La₂CaB₁₀O₁₉ (LCB) crystal has a large effective nonlinear optical coefficient (1.05 pm/V), moderate birefringence (0.053 at 1064 nm), high laser damage threshold (11.5 GW/cm²), wide transmittance range (173-3000 nm), good thermal and mechanical properties^{[1⇓⇓⇓-5]}. Besides its application for laser frequency conversion, LCB has been proved a good host crystal, because La³⁺ ions in LCB crystal can be substituted by rare-earth (RE) ions, including Nd³⁺, Sm³⁺, Pr³⁺, Yb³⁺, Tb³⁺ and Er^{3+[6⇓⇓⇓⇓-11]}. These RE doped LCB crystals exhibit excellent optical and thermal properties, which illustrate that RE doped LCB crystals may be promising laser materials. For example, green laser radiation with the output of 26.64 mW were obtained by self-frequency doubling using a Nd³⁺:LCB crystal^[12].

On the other hand, Ce³⁺ doped laser crystals have been widely used in efficient and convenient all-solid-state ultraviolet (UV) lasers. It is reported that the main emission bands of Ce³⁺ ions in strong electronegativity fluoride and oxide matrix materials mainly locate in the UV range^{[13⇓⇓⇓⇓-18]}. Since La³⁺ ions in a LCB crystal are surrounded by ten O^2- ions, the main fluorescence bands of Ce³⁺:LCB crystal may occur in the UV region. To our knowledge, the optical and thermal properties of Ce³⁺ doped La₂CaB₁₀O₁₉ (Ce³⁺:LCB) crystal have not been reported yet. Thus, it is an essential work to explore the optical and thermal properties of the Ce³⁺:LCB crystal.

In this work, Ce³⁺:LCB crystal has been grown by the top-seeded solution growth (TSSG) method. The XRD pattern and X-ray rocking curve of (010) face were collected. The transmittance, absorption, excitation, emission and radioluminescence spectra were studied at room temperature. Sellmeier equations for the refractive indices were calculated. Thermal conductivity, thermal diffusivity and thermal expansivity of Ce³⁺:LCB crystal were investigated. All of the above results demonstrate that Ce³⁺:LCB crystal is promising for the lasing device in the UV range.

Ce³⁺:LCB crystal, as large as 40 mm×21 mm×6 mm (Fig. 1(a)), was grown by TSSG method in a Li₂O-CaO-B₂O₃ flux system. The molar ratio of Ce₂O₃, La₂O₃, CaCO₃, Li₂CO₃ and H₃BO₃ was 0.03 : 0.97 : 2.00 : 2.30 : 28.00. The ground mixture was slowly heated and sintered at 950 ℃ for 20 h. Then, the charge materials were placed in a platinum crucible with a diameter of 60 mm and melted at 1020 ℃. The charge materials were transferred into a one-zone Kanthal A1 resistance furnace with an accurate Eurotherm controller after cooling. After transferred into a resistance heated furnace, Ce³⁺:LCB crystal was grown by TSSG. The [110]-oriented seed was slowly dipped into the melting solution and kept rotating at 30 r/min in the whole growth. The melting solution was cooled at the rate of 1 ℃/d during the first 10 d and 1.5 ℃/d till the growth end. After about 30 d, the crystal was slowly pulled out of the solution and cooled down to the room temperature within one week with the initial cooling rate of 10 ℃/d. The grown crystal was cut and polished into dimensions of 10 mm×6 mm×1 mm with (010) surface (Fig. 1(b)).

The concentration of Ce³⁺ ions in a grown Ce³⁺:LCB crystal was determined by Thermo Fisher iCAP RQ inductively coupled plasma atomic emission spectrometry (ICP-AES). X-ray diffraction data was collected using a Rigaku SmartLab 9 kW diffractometer with monochromatized with CuKα1 radiation (λ=0.15406 nm). The lattice parameters of the Ce³⁺:LCB crystal were collected by Bruker SMART APEX II 4K CCD diffractometer with MoKα radiation (λ=0.071073 nm). The transmittance and absorption spectra of Ce³⁺:LCB crystal was collected by Hitachi U4100 UV-VIS-NIR spectrophotometer over the wavelength range from 200 nm to 500 nm at room temperature. The refractive index was measured by Wedel UV-VIS-IR SpectroMaster refractive index meter with mercury and helium lamps light sources. The excitation and emission spectra of Ce³⁺:LCB crystal were obtained by Edinburgh Analysis Instruments FLS980 spectrophotometer with Xenon lamp light source in the range from 230 nm to 400 nm with a step width of 0.1 nm. The thermal conductivity and thermal diffusivity were measured by NETZSCH LFA 457 NanoFlash analyzer in the temperature range from 300 K to 773 K. The thermal expansion coefficient was measured by NETZSCH DIL 402 thermal mechanical analyzer in the temperature range from 358 K to 773 K.

The concentration of Ce³⁺ ions in Ce³⁺:LCB crystal was 1.05×10²⁰ ions/cm³. According to the effective segregation coefficient equation K_eff = C/C₀, where C is the dopant concentration in the crystal and C₀ is the dopant concentration in the mixture. The K_eff and molar ratio of Ce³⁺ to La³⁺ are calculated to be 0.58 and 1.74%, respectively.

Fig. 2(a) shows XRD pattern of an as-grown Ce³⁺:LCB crystal with a standard LCB crystal. It reveals that the diffraction peaks of Ce³⁺:LCB are consistent with that of LCB crystal. The lattice parameters of the Ce³⁺:LCB crystal are a=1.1061 nm, b=0.6577 nm, c= 0.9124 nm, α=γg=90°, β=91.5°, which are slightly different from those of the LCB crystal (a=1.1043 nm, b=0.6563 nm, c=0.9129 nm, α=γ=90°, β=91.5°). The Ce³⁺ ions are considered to substitute the La³⁺ ions due to the similar properties and radius of Ce³⁺ (0.1196 nm) and La³⁺ (0.1216 nm), leading to a slight change of the lattice after doping. Fig. 2(b) shows the X-ray rocking curve of (010) surface for the Ce³⁺:LCB crystal. The full-width at half-maximum (FWHM) of (010) surface for Ce³⁺:LCB crystal is 0.01473", indicating its very high quality.

The UV-Vis-NIR transmittance spectrum of a Ce³⁺: LCB crystal is shown in Fig. 3(a). The transmittance exhibits a sharp decrease in the UV region due to characteristic absorption bands of Ce³⁺ ions. For the wavelength above 350 nm, the transmittance of Ce³⁺:LCB crystal has almost constant value about 94%, indicating that its transparency in Vis and NIR regions is not obviously affected by Ce³⁺ ions doping.

Fig. 3(b) shows optical absorption spectrum of a Ce³⁺:LCB crystal in the wavelength range from 200 nm to 500 nm. In the UV region, two main absorption peaks are obtained at 280 and 316 nm, corresponding to typical characteristic absorption peaks of Ce³⁺ ions. Different from other trivalent rare-earth ions, Ce³⁺ ions in host crystals exhibit broad absorption bands. The absorption cross section (σ) can be calculated by Eq. (1):

where OD(λ) is optical density function, N₀ is concentration of Ce³⁺ ions and L is optical length. The values of absorption cross section at 280 and 316 nm are 1.85×10⁻¹⁹ and 2.17×10⁻¹⁹ cm², respectively.

Since LCB is a biaxial crystal, the refractive indices of Ce³⁺:LCB crystal was measured by two right-angle prisms with different light-pass surfaces. The orientations of the two prisms are similar to those of La₂CaB₁₀O₁₉ crystals, as reported in Ref. [19]. The cut prisms of Ce³⁺:LCB crystal are shown in Fig. 3(c). Its refractive indices are obtained by improved Sellmeier equations:

where n is refractive index, λ is the incident wavelength and the constant of A, B, C, D can be obtained in terms of least-squares fit. The measured three principal refractive indices (n_x, n_y, n_z) are listed in Table 1. The n_x, n_y, n_z at other wavelengths can be calculated by Eq. (3):

Based on Sellmeier equations, refractive indices at UV regions can be calculated, which provides essential parameters of Ce³⁺:LCB crystal for UV laser applications. Fig. 3(d) shows the fitted curves of three principal refractive indices. It can be seen that the Ce³⁺:LCB crystal belongs to a positive biaxial optical crystal with a birefringence ∆n =0.051 at 1064 nm, which is close to that of LCB crystal (∆n = 0.053 at 1064 nm)^[20]. These results indicate that the birefringence of the grown crystal is almost not affected by small amounts of RE ions doping.

Excitation and fluorescence spectra of Ce³⁺:LCB crystal at 300 K are shown in Fig. 3(e). The excitation and fluorescence bands of Ce³⁺:LCB crystal are broad, which is determined by parity-allowed electric dipole transitions 4f→5d. Two broad excitation bands are observed in the UV regions of 240-290 nm and 290- 340 nm, which correspond to transitions from 4f (²F_5/2 and ²F_7/2) ground states to 5d excited states. The fluorescence bands are inhomogeneously broadening. Four obvious emission bands are obtained at 290, 304, 331, and 353 nm by Gauss fitting, which correspond to transitions from 5d states to ²F_5/2 and ²F_7/2 states (Fig. 3(f)). The f-d transition energy of Ce³⁺ ions in the Ce³⁺:LCB crystal is calculated to be 33557, 15783 cm^-1, lower than that of free Ce³⁺ ions (49340 cm^-1)^[21]. The separation between ⁴F_7/2 and ⁴F_5/2 states is 2134 cm^-1, which agrees well with the theoretical value of 2250 cm^-1[22]. It is worth to mention that main fluorescence range (280-380 nm) of Ce³⁺:LCB crystal is located in the UV region due to the reduction of 5d states. The results can provide essential parameters of Ce³⁺:LCB crystal for the application of UV lasers.

The temperature dependence of thermal conductivity and thermal diffusivity for Ce³⁺:LCB crystal are shown in Fig. 4(a, b). Thermal conductivity and thermal diffusivity are 6.45 W/(m·K) and 1.76 mm²/s at 300 K, respectively, and then gradually decrease to 2.254 W/(m·K) and 0.583 mm²/s as the temperature rises to 773 K. Thermal conductivity and thermal diffusivity of Ce³⁺:LCB crystal only decrease about 45% when temperature increases from 300 K to 423 K, less than those of Nd³⁺-doped LiLuF₄ (Nd³⁺:LLF) crystal (50%)^[23]. Therefore, Ce³⁺:LCB crystal exhibits better thermal stability and wider operational temperature range. The temperature dependence of thermal conductivity k(T) is approximated by the polynomial:

A₁, B₁, C₁, and D₁ coefficients for Ce³⁺:LCB crystal are calculated to be -9.67×10^-8 W/(m·K⁴), 1.87×10^-4 W/(m·K³), -0.12 W/(m·K²) and 28.50 W/(m·K), respectively.

For Ce³⁺:LCB crystal along the c direction, temperature-dependent thermal expansion coefficient and lattice parameters of c direction are shown in Fig. 4(c, d). The expansion coefficient and lattice parameter are 2.94×10^-6 /K and 0.91240 nm at 358 K, and then linearly enlarged to 5.3×10^-5 /K and 0.91246 nm at 773 K. The Ce³⁺:LCB crystal with smaller lattice distortion tends to sustain a larger thermal shock under laser irradiation. These results demonstrate that Ce³⁺:LCB crystal exhibits better thermal stability over a wider temperature, which benefits for its application for UV lasers.

In summary, Ce³⁺:LCB crystals have been grown by TSSG method in Ca₂O-Li₂O-B₂O₃ flux system. The lattice parameters of Ce³⁺:LCB are a=1.1061 nm, b= 0.6577 nm, c=0.9124 nm, α=γ=90°, β=91.5°. The FWHM of X-ray rocking curve is 0.01473", indicating good crystalline quality of the grown Ce³⁺:LCB crystal. The main absorption bands and excitation bands of Ce³⁺:LCB crystal are located in 200-288 nm and 305-330 nm regions. Four emission peaks are obtained at 290, 304, 331, and 353 nm, corresponding to transitions from 5d states to ²F_5/2 and ²F_7/2 states. The values of thermal conductivity and thermal diffusivity are 6.45 W/(m·K) and 1.76 mm²/s at 300 K, respectively. Furthermore, the thermal stability of Ce³⁺:LCB crystal is slightly higher than that of Nd³⁺:LLF crystal. The lattice parameter of Ce³⁺:LCB crystal along c direction extremely slowly increases from 0.91240 to 0.91246 nm with increasing the temperature from 358 K to 773 K, exhibiting good temperature stability. With excellent optical property and enhanced thermal stability, Ce³⁺:LCB crystal is suggested to be a potential candidate for high performance and high reliability UV laser materials.