Toward Optical Ceramics Based on Cubic Yb3+ Rare Earth Ion-Doped

Article pubs.acs.org/JPCC

Toward Optical Ceramics Based on Cubic Yb3+ Rare Earth Ion-Doped Mixed Molybdato-Tungstates: Part I - Structural Characterization Magdalena Bieza,† Malgorzata Guzik,*,† Elzbieta Tomaszewicz,‡ Yannick Guyot,§ Kheirreddine Lebbou,§ Eugeniusz Zych,† and Georges Boulon§ †

Faculty of Chemistry, University of Wroclaw, ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Al. Piastów 42, 71-065 Szczecin, Poland § Univ Lyon, Université Claude Bernard Lyon1, CNRS, Institut Lumière Matière, F-69622, Lyon, France

J. Phys. Chem. C 2017.121:13290-13302. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/03/18. For personal use only.

‡

ABSTRACT: Yb3+-doped mixed molybdato-tungstate powders of chemical solution La2−xYbxMoWO9, which crystallize in the cubic system with the space group P213 (No. 198), were studied so that in further research they could serve to the fabrication of transparent optical ceramics. The powders were activated by Yb3+ ions with the concentration of (0.1, 1, 3, 10, 20 mol %) and were prepared by three various techniques: high-temperature solid-state reaction, Pechini, and combustion methods. The main goal of this Part 1 is primarily concerned with the structural characterizations of the cubic structure by XRD analysis, FT-IR, and Raman spectroscopies and of the morphology by SEM and TEM techniques, as well as the calculation of the optical band gap by diffuse reflectance spectroscopy. We have also checked the thermal stability of the samples prepared by combustion and solid-state methods. The TEM images indicated that via combustion method at 600 °C we succeeded to obtain the smallest nanocrystallites with average diameters between 60 and 80 nm. Obviously, the grain sizes increased with the rise of the annealing temperature applied during various types of synthesis. The morphology also differed according to combustion, Pechini, and solid-state methods used for fabrication. The first synthesis of translucent microceramics are also shown.

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Cd1−3xEu2x□xMoO4 (cationic vacancy is denoted by □) was investigated as a strong, pure, red-emitting phosphor for white light emitting diodes (WLEDs) by taking advantage of the Eu3+ spectroscopic probe ion to analyze in detail the structural properties as a continuation of our previous analysis on both, Cd1−3xNd2x□xMoO4 and Cd1−3xYb2x□xMoO4 characterized by very strong emission in the NIR region.25−27 In last years increasing interest of development of the ceramic laser materials has been observed as the most important innovation of laser material fabrication technology. The polycrystalline ceramics possess many advantages in comparison to single-crystals; large size, a great number of varieties of shapes, better mechanical strength, higher content of doping activators, lower temperature of synthesis, less timeconsuming, cheaper manufacturing process, and ability to engineer profiles and structures, which, moreover, do not require expensive equipment.28−31 Indeed, two conditions must be fulfilled to obtain transparent ceramics: the compounds should crystallize in the cubic system and the size of the crystallites must be in the order of tens of nanometers. The above-mentioned advantages of the production of transparent ceramics are the reasons why a tendency to

INTRODUCTION For over 50 years the object of many research programs concerns new optical materials. Examples are well-known tungstates and molybdates that belong to important families of inorganic oxide materials used in the optical field due to their potential applications. In the literature, the most investigated groups are CaWO4 and MgWO4 as phosphors, as well as ZnWO4, CdWO4, and PbWO4 as scintillators.1−8 Additionally, Nd3+-doped CaWO4 was the first continuously operating crystal laser reported in 1961. Well known laser materials are MRE(WO4)2 (M = alkali metal, RE = Y, Gd, Lu) doped with optically active trivalent rare-earth ions such as Nd, Dy, Ho, Er, Tm, or Yb trivalent ions. Both Nd3+- and Yb3+-doped KGd(WO4)2 and KY(WO4)2 crystals became very important laser materials for near-infrared region.9−12 Also, phosphors like Eu-doped MgWO4,13 Eu-doped ZnWO4,14 or Ce-doped MWO4 (M = Ca, Sr, Ba) phosphors15 have been recently published. Moreover, like for the tungstate family, undoped and rare earth-doped MMoO4 (M = Ca, Sr, Ba, Pb, and Cd) also form a wide class of materials used in different fields such as scintillators, phosphors, photoconductive and photocatalytic materials, as can be seen in these mentioned references as well as therein.16−24 Our scientific program involves with trivalent rare earth ionsdoped CdMoO4 scheelite-type cadmium molybdate. As an example, materials with the chemical formula of © 2017 American Chemical Society

Received: January 23, 2017 Revised: May 19, 2017 Published: May 19, 2017 13290

DOI: 10.1021/acs.jpcc.7b00746 J. Phys. Chem. C 2017, 121, 13290−13302

Article

The Journal of Physical Chemistry C

Part 1 in this article is concerned with the results of nanoand microcrystalline Yb3+-doped mixed La2MoWO9 molybdato-tungstate powders prepared by three different ways (solidstate, Pechini, and combustion methods) and the first synthesis of the translucent micro-grained ceramics. Our interest is primarily concerned with the structural characterization of the cubic structure by XRD analysis, the morphology and the particle sizes by SEM and TEM techniques, as well as, the Mo− O bonds in MoO4 and W−O bonds in WO4 tetrahedra by FTIR and Raman spectroscopies and, at last, the determination of the optical band gap by reflectance spectroscopy. Part 2, in a second article, will be mainly dedicated to spectroscopic properties as absorption, transmission, site selective spectroscopy, cooperative luminescence, and decay times measurements of both Yb3+-doped mixed La2MoWO9 molybdato-tungstate powders and Yb3+-doped La2MoWO9 translucent ceramics.

replace single crystals by transparent ceramics is observed. Surprisingly, today available rare earth (RE3+) luminescent ionsdoped cubic optical transparent ceramics used as laser sources or phosphors for lighting are limited to a very small number. Until now, only few transparent ceramics are known as optical materials with oxides (garnets, sesquioxides, spinels) and fluorides. Especially, we can mention the following references for laser materials: • Nd3+-doped Y3Al5O12 garnets.32 In fact, Nd3+:YAG ceramics has been fabricated and used to demonstrate an output power at 1.06 μm of 6733 and >100 kW,34 respectively. • Yb3+-doped Y3Al5O12 ceramics.35,36 • Yb3+-doped Y2O3 ceramics made by hot pressing of high submicron purity coprecipitated powder for high power solid-state lasers exploiting hosts with higher thermal conductivity than YAG.31 • Nd3+-doped Lu2O3 sesquioxide ceramics fabricated by Hot Isostatic Pressure (HIP) procedure. 37 Laser oscillation has been observed in hot pressed 10% Yb3+doped Lu2O3 ceramics.38 • Nd3+-doped Lu2O3 sesquioxide ceramics fabricated by the Spark Plasma Sintering (SPS) procedure.39,40 • Nd3+, Yb3+-co-doped SrF2 laser ceramics.41 Due to many advantages of rare earth-doped molybdate and tungstate compounds which fit several requests for optical materials, such as good mechanical strength, thermal property and chemical stability, our research was carried out toward new cubic ceramic optical materials different from those already known. The main objective is to succeed the challenge of synthesis of some rare earth ions (Ce3+, Nd3+, Eu3+, Yb3+)doped cubic tungstate and molybdate inorganic materials accompanied by structural and spectroscopic characterizations, within the expected goal of future innovative optical transparent ceramics. We have started with Nd3+-doped La2Mo2O9 as described in two recent papers42,43 and have demonstrated that Nd3+-doped monoclinic structure (α-form) were observed for the concentration of Nd3+ ion up to 15%. Pure cubic phase (βform), necessary to get transparent ceramics, was obtained when the Nd3+ contents had reached 50%. So, among a whole series of Nd3+-doped La2Mo2O9 with a wide range of Nd3+ ion concentrations, only part of the compounds with cubic structure, over 50% of the Nd3+, could be useful to fabricate transparent ceramics. However, in such a case of compounds with high concentration of Nd3+ ions, unfavorable and very strong effect known as concentration quenching process takes place due to the clustering of Nd3+ ions and energy transfer by down- and up-conversion processes. Continuing research, we have noticed that the partial substitution of Mo6+ ions by tungsten W6+ ones (ratio 1:1) can stabilize the cubic phase of mixed Yb3+-doped La2MoWO9 molybdato-tungstate. In the literature there is no spectroscopic result on rare earth ions-doped La2MoWO9 mixed molybdatotungstates with the exception of Sm3+-doped La2WxMo2−yO9 as a new promising orange-red emitting phosphor for white light emitting diode application.40,44 We have first selected Yb3+-doped La2MoWO9 as laser material, but also because we can play with the presence of the 2 F7/2 ↔ 2F5/2 0-phonon line of Yb3+ ions used as a structural probe in solids. We present results of our research into two distinct articles:

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MATERIALS Synthesis of Nano- and Micropowders. A series of nano- and microcrystalline molybdato-tungstate powders with chemical formula of La2−xYbxMo2−yWyO9 (later labeled as Yb3+doped La2MoWO9) with different concentrations of Yb3+ optically active ions were synthesized by combustion, Pechini, and solid-state reaction methods. The concentration of the Yb3+ activator was set to 0.1, 1, 3, 10, and 20 mol %, calculated in respect to La3+ substitution. The substitution of molybdenum Mo6+ by tungsten W6+ (in the molar ratio of 1:1) allows to obtain molybdato-tungstate type of La2MoWO9. Solid-State Reaction. Commercial powders of La2 O3 (99.999%, Stanford Materials), Yb2O3 (99.995%, Stanford Materials), MoO3 (99.5%, Alfa Aesar), and WO3 (99.9%, Fluka) were used as the starting materials for synthesis of Yb3+doped La2MoWO9 solid solutions. The appropriate molar ratios of preheated oxides were mixed using agate mortar in acetone to ensure the best homogeneity and reactivity. The mixtures were then placed in corundum crucibles and heated at 1100 °C for 2 h under air atmosphere in order to produce desirable powders. The reaction equation of the solid-state synthesis can be given as follows in eq 1. (1 − x /2)La 2O3(s) + x /2Yb2 O3(s) + MoO3(s) + WO3(s) → La 2 − xYbx MoWO9(s)

(1)

Pechini and Combustion Methods. Stoichiometric quantities of La(NO3)3·6H2O (99%, Alfa Aesar), Yb(NO3)3·5H2O (99.999%, Aldrich), (NH4)6Mo7O24·H2O (99.99%, Aldrich), (NH4)10W12O41·H2O (99.99%, Aldrich), and C6H8O7 (pure p.a.%) were used as starting materials. The Yb3+-doped La2MoWO9 powders were prepared by using a wet chemical route. Initially, a small amount of aqueous ammonia solution (NH3·H2O, 25 mass%) was added to the aqueous solution of metals nitrates and the mixture was mixed at 80 °C for 1 h. Solutions of metal nitrates were mixed in stoichiometric amounts with citric acid (CA) in deionized water at 80 °C, obeying the molar proportion 1:4; [La3+ + Yb3+ + Mo6+ + W6+]/[CA]. Afterward, the two solutions were mixed together and stayed under continuous stirring and heating at 80 °C for solvent evaporation. According to the Pechini method, the obtained gel was heated slowly and progressively from 80 to 400 °C to obtain black powder. Then the obtained material was fired in air at 900 °C for 5 h in a resistance furnace. However, 13291


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Table 1. Content of Yb2O3/La2O3/MoO3/WO3 Initial Mixtures, Formula of Solid Solutions, Calculated Lattice Parameters, Calculated and Experimental Value of Density of Some Solid Solutions Obtained by Solid-State, Pechini, and Combustion Methods content of initial mixtures Yb2O3/ La2O3/MoO3/WO3 (mol %) formula of solid solution, value of x parameter in La2−xYbxMoWO9

Yb2O3

La2O3

MoO3

La1.60Yb0.40MoWO9 x = 0.40 La1.80Yb0.20MoWO9 x = 0.20 La1.94Yb0.06MoW O9 x = 0.06 La1.98Yb0.02MoWO9 x = 0.02 La1.998Yb0.002MoWO9 x = 0.002 La2MoWO9

6.67 3.34 1.00 0.34 0.04 x

26.67 30.00 32.34 33.00 33.30 33.34

33.33 33.33 33.33 33.33 33.33 33.33


6.67 3.34 1.00 0.34 0.04 x

26.67 30.00 32.34 33.00 33.30 33.34

33.33 33.33 33.33 33.33 33.33 33.33


6.67 3.34 1.00 0.34 0.04 x

26.67 30.00 32.34 33.00 33.30 33.34

33.33 33.33 33.33 33.33 33.33 33.33

WO3

concd of Yb3+ ions (mol %)

a (Å)

Solid-State Method 33.33 20.00 7.103(4) 33.33 10.00 7.146(1) 33.33 3.00 7.161(1) 33.33 1.00 7.161(0) 33.33 0.10 7.160(7) 33.33 x x Pechini Method 33.33 20.00 7.102(2) 33.33 10.00 7.143(2) 33.33 3.00 7.160(6) 33.33 1.00 7.160(8) 33.33 0.10 7.159(5) 33.33 x x Combustion Method 33.33 20.00 7.102(8) 33.33 10.00 7.144(1) 33.33 3.00 7.160(1) 33.33 1.00 7.160(9) 33.33 0.10 7.160(1) 33.33 x 7.160(4)

3

density calcd (g/cm3)

volume (Å )

M (g/mol)

Z

358.425(4) 364.937(3) 367.243(2) 367.224(7) 367.169(4)

715.2494 708.4225 703.6437 702.2783 701.6639 701.5956

2 2 2 2 2 2

6.63 6.45 6.36 6.35 6.35

358.257(4) 364.485(5) 367.161(7) 367.198(6) 366.994(0)

715.2494 708.4225 703.6437 702.2783 701.6639 701.5956

2 2 2 2 2 2

6.63 6.45 6.36 6.35 6.35

358.348(2) 364.624(8) 367.078(6) 367.212(4) 367.080(2) 367.132(4)

715.2494 708.4225 703.6437 702.2783 701.6639 701.5956

2 2 2 2 2 2

6.63 6.45 6.37 6.35 6.35 6.35

density expl (g/cm3)

6.33

6.40

6.39 6.27

(ICSD #420672). XRD patterns were analyzed by High Score Plus 4.0 software and lattice parameters were calculated using the least-squares refinement procedure by POWDER software.45 FT-IR and Raman Measurements. FT-IR spectra of powdered samples in the 1000−80 cm−1 spectral range were measured by using the Specord-M-80 spectrometer (Carl Zeiss Jena). The powdered samples were mixed with Nujol oil (a mixture of liquid hydrocarbons) and then pressed into pellets. The spectral resolution of FT-IR measurement was 2 cm−1. Raman spectra were recorded on Nicolet Magna 860 FTIR/ FT Raman spectrometer at an excitation line of 1.064 μm with a capacity of about 400−500 mW. The 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 measurement was 1 cm−1. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Scanning electron microscopy (SEM) studies were carried out on Hitachi S3400N equipped with an energy dispersive X-ray spectroscopy (EDX) EDAX analyzer. The powders were coated with thin gold alloy layer to facilitate conductivity, while the ceramics were analyzed without the gold coating. Transmission electron microscopy (TEM) measurements were carried out on a Tecnai G2 20 X-Twin TEM (Fei, Hillsboro, U.S.A.) or a H-8100 TEM of the Hitachi company (Tokyo, Japan) equipped with a Penta FET EDX detector (Oxford Instruments, U.K.). Acceleration voltage of 200 kV was applied. To prepare the samples, a suspension of the sample in ethanol was dropped on a copper or gold grid coated with carbon and then dried in air. Density Measurements. The density of samples under study was measured using an Ultrapycnometer 1000

the conditions of annealing during the combustion method is more violent and faster. The obtained resin was fired in air at 600 °C for 3 h in a resistance furnace. Synthesis of Microceramics. All microceramics investigated in this work were prepared from the nanocrystalline powders of Yb3+-doped La2MoWO9 solid solutions. The initial nanopowders were obtained by using the combustion method (600 °C/3 h) described in the previous chapter. Before sintering, the nanopowders were formed into tablets by pressing under pressure of 4 atm/5 min. The pellets were sintered at high temperature of 1200 °C/2 h or 1200 °C/6 h under vacuum or in nitrogen atmosphere. All microceramics are characterized by a light yellowish color, which may be caused by the presence of vacancies in the structure. This color appeared after annealing. First annealing at 1200 °C/2 h or 1200 °C/6 h under vacuum or nitrogen atmosphere led to obtain the translucent ceramics. So, to improve the transparency and get rid of the yellow color we have performed the oxidation process at 1000 °C/12 h under the pure oxygen at a pressure of 1 bar.

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EXPERIMENTAL METHODS XRD Phase Analysis. The powder X-ray diffraction patterns were collected at room temperature by using a D8 Advance X-ray diffractometer (Bruker). The measurements were performed within the range 10−60° 2θ with a scan rate of 0.008° per step and a counting time of 5 s per step. For the experiments nickel-filtered Cu Kα radiation (Kα1+2, λ = 0.15418 nm) was used. Powder diffraction patterns of some Yb3+-doped La2MoWO9 solid solutions with different contents of the optically active ion and 3% -doped microceramics were compared with the simulated XRD pattern of cubic La2Mo2O9 from the database of inorganic crystal structures 13292


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last article devoted to Nd3+-doped La2Mo2O9, which can crystallize either in the α-monoclinic or in the β-cubic form, an overview devoted to the structural considerations have been done.42,43 In this oxide family with lanthanum, the polymorphic change from the α-monoclinic form to the β-cubic one of La2Mo2O9 takes place at 560 °C. Goutenoire et al. concluded that the high temperature β-polymorph presents a cubic symmetry (s.g. P213) with cell parameter ac ∼ 7.20 Å.51 The low temperature α-polymorph has been described as a 2ac × 3ac × 4ac superstructure of the β-polymorph with very subtle monoclinic distortion.52 Although there are a lot of reports on the crystal structure including the studies based on single crystals, the knowledge is still limited and we cannot say that the crystal structure of β-La2Mo2O9 is well understood. In fact, this crystal structure is unusual and quite different from “typical” inorganic oxide because it is closer to small protein than to normal inorganic material due to 312 crystallographically independent atoms: 48 La, 48 Mo, and 216 O, which create the La2Mo2O9 structure.53 It is very difficult to find true information about bond lengths between, for example, the La−La atoms, which is very important from the spectroscopic point of view. Basing on the report of Alekseeva from 2011 on the structure of β-phase of Sb-doped La2Mo2O9 we could believe that this distance (La−La) is equal to 3.15 Å.54 However, it is very difficult to determine the coordination of polyhedra of lanthanum and molybdenum because of a large number of partially occupied positions. The La−O distances vary from 2.3 to 2.93 Å and the Mo−O distances vary from 1.53 to 1.97 Å.55 The unit cell parameters fixed at 33 K for the high temperature cubic symmetry is 7.1377(2) Å with the space group P213 and Z = 2. Here, the La atom is surrounded by 15 oxygen positions (12 of them are under occupied O(2) and O(3) positions), while the Mo atoms are surrounded by seven oxygen positions, six of which are also under occupied.56 These values slightly differ from that reported by Chun-Ju who reported that the La cations are surrounded by eight oxygen ions or seven oxygen ions, forming LaO8 or LaO7 polyhedra.57 So, to complete the knowledge on structure of both polymorphs, it is still necessary to perform either the Rietveld refinement or structural analysis basing on the single crystals. As we have mentioned in the introduction, our current interest is focused only on the compositions representing the cubic phase. Several possible substitutions on both cationic51 and anionic58 sites of La2Mo2O9 stabilize the high temperature β-form. Recently, we tried to obtain the cubic β-phase composition by doping the system with the Nd3+ ions. However, our research shown that in order to get the cubic Nd3+-doped La2MoWO9 solid solutions is necessary high activators amount (50−60 mol % Nd3+), which excludes their potential applications as laser materials.42,43 Such compounds are not useful as optical materials because of strong concentration quenching. One of possible solutions of this problem could be partial substitution of Mo6+ ions by the W6+ ones. According to Marrero-Lopez, W6+-substitution with appropriate amount of this ion stabilizes the cubic βpolymorph.59 Tungsten W6+ is an appropriate cation for partial substitution, because RE tungstates are more stable than analogous molybdates. Both ions belong to the same chemical group and consequently possess a similar ionic radius (0.59 and 0.60 Å for Mo6+ and W6+, respectively with the respect to the six coordination number of the metal ion), which permits high levels of substitutions. The results offered by Marrero-López et al. in ref 59 suggest that the structure of La2Mo2−xWxO9 with

Quantachrome Instrument (Model Ultrapyc 1200e, U.S.A.) using argon (99.999%) as a pycnometric gas. DTA-TG Measurements. Measurements were carried out on the TG-DTA thermoanalyzer (Model Setaram SETSYS 16/ 18) at a heating rate of 10 °C min−1 in air (maximum temperature 1250 °C, air flow rate 110 mL h−1) and using alumina crucibles. Diffuse Reflectance Measurements. Reflection spectra were recorded at room temperature in the spectral region 190− 1000 nm by using a spectrophotometer JASCO-V670.

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RESULTS AND DISCUSSION Structural Characterization of La2MoWO9. The crystal structure of compounds with Ln2M2O9 formula (M = Mo and

Figure 1. X-ray powder diffraction of undoped La2MoWO9 and Yb3+doped La2MoWO9 solid solutions with different concentration of Yb3+ ions obtained via combustion method and simulated XRD patterns of La2Mo2O9 cubic form (ICSD #420672).

Figure 2. (a) X-ray powder diffraction patterns of 3 mol % Yb3+-doped La2MoWO9 obtained by different methods: solid-state reaction, combustion, and Pechini methods, as well as simulated XRD patterns of cubic La2Mo2O9 (ICSD #420672); (b) pseudocubic line (231).

W with Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Er, and Lu46−52 has been a subject of investigations for more than 40 years. In our 13293


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Figure 3. XRD patterns and pictures of sintered ceramics of 3 mol % Yb3+-doped La2MoWO9 annealed in different conditions and prepared from nanopowder obtained by combustion method and pressed under 4 atm/5 min.

Figure 4. Representative TEM micrographs of 20 mol % Yb3+-doped La2MoWO9 solid solution obtained by combustion method.

the content of tungsten; x = 1 becomes closer to cubic than molybdates structures with smaller and bigger amount of tungsten. It is important to note that higher tungsten content (x = 1.8) leads to stabilization of the triclinic phase (s.g. P1, No. 2) isostructural with La2W2O9.60 In the literature we can find only few articles devoted to the structural analysis of La2Mo2−xWxO9 with different molar ratio of Mo6+ ion to W6+ one60−73 and complete interpretation of the La2Mo2−xWxO9 structure was not successful because of the lack of the corresponding crystals. So, in order to receive cubic materials based on La2MoWO9 molybdato-tungstates, we have applied partial substitutions of molybdenum ions by tungsten ions in the molar ratio 1:1. The lattice parameters calculated by cell refinement fitting on the basis of XRD data are presented in Table 1. The substitution of La3+ (1.10 Å for CN = 7 and 1.16 Å for CN

Figure 5. SEM micrographs of 1 mol % Yb3+-doped La2MoWO9 obtained by Pechini method.

= 8) ions in La2MoWO9 by smaller ones (Yb3+, 0.925 Å for CN = 7, and 0.985 Å for CN = 8) leads to a decrease in the lattice parameters of Yb3+-doped La2MoWO9 solid solutions. The lattice constants decrease with increasing of Yb3+-content. The calculated by us lattice parameters are bigger than unit cell constant of high-temperature cubic structure of La2Mo2O9 (a = 7.1377(2) Å) which was investigated by Voronkova et al.56 Recently, Y. Deng et al. published results concerning of Sm3+13294


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Table 2. Contents (in at. %) of Yb, La, Mo, W, and O Elements on Area Presented in Figure 7A Evaluated for 3 mol % Yb3+-Doped La2MoWO9 Pellet elements

Yb

La

Mo

W

O

position position position position

1.66 1.39 1.46 1.50

8.45 8.52 8.04 9.06

7.16 7.04 7.07 7.57

5.24 5.18 4.93 5.44

77.49 77.88 78.50 76.43

1 2 3 4

Table 3. Contents (in at. %) of Yb, La, Mo, W, and O Elements on Area Presented in Figure 7B Evaluated for 3 mol % Yb3+-Doped La2MoWO9 Microceramic Figure 6. SEM micrographs of 3 mol % Yb3+-doped La2MoWO9 obtained by solid-state reaction.

elements position position position position

doped La2MoWO9.44 According to them, the materials crystallize in cubic system and demonstrate β-La2Mo2O9 structure (space group P213, No. 198). The lattice constant of cubic modification of La2Mo2O9 was found to be 7.2351 Å44 and it is significantly larger than the values calculated by us. XRD Analysis of Nano- and Micropowders. The XRD studies performed by us have shown that all samples obtained by using three different synthetic routes crystallized as a pure phase of cubic symmetry. As an example, we present in Figure 1, the room-temperature X-ray powder diffraction (XRD) patterns of La2MoWO9 and Yb3+-doped La2MoWO9 solid solutions obtained by a combustion method. The lattice parameters (see Table 1) calculated by cell refinement fitting on the basis of XRD data obtained in the large compositional range clearly confirmed that the pure cubic phase forms both for undoped sample as well as for the Yb3+ low concentration compositions. All XRD patterns were compared also with the

1 2 3 4

Yb

La

Mo

W

O

11.58 10.81 1.30 1.40

7.87 8.11 11.12 12.13

4.02 3.99 7.65 7.91

6.57 6.66 7.26 7.97

69.96 70.44 72.67 70.59

standard of the cubic β-La2Mo2O9 (ICSD #420672), which belongs to the P213 (No. 198) space group. In Figure 2 we have compared the X-ray powder diffraction patterns recorded for samples obtained by three different synthesis methods. It is clearly seen that the positions of adequate diffraction lines are very close to each other and no line from impurities was observed. However, some differences in the width reflections of the samples from different methods were noticed. The broadest reflections occur for the nanocrystalline solid solution obtained by the combustion method, while for the microcrystalline samples the reflections are very sharp and more intensive. XRD results are in a good agreement with those obtained by using SEM and TEM techniques. In the

Figure 7. SEM images with the elemental analysis (with EDX) performed for 3 mol % Yb3+-doped La2MoWO9 pellet compressed at pressure of 4 atm/5 min (A), microceramic annealed at 1200 °C/6 h in vacuum with clearly visible white points (B), and general view of microceramic annealed at 1200 °C/6 h in vacuum (C). 13295


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Figure 8. EDX spectra of Yb, La, Mo, W, and O elements on area presented in Figure 7B evaluated for 3 mol % Yb3+-doped La2MoWO9 microceramic.

Table 1. The bulk density increases with doping content of Yb3+ increasing, can be related to the large atomic mass of Yb3+ in comparison with small atomic mass of La3+. The same as for the unit cell parameter, the dependence of density versus Yb3+ concentration is not linear and the Vegard law is not fulfilled. XRD of Microceramics. The nanopowdered cubic samples synthesized by combustion method were used to the first attempts of the transparent ceramic fabrication. Figure 3 presents the photos of our first trials which showed quite good results with not yet transparent but translucent ceramics. The best transparency we were able to obtain for the sample annealed at 1200 °C during 6 h under vacuum atmosphere. All microceramics are characterized by a light yellowish color, which may be caused by the presence of the oxygen vacancies in the structure. Figure 3 presents also the XRD patterns of sintered ceramics were all main peaks corresponding to the cubic phase of Yb3+-doped La2MoWO9 are present. We should mention that two very small reflections (marked with stars) have been observed in case of all samples. The identification of this phase is very difficult due to the lack in crystallographic data based on the standards. In the result of comparison it is highly probable that the second phase belongs to the tetragonal molybdato-tungstates containing richer Yb3+ concentration than the main phase as it will be seen in next section. Formation of this phase can be the result either of the application of high temperature at 1200 °C or the application of vacuum. Here we have to remind that we did not observe any additional phases in the XRD patterns of micropowders obtained by solid-state reaction at 1100 °C/2 h. This is a drawback of used conditions for the fabrication of our first

analysis of the X-ray powder diffraction patterns performed for the LAMOX compounds,42,43 we should remind that a very important and informative observation was a pseudocubic [2 3 1] peak (Figure 2b), which by change of the shape of reflection at 47.5−48° of 2Θ allowed to detect the phase transformation from α-monoclinic to β-cubic. We would like to pay attention on this subtle change in the reflections of both phases observed in the case of La2Mo2O9 matrix, the shape of the mentioned peak for the monoclinic phase is complex. Here, for La2MoWO9 and Yb3+-doped La2MoWO9, this reflection possesses only one component, and we clearly see that the corresponding peak is much larger for the sample from the combustion method than for those from Pechini and solidstate reaction ones. Basing on the XRD analysis, we observe that both, Yb3+ ion doping (x = 0.002−0.40) and the partial substitution of Mo6+ ion by W6+ one with the molar proportion 1:1, leads to the cubic product, as expected. As it is evident from Table 1, the lattice parameter calculated for each sample significantly depends on the Yb3+ ion concentration in La2MoWO9 matrix. Regardless of the kind of synthesis, the a parameter initially increases and then decreases as increasing the concentration of Yb3+ optically active ion. It means that the dependence of a parameter on Yb3+ concentration is not linear (parabolic) and Yb3+-doped solid solutions do not fulfill the Vegard law. A similar (parabolic) dependence of lattice constant versus R3+ and W6+ concentration was observed for cubic La2−xRxMo2−yWyO9 solid solutions where R3+ is Nd3+ or Gd3+.48 Calculated and experimental bulk density of Yb3+doped La2MoWO9 phases containing various Yb3+ concentrations prepared by three different techniques are showed in 13296


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Morphology and Particle Size by SEM and TEM Analysis. The morphology of Yb3+-doped La2MoWO9 nanoand microcrystallites was checked by application of Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). TEM of Yb3+-Doped La2MoWO9 Obtained by Combustion Method (Nanometer Size). Figure 4 presents the TEM micrographs of the sample activated by 20 mol % of Yb3+, obtained by combustion method. According to the HRTEM study the particles of Yb3+-doped La2MoWO9 solid solutions, sintered at 600 °C/3 h in the air, possess ellipsoid shape with the average size ∼50 nm, due to the violent reaction that occurs in method and the temperature applied during the synthesis. Here, as a result of reaction, we obtained a large volume of sample. The overview pictures show homogeneous particles, which form large groups of agglomerates. The grain boundaries between the nanocrystallites are clearly seen. We did not observe any grain size dependence on the increase of the activator concentration. The energy-dispersive X-ray microanalysis (EDX) of the nanocrystallites of the Yb3+-doped La2MoWO9 phase (not presented here) exhibits the peaks related to La, Yb, Mo, W, and O elements as well as the intensity ratios of recorded peaks, which confirm the quantitative compositions of the samples under investigation. For all powdered samples, the distribution of elements has been detected very homogeneous. SEM of Yb3+-Doped La2MoWO9 Obtained by Pechini Method and High Temperature Solid-State Reaction (Micrometer Size). According to higher sintering temperatures applied in both Pechini and high temperature solid-state reaction methods than for combustion method, for all Yb3+doped compositions the grains have much larger sizes. A large impact on the size is also the fact that the reactions do not occur as quickly and violent as in the case of the combustion method. As an example, we present in Figure 5 the SEM micrographs of 1 mol % Yb3+-doped La2MoWO9, which represents the sample from the series of microcrystalline cubic molybdatotungstates obtained by Pechini method. The grains have the average size of about 0.6−1.7 μm, quite large due to the higher synthesis temperature and longer annealing time (900 °C/5 h) in this synthesis method. The overview pictures show a good quality homogeneous product crystallizing in the compact form with the grains of circular shape and clear grain boundaries. Less homogeneous morphology exhibits the samples from high-temperature solid-state reaction (1100 °C/2 h) as one can observe it in Figure 6 from the loose clusters with various sizes, composed by aggregation of the smaller grains of irregular form. Here, the average grain size of particles is in the range of 1−10 μm, but not at the nanometer scale. SEM of Yb3+-Doped La2MoWO9 Microceramics. Also, for the ceramics, we have performed analysis of the morphology, including elemental analysis, by using EDX technique. Figure 7 presents SEM micrographs obtained for 3 mol % Yb3+-doped La2MoWO9 pellet compressed at a pressure of 4 atm/5 min from the nanopowder obtained from the combustion method (600 °C) and microceramic annealed at 1200 °C/6 h in vacuum. The morphology of the compressed pellet without annealing (Figure 7a) resembles very much that of the nanopowder obtained by the combustion method used to fabricate the ceramic material. The elemental analysis shows

Figure 9. (a) Room temperature FT-IR spectra of cubic La2MoWO9 and 0.1, 3 mol % Yb3+-doped La2MoWO9. (b) Room temperature Raman spectra of cubic La2MoWO9 to compare with monoclinic 0.2, 50 mol % Nd3+-doped La2Mo2O9.

Figure 10. UV−vis-NIR diffused reflectance spectra of Yb3+-doped La2MoWO9 and undoped La2MoWO9 solid solutions.

translucent ceramics; however, only such conditions allow to obtain the samples with such good translucency. Application of lower temperature leads to samples with very limited transparency. 13297


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Figure 11. Plots of [F(R)·hν]2 vs the energy of the incident photon hν for nano and microcrystalline 10 mol % Yb3+-doped La2MoWO9 and undoped La2MoWO9 solid solutions obtained by three synthesis method, Eg is the band gap energy.

rather homogeneous distribution of all ions and also the Yb3+ one (Table 2). On the contrary, the SEM analysis performed for the 3 mol % Yb3+-doped La2MoWO9 microceramic annealed at 1200 °C/ 6 h in vacuum has shown the segregation of the second phase, as white points at the grain boundary. The EDX analysis (Table 3 and Figure 8) pointed out that the content of Yb3+ ions in the white points is much higher than at the grain surface. The same results we obtained making the cross-section analysis of microceramic sample where a large number of white points is clearly seen between the grains. The supposition of the presence of the second phase in our translucent ceramics corresponds well with the results of XRD analysis, where two additional reflections were observed. Let’s say that this attempt is only an intermediate step before the fabrication of ceramics by using a standard technique like SPS (Spark Plasma Sintering), one which is under progress at the MATEIS Laboratory of INSA-Lyon. Characterization of Bonds by FT-IR and Raman Spectroscopies. The high-temperature cubic phase of La2Mo2O9, the same as β-SnWO4, belongs to the space group P213. In β-SnWO4 structure, tungsten ions are surrounded by four oxygens ions with one O2− on the 3-fold axis (site 4a), and the three others away from it (site 12b). Tungsten ions with four oxygen ions form WO4 tetrahedra. In the structure of β-La2Mo2O9, the molybdenum ions coordination is significantly modified as follows: while the 4a site remains fully occupied, the 12b site is only partially occupied (2/3) and molybdenum ion is moved toward the center of a corresponding triangular face. Additional oxygen ion, O3, split on a 12b position (1/3 occupancy), is located on

the other side of the triangular plane relative to the fully occupied site 4a.52,64 Other authors have suggested a presence of MoO4 tetrahedra as well as MoO5 trigonal bipyramids in the structure of β-La2Mo2O9.57 In view of the complex structure of cubic La2Mo2O9, their FT-IR and Raman spectra were presented in our earlier paper and they were only preliminary discussed.43 Tungsten ion seems to be a suitable cation for partial substitution of the molybdenum ion; the oxidation state +6 for tungsten is more stable than for molybdenum under reducing conditions and it possesses very similar ionic radius (CN = 4, Mo6+, 41 pm, W6+, 42 pm; CN = 5, Mo6+, 50 pm, W6+, 51 pm).65 Large similarity of Mo6+ and W6+ ions allows high level of substitution of Mo6+ by W6+. Thereby, the phases with the cubic structure of high-temperature polymorph of La 2 Mo 2 O 9 may exist at room temperature. 66,67 Solid molybdates and tungstates with isolated MoO4 and WO4 tetrahedra have Td symmetry. The internal vibrations under Td symmetry transforms as A1 (ν1 symmetric stretching), E (ν2 symmetric bending), and 2F2 (ν3, ν4 asymmetric stretching and bending).68 Figure 9a presents the FT-IR spectra of La2MoWO9 and Yb3+-doped La2MoWO9 solid solutions with different concentrations of optically active ions. The spectra were recorded at room temperature and in the wavenumber range of 1000−500 cm−1. As it is seen from Figure 9a, the spectra show big similarities to each other. They contain broad absorption bands with maxima at about 917 cm−1 that are due to the symmetric stretching of ν1 modes of Mo−O bonds in MoO4 as well as W−O bonds in WO4 tetrahedra.64−66,68−70 The broad bands situated at 847, 722, and 632 cm−1 are related to the ν3 internal modes originated from the asymmetric stretching vibrations of Mo−O−Mo bonds in MoO4 as well as 13298


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due to asymmetric stretching vibrations (ν3) of MoO4 (cubic Nd3+-doped La2Mo2O9) as well as MoO4 and WO4 (undoped La2MoWO9).68,69 Diffuse Reflectance Spectra and Band Gap Determination. The band gap energy, Eg, is a key attribute of materials that determines their applications in optoelectronics. Optical absorption measurements are widely used to characterize the electronic properties of ceramics, through the determination of such parameters describing the electronic transitions as the band gap. The band gaps for bulk undoped La2MoWO9 and Yb3+-doped La2MoWO9 powders prepared by three different methods were deduced from their diffuse reflectance spectra shown in Figures 10 and 11. The diffuse reflectance spectrum recorded the ratio of the light scattered from a thick layer of each sample and a nonabsorbing reference sample as a function of the wavelength. The reflectance data were converted into equivalent absorption ones using the Kubelka−Munk transformation according to the following equation:71 F(R ) = (1 − R )2 /2R

(2)

where R is reflectancy (%) and F(R) is the Kubelka−Munk function. It is well-known that the band gap, Eg, and absorption coefficient are related, as in the following equation68,72 αhν = F(R )hν = A(hν − Eg )n

(3)

where A is a constant characteristic for material under study and ν is light frequency. According to the Tauc model, the n parameter is a constant characterizing the transition process that may take the value 1/2, 3/2, 2, or 3, depending on the nature of the electronic transitions responsible for the absorption.73 The Tauc optical gap is determined through an extrapolation of the linear trend observed in the spectral dependence of (αhν)1/n, that is, [F(R)·hν]1/n over a limited range of photon energies hν. Plots of [F(R)·hν]2 versus the energy of the incident photon hν, first for undoped La2MoWO9 solid solutions and second for nano and microcrystalline of 10 mol % Yb3+-doped La2MoWO9 solid solutions and prepared by three synthesis methods shown in Figure 11. The band gap energy for n = 1/2, that is, for direct allowed transition of an electron from the valence to the conduction band was found to be 3.54 eV in undoped La2MoWO9 and 4.04 eV (combustion), 3.59 eV (Pechini), and 3.40 eV (solid-state method), according to the change of the grain size starting from 50 to 80 nm through 0.6−1.7 μm until 1−10 μm, respectively, to the applied methods. Further addition of Yb3+ does not change the host La2−xYbxMoWO9 matrix, suggesting that this absorption may be related to the formation of both tungstate and molybdate complex. This band gap increases with the decreasing of the size of the particles as expected. The same technique of measurement for Eg was used in La2(MoO4)3:Tb3+ and Gd2(MoO4)3:Tb3+ phosphors.77,78 The values of Eg are typical for insulators, for which electrons can not only be activated by thermal energy kT. These similar curves and the band gap values on both, undoped and Yb3+-doped La2MoWO9, make sense in the characterization of the fundamental absorption of these materials composed of Yb3+-doped mixed molybdato-tungstate powders. Consequently, here we have both absorption of tungstate groups by the excitation from the p orbital of O2− to Wt2g in the (WO42−) group and absorption of molybdate groups from the charge transfer band between the p orbital of

Figure 12. DTA−TG curves of cubic La2MoWO9 obtained in two different manners, that is, by using solid-state (a) and combustion methods (b), respectively.

W−O−W bonds in WO4 tetrahedra.68,69 In turn, the absorption bands observed in the IR spectra at 520 cm−1 are due to the asymmetric bending of ν4 modes of Mo−O and W− O bonds in adequate tetrahedra.68,69 The Raman spectra of cubic symmetry La2MoWO9 and Nd3+-doped La2Mo2O9 phases with different concentrations of Nd3+ active ions and different symmetry (cubic or monoclinic La2Mo2O9 type) are shown in Figure 9b. Raman spectrum of low-concentrated monoclinic La2Mo2O9:Nd3+ solid solution shows significant differences in the number and intensity of recorded absorption bands compared to Raman spectra of cubic phases. The Raman band in the La2MoWO9 spectrum observed around 920 cm−1 and active also in IR can be assigned to the symmetric stretching vibrations (ν1) of MoO4 as well as WO4 tetrahedra.68,69 In the case of cubic Nd3+-doped La2Mo2O9: this band related solely to the symmetric stretching vibrations (ν1) of MoO4 is slightly shifted toward of smaller wavenumbers. Very intensive Raman band observed for cubic doped lanthanum molybdate as well as weak one for La2MoWO9 and identified at 850 cm−1 are due to asymmetric stretching vibrations (ν3) of MoO4 (cubic La2Mo2O9:Nd3+) and simultaneously of MoO4 and WO4 in the case of lanthanum molybdato-tungstate.68,69 Broad Raman bands identified at around 775 cm−1 and not observed in IR are 13299


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The Journal of Physical Chemistry C O2− and the 4d orbital of Mo6+ within the octahedralcoordinated group of (MoO66−). We got intermediate values between rather high values of tungstates and lower values of molybdates, as can be seen from the following examples: in Y6MoO12, Eg = 3.05 eV,74 in RE2W2O9 Eg = 3.53 eV (Sm2W2O9), 3.65 eV (Eu2W2O9), 3.67 eV, (Pr2W2O9), and 3.73 eV (Gd2W2O9), respectively.75 Moreover, the Eg band gaps of scheelite compounds are 3.1 eV in (NaBi)0.5MoO4, 3.0 eV in (AgBi)0.5MoO4, and 3.5 eV in (NaBi)0.5WO4.76 DTA−TG Studies. Our fundamental studies covered also the thermal analysis. Figure 12a,b shows DTA-TG curves recorded during controlled heating up to 1260 °C of La2MoWO9 obtained in to different manners, that is, by using solid-state (Figure 12a) and combustion (Figure 12b) methods, respectively. No thermal effects were observed on DTA curves of both samples. Slight mass losses, higher for the sample obtained by a combustion method, are observed on TG curves of both samples. These small mass losses are the results of oxidation of organic residues as well as a removal of small amounts of both samples by flowing gas through the furnace chamber during DTA-TG measurements. The above results indicate that La2MoWO9 is thermally stable in air up to 1260 °C. It does not undergo polymorphic changes and exists in the cubic form only.

oxygen vacancies in the structure. The best results and the most translucent ceramics were obtained by annealing at 1200 °C for 6 h under vacuum. SEM images of 3 mol % Yb3+-doped La2MoWO9 microceramic annealed at 1200 °C/6 h in vacuum clearly show visible white points in the surface of grains with much higher Yb3+ concentration than inside the grains. The fabrication of ceramics by using SPS (Spark Plasma Sintering) technique is under progress at the MATEIS Laboratory of INSA-Lyon. A second part of this research will characterize spectroscopic properties of both powders and microceramic samples.79

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 71 375 73 73. E-mail: [email protected]. ORCID

Malgorzata Guzik: 0000-0002-5963-6316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank the Minister 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.

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CONCLUSIONS In this paper, we show that Yb3+-doped mixed La2MoWO9 molybdato-tungstate powders constitute a new type of cubic (the space group P213) material. Using high-temperature solidstate reaction, Pechini, and combustion methods, we have obtained the series activated in a wide Yb3+ concentration range (0.1, 1, 3, 10, 20 mol %). The structural studies have demonstrated that Yb3+-doped solid solutions and undoped La2MoWO9 crystallize in the cubic system. If we compare these results with our previous ones on Nd3+-doped La2Mo2O9, we can easily notice the benefits of Yb3+-doped La2MoWO9 because, to obtain the cubic β-form of Nd 3+ -doped La2Mo2O9, we had to add 50 mol % of active ions that is resulting in strong concentration quenching of the Nd3+ emission. Yb3+-doped mixed La2MoWO9 molybdato-tungstates are thermally stable in air up to 1260 °C and exists in the cubic form only. Nanocrystalline powders of Yb3+-doped La2MoWO9 with the smallest grain size of around 50−80 nm were obtained by the combustion method. The TEM images of nanopowders show homogeneous particles with ellipsoid shape that created bigger clusters. The other two methods of synthesis, Pechini and solidstate reaction, allowed us to obtain microcrystalline samples. The SEM images show that particles by Pechini possess irregular shape and their size is equal to 0.6−1.7 μm. The biggest particles were prepared by solid-state reaction, and they created loose clusters with various sizes that are composed by aggregation of the smaller grains. We noticed that the grain size increases with increasing annealing temperature of the products. Moreover, the absorption edge may be related to the formation of both tungstate and molybdate complexes and Eg values have been evaluated in each sample of undoped and Yb3+-doped mixed La2MoWO9 molybdato-tungstate powders. These nanocrystalline powders from combustion method were used to obtain the first microceramics of Yb3+-doped La2MoWO9. All microceramics are characterized by a light yellowish color, which it may be caused by the presence of the

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Toward Optical Ceramics Based on Cubic Yb3+ Rare Earth Ion-Doped

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