Figures
Abstract
This paper reports the growth, mechanical, thermal and spectral properties of Cr3+∶MgMoO4 crystals. The Cr3+∶MgMoO4 crystals with dimensions up to 30 mm×18 mm×14 mm were obtained by TSSG method. The absorption cross-sections of 4A2→4T1 and 4A2→4T2 transitions are 12.94×10−20 cm2 at 493 nm and 7.89×10−20 cm2 at 705 nm for E//Ng, respectively. The Cr3+∶MgMoO4 crystal shows broad band emission extending from 750 nm to 1300 nm with peak at about 705 nm. The emission cross-section with FWHM of 188 nm is 119.88×10−20 cm2 at 963 nm for E//Ng. The investigated results showed that the Cr3+∶MgMoO4 crystal may be regarded as a potential tunable laser gain medium.
Citation: Li L, Huang Y, Zhang L, Lin Z, Wang G (2012) Growth, Mechanical, Thermal and Spectral Properties of Cr3+∶MgMoO4 Crystal. PLoS ONE 7(1): e30327. https://doi.org/10.1371/journal.pone.0030327
Editor: Gerardo Adesso, University of Nottingham, United Kingdom
Received: June 6, 2011; Accepted: December 13, 2011; Published: January 24, 2012
Copyright: © 2012 Li et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work is supported by the National Natural Science Foundation of China (No. 61008060) and the National Natural Science Foundation of Fujian Province (No. 2011J01376), respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Tunable solid-state lasers have a wide field of applications in medicine, military, ultra short pulse generation and communication [1], [2]. Since 1960 many Cr3+-doped tunable laser crystals have been investigated, such as BeAl2O4, LiCaAlF6, Be3Al2(SiO3)6, GdScGa-Garnet and LaSc(BO3)4 [3]–[9]. Some of them have been became commercial material, such as Cr∶LiSrAlF6. Recently, research is focusing on the tunable solid-state laser crystals in visible and near infrared spectrum region for flash lamping and diode laser pumping. The Cr3+-doped molybdate crystals have gained interest because they have broad emission bands and larger absorption and emission cross-sections [10]–[12].
Metal molybdates of the general formula AMoO4 (A = Mg, Cd, Pb, Zn and Ca) have been attracted much attention owing to their important application of optoelectronic devices [13]–[16]. MgMoO4 is a member of this family, it belongs to monoclinic system with C2/m space group and cell parameters a = 10.273, b = 9.288, c = 7.025, β = 106.96°, z = 8. Recently, the MgMoO4 and Yb3+-doped MgMoO4 crystal were reported as a cryogenic phonon-scintillation detector [13]. The spectral properties of the Cr3+∶MgMoO4 crystal were reported such as crystal field strength and Racah parameters [17]. In this paper we further report the growth, mechanical, thermal and polarized spectral characteristics of the Cr3+∶MgMoO4 crystal.
Materials and Methods
1. Crystal Growth
Since the MgMoO4 crystal melts congruently at 1320°C, it can generally be grown by the Czochralski method, i. e. pulling directly from melt of the MgMoO4 crystal. However the MgMoO4 crystals with good quality were difficulty obtained because of the strong evaporation of MoO3 component under high temperature [18], [19]. Therefore, in order to reduced the growth temperature we selected the top seeded solution growth (TSSG) method to grow the Cr3+∶MgMoO4 crystals. The Cr3+∶MgMoO4 crystals were grown by the top seeded solution growth (TSSG) method from a flux of K2Mo2O7. The chemicals used were MgO, K2CO3, Mo2O3 and Cr2O3 with purity of 99.99%. The crystal growth was carried out in a vertical tubular furnace with a nickel-chrome wire as the heating element, as shown in Fig. 1. An AL-708 controller with a Pt-PtRh thermocouple controlled the furnace temperature and the cooling rate [20]. The temperature gradient in the furnace chamber was measured before performing of crystal growth, as shown in Fig. 2. The longitudinal temperature field in the furnace chamber could be divided into three parts: the flat zone ranging from B to C and the gradient zone ranging from A to B and C to D. The crystal growth was performed in the flat temperature zone in the furnace, which is available to grow large size crystal.
In order to select the suitable composition of the solution, the solubility curve of the MgMoO4 in the solution of MgMoO4-K2Mo2O7 was determined by the trial seeding method. The saturation temperatures were determined for various compositions in the range of 60–75 mol% by adjusting the temperature of the solution until a trial seeding showed no change in weight or surface micro topography after 3–4 h immersion. Fig. 3 shows the solubility curve of the MgMoO4 in the solution.
The crystal growth was performed by TSSG method. The procedure is as follows: firstly, the starting materials of 2 at% Cr3+-doped MgMoO4 and K2Mo2O7 were weighed according to the ratio of MgMoO4∶ K2Mo2O7 = 2∶3 mol. The weighed materials were mixed and put into the platinum crucible with dimension of Ø50 mm×60 mm. The full charged crucible was placed into the furnace and kept at 950°C for 48 h to make the solution melt completely and homogeneously. Secondly, a platinum wire was as seed crystal was soaked into the solution, and the temperature was cooled down from 950°C to 835°C at a cooling rate of 2°C/d. Then, the crystals grown on the platinum wire were drawn out of the solution surface and cooled down to room temperature at a cooling rate of 20°C/h. Finally, after obtained small crystals, a seed cut from the as-obtained crystal was used to grow large size crystals. The saturation temperature of the solution was exactly determined to be 865°C by repeated seeding. Then the seed was dipped into the solution at a temperature 20°C above saturation temperature and was kept at this temperature for 20 min to dissolve the surface of the seed. The crystals were grown at a cooling rate of 1°C/d and rotated at a rotating rate of 15 rpm in the range of 865∼835°C. In comparison with the Czochralski method, the starting growth temperature was reduced from 1320°C to 865°C, which greatly reduced the evaporation of MoO3 component. When the growth process ended, the crystals were pulled out of the solution and cooled to room temperature at a cooling rate of 20°C/h. The grown crystals with few inclusions were shown in Fig. 4, in which the Cr3+∶MgMoO4 crystals were grown along [001], [010] and [] directions, respectively. The maximum size is up to 30 mm×18 mm×14 mm. The morphology of the grown crystals depends on the growing direction. The morphology of the crystal grown along [001] direction appeared a regular hexagonal shape with families of crystal planes {001}, {110} and {111}, as shown in Fig. 4(c and d). A sample with dimensions of 5.39 mm×4.82 mm×3.92 mm and free inclusion was cut from the as-grown crystal (Fig. 4(e)). The optical homogeneity of the Cr3+∶MgMoO4 crystal was determined to be 3.9×10−5 using Tyman-Green optical interferometer, as shown in Fig. 4(f). This result shows that the grown crystal has good quality.
]; (c) crystal grown along [001] direction; (d) predicted hexagonal shape of Cr3+∶MgMoO4); (e) a cut sample with free inclusion ; (f) interference fringe of crystal. (where the color is reflecting light).
The concentration of Cr3+ ion in the Cr3+∶MgMoO4 crystal was determined to be 0.48 at% by inductively coupled plasma atomic emission spectrometry (ICP-AES). Then, the segregation coefficient of Cr3+ ion in crystal is defined as following formula:(1)Thus, the segregation coefficient of Cr3+ ion in the Cr3+∶MgMoO4 crystal is 0.24. The X-ray powder diffraction pattern of the Cr3+∶MgMoO4 was collected by MiniFlex II powder diffractometer with Cu Kα radiation, and the result was consistent with that reported by V.V. Bakakin [21].
2. Mechanical and Thermal Properties
The hardness is an important mechanical property of the crystal materials. The hardness of the Cr3+∶MgMoO4 crystal was measured using a Vickers microhardness tester (WILSON-WOLPERT 401MVA™) which equipped with a diamond square pyramid indenter attached to an incident-light microscope. Three samples with [100], [010] and [001] directions were cut from the as-grown Cr3+∶MgMoO4 crystal and polished for the test. The load applied for indenting was 200 kg and the indentation time was kept at 10 s for all samples. The value of the Vickers microhardness HV is calculated using the following expression(2)where P is the applied load and D is the diagonal length of the indentation.
For a crystal with well-defined cracks, the resistance to fracture indicates the toughness of a material. According to Ref. 20, fracture toughness Kc is dependent on the ratio of c/a, where c is the crack length and a is the half-diagonal length of the square indentation. When c/a≤2.5, the cracks have the Palmqvist's configuration, Kc is calculated using the equation(3)where l = c−a is the mean Palmqvist's crack length, the constant k is 1/7 for the Vickers indenter.
The brittleness index Bi is calculated using the relation(4)
The calculated results are listed in Table 1.
The thermal expansion of crystal is another important thermal factor for the crystal. Since the Cr3+∶ MgMoO4 crystal with monoclinic system is of anisotropy, three samples with dimensions of 6.0 mm×6.0 mm×20 mm used for thermal expansion coefficient measurement were cut from the Cr3+∶ MgMoO4 crystal along a-axis, b-axis and c-axis, respectively. The thermal expansion coefficients were measured using a DIL 402PC type thermal expansion dilatometer instrument at a heating rate of 10°C/min and the result in the range of 100∼800°C. Fig. 5 shows the linear expansion versus the temperature. The linear thermal expansion coefficient is defined as:(5)where L0 is the initial length of the sample at room temperature,ΔL is the change in length when temperature changes ΔT. The thermal expansion coefficient was calculated from the slope of the linear fitting of the linear relation between ΔL/L and temperature. The thermal expansion coefficients were calculated and listed in Table 2. The results show that the thermal expansion coefficient exhibits strongly direction dependence, the thermal expansion coefficient along b-axis is 3.3 times than that along c-axis.
3. Spectral Properties
Since the MgMoO4 crystal belongs to monoclinic system, there are three refractive indices along the optical indicatrix axis (Ng, Nm, Np) which do not coincide with the crystallographic axes (a, b, c). Ng and Nm are located in the ac plane, while Np is parallel to b-axes. The angular relation between the two sets axes of MgMoO4 crystal was determined by a polarizing microscope. Fig. 6 shows the relative orientation of the optical indicatrix axis (Ng, Nm, Np) relative to the crystallographic axes (a, b, c) of MgMoO4, Nm was located at about 7°51′ to –c axis, therefore Ng was located at about 24°25′ to a axis.
Based on the results obtained above, a sample of the Cr3+∶MgMoO4 crystal with dimension of 5.39 mm×4.82 mm×3.92 mm was cut from as-grown crystal and polished for the spectroscopic experiments,as shown in Fig. 4(e). The edges of cuboid were parallel to the optical indicatrix axis Ng, Nm, Np, respectively. The polarized absorption spectrum was measured using a Perkin-Elmer UV-VIS-NIR spectrometer (Lambda-900) in the range of 300–1100 nm at room temperature. The polarized fluorescence spectra were measured using the Edinburgh Analysis Instruments FLS920 spectrophotometer with Xenon lamp as light source. The fluorescence lifetime was measured by by Lifespec-ps system of Edinburgh Instruments Ltd. The light source is continuous tunable picosecond pulsed Ti: sapphire (Tsunami+GWU). In experiment of lifetime measurement, the pulse duration of the incident light is 2∼100 ps, the time resolution of the MCP-PMT detector is about 50 ps, the resolution of the monochromater is 0.5∼2 nm, and the signal-to-noise ratio of Lifespec-ps system is 6000∶1. The wavelength of the excited light is 700 nm, and the detection wavelength is 820 nm.
Fig. 7 shows the polarized absorption spectra of the Cr3+∶MgMoO4 crystal measured at room temperature. The spectrum consists of two broad absorption bands centered at about 492 nm and 703 nm, corresponding to the electron-vibronic transitions from the 4A2 ground state to the excited 4T2 and 4T1 states. The dip presented at 726 nm on the low energy band is caused by Fano-type antiresonance due to the spin forbidden transition from 4A2 to 2E and the R-lines were not observed [22], [23]. The structure of the absorption band was characteristic of the Cr3+ ion in a weak crystal field as well as the absorption spectrum of the Cr3+∶LiSrAlF6 and Cr3+∶LaSc(BO3)4 [5], [8]. The absorption cross-sections σa were determined to be 12.9×10−20 cm2 at 491 nm and 7.89×10−20 at 705 nm for E//Ng, respectively.
Fig. 8 presented the polarized and unpolarized fluorescence spectra measured at room temperature and 10K. It is shown that the main feature of the fluorescence spectra is a broad band extending from 750 nm to 1300 nm, corresponding to the transition from 4T2 excited level to 4A2 ground level. Even at 10K it is still a broad emission band. The luminescence spectra of Cr3+∶MgMoO4 crystals are strongly polarized at room temperature. The broadest emission band was observed with a peak at 963 nm with a full width at half maximum (FWHM) of 188 nm for E//Ng. Such broad absorption and emission bands were caused by the structure of the MgMoO4 crystal, except for its broad and emission transitions of the Cr3+ ions. It is reason that the structure of the MgMoO4 crystal consists of two types of MgO6 tetrahedra [24], the Cr3+ ions occupied the different Mg2+ sites in the two types of MgO6 tetrahedra when the Cr3+ ions were doped into the MgMoO4 crystal and replaced the Mg2+ ions. In other word, the Cr3+ ions occupied the two luminous centers, which results in broad absorption and emission bands.
The absorption and fluorescence spectra of the Cr3+∶MgMoO4 crystal indicated that the Cr3+ ions in the Cr3+∶MgMoO4 crystal occupied a weak-field site, in which the 4T2 level is below the 2E level. According to Tanabe-Sugano diagram [25], the signification of Cr3+ ions occupying strong-field or weak-field sites is immediately apparent from the luminescence spectrum. In the strong-field the luminescence spectrum consists of broadband emission of 4T2→2A2 transition and sharp line emission of 2E2→2A2 transition. The weak-field sites in the Cr3+∶MgMoO4 crystal give rise to the Cr3+ luminescence in the 4T2→2A2 transition band alone, even at 10K dominant feature of photoluminescence spectrum is still broadband emission, which is available for tunable laser crystal. The fluorescence lifetime was determined to be 1 μs measured at room temperature. Since the radiation lifetime is mainly derived from the parity-forbidden 4T2→2A2 transition with short lifetime in the weak-field, the Cr3+∶MgMoO4 crystal has a very short fluorescence lifetime.
The emission cross-section σe was calculated using the formula [26](6)where λ is the wavelength of the emission peak, n is the refractive index of the Cr3+∶MgMoO4 crystal, which was determined by the method of minimum deviation and the value was listed in Table 3, and
the frequency of FWHM. The
is the fluorescence lifetime, which was determined to be 1 µs. Thus, the emission cross-section of 4T2→4A2 transition is 119.88×10−20 cm2 at 963 nm for E//Ng.
Results and Discussion
The Cr3+∶MgMoO4 crystals with dimensions up to 30 mm×18 mm×14 mm were obtained by TSSG method. The mechanical and thermal properties and polarized optical characteristic of the Cr3+∶MgMoO4 were investigated. The thermal expansion coefficient of the Cr3+∶MgMoO4 crystal along c-axis is smaller than that of the other directions. The investigated results of spectral properties of the Cr3+∶MgMoO4 crystal showed that its absorption and emission spectra exhibit strong polarized and depend on the optical indicatrix axis Ng, Nm, Np at room temperature. The Cr3+∶MgMoO4 crystal has large absorption cross-section at about 705 nm, which is available for the diode laser pumping. The Cr3+∶MgMoO4 crystal exhibits a broad band emission extending from 750 nm to 1300 nm with peak at about 705 nm. The emission cross-section with FWHM of 188 nm is 119.88×10−20 cm2 at 963 nm for E//Ng. In comparison with other Cr3+ doped materials (Table 4), the Cr3+∶MgMoO4 crystal has large emission cross-section and FWHM of the fluorescence. The fluorescence lifetime was determined is 1 μs, which is available to apply to short pulse laser. To sum up above the results, the conclusion was drawn that the Cr3+∶MgMoO4 crystal may be regarded as a potential tunable laser gain medium.
Author Contributions
Conceived and designed the experiments: LL GW. Performed the experiments: LL ZL. Analyzed the data: LL GW. Contributed reagents/materials/analysis tools: YH LZ. Wrote the paper: LL GW.
References
- 1. Kück S (2001) Laser-related spectroscopy of ion-doped crystals for tunable solid state-lasers. Appl Phys B 72: 515–562.
- 2. Samtleben TA, Hulliger J (2005) LiCaAlF6 and LiSrAlF6: tunable solid state laser host materials. Opt Laser Eng 43: 251–262.
- 3. Kenyon PT, Andrews L, McCollum B, Lempicki A (1982) Tunable infrared solid-state laser materials based on Cr3+ in low ligand fields. IEEE J Quantum Electron QE-18: 1189–1197.
- 4. Payne SA, Chase LL, Newkirk HW, Smith LK, Krupke WF (1988) LiCaAlF6∶Cr3+: a promising new solid-state laser material. IEEE J Quantum Electron 24: 2243–2252.
- 5. Payne SA, Chase LL, Smith LK, Kway WL, Newkirk HW (1989) Laser performance of LiSrAlF6∶Cr3+. J Appl Phys 66: 1051–1056.
- 6. Kisliuk P, Moore CA (1967) Radiation from the 4T2 state of Cr3+ in ruby and emerald. Phys Rev 160: 307–312.
- 7. Struve B, Huber G, Laptev VV, Shcherbakov IA, Zharikov EV (1983) Tunable room-temperature cw laser action in Cr3+: GdScGa-Garnet. Appl Phys B 30: 117–120.
- 8. Long XF, Lin ZB, Hu ZS, Wang GF (2004) Polarized spectral characteristics and energy levels of Cr3+∶LaSc3(BO3)4 crystal. Chem Phys Lett 392: 192–195.
- 9. Lai ST, Chai BT, Long M, Morris RC (1986) ScBO3∶Cr–A room temperature near-infrared tunable laser. IEEE J Quantum Electron 22: 1931–1933.
- 10. Wang GJ, Long XF, Zhang LZ, Wang GF, Polosan S, et al. (2008) Spectroscopic characteristic and energy levels of Cr3+ in Cr3+∶KAl(MoO4)2 crystal. J Lumin 128: 1556–1560.
- 11. Wang GJ, Zhang LZ, Lin ZB, Wang GF (2010) Growth and spectroscopic characteristics of Cr3+∶CsAl(MoO4)2 crystal. J Alloys Compd 489: 293–296.
- 12. Wang GJ, Lin ZB, Zhang LZ, Huang YS, Wang GF (2009) Spectral characterization and energy levels of Cr3+∶Sc2(MoO4)3 crystal. J Lumin 129: 1398–1400.
- 13. Spasskii DA, Kolobanov VN, Mikhailin VV, Berezovskaya LYu, Ivleva LI, et al. (2009) Luminescence peculiarities and optical properties of MgMoO4 and MgMoO4∶Yb crystals. Opt Spectro 106: 556–563.
- 14. Mikhailik VB, Kraus H, Wahl D, Mykhaylyk MS (2005) Studies of electronic excitations in MgMoO4, CaMoO4 and CdMoO4 crystals using VUV synchrotron radiation. Phys Status Solidi B 242: R17–R19.
- 15. Preifer E, Rudolph P (1990) Investigations of the crystal growth of PbMoO4 by the Czochralski method. Crystal Res Technol 25: 3–9.
- 16. Mikhailik VB, Kraus H, Wahl D, Ehrenberg H, Mykhaylyk MS (2006) Optical and luminescence studies of ZnMoO4 using vacuum ultraviolet synchrotron radiation. Nucl Instrum Methods Phys Res Sect A 562: 513–516.
- 17. Cavalli E, Belletti A, Brik MG (2008) Optical spectra and energy levels of the Cr3+ ions in MWO4 (M = Mg, Zn, Cd) and MgMoO4 crystals. J Phys Chem Solids 69: 29–34.
- 18. Spasskii DA, Kolobanov VN, Mikhailin VV, Berezovskaya LYu, Ivleva LI, et al. (2009) Luminescence peculiarities and optical properties of MgMoO4 and MgMoO4∶Yb crystals. Opt Spectro 106: 556–563.
- 19. Mikhailik VB, Kraus H, Wahl D, Mykhaylyk MS (2005) Studies of electronic excitations in MgMoO4, CaMoO4 and CdMoO4 crystals using VUV synchrotron radiation. phys stat sol (b) 242, No. 2: R17–R19.
- 20. Wang GF, Luo ZD, Chen JM (1990) Flux growth of Cr3+ ∶ LiGaW2O8 crystals and phase equilibrium of LiGaW2O8-Li2B2O4. J Crystal Growth 100: 447–449.
- 21. Bakakin VV, Klevtsova RF, Gaponenko LA (1982) Crystal-structure of magnesium molybdate MgMoO4-An example of the modified closed packing with 2 types of tetrahedra. Kristallografiya 27: 38–42.
- 22. Fano U (1961) Effects of configuration interaction on intensities and phase shifts. Phys Rev 124: 1866–1878.
- 23. Fano U, Cooper JW (1965) Line profiles in far-uv absorption spectra of rare gases. Phys Rev 137: A1364–A1379.
- 24. Bakakin VV, Klevtsova RF, Gaponenko LA (1982) Crystal structure of magnesium molybdate MgMoO4-an example of modified closest packing with two types of tetrahedra. Kristallografiya 27: 38–42.
- 25.
Sugano S, Tanabe Y, Kamimura H (1970) Multiplets of transition metal ions in crystals, Academy Press, New York.
- 26. Huber G, Krühler WW, Bludau W, Daniemeyer HG (1975) Anisotropy in the laser performance of NdP5O14. J Appl Phys 46: 3580–3584.
- 27. Wang GJ, Huang YS, Zhang LZ, Lin ZB, Wang GF (2011) Growth and spectral properties of Cr3+∶RbAl(MoO4)2 crystal. Mat Rers Innovat 15: 167–171.
- 28. Wang GJ, Zhang LZ, Lin ZB, Wang GF (2010) Growth and spectroscopic characteristics of Cr3+∶CsAl(MoO4)2 crystal. J Alloys Compd 489: 293–296.