1. Introduction

Since the discovery of laser in the sixties very intense research has been carried out in the field of nonlinear optics aimed to expand the frequency range provided by the known laser materials. New laser sources based on nonlinear optical (NLO) properties of different materials are of common use today, not only in laboratory research but in other fields such as laser diagnosis and therapy, optical telecommunications and signal processing, integrated optics, and many other related fields. Moreover, the development of powerful laser pump diodes has increased the interest to investigate new nonlinear materials for laser applications.

Among NLO materials, the interest in borate compounds has increased in recent years due to their good optical properties such as good transparency in the ultraviolet, high damage threshold, and good nonlinearity which make them promising materials not only for NLO devices [1,2] but also for potential applications in the field of lasers [3-8].

The extraordinary versatility of the borate structure facilitates the design of new compounds. Recently a new oxyborate of formula Na₃La₉O₃(BO₃)₈ has been discovered in the ternary Na₂O-La₂O₃-B₂O₃ diagram and its structure resolved [9]. The unit cell is hexagonal with space group P- 6̄ 2m (189) and the lanthanum occupies two different crystallographic sites in the structure with coordinations eight and nine. In a very recent work, the authors have presented the first spectroscopic characterization of Nd³⁺ ions in this Na₃La₉O₃(BO₃)₈ crystal by using steady-state and time resolved laser spectroscopy [10]. This study shows the existence of at least two different crystal field sites for Nd³⁺ ions in this material in accordance with the existence of two non equivalent crystallographic lanthanum sites. However, a careful examination of the excitation spectra of these ions shows the presence of a complex structure which suggests the existence of other possible crystal field sites for the rare-earth (RE) in this crystal.

It is worthy to mention that the optical properties of rare-earth doped crystals are closely related to local structure and bonding at the ion site. The existence of different crystal field sites may produce spectral broadening and/or multiple emission lines which can influence the energy extraction from the material as well as the wavelength tuning capability when it is used as a lasing medium. As a consequence, the knowledge of the precise crystal field structure of the rare-earth in a given material is of paramount importance to understand its potentialities for lasing applications.

In order to clarify the nature of the RE environments in Na₃La₉O₃(BO₃)₈ crystal we have undertaken the study of the site-resolved luminescence of Eu³⁺ in this crystal, taking into account the adequacy of the dopant ion as a structural probe. Since the ⁵D₀ state is nondegenerate under any symmetry, the structure of the ⁵D₀→⁷F_J emission is only determined by the splitting of the terminal levels caused by the local crystal field. Moreover, as the ⁷F₀ level is also nondegenerate, site-selective excitation within the inhomogeneous broadened ⁷F₀→⁵D₀ absorption band can be performed by using the fluorescence line narrowing (FLN) technique to distinguish among different local environments around the rare-earth ions [11,12]. On the ground of the experimental results a crystal-field analysis and simulation of the energy level schemes have also been performed in order to parametrize the crystal-field around the Eu³⁺ ions. As a conclusion, we found evidences about the existence of at least four symmetry independent crystal field sites for the RE ions in this crystal. A plausible argument about the crystallographic nature of these sites is finally given.

2. Experimental techniques

Single crystals were grown by a self flux method, using an excess of the constituents as solvent in the pseudo ternary phase diagram Na₂O-La₂O₃(Eu₂O₃)-B₂O₃. Analytical grade purity of Na₂CO₃-La₂O₃(Eu₂O₃)-H₃BO₃ reactants with molar ratio 28.51%, 21.52% and 49.95% were weighed (about 100g), ground and mixed, sintered at successively 400°C and 650°C, then melted at 1160°C in a 50 cm³ Pt crucible in several batches. 0.5 mol% Eu³⁺ doping was chosen.

The growth experiments were carried out in a Kanthal resistance furnace equipped with an Eurotherm controller for temperature and cooling rate regulation. Melting and crystallization temperatures were first determined by using the dipping of a Pt wire. After homogenization of the melt at 1180°C during 24 hours, the temperature was slowly decreased to 1130°C (10°C/h) while the cooling rate was decreased to 0.2°C/h until total solidification took place, and then the furnace was cooled down to room temperature (30°C/h). Crystals grown on the surface of the melt were separated mechanically.

Resonant time-resolved FLN spectra were performed by exciting the sample with a pulsed frequency doubled Nd:YAG pumped tunable dye laser of 9 ns pulsed width and 0.08 cm^-1 linewidth and detected by an EGG&PAR Optical Multichannel Analyzer. The measurements were carried out by keeping the sample temperature at 10 K in a closed cycle helium cryostat.

For lifetime measurements, the fluorescence was analyzed with a 0.25 m Jobin-Ybon monochromator and the signal detected by a Hamamatsu R636 photomultiplier. Data were processed by a Tektronix oscilloscope.

3. Experimental results

3.1 FLN spectra

Time-resolved line-narrowed fluorescence spectra of the ⁵D₀→⁷F_0-6 transitions of Eu³⁺ doped Na₃La₉O₃(BO₃)₈ crystal were obtained at 10 K by using different resonant excitation wavelengths into the ⁷F₀→⁵D₀ transition, and at different time delays after the laser pulse. Depending on the excitation wavelength the emission spectra present different characteristics concerning the number of observed ⁵D₀→⁷F_J transitions, their relative intensity, and the magnitude of the observed crystal-field splitting for each ⁷F_J state. Figure 1 shows the spectra corresponding to the ⁵D₀→⁷F_0,1,2 transitions obtained with a time delay of 10 μs after the pump pulse, at four different pumping wavelengths 581.9, 581.7, 580.4 and 580 nm, which selectively show the presence of four main isolated Eu³⁺ sites.

We shall hereafter refer to the optical features of these spectra as originating from sites A (λ_exc=581.9 nm), B (λ_exc=581.7 nm), C (λ_exc=580.4 nm), and D (λ_exc=580 nm). The presence of the line for the ⁵D₀→⁷F₀ transition in each spectrum indicates a site of C_nv, C_n or C_s symmetry for Eu³⁺. These symmetries allow the transition as an electric dipole process, according to the group theory selection rules, with a linear term in the crystal-field expansion [13]. The symmetry characteristics of these Eu³⁺ optical centers can be inferred through the comparison among the number of possible and experimentally observed ⁵D₀→⁷F_0-6 transitions [14], and thus some symmetry point groups can be initially supposed for these Eu³⁺ optical centers.

The spectra obtained with excitation wavelengths 581.9 and 580.0 nm display, in each case, two Stark levels for the ⁵D₀→⁷F₁ transition and four levels in the hypersensitive ⁵D₀→⁷F₂ region. These results indicate that Eu³⁺ in A and D sites can be in the presence of a rather higher hexagonal, trigonal or tetragonal symmetry. Given the scarce number of energy levels observed for the ⁵D₀→⁷F_J transitions with J>2 we can reasonably extract no more information about specific symmetry point groups from these spectra, a task that must be undertaken under the detailed consideration of the Na₃La₉O₃(BO₃)₈ crystal structure, as will be developed in the following Section. On the contrary, the spectrum obtained with the excitation wavelength 581.7 nm shows three Stark levels for the ⁵D₀→⁷F₁ transition, and five and seven levels for the ⁵D₀→⁷F₂ and ⁵D₀→⁷F₃ emissions, respectively, which means that the degeneracy of these three states is completely lifted, that is, the Eu³⁺ B optical center is located in a crystal site with C_2v or lower symmetry. Finally, in the spectrum collected with excitation wavelength 580.4 nm, the two and three energy levels for Eu³⁺ site for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂, respectively, indicate a trigonal symmetry for the Eu³⁺ C site, which together with the observation of the ⁵D₀→⁷F₀ transition, reduces the possibilities to C_3v or C₃ point group symmetries. It is worth noticing that some minor peaks appearing in the spectra of the less intense emissions from sites C and D, probably associated to contributions from Eu³⁺ ions placed in residual/and or interstitial sites have been disregarded.

Table 1 (see Appendix) summarizes the FLN spectral characteristics of A, B, C, and D crystal field sites together with a plausible assignment of crystallographic cationic site for Eu³⁺. Energy levels observed for transitions from ⁵D₀ to the ground ⁷F_J manifold for these main four Eu³⁺ sites are included in Table 2 (see Appendix).

Regarding the relative intensity of the emission coming from the different sites, it is worthy to mention that the highest intensities corresponds to sites A and B, being the intensity from site A around 100 times higher than the one from site B and around three orders of magnitude higher than intensities from sites C and D.

 

Fig. 1. ⁵D₀→⁷F_0,1,2 emissions of Eu³⁺ in Na₃La₉O₃(BO₃)₈ crystal.

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3.2 Lifetimes

As could be expected, if there are different sites for the Eu³⁺ ion, the lifetime of state ⁵D₀ should depend on the excitation wavelengths. We have measured the lifetime of the ⁵D₀ state at different excitation wavelengths which correspond to those at which the Eu³⁺ sites are selectively resolved, and collected the luminescence at the highest intensity Stark component of the ⁵D₀→⁷F₂ transition. The experimental decays are well described by a single exponential function to a good approximation. The values of the measured lifetime are 1.87 ms, 1.73 ms, and 1.32 ms for sites A, B, and C respectively. The low intensity of the emission from site D makes it difficult to measure its lifetime accurately.

4. Crystal-field analysis and simulation of the energy level schemes

The detailed description of the theoretical background of the crystal field analysis and the methods followed to reproduce the experimental sequences of energy levels for Eu³⁺ in A, B, C, and D sites have been previously described [11,12]. In each case, the one-electron crystal field Hamiltonian can be expressed [15] as a sum of products of tensor operators (c^k_q)_i, with real B^k_q and complex S^k_q parameters as coefficients, these later appropriated to the Eu³⁺ site symmetry in the host, $H_{\mathit{CF}}=\sum _{k=2}^{4,6}\sum _{q=0}^k\left[B_q^k\mathit{\left(}{C}_q^k+{\mathit{\left(}-1\mathit{\right)}}^q{C}_{-q}^k\mathit{\right)}+i{S}_q^k\mathit{(}{C}_q^k-{\mathit{\left(}-1\mathit{\right)}}^q{C}_{-q}^k\mathit{)}\right]$ detailed as follows:

for A and D sites (Eq. 1), B site (Eq. 2) and C site (Eq. 3).

Schemes of 19, 34, 19, and 18 observed Stark levels, included in Table 2, from the total number of 37, 49, 33, and 37, were considered in the simulation of the sequence of Eu^{3+ 7}F_J energy levels in sites A, B, C, and D, with C_4v, C₂, C_3v, and C_4v crystal fields, respectively. Resulting simulated energy levels are also collected in Table 2, and values of their corresponding crystal field parameters and figures of merit of respective fits are included in Table 3 (see Appendix).

5. Correlation of FLN isolated Eu³⁺ sites with the crystal structure

The presence of the above observed Eu³⁺ optical centers must be explained by considering which sites of the Na₃La₉O₃(BO₃)₈ crystal structure can accommodate Eu³⁺ cations. Thus, the assignment of each A, B, C or D site to a specific site in the crystal structure must be led by the symmetry-related characteristics of the optical centers resolved in the FLN spectra. Though Eu³⁺ ion usually substitutes lanthanide cations in most of lanthanide-based compounds, in some mixed oxides, containing monovalent cations, these ions may have the same, or nearly the same, oxygen coordination than the one at the lanthanide site giving rise to some structural disorder [16] which facilitates the occupancy of these sites by the RE ions if charge compensation is allowed; therefore, we start this correlation with the inspection of the symmetry characteristics of their oxygen environments.

Following the previous structure description [9], from which the same numbering of atoms has been kept in the subsequent text and in Figs. 2 to 4, Na₃La₉O₃(BO₃)₈ crystals present the symmetry of the hexagonal space group P6̄ 2m (No. 189), with lattice parameters (Å) a=8.9033(3), c=8.7131(3), V=598.14(4), and Z=1; (see Fig. 2). In this oxyborate host La atoms occupy two different crystal sites, 3g and 6i, coordinated to eight and nine oxygen atoms, respectively. The La1O₈ polyhedron can be described as a distorted square antiprism (SAP), with C_4v symmetry, and La2O₉ is a distorted monocapped square antiprism (MSAP) with C_2v (or lower) symmetry; [see Figs. 3(a) and 3(b)]. The Na⁺ cations, of only one type, are surrounded by six oxygen atoms, being the NaO₆ coordination polyhedron described as a highly distorted octahedron; [see Fig. 3(c)]. This coordination is quite unusual for trivalent lanthanides [14], and therefore we have considered an extended oxygen environment which includes oxygen atoms that are not only the nearest neighbors indicated above. Four additional O3, from close La1O₈, La2O₉ and B3O₃ polyhedra, are found at a distance of 3.185(3) Å from Na⁺, in such a way that the oxygen distribution of the current NaO₁₀ polyhedron can be described as a tetracapped trigonal prism (TTP), with C_3v symmetry, which is one of the most frequently observed coordination polyhedra for lanthanide systems; [see Fig. 3(d)].

 

Fig. 2. Projection of the structure of Na₃La₉O₃(BO₃)₈ on the ab plane. Larger blue and yellow spheres represent La1 and La2 cations, respectively, medium cyan spheres stand for Na cations, red, pink, and violet triangles are indicating B1O₃, B2O₃ and B3O₃ groups, respectively, and the smallest green spheres are the oxygen atoms.

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Fig. 3. Coordination polyhedra for cationic sites in the Na₃La₉O₃(BO₃)₈ crystal: a) La1O₈ C_4v distorted square antiprism, b) La2O₉ C ₂ distorted monocapped square antiprism, c) distorted octahedron NaO₆, and d) extended NaO₁₀ C _3v tetracapped trigonal prism.

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From the above mentioned crystallographic symmetries, the characteristics of which are included in Table 1, it seems reasonable to attribute the spectra of sites A and B to Eu³⁺ located in environments derived from the replacement of La³⁺ in La1O₈ and La2O₉ polyhedra, respectively. Moreover, the C_3v symmetry of the extended NaO₁₀ coordination polyhedra could account for the crystal field characteristics found for Eu³⁺ at site C where some kind of additional charge compensation should be expected.

Up to now the La and Na crystal sites can explain the main three among four isolated Eu³⁺ sites in the FLN spectra. Therefore, an additional cationic site possessing the C_4v symmetry suggested by the spectroscopic characteristics of the remaining D spectrum, should be identified in the Na₃La₉O₃(BO₃)₈ crystal structure.

Returning to the structure description of the Na₃La₉O₃(BO₃)₈ crystal [9], it consists of an alternative stacking of layers along the c-axis containing Na-B(2)O(2)₃ (z=0), B(1)O(1)₃ (z=0.21), La2 (z=0.23), B(3)O(3)₃ (z=0.32) and La1O4 (z=1/2, on the mirror plane); (see Fig. 4). Within this picture, the three B1O₃, B2O₃ and B3O₃ triangles are running in rows along of the c axis as shown in Fig. 4. If the sites of boron cations act as perturbed Eu³⁺ sites induced by the Eu³⁺-doping itself, they would manifest the C_4v symmetry corresponding to the remaining D site in the FLN spectrum. Let us revise the local extended oxygen environments around the three B cations: For Eu³⁺ in the B1 site, three O4 at a distance 3.420(3) Å, and three O1 at 3.917(3) Å, will constitute its extended environment with C_3v local symmetry. Correspondingly, Eu³⁺ in the B2 site is surrounded by six O3, all of them at 3.084(3) Å. When the substitution in the B3 site is considered, Eu³⁺ is surrounded by three O2 at 3.145(3) Å, three O3 at 3.419(3) Å, and three O4 at 3.694(3) Å, which could result in a C_4v local symmetry. This last perturbed Eu³⁺ site, surrounded by nine oxygen atoms, can be thought of as the origin of the D center, which moreover can be distributed in an ordered way through out the crystal, and thus require nearby cationic vacancies for charge compensation.

 

Fig. 4. View of the structure of the Na₃La₉O₃(BO₃)₈ crystal, showing the alternate layers of Na-B(2)O(2)₃ (z=0), B(1)O(1)₃ (z=0.21), La2 (z=0.23), B(3)O(3)₃ (z=0.32) and La1O₄ (z=1/2, on the mirror plane), with BO₃ triangles aligned in rows along the c axis.

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In conclusion, according to the above mentioned symmetry characteristics of the FLN spectra for Eu³⁺ located in the A, B, C, and D sites, the simulations of the corresponding energy level sequences performed for C_4v, C₂, C_3v, and C_4v crystal field potentials, respectively, yield ⁷F_J schemes in very good agreement with the experimental data, as can be seen in Table 1.

Initially the spectrum for Eu³⁺ in site B was simulated by considering the C_2v potential, but the agreement between observed and calculated energy levels was found to improve by introducing the complex S^k _q parameters of symmetry C₂, which in turn agrees with the fact that the La2O₉ site, to which the Eu³⁺ B-spectrum corresponds, is a very distorted MSAP.

The C_4v characteristics of the Eu³⁺ spectrum in site A, the most abundant one, fully reflect the nature of the La1O₈ environment, with La1 located on the mirror plane in the c-axis. The crystal field parameters involved in the description of C_4v are the same as for D_4h and D₄ potentials, but the presence of the ⁵D₀→⁷F₀ transition undoubtedly discards these latter symmetries.

Spectra for Eu³⁺-A and D sites, both with C_4v symmetry characteristics, have been reproduced through very different sets of crystal field parameters (see Table 3), which lead to very different Eu³⁺ local environments.

On the other hand, the inferred existence of an extended NaO₁₀ environment for Eu³⁺ in site C can be understood on the basis of the poor or incomplete effective shielding of Eu³⁺ by the six nearest coordinated oxygen atoms that form the distorted octahedral coordination in the crystallographic description of the structure. Thus, the crystal field generated by the ligands in the first coordination sphere is, in this case, not a good enough approximation for the crystal field perturbation felt by the Eu³⁺ doping cation, an effect which was previously described in other well known Eu³⁺-doped borate layered crystal, YAl₃(BO₃)₄ [17].

6. Conclusion

By using the fluorescence line narrowing technique we have demonstrated the existence of four different local environments around the RE ions in Na₃La₉O₃(BO₃)₈ crystal. On the ground of the experimental results, the crystal-field analysis and simulation of the energy level schemes allow to connect the predicted symmetry of the resolved sites with the crystal structure. In conclusion, though RE ions may occupy the crystallographic sites for La1, La2, Na, and B3 the luminescence results suggest that the first possibility is the most likely to occur.

Appendix

Table 1. Summary of spectroscopic results and assignment of Eu³⁺ positions in Na₃La₉O₃(BO₃)₈ crystal

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Table 2. Observed and calculated energy levels (cm^-1) of Eu³⁺ optical centers observed in Na₃La₉O₃(BO₃)₈ crystal

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Table 3. Phenomenological crystal-field parameters (cm^-1) for Eu³⁺ optical centers observed in Na₃La₉O₃(BO₃)₈

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Acknowledgments

This work was supported by the Spanish Government MEC (MAT2004-03780) and the Basque Country Government (IT-331-07).

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