The role of Yb 3+ concentrations on Er 3+ doped SrLaMgTaO 6 double perovskite phosphors

Er 3+ , Yb 3+ co-doped SrLaMgTaO 6 phosphors have been synthesized by a solid state reaction method. Under a 975 nm excitation, the SrLaMgTaO 6 :Er 3+ /Yb 3+ phosphors exhibited strong green luminescence centered at 525 nm and 545 nm and relatively weak red anti-stokes luminescence centered at 660 nm. Herein, we have established the upconversion mechanism based on the down conversion and upconversion photoluminescence (UCPL) properties. The sensitized energy upconversion [s-ETU2: Yb 3+ ( 2 F 5/2 ) + Er 3+ ( 4 I 11/2 ) / Yb 3+ ( 2 F 7/2 ) + Er 3+ ( 4 F 7/2 )] is a main route of green UCPL, and the energy back transfer [EBT: Er 3+ ( 4 S 3/2 ) + Yb 3+ ( 2 F 7/2 ) / Er 3+ ( 4 I 13/2 ) + Yb 3+ ( 2 F 5/2 )] is a main route of red UCPL, which reduces the green UCPL to a certain degree. The reduced green and red UCPL of the sample with a high concentration of Yb 3+ is mainly attributed to the concentration quenching of the Yb 3+ ion.


Introduction
It is expected that the luminous behavior of lanthanide ions will play a signicant role for luminescent materials in specic applications, while the structure of the host matrix imposes great effects on the behavior of the lanthanide phosphors. 1,24][5][6][7][8] Among the lanthanide ions, Er 3+ ion is a promising competitor for UC luminescence centers due to its abundant energy levels with a long lifetime. 9Moreover, Yb 3+ ion is usually employed as the sensitizer for Er 3+ ion due to its large absorption cross section around 975 nm wavelength and the advantage of an efficient energy transfer process between Yb 3+ and Er 3+ ions.However, host materials are also important for obtaining efficient luminescent properties by the upconversion process.In order to obtain the efficient UC phosphors, it is generally believed that it requires lower phonon energies of the host materials, which leads to minimizing non-radiative losses.Additionally, the upconversion photoluminescence (UCPL) properties depend on many factors of the host materials such as the symmetry, lattice parameters, doping concentration of the activator and sensitizer ions. 10,11However, the knowledge related to the inuence of crystal structure on the UCPL is quite limited.
4][15] Since the energy transfer mechanism between donor and acceptor species has been treated within the framework of the Förster and Dexter approaches, 16,17 it is important to know the symmetry site of the donor and acceptor in the phosphors.In a typical double perovskite structure, both B and B 0 are coordinated by orderly arranged six oxygen atoms.A (or A 0 ) site has various coordination numbers from eight to twelve according to the distortion degree of the crystal structure.
In the several mechanisms for the upconversion processes, the main UC processes are ground state absorption followed by excited state absorption (GSA/ESA), energy transfer UC between same kinds of ions (GSA/ETU), and energy transfer UC between activator and sensitizer ion (GSA/s-ETU).The rates of energy transfer (ET) between donor and acceptor are strongly dependent on the crystal structure.In the SrLaMgTaO 6 crystal, two kinds of B ions are ordered and formed a three-dimensional network of alternating MgO 6 and TaO 6 octahedra.Since the tolerance factor (t) of complex perovskite SrLaMgTaO 6 are related to the ionic radii of Mg and Ta cations, t ¼ 0.952 < 1, 18 both of the MgO 6 and the TaO 6 octahedral tilting distortions exist in this crystal.To the best of our knowledge, there are no reports available on the synthesis and upconversion properties of the SrLaMgTaO 6 except our recent work. 19he present paper deals with the inuence of Yb 3+ concentration on the luminescence properties in SrLaMgTaO 6 phosphors with xed Er 3+ concentration to 7 mol% and inducing Yb 3+ concentration from 0% to 10.5 mol%.The crystal structure, optical properties and electronic structure of SrLaMgTaO 6 were investigated experimentally and theoretically.The upconversion luminescence behaviors of the phosphor have been discussed in detail with luminescence mechanisms.

Sample preparation
Crystalline powder samples of SrLaMgTaO 6 :Er 3+ (7%) codoped with Yb 3+ (0%, 3.5%, 7%, 10.5%) were prepared by a solid state reaction method.The SrCO 3 (99.994%),La 2 O 3 (99.99%),(MgCO 3 ) 4 Mg(OH) 2 $5H 2 O (99.99%), Ta 2 O 5 (99.99%),Er 2 O 3 (99.9%)and Yb 2 O 3 (99.99%)were used as starting materials.All the reagents were purchased from Sigma-Aldrich.The required amounts were weighed according to the stoichiometric ratio and ground to a ne powder using agate mortar and pestle.The mixture of starting materials was preheated at 600 C for 2 h and then calcined at 900 C for another 2 h and slowly cooled down naturally to room temperature.Finally, these materials were annealed at 1400 C at a rate of 5 C min À1 and kept for 12 h before cooling down to room temperature.

Characterization and optical measurements
The phase identication of the prepared phosphors was characterized by powder X-ray diffraction (XRD) analysis on a Philips X'Pert MPD diffractometer (Philips, Netherlands) using CuK a radiation (l ¼ 1.546 Å).The UCPL emission spectra were recorded with a uorescence spectrophotometer (Acton Spec-traPro 750-Triplet Grating Monochromator) from CCD detector (Princeton EEV 10241024 and PI-Max 133 Controller).All the measurements except photoluminescence (PL) spectra of SrLaMgTaO 6 were performed at room temperature.The sample was placed in a liquid helium ow cryostat for the measurements of PL spectra of SrLaMgTaO 6 at 20-300 K.

XRD patterns and crystal structure
The ionic radii of Er 3+ and La 3+ (R ¼ 1.17 Å, CN ¼ 12).Therefore, Er 3+ /Yb 3+ can be easily substituted into the Sr 2+ and La 3+ site, while the Mg 2+ (0.72 Å, CN ¼ 6) and Ta 5+ (0.64 Å, CN ¼ 6) sites are too small to occupy Er 3+ /Yb 3+ ions.The dopant ions were expected to occupy the site of La 3+ ion due to its similar oxidation state and larger ionic radius in the SrLaMgTaO 6 :Er 3+ /Yb 3+ , thus, the nominal formula of phosphors is written as SrLa (1ÀxÀy) -MgTaO 6 :xEr 3+ ,yYb 3+ .The structure of synthesized samples was examined by the measuring the XRD patterns, and all the samples exhibited monoclinic structure with space group P2 1 / n (Fig. 1).Within the synthesized dopant limits, all the diffraction peaks are well matched with those in the standard card NIMS (National Institute for Materials Science) without any other characteristic peaks related to raw materials or impurities.

Photoluminescence (PL) of SrLaMgTaO 6
The sample of SrLaMgTaO 6 was placed in a liquid helium ow cryostat for the measurements of PL spectra at 20-300 K. Fig. 2 shows the PL spectra of SrLaMgTaO 6 under an excitation wavelength of 355 nm.The broad and strong emission bands centered at 691 nm and 704 nm were observed at low temperature, which might be attributed to the existence of TaO 6 emission from SrLaMgTaO 6 host crystal.As increasing of the sample temperature, the total PL intensity was slightly reduced, and the PL intensity of the band centered 691 nm was highly reduced compared to that of the band centered 704 nm.To our knowledge, the strong red emission at room temperature has not been reported on tantalum based host lattices so far.However, the TaO 6 emission from SrLaMgTaO 6 host lattice was observed for the rst time.It has been reported that perovskite-  like tantalate powders have an intrinsic emission from TaO 6 group ranging from 340 nm to 490 nm at low temperature, and the emission is quenched by improving temperature.Some reports about PL emitted from Ta-O bond and Mo-O bond are listed in Table 1.

Upconversion photoluminescence of Er 3+ in SrLaMgTaO 6
The UCPL spectra of SrLaMgTaO 6 :xEr 3+ with different concentration of Er 3+ ions under a 975 nm excitation are shown in Fig. 3. Since the UCPL properties depend on the concentrations of the dopant ions of the activator, it is important to know the relative UCPL intensity emitted by a single Er 3+ ion as function of Er 3+ concentration.In order to compare the green intensities of SrLa (1Àx) MgTaO 6 :xEr 3+ (x ¼ 0.01, 0.03, 0.05, 0.07, 0.1, 0.12), same experimental condition was maintained.The PL intensity per mol is the intensity of green UCL emitted from x mol% sample divided by x.We obtained the value for 5 times, and the average values were displayed in inset of Fig. 2. Since the values nearly remained constant up to 7 mol% and then decreased rapidly above 10 mol% of Er 3+ concentration, we xed the Er 3+ concentration to 7 mol%.The decrement may be attributed to the concentration quenching effect.
3.4 Photoluminescence of SrLaMgTaO 6 :Er 3+ /Yb 3+ Fig. 4(a) shows the photoluminescence (PL) spectra of SrLaMgTaO 6 :Er 3+ /Yb 3+ under an excitation wavelength of 355 nm.Two typical green emission bands centered at 525 and 545 nm are assigned to the 2 H 11/2 / 4 I 15/2 , and 4 S 3/2 / 4 I 15/2 transitions, respectively.A relatively weak red emission band centered at 660 nm assigned to 4 F 9/2 / 4 I 15/2 transition.The strong emission band centered at 704 nm from Er 3+ single doped SrLaMgTaO 6 host crystal was observed, which was due to the TaO 6 emission.By incorporation of Yb 3+ ion, dramatically weaken PL seems to be due to the energy transfer from the TaO 6 band to 2 F 7/2 state of Yb 3+ which has high absorption crosssection.
Fig. 4(b) shows the PLE spectra of SrLaMgTaO 6 :Er 3+ /Yb 3+ monitored at 704 nm host emission.The wide band centered at 348 nm seems to be emitted from exciton of the crystal related to Ta-O charge transfer band.The excitation energy of SrLaMgTaO 6 is lower than those of other tantalate as can be seen in Table 1.It is well known that the energy of charge transfer transition from ligand to central metal ion is dependent on the coordination number. 24The orbital mixing of Ta 5+ and O 2À of TaO 6 octahedron is larger than that of TaO 4   tetrahedron.Since the required energy decreases to transfer an electron from O 2À to Ta 5+ ion, the CT band will move to lower energy region.And three sharp peaks at 351 nm, 368 nm and 385 nm comes from 4  Fig. 5 shows the UCPL spectra of SrLaMgTaO 6 :Er 3+ /Yb 3+ under an excitation wavelength of 975 nm.Two typical green UCPL bands and a relatively weak red UCPL band are observed, which are similar to those of down conversion luminescence (DCPL) bands shown in Fig. 4(a).However, the intensity of green DCPL is nearly independent on the concentration of Yb 3+ , the overall UCPL intensities are greatly enhanced with introducing sensitizer Yb 3+ ion owing to the efficient energy transfer from Yb 3+ to Er 3+ , which is also known as sensitized energy transfer upconversion (s-ETU).The intensity of UCPL has been continuously enhanced with increasing the Yb 3+ ion and reaches a maximum value at a concentration of 7 mol%.As the concentration of Yb 3+ has further increased, the UCPL intensity had reduced.Both of the concentration quenching rate among Yb 3+ ions and the rate of the energy back transfer (EBT) from Er 3+ to Yb 3+ [Er 3+ ( 4 S 3/2 ) + Yb 3+ ( 2 F 7/2 ) / Er 3+ ( 4 I 13/2 ) + Yb 3+ ( 2 F 5/2 )] become active with further doping of Yb 3+ , which seems to be the main reasons of the reducing UCPL intensity.Fig. 6 shows the temporal behaviours of the green emission of the 4 S 3/2 level under a 355 nm excitation with pulse duration of 6 ns.The decay times of the 4 S 3/2 level become shorter from 122 to 92 ms as the Yb 3+ concentrations increases from 0 to 10.5 mol%.It is attributed to the energy transfer from 4 S 3/2 state of Er 3+ to Yb 3+ ion.Since Yb 3+ ion has only two levels, the energy transfer from Er 3+ to Yb 3+ , so called EBT [EBT: Er 3+ ( 4 S 3/2 ) + Yb 3+ ( 2 F 7/2 ) / Er 3+ ( 4 I 13/2 ) + Yb 3+ ( 2 F 5/2 )] can be possible.The efficiency of EBT dened as the ratio of EBT rate to total emission rate of 4 S 3/2 state was calculated by the following equation: ; where, s(x) is the decay time of the 4 S 3/2 level in SrLaMgTaO 6 with x mol% concentration of Yb 3+ and 7 mol% of Er 3+ .The efficiency of EBT that has calculated from decay times is displayed in Fig. 7(a).Since the EBT plays a role to reduce the green UCL intensity, it is important to know EBT efficiency.It has increased linearly from 0 to 24% as the Yb 3+ concentration increases from 0 to 10.5 mol%.
If the UCPL green emission is only due to the ESA process, the decay times of the 4 S 3/2 under a 975 nm excitation will be similar with that of 355 nm excitation.However, by incorporation of sensitizer such as Yb 3+ , the sensitized energy transfer upconversion (s-ETU) process that has additionally participated in the UCPL process makes the decay curves delayed.Two different variations of the decay time as function of the Yb 3+ concentration are shown in Fig. 7(b).The pulsed light of a 975 nm was generated from the optical parametric oscillator pumped from the 3 rd harmonics of Nd:YAG laser with pulse duration of 6 ns.Small differences of decay time between UCPL and DCPL at x ¼ 0 is attributed to the ETU process which is more active in the highly Er 3+ dopant sample.The concentration of Er 3+ of all samples is xed to 7.0 mol%.By addition of sensitizer Yb 3+ ion, the DCPL decay times has reduced due to the EBT, however, the UCPL decay times had increased due to the s-ETU from Yb 3+ to Er 3+ .As seen in Fig. 5, increase of the UCPL intensities up to 7 mol% of Yb 3+ concentration are attributed to the enhanced s-ETU.At highly dopant sample, the excited state of Yb 3+ ( 2 F 5/2 ) will be quenched by the highly enhanced energy migration among Yb 3+ , which makes the  reduction of s-ETU rate.The rate of s-ETU which is dependent on the concentration of sensitizer is limited by the concentration quenching of sensitizer.In short, lower value of the UCPL decay time at x ¼ 10.5 sample are resulted from the concentration quenching of Yb 3+ and the EBT from Er 3+ to Yb 3+ .
The possible populating process of the Er 3+ and Yb 3+ for the UCPL under the excitation of 975 nm are shown in Fig. 8.By absorbing NIR photons, GSA of Yb 3+ ion and Er 3+ ion takes place as following: 2 F 7/2 + a photon (975 nm) / 2 F 5/2 in Yb 3+ and 4 I 15/2 + a photon (975 nm) / 4 I 11/2 in Er 3+ .Most of incident energy is absorbed by the Yb 3+ ions due to the larger absorption cross section around 975 nm wavelength comparing to that of Er 3+ .Therefore, the population at excited level 4  ( 2 F 7/2 ) / Er 3+ ( 4 I 13/2 ) + Yb 3+ ( 2 F 5/2 )].As can be seen in Fig. 5, the red UCPL intensity of the sample doped with 7 mol% Er 3+ is very smaller than that of co-doped with 7 mol% Er 3+ and 7 mol% Yb 3+ .Considering the multiphonon relaxation and the CR processes are independent of the concentration of Yb 3+ ion, the UCPL red emission is mainly attributed to the EBT process.As increasing Yb 3+ concentration, the enhanced rate of EBT results in higher red UCPL emission.As further doping of Yb 3+ up to 7 mol%, the population of 2 F 5/2 state that is an initial state of s-ETU2 process will be quickly reduced owing to the concentration quenching of Yb 3+ .Since the initial state of EBT, Er 3+ ( 4 S 3/2 ), is easily populated via a multiphonon relaxation from the nal state of s-ETU2, Er 3+ ( 4 F 7/2 ), the reduction of s-ETU2 process is followed by reduction of EBT.Consequently, the reduced green and red UCPL of the sample with 10.5 mol% Yb 3+ is chiey attributed to the concentration quenching of Yb 3+ .
The logarithmic scale dependence of the UC emission intensities of 4 S 3/2 / 4 I 15/2 (green) transition on the working pump power over the range of 100 to 1000 W in the SrLaMgTaO 6 :Er 3+ /Yb 3+ phosphor is shown in Fig. 9.The number of photons required to populate the upper level under unsaturated condition can be described by relation: I up ¼ (P pump ).Where I up is the UCPL emission intensity, P pump is the power of the pump laser and n is the number of pumped photons required in the mechanism which is indicated by the slope.The value of n obtained in SrLaMgTaO 6 :Er 3+ /Yb 3+ phosphors with the monoclinic structure were 1.56 for green UCL.The slope value of 4 S 3/2 level of Er 3+ indicate that the two photon processes are mainly responsible for green emission.Fig. 8 The schematic energy level diagrams of Er 3+ and Yb 3+ ions in the SrLaMgTaO 6 :Er 3+ /Yb 3+ and the possible mechanisms of the upconversion.

Fig. 2
Fig. 2 Photoluminescence spectra of SrLaMgTaO 6 at different temperature under an excitation wavelength of 355 nm.

Fig. 3
Fig.3The UCPL spectra of SrLaMgTaO 6 :xEr 3+ with different concentration of Er 3+ under a 975 nm excitation.The inset shows the UCPL intensity variation per mol as function of Er 3+ concentration.

Fig. 7
Fig. 7 (a) Decay time of the 4 S 3/2 EBT rate from Er 3+ to Yb 3+ ions in SrLaMgTaO 6 phosphors with different concentration of Yb 3+ ion.(b) The decay time of downconversion and upconversion as function of the Yb 3+ concentration.