Spectroscopic properties, concentration quenching and Yb3+ site occupations in vacancied scheelite-type molybdates

Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Spectroscopic properties, concentration quenching and Yb3 þ site occupations in vacancied scheelite-type molybdates M. Guzik a,n, E. Tomaszewicz b, Y. Guyot c, J. Legendziewicz a, G. Boulon c a

Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Al. Piastów 42, 71-065 Szczecin, Poland c Institute Light Matter (ILM), UMR5306 CNRS-University of Lyon 1, University of Lyon, 69622 Villeurbanne, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 8 January 2015 Received in revised form 18 February 2015 Accepted 19 February 2015

A series of micro-crystalline Yb3 þ -doped vacancied Cd1 3xYb2x□xMoO4 solid solutions has been prepared by a high-temperature solid state reaction method. The structural studies performed by X-ray powder diffraction measurements have shown that the samples are monophasic and crystallize in the tetragonal scheelite-type structure (the space group I41/a, with point symmetry close to D2d) when the x parameter is greater than 0 and does not reach 0.1430 (33.36 mol% of Yb3 þ ions). The substitution of divalent Cd2 þ by trivalent Yb3 þ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □), due to the charge compensation: 3Cd2 þ -2Yb3 þ þ □ vacancy. Direct excitation of Yb3 þ by means of 2F7/2-2F5/2 absorption at 940–980 nm leads to reversed 2F5/2-2F7/2 transitions giving Yb3 þ emission in the range of 970–1130 nm. The intense and broad emission lines of Yb3 þ ions, which are also used as a structural probe at 77 K have been observed. The existence of more than one component of the 0-phonon line at 975 nm and 976.6 nm indicate two Yb3 þ distribution sites, which is in agreement with results obtained for the Nd3 þ ion. Basing on the absorption and emission spectra the Yb3 þ electronic energy levels have been proposed. The effect of dopant concentration had an influence on luminescent properties but had no influence on the powder morphology. Yb3 þ concentration dependences of the 2F5/2 experimental decay time were analyzed in order to attempt the understanding of the concentration quenching mechanism and estimate the main parameters useful for a theoretical approach of laser potential. & 2015 Elsevier B.V. All rights reserved.

Keywords: Yb3 þ dopant Cadmium molybdate Vacancied micro-powders Photoluminescence Concentration quenching Self-trapping

1. Introduction Among the rare-earth ions, Yb3 þ is particularly often studied because of its simple 4f13 electronic configuration consisting of the 2F7/2 ground and 2F5/2 excited state manifolds separated by approximately 10,000 cm  1 [1,2]. The Yb3 þ ion exhibits efficient luminescence in a whole variety of hosts, since the large inter-manifold energy gap and the lack of intermediate levels reduce the probability of a non-radiative decay. Potential of Yb3 þ rare earth ions in optical materials has been searched for various kinds of applications such as CW and pulsed lasers, scintillators or structural probes in solids. The intense absorption lines are well suited for laser diode pumping near 980 nm and the small quantum defect between absorption and emission (about 650 cm  1), much smaller than Nd3 þ -doped laser hosts, reduces the thermal loading of the material during laser operation [1]. It has been shown that Yb3 þ -doped laser crystals have many advantages over Nd3 þ -doped crystals (for example YAG), mainly under n

Corresponding author. E-mail address: [email protected] (M. Guzik).

laser diode-pumping, such as a small quantum defects between the pump and the laser photons resulting in low thermal loading, long radiative life time of the upper laser level, broad absorption band width, and no excited-state absorption [3]. The important goal of research is to understand physical processes in solids doped or co-doped with tivalent ytterbium ions, and the large band gaps of inorganic hosts are very appropriate for this kind of study [4]. While searching for interesting laser host materials for solid state lasers, alkali rare-earth double molybdates, tungstates, phosphates or alkali metal halides were studied [5–9]. In this case it is possible to analyze the electronic structure with two multiplets, one excited state (2F5/2) above the ground state (2F7/2), and also the charge transfer bands, involving allowed transitions between the p levels of ligands (oxygens, halides) forming the valence band of the crystal and the 4f states. During the last decade, with advances of high performance GaAs and InGaAs laser diodes emitting wavelengths between 900 and 1100 nm, interest on trivalent ytterbium (Yb3 þ ion)-doped crystals has been renewed for applications in high-efficiency (450%) and high power ( 450 W/cm) diode-pumping laser

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systems [1]. Both neodymium- and ytterbium-doped KGd(WO4)2 (KGW) and KY(WO4)2 (KYW) single crystals have become very important laser materials for the near-infrared region. Their emission cross-sections are higher than that of YAG at 1.067 μm. Such laser sources emitting at 1.351 μm can be used for medical applications. Thus, especially KGW and KYW tungstates doped with Yb3 þ ions show high potential for light generation or amplification [10–12]. Tungstates and molybdates are very wellknown compounds widely applied as matrices for lanthanide based laser and phosphors and even though are studied since a more than 50 years, they are still the object of study of many researchers. Since several years our team has been performing a systematic study of new RE3 þ -doped tungstates and molybdates for application as optical materials [13–18]. In the last period our attention was focused on RE3 þ -doped CdMoO4 matrix, which is an interesting task to search for the optical and laser materials with improved optical parameters. Already performed studies of solid solutions containing Nd3 þ ions have shown that both short lifetimes and high emission intensities suggest that the phases investigated here could be especially promising for an ultra-short pulse laser generation [19]. In this work, Yb3 þ laser ions have been introduced. Our interest is primarily concerned with characterization of structure and morphology, as well as, the absorption, luminescence spectra, and decays of Yb3 þ activated molybdate micro-powders. The effect of dopant concentration had an influence on luminescent properties but had no influence on the powders morphology. Identifying the origin of the concentration dependence of the observed experimental lifetime, the excited state dynamics should be divided into two regimes: first, the indication of fluorescence re-absorption, socalled self-trapping process, and, secondly, the usual quenching energy transfer process [20–23]. We should not forget that in case of the structure of Cd1  3xYb2x□xMoO4 solid solutions it is possible to create the Yb3 þ pair ions due to the substitution of three divalent Cd2 þ ions by two trivalent Yb3 þ ions in order to respect the charge compensation law, which consequently leads to the formation of cationic vacancies (□).This is the general way of substitution in luminescent materials when a divalent cation is substituted by a trivalent one. Moreover, according to our data on Nd3 þ -doped molybdates, the

creation of Nd3 þ –Nd3 þ pairs with short distance (3.908 Å) [19] and even shorter Yb3 þ –Yb3 þ pairs (3.66 Å) can be considered as dimer states which affect the spectral properties of the system by giving rise to cooperative emission around 500 nm. The emission from this state could be expected. That process will be a subject of our next paper. The excess charge due to Yb3 þ -Cd2 þ substitution could be compensated by the formation of Cd2 þ vacancy or interstitial oxygen Oi2  on two Yb3 þ ions, ̇ which forms the dipole complexes of [2(Yb3Cdþ ) –V″ Cd] and ̇ [2(Yb3Cdþ ) –O ″], respectively [24]. An another approach of charge i compensation is an addition of monovalent alkali metal like Li þ , Na þ or K þ [25–27]. In case of CdMoO4–Yb2(MoO4)3 solid solutions , the system is completely different than M′M″O4–RE2O3, (M′¼ Cd, Pb; M″ ¼ Mo, W). As cadmium molybdates and ytterbium molybdates are stoichiometric compounds, the probability of presence of interstitial oxygen Oi2  is very low. Also, we decided not to compensate charge by monovalent ions because in some examples the vacancies can increase emission intensities. Thus, in case of Cd1  3xYb2x□xMoO4 solid solutions presented in this paper only the “3Cd2 þ -2Yb3 þ þ vacancy” mechanism is taken into account, so that concentration dependence of vacancies on the structural and optical properties are investigated for the first time.

2. Experimental section 2.1. Synthesis CdMoO4 and Yb2(MoO4)3 were used as the starting materials for synthesis of Cd1  3xYb2x□xMoO4 solid solutions. Cadmium molybdate was prepared by heating an equimolar mixture of CdO with MoO3 (both reactants with purity 99.9%, Aldrich) in the thermal conditions described previously [28,29]. Ytterbium molybdate was obtained by sintering of Yb2O3 (99.998%, Stanford Materials) with MoO3 mixed in the 1:3 molar ratio, and in analogously thermal conditions used by us in the synthesis of other rare-earth molybdates [28,29]. CdMoO4 and Yb2(MoO4)3 mixed in appropriate molar ratios were sintered in static air, in corundum

Table 1 Crystallographic characteristic of Cd1  3xYb2x□xMoO4 for different values of x parameter. No.

Composition of initial samples

Formula of Cd1  3xYb2x□xMoO4 solid solution, value of x parameter

Conc. of Yb3 þ (mol%)

Number of Yb3 þ ion per volume (ion/cm3)

1

CdMoO4

x ¼0

0

0

2

0.25% Yb2(MoO4)3 99.75% CdMoO4 0.50% Yb2(MoO4)3 99.50% CdMoO4 2.00% Yb2(MoO4)3 98.00% CdMoO4 2.50% Yb2(MoO4)3 97.50% CdMoO4 5.00% Yb2(MoO4)3 95.00% CdMoO4 7.50% Yb2(MoO4)3 92.50% CdMoO4 10.00% Yb2(MoO4)3 90.00% CdMoO4 12.50% Yb2(MoO4)3 87.50% CdMoO4 15.00% Yb2(MoO4)3 85.00% CdMoO4

Cd0.9925Yb0.0050□0.0025MoO4 x ¼0.0025 Cd0.9850Yb0.0100□0.0050MoO4 x ¼0.0050 Cd0.9424Yb0.0384□0.0192MoO4 x ¼0.0192 Cd0.9286Yb0.0476□0.0238MoO4 x ¼0.0238 Cd0.8635Yb0.0910□0.0455MoO4 x ¼0.0455 Cd0.8044Yb0.1304□0.0652MoO4 x ¼0.0652 Cd0.7483Yb0.1678□0.0839MoO4 x ¼0.0839 Cd0.7000Yb0.2000□0.1000MoO4 x ¼0.1000 Cd0.6538Yb0.2308□0.1154MoO4 x ¼0.1154

0.50

6.46  1019

1.01

1.43  10

20

3.92

4.96  1020

4.88

6.14  1020

9.53

21

3 4 5 6 7 8 9 10

1.17  10

21

13.95

1.67  10

18.32

2.13  1021

22.22

2.51  1021

26.09

21

2.88  10

Lattice parameters (Å)

Density (g cm  3)

a

c

exp.

cal.

0.5155 (7) 0.5156 (2) 0.5157 (3) 0.5159 (7) 0.5160 (2) 0.5162 (3) 0.5164 (3) 0.5167 (8) 0.5167 (8) 0.5169 (4)

1.1210 (9) 1.1213 (5) 1.1216 (2) 1.1220 (5) 1.1224 (2) 1.1233 (3) 1.1244 (1) 1.1253 (2) 1.1261 (3) 1.1269 (1)

6.05

6.06

6.06

6.06

6.05

6.06

6.05

6.06

6.04

6.06

6.02

6.05

5.96

6.03

5.92

6.00

5.85

5.97

5.80

5.96

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crucibles, and the following heating treatment was used: 1123 K (12 h), 1223 K (12 h), 1273 K (12 h), 1323 K (12 h), and 1353 K (12 h). After each heating period, all samples were cooled slowly down to ambient temperature, and for better reactivity ground in an agate mortar, and next examined for their content by the powder XRD method. The synthesis of samples under study can be described by the following equation: (1–3x) CdMoO4(s) þ x Yb2(MoO4)3(s) ¼Cd1  3xYb2x□xMoO4(s)

(1)

Table 1 shows the chemical formula of Cd1  3xYb2x□xMoO4 solid solutions under study and the corresponding concentration of Yb3 þ ions.

3

900 l/mm grating blazed at 1300 nm. For the comparative measurements of the integral intensity, a LED source at 590 nm was used. Emission spectra were recorded at room and liquid nitrogen temperature. 2.2.8. Luminescence decay measurements The luminescence decay curves were recorded under pulsed laser excitation (OPO laser, EKSPLA NT342, 10 Hz, 7 ns), the fluorescence intensity around 1.06 μm being detected with a R1767 Hamamatsu photomultiplier through a HRS1 Jobin-Yvon monochromator equipped with a 1 μm blazed grating and coupled to a LECROY LT 342 digital oscilloscope. The luminescence decay curves were recorded at RT and 77 K.

2.2. Apparatus 2.2.1. XRD phase analysis Room temperature (RT) powder XRD method was used to identify the phase purity and the crystal structure of solid solutions. X-ray diffraction patterns were collected over the angular range 2Θ ¼ 10–70° in continuous scanning mode with the scan rate of 0.013° per step on EMPYREAN II diffractometer (PANalytical) using Cu Kα1,2 radiation (λ ¼ 0.15418 nm). XRD patterns were analyzed by HighScore Plus 4.0 software and lattice parameters were calculated using the least squares refinement procedure by POWDER software [30]. 2.2.2. DTA–TG measurements Simultaneous DTA and TG measurements were carried out on a TA Instruments thermoanalyzer (model SDT 2960, USA) at the heating and cooling rates of 10 K/min to the maximum temperature of 1573 K in air (flow of 110 ml/h). 2.2.3. Density measurements The density of samples under study was measured on a Ultrapycnometer 1000 Quantachrome Instruments (model Ultrapyc 1200e, USA) using argon (99.999%) as a pycnometric gas. 2.2.4. FT-IR and Raman mesurements FT-IR spectra of powdered samples in the 1000–80 cm  1 spectral range were measured using the Specord-M-80 spectrometer (Carl Zeiss Jena). Raman spectra were recorded with Nicolet Magna 860 FTIR/FT Raman spectrometer at an excitation line of 1.064 microns with a capacity of about 400–500 mW. The powdered samples were mixed with Nujol oil (a mixture of liquid hydrocarbons) and then pressed into pellets. For Raman studies an apparatus was equipped with a CaF2 beam splitter and a detector made of InGaSe. The polycrystalline samples were placed in a quartz tube. The spectral resolution of Raman and IR measurements was 2 cm  1.

3. Results and discussion 3.1. XRD analysis Room temperature XRD analysis was used for the identification of samples obtained after last period of annealing of initial CdMoO4/Yb2(MoO4)3 mixtures. Fig. 1 shows the representative X-ray powder diffraction patterns of Cd1  3xYb2x□xMoO4 stable solid solutions containing different concentrations of Yb3 þ ions, compared with standard from ICSD (#084455). No extra diffraction lines corresponding to starting materials, CdO as well as other known ytterbium molybdates (i.e. Yb2MoO6 or Yb6MoO12) were observed. Samples of Cd1  3xYb2x□xMoO4 solid solution are monophasic, and crystallize in the tetragonal scheelite type structure (the space group I41/a) when the x parameter is greater than 0 and does not reach 0.1430 (33.37 mol% of Yb3+). The lattice parameters calculated by cell refinement fitting on the basis of XRD data for monophasic samples under study successively increase with increase of x value, which corresponds to an increasing content of Yb3 þ ions (0.5–26 mol%). The dependence of lattice constants (a and c) as well as unit cell volume (V) vs. the value of x parameter are nearly linear, i.e. both unit cell parameters and V fulfill the Vegard law. We have observed a slight shift of the diffraction peaks towards higher 2Θ angles with increasing amounts of Yb2(MoO4)3 in the initial mixtures (difference in the ionic radii CN-8 Cd2 þ ¼1.1 Å and Yb3 þ ¼0.985 Å). Basing on differences of Nd3 þ and Yb3 þ ionic radii and Nd3 þ –Nd3 þ distance

2.2.5. Scanning electron microscopy (SEM) Scanning electron microscopy studies were carried out on Hitachi S-3400N equipped with an energy dispersive X-ray spectroscopy (EDX) EDAX analyzer. The powders were coated with thin gold alloy layer to facilitate conductivity. 2.2.6. Absorption measurements Absorption spectra in the 200–2500 nm spectral range were recorded at 4 and 293 K with a Cary-Varian 5000 Scan spectrometer equipped with an Oxford CF 1204 helium flow cryostat. The pellets used for the absorption measurements were prepared under 20 MPa pressure. 2.2.7. Emission measurements Emission measurements under CW titanium sapphire laser were performed with the help IR Hamamatsu CCD camera with a

Fig. 1. X-ray powder diffraction patterns of selected Cd1  3xYb2x□xMoO4 solid solutions with different contents of the optically active ion. Simulated XRD pattern of tetragonal CdMoO4 according to ICSD#084455 is presented by ticks marks at the bottom of the figure.

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(3.908 Å) in Cd1  3xNd2x□xMoO4 molybdates, [14] the Yb3 þ –Yb3 þ distance was recalculated and it is equal to 3.66 Å. The shifting of all diffraction lines towards lower 2Θ angles, according to the Vegard law, was clearly observed for similar compositions Cd1  3xNd2x□xMoO4 where Nd3 þ ions were slightly larger (1.109 Å) than Cd2 þ (1.1 Å) [19]. 3.2. DTA studies Our XRD studies have shown that the range of existence of Cd1 3xRE2x□xMoO4 solid solutions decreases with decreasing of RE3 þ ionic radii in the lanthanide series. It means that from Pr3 þ (for which it is possible to obtain 50 mol% of active ion in the CdMoO4 matrix) towards the elements with smaller radii like Yb3 þ the maximum concentration of this ion for possible incorporation into CdMoO4 matrix network is equal to only 33.36 mol%. We have found that a melting point of the solution within the series (i.e. for same RE3 þ ) decreases with increasing the RE3 þ content in the solution. For these solid solutions their melting point is always lower than for pure CdMoO4. Fig. 2 presents DTA curves recorded during heating and cooling of the pure matrix and Cd1  3xYb2x□xMoO4 solutions under study. The endothermic effect starting at 1405 K is due to congruent melting of cadmium molybdate. The crystallization process from the resulting melt starts at little higher temperature, i.e. 1417 K. The melting point of CdMoO4 is very close to the values obtained in our previous studies [28,29]. Additionally, DTA studies of Cd1  3xYb2x□xMoO4 solid solutions with concentrations of Yb3 þ ions of 1 and 4 mol% were carried out. The thermal studies show that only one endothermic effect is observed on DTA curves of each solid solution. This effect is connected with melting of the samples. As in the case of analogous solid solutions for other rare earth ions [29], their melting point is lower than that of the pure matrix and decreases with increasing the content of Yb3 þ ions. Fig. 3. Scanning electron micrographs presenting the morphology and particle size of Cd0.9424Yb0.0384□0.0192MoO4 (3.92 mol%) solid solutions.

heating

I1405

DTA signal (a.u.)

matrix

cooling

matrix 3+

1.01%Yb

1417 I

1402 3+

3.92%Yb

I

1400

exo up 1000

1200 1400 Temperature (K)

Fig. 2. DTA curves of CdMoO4 matrix: heating and cooling); CdMoO4:Yb3 þ (1.01 mol%) (Cd0.9850Yb0.0100□0.0050MoO4 solid solution); and CdMoO4:Yb3 þ (3.92 mol%) (Cd0.9556Yb0.0296□0.0148MoO4).

3.3. Morphology and particle size by SEM The annealing temperatures applied to obtain the solid solutions with Yb3 þ ions were more than 100 K higher than that for the analogs with Nd3 þ ones. This is immediately reflected in the morphology of the samples because the grain size is much larger than for the recently observed for Nd3 þ -doped solid solutions (1–10 μm) [19]. Fig. 3 presents the morphology of microcrystallites of cadmium and ytterbium molybdates with 3.92 mol% of optically active ion. The micrographs show homogenous particles due to the high temperatures of their synthesis. The product crystallizes in the compact form of an average size of about 60–135 μm. The particles are well separated and the grain boundaries between the microcrystals can be clearly seen. The grains are of irregular with sharp ends and do not resemble to spherical-like particles which were previously observed for the Cd1  3xNd2x□xMoO4. The energy dispersive X-ray microanalysis (EDS) (not presented here) of the Cd1  3xYb2x□xMoO4 phases exhibit the peaks related to Cd, Mo, O, and Yb elements and the intensity ratio of recorded peaks confirms the quantitative composition of the samples under investigation. 3.4. Spectroscopic analysis 3.4.1. FT-IR spectra Fig. 4 shows the FTIR spectra of CdMoO4 and Cd1 3xYb2x□xMoO4 with different concentrations of Yb3 þ ions recorded at RT. The fundamental frequencies of the MoO42 anions in an aqueous solution

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are: 894 cm  1 (ν1), 407 cm  1 (ν2), 833 cm  1 (ν3), and 318 cm  1 (ν4) [7–9,31]. For solid molybdates with the scheelite-type structure (isolated MoO4 tetrahedra, Cd1 3xRE2x□xMoO4, RE¼Pr, Nd, Sm–Dy, and for 0oxr0.25) the stretching multiples (ν1 and ν3) appear with vibration frequencies of 908–675 cm  1 as well as bending modes (ν2 and ν4) in the 440–260 cm  1 range [32,33]. The vibrational spectra of pure cadmium molybdate and ytterbium doped solid solutions are very similar to each other because of the number, location and intensity of registered absorption bands (Fig. 4). Analysis of these spectra clearly confirms the presence of MoO4 in the structure of analyzed samples and thereby the scheelite-type structure. 3.4.2. Yb3 þ energy level diagram and structure The Yb3 þ ions have only two Stark-split energy manifolds: the 2 F7/2 ground state and the 2F5/2 excited state. In Fig. 5, Stark levels are distributed in these manifolds and labeled from 1 to 4 in the ground state and from 5 to 7 in the excited state from the lowest energy to the highest energy, respectively. Depending of the local symmetry the two manifolds are split by the crystal field. In the CdMoO4 tetragonal lattice, with lattice parameters a¼0.5155(7) Å and c¼1.1210(9) Å (see Table 1), cadmium ions are occupying octahedral sites very close to D2d symmetry. Yb3 þ ions, characterized by an ionic radius of 98.5 pm, can substitute Cd2 þ (1.10 Å) sites. Recently, we have presented the crystal structure for the analog with Nd3 þ ion (1.109 Å) [19], which consists of CdMoO4 with CdO8 dodecahedra and MoO4 tetrahedra. The scheelite-type structure was made up from CdO8 polyhedra and MoO4 tetrahedra

ν

ν

ν

5

connected by common vertices. CdO8 polyhedra connect via the edges and form a 3D framework. Each Mo6 þ atom is surrounded by four equivalent O2 atoms at distances of 1.75 Å in approximately tetrahedral symmetry. Each Cd2 þ atom is surrounded by eight O atoms at distances of 2.40 Å and 2.44 Å in approximately octahedral symmetry. The Cd2 þ cations in CdMoO4 can be substituted by trivalent rare earth ions. The phases show a random distribution of Nd3 þ . The distance between the Cd2 þ /Nd3 þ ions is equal to 3.908 Å. Indeed this is the distance between two Nd3 þ pair ions due to the substitution of three divalent Cd2 þ ions by two trivalent Nd3 þ ions in order to respect the charge compensation law and consequently leads to the formation of cationic vacancies (□). As we mentioned before, in case of the system presented now, the recalculated Yb3 þ –Yb3 þ distance is even shorter (3.66 Å). The spectroscopic studies performed for Nd3 þ ions in this matrix have highlighted two main sites in the description of the local structure for CdMoO4 doped by different quantities of neodymium ions. Each of such Nd3 þ ions has not only Cd2 þ atoms but also □ cationic vacancies in their neighborhood. This should give one type of local symmetry for all Nd3 þ foreign ions if the □ vacancy was located always at the same place. But in case of these solid solutions it seems that the vacancy should be located mainly in two positions to create two main distributions of Nd3 þ sites. Yb3 þ ions, characterized by ionic radius of 0.985 Å, can substitute Cd2 þ (1.10 Å) sites and a similar situation is observed now for Cd1 3xYb2x□xMoO4 solid solutions where the number of components observed in the emission spectra suggests two types of local symmetries of Yb3 þ ions. For both Nd3 þ and Yb3 þ ions the probability to create pairs and aggregates is high. 3.4.3. Assignment of absorption spectra Fig. 6 presents the absorption spectra for 9.53 mol% Yb3 þ in Cd1  3xYb2x□xMoO4 recorded at room and liquid helium temperature. At RT the spectra consist of broad weakly resolved bands located between 850 nm and 1050 nm corresponding to the transition from the 2F7/2 ground state of Yb3 þ to the three Stark components of the 2F5/2 excited state. The absorption line corresponds to the so-called zero phonon line located at around 974.3 nm (10,264 cm  1) and is not the strongest absorption line. This is the transition from the lowest Stark component of the 2F7/2(1) ground state to the lowest Stark level of the 2F5/2(5). Two others lines of comparable intensities are located at around 963 nm (10,384 cm  1) corresponding to 2F7/2(1)-2F5/2(6) and 933 nm (10,718 cm  1) to 2 F7/2(1)-2F5/2(7) transitions. An additional component observed at RT at 993 nm (10,070.5 cm  1) corresponds to the absorption from

Fig. 4. FT-IR spectra of CdMoO4 and Cd1  3xYb2x□xMoO4 with different concentrations of Yb3 þ ions recorded at RT.

Fig. 5. Electronic energy level diagram and optical transitions expected with Yb3 þ ions at low temperature.

Fig. 6. Absorption spectra of Cd0.8635Yb0.0910□0.0455MoO4 (9.53 mol%) solid solution recorded at RT and 4.2 K.

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Fig. 8. Emission spectra of Cd1  3xYb2x□xMoO4 solid solutions recorded at RT and 77 K.

the higher energy Stark components of the ground state. The measurements performed at 4.2 K lead to narrowing and better resolution of the main three electronic Stark components. According to the crystal-field theory of Kramer's ions, the maximum of allowed components splitting for J¼5/2 state is three; therefore, for one symmetry site the absorption spectrum at liquid helium temperature should be resolved into three bands. The 0-phonon line is a sharp one and contains only one component at 4.2 K. The total splitting of the excited 2F5/2 state 454 cm  1 is smaller than that observed for oxide matrices (about 793 cm  1) [34], K5Bi(MoO4)4 molybdates [35], or double phosphates (750–800 cm  1) [5,6] and is similar to fluoride hosts (about 420 cm  1) in which vacancies also occur [36]. Fig. 7 shows the 2F7/2-2F5/2 transition in the absorption spectra of Cd0.8635Nd0.0910□0.0455MoO4 (9.53 mol%) recorded at RT. The s absorption and emission cross-sections are important parameters

Fig. 7. The 2F7/2-2F5/2 transition of the absorption spectra of Cd0.8635Yb0.0910□0.0455MoO4 (9.53 mol%) recorded at RT.

for laser application. The assigned absorption cross-section of the 1– 5 transition at 974.3 nm is equal to 1.45  10  19 cm2 with number of Yb3 þ ions per volume N0 ¼6.32  1020 ions/cm3 (for 9.32 mol%). This parameter was calculated in the same way as for analogs samples with Nd3 þ [19] or Nd3 þ -doped tungstates [16]. For double tungstates these values are as follows: 1.28  10  20 cm2 for Nd3 þ -doped KGdW, while for Yb3 þ -doped KGdW is 1.2  10  19 cm2 and 1.33  10  19 cm2 for Yb3 þ -doped KYW [37,38]. 3.4.4. Photoluminescence spectra The selected emission spectra of the samples of Cd1  3xYb2x□xMoO4 registered at different temperatures and doped with different Yb3 þ contents are plotted in Fig. 8. The studied materials exhibit the luminescence in the near-infrared region from the 2F5/2-2F7/2 transitions of the Yb3 þ ions. At RT the bands are very broad and the components are very difficult to identify. In the case of the Yb3 þ ion, one should observe transitions into four Stark components of the ground 2F7/2 split by the crystal field. It corresponds to the transitions from the lowest Stark level of the 2F5/2 excited state (5) to each of the four Stark levels of the 2F7/2 state (1, 2, 3, 4) respectively. The (5-1) component is the so-called 0-phonon line at around 975–977 nm. In case of Cd1 3xYb2x□xMoO4 solid solutions at 4.2 K the intensity of this line for all investigated concentrations of Yb3 þ ions is not very high due to the well- known reabsorption effect by energy transfer between resonant 0-phonon lines. The charge compensation by association of two Yb3 þ cations in place of three Cd2 þ cations perturbs the local structure of optical centers since a vacancy has been created. The slight shift of the 0-phonon emission line seen only in luminescence spectra of the obtained samples when Yb3 þ concentration increases is probably connected to the multisite distribution of Cd1 3xYb2x□xMoO4. This line localized at 974.3 nm splits into two well resolved components at 974.3 nm (10,264 cm  1) and 976.7 nm (10,238.5 cm  1), with a splitting of 25.5 cm  1 and the full width at half maximum (FWHM) equal to 21 cm  1. This indicates clearly two main Yb3 þ distributions of sites in the lattice of D2d point symmetry with slightly lower symmetry (see

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Transmitance (%)

7

IR RT

RAMAN RT 0

200

400

600

800

1000

Intensity (a.u.)

1.0

EMISSION 77 K

0.8 0.6 0.4 0.2 0.0

400

600

Absorbance (a.u.)

0 200 5 1 emission line

0 200 1 5 absorption line

800

1000

ABSORPTION 4.2 K

400

600

800

1000

cm

-1

Fig. 10. Superposition of the absorption spectra at 4.2 K, emission spectra at 77 K, Raman and IR spectra at RT.

Fig. 9. Emission spectra of Cd0.9424Yb0.0384□0.0192MoO4 (3.92%) solid solution at 77 K under different excitation lines of Ti-Sapphire laser.

inset Fig. 8). The line did not shift with the Yb3 þ content. The position of zero-phonon line from the absorption spectra stays in good agreement with that from the emission. Contrary to emission spectra at RT high resolution, emission spectra are reached at low temperature (4.2 K) and we observe doubling of almost each line, so there are more than four components expected for one symmetry site. In fact seven intense components are clearly resolved. The main bands corresponding to the four Stark emission components are located at 974.3 nm (10,264 cm  1) (5-1), 996 nm (10,040 cm  1) (5-2), 1012 nm (9881 cm  1) (5-3) and 1023 nm (9 775 cm  1) (5-4), respectively. In the Yb3þ -doped molybdates under investigation, not only the lowest level 5 of the 2F5/2 excited state is emitting, but one can observe the 6-1 transition at 962.4 and 964 nm in Figs. 8 and 9. Additional weak components may be a result of electron– phonon coupling with M–O modes. Among all compounds under investigation, the highest emission intensity is shown by the material containing 4 mol% of Yb3 þ ions. The comparison of the luminescence intensity of all samples was performed by recording the spectra under the same conditions. Fig. 9 presents emission spectra of Cd0.9424Yb0.0384□0.0192MoO4 (3.92 mol% of Yb3 þ ) recorded at 77 K under different excitation lines of the Ti-Sapphire laser. The number of zero-phonon line components varies when the excitation wavelength changes, confirming the distribution of Yb3 þ ions in more than one nonequivalent site in the lattice. The resonant transition is the best adapted to give vibronic side

lines. Thus, by superposition of the absorption, emission IR and Raman spectra we distinguish between the vibronic components (see Fig. 10). We can see that mainly the translatory Tʹ(Cd2 þ ) and Tʹ(MoO42 ) lattice modes and libratory L(MoO4) lattice modes with low energy can be responsible for effective phonon coupling [32]. 3.4.5. Energy level diagram of Yb3 þ ions in Cd1  3xYb2x□xMoO4 Fig. 11 represents a summary of the spectroscopic results by combining experimental absorption and emission spectra of the highest population of Cd0.8635Yb0.0910□0.0455MoO4 solid solution (9.53 mol% of Yb3 þ ions). An energy level diagram can be derived from low temperature measurements because at RT the spectra were not structured enough to assign the transitions. 3.4.6. Decay curves and decay times In order to see how the Yb3 þ concentration influences the luminescence quenching, decays for samples doped with different concentrations of Yb3 þ ions were recorded at RT and at 77 K. The collected decays normalized to the signal intensity are shown in Fig. 12. As can be seen, the decay curves strongly dependent on Yb3 þ concentration. Indeed, the slight curvatures show both Yb3 þ multisite effect (at least two sites) and some energy transfer between these multisites mainly due to the self-trapping effect and the overlapping between resonant lines, so that this is not possible to assign precisely individual values of each decay. We can only observe the global quenching of the decays when concentration increases. Table 2 presents the integrated luminescence lifetimes for the Cd1  3xYb2x□xMoO4 solid solutions with different concentrations of Yb3 þ ions, λex ¼ 925 nm.

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1

0.5% 1.01% 3.93% 9.53% 13.95% 18.32%

Cd1-3xYb2x xMoO4

Normalized Intensity (a.u.)

RT

0.1

0.01

1E-3 0.0

0.5

1.0

1.5

2.0

Time (ms)

Normalized Intensity (a.u.)

1

Fig. 11. a) Absorption and emission spectra at low temperatures of Cd0.8635Yb0.0910□0.0455MoO4 (9.53 mol% of Yb3 þ ). (b) Low temperature Stark splitting levels of Yb3 þ ions calculated from the experimental data.

0.5% 1.01% 3.92% 9.53% 13.95% 18.32%

Cd1-3xYb2x xMoO4 77K

0.1

0.01

1E-3 0.0

0.5

1.0

1.5

2.0

Time (ms)

3.4.7. Concentration quenching 3.4.7.1. Self-trapping and self-quenching processes. To present the combined case of self-trapping and self-quenching in the lifetime analysis of Yb3 þ in Cd1 3xYb2x□xMoO4 solid solutions, we have applied the model proposed earlier [20] for Yb3 þ -doped cubic oxides such as Y2O3, Sc2O3, and Lu2O3 sesquioxides, YAG, GGG and LuAG garnets, and Yb3 þ -doped cubic fluorides (CaF2 and KY3F10) [21]. It has been shown that self-quenching, for a rather large doping range, is well described by a limited diffusion process within the doping ion subsystem towards impurities analogous to the doping ions themselves. Fast diffusion towards intrinsic non-radiative centers cannot explain the observed results. Assuming an electric dipole–dipole interaction (s¼6) between ions, the self-quenching behavior can be simply described by

τ (N) = τ (rad)/[1 + (9/2π)(N /N0 )2]

(2)

where τ (rad) is the measured radiative lifetime at weak concentration, N is the ion doping concentration, N0 is the doping concentration corresponding to the critical distance R0 for which the non-radiative energy transfer is as probable as photon emission:

R 0 = (3/4πN0 )1/3 In case of photon trapping Eq. (2) should be multiplied by:

τtrapping = τ (rad)(1 + σNl)

(3)

where l is the average absorption length and s is the transition cross-section. Then the general formula is

Fig. 12. Luminescence decay curves of Cd1  3xYb2x□xMoO4 solid solutions at RT and 77 K, λex ¼ 925 nm, λem ¼ 1000 nm. Table 2 Luminescence decay times for the Cd1  3xYb2x□xMoO4 solid solutions, λex ¼ 925 nm, λem ¼ 1000 nm. Yb3 þ ions (mol%)

DT Tint (µs) RT

DT Tint (µs) 77 K

0.5 1.01 3.92 9.53 13.95 18.32

297 313 341 112 63 42

269 277 285 162 120 83

τ (N) =

τ (rad)(1 + σNl) 1 + (9/2π)(N /N0 )2

(4)

This model fits well with the experimental data, as can be seen in Fig. 13a. 3.4.7.2. Laser optimization. A simple quantitative method for optimizing the concentration dependence of the gain for amplifiers and lasers has been proposed and performed in [20]. Since we now have continuous reliable mathematical curves for self-quenching, it is possible to determine in a simple and unambiguous way the material optimum concentration for its active optical use. From the steady state rate equation, the material gain is simply given by

G = exp[σg σa Nτ′ (N) l]

(5)

Here sg is the gain cross-section taking care of the quasi three-

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It is plotted, as an example, in Fig. 13b for Yb3 þ in Cd1  3xYb2x□xMoO4. The optimized concentrations for gain are found at roughly 6 mol% at 77 K and 3 mol% at RT.

a

4. Conclusions

.

Fig. 13. a) Concentration dependence of experimental decay times in Cd1  3xYb2x□xMoO4 solid solutions with different concentrations of Yb3 þ ions fitted with the self-trapping and self-quenching effect according to Eq. (3). (b) Concentration dependence of experimental decay times corrected by the selftrapping effect according to Eq. (1), compared in the right scale with the optimization of the optical gain by the product τ′(N) N. The Yb3 þ optimum theoretical concentration can be read at the maximum: 3 at RT and 6 mol% at 77 K, respectively.

level situation for lasing between the first excited and ground state, sa is the pump absorption cross-section for the pumping wavelength, N is the chemical concentration of active ions, τ′(N) is the ions excited state lifetime corrected from the self-trapping at the considered concentration N, and l is the amplification length. From Eq. (5), the product τ′(N)N, can be optimized easily in Fig. 13b. Since this maximum value is unique, we propose to consider such maximum value as an absolute scale for self-quenching characterizing for any given host-doping couple. Interestingly, it is verified that the optimum concentration for gain, Nm, is found equal to 0.83N0. Then the critical concentration itself, which from Eq. (5) can be simply defined as the concentration reducing τw to 0.41τw is a good indication of the self-quenching magnitude and can also easily provide the optimum concentration. This simply comes from the fact that the derivative

Using a high-temperature solid state method we have obtained a series of micro-crystalline Yb3 þ -doped vacancied scheelite-type cadmium molybdate Cd1 3xYb2x□xMoO4 solid solutions, which can exist as the monophasic tetragonal scheelite-type samples (the space group I41/a, No. 88) when the Yb3 þ concentration is lower than 33.36 mol%. The substitution of divalent Cd2þ by trivalent Yb3 þ cations leads to the formation of cationic vacancies in the framework, with respect to the charge compensation effect: 3Cd2 þ - 2Yb3þ þ□ vacancy. Depending on the position of the vacancy, some disordering is manifesting by broad absorption and emission lines. Structural studies, performed by using Yb3 þ ions as a structural probe, have clearly indicated two Yb3 þ distribution sites, most probably due to two main positions of □ cationic vacancies around Yb3 þ pairs, which stays in good agreement with results obtained for analogous samples doped with Nd3 þ ions. Promising optical parameters have been evaluated. The value of absorption cross-section, calculated for 9.53% Yb3 þ -doped Cd1  3xYb2x□xMoO4 (1.45  10  19 cm2 at λ ¼ 974.3 nm with N0 ¼6.32  1020 ion/cm3) similar to tetragonal 5% Nd3 þ -doped Cd1  3xNd2x□xMoO4 (1.44  10  19 cm2 at λ ¼804.8 nm with N0 ¼6.5  1020 Nd3 þ ions/cm3), is relatively high, when comparing with well-known laser double tungstates. Luminescence decays are strongly influenced by the Yb3 þ concentration. A self-trapping process has been seen up to 3.93 mol% Yb3 þ and a value of the experimental radiative lifetime has been measured and is equal to about 300 μs. Then, the decay curves show strong concentration quenching. The model of laser optimization shows that the optimized concentrations for gain are roughly 6 mol% at 77 K and 3 mol% at RT. The broad emission band due to the intrinsic disorder around Yb3 þ ions, could be suitable for ultra-short laser pulses.

Acknowledgements We wish to thank the Ministry of Science and Higher Education in Poland and in France for the Grant POLONIUM for scientific exchange between Institute Light Matter (ILM), UMR5306 CNRS-University Lyon1, University of Lyon, France and Faculty of Chemistry, University of Wrocław in Poland. We also wish to thank the National Science Center of Poland for the Grant HARMONIA No. UMO013/08/M/ST5/ 007484700/PB/WCH/13 and the French Embassy in Warsaw for a French Government scholarship for research stage in Lyon. These financial supports are gratefully acknowledged.

References [1] [2] [3] [4] [5]

d (τ′N /N0 ) 1 − (9/2π)(N /N0 )2 = τ (rad) dN ⎡1 + (9/2π)(N /N )2⎤2 0 ⎦ ⎣ is zero for

N /N0 =

2π /9 = 0.83.

(6)

[6] [7] [8] [9] [10]

G. Boulon, J. Alloy. Compd. 451 (2008) 1. J. Legendziewicz, J. Alloy. Compd. 341 (2002) 34. J. Amami, D. Hreniak, Y. Guyot, W. Zhao, G. Boulon, J. Lumin. 130 (2010) 603. A.A. Kaminskii, Excited States of Transition Elements, World Scientific Publishing Co. Pte. Ltd., Singapore (1989) 669. J. Legendziewicz, J. Cybinska, M. Guzik, G. Boulon, G. Meyer, Opt. Mater. 30 (2008) 1655. J. Legendziewicz, M. Guzik, W. Szuszkiewicz, J. Alloy. Compd. 451 (2008) 165. L. Macalik, J. Hanuza, J. Legendziewicz, Acta Phys. Polonica A 84 (1993) 909. L. Macalik, J. Hanuza, J. Sokolnicki, J. Legendziewicz, Spectrochim. Acta Part A 55 (1999) 251. L. Macalik, J. Hanuza, B. Macalik, W. Stręk, J. Legendziewicz, Eur. J. Solid State Inorg. Chem. 33 (1996) 397. G. Boulon, G. Metrat, N. Muhlstein, A. Brenier, M.R. Kokta, L. Kravchik, Y. Kalisky, Opt. Mater. 24 (2003) 377.

Please cite this article as: M. Guzik, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.02.043i

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[11] A. Brenier, F. Bourgeois, G. Metrat, N. Muhlstein, G. Boulon, Opt. Mater. 16 (2001) 207. [12] G. Metrat, N. Muhlstein, A. Brenier, G. Boulon, Opt. Mater. 8 (1997) 75. [13] E. Tomaszewicz, M. Guzik, J. Cybinska, J. Legendziewicz, Helv. Chim. Acta 9 (2009) 2274. [14] M. Guzik, J. Cybinska, E. Tomaszewicz, Y. Guyot, J. Legendziewicz, G. Boulon, W. Strek, Opt. Mater. 34 (2011) 487. [15] M. Guzik, E. Tomaszewicz, S.M. Kaczmarek, J. Cybinska, H. Fuks, J. Non-Cryst. Solids 356 (2010) 1902. [16] M. Guzik, E. Tomaszewicz, Y. Guyot, J. Legendziewicz, G. Boulon, J. Mater. Chem. 22 (2012) 14896. [17] M. Guzik, M. Bieza, E. Tomaszewicz, Y. Guyot, G. Boulon, Z. Naturforsch. 69b (2014) 193. [18] M. Guzik, M. Bieza, E. Tomaszewicz, Y. Guyot, E. Zych, G. Boulon, Opt. Mater. 41 (2015) 21. [19] M. Guzik, E. Tomaszewicz, Y. Guyot, J. Legendziewicz, G. Boulon, J. Mater. Chem. C (2015), http://dx.doi.org/10.1039/C4TC02963A, accepted for publication. [20] F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103. [21] G. Boulon, Y. Guyot, A. Yoshikawa, J. Rare Earths 27 (2009) 616. [22] G. Boulon, Y. Guyot, H. Canibano, S. Hraiech, A. Yoshikawa, J. Opt. Soc. Am. B 25 (2008) 884. [23] Y. Guyot, H. Canibano, C. Goutaudier, A. Novoselov, A. Yoshikawa, T. Fukuda, G. Boulon, Opt. Mater. 28 (2005) 1.

[24] H.J. Seo, T. Tsuboi, K. Jang, Phys. Rev. B 70 (2004) 205113. [25] S. Hraiech, A. Jouini, K.J. Kim, Y. Guyot, A. Yoshikawa, G. Boulon, Radiat. Meas. 45 (2010) 323. [26] M. Puchalska, M. Sobczyk, J. Targowska, A. Watras, E. Zych, J. Lumin. 143 (2013) 503. [27] X. Cao, T. Wei, Y. Chen, M. Yin, C. Guo, W. Zhang, J. Rare Earths 29 (2011) 1029. [28] E. Tomaszewicz, S.M. Kaczmarek, H. Fuks, Mater. Chem. Phys. 122 (2010) 595. [29] E. Tomaszewicz, E. Filipek, H. Fuks, J. Typek, J. Eur. Ceram. Soc. 34 (2014) 1511. [30] D. Taupin, J. Appl. Crystallogr. 6 (1973) 380. [31] R.G. Brown, J. Denning, A. Hallett, S.D. Ross, Spectrochim. Acta A 26 (1970) 963. [32] P. Godlewska, E. Tomaszewicz, L. Macalik, J. Hanuza, M. Ptak, P. Tomaszewski, M. Mączka, P. Ropuszynska-Robak, Mater. Chem. Phys. 139 (2013) 890. [33] P. Godlewska, E. Tomaszewicz, L. Macalik, J. Hanuza, M. Ptak, P.E. Tomaszewski, P. Ropuszyńska-Robak, J. Mol. Struct. 1037 (2013) 332. [34] G. Boulon, L. Laversenne, C. Goutaudier, Y. Guyot, M.T. Cohen-Adad, J. Lumin. 102/ 103 (2003) 417. [35] H. Canibano, G. Boulon, L. Palatella, Y. Guyot, A. Brenier, M. Voda, R. Balda, J. Fernandez, J. Lumin. 102–103 (2003) 318. [36] A. Bensalah, Y. Guyot, A. Brenier, H. Sato, T. Fukuda, G. Boulon, J. Alloy. Compd. 380 (2004) 15. [37] Specification of Nd:KGW, Dohrer Electrooptic, 〈www.doehrer.com〉. [38] Specification of Yb:KGW and Yb:KYW, Eksma Optics, 〈www.eksmaoptics.com〉.

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