected by using a correction file obtained from a tungsten incandescent lamp certified by the National Physical Laboratory, UK.

For photoluminescence decay (PLD) measurements on Eu²⁺, the spectrometer was equipped with a picosecond pulsed laser (5 mW, pulse width = 76.6 ps, λ_em = 375 nm). For PLD measurements on Mn²⁺ a microsecond pulsed Xe lamp (100 W, pulse width = 1 μs) was attached to the spectrometer.

For temperature dependent measurements the spectrometer was equipped with a cryostat (Oxford Instruments). Liquid nitrogen was used as a cooling agent. Temperature stabilization time was 60 s and tolerance was set to ±3 K.

Diffuse reflectance (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.¹⁴ To this end, PL spectra of the sample as well as of the excitation source were recorded in the integration sphere. From this, η_ext can be calculated by the following equation:

where I_em(λ) is the emission intensity of the sample. I_ex(λ) and I^′_ex(λ) are the intensities of the excitation source in the absence and presence of the sample.

3. Results and discussion

Collected XRD patterns of doped and also of undoped BaCa₂Si₃O₉ samples as well as the ICDD reference cards of BaCa₂Si3O₉ and SiO₂ are presented in Fig. 1. The XRD patterns indicate the formation of the triclinic BaCa₂Si₃O₉ phase. Furthermore, with increasing doping concentrations of Eu²⁺ and Mn²⁺, no change in the host structure can be observed. However, samples with x > 0.1 show an additional reflex at about 25.9° which can be assigned to SiO₂. This proves the successful synthesis of the investigated BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ phosphors with a Mn²⁺ concentration up to x = 0.1.

Fig. 1 XRD patterns of pure BaCa₂Si₃O₉ as well as of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ and Ba(Ca_0.95Mn_0.05)₂Si₃O₉.

Fig. 2 shows DR spectra of undoped BaCa₂Si₃O₉ host material and also of Ba_0.99Eu_0.01Ca₂Si₃O₉ and Ba(Ca_0.95Mn_0.05)₂Si₃O₉. Undoped BaCa₂Si₃O₉ exhibits high reflectance between 250 and 800 nm. Thus, undoped BaCa₂Si₃O₉ possess a white body colour. The DR spectrum of Ba_0.99Eu_0.01Ca₂Si₃O₉ shows a broad absorption band between 250 and 450 nm due to the allowed [Xe]4f⁷–[Xe]4f⁶5d¹-transition of Eu²⁺. Because of the absorption band in the blue range of the electromagnetic spectrum, Ba_0.99Eu_0.01Si₃O₉ exhibits a slightly yellow body colour. The DR spectrum of Ba(Ca_0.95Mn_0.05)₂Si₃O₉ shows absorption in the UV region between 250 and 350 nm which can be tentatively assigned to a Mn²⁺ charge transfer transition.¹⁵

Fig. 2 DR spectra of pure BaCa₂Si₃O₉ as well as of Ba_0.99Eu_0.01Ca₂Si₃O₉ and Ba(Ca_0.95Mn_0.05)₂Si₃O₉.

The PLE spectrum as well as the PL spectrum of Ba_0.99Eu_0.01Ca₂Si₃O₉ is depicted in Fig. 3a. The PLE spectrum was recorded monitoring the 460 nm emission of Eu²⁺ and exhibits a broad band consisting of various unresolved bands. This band is due to excitation from the [Xe]4f⁷ ground state to the [Xe]5d¹ multiplet of Eu²⁺. The PL spectrum of Ba_0.99Eu_0.01Ca₂Si₃O₉ (λ_ex = 320 nm) shows an emission band peaking at about 444 nm with FWHM of 47 nm (2333 cm⁻¹). This emission originates from relaxation of the excited [Xe]5d¹ state to the [Xe]4f⁷ ground state. Further, Ba_0.99Eu_0.01Ca₂Si₃O₉ possesses an emission band in the red region of the spectrum. The PLE spectrum of this emission is shown as a dotted line. Fig. 3b illustrates the PLE and PL spectra of Ba(Ca_0.95Mn_0.05)₂Si₃O₉. Monitoring the 590 nm emission of Mn²⁺, the PLE spectrum of Ba(Ca_0.95Mn_0.05)₂Si₃O₉ shows various bands centred at 326, 348, 359, 405, 432, and 510 nm. These peaks can be assigned to transitions from the ⁶A₁(⁶S) ground state to ⁴T₁(⁴P), ⁴E(⁴D), ⁴T₂(⁴D), [⁴E(⁴G)⁴A₁(⁴G)], ⁴T₂(⁴G), and ⁴T₁(⁴G), respectively. The PL spectrum of Ba(Ca_0.95Mn_0.05)₂Si₃O₉ was recorded under an excitation wavelength of λ_ex = 405 nm and exhibits a broad band with a maximum at about 594 nm resulting from a transition from the ⁴T₁(⁴G) excited state to the ⁶A₁(⁶S) ground state. This observation is in accordance with the results given by Gaft et al. regarding Mn²⁺ emission in natural walstromite.¹² In addition, the Mn²⁺ emission band comprises a tailing to the red range of the visible spectrum indicating emission from different crystallographic sites. PLE as well as PL spectra of co-doped Ba_0.99Eu_0.01(Ca_0.95Mn_0.05)₂Si₃O₉ are depicted in Fig. 3c. The PL spectrum of Ba_0.99Eu_0.01(Ca_0.95Mn_0.05)₂Si₃O₉ shows two bands peaking at about 445 and 590 nm which can be assigned to electronic transitions of Eu²⁺ ([Xe]5d¹ to [Xe]4f⁷) and Mn²⁺ (⁴T₁(⁴G) to ⁶A₁(⁶S)), respectively. PLE spectra of Ba_0.99Eu_0.01(Ca_0.95Mn_0.05)₂Si₃O₉ were recorded, monitoring the 460 nm emission of Eu²⁺ as well as the 590 nm emission of Mn²⁺. These PLE spectra appear similar to the PLE of singly doped Ba_0.99Eu_0.01Ca₂Si₃O₉ monitoring the blue emission of Eu²⁺. This observation strongly implies the occurrence of ET from Eu²⁺ to Mn²⁺.

Fig. 3 Room temperature PLE and PL spectra of Ba_0.99Eu_0.01Ca₂Si₃O₉ (a), Ba(Ca_0.95Mn_0.05)₂Si₃O₉ (b), and Ba_0.99Eu_0.01(Ca_0.95Mn_0.05)₂Si₃O₉ (c).

Fig. 4 illustrates the PL spectra of the synthesized Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ series with x = 0, 0.005, 0.010, 0.015, 0.020, 0.050, 0.100, 0.150, and 0.200. As can be concluded from the spectra, with increasing Mn²⁺ concentration the PL intensity of Eu²⁺ decreases. This behaviour reflects the assumption provided above regarding ET from Eu²⁺ to Mn²⁺. The inset in Fig. 4 depicts the relative PL intensities of Eu²⁺ and Mn²⁺. It was found that the PL intensity of Mn²⁺ increases continuously up to a Mn²⁺ concentration of x = 0.15 where a saturation effect begins. Concentration quenching of the Mn²⁺ emission was not observed for the synthesized samples.

Fig. 4 PL spectra of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ and integrated PL intensity in dependence of the Eu²⁺ and Mn²⁺ concentration (inset).

Furthermore, PLD measurements were performed monitoring the Eu²⁺ emission to investigate the ET from Eu²⁺ to Mn²⁺ in more detail. Fluorescence lifetimes τ of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ samples with increasing Mn²⁺ content were determined, monitoring the 445 nm emission of Eu²⁺ under an excitation wavelength of λ_ex = 375 nm. Fig. 5 illustrates the obtained PLD curves. The PLD curves can be best fitted with a bi-exponential function applying the following equation:

Fig. 5 PLD curves of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ and ET efficiency η_T in dependence of the Mn²⁺ concentration (inset).

Here, I is the luminescence intensity, A₁ and A₂ are parameters and t is the time. The terms τ₁ and τ₂ are the partial lifetimes for the exponential components. The bi-exponential shape of the PLD curves originates from two different emitting centres and suggests Eu²⁺ emission from two distinct crystallographic sites. This is thoroughly possible since BaCa₂Si₃O₉ provides two different Ca sites and one Ba site, as mentioned above. This assumption is backed by PLD measurements monitoring the emission at 415 and 495 nm which yield different luminescence lifetimes (τ at 415 nm = 602 ns, τ at 415 nm = 702 ns) (Fig. S1 in the ESI†). The average luminescence lifetimes τ of Eu²⁺ in BaCa₂(Si₃O₉):Eu²⁺,Mn²⁺ can be derived using the above mentioned parameters and the following equation:

Emission fractions frac₁ and frac₂, partial lifetimes τ₁, and τ₂ and also the calculated average lifetimes are summarized in Table 1. The results show a decreasing lifetime τ of Eu²⁺ with increasing Mn²⁺ concentration. Since ET processes are usually faster than radiative transitions, this circumstance clearly indicates the occurrence of an ET from Eu²⁺ to Mn²⁺. The ET efficiency η_T be calculated using following equation:¹⁶

Table 1 Fluorescence lifetimes τ as well as partial lifetimes τ₁ and τ₂ and the emission fractions frac₁ and frac₂ of Eu²⁺ in Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉

Sample (x)	frac1 (%)	τ 1 (ns)	frac2 (%)	τ 2 (ns)	τ (ns)
0.000	13	212	87	698	634
0.005	14	208	86	697	629
0.010	13	206	87	690	625
0.015	13	206	87	693	630
0.020	12	204	88	687	628
0.050	13	201	87	680	617
0.100	13	193	87	673	609
0.150	15	175	85	636	569
0.200	16	176	84	629	555

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In this equation τ_S and τ_S0 are the average lifetimes of Eu²⁺ in the presence and absence of Mn²⁺. The inset of Fig. 5 illustrates the calculated values for η_T for the investigated BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ samples as a function of the Mn²⁺ content. This shows that the ET efficiency η_T continuously increases with increasing Mn²⁺ content. For the highest Mn²⁺ concentration with x = 0.20, η_T amounts to 12% whereas the PL intensity of Eu²⁺ decreases around 70%. Due to faster competitive ET processes, e.g. migration to defect sites, a significant amount of emission energy is lost before it can be transferred to the Mn²⁺ ions. This behaviour increases with increasing Mn²⁺ concentration.

The average distance between Eu²⁺ and Mn²⁺ (R_Eu–Mn) in the host material at which ET occurs can be estimated by applying the following formula published by Blasse:¹⁷

Here, V is the volume of the unit cell, x the concentration of Eu²⁺ and Mn²⁺ in the host, and N is the number of available sites for the dopants in the unit cell. For BaCa₂Si₃O₉, V and N are 396.01 ų and 6, respectively. Using this equation for x = 0.005, 0.010, 0.015, 0.020, 0.050, 0.100, 0.150, and 0.200, (R_Eu–Mn) is calculated to be 20.3, 18.5, 17.1, 16.1, 12.8, 10.5, 9.2, and also 8.4 Å, respectively. The Mn²⁺ concentration where the Eu²⁺ PL intensity is one-half of that in the sample without Mn²⁺ is called critical concentration x_c. Plugging in x_c into formula (5), the critical distance R_c can be derived. R_c is defined as the distance where the probability of ET from sensitizer to activator equals the probability of radiative transition in the sensitizer. For Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉, x_c was determined to be 0.134. From this, R_c is calculated to be 9.6 Å.

To elucidate the characteristics of the ET process in Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉, the obtained decay curves were analysed by applying the Inokuti–Hirayama model. Assuming a uniform distribution of Eu²⁺ and Mn²⁺ in the host structure, and neglecting further energy migration processes besides the ET from Eu²⁺ to Mn²⁺, the time-dependent course of the PL intensity of Eu²⁺ can be described by following equation:¹⁸

In this equation, t is the time, I(t) is the PL intensity after excitation, I₀ is the PL intensity at t = 0, τ₀ is the intrinsic luminescence lifetime of the sensitizer in the absence of the activator, α is a parameter for the probability of ET. S is related to the electronic multipole character of the ET. S = 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Modifying eqn (6) leads to following relation:

Plotting the PLD curves using this relation yields a straight line with a slope equal to 1/S. The obtained curves for the measured Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ samples with x = 0.005, 0.010, 0.015, 0.020, 0.050, 0.100, 0.150, and 0.200 are depicted in Fig. 6. The calculated S values are roughly coincident with the theoretical value of S = 8. This result suggests dipole–quadrupole interactions as the dominant mechanism of ET from Eu²⁺ to Mn²⁺ in co-doped BaCa₂Si₃O₉:Eu²⁺,Mn²⁺.

Fig. 6 Plots of ln[−ln(I(t)/I₀) − (t/τ₀)] versus ln(t/τ₀)³. Experimental data are represented by the dots. Grey lines represent the fitting functions.

This finding is quite reasonable since the electronic [Xe]4f⁷–[Xe]4f⁶5d¹-transition in Eu²⁺ is allowed whereas the electronic [Ar]3d⁵–[Ar]3d⁵-transition in Mn²⁺ is forbidden. Therefore, the critical distance R_c for ET from Eu²⁺ to Mn²⁺ can be calculated using the spectral overlap method:^17,19

In this formula f_q = 10⁻¹⁰ and f_d = 10⁻⁷ are the oscillator strengths of dipole and quadrupole electronic absorption transitions, respectively, for Mn²⁺. The absorption cross section Q_A of Mn²⁺ can be expressed using the relationship Q_A = 4.8 × 10⁻¹⁶f_d derived by Blasse. λ_S is the emission wavelength of Eu²⁺. E is the energy of maximal spectral overlap. From this, the critical distance R_c is calculated to be 8.9 Å. This value is in accordance with the one calculated using Blasse's approach (9.6 Å).

To investigate the temperature dependence of the PL of co-doped Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉, PL spectra were recorded from 100 to 800 K. The obtained PL spectra are depicted in Fig. 7. The spectra demonstrate that with increasing temperature PL intensity of Eu²⁺ and Mn²⁺ decreases due to thermal quenching. The inset of Fig. 7 illustrates the normalized PL integrals versus their temperature. From this, activation energy E_A for thermal quenching can generally be estimated by fitting the data points by a Fermi–Dirac distribution:

Fig. 7 PL spectra of Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉ from 100 to 800 K and PL integral in dependence of the temperature (inset).

In this equation, I(T) is the luminescence intensity at a certain temperature and I₀ is the luminescence intensity at zero Kelvin. B is the quenching frequency factor, T is the temperature and k is the Boltzmann constant. However, due to an additional turning point, the present data points cannot be fitted by ordinary Fermi–Dirac distribution. Therefore, eqn (9) is expanded to a sum of two weighed Fermi–Dirac distributions:

Here, I_0,a and I_0,b are the PL intensities at zero Kelvin. A describes the fraction of the first term. From this, E_A,a and E_A,b were determined to be about 0.07 and 0.84 eV, respectively. Plugging in the values for E_A,a and E_A,b in eqn (11) yields T_1/2,a and T_1/2,b:

T _1/2 is the temperature at which PL intensity of a luminescent centre is decreased to one-half of its maximum. For Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉, T_1/2,a and T_1/2,b were calculated to be 242 and 666 K, respectively. The PL intensities originating from Eu²⁺ and Mn²⁺ were plotted separately (Fig. S2 and S3 in the ESI†). From that it can be seen that the bi-sigmoidal temperature behaviour strongly decreases if investigating the PL intensities of Eu²⁺ solely. Due to overlapping of the red emission band the bi-sigmoidal shape does not vanish completely. To examine the origin of this double sigmoidal quenching behaviour, PL of singly doped Ba_0.99Eu_0.01Si₃O₉ was measured in the temperature range from 100 to 800 K as well (Fig. S4 in the ESI†). At low temperatures an additional emission band extending from about 600 to 750 nm arises. PLD measurements of this emission band revealed a fluorescence lifetime of about 945 ns (Fig. S5 in the ESI†). This value matches well with the decay time of Eu²⁺. Furthermore, the obtained PLD curve shows mono-exponential decay behaviour, indicating emission from one particular crystallographic site. These Eu²⁺ ions presumably occupy the 6-fold coordinated Ca2 sites in BaCa₂Si₃O₉. On this small site, the crystal field splitting of the d-orbitals of Eu²⁺ is rather high because of which the lower edge of the [Xe]5d multiplet is moved towards the ground state. Due to relatively low emission energies in the red spectral range and consequently strong multi-phonon relaxation as well as large Stokes shift, this emission exhibits a lower quenching temperature. Furthermore, with increasing temperature a blue shift of Mn²⁺ emission can be observed. This behaviour is due to an increasing average distance between the Mn²⁺ ions and their oxygen ligands with increasing temperatures. As a consequence, the crystal field splitting of the 3d orbitals of Mn²⁺ decreases resulting in a shift of the ⁴T₁(⁴G) → ⁶A₁(⁶S)) transition to higher energies.²⁰

To obtain more detailed information regarding the temperature dependent behaviour of Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉, PLD measurements were performed from 100 to 500 K. Fig. 8 depicts the PLD curves of Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉ monitoring the 445 nm emission of Eu²⁺. The curves can be best fitted with a bi-exponential function, suggesting Eu²⁺ emission from two distinct crystallographic sites as mentioned previously. The derived fluorescence lifetimes are presented in Table 2 and show an unusual increase with increasing temperatures. This phenomenon was only reported by a few authors and is still uncommon.^21,22 According to Meijerink and Blasse, this behaviour is presumably caused by thermal population of higher energetic 4f ⁶5d states. Excited states in Eu²⁺ can either be (spin) octets or sextets. The sextet states are at higher energies than the octet states according to Hund's rule. With increasing temperature the higher energetic sextet states will be populated. For these states, transition to the ⁸S_7/2 ground state is spin-forbidden resulting in lower decay rates and thus longer fluorescence lifetimes.²² Fluorescence lifetimes of Mn²⁺ emission in dependency of temperature were measured, too. The obtained PLD curves are depicted in Fig. 9 and can be well fitted with a bi-exponential function. This result is consistent with the assumption made above that Mn²⁺ is located on two different crystallographic sites. The calculated average lifetimes of Mn²⁺ are summarized in Table 2. With increasing temperature, Mn²⁺ fluorescence lifetimes decrease continuously due to an increase in non-radiative relaxation to the ground state. Therefore, it can be concluded that thermal quenching of emission in BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ is primarily caused by Mn²⁺ ions. As deduced from PLD curves of Mn²⁺ emission as well as from ionic radii of Ba²⁺, Ca²⁺ and Mn²⁺, it can be concluded that Mn²⁺ ions preferably occupy the two Ca sites in BaCa₂Si₃O₉. Since the ionic radii of 6- and 8-fold coordinated Mn²⁺ are smaller compared to those of Ca²⁺, the excited states of Mn²⁺ can strongly be polarized. This leads to an increase in the equilibrium distance between Mn²⁺ and its oxygen ligands and therefore to a shift of the ground and excited state parabolas in the configurational coordinate diagram. Further, the crossing point of the parabolas is shifted to lower energy, and thus to a lower activation energy for thermal quenching.

Fig. 8 PLD curves of Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉ at 100, 300 and 500 K monitoring the emission of Eu²⁺. Fluorescence lifetimes in dependency of temperature (inset).

Fig. 9 PLD curves of Ba_0.99Eu_0.01(Ca_0.90Mn_0.10)₂Si₃O₉ at 100, 300 and 500 K monitoring the emission of Mn²⁺. Fluorescence lifetimes in dependency of temperature (inset).

Table 2 Fluorescence lifetimes τ as well as partial lifetimes τ₁ and τ₂, and the emission fractions frac₁ and frac₂ of Eu²⁺ and Mn²⁺ in Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ from 100 to 500 K

	Eu^2+					Mn^2+
T (K)	frac1 (%)	τ 1 (ns)	frac2 (%)	τ 2 (ns)	τ (ns)	frac1 (%)	τ 1 (ms)	frac2 (%)	τ 2 (ms)	τ (ms)
100	16	214	84	637	570	9	19	91	47	44
125	15	205	85	637	571
150	14	190	86	635	574	9	17	91	45	42
175	13	191	87	638	578
200	12	184	88	644	587	6	13	94	41	39
225	12	190	88	645	591
250	13	205	87	656	597	8	15	92	38	36
275	13	199	87	656	597
300	12	199	88	659	602	10	16	90	35	33
325	13	187	87	663	600
350	15	215	85	681	612	5	10	96	31	30
375	15	216	85	684	615
400	14	216	86	687	621	8	12	92	29	28
425	14	195	86	689	619
450	13	199	87	688	622	4	7	96	26	26
475	15	211	85	701	628
500	13	201	87	687	622	4	7	96	24	24

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To investigate the PL efficiency of the Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ samples, external quantum efficiencies η_ext were measured as mentioned above. To the best of our understanding, external quantum efficiency η_ext is defined as:

where N_emission is the number of photons emitted by the phosphor and N_absorption is the number of photons absorbed by the phosphor. The resulting quantum efficiencies η_ext are summarized in Table 3. For the investigated samples η_ext is around 30%. The low quantum efficiency of BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ can be ascribed to the overlap of the blue emission band and the excitation band of the red emission of Eu²⁺. Due to ET from the blue Eu²⁺ site to the red Eu²⁺ site which is thermally quenched, η_ext decreases.

Table 3 External quantum efficiencies η_ext and CIE colour coordinates of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉

Sample	η ext	CIE (x|y)
x = 0.000	0.31	(0.151|0.043)
x = 0.005	0.31	(0.163|0.053)
x = 0.010	0.30	(0.170|0.060)
x = 0.015	0.31	(0.182|0.073)
x = 0.020	0.32	(0.190|0.080)
x = 0.050	0.34	(0.224|0.112)
x = 0.100	0.36	(0.260|0.146)
x = 0.150	0.26	(0.323|0.210)
x = 0.200	0.30	(0.354|0.240)

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According to the relevant PL spectra, Commission International de l'Eclairage 1931 (CIE) chromaticity coordinates were calculated. Fig. 10 depicts the CIE chromaticity diagram for Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ with x = 0, 0.005, 0.010, 0.015, 0.020, 0.050, 0.100, 0.150, and 0.200. By increasing the Mn²⁺ concentration, the chromaticity coordinates can be shifted from blue to magenta colour tones. Due to the lack of emission in the green spectral range, BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ has to be combined with an additional green emitting phosphor to generate white light, such as (Ba,Sr)₂SiO₄:Eu²⁺, Sr₁₂Al₁₄O₃₂Cl₂:Eu²⁺ or KBaBP₂O₈:Tb³⁺.²³ Fig. S6† shows a simulated white emission spectrum consisting of Ba_0.99Eu_0.01Ca(_0.80Mn_0.20)₂Si₃O₉ and (Ba_0.49Sr_0.49Eu_0.02)₂SiO₄. The CIE 1931 colour coordinates of the spectrum are x = 0.327 and y = 0.333 which result in a correlated colour temperature (CCT) of 5745 K. The colour rendering index (CRI) R_a of the simulated spectrum is equal to 82. This CCT is similar to that of a phosphor-converted white LED consisting of a blue LED chip and YAG:Ce³⁺ (CCT = 5600 K) whereas the CRI is higher compared to that of this phosphor-converted LED (R_a = 71).²⁴

Fig. 10 CIE chromaticity diagram of Ba_0.99Eu_0.01(Ca_1−xMn_x)₂Si₃O₉ with different Mn²⁺ concentrations.

4. Conclusions

A series of BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ powder samples with various Mn²⁺ concentrations was prepared via high temperature solid state synthesis. Excited by UV radiation, BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ shows two emission bands located at about 444 and 594 nm originating from Eu²⁺ and Mn²⁺, respectively. It turned out that Eu²⁺ ions in BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ occupy the 8-fold coordinated Ba and Ca1 sites as well as the 6-fold coordinated Ca2 sites. Mn²⁺ ions tend to occupy both Ca sites. These results are confirmed by PLD measurements. The highest PL intensity of Mn²⁺ was found at a Mn²⁺ concentration of x = 0.15. It was further demonstrated that ET from Eu²⁺ to Mn²⁺ in BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ is of resonant type and occurs via dipole–quadrupole interaction. The critical distance R_c between Eu²⁺ and Mn²⁺ was derived by using Blasse's concentration quenching method as well as the spectral overlap method and was calculated to be 9.6 and 8.9 Å, respectively. Temperature dependent PL measurements prove a “double” sigmoidal decrease of the PL intensity of Eu²⁺. Therefore, BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ has two T_1/2 values, T_1/2,a and T_1/2,b, which were calculated to be 242 and 666 K, respectively. In addition to that, PLD measurements revealed an unusual increase of Eu²⁺ fluorescence lifetime, which presumably originates from thermal population of higher excited energy levels. Temperature dependent PLD measurements on the Mn²⁺ emission suggest that thermal quenching in BaCa₂Si₃O₉:Eu²⁺,Mn²⁺ is primarily caused by Mn²⁺ ions. CIE 1931 colour points can be tuned from the blue to the magenta colour range.

Acknowledgements

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

Notes and references

1. S. Nakamura, P. Stephen and F. Gerhard, The Blue Laser Diode: The Complete Story, Springer-Verlag, Berlin/Heidelberg, 1997.
2. (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.
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) W. Lv, M. Jiao, Q. Zhao, B. Shao, W. Lü and H. You, Inorg. Chem., 2014, 53, 11007–11014; (b) W.-J. Yang and T.-M. Chen, Appl. Phys. Lett., 2006, 88, 101903.
7. P. Eskola, Am. J. Sci., 1922, 4, 331–375.
8. J. T. Alfors, M. C. Stinson and R. A. Matthews, Am. Mineral., 1965, 50, 314–340.
9. L. S. Dent Glasser and F. P. Glasser, Am. Mineral., 1968, 53, 9–13.
10. M. C. Barkley, R. T. Downs and H. Yang, Am. Mineral., 2011, 96, 797–801.
11. S. Yao, L. Xue and Y. Yan, Opt. Laser Technol., 2011, 43, 1282–1285.
12. M. Gaft, H. Yeates and L. Nagli, Eur. J. Mineral., 2013, 25, 71–77.
13. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Cryst., 1976, 32, 751–767.
14. Y. Kawamura, H. Sasabe and C. Adachi, Jpn. J. Appl. Phys., 2004, 43, 7729–7730.
15. (a) A. Mehra, Jpn. J. Appl. Phys., 1968, 7, 963–964; (b) E. van der Kolk, P. Dorenbos, C. W. van Eijk, A. P. Vink, M. Weil and J. P. Chaminade, J. Appl. Phys., 2004, 95, 7867.
16. (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.
17. G. Blasse, Philips Res. Rep., 1969, 24, 131–144.
18. M. Inokuti and F. Hirayama, J. Chem. Phys., 1965, 43, 1978.
19. D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
20. Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn., 1954, 9, 766–779.
21. (a) J. P. Spoonhower and M. S. Burberry, J. Lumin., 1989, 43, 221–226; (b) T. Tsuboi and P. Silfsten, J. Phys.: Condens. Matter, 1991, 3, 9163–9167; (c) C. K. Duan, A. Meijerink, R. J. Reeves and M. F. Reid, J. Alloys Compd., 2006, 408–412, 784–787.
22. A. Meijerink and G. Blasse, J. Lumin., 1990, 47, 1–5.
23. (a) B. Han, J. Zhang, P. Li and H. Shi, Phys. Status Solidi A, 2014, 211, 2483–2487; (b) Z. Jiang, Z. Sun, X. Su, L. Duan and X. Yu, J. Alloys Compd., 2013, 577, 683–686; (c) T. L. Barry, J. Electrochem. Soc., 1968, 115, 1181–1184.
24. M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers and M. G. Craford, J. Disp. Technol., 2007, 3, 160–175.

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Footnote

† Electronic supplementary information (ESI) available: PLD curves of Ba_0.99Eu_0.01Ca₂Si₃O₉ monitored at 415 and 495 nm, PL integrals of Eu²⁺ and Mn²⁺ in dependence of temperature, PL spectra of Ba_0.99Eu_0.01Ca₂Si₃O₉ from 100 to 800 K and PL integral in dependence of temperature, PLD curve of Ba_0.99Eu_0.01Ca₂Si₃O₉ monitored at λ_em = 700 nm and T = 77 K, simulated white emission spectrum. See DOI: 10.1039/c5dt00591d

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