ARTICLE pubs.acs.org/crystal
Growth, Structure, and Optical Properties of the Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 Crystal Guojian Wang, Yisheng Huang, Lizhen Zhang, Shengping Guo, Gang Xu, Zhoubin Lin, and Guofu Wang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, P.O. Box 143, Fuzhou, Fujian 350002, China
bS Supporting Information ABSTRACT: The Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal was grown from a flux of K2Mo3O10 by the top seeded solution growth method. The K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal crystallizes in the rhombohedral system in space group R3c. The K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal has a highly disordered structure. Investigation of the spectral properties of the Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal shows that it exhibits broad absorption and emission bands, which are caused by the disordered structure of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal, except for the broad emission of Cr3+ ions. On the basis of the absorption and emission spectra, the crystal field strength Dq/B and Racah parameter C have been calculated. The Dq/B value of 2.18 implies that Cr3+ ion occupies the low crystal field site in the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. These results suggest that the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal is a potential tunable laser crystal material.
1. INTRODUCTION Tunable solid-state lasers have a wide range of application in medicine, ultrashort pulse generation, environmental wotk, and communication.1,2 With the development of diode-pumped solid-state lasers, the discovery of more efficient Cr3+-doped materials for diode-pumped tunable solid-state lasers has generated much interest.3,4 Recently, Cr3+-doped molybdate crystals have received much attention because of their interesting properties in tunable laser applications.514 In 1988, Kozhevnikov et al. discovered a new compound, K1xMg1xSc1+x(MoO4)3 (x = 00.6).15 To the best of our knowledge, its structural details and crystal growth have not been reported. After exploring new Cr3+-doped tunable laser host materials, we report the growth, structure, and optical properties of the Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. 2. EXPERIMENTAL PROCEDURES 2.1. Crystal Growth. Because K0.6(Mg0.3Sc0.7)2(MoO4)3 melts incongruently,15 it is grown only by the flux method. The pure and Cr3+doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals were grown from a flux of K2Mo3O10 by the top seeded solution growth (TSSG) method, where the molar ratio of K0.6(Mg0.3Sc0.7)2(MoO4)3 to K2Mo3O10 is 1:2. The raw materials of pure and 1 atom % Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals and K2Mo3O10 were weighed according to their stoichiometric composition. The weighed materials were mixed and put into a platinum crucible with a volume of 100 mm3, and then the platinum crucible was placed into a vertical tubular furnace. The mixtures were heated to 850 °C for 2 days to make the solution melt completely and homogeneously. The saturation temperature of the r 2011 American Chemical Society
Figure 1. Pure (a) and Cr3+-doped (b) K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals grown by the TSSG method. solution was exactly determined by repeated seeding trials. The crystals were grown at a cooling rate of 12 K/day and a rotating rate of 4.5 rpm. When the growth ended, the grown crystals were pulled out of the solution and cooled slowly to room temperature. Pure and Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals with dimensions 19 mm 18 mm 5 mm and 27 mm 26 mm 8 mm, respectively, were obtained, as shown in Figure 1. The contents of K, Mg, Sc, Cr, and Mo ions in Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals were measured by ionic coupled plasma (ICP) spectrometry, and the results are listed in Table 1. The measured K:Mg:(Sc+Cr):Mo molar ratio is close to 0.6:0.6:1.4:3, which agrees with the composition of K0.6(Mg0.3Sc0.7)2(MoO4)3. The ICP Received: April 8, 2011 Revised: July 8, 2011 Published: July 11, 2011 3895
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Table 1. Elemental Analysis Using the ICP Technique (Sc+Cr)a K
Mg
Sc
Cr
Mo
required for 0.61 atom % Cr : 4.04
2.51
10.726
0.125
49.548
10.53
0.12
49.25
3+
K0.6(Mg0.3Sc0.7)2(MoO4)3 (wt %) measured (wt %)
4.01
2.56
calculated molar number
0.103
0.105 0.234
0.0023 0.513
molar ratio
0.595
0.612 1.36
0.013
2.978
a
Because the Sc3+ and Cr3+ ions have the same valence, the Sc3+ ions were generally replaced with Cr3+ ions in the Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal.
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters atom
x
site
y
Ueq (Å2)
z
SOF
Mo(1)
18e
0.38405(3)
0.3333
0.0833
0.0094(2)
1
Mg(1)
12c
0.3333
0.6667
0.02398(3)
0.0093(2)
0.30
Sc(1) K(1)
12c 6b
0.3333 0.3333
0.6667 0.6667
0.02398(3) 0.1667
0.0093(2) 0.0353(7)
0.70 0.6
O(2)
36f
0.3627(3)
0.5065(3)
0.07413(9)
0.0227(4)
1
O(1)
36f
0.4734(3)
0.3015(3)
0.02551(9)
0.0291(5)
1
Figure 2. View of the structural unit of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal.
Table 3. Selected Bond Lengths (angstroms) for the K0.6(Mg0.3Sc0.7)2(MoO4)3 Crystala Mo(1)O(1)#1
1.742(2)
Mg(1)O(1)#7
2.077(2)
Mo(1)O(1)
1.742(2)
K(1)O(2)#4
2.802(2)
Mo(1)O(2)#1
1.756(2)
K(1)O(2)#3
2.802(2)
Mo(1)O(2)
1.756(2)
K(1)O(2)
2.802(2)
Mg(1)O(2)#3
2.066(2)
K(1)O(2)#8
2.802(2)
Mg(1)O(2)#4
2.066(2)
K(1)O(2)#9
2.802(2)
Mg(1)O(2)
2.066(2)
K(1)O(2)#10
2.802(2)
Mg(1)O(1)#5 Mg(1)O(1)#6
2.077(2) 2.077(2)
O(1)Sc(1)#7 O(1)Mg(1)#7
2.077(2) 2.077(2)
a
Figure 3. View of the coordination environment of the K, Sc, Mo, and Mg atoms in the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal.
analysis confirmed that the grown crystals belong to the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. 2.2. Structure Analysis. A single crystal of K0.6(Mg0.3Sc0.7)2(MoO4)3 with dimensions of 0.4 mm 0.4 mm 0.3 mm was cut from the as-grown pure crystal and subjected to X-ray diffraction analysis. The diffraction data were collected on a Rigaku Mercury CCD diffractometer equipped with graphite-monochromated Mo KR (λ = 0.71073 Å) radiation at 293 K. A total of 477 independent reflections were collected in the θ range of 325.48°, of which 469 with I g 2σ(I) were independent. The absorption correction based on the empirical PSI scan technique was applied. The structure was determined by direct methods and refined by the full-matrix least-squares technique. The final R = 0.0156, and wR = 0.0610 with the following parameters: w = 1/[σ2Fo2 + (0.0485P)2 + 2.9109P], where P = (Fo2 + 2Fc2)/3, (ΔF)max = 0.696, (ΔF)min = 0.364 e Å3, (Δ/σ)max = 0.000, and S = 1.027. All calculations were conducted with SHELXTL. The atomic coordinates
and equivalent isotropic displacement parameters are listed in Table 2, and the selected bond lengths are listed in Table 3. The structure of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal is illustrated in Figures 2 and 3. This crystal has a high degree of structural disorder. The magnitude of the temperature factors and valence charge equilibrium after least-squares refinement indicates the existence of a statistical distribution of Mg and Sc atoms with a Mg:Sc occupancy ratio of 0.3:0.7. Such disordered structure can result in broad absorption and emission bands in the laser materials.16,17 K0.6(Mg0.3Sc0.7)2(MoO4)3 possesses a framework that includes Mg/ScO6 octahedra and MoO4 tetrahedra. Each Mg/ScO6 octahedron shares its six corners with six MoO4 tetrahedra. As an interstitial atom, a potassium atom occupies the 6b site. As for the Cr3+-doped laser host crystal, the Sc3+ ion in the ScO6 octahedron can be replaced with the Cr3+ ion because the ion radius of the Cr3+ ion (0.755 Å) approaches that of the Sc3+ ion (0.885 Å). The disordered structure will result in the absorption and emission bands broadening homogeneously when the Cr3+ ions take the place of the Sc3+ ions in the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. Figure 4 shows the powder X-ray diffraction pattern of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal,
Symmetry transformations used to generate equivalent atoms: (#1) x y + 1/3, y + 2/3, z + 1/6; (#2) y 2/3, x 1/3, z + 1/6; (#3) y + 1, x y + 1, z; (#4) x + y, x + 1, z; (#5) y, x + y + 1, z; (#6) x y, x, z; (#7) x + 1, y + 1, z; (#8) x y + 2/3, x + 1/3, z + 1/3; (#9) y 1/3, x + y + 1/3, z + 1/3; (#10) x + 2/3, y + 4/3, z + 1/3.
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Figure 4. Powder X-ray diffraction pattern of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal.
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Figure 6. Absorption spectrum of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal at 300 K.
Table 4. Comparison of Crystal Field Parameters for Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 with Those of Other Cr3+-Doped Crystals Dq (cm1) B (cm1) Dq/B C (cm1) C/B
material
Figure 5. Growth morphology of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. which was determined using a D-max-rA type diffractometer with Cu KR radiation (λ = 1.54056 Å). The faces of the grown crystal of pure and Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystals were determined with the YX-200 X-ray diffraction orienting instrument, which belong to (0 1 2). On the basis of the structure of K0.6(Mg0.3Sc0.7)2(MoO4)3, the morphological scheme of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal is drawn, as shown in Figure 5. 2.3. Spectral Properties. The absorption spectrum at room temperature was recorded with a Perkin-Elmer UVvisNIR spectrometer (Lambda-35). The fluorescence spectrum and fluorescence lifetime were measured with an Edinburgh Instruments FLS920 spectrophotometer with a continuous Xe-flash lamp at 300 K. Figure 6 shows the absorption spectrum of the Cr3+ :K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal at 300 K. The absorption band with a peak at 489 nm between 420 and 600 nm is attributed to the 4A2 f 4T1 transition of the Cr3+ ion. The absorption region (600850 nm) has properties similar to those of Cr3+-doped Sc2(MoO4)3 and Sc2(WO4)3 crystals.13,18 The two dips near 705 nm in the absorption band of the 4 A2 f 4T2 transition were caused by an interaction between the sharp intra-t32 level (2E, 2T1, and 2T2) and the vibrationally broadened t22e (4T2) quasi-continnum, resulting in Fano-antiresonance structures.19,20 We take the dips in the absorptions as approximate values of the positions of the 4A2 f 2T1 and 2E transitions. The absorption cross sections (σR) of the 4A2 f 4T1 and 4A2 f 4T2 transitions were estimated to be 1.31 1019 cm2 at 489 nm and 1.06 1019 cm2 at 705 nm, respectively. The crystal field strength Dq and the Racah parameters B and C can be calculated using the absorption spectrum of the Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. The value of Dq was obtained from the
Al2O323
1664
640
2.6
3300
5.2
YAB24
1680
672
2.5
3218
4.8
GAB25
1695
673
2.52
3380
YSB25
1539
644
2.39
GSB25
1563
638
2.45
LSB26 GSGG27
1529 1563
675 638
2.27 2.45
3448
5.1
NaAl(WO4)228
1548
615.6
2.51
3083
5.0
NaMg3Al(MoO4)529
1440
676
2.13
2945
4.36
KAl(MoO4)29
1494.8
585.5
2.55
3049
5.2
Sc2(MoO4)313
1410.4
604.4
2.33
3000
4.96
CsAl(MoO4)214
1492.5
582.1
2.56
3066
5.25
K0.6(Mg0.3Sc0.7)2(MoO4)3
1418.4
650.8
2.18
2994
4.6
5.0
Figure 7. Photoluminescence spectrum of Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 excited with 705 nm radiation at 300 K. peak energy of the 4A2 f 4T2 transition:21 Dq ¼ 3897
Eð4 T 2 Þ Eð4 A 2 Þ ¼ 1418:4 cm1 10
ð1Þ
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Table 5. Spectral Parameters of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 Crystal and Other Cr3+-Doped Materials A2 f 4T1
4
material
4
A2 f 4T2
T2 f 4A2
4
λ (nm) σR (1020 cm2) λ (nm) σR (1020 cm2) λ (nm)
fwhm
σe (1020 cm2) τf (μs)
ref
BeAl2O4
420
10.0
600
20.0
750
0.5
260
K2NaScF6
430
1.4
630
0.7
760
1.3
285
32
GSGG
488
5.1
647.1
3.3
777
0.75
114
33
LiCaAlF6
425.5
625
780
2000 cm1
1.23
175
34
33
12
0.2 21
13 14
10
this work
KAl(MoO4)2 σ-polarization
480
π-polarization Sc2(MoO4)3 CsAl(MoO4)2 K0.6(Mg0.3Sc0.7)2(MoO4)3
8.44
669
3.72
823
146 nm
481
5.03
499 481
37.4 5.05
668
2.25
823
135 nm
710 670
32.1 3.06
880 818
176 nm (2179 cm1) 147 nm
375 4.27
489
13.1
705
10.6
870
189 nm (2383 cm1)
8.46
The magnitude of B can be calculated using the equation22
B ¼ Dq
!2 ! ΔE ΔE 10 Dq Dq ! ΔE 8 15 Dq
ð2Þ
where ΔE is the peak energetic difference between the 4T2 and 4T1 states. Upon substitution of the value of Dq and a ΔE of 6265 cm1, B was calculated to be 650.8 cm1. C can be determined from an approximate expression given by Henderson and Imbush:22 Eð2 EÞ=B≈3:05ðC=BÞ 1:80ðB=DqÞ þ 7:90
ð3Þ
Thus, C was calculated to be 2994 cm1. The calculated results are listed in Table 4. The fluorescence spectrum of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal is shown in Figure 7. It exhibits a broad band emission extending from 730 to 1280 nm, corresponding to the transition from the 4T2 excited level to the 4A2 ground level. The emission band with a peak at 870 nm and a full width at half-maximum (fwhm) of 189 nm corresponds to the 4T2 f 4A2 transition. The emission cross section σe was calculated using the formula31 λ 4π2 τf n2 Δν 2
σe ¼
ð4Þ
where λ is the wavelength of the emission peak, n is refractive index that was estimated to be 1.78 by an Abbe retractometer at 589 nm, Δν is the frequency at fwhm (Δν = 7.15 1013 s1), and τf is the fluorescence lifetime. The fluorescence lifetime was measured to be 10 μs at 300 K. Thus, the emission cross section σe was calculated to be 8.46 1020 cm2 at 870 nm and 300 K.
3. RESULTS AND DISCUSSION The Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal with dimensions of 27 mm 26 mm 8 mm has been grown from a flux of K2Mo3O10 by the TSSG method. The K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal belongs to the rhombohedral system with space group R3c and the following cell parameters: a = 9.4300(8) Å, c = 24.337(4) Å, and Z = 6. This crystal has a highly disordered structure. The absorption and emission spectra of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal exhibit broad absorption and emission bands. Such broad absorption and emission bands were caused by the highly disordered structure of the K0.6(Mg0.3Sc0.7)2(MoO4)3
32
2.74 2.93
crystal, except for its broad absorption and emission transitions of the Cr3+ ion. On the basis of the absorption and emission spectra, the crystal field strength and Racah parameter have been calculated: Dq = 1418.4 cm1, B = 650.8 cm1, and C = 2933.1 cm1. Compared with the Cr:LiCaAlF6 crystal and other Cr3+-doped molybdate crystals with ordered structure (see Tables 4 and 5), the Cr3+: K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal has a low value of Dq/B (2.18). When the value of Dq/B is between 2.18 and 2.3, it implies that the Cr3+ ion occupies the low crystal field site.30 The Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal has a broad emission fwhm of 189 nm and larger absorption and emission cross sections.
4. CONCLUSION Because the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal with disordered structure can have broad absorption and emission bands, the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal may be regarded for potential tunable laser medium applications. ’ ASSOCIATED CONTENT
bS
Supporting Information. CIF data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-59183714636. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61008060) and the Young Scientists Innovation Foundation of Fujian Province (2008F3113). ’ REFERENCES (1) K€uck, S. Appl. Phys. B: Lasers Opt. 2001, 72, 515–562. (2) Samtleben, T. A.; Hulliger, J. Opt. Laser Eng. 2005, 43, 251–262. (3) Scheps, R.; Gately, B. M.; Myers, J. F.; Krasinski, J. S.; Heller, D. F. Appl. Phys. Lett. 1990, 56, 2288–2290. (4) Skidmore, J. A.; Emanuel, M. A.; Beach, R. J.; Benett, W. J.; Freitas, B. L.; Carlson, N. W.; Soalrz, R. W. Appl. Phys. Lett. 1995, 66, 1163–1165. (5) Hermanowicz, K. J. Alloys Compd. 2002, 341, 179–182. 3898
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