Growth, Mechanical, Thermal and Spectral Properties of Cr3+∶MgMoO4 Crystal

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.

Metal molybdates of the general formula AMoO 4 (A = Mg, Cd, Pb, Zn and Ca) have been attracted much attention owing to their important application of optoelectronic devices [13][14][15][16]. MgMoO 4 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, b = 106.96u, z = 8. Recently, the MgMoO 4 and Yb 3+ -doped MgMoO 4 crystal were reported as a cryogenic phonon-scintillation detector [13]. The spectral properties of the Cr 3+ :MgMoO 4 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 Cr 3+ :MgMoO 4 crystal.

Crystal Growth
Since the MgMoO 4 crystal melts congruently at 1320uC, it can generally be grown by the Czochralski method, i. e. pulling directly from melt of the MgMoO 4 crystal. However the MgMoO 4 crystals with good quality were difficulty obtained because of the strong evaporation of MoO 3 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 Cr 3+ :MgMoO 4 crystals. The Cr 3+ :MgMoO 4 crystals were grown by the top seeded solution growth (TSSG) method from a flux of K 2 Mo 2 O 7 . The chemicals used were MgO, K 2 CO 3 , Mo 2 O 3 and Cr 2 O 3 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 MgMoO 4 in the solution of MgMoO 4 -K 2 Mo 2 O 7 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 MgMoO 4 in the solution.
The crystal growth was performed by TSSG method. The procedure is as follows: firstly, the starting materials of 2 at% Cr 3+doped MgMoO 4 and K 2 Mo 2 O 7 were weighed according to the ratio of MgMoO 4 : K 2 Mo 2 O 7 = 2:3 mol. The weighed materials were mixed and put into the platinum crucible with dimension of Ø50 mm660 mm. The full charged crucible was placed into the furnace and kept at 950uC 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 950uC to 835uC at a cooling rate of 2uC/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 20uC/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 865uC by repeated seeding. Then the seed was dipped into the solution at a temperature 20uC 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 1uC/d and rotated at a rotating rate of 15 rpm in the range of 865,835uC. In comparison with the Czochralski method, the starting growth temperature was reduced from 1320uC to 865uC, which greatly reduced the evaporation of MoO 3 component. When the growth process ended, the crystals were pulled out of the solution and cooled to room temperature at a cooling rate of 20uC/h. The grown crystals with few inclusions were shown in Fig. 4 Fig. 4(c and d). A sample with dimensions of 5.39 mm64.82 mm63.92 mm and free inclusion was cut from the as-grown crystal ( Fig. 4(e)). The optical homogeneity of the Cr 3+ :MgMoO 4 crystal was determined to be 3.9610 25 using Tyman-Green optical interferometer, as shown in Fig. 4(f). This result shows that the grown crystal has good quality.
The concentration of Cr 3+ ion in the Cr 3+ :MgMoO 4 crystal was determined to be 0.48 at% by inductively coupled plasma atomic emission spectrometry (ICP-AES). Then, the segregation coefficient of Cr 3+ ion in crystal is defined as following formula: g~C r 3z concentration in the crystal Cr 3z concentration in the initial charge ð1Þ Thus, the segregation coefficient of Cr 3+ ion in the Cr 3+ :MgMoO 4 crystal is 0.24. The X-ray powder diffraction pattern of the Cr 3+ :MgMoO 4 was collected by MiniFlex II powder diffractometer with Cu K a radiation, and the result was consistent with that reported by V.V. Bakakin [21].   was kept at 10 s for all samples. The value of the Vickers microhardness HV is calculated using the following expression

Mechanical and Thermal Properties
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 K c 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, K c is calculated using the equation where l = c2a is the mean Palmqvist's crack length, the constant k is 1/7 for the Vickers indenter. The brittleness index B i is calculated using the relation The calculated results are listed in Table 1.
The thermal expansion of crystal is another important thermal factor for the crystal. Since the Cr 3+ : MgMoO 4 crystal with monoclinic system is of anisotropy, three samples with dimensions of 6.0 mm66.0 mm620 mm used for thermal expansion coefficient measurement were cut from the Cr 3+ : MgMoO 4 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 10uC/min and the result in the range of 100,800uC. Fig. 5 shows the linear expansion versus the temperature. The linear thermal  expansion coefficient is defined as: where L 0 is the initial length of the sample at room temperature,DL is the change in length when temperature changes DT.
The thermal expansion coefficient was calculated from the slope of the linear fitting of the linear relation between DL/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 caxis.

Spectral Properties
Since the MgMoO 4 crystal belongs to monoclinic system, there are three refractive indices along the optical indicatrix axis (N g , N m , N p ) which do not coincide with the crystallographic axes (a, b, c). N g and N m are located in the ac plane, while N p is parallel to b-axes. The angular relation between the two sets axes of MgMoO 4 crystal was determined by a polarizing microscope. Fig. 6 shows the relative orientation of the optical indicatrix axis (N g , N m , N p ) relative to the crystallographic axes (a, b, c) of MgMoO 4 , N m was located at about 7u519 to -c axis, therefore N g was located at about 24u259 to a axis.
Based on the results obtained above, a sample of the Cr 3+ :MgMoO 4 crystal with dimension of 5.39 mm64.82 mm 63.92 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 N g , N m , N p , 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 (Tsuna-mi+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 Cr 3+ :MgMoO 4 crystal measured at room temperature. The spectrum consists of two broad absorption bands centered at about 492 nm and 703 nm, corresponding to the electronvibronic transitions from the 4 A 2 ground state to the excited 4 T 2 and 4 T 1 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 4 A 2 to 2 E and the R-lines were not observed [22,23]. The structure of the absorption band was characteristic of the Cr 3+ ion in a weak crystal field as well as the absorption spectrum of the Cr 3+ :LiSrAlF 6 and Cr 3+ :LaSc(BO 3 ) 4 [5,8]. The absorption cross-sections s a were determined to be 12.9610 220 cm 2 at 491 nm and 7.89610 220 at 705 nm for E// N g , 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 4 T 2 excited level to 4 A 2 ground level. Even at 10K it is still a broad emission band. The luminescence spectra of Cr 3+ :MgMoO 4 crystals are strongly polarized at room tempera-ture. 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//N g . Such broad absorption and emission bands were caused by the structure of the MgMoO 4 crystal, except for its broad and emission transitions of the Cr 3+ ions. It is reason that the structure of the MgMoO 4 crystal consists of two types of MgO 6 tetrahedra [24], the Cr 3+ ions occupied the different Mg 2+ sites in the two  types of MgO 6 tetrahedra when the Cr 3+ ions were doped into the MgMoO 4 crystal and replaced the Mg 2+ ions. In other word, the Cr 3+ ions occupied the two luminous centers, which results in broad absorption and emission bands.
The absorption and fluorescence spectra of the Cr 3+ :MgMoO 4 crystal indicated that the Cr 3+ ions in the Cr 3+ :MgMoO 4 crystal occupied a weak-field site, in which the 4 T 2 level is below the 2 E level. According to Tanabe-Sugano diagram [25], the signification of Cr 3+ ions occupying strongfield or weak-field sites is immediately apparent from the luminescence spectrum. In the strong-field the luminescence spectrum consists of broadband emission of 4 T 2 R 2 A 2 transition and sharp line emission of 2 E 2 R 2 A 2 transition. The weak-field sites in the Cr 3+ :MgMoO 4 crystal give rise to the Cr 3+ luminescence in the 4 T 2 R 2 A 2 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 t f was determined to be 1 ms measured at room temperature. Since the radiation lifetime is mainly derived from the parity-forbidden 4 T 2 R 2 A 2 transition with short lifetime in the weak-field, the Cr 3+ :MgMoO 4 crystal has a very short fluorescence lifetime.
The emission cross-section s e was calculated using the formula [26] s e~l p 2 where l is the wavelength of the emission peak, n is the refractive index of the Cr 3+ :MgMoO 4 crystal, which was determined by the method of minimum deviation and the value was listed in Table 3, and Dn the frequency of FWHM. The t f is the fluorescence lifetime, which was determined to be 1 ms. Thus, the emission cross-section of 4 T 2 R 4 A 2 transition is 119.88610 220 cm 2 at 963 nm for E//N g .

Results and Discussion
The Cr 3+ :MgMoO 4 crystals with dimensions up to 30 mm618 mm614 mm were obtained by TSSG method. The mechanical and thermal properties and polarized optical characteristic of the Cr 3+ :MgMoO 4 were investigated. The thermal expansion coefficient of the Cr 3+ :MgMoO 4 crystal along c-axis is smaller than that of the other directions. The investigated results of spectral properties of the Cr 3+ :MgMoO 4 crystal showed that its absorption and emission spectra exhibit strong polarized and depend on the optical indicatrix axis N g , N m , N p at room temperature. The Cr 3+ :MgMoO 4 crystal has large absorption cross-section at about 705 nm, which is available for the diode laser pumping. The Cr 3+ :MgMoO 4 crystal exhibits a broad band emission extending from 750 nm to 1300 nm with peak at about 705 nm. The emission crosssection with FWHM of 188 nm is 119.88610 220 cm 2 at 963 nm for E//N g . In comparison with other Cr 3+ doped materials (Table 4), the Cr 3+ :MgMoO 4 crystal has large emission cross-section and FWHM of the fluorescence. The fluorescence lifetime t f was determined is 1 ms, which is available to apply to short pulse laser. To sum up above the results, the conclusion was drawn that the Cr 3+ :MgMoO 4 crystal may be regarded as a potential tunable laser gain medium.