Rare Earth Ions-Doped Mixed Molybdato-Tungstates

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article 3+

Towards Optical Ceramics Based on Yb Rare Earth Ions-Doped Mixed Molybdato-Tungstates – Spectroscopic Characterization Magdalena Bieza, Malgorzata Guzik, Elzbieta Tomaszewicz, Yannick Guyot, Kheirreddine Lebbou, and Georges Boulon J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Towards Optical Ceramics Based on Yb3+ Rare Earth Ion-Doped Mixed Molybdato-Tungstates: Part II - Spectroscopic Characterization Magdalena Bieza1, Małgorzata Guzik*,1, Elżbieta Tomaszewicz2, Yannick Guyot3, Kheirreddine Lebbou3, Georges Boulon3 1

Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław,

Poland 2

Department of Inorganic and Analytical Chemistry, West Pomeranian University of

Technology, Al. Piastów 42, 71-065 Szczecin, Poland 3

Univ Lyon, Université Claude Bernard Lyon1, CNRS, Institut Lumière Matière, F-69622,

Lyon, France

ABSTRACT Investigations of spectroscopic properties were performed for Yb3+-doped solid solutions of chemical formulas based on mixed La2MoWO9 molybdato-tungstate powders which crystalize in the cubic system with the space group P213 (no. 198) and from which it was also possible to prepare the first translucent optical ceramics. Samples activated by the Yb3+ ion in a large concentration range were synthesized by three various techniques: high-temperature solid-state reaction, the Pechini method, and the combustion method. The micro-crystalline solid solutions obtained by the high-temperature solid-state reaction characterized by intense

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

luminescence are useful for detailed fundamental analysis. The direct excitation of Yb3+ into 2

F7/2→2F5/2 absorption at 940–980 nm leads to reversed 2F5/2→2F7/2 transitions giving Yb3+

emission lines in the 970–1100 nm range. The absorption and emission 0-phonon lines of Yb3+ ions were also used as structural probes at a low temperature, and the conjugation with SEM and TEM techniques was particularly useful here. The multisite character of Yb3+ was confirmed in high-resolution site-selective emission spectra. In the case of micro-crystalline ceramics, the grains are characterized by a wide 0-phonon line around 976 nm and a high number of multisites and white points by another sharper line around 968 nm. Based on the absorption and emission spectra, the Yb3+ electronic energy level diagram was proposed for the main site. The effect of dopant concentration as well as the grain size influence on the luminescent properties and the decay times were analyzed in order to attempt to understand the concentration quenching mechanism and estimate the parameters useful for a theoretical approach to laser potential first with cubic single crystals and then with cubic transparent ceramics. The second part is related to the spectroscopic properties of powders and microceramic samples analyzed in the first part. INTRODUCTION This paper is a continuation of our research that are carried out towards new ceramic optical materials based on Yb3+-doped solid solutions of chemical formulas based on mixed La2MoWO9 molybdato-tungstates which crystalize in the cubic system. This second part is devoted to the spectroscopic properties of powders and micro-ceramic samples analyzed in the first part1. Yb3+ ions play an important role as active ions in doped materials for laser applications. The advantages of Yb3+ ions derive from their simple electronic structure consisting of two manifolds: the 2F7/2 ground and the 2F7/2 excited state, separated by about 10 000 cm-1. The intense and broad Yb3+ absorption lines are well suited for IR InGaAs diode laser


2

Page 3 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pumping between 900 and 980 nm, and the small Stokes shift between absorption and emission spectra is in favor of efficient laser materials. Cooperative luminescence upconversion towards green wavelengths can occur when Yb3+ ions are in close proximity. Its efficiency is weak, but these spectra allow to detect the presence of Yb3+ pairs in the lattice. The Yb3+ activator ion is used as a structural probe ion, as a sensitizer for other rare earth ions or for direct laser ions. The state-of-the-art Yb3+-doped inorganic materials in both basic and applied research were described by Boulon2. For several years, our team has been carried out a systematic research into RE3+-doped molybdates and tungstates which could find a potential application mainly as laser materials. The most recent object of our studies is RE3+-doped scheelite-type cadmium molybdate with the chemical formula Cd1-3xRE2xxMoO4 (RE=Nd3+, Yb3+ or Eu3+, =vacancy). The substitution of divalent Cd2+ by trivalent lanthanide ions (Nd3+, Yb3+ or Eu3+) leads to the formation of cationic vacancies in the framework with respect to the charge compensation effect: 3Cd2+→2RE3++ vacancy. Our studies have demonstrated that the Cd1-3xEu2xxMoO4 solid solution with 0.05 mol% of Eu3+ presents an emission very close to the ideal red light and should be a promising phosphor. Cd1-3xNd2xxMoO4, in turn, is characterized by several times higher integral intensities of emission transitions with broader lines than in the case of Nd3+-doped YAG. These neodymium- and ytterbium-doped solid solutions could be promising for ultra-short pulse laser generation due to short lifetimes and high emission intensities3–5. Our previous studies have been concerned with Eu3+-doped ZnY4W3O16 and Eu3+-doped Cd0.25Gd0.50.25WO4 for visible-light-emitting phosphors, and Nd3+-doped ZnY4W3O16 and Nd3+-doped Cd0.25Gd0.50.25WO4 were considered as possible laser materials 6–9

. We have also investigated Nd3+-doped La2Mo2O9 as described in two recent papers10,11,

which demonstrated that the low concentration of Nd3+ ions in the La2Mo2O9 host lattice leads


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 39

to a monoclinic structure (α-form), while the pure cubic structure (β-form) can be achieved when the concentration of Nd3+ reaches 50%, but for this sample we observe a very strong effect of the concentration quenching process, which takes place due to the clustering of Nd3+ ions and energy transfer by down- and up-conversion processes. As a continuation of our research to find good cubic materials, we have noticed that the partial substitution of Mo6+ ions by tungsten W6+ ones (1:1 ratio) can stabilize the cubic phase of the mixed La2MoWO9 molybdato-tungstate. For molybdato-tungstate mixed systems, only Deng et al.

12

have

described the spectroscopic results of Sm3+-doped La1.97WyMo2-yO9 as a new promising orange-red emitting phosphor for white light emitting diode applications. The authors claim that compounds with the chemical composition of 3% Sm3+-doped La1.97WyMo2-yO9 (y=0, 0.5, 1.0) crystallize in the cubic system, but they show no information about the disordered structure and distribution of the crystallographic positions occupied by Sm3+ ions. No other paper has so far reported spectroscopic results for rare earth-doped La2MoWO9. This is why Yb3+-doped La2MoWO9 is becoming more interesting. In the first part of this research, we presented results concerning structural properties, morphology, FT-IR, and Raman spectroscopy that play an important role in the fundamental research. In continuation of prior research into nano- and micro-crystalline Yb3+-doped La2MoWO9 powders, our main purpose now concerns spectroscopic properties determined by absorption, site selective emission spectroscopy, cooperative luminescence, and decay times of Yb3+-doped La2MoWO9 powders. The influence of Yb3+ dopant concentration on luminescent properties was defined. We consistently develop the technique of sintering nanocrystalline powders to optically transparent ceramics. Consequently, this research also presents spectroscopic results for Yb3+-doped La2MoWO9 translucent micro-ceramics.


4

Page 5 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MATERIALS AND CHARACTERIZATIONS SAMPLE PREPARATION The samples activated by the Yb3+ ion in a large (0.1, 1, 3, 10, 20 mol%) concentration range were prepared by three various techniques: the high-temperature solid-state reaction, the Pechini method, and the combustion method. The synthesis of all samples is described in detail in the first part of our work entitled “Towards Optical Ceramics Based on Yb3+ Rare Earth Ion-Doped Mixed Molybdato-Tungstates: Part I - Structural Characterization”1. SAMPLE CHARACTERIZATION ABSORPTION MEASUREMENTS Absorption spectra in the 800–1150 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 the pressure of 20 MPa for 5 min. EMISSION MEASUREMENTS Emission measurements under a CW titanium sapphire laser were performed with the help of an IR Hamamatsu CCD camera with a 900 l/mm grating blazed at 1300 nm. LUMINESCENCE DECAY MEASUREMENTS Luminescence decay curves were recorded under pulsed laser excitation (OPO laser, EKSPLA NT342, 10 Hz, 7 ns), the fluorescence intensity around 1 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. Both luminescence emission spectra and decay curves were recorded at room and liquid-nitrogen temperatures.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 39

RESULTS AND DISCUSSION Yb3+ ENERGY LEVEL DIAGRAM AND ASSIGNMENT OF ABSORPTION SPECTRA The spectral term for the Yb3+ ion 2FJ is split by the spin-orbit interaction into two energy manifolds: 2F7/2 (ground state) from the 1 to the 4 level and 2F5/2 (excited state) from the 5 to the 7 level, according to the increasing energy. In comparison to other lanthanide ions, the energy level diagram of the Yb3+ ion is simple, due to the crystal-field theory of Kramer’s ions, where the maximum allowed component splitting is three in the absorption spectra for the J=5/2 state and four in the emission spectra for the J=7/2 state, for one symmetry site in the structure (see Fig. 1). This very simple energy level structure prevents the existence of several de-excitation processes that influence the dynamics of energy level populations in other rare earth laser ions with more complex electronic structures, such as the excited state absorption or the concentration quenching by down-conversion cross-relaxation or up-conversion inside the system of active ions2. Low-temperature spectra are very useful to get important information about the environment of the Yb3+ ion in the host lattice. The resonance (0-phonon) lines 1→5 and 5→1 in both absorption and emission spectra are usual structural probes to assign all transitions in materials, with intensities depending on the radiative energy transfer inside the volume of the materials. Figure 1 presents high-resolution absorption spectra recorded at room and liquid-helium temperatures for solid solutions activated with the same amount of the Yb3+ ion, i.e. 10 mol%, and fabricated by three methods: solid-state reaction, the Pechini method, and the combustion method. The absorption spectra at room temperature present broad weakly resolved bands located between 880 nm and 1020 nm. Only one broad zero-phonon line at ca. 975.5 nm is well-formed. The remaining components are barely visible for all samples. When the


6

Page 7 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature lowers, the absorption bands should narrow. However, only for micro-crystalline solid solutions from the solid-state reaction (Fig. 1a) is there a nice formation of bands corresponding to transitions from the first Stark level (1) of the 2F7/2 Yb3+ ion ground state to the 5, 6, and 7 Stark levels of the 2F7/2 excited states, respectively. Nevertheless, these lines are broad even at 4 K, which suggests the disordering of active ions in the structure. We already proposed the existence of such a disorder in the structure for the similar compositions of La2Mo2O9 dilanthanum dimolybdate doped with Nd3+ ions, which may crystalize as either monoclinic or cubic phase, depending on the activator concentration10, 11. Here, the difference between the ionic radii of La and Yb is even bigger than in case of La and Nd ions, so the disorder in the framework may also be bigger. Comparing samples prepared using the three methods, one can see that the best split components are observed for the micro-crystalline materials obtained by the solid-state reaction, due to the highest temperature (1100 °C / 2 h) applied during the synthesis (see Fig. 1a), for which the crystallites possess the largest grain size (1–10 µm). Low-temperature absorption spectra illustrate four well-formed absorption lines with maxima at 913 nm (10 953 cm-1), 932 nm (10 730 cm-1), 961 nm (10 406 cm-1), and 974 nm (10 267 cm-1), which correspond to transitions from the first (1) Stark level of the 2F7/2 ground state of the Yb3+ ion to the 5, 6, and 7 ones of the 2F7/2 excited states, respectively. In the case of the Yb3+ ion, vibronic components very often assist electronic ones. As the maximum allowed component splitting for the J=5/2 state is three (5/2 + ½ = 3), at least one of these four components may have a vibronic character. By comparing other Yb3+-doped molybdates, we can conclude that the vibronic component might be the line of smallest intensity observed at 913 nm (10 953 cm-1).


7


3+

10 mol% Yb -doped La2MoWO9 (a) solid-state, 1100oC/2h 16 1 - 10 µm

7

15

961 nm

17 932 nm

2F 5/2

6

974.25 nm 975.5 nm

5

4

913 nm

3

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 39

2

F7/2 F5/2

900

920

940

960

o

(b) Pechini, 900 C/5h 1 5 0.7 - 1 µm

974.5 nm 975.3 nm

980

2F 7/2

2

2

1

Yb3+

1000 1020

(c) combustion, 600oC/3h ~ 50 nm

16 17 954nm 930nm

17 926nm

15

976.3 nm 976.3 nm

16 953nm

4K RT 900

920

940

960

980 1000 1020 900

920

940

960

980

1000 1020

Wavelength (nm)

Figure 1. Absorption spectra of 10 mol% Yb3+-doped La2MoWO9 solid solutions from combustion, Pechini method and solid-state reaction, recorded at RT (blue line) and 4 K (black line). Figures 1b and 1c present the absorption spectra of 10 mol% Yb3+-doped La2MoWO9 solid solutions prepared by the Pechini (Fig. 1b) and combustion (Fig. 1c) methods. Three absorption lines are also clearly seen, but their resolution is not so good as in the case of the sample from the solid-state reaction. At this point, it should be reminded that the samples are of different particle sizes depending on the preparation method: for the Pechini method (900 °C / 5 h), the grain size is 0.7–1 µm, and for the combustion method (600 °C / 3 h), the grain size is ~50 nm. Broad absorption bands indicate different distributions of the crystallographic Yb3+ sites. For all the samples under investigation, the 0-phonon line corresponding to the 2F7/2(1)→2F5/2(5) transition is the strongest absorption line and has only one broad component, which could indicate the lack of differentiation of crystallographic sites


8

Page 9 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


occupied by the Yb3+ ions, and thereby the presence of the same average environment of all Yb3+ ions in the structure. However, the band is relatively wide: the full width at half maximum (FWHM) is 110.3 cm-1 at RT and 96.9 cm-1 at 4 K for the material from the reaction in the solid-state, 94.5 cm-1 at RT and 96.7 cm-1 at 4 K for the sample from the Pechini method, and 108 cm-1 at RT and 77.5 cm-1 at 4 K for the nano-crystallites obtained by using the combustion method. This broadening suggests a distribution of Yb3+ crystallographic sites of similar symmetry in the crystal lattice. The analysis of the 0-phonon line also shows that the size of the grains influences the position of the 0-phonon line shifted to 976.3 nm in the case of nano-materials obtained by the combustion method instead of 974.5 nm in the solid-state reaction and 974.5 nm in the Pechini method. The calculated value of total splitting of the 2F5/2 excited state is 474 cm-1, similar to that observed for other molybdates like (Gd0.9Yb0.1)2(MoO4)3, 472 cm-1, or Yb2(MoO4)3, 494 cm-1 13. The weak lines present in the spectra might be a result of electron-phonon coupling with M-O or W-O modes, but an analysis of this phenomenon is very difficult. Figure 2 presents the superposition of the absorption spectra for 10 mol% Yb3+-doped La2MoWO9 at 4 K, the emission spectra for 3 mol% Yb3+-doped La2MoWO9 at 77 K, and the Raman and IR spectra for La2MoWO9 at RT, which could help to distinguish the vibronic components, but this analysis is not trivial.


9

Page 10 of 39

IR RT

La2MoWO9

Raman RT 0

200

400

600

800

1000

1200

1400

1600

1800

2000

3+

3% Yb -doped La2MoWO9 Emission 77 K

0

200

400

600

800

1000

1200

1400

1600

1800

2000

51 emission line

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a.u.) Transmittance (%)


3+

10 % Yb -doped La2MoWO9 Absorption 4 K

0

200

400

600

800

1000

1200

1400

15 absorption line

1600

1800

2000 -1

Wavenumber cm

Figure 2. Superposition of the absorption spectra for 10 mol% Yb3+-doped La2MoWO9 at 4 K, emission spectra for 3 mol% Yb3+-doped La2MoWO9 at 77 K, Raman and IR spectra for La2MoWO9 at RT. ABSORPTION SPECTRA OF TRANSLUCENT MICRO-CERAMICS Figure 3 presents room- and low-temperature absorption spectra recorded for micro-ceramics of 3 mol% Yb3+-doped La2MoWO9 prepared under different annealing conditions (in vacuum, N2, and O2). In the room-temperature absorption spectra shown in Figure 3a, the differences of the shape and the number of components is clearly visible if one compares the ceramics obtained under different conditions and also the nano-powder (from the combustion method) compressed into pellets but not annealed. The absorption spectrum presenting the 2F7/2 →2F5/2 transition recorded for pressed powder without any heat treatment is identical to the spectrum for powder from the combustion method presented previously in Figure 1.


10

Page 11 of 39

2,8

compressed tablet from powder obt. by combustion m.at 600oC/3h

2

F7/22F5/2

RT

Absorbance (a.u.)

16

17

2,6

2,4

15

1200oC/2h N2+ 2 x 1000oC/12h in O2

o

1200 C/6h vacuum

(1 bar)

2,2

o

o

o

o

1200 C/2h vacuum + 2 x 1000 C/12h in O2 (1 bar) 2,0

1200 C/6h vacuum + 2x 1000 C/12h in O2 (1 bar)

1,8 800 820 840 860 880 900 920 940 960 980 1000 1020 1040 1060 1080 1100

(a)

Wavelength (nm)

2

2

F7/2 F5/2

16

17

976.2

1,05

968.9

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RT 4K

15

930.6 916.5 924 935.6

957.7 945 940.4

0,98

3+

ceramic 3 mol% Yb -doped La2MoWO9 o o 1200 C/6h vaccum + 2x 1000 C/12h in O2 (1 bar)

(b)

880

900

920

940

960

980

1000

1020

1040

1060

1080

Wavelength(nm)

Figure 3. Absorption spectra of 3 mol% Yb3+-doped La2MoWO9 sintered ceramics.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 39

In turn, the spectra for ceramics contain many components, sometimes even 9 (1017.5, 995.5, 976, 970, 960.5, 944, 930, 918, 866 nm), with 0-phonon line clearly at 976.2 and 968.9 nm. The line located at 976.2 nm is more intense than that at 968.9 nm. It is important to notice that the spectrum for ceramics obtained under nitrogen atmosphere differs from that for the ceramics obtained under vacuum. The intensity ratio of the two components corresponding to the 0-phonon line is reverse to that for other ceramics. From Figure 3a, which presents directly registered absorption spectra, one can conclude that two ceramics obtained in vacuum and in the air annealed for 2 hours are characterized by the best translucency after 6 hours, which is in accordance with the photos of ceramics presented in the first part of this research1. The absorption spectra at liquid helium were recorded for all ceramics, but we decided to present the results only for the best sample. In the lowtemperature absorption spectra (Fig. 3b), the 0-phonon line is very well split into two components. We suppose that the line situated at 976.2 nm corresponds to the cubic Yb3+doped La2MoWO9 phase, while the line located at 968.9 nm corresponds to the unknown tetragonal Yb3+-rich phase, which is observed as white points on the grain boundary. At low temperature, the bands of the 2F7/2 →2F5/2 transition contain additional lines in comparison with the spectra at room temperature. Thus, this number of components points out a different distribution of Yb3+ crystallographic sites. PHOTOLUMINESCENCE STUDIES EMISSION SPECTRA OF POWDERED SAMPLES The emission spectra of Yb3+ in La2MoWO9 solid solutions recorded in the 2F5/2→2F7/2 transition region under λex=932 nm by pumping with a tuneable Ti-sapphire laser were measured at room and liquid-nitrogen temperatures. Figure 4 presents selected emission spectra for the micro-powders from the solid-state reaction doped with different Yb3+ ion contents. For the sample containing 3 mol%, the most intense line, the so-called zero-phonon


12

Page 13 of 39

line, corresponding to the 5→1 transition from the lowest level of the 2F5/2 excited state to the lowest level of the 2F7/2 ground state, is located at around 976.3 nm at RT and 977 nm at 77 K, respectively. Independently of the temperature, the lines are very broad due to the multisite character of Yb3+ ions depending on the random neighborhood of either W6+ or Mo6+ cations. Going to the liquid-nitrogen temperature (for sample 3%), one can distinguish five lines. According to the crystal-field theory of Kramer’s ions for one site, we expect only four 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, and 4), respectively. The assignment of Yb3+ Stark levels is not easy due to the appearance of a strong electron–phonon coupling between both electronic and vibronic transitions of the main site.

24000

2

F5/22F7/2

51

22000

977.5 nm

20000

Luminescence intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


52

1000 nm

18000

3+

Yb -doped La2MoWO9 solid-state reaction

53

RT

1010 nm 1020 nm

0.1 mol% 1 mol% 3 mol% 10 mol% 20 mol%

16000 14000 12000

77 K 3 mol% laser Ti-sapphire 54 λex=932 nm

10000 8000 6000 4000

1060 nm 976.2 nm

2000 0 944

960

976

992

1008 1024 1040 1056 1072 1088 1104 1120

Wavelength (nm)

Figure 4. Emission spectra of Yb3+-doped La2MoWO9 from solid-state reaction with different concentration of Yb3+ ions, measured at RT and 77 K under Ti-sapphire laser excitation, λex= 932 nm.


13


25000

3+

2

51 20000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 39

52

2

F5/2 F7/2

Yb -doped La2MoWO9 3 mol% solid-state 3 mol% Pechini 1 mol% combustion

53

77K λex=932 nm

15000

laser Ti-sapphire 10000

54 5000

0 960

980

1000

1020

1040

1060

1080

1100

Wavelength (nm)

Figure 5. Low temperature emission spectra of 1 and 3 mol% Yb3+-doped La2MoWO9 solid solutions from three methods under Ti-sapphire laser excitation, λex= 932 nm.

The 5→1 line can be associated with the 0-phonon line at 977 nm (10 235.4 cm-1) and the 5→4 line at around 1060 nm (9434 cm-1) with high probability. Two of the three wellseparated emission lines peaking at 1000 nm (10 000 cm-1), 1010 nm (9901 cm-1), and 1020 nm (9804 cm-1) should correspond to the 5→2 and 5→3 electronic transitions, respectively. Consequently, one of these lines should have a vibronic character. We also checked how the emission intensity changes depending on the applied synthesis route. Figure 5 presents the low-temperature emission spectra of 1 and 3 mol% Yb3+-doped La2MoWO9 solid solutions from the three methods under Ti-sapphire laser excitation at λex=932 nm. Similarly as in the case of the absorption spectra, only for the micro-crystalline


14

Page 15 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solid solutions from the solid-state reaction do we see a nice resolution of the bands corresponding to the transitions from the lowest Stark level of the 2F5/2 excited state (5) to Stark levels 1, 2, 3, or 4 of the 2F7/2 ground state of Yb3+ ions at 977 nm, 1000 nm, 1010 nm, 1020 nm, and 1060 nm, respectively. The full width at half maximum (FWHM) of the 0-phonon line stays almost the same at 73 cm-1 for all the methods. The emission intensity decreases from the solid-state method to the combustion one.

SITE-SELECTIVE 2F5/22F7/2 EMISSION By using the site-selective excitation method, we also tried to precisely characterize the Stark components of Yb3+ in this matrix. The low-temperature 2F5/2→2F7/2 emission spectra for excitation into the 5 ↔1 resonant line, by pumping with a tuneable Ti-sapphire laser and by changing the excitation lines every 1 nm in the range from 972 to 977 nm, are given in Figure 6. This analysis was done with the sample from the solid-state reaction because of the most intense luminescence signal. For all excitations, we clearly observe three lines at 1000 nm (5→2), 1020 nm (5→3), and 1060 nm (5→4), respectively. At last, we would like to mention some disorder of the active ions in the host, similar to what was observed for LAMOX doped with Nd3+ ions. The site-selective spectroscopy technique confirms the multisite character related mainly to the existence of the two main LaO8 and LaO7 polyhedra, similar as in the LAMOX host lattice10, 11, and especially in the La2MoWO9 compound to the presence of both disordered W6+ and Mo6+ cations around La3+ ions.


15


80000

3 mol% Yb3+-doped La2MoWO9

5 2 70000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

solid-state reaction

5 3

60000

2

2

F5/2 F7/2

50000 40000

5 4

30000

laser Ti-sapphire 972 nm λex= 973 nm 974 nm 975 nm 976 nm 977 nm 77 K

20000 10000 0 1000

1010

1020

1030

1040

1050

1060

1070

1080

1090

1100

Wavelength (nm) Figure 6. Site selective emission spectra of 3 mol% Yb3+-doped La2MoWO9 from solid-state

reaction, measured at 77 K under different excitation lines of the Ti-sapphire laser.

ENERGY LEVEL DIAGRAM OF Yb3+ IONS IN La2MoWO9

Figures 7a and 7b present room- and low-temperature results of absorption and emission spectra of 10 mol% Yb3+-doped La2MoWO9 micro-powder. The spectra recorded at low temperature were necessary for the calculation of the energy level diagram of Yb3+ ions in La2MoWO9 and allowed us to indicate bands with maximum positions (see Fig. 8). From a comparison of the total splitting levels for Yb3+ in different kinds of matrices, we noticed similar splitting values for Yb3+-doped La2MoWO9 to 10 718 cm-1 in the case of molybdates Cd0.8635Yb0.04760.0238MoO4 (9.53 mol%)4 and the same value as 10 730 cm-1 for 19.6 at% Yb3+-doped KY(WO4)2 tungstates14.


16

3+

10 mol% Yb -doped La2MoWO9 absorption, RT 51 15 emission, RT λex= 932 nm 53 52 16

Yb3+

12000 2F 5/2

7 6 5

10000 4 3 2 1

2F 7/2

8000

17

6000

54 4000

2,35

2000

(a)

0

(a)

880

920

absorption, 4 K emission, 77 K λex= 932 nm

960

1000

2

1080

51 15 974.5 nm 52 16

50000

53

laser Ti-sapphire 17 2,28

1040

40000

2

2

2

F5/2 F7/2

F7/2 F5/2

30000

20000

54

2,24

10000


Absorbance (a.u.)

2,40

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 17 of 39

(b) 0 880

(b)

920

960

1000

1040

1080

Wavelength (nm)

Figure 7. Absorption and emission spectra of 10 mol% Yb3+-doped La2MoWO9 recorded at

RT (a) and low temperatures: 4, 77 K (b).


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 39

Figure 8. Low temperature Stark splitting levels of Yb3+ ions calculated from the experimental data. EMISSION SPECTRA OF MICRO-CERAMICS The ceramic sample of 3 mol% Yb3+-doped La2MoWO9 was also investigated in respect of fluorescence properties. We performed luminescence measurements by using the selective lines of Ti-sapphire laser as excitation sources. Figure 9 illustrates the room-temperature emission spectra of the lines corresponding to the 2F5/2 →2F7/2 transitions for three powdered samples prepared by different methods and for one ceramic sample. As can be seen, there is one broad line corresponding to the 0-phonon line with a maximum at 975.7 nm, well coinciding for all the samples. For the ceramic sample, one more line is observed at 968.7 nm, which shows a very low intensity. It is worth noting that the positions of both the peaks observed very well match two 0-phonon lines, at 976.2 and 968.9 nm, observed in the low-temperature absorption spectra recorded for the same ceramic sample. For the absorption spectra, we assumed that the line situated at 976.2 nm corresponds to grains of the cubic Yb3+-doped La2MoWO9 phase, while the line located at 968.9 nm


18

Page 19 of 39

corresponded to the unknown tetragonal Yb3+-rich phase, which is observed as white points on the grain boundaries.

3+

3 mol% Yb -doped La2MoWO9

975.7 51 2

Combustion Pechini Solid state Ceramic

2

F5/2 F7/2

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


λex= 910 nm RT

52

5 3

968.7

54

Hg-Line

950

975

1000

1025

1050

1075

1100

1125

Wavelength (nm)

Figure 9. Room temperature emission spectra of nano and micropowder of 3 mol % Yb3+doped La2MoWO9 prepared by different methods and for ceramic sample λex= 910 nm of Ti-doped sapphire laser. The spectra are calibrated by Hg-lamp.


19


ceramic 3 mol% Yb3+-doped La2MoWO9 77 K 51

1,0

52

975

978 910 nm 937 nm 950 nm

53 0,5

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

0,0

970

980

2

54

F5/2 F5/2

λex=

51

950

975

1000

1025

1050

1075

990

2

1100

900 nm 910 nm 937 nm 950 nm 975,8 nm 980,6 nm 981,6 nm 1125

1150

Wavelength (nm) Figure 10. Emission spectra of 3 mol % Yb3+-doped La2MoWO9 powdered and ceramic sample measured at 77K under Ti-doped sapphire laser excitation. Here, the second 0-phonon component at 968.7 nm, characterized by a very low luminescence intensity, is also observed in the low-temperature emission spectra under Tisapphire laser (910 and 950 nm) excitation (Fig. 10). In addition, for low-temperature fluorescence, we observe the splitting of the main 0-phonon line (975.76 nm at RT) into two components at 975 nm (10 256.4 cm-1) and 978 nm (10 225.9 cm-1) with a splitting of 30.5 cm-1. This phenomenon is clearly observed using different excitation lines, i.e. 910, 937, and 950 nm (see the insert in Fig. 10), and it was observed neither for nano- nor for micropowders. In addition, by changing the excitation lines in the range from 900 to 981.6 nm, it is easy to notice intensity ratio changes of the bands corresponding to the 5→2 and 5→3 transitions.


20

Page 21 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In the case of the excitation line at 937 nm, we observed more than three bands in the range from 990 to 1100 nm, which should correspond to transitions from the lowest Stark level of the 2F7/2 excited state (5) to Stark levels 2, 3, or 4. The multiplicity of emission lines can point to a multisite character of the samples. As was proposed previously for powdered samples, there was some disorder of the active ions in the host, similar as that observed for LAMOX doped with Nd3+ ions. The site-selective spectroscopy technique most probably confirms the multisite character related mainly to the existence of the two main LaO8 and LaO7 polyhedra as observed in the LAMOX host lattice 10, 11

. Consequently, the splitting of the 0-phonon lines at 975 and 978 nm, seen in Figure 10,

is connected with the LaO8 and LaO7 polyhedra. However, the exact assignment to these polyhedra is not possible from all the results we have obtained so far. The disorder in the La2MoWO9 host lattice is even higher than in LAMOX due to the presence of both Mo6+ and W6+ cations.


21


EXCITATION

SPECTRA

OF

NANO-POWDERED

AND

MICRO-CERAMIC

SAMPLES

2

2

2F 5/2

6 5

1-5

F7/2 F5/2

Yb3+

7

3+

3 mol% Yb -doped La2MoWO9

Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

(white point)

ceramic abs. 4K 1-7

4

1-5

1-6

3 2

2F

7/2

1

1-7 nanopowder abs. 4K

1-5

1-6

vibr.

nanopowder exc. 77K vibr.

ceramic exc. 77K

vibr.

880

920

900

940

960

980

1000

nm

Figure 11. Absorption and excitation spectra of 3 mol % Yb3+-doped La2MoWO9 powdered and ceramic sample measured at 4 and 77K. The excitation were recorded with a Xe-lamp by monitoring the emission at 1070 nm corresponding to the 5-4 transition.

As shown in Figure 11, the common graph of low-temperature absorption and excitation spectra with emission monitored at 1070 nm makes it possible to compare the obtained results for 3 mol% Yb3+-doped La2MoWO9 in the form of nano-powder and ceramic. Both absorption and excitation spectra were measured in the same range from 880 to 1010 nm. As is clear, the most intense line situated around 976 nm is the 0-phonon line that corresponds to the 2F7/2 (1)→2F5/2 (5) transition. It is important to note that the full width at half maximum (FWHM) of this 0-phonon line in the excitation spectra is almost the same for both types of


22

Page 23 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples. In addition, the absorption spectrum of the ceramic shows two components located at 968.9 nm (10 320.9 cm-1) and 976.2 nm (10 243.8 cm-1), respectively. These spectra make sense. The absorption spectrum of Yb3+ ions in the ceramic sample corresponds to the mixing of both the main phases of La2MoWO9 in the grains and the so-called white points richer in Yb3+ ions. The high Yb3+ concentration in the white points can explain a strong quenching of the luminescence and the differences observed between absorption and excitation spectra in Figure 11 as shown by arrows: some absorption spectra maxima coincide with excitation spectra minima. Let us say that articles on Yb3+ dopants usually do not show excitation spectra within this IR spectral range. COOPERATIVE LUMINESCENCE OF Yb3+ PAIRS Since its first observation by Nakazawa and Shionoya in YbPO415, cooperative luminescence of ytterbium ions has been reported in several hosts, crystals and glasses16, 17. The cooperative luminescence results from the simultaneous de-excitation of two Yb3+ ions in the blue range at around 500 nm (see Fig. 12). This corresponds to twice the energy of the IR emission spectrum of isolated Yb3+ ions18. Cooperative luminescence is indeed an effective way to indicate the formation of Yb3+ pairs and clusters in crystals and glasses.


23


Figure 12. The scheme presenting the cooperative luminescence. The Yb3+ ion is a unique case among rare-earth ions due to the simplicity of the energy level diagram in the near IR range. The absorption spectrum occurs between 900 and 1100 nm and the emission spectrum between 950 and 1150 nm. The theoretical cooperative emission spectrum can be calculated taking into account all of the energetic combinations between Stark levels of the 2F5/2 and 2F7/2 manifolds. 3+

20 mol% Yb -doped La2MoWO9 energy -transfer 3+ 3+ Yb -Tm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

solid-state

IR convolution cooperative emission λex=976 nm, RT

0-phonon line 488 nm 3+

475 nm

Yb pairs 505 nm

511 nm

energy transfer 3+ 3+ Yb -Er

(a)

450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600

Wavelength (nm)


24

Page 25 of 39

3+

20 mol% Yb -doped La2MoWO9 Pechini method

0-phonon line 488 nm



500 nm

energy transfer 3+ 3+ Yb -Tm 478 nm

3+

Yb pairs

520 nm


450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600

(b)

Wavelength (nm)

3+

20 mol% Yb -doped La2MoWO9 3+

475.8 nm

combustion method


3+

energy transfer Yb -Tm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0-phonon line 488 nm 3+

Yb pairs 505 nm


450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600

(c)

Wavelength (nm)


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

Figure 13. Cooperative luminescence of Yb3+ pairs in 20 mol% Yb3+-doped La2MoWO9 obtained by three methods. The pumping excitation is a CW laser diode at 973 and 976 nm, 298 K.

Fig. 14 Energy level diagram of energy transfer from Yb3+ to Er3+ and Tm3+ ions.

The cooperative luminescence spectrum F(E) is then related to the infrared one f(E) by: ()=

(1)

f (E’) is the infrared spectrum, while f (E) is the visible spectrum if the cooperative luminescence mechanism operates17–19. All Yb3+-doped La2MoWO9 samples synthetized by the three different methods show a weak visible blue-green luminescence when isolated Yb3+ ions are excited by a pulsed


26

Page 27 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ti-sapphire laser into the 0-phonon line at around 973 nm or 976 nm. Figure 13 presents the room-temperature visible emission spectrum of 20 mol% Yb3+-doped La2MoWO9. Such a high Yb3+ concentration was selected to clearly increase the intensity of the cooperative emission phenomena. The convolution spectra fit well with the experimental spectra of the cooperative emission and help to detect the signatures of pairs by observing the convolution of both the 0-phonon line at around 975 nm and the unresolved broad band at around 1000 nm. The convoluted 0-phonon line appears at around 488 nm and the broad band at around 500 nm. In addition of this signal, a strong energy transfer can be seen, as usual, between Yb3+ ions and both Tm3+ and Er3+ ppm impurities of the raw materials. 530–560 nm green lines (2H11/2, 4S3/2→4I15/2) and 660 nm red lines (4F9/2→4I15/2) correspond to the fluorescence emission of the Er3+ ions, and 476 nm blue signals (1G4→3H6) are ascribed to Tm3+ ions20. Depending on the initial concentration, the Tm3+ line at 476 nm appears almost alone with a high intensity in the solid-state reaction method, in which the initial quality of the Yb2O3 Stanford materials was 99.995%, whereas the intensity is much weaker with the Pechini and the combustion methods from the highest quality of Yb(NO3)3·5H2O (99.999%, Aldrich). Due to resonant energy transfers between Yb3+, Er3+, and Tm3+ ions, cooperative luminescence is a powerful tool to point out the presence of impurities in the raw materials (see Fig. 14). The importance of this knowledge in Yb3+-doped materials is connected with the contribution of these impurities to the concentration quenching observed for Yb3+ ions especially in laser materials16, 21. Another useful piece of information obtained from cooperative luminescence is the shortest distance between Yb3+ ions and La3+ ions in the cubic structure of La2MoWO9 we do not know from XRD analysis. Cooperative luminescence is naturally favored towards the shortest distance between Yb3+ ions. The calculation of the shortest distance, d, between Yb3+


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

ion pairs in the dipole–dipole interaction was estimated as d=4.5 Å16. As mentioned in the first part of our research1, based on the 2011 report by Alekseeva on the structure of the β phase of Sb-doped La2Mo2O9, we believed that this distance (La-La) is 3.15 Å. Our results are in agreement with that, which means that two La3+ cations substituted by two Yb3+ dopants are located at less than 4.5 Å for the cubic structure of La2MoWO9 in which we do not know the exact positions of cations. DECAY CURVES We noticed that two main factors, i.e. the activator concentration and the grain size, play a very important role in the decay and luminescence quenching processes occurring in the series of samples under investigation. Decay measurements of cubic Yb3+-doped La2MoWO9 solid solutions obtained by the three different methods were performed at room temperature and at 77 K under Ti-sapphire laser pulsed excitation at λex=925 nm by monitoring 2F5/2 →2F7/2 luminescence at λem=1000 nm. As an example, Figure 15 presents the decays collected for solid solutions obtained by the solid-state method (grain size 1–10 µm), recorded at room temperature. The decays were normalized to signal intensity. As can be seen, the decays strongly depend on optically active ion concentration, which is confirmed by the values of calculated lifetimes presented in Table 1. For the lowest concentration (0.1 mol%), the decay curve is almost exponential with a fitted lifetime of about 340 µs at RT and 375 µs at 77 K, which corresponds roughly to the radiative lifetime. Similar curve shapes, almost exponential, were observed for samples containing 1 and 3 mol% of Yb3+ ions.


28

Page 29 of 39

1

3+

0.1% 1% 3% 10% 20%

Yb -doped La2MoWO9 solid-state method

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,1

RT

0,01

1E-3 0

1

2

3

4

Time (ms)

Fig. 15 Room temperature luminescence decays curves of cubic Yb3+-doped La2MoWO9 from solid-state reaction, λex= 925 nm and λem= 1000 nm. The values of calculated lifetimes increase to 490 µs at RT and 425 µs at 77 K for 3 mol% of Yb3+ ions, due to the familiar self-trapping effect, and then decrease to 210 µs at RT and 240 µs at 77 K for 20 mol%, due to the also familiar self-quenching effect. When the Yb3+ ion concentration increases, decays become non-exponential as a result of both the Yb3+ multisite effect (at least two sites) and some energy transfer between these multisite lines, so that it is not possible to provide the precise individual value of each decay, and we only observe the global quenching of the decays. We observed a very similar trend of fluorescence decay changes for other Yb3+-doped molybdates with scheelite-type structures, crystallizing in the tetragonal structure with the grain size around 1–10 µm4. However, for cubic samples, decay lifetimes are much longer. For example, at RT for 3 mol%, 490 µs, and for 10 mol%, 320 µs, while for tetragonal Cd1-3xYb2xxMoO4 solid solutions 3.92 mol%, 341 µs, and for 9.53


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

mol%, only 112 µs. Even greater differences are clearly seen for the highest Yb3+ concentrations: for 20 mol% Yb3+ with a cubic structure, 210 µs at RT, while for 18.32 mol% Yb3+ with a tetragonal structure, only 42 µs at RT. The same tendency was also observed at 77 K. The size of the grain strongly depends on the decay. For the samples obtained by the Pechini method with a lower grain size (0.6–1 µm) than in the solid-state method (1–10 µm), integrated lifetimes systematically decrease. Using the combustion method with a still much lower grain size (~50 nm), the same observation was made. It means that the self-trapping effect disappears in agreement with the shorter number of resonant transitions of the 2F5/2 ↔ 2F7/2 0-phonon line, and then the shortest distances of light inside the small grains. Decay time (µs)

Decay time (µs) at

RT

77K

0.1%

340

375

1%

440

415

3%

490

425

10%

320

330

20%

210

240

0.1%

350

385

1%

320

-

3%

230

275

10%

135

190

20%

130

160

0.1%

300

340

1%

290

240

20%

21

21

Synthesis method

mol% Yb3+

SOLID STATE grain size 1 – 10 µm

PECHINI grain size 0.6 – 1 µm

COMBUSTION grain size ~50 nm

Table 1 Luminescence decays times of cubic Yb3+-doped La2MoWO9 solid solutions obtained by three methods, recorded at room temperature and 77 K, λex= 925 nm and

λem= 1000 nm


30

Page 31 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CONCENTRATION QUENCHING ANALYSIS FOR LASER APPLICATIONS An efficient host for laser operations should also incorporate a high concentration of the trivalent rare-earth ion as laser gain is proportional to doping concentration. However, quenching processes happen and limit the performance of the emission output. This is why we analyzed Yb3+ concentration quenching processes in La2MoWO9 as described below. A. SELF-TRAPPING AND SELF-QUENCHING PROCESSES To present the combined case of the self-trapping process (re-absorption of the resonant transition 1→5, 5→1 and thus increasing τ exp) and the self-quenching process (decreasing τ exp by the energy transfer to impurities) to the lifetime analysis of cubic Yb3+-doped La2MoWO9, Figure 16a presents the application of the model previously proposed for Yb3+doped cubic oxides such as Y2O3, Sc2O3, Lu2O3 sesquioxides, YAG, GGG, LuAG garnets, and Yb3+-doped cubic fluorides (CaF2 and KY3F10)17, 21. It has been shown that self-quenching, for a rather large doping range, is well described by the 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 (s=6) between ions, the self-quenching behavior can be described simply as follows:

τ (N ) = τ (rad ) /[1 + (9 / 2π )( N / N 0 ) 2 ]

(2)

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

R0 = (3 / 4πN 0 )

1/ 3

In the case of photon trapping, Eq. (2) should be multiplied by 1+σ Nl:


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

τ (N ) =

τ (rad )(1 + σNl ) 1 + (9 / 2π )( N / N 0 ) 2

Page 32 of 39

(3)

where: - l is the average absorption length, - σ is the transition cross-section. The experimental data on the concentration dependence of Yb3+-doped La2MoWO9 at RT fit well with this model as can be seen both in Figure 16a, when we take into account selftrapping and self-quenching effects (Eq. (3)), and in Figure 16b, where only self-quenching is considered (Eq. (2)). B. LASER OPTIMIZATION A simple quantitative method for optimizing the concentration dependence of the gain for amplifiers and lasers was proposed and performed in17. Since we now have continuous reliable mathematical curves for self-quenching, it is possible to determine the material optimum concentration for its active optical use in a simple and unambiguous way. From the steady state rate equation, the material gain is simply as follows:

G=exp [σg σa N τ'(N) l]

(4)

Here σg is the gain cross-section taking care of the quasi three-level situation for lasing between the first excited state and the ground state, σa is the pump absorption cross-section for the pumping wavelength, N is the chemical concentration of active ions, τ'(N) is the ion excited state lifetime corrected from self-trapping at the considered concentration N, and l is the amplification length. From Eq. (4), the product τ'(N)N can be easily optimized in Figure 16b. Since this maximum value is unique, we propose to consider it as an absolute scale for self-quenching characterization for any given host-doping couple. Interestingly, it is verified that the


32

Page 33 of 39

optimum concentration for gain, Nm, is 0.83N0. This simply comes from the fact that the derivative: d (τ ' N / N 0 ) 1 − (9 / 2π )( N / N 0 ) 2 = τ ( rad ) 2 dN 1 + (9 / 2π )( N / N 0 ) 2

(5)

is zero for: N / N 0 = 2π / 9 = 0.83

(6)

[

]

Then the critical concentration itself is a good indication of the self-quenching magnitude and can also easily provide the optimum concentration. It is plotted, as an example, in Figure 16b for Yb3+-doped La2MoWO9 at RT. The optimized concentrations for gain are found at roughly 6 mol%.

RT 77 K

500 450

La2MoWO9 Solid-State Reaction

400 350

Lifetime (µs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 250 200

Self-quenching

150 100 50

Yb3+

Impurities: Er3+, Tm3+… OH-

12,5

15,0

0

a

0,0

2,5

5,0

7,5

10,0

17,5

20,0

Yb concentration (mol %)


33


350

La2MoWO9 RT

300

1000

Nxτ(N)

250

Lifetime (µs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

200

500

150

( self trappping corrected) Solid State Reaction (cor) Pechini (exp) Combustion (exp)

100 50

0

0

b

0,1

1

10

100

Yb concentration (mol %)

Figure 16. a) Concentration dependence of experimental decay times in La2MoWO9 prepared by solid-state reaction with different concentration of Yb3+ ions fitted with the self-trapping and self-quenching effects according to Eq. (3) at RT presented on the schemes in Fig. a, b) Concentration dependence of experimental decay times corrected by the self-trapping effect according to Eq. (3), compared in the right scale with the optimization of the optical gain by the product τ(N) N. The Yb3+ optimum theoretical concentration can be read at the maximum: 6 mol% at RT. CONCLUSIONS In this paper we have shown that Yb3+-doped La2MoWO9 constitutes a new type of cubic (space group P213) molybdates. Using combustion, the Pechini method, and the solid-state reaction, we obtained the series of molybdates activated by the Yb3+ ion in a large concentration range (0.1, 1, 1.5, 3, 10, 20 mol%). Structural studies have demonstrated that even solid solutions with small amounts of Yb3+ ions, such as 0.1 mol%, and undoped


34

Page 35 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


La2MoWO9 crystallize in the cubic system. If we compare the research with our previous results on Nd3+-doped La2Mo2O9, we can easily notice the benefits of Yb3+-doped La2MoWO9 because, to obtain the cubic β-form of Nd3+-doped La2Mo2O9, we had to add 50 mol% of the active ion, which resulted in strong concentration quenching of Nd3+ luminescence. Micro-crystalline solid solutions obtained by the high-temperature solid-state reaction characterized by intense luminescence are useful for detailed fundamental analysis. The absorption and emission 0-phonon lines of Yb3+ ions were also used as structural probes at low temperatures. The multisite character of Yb3+ was confirmed in the high-resolution site-selective emission spectra. The obtained results are in accordance with our previous research on Nd3+-doped La2Mo2O9. It is related to the existence of two main LaO8 and LaO7 polyhedra. Our new studies show that Yb3+-doped La2MoWO9 is characterized by broad bands both of absorption and emission spectra, which suggests the disordering of Yb3+ ions in the host structure. The broad spectral emission band in the investigated solid solutions might allow the generation of ultra-short pulses, which could find application in pico- or even femtosecond lasers. In addition, all Yb3+-doped La2MoWO9 samples show weak visible blue-green luminescence when Yb3+ isolated ions are excited by a pulsed Ti-sapphire laser into the 0phonon line at around 973 nm or 976 nm. We noticed a strong energy transfer between Yb3+ ions and both Tm3+ and Er3+ ppm impurities of the raw materials. By means of convolution spectra, we observed additional lines that were connected with the contribution of these impurities in the raw materials. Due to the higher quality of substrates used for the Pechini and the combustion methods, the Tm3+ line at 476 nm is less intense than in the case of the solid-state reaction, in which the initial quality of the Yb2O3 Stanford materials was of 99.995%. Another useful piece of information obtained only from cooperative luminescence


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

is the distance between Yb3+ ions and La3+ ions in the cubic structure of La2MoWO9 that are located at less than 4.5 Å. Measurements of photoluminescence spectra and decay times made it possible to note the size effect on concentration quenching. Emission intensity decreases from the solid state method, with a bigger grain size (1–10 µm), to the combustion method, with the smallest grain size (~50 nm). The size of the grain also has a strong dependence on the decays. For Yb3+-doped La2MoWO9 obtained by the Pechini method with a lower grain size (0.7 – 1 µm) than in the solid-state method, the integrated lifetimes systematically decrease. When using the combustion method with a still much weaker grain size (~50 nm), the same observation holds. For the sample from the solid-state reaction, the values of calculated lifetimes increase to 490 µs at RT and 425 µs at 77 K for 3 mol% of Yb3+ ions due to the familiar self-trapping effect and subsequently decrease to 210 µs at RT and 240 µs at 77 K for 20 mol% due to the likewise familiar self-quenching effect, not observed for the samples from the other methods. This is strongly correlated with the distance between the Yb3+-Yb3+ ions. For these samples, the optimum theoretical concentration was also calculated at the maximum: 6 mol% at RT. The nano-crystalline powders from the combustion method, in turn, were used to obtain the micro-ceramics of Yb3+-doped La2MoWO9. All the micro-ceramics are characterized by a light yellowish color, which may be caused by the presence of vacancies in the structure. The best results and the most translucent ceramic was obtained by annealing at 1200 °C for 6 h under vacuum. ACKNOWLEDGEMENTS

We wish to thank the Minister of Science and Higher Education in Poland and the Minister of National Education in France for the POLONIUM Grant for scientific exchange between the Institute of Light and Matter (ILM), UMR5306 CNRS,


36

Page 37 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


University of Lyon, France, and the Faculty of Chemistry, University of Wrocław, Poland. We also wish to thank the National Science Center of Poland for the HARMONIA grant, No. UMO013/08/M/ST5/007484700/PB/WCH/13, and the French Embassy in Warsaw for a French government scholarship for research in Lyon. All the above financial support is gratefully acknowledged.

AUTHOR INFORMATION Corresponding Author: Malgorzata Guzik *Tel.: +48 71 375 73 73. E-mail: [email protected] REFERENCES (1) Bieza, M.; Guzik, M.; Tomaszewicz, E.; Guyot, Y.; Lebbou, K.; Zych, E.; Boulon, G. Towards Optical Ceramics Based on Yb3+ Rare Earth Ions-Doped Mixed MolybdatoTungstates –Spectroscopic Characterization. J. Phys. Chem. C, 2017, Submitted. (2) Boulon, G. Why so Deep Research on Yb3+-Doped Optical Inorganic Materials? J. Alloys Compd. 2008, 451, 1-11. (3) Guzik, M; Tomaszewicz, E.; Guyot, Y.; Legendziewicz, J.; Boulon, G. Structural and Spectroscopic Characterizations of New Cd1−3xNd2x□XMoO4 Scheelite-Type Molybdates with Vacancies as Potential Optical Materials. J. Mat. Chem. C 2015, 3, 4057 – 4069. (4) Guzik, M.; Tomaszewicz, E.; Guyot, Y.; Legendziewicz, J.; Boulon, G. Spectroscopic Properties, Concentration Quenching and Yb3+ Site Occupations in Vacancied Scheelite-Type Molybdates. J. Lumin. 2016, 169, 755–764. (5) Guzik, M.; Tomaszewicz, E.; Guyot, Y.; Legendziewicz, J.; Boulon, G. 3+ Eu Luminescence from Different Sites in a Scheelite-Type Cadmium Molybdate Red Phosphor with Vacancies. J. Mat. Chem. C 2015, 33, 8582-8594. (6) Guzik, M.; Tomaszewicz, E.; Guyot, Y.; Legendziewicz, J.; Boulon, G. Structural and Spectroscopic Characterizations of Two Promising Nd-Doped Monoclinic or Tetragonal Laser Tungstates. J. Mater. Chem., 2012, 22, 14896-14906. Guzik, M; Cybińska, J.; Tomaszewicz, E.; Guyot, Y.; Legendziewicz, J.; Boulon, G. (7) Spectroscopic Behavior of Nd3+ in a New Microcrystalline ZnY4W3O16 Tungstate. Opt. Mater., 2011, 34, 487-495. (8) Guzik, M.; Tomaszewicz, E.; Kaczmarek, S. M.; Cybińska, J.; Fuks, H. Spectroscopic Investigations of Cd0.25Gd0.50□0.25WO4:Eu3+ - A New Promising Red Phosphor. J. Non-Cryst. Solids, 2010, 356, 1902-1907. (9) Tomaszewicz, E.; Guzik, M.; Cybińska, J.; Legendziewicz, J. Spectroscopic Investigation of the Europium(3+) Ion in a New ZnY4W3O16 Matrix. Helv. Chim. Acta, 2009, 92, 2274-2290.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 39

(10) Guzik, M.; Bieza, M.; Tomaszewicz, E.; Guyot, Y.; Boulon, G. Nd3+ Dopant Influence on the Structural and Spectroscopic Properties of Microcrystalline La2Mo2O9 Molybdate. Z. Naturforsch. 2014, 69b, 193-204. (11) Guzik, M.; Bieza, M.; Tomaszewicz, E.; Guyot, Y.; Zych, E.; Boulon, G. Nd3+ Dopant Influence on the Structural and Spectroscopic Properties of Microcrystalline La2Mo2O9 Molybdate. Opt. Mater. 2015, 41, 21-31. (12) Deng, Y.; Yi, S.; Huang, J.; Zhao, W.; Xian, J. Synthesis and Luminescence Properties of Sm3+-doped La2WyMo2-yO9 Orange-Red Phosphors. Physica B 2014, 433, 133–137. (13) Lin, Z.; Han, X.; Zaldo, C. Solid State Reaction Synthesis and Optical Spectroscopy of Ferroelectric (Gd1−xLnx)2(MoO4)3; with Ln = Yb or Tm. J. Alloys Compd, 2010, 492, 77-82. (14) Tang, L.; Lin, Z.; Zhang, L.; Wang, G. Phase Diagram, Growth and Spectral Characteristic of Yb3+:KY(WO4)2 Crystal. J. Cryst. Growth 2005, 282, 376-382. (15) Nakazawa, E.; Shionoya, S. Cooperative Luminescence in YbPO4. Phys. Rev. Lett., 1970, 25, 1710-1712. (16) Boulon, G. Frontiers Developments in Optics and Spectroscopy, Di Bartolo, B. and Forte, O. Eds.; September, 2007, 5-1–5-22. Book available free on link below: http://www.bc.edu/schools/cas/physics/spectroscopy/recentbooks.html (17) Auzel, F.; Goldner, P. Towards Rare-Earth Clustering Control in Doped Glasses. Opt. Mater., 2001, 16, 93-103. (18) Boulon, G.; Guyot, Y.; Guzik, M.; Epicier, T.; Gluchowski, P.; Hreniak, D.; Strek, W.; Yb3+ Ions Distribution in YAG Nanoceramics Analyzed by Both Optical and TEM-EDX Techniques, J. Phys. Chem. C. 2014, 118, 15474-15486. (19) Wiglusz, R. J.; Boulon, G.; Guyot, Y.; Guzik, M.; Hreniak, D.; Strek, W. Structural and Spectroscopic Properties of Yb³⁺-Doped MgAl₂O₄ Nanocrystalline Spinel. Dalton Trans., 2014, 43, 7752-7759. (20) dos Santos, P. V.; Vermelho, M. V. D.; Gouveia, E. A.; Araujo, M. T.; Gouveia-Neto, A. S.; Cassanjes, F. C.; Ribeiro, S. J. L.; Messaddeq, Y. Blue Cooperative Luminescence in Yb3+-Doped Tellurite Glasses Excited at 1.064 µm. J. Chem. Phys., 2002, 116, 6772-6776. (21) Boulon, G.; Guyot, Y.; Yoshikawa, A. Optimization of the Gain in Yb3+-Doped Cubic Laser Crystals of 99.99% Purity. J. Rare Earths, 2009, 27, 616-618.


38

Page 39 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


39

Rare Earth Ions-Doped Mixed Molybdato-Tungstates

Recommend Documents