WANG Dong-Mei LIU Guo-Jiao LIU Le-Hui YUAN Fei-Fei ZHANG Li-Zhen LIN Zhou-Bin②

a (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

b (Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

c (Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, China)

d (State Key Laboratory of Structural Chemistry, Fuzhou 350002, China)

ABSTRACT A new crystal, Ca2(Al1.8Ga0.2)SiO7, was obtained by substituting Ga3+ ions for some Al3+ ions in Ca2Al2SiO7 crystal. The growth, structure and optical spectroscopic properties of Cr4+-doped Ca2(Al1.8Ga0.2)SiO7 were studied. It shows strong absorption at 693 and 762 nm and a broad emission band with peak wavelength at 1223 nm. Both the absorption and emission peaks of Cr4+-doped Ca2(Al1.8Ga0.2)SiO7 crystal are red-shifted in comparison with that of Cr4+-doped Ca2Al2SiO7 crystal due to its weaker lattice field. The investigation results show that there is only one kind of tetrahedral site for Cr4+ occupation in the lattice of Ca2(Al1.8Ga0.2)SiO7 crystal.

Keywords: crystal structure, optical properties, crystalline field, single crystal growth;

1 INTRODUCTION

Tunable solid-state laser, whose output laser wavelength is continuously adjustable in a certain range, has been widely used in the medicine field, ultra-short pulse generation, and communication[1,2]. It is widely thought that the Cr4+ion is a desired broadband near infrared luminescent center in the weak crystal field materials[3], since its broad emission band ranging from 1.13 to 1.63 μm meets the optical telecommunication windows[4]. Besides broadband emission, it also has a wide absorption band, which is beneficial to the absorption of pump energy. In addition, the four-level energy band structure benefits the near infrared laser output with continuous pump or pulse laser pump[4]. These interesting characteristics make Cr4+-doped crystals apply in novel broad-band amplifier[5,6], optical communications, eye-safe imaging, and spectroscopy[4]. Benefiting from its interesting characteristic in tunable laser applications, Cr4+ion doped crystals have received increasing attention[6-9]and a number of Cr4+ion doped crystals have been grown and studied, such as Mg2SiO4[10], YAG[11], Y2SiO5[12], Li2MgSiO4[13],SrAl2O4[12], CaGd4(SiO4)3O[14]and so on. Among them,Cr4+:Mg2SiO4and Cr4+:YAG crystals have been commercialized. Nonetheless, these materials have several drawbacks. One is the serious nonradiative transition which results in the low laser output efficiency. Another problem is the undesired presence of Cr3+along with Cr4+, leading to rather complicated optical spectra and low laser output efficiency[8]. Therefore, it is necessary to explore new tunable laser materials which exhibit only Cr4+ions and a weaker crystal field.

In the melilite crystals of Ca2Al2SiO7(CAS) and Ca2Ga2SiO7(CGS), only the tetrahedral site is available,which makes the substitution of Cr4+easy to happen[15,16].Sugimoto A. et al.[15]have reported the spectroscopic properties of Cr-doped melilite crystals. Larry D. Merkle et al.[16]have studied the crystal growth and spectroscopic properties of Cr4+:CAS and Cr4+:CGS. The corresponding peaks in absorption and emission spectra of CAS and CGS have been ascribed to Cr4+in the tetrahedral sites and a weak crystal field[17]. Since CAS and CGS are analogous to the melilite group of minerals[15], it may not fundamentally change the structure by substituting Ga3+ions for some Al3+ions in CAS crystal. But the distortion of tetrahedron may be increased by doping Ga3+ions in Ca2Al2SiO7to increase the ionic size in this site, so that the average cation-ligand distance at tetrahedron site may be large, which might lead to a weaker crystal field[18]. With this in mind, a new tunable laser crystal was obtained, in which tetrahedron is the only site, and Cr4+ions in that site should encounter a weaker crystal field.

In this paper, a large single crystal of Cr4+:Ca2(Al1.8Ga0.2)SiO7with dimensions of Φ 20×30 mm3was grown successfully by the Cz technology. The crystallographic structure of Cr4+:Ca2(Al1.8Ga0.2)SiO7was researched. Furthermore, more details of crystal spectra,including absorption and fluorescence spectra and fluorescence decay kinetics, were reported.

2 EXPERIMENTAL

2. 1 Synthesis polycrystalline

The chemicals CaCO3, Ga2O3, SiO2, Cr2O3and Al2O3(purity 99.99%) used as raw materials were calculated in the stoichiometric amounts of xCr4+:Ca2(AlGa)SiO7(x = 0, 1, 2, 3 at.%) (Al2O3:Ga2O3= 1:1). The powder of xCr4+:Ca2(AlGa)SiO7(x = 0, 1, 2, 3 at.%) was synthesized by conventional solid-state reaction. The mixture was evenly ground in an agate mortar, and pressed to pellets which were placed into an alumina crucible and calcined at 980 ºC for 5 hours in the furnace in order to eliminate the absorbed water in the materials and decompose the carbonate. Then put them into the muffle furnace and heat again at 1300 ºC for 10 hours to perform the solid state reaction. Finally,Cr4+:Ca2(AlGa)SiO7powders were obtained.

2. 2 Growth crystal

Ca2(AlGa)SiO7could melt congruently, which makes it possible to be grown by the Cz method. The molar concentration of doped Cr ions was 0.5 at% in the Cr4+:Ca2(AlGa)SiO7polycrystalline material which was then loaded into an Ir crucible with the size of Φ 45×50 mm3and heated by a 2-kHz radio 25 kHz mid-frequency induction furnace (DJL-400) in the N2atmosphere. An [100] oriented Ca2(AlGa)SiO7single crystal was used as the seed. The crystal was grown at a pulling rate of 0.5～1 mm/h and a rotating rate of 5～15 rpm. At the end of growth process, the crystal was pulled slowly out from the melt, followed by cooling to room temperature at an annealing rate of 15～20ºC/h. The concentration of Cr4+ions in Ca2(AlGa)SiO7crystal was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Meanwhile, the rough contents of Ca, Al, Ga and Si were measured by the energy-dispersive spectrometry (EDS).

2. 3 Phase identification

The powder X-ray diffraction (XRD) patterns of the traditional solid state synthesis products and as-grown crystal were measured by Miniflex 600 power diffractometer with Cu-Kα radiation. The data were collected in the angular range of 2θ = 10～80° with a scan step width of 0.02° and a scan speed of 5 °/min. The XRD data were also collected in the 2θ range of 10～80° with a step of 0.02° and an exposure of 7 s at every point for further Rietveld refinements. And the HighScore Plus software was used to perform Rietveld refinements. A pseudo-Voigt function was selected to describe the line profiles. The structure parameters and atomic coordinates of Ca2Al2SiO7crystal were used as the starting model for the iteration procedure.

2. 4 Structure determination and element analysis

The structure of the grown crystal was determined based on its single-crystal XRD data which were collected by Rigaku ROD, Synergy Custom system, HyPix diffractometer equipped with mirror-monochromatic GaKα radiation at 100 K. A single crystal with dimensions of 0.3mm × 0.3mm ×0.96mm was selected and used for data collection. The SHELXT[19]structure solution program using Intrinsic Phasing and the SHELXL[20]refinement package using least-squares minimization were used to solve the structure.

2. 5 Spectra properties

A polished rectangular wafer (3.12mm × 3.23mm ×0.96mm) was used for spectral measurements. Since the crystal cleavages seriously, the optical spectra were recorded with the incident light only perpendicular to the optic c-axis(E⊥c). The absorption spectra were measured by a Perkin Elmer UV-VIS-NIR spectrometer (Lambda-950) in the range of 250～1200 nm at room temperature. The emission spectra with 694 nm excitation and fluorescence lifetime were measured using the Edinburgh Analysis Instruments FSL980 spectrophotometer with Xenon lamp as the light source at 77 K.

2. 6 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) measurement was carried out using AlKα X-ray radiation operated at 150 W(Thermo Scientific Escalab 250Xi, USA). The shift of the bonding energy due to the relative surface charging was corrected using the C 1s level at 284.8 eV as an internal standard.

3 RESULTS AND DISCUSSION

3. 1 X-ray powder diffraction

In order to investigate the influence of Cr4+substitution on the microstructure and phase composition of Cr4+:Ca2(AlGa)SiO7compounds, the powder XRD patterns of xCr4+:Ca2(AlGa)SiO7(x = 0, 1, 2, 3 at.%) were analyzed and shown in Fig.1a. As displayed in this figure, all of the diffraction patterns are consistent with the standard XRD patterns of Ca2Al2SiO7(PDF#79-1726), indicating that the Cr doped in the crystal with different concentrations did not change the structure. The lattice cell parameters of Ca2Al2SiO7powders with different doping concentrations were calculated and shown in Fig.1b. It is clear that the lattice cell of xCr4+:Ca2(AlGa)SiO7shrinks gradually with the elevating of Cr4+content. In a tetrahedral coordination system,the ionic radii of Cr4+, Ga3+, Al3+and Si4+are 0.41, 0.47, 0.39 and 0.26 Å, respectively[21,22]. Once introduced into Ca2(AlGa)SiO7, the Cr4+ions with smaller ionic radius replace the Ga3+ones, thus causing the shrinkage of lattice cell.

Fig.1. (a) XRD patterns of powdered xCr4+:Ca2(AlGa)SiO7 (x = 0, 1, 2, 3 at.%) compared with the standard pattern of Ca2Al2SiO7(PDF#79-1726); (b) Lattice parameters of a, c and v versus the content of Cr4+ ion

3. 2 Growth of the single crystal

The as-grown crystal of Cr4+:Ca2(AlGa)SiO7with dimensions of Φ 20 × 30 mm3was obtained, as shown in Fig.2a.It can be seen that the grown crystal is deep blue without obvious impurity. The XRD pattern of the grown Cr4+:Ca2(AlGa)SiO7crystal is shown in Fig.2b, which matches the standard XRD pattern (PDF#79-1726) of Ca2Al2SiO7very well.

Fig.2. (a) Cr4+:Ca2(Al1.8Ga0.2)SiO7 crystal grown by the Czochralski method;(b) Crystal X-ray diffraction patterns of Cr4+:Ca2(Al1.8Ga0.2)SiO7 crystal

In order to confirm the chemical composition of the grown crystal, the contents of Ca:Al:Ga:Si and Cr atoms in the as-grown crystal were measured by ICP-AES and EDS tests.The measured molar ratio of Ca:Al:(Ga + Cr):Si is close to 2:1.8:0.2:1, which means this crystal is inconsistent with the stoichiometric amounts of Ca2(AlGa)SiO7. There are mainly two reasons why the Ga content decreased during the crystal growth procedure. One is that Ga2O3is volatile during the crystal growth, which results in the loss of gallium. The other lies in that the Ga3+ion has bigger radius than Al3+and the segregation coefficient is small. The real chemical formula may be changed into Cr4+:Ca2(Al1.8Ga0.2)SiO7, which will be confirmed latter. The concentration of Cr ions in this crystal was measured to be 0.33 at.% (2.2 × 1019ions/cm3). As the concentration of Cr4+ions in the melt is 0.5 at.%, the segregation coefficient η of Cr4+ion is 0.66.

3. 3 Crystal structure

Using Olex2, the structure was solved with the SHELXT[19]structure solution program using Intrinsic Phasing and refined with the SHELXL[20]refinement package using least-squares minimization. The structures were also checked for possible missing symmetry with PLATON. The calculated molar ratio of Ca:Al:Ga:Si is 2:1.8:0.2:1, which agrees with the measurement data of ICP and EDS.

Fig.3. Ball-and-stick and polyhedral representations for Ca2(Al1.8Ga0.2)SiO7. (a) The (Al/Ga)O4 tetrahedron and (Al/Si)O4 tetrahedron, (b)[(Al/Ga)3Si2O15] structural unit, (c) 2D [(Al/Ga)2SiO7] layers in the ab plane composed of (Al/Ga)O4 and SiO4 tetrahedra, and (d) the[(Al/Ga)2SiO7] layers stacked along the c axis while Ca2+ located at interlayers to balance the charge

3. 4 Spectra properties

3. 4. 1 Absorption spectra

Since the as-grown crystal Cr4+:Ca2(Al1.8Ga0.2)SiO7cleaved seriously, we only measured the absorption spectra recorded at room-temperature with the incident light perpendicular to its optic c-axis (Fig.4). As can be seen from this figure, the grown crystal shows strong and broad band optical absorption in the visible and near infrared ranges with two peaks at about 693 and 762 nm, corresponding to the transition of Cr4+ions from the ground state3A2to the excited state3T1[15]. The absorption cross sections (σa) were estimated to be 5.3 × 10-19cm2at 693 nm and 4.7 × 10-19cm2at 762 nm, respectively, which are very suitable for the commercial AlGaIn LD pumping. Compared with CAS, the absorption peaks have a slight red shift in this crystal.

3. 4. 2 Emission spectra

As shown in Fig.4, the emission spectrum of Cr4+:Ca2(Al1.8Ga0.2)SiO7crystal recorded at 77 K is dominated by a broad band emission extending from 1100 to 1500 nm with a full width at half-maximum (FWHM) of 218 nm and the emission peak was at about 1223 nm, ascribed to the3T2to3A2transition of Cr4+ions in a tetrahedral crystal field[15]. Its peak wavelength is about 30 nm longer than that in the CAS crystal, which means this crystal has weaker crystal field than the CAS one[16].

Fig.4. Absorption spectra (black line) at room temperature and emission spectra (red line) at 77 K with the incident light perpendicular to the optic c-axis of the Cr4+:Ca2(Al1.8Ga0.2)SiO7 crystal

Fig.5 shows the decay curve of Cr4+:Ca2(Al1.8Ga0.2)SiO7crystal with 694 nm excited at 77 K, which exhibits single exponential character. According to the single-exponential fitting results, the fluorescence lifetime was 22.2 μs.

Fig.5. Fluorescence decay curve of Cr4+:Ca2(Al1.8Ga0.2)SiO7 crystal

3. 5 X-ray photoelectron spectroscopy analysis

The absorption and emission spectra demonstrate that only Cr4+ion is doped in the Ca2(Al1.8Ga0.2)SiO7crystal. Further research is required to confirm only Cr4+ions exist in the as-grown crystal. We investigated the XPS spectra of Cr4+:Ca2(Al1.8Ga0.2)SiO7crystal and Cr2O3. The Cr 2p3/2of Cr4+:Ca2(Al1.8Ga0.2)SiO7crystal and Cr2O3XPS spectra are presented in Fig.6. As displayed in this figure, the binding energy of Cr 2p3/2in Cr2O3is centered at 576.69 eV, which is close to the Cr2O3(Cr3+) state[24], and that of the broad Cr 2p3/2peak at 576.98 eV corresponds to the Cr4+state[25]. In general, the corresponding binding energy will increase when the valence state increases. All of those indicate that only the Cr4+ions occur in the Cr4+:Ca2(Al1.8Ga0.2)SiO7crystal.

Fig.6. XPS spectrum of Cr4+:Ca2(Al1.8Ga0.2)SiO7 crystal and Cr2O3

4 CONCLUSION

In this work, the replacement of Ga3+for part Al3+ions does not fundamentally change the framework of the CAS structure, but the tiny difference of ionic radius and electronegativity between Ga3+and Al3+makes slight alteration on the arrangement and local crystal circumstance of AlO4tetrahedra, which results in a longer average cation-ligand distance, so that dopants in that site should encounter a weaker crystal field in its crystal. By this way, a Cr4+-doped Ca2(Al1.8Ga0.2)SiO7single crystal with dimensions of Φ 20 × 30 mm3was successfully grown by the Cz method, and then the segregation coefficient of Cr4+ions was calculated to be about 0.66. The lattice site with tetrahedral coordination is the only site for Cr4+ions to occupy in this crystal. The refinement analysis indicated that Cr4+ions most probably replaced the sites of Ga3+ions in GaO4tetrahedron. And the Ga2O3is easily volatilize so that the crystal is inconsistent with the stoichiometric amounts and the chemical formula is Cr4+:Ca2(Al1.8Ga0.2)SiO7. The absorption cross sections are 5.3×10-19cm2and 4.7×10-19cm2at 693 and 762 nm, respectively, which is very suitable for the commercial AlGaIn LD pumping. A broad emission band locates at about 1223 nm with a full width at half-maximum of 218 nm and the fluorescence lifetime is about 22.2 μs at 77 K. Compared with CAS crystal, the emission peak wavelength is about 30 nm longer, which means the crystal field in this crystal is weaker than that in CAS. The investigations of spectra and XPS spectrum show that only Cr4+ion is doped in the Ca2(Al1.8Ga0.2)SiO7crystal.