Synthesis and Optical Characterizations of Yb³⁺: Ca_xSr_1−xF₂ Transparent Ceramics

Abstract

In this study, 3 at.% Yb³⁺: Ca_xSr_1−xF₂ nanopowders were synthesized via the chemical co-precipitation method. Highly transparent 3 at.% Yb³⁺: Ca_xSr_1−xF₂ ceramics with various CaF₂ concentrations were fabricated by hot-pressed sintering. The 3 at.% Yb³⁺: Ca_xSr_1−xF₂ nanopowders exhibited a spherical shape with slight agglomeration, and their particle size ranged from 26 nm to 36 nm. With an increase of the CaF₂ concentration, the peak shape changed significantly and the width of the emission band increased inhomogeneously. The minimal fluorescence lifetime at the wavelength of 1011 nm of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics with various CaF₂ concentrations was higher than 3.25 ms, which was longer than that of the 3 at.% Yb³⁺: CaF₂ (2.6 ms) and the 3 at.% Yb³⁺: SrF₂ (3.22 ms) reported in previous literature. The results indicate that incorporating Ca²⁺ ions into the SrF₂ is an effective method to modulate the optical properties of transparent ceramics.

1. Introduction

Since the first Dy: CaF₂ transparent ceramics were prepared by Hatch et al. in 1964 [1], alkaline-earth fluorides (AeF₂, Ae = Ca, Sr, Ba) have attracted much attention in the field of solid-state lasers because of their low phonon energy, high transmittance, and wide range of light transmission [2,3,4]. For instance, Su et al. prepared Tm³⁺: CaF₂ single crystals and their slope efficiency reached up to 64.4%, which was higher than Tm³⁺: YAG single crystals with 31.8% [5,6]. Basiev, T. T. et al. fabricated Nd³⁺: SrF₂ laser ceramics with a laser slope efficiency of 19% [7]. On the other hand, AeF₂ possessed the cubic structure (${\mathrm{F}}_{\mathrm{3}\mathrm{m}}^{\mathrm{m}}$) with fluoride ions occupying the centers of the octants and Ae²⁺ ions occupying the nodes in a face-centered lattice, which made it possible for the Re³⁺ (rare earth) ions to incorporate in the AeF₂ crystal structure and achieve high dopant concentrations. Moreover, various Re³⁺ ions have been successfully doped into alkaline earth fluorides, such as Nd³⁺: CaF₂ crystals [8], Eu³⁺: CaF₂ transparent ceramics [9], Er³⁺: CaF₂ glass-ceramics [10], and Nd³⁺: SrF₂ nanoparticles [11].

Among the Re³⁺ ions, Yb is one of the most promising elements for solid-state laser materials like high power or ultrafast lasers. Yb³⁺ ions possess a simple electronic-level structure with only two multiples ²F_7/2 and ²F_5/2, enabling efficient diode-pump laser systems [12,13,14]. A. Lucca et al. prepared the first diode-pumped laser based on the Yb³⁺: CaF₂ single crystal and achieved a laser output of 5.8 W at 1053 nm [15]. Subsequently, a Yb³⁺: CaF₂ single crystal achieved a 150 femtosecond, 880 mW laser output at the central wavelength of 1043 nm [16]. However, the local environment of Yb³⁺ ions affects its optical properties [17].

One of the effective methods to alter the local environment of Yb³⁺ ions is introducing non-active ions, like Ga³⁺ ions. Y.J. Wu and his co-workers modified their lattice structure by co-doping Ga³⁺ ions into Yb³⁺: SrF₂ to form a disordered lattice site, broadening the absorption and emission cross-sections, and explored the potential of the Yb³⁺, Ga³⁺: SrF₂ crystal in tunable and passively mode-locked lasers [18]. Further, due to the different radii of Ca²⁺ ions and Sr²⁺ ions, the local environment of rare earth ions in Ca_xSr_1−xF₂ ceramics varies with the concentration of CaF₂. Recently, a few studies have researched the CaF₂-SrF₂ mixed matrix because incorporating the Ca²⁺ ions into the SrF₂ destroys the symmetry of the crystal structure, thereby affecting its luminescence performance [19,20,21]. Compared with the Yb³⁺: SrF₂ single crystal, the slope efficiency of CaF₂-SrF₂-YbF₃ nanoceramics is 1.7 times higher than the former [22]. Therefore, it is meaningful to investigate the influence of CaF₂ concentration on the optical properties of Ca_xSr_1−xF₂ transparent ceramics. However, according to the previous literature on Ca_xSr_1−xF₂ transparent ceramics, the ratio of SrF₂ to CaF₂ is relatively small [23]. When the proportion of SrF₂ is higher than 25 at.%, the Sr²⁺ cannot incorporate into the CaF₂ structure, and two single phases (CaF₂ and SrF₂) exist in the products [24]. In this paper, high purity of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) nanopowders were synthesized via the coprecipitation method and the corresponding transparent ceramics were prepared by hot-pressed sintering. The morphology of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ nanopowders was observed by a scanning electron microscope (SEM). The optical quality, microstructure, and photoluminescence properties of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics were also characterized and discussed.

2. Experimental Procedure

The 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) nanopowders were synthesized with the coprecipitation method by dropping the KF·2H₂O aqueous solution into the cationic aqueous solution (Sr(NO₃)₂, Ca(NO₃)₂·4H₂O and Yb(NO₃)₃·5H₂O) under stirring. The 3 at.% Yb³⁺: Ca_xSr_1−xF₂ nanopowders were prepared following this formulation:

0.03Yb(NO₃)₃ + xCa(NO₃)₂ + (1 − x)Sr(NO₃)₂ + 2.09KF → Yb_0.03Ca_xSr_1−xF_2.09↓ + 2.09KNO₃

The obtained precipitates were aged for 3 h, and finally, the precipitates were washed with deionized water, filtered, and freeze-dried under vacuum for 8 h. The fully dried powder was ground in a mortar to remove large aggregates.

The 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics were fabricated by hot-pressing the as-synthesized nanopowders in vacuum. The nanopowders were calcined at 400 °C for 2 h to remove residual water and nitrates from the powder, then sintered at 800 °C under an axial pressure of 40 MPa for 2 h followed by cooling down to room temperature. Then, the 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics were both-side polished to 2 mm for further measurements.

The phase compositions of 3 at.% Yb: Ca_xSr_1−xF₂ ceramics were identified by X-ray diffraction (XRD, D8 Advance, Bruker, Karlsruhe, Germany). The morphology of nanopowders and the fracture microstructure of transparent ceramics of Yb³⁺: Ca_xSr_1−xF₂ were observed by a scanning electron microscope (SEM, S4800, Hitachi, Tokyo, Japan). The average particle size of the Yb³⁺: Ca_xSr_1−xF₂ transparent ceramic was calculated by scanning software. The in-line transmittance and absorption spectra of the Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics were measured by a spectrophotometer (U-3500, Hitachi, Tokyo, Japan). The photoluminescence (PL) spectra and fluorescence lifetimes were recorded by a spectrofluorometer (FLS980, Edinburgh, Livingston, UK) at room temperature.

3. Results and Discussion

The XRD patterns of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics with various CaF₂ concentrations and standard XRD patterns of CaF₂ (JCPDS file number 65-0535) and SrF₂ (JCPDS file number 06-0262) are shown in Figure 1a. All the diffraction peaks of 3 at.% Yb: Ca_xSr_1−xF₂ transparent ceramics corresponded well to the cubic CaF₂ and/or SrF₂ phase without redundant peaks. With the increase of CaF₂ concentration, the position of all diffraction peaks shifted to a high angle, and was closer to the peak of pure CaF₂, indicating that the position of Sr²⁺ ions was replaced by the Ca²⁺ ions in the SrF₂ unit cell and formed a solid solution structure instead of a two-phase composite structure of CaF₂ and SrF₂. According to the Hume-Rothery solid solution theory, taking r₁ and r₂ to represent the ionic radius of different ions, there is the empirical formula [25]

$$\mathsf{\Delta }r=\left|\frac{{\mathrm{r}}_1-{\mathrm{r}}_2}{{\mathrm{r}}_1}\right|$$

According to the formula, the obtained value of $\mathsf{\Delta }r$ was 10%. Further, the two ions had the same valence state and CaF₂ and SrF₂ possess the same fluorite structure. Therefore, Ca²⁺ ions can be incorporated into the SrF₂ unit cell at any ratio to form a continuous solid solution instead of destroying the unit cell structure. In addition, the lattice constant of Yb³⁺: Ca_xSr_1−xF₂ was calculated by the Barrage equation and is presented in Figure 1b. At lower CaF₂ concentrations, the lattice constant was close to that of SrF₂, while at higher CaF₂ concentrations, the lattice constant was close to that of CaF₂. The linear relationship between the lattice constant and CaF₂ concentration was corresponded well to the 2θ shift trend in Figure 1b. In the condition of the expansion of the lattice constant when larger Sr²⁺ ions were substituted, the stability of the unit cell structure was still maintained.

The SEM images of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) nanopowders are shown in Figure 2. All 3 at.% Yb³⁺: Ca_xSr_1−xF₂ nanopowders exhibited a spherical shape and uniform distribution in the field of vision with slight agglomeration. With the increase of the CaF₂ concentration, there was nearly no significant change in the shape and particle size of the powder. The average particle size of the powders ranged from 26 nm to 34 nm.

The in-line transmittance spectra and photographs of as-fabricated 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) transparent ceramics are presented in Figure 3. The 3 at.% Yb³⁺: Ca_0.5Sr_0.5F₂ transparent ceramics had the highest optical transmittance among the five samples. The text on the paper can be clearly identified through the ceramics. The transmittance of all ceramics was higher than 80% over the wavelength of 1130 nm and reached 90% around 2000 nm, which might be attributed to the well-dispersed nanopowders. The small and spherical-shaped nanopowders had relatively large surface energy and high sintering activity, which was favorable for the sintering process. In addition, well dispersion of the nanopowders was conducive to the discharge of pores. However, the incorporation of CaF₂ caused the lattice distortion of the matrix material, which affected the sintering activity of the synthesized nanopowders. Then, the diffusion rate of the powder was affected by the nanopowders which obsess different sintering activity, resulting in different pores in the sample. At ultraviolet wavelength range, the 3 at.% Yb³⁺: Ca_0.5Sr_0.5F₂ transparent ceramic possessed the highest in-line transmittance, which might have been caused by the absence of impurities and fewer micropores in the ceramic. However, a significant transmittance loss was observed at the ultraviolet wavelength range similar to the CaF₂ and SrF₂ transparent ceramics [26,27], which might have been caused by fewer micro-obturator pores existing in the sample. When the size of the scattering source was much smaller than the wavelength of the incident light, the scattering intensity and the incident light wavelength conformed to Rayleigh’s law of scattering [28]:

$$S=\left(128{\pi }^5{d}^6/3{\lambda }^4\right){\left\{\left[{\left(\frac{n_2}{n_1}\right)}^2-1\right]/\left[{\left(\frac{n_2}{n_1}\right)}^2-2\right]\right\}}^2$$

where $\mathrm{S}$,$d$, $\lambda$, and $n_2$ and $n_1$ are the cross section for the scattering of a particle, the radius of the scattering body, the measuring wavelength, and the refractive indexes for the host materials and scattering body, respectively. According to Rayleigh’s law of scattering, scattering intensity is inversely proportional to the fourth power of the wavelength of the incident light. Therefore, the micropores inside the ceramic strongly led to the scatter of short-wavelength light such as ultraviolet light, which affected the transmittance of transparent ceramics in the ultraviolet wavelength range.

As can be seen in SEM micrographs of the fracture surface of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics (Figure 4), at a low CaF₂ concentration (x = 0.1, 0.3), transgranular fractures accounted for a large proportion. With an increase of the CaF₂ concentration, the transgranular fractures gradually decreased and tended to disappear, and intergranular fractures were the main fracture mode. Intracrystalline pores can affect the fracture mode of ceramic samples. It can be clearly seen that there were some intracrystalline pores inside the ceramics when the CaF₂ concentration was low. The intracrystalline pores were the starting point of the crack propagation during the fracture process, which extended to the grain boundary to form transgranular fractures. When the intracrystalline pores gradually decreased, the proportion of transgranular fractures also decreased.

Figure 5 shows the relative absorption (divide intensity by Yb concentration) spectra of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) transparent ceramics. The absorption spectra of all samples possessed a similar shape with two absorption peaks, located at 922 nm and 972 nm, respectively. This was caused by the stark split of ground state ²F_7/2 energy levels and the excited state ²F_5/2 energy levels. In general, the absorption intensity increased with an increase of the Yb³⁺ concentration. Considering the influence of the Yb³⁺ concentration on the absorption intensity, we conducted an ICP test (within ±0.01 margin of error) to determine the practical doping concentration of Yb^3+, and have attached the test result in

Table 1. Designed concentration and practical concentration of Yb³⁺ in 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) transparent ceramics.

Sample	Yb3+ (mol%)
	Designed Value	ICP Result (±0.01)
3 at.% Yb: Ca0.1Sr0.9F2	3	2.82
3 at.% Yb: Ca0.3Sr0.7F2	3	2.67
3 at.% Yb: Ca0.5Sr0.5F2	3	2.84
3 at.% Yb: Ca0.7Sr0.3F2	3	2.63
3 at.% Yb: Ca0.9Sr0.1F2	3	2.60

. The test result was consistent with the result of the absorption spectrum. Thus, we attributed the change in absorption intensity to the change in the practical doping concentration of Yb³⁺.

The emission spectra of the 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics with different concentrations of CaF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) are shown in Figure 6. With the increase of CaF₂ concentrations, the peak shape changed significantly and the width of the emission band increased inhomogeneously. It can be seen in Figure 6 that there were three different emission peaks located at 976 nm, 1011 nm and 1027 nm, respectively. The intensity of the emission peaks at 976 nm and 1011 nm gradually increased with the increase of the CaF₂ concentration. The emission peak at 1027 became much clearer when the CaF₂ concentration was higher than 0.7, which might be attributed to the tendency of Yb³⁺ ions in the AeF₂ matrix to form different centers of symmetry. With the increase of the cation radius (Ca → Sr → Ba), the symmetric system gradually changed from C_4v to C_3v. The emission peak of the tetragonal symmetry center of Yb³⁺ in AF₂ (A = Ca, Sr) was located at 1025 nm [29]. Therefore, when the concentration of CaF₂ increased, the proportion of the symmetry center of C_4v gradually increased, and the emission peak near 1027 became much clearer. Further, Youngman et al. have proved that the substitution of Ca ²⁺ ions for Sr²⁺ ions is random in Ca_1−xSr_xF₂ single crystals [30]. Similarly, the linear relationship in Figure 1b proved that it also conformed to Vegard’s law in Ca_xSr_1−xF₂ transparent ceramics. The substitution of Ca²⁺ ions in Ca_xSr_1−xF₂ transparent ceramics is random, too [31]. Therefore, on the one hand, when Ca²⁺ ions replaced Sr²⁺ ions, the distance between Yb³⁺ decreased, leading to an increase in the degree of clustering of Yb³⁺. A wealth of lattice sites formed, combining the Yb³⁺ cluster with different symmetry, resulting in the broadening of the spectrum. On the other hand, the substitution process caused lattice distortion and affected the Yb³⁺ crystal field environment. The combined effect of the two aspects caused the inhomogeneous broadening of the emission spectrum [24].

The room-temperature luminescence decay curves of 3% at. Yb³⁺: Ca_xSr_1−xF₂, with different concentrations of CaF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) at 976 nm and 1011 nm pumped by the xenon lamp of 896 nm, are shown in Figure 7a,b, respectively. To visualize the change of lifetime with the concentration of CaF₂, the change curve of the fluorescence lifetime at 976 nm and 1011 nm as a function of the CaF₂ concentration is presented in Figure 8. A single exponential function was used to fit the decay curve of all ceramics:

I(t) = A1exp(−t/τ1) + I0

where I(t) and I₀ represent the luminescence intensity at time t and t₀, respectively; t is time; A₁ is a constant; τ₁ is the decay time.

The fluorescence lifetime at 976 nm gradually increased from 3.69 ms to 4.34 ms and then decreased to 3.89 ms with the increase of the CaF₂ concentration. Similarly, the fluorescence lifetime at 1011 nm increased from 3.25 ms to 3.84 ms and then decreased to 3.82 ms (the fluorescence lifetime value is the average value of the four test results). The fluorescence lifetime at 976 nm was longer than that at 1011 nm, which may have been caused by the re-absorption at 976 nm. This is because there was an overlap area between the absorption peak at 972 nm and the emission peak at 976 nm. Therefore, when Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics were excited by the 896 nm xenon lamp to emit 976 nm light, this wavelength was in the absorption band, so it was reabsorbed. Similarly, there was no reabsorption at 1011 nm because Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics have extremely weak absorption at 1011 nm. Moreover, compared with Yb³⁺: CaF₂ [32] (about 2.6 ms) and Yb³⁺: SrF₂ [33] (about 3.22 ms), the fluorescence lifetime of Yb³⁺: Ca_xSr_1−xF₂ with the same rare earth doping concentration was longer than both of them. This can be attributed to the incorporation of CaF₂. On the one hand, the incorporation of Ca²⁺ broke the clusters of Yb³⁺ ions, thereby increasing the fluorescence lifetime. On the other hand, when the doping ratio was higher, the crystal lattice tended to be CaF₂, and the fluorescence lifetime of Yb³⁺: CaF₂ transparent ceramics were lower than that of Yb³⁺: SrF₂ transparent ceramics, so the fluorescence lifetime was reduced. Under the combined effect of two factors, the fluorescence lifetime of Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics presented a curve that first increased and then decreased.

4. Conclusions

Highly pure 3 at.% Yb³⁺: Ca_xSr_1−xF₂ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) nanopowders with various CaF₂ concentrations were successfully synthesized by the chemical co-precipitation method. The phase analysis showed that CaF₂ can be incorporated into the crystal lattice of SrF₂ at any ratio. With an increase of the CaF₂ concentration, the lattice constant of 3 at.% Yb³⁺: Ca_xSr_1−xF₂ decreased. All the as-fabricated 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics possessed high optical quality, with their in-line transmittance higher than 80% at the wavelength of 1130 nm. The absorption spectra of all 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics samples had similar spectra shapes. With the increase of CaF₂ concentrations, the peak shape changed significantly and the width of the emission bands gradually increased. The fluorescence lifetime of all 3 at.% Yb³⁺: Ca_xSr_1−xF₂ transparent ceramics at 1011 nm exceeded 3.25 ms, which is suitable for high-power laser output. Above all, incorporating Ca²⁺ ions into the SrF₂ is an effective method to modulate the optical properties of transparent ceramics. The present results can stimulate further studies on the modulation of optical spectra and the performance of solid-state laser materials.