1. Introduction

Strontium barium niobate Sr_xBa_1-xNb₂O₆ (SBN) crystals (0.25 ≤ x ≤ 0.75) belong to a family of ferroelectric materials with relevant piezo-electric, pyroelectric, and non-linear electro-optic properties [1]. Potential applications cover photorefractive memories, optical amplification, light waveguiding, lasers, temperature sensors and fibers [2–6]. The incorporation of rare earth (RE³⁺) ions additionally produces an enhancement of its electro-optic and photorefractive coefficients [7]. As a result, the synthesis of RE³⁺-doped SBN structures on the nanoscale has received significant attention for photonics and optoelectronics devices. In this context, one of the most promising methods is the crystallization of glasses using either laser irradiation or a conventional furnace [8,9]. The formation of Sr_0.5Ba_0.5Nb₂O₆ nanocrystals is triggered by a thermal treatment near the crystallization temperature of the glass. Using spatially selective cw Ar⁺ laser irradiation or controlling the heating time of a furnace, a glass-ceramic is obtained with embedded SBN nanocrystals [5,10,11]. Owing to the ease of fabrication, the remarkable macroscopic properties and the ability to introduce RE³⁺ ions in the nanocrystals, transparent nanostructured glass-ceramics have become a unique material for optical engineering.

The incorporation of RE³⁺ ions in the SBN low-phonon energy nanocrystalline environment is paramount to increase the quantum yield of the RE³⁺ luminescence by reducing the non-radiative de-excitation rates [12]. Previous reports confirmed the inclusion of the RE³⁺ ions into the SBN nanocrystals by monitoring the emissions of Nd³⁺ at 900 nm and Er³⁺ ions at 1550 nm in the glass-ceramic compared to those of the precursor glass. About 75% of the RE³⁺ ions were incorporated in the SBN nanocrystals [13,14], probably playing the role of heterogeneous nucleating agents as in other glass-ceramics [15]. However, further studies with nanoscale spatial resolution are necessary in order to elucidate the real distribution of SBN nanocrystals. In particular, their agglomeration and/or clustering pathways could favor the non-radiative energy transfer among RE³⁺ ions, causing even quenching of the luminescence.

Although X-ray fluorescence (XRF) and diffraction (XRD) can shed light on the composition and crystal structures that coexist in the glass-ceramic (e.g. nanocrystalline and amorphous) respectively, nano-X-ray excited optical luminescence (nano-XEOL) is the only technique that makes possible the optical study of the spatial and site distributions of the RE³⁺ ions concurrently in the glass-ceramic. Based on a hard X-ray nanoprobe, this imaging tool is ideal for those applications where light emission plays the functional role. So far, XEOL has been used to investigate optical heterogeneities, defects, impurities and radiative de-excitations of optically active centers in nano-LEDS [16,17], photovoltaic materials [18], nanocrystals [19] and RE³⁺-doped nanomaterials [20]. Unlike an UV source, which excites valence to conduction band transitions, X-ray photons excite core electrons to bound, quasi-bound or continuum states, allowing more pathways to be involved in the decay. In this work, we use nano-XEOL to analyze the distribution of SBN nanocrystals and to elucidate the role played by Nd³⁺ ions in the formation of a borate-based glass-ceramic. To our knowledge, this is the first time nano-XEOL imaging is applied to nanostructured glass-ceramics.

2. Experimental

Figure 1 shows a sketch of the experimental setup and beam profiles: a scanning XRF coupled with an XEOL setup that supplies simultaneous compositional and optical information at the nanometer scale. Developed at the hard X-ray nanoprobe end station located at 60 meters from the ESRF source (presently upgraded and relocated at 165 meters), Si mirrors mounted in crossed Kirkpatrick-Baez configuration highly focus an intense hard X-ray beam [21]. The emission of characteristic X-rays is recorded with a Si drift detector whereas the luminescence using a microscope objective lens. The latter is then focused into a multimode optical fiber and carried out of the experimental hutch to be analyzed by an optical bench consisting of a monochromator and NIR InGaAs detector.

 

Fig. 1 (a) Schematic of the multimodal experimental setup at the hard X-ray nanoprobe beamline ID22NI (presently ID16B) at the European Synchrotron Radiation Facility. (b),(c) Focused beam profiles (vertical and horizontal) taken at 17.18 keV by means of the Au knife-edge scans.

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The SBN precursor glass has a composition (in mol%) of 50 B₂O₃ − 22.5 Nb₂O₅ −11.25 SrO − 11.25 BaO − 5 Nd₂O₃. The methods of synthesis of the glass and glass-ceramic are described elsewhere [14]. XRD patterns were measured in a diffractometer (PANalytical X’Pert PRO) using the Cu K_α1 radiation. Statistical relevant XEOL data were collected in different regions of the SBN glass-ceramic monitoring the NIR emissions of the Nd³⁺ ions in the 1-1.5 eV energy range with 125 × 125 nm² per pixel in order to match the beam size.

3. Results and discussion

XRD patterns of both the precursor SBN-borate glass and the SBN glass-ceramic are compared in Fig. 2. The diffractogram of the glass shows two broad bands associated with the lack of long range order of the amorphous glass network, whereas that of the glass-ceramic also displays intense narrow peaks associated with a crystalline phase. The Rietveld refinement analysis suggests the crystalline phase to be the tetragonal tungsten–bronze structure of the Sr_0.5Ba_0.5Nb₂O₆ crystal [22] in the space group P4/mmm (Laue group 4/mmm), with cell parameters a = 12.4443(6) Å, b = 12.4443(6) Å, and c = 3.9010(3) Å. An average diameter of around 60 nm has been estimated for the precipitated SBN nanocrystals in this glass-ceramic using the Scherrer formula, similar to those results obtained applying AFM techniques [23].

 

Fig. 2 XRD patterns of an SBN-borate glass (bottom) and SBN nanostructured glass-ceramic (top). Rietveld fitting of the XRD pattern for the glass-ceramic in tetragonal space group P4/mmm is also shown (in red). A projection of Sr_0.5Ba_0.5Nb₂O₆ structure onto the plane xy is also included, where different cationic sites are indicated.

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The glassy SBN-borate phase is composed of boron glass-former and strontium, barium and niobate glass-modifiers. The 3D network is formed by the gathering of BO₃ triangles and BO₄ tetrahedral units into well-defined borate groups [24]. The modifier cations break the borate network, and non-bridging oxygens seems to be bonded to Nd³⁺ ions in the vicinity of the breakdown, replacing the Sr²⁺ and Ba²⁺ ones with similar ionic radius. As a result, there is a large variety of environments with different distances and angles for the Nd³⁺ ions [25]. The SBN crystalline phase is a 3D network of sharing oxygen vertices of NbO₆ octahedron with three kinds of channels along the four fold axis that generate different cationic sites (see Fig. 2) [22,26]: The A₁ sites (C₄ symmetry) are partially occupied only by Sr²⁺ ions, although they also replace Ba²⁺ ions at the A₂ sites (C_S symmetry); two distorted octahedral sites, B₁ (C_2V symmetry) and B₂ (C₁ symmetry), are completely filled by Nb⁵⁺ cations with sixfold coordination; and finally, an interstitial empty C site with ninefold coordination, as well as randomly distributed oxygen vacancies. Such configuration generates a deviation from a regularly ordered structure, what is claimed to be responsible for the optical properties of the SBN crystals [22].

To obtain preliminary compositional information, XRF data were acquired with a 17.18 keV X-ray nanobeam (see Fig. 3(a)). Following the X-ray absorption events, the subsequent filling of the core holes is fulfilled by higher energy core-level electrons, causing series of radiative and non-radiative processes [27]. Part of the radiative processes give rise to XRF (K, L and M lines). Since the energies of these emissions are characteristic of individual elements, they can be used to identify the composition using PyMca code [28] (see Fig. 3). The XRF analysis also reveals the presence of argon due to air and other unintentional dopants or impurities, such as lead, as well as traces of palladium, gold and thulium. These unexpected elements do not seem to play a relevant role in the formation of neither the SBN nanocrystalline nor the SBN-borate glass coexisting phases. But, they may have a strong influence in the optical properties of the RE³⁺ ions acting as local optical traps. Through resonant and/or phonon-assisted energy transfer mechanisms they may produce an inhibition of the UV-Vis-NIR luminescence of the RE³⁺ ion, especially in the glassy phase.

 

Fig. 3 (a) XRF experimental data (black dots) and fitting curves using PyMca code (color lines). X-ray fluorescence background (light green line) is also shown. The broad background results from the X-ray scattering by the surrounding environment (e.g., air and compact setup). (b) ⁴F_3/2→⁴I_J (J = 9/2 and 11/2) NIR XEOL luminescence of an SBN ferroelectric glass-ceramic doped with 5 mol% of Nd₂O₃ under non-selective excitation with 17.18 keV hard X-ray energy of a synchrotron radiation.

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Exciting the glass-ceramic with an X-ray nanobeam of 17.18 keV, in addition to the X-ray photon absorption and XRF processes, a fraction of radiation is emitted as luminescence in the optical (UV-Vis-NIR) range. Thus, varying the beam position on the sample we can obtain XEOL spectra with about 150 nm spatial resolution. Figure 3(b) shows a representative XEOL spectrum, where two bands in the NIR range centered at 890 nm (1.39 eV) and 1065 nm (1.16 eV) dominate. They can be straightforwardly associated, according to the well-known Dieke’s diagram, with the electric-dipole ⁴F_3/2→⁴I_J (J = 9/2, 11/2) intra-configurational 4f ³−4f ³ electronic transitions of the Nd³⁺ ions. In the glass-ceramic, Nd³⁺ ions mainly occupy the A₁ and/or A₂ sites replacing Sr²⁺ and/or Ba²⁺ with C₄ and C_S local point symmetries, respectively. Thus, the crystal-field interaction between Nd³⁺ ions and their oxygen ligands will completely break the degeneracy of the ^{2S + 1}L_J multiplets of the free-Nd³⁺ ions giving rise to a fine structure of crystal-field, or Stark, levels [12]. Since this interaction also rules the probabilities of the absorption and emission processes of the RE³⁺ ions, the structural distortions generate small variations in the energy level scheme and in the optical properties of the Nd³⁺ ions. Accordingly, the inhomogeneous broadening of the absorption and emission bands is also affected. This effect is also suffered in a larger extent by those Nd³⁺ ions that remain in the amorphous glassy phase. Thus, the NIR emission bands would be a convolution of the emissions from the Nd³⁺ ions in the SBN nanocrystals and those remaining in the surrounding SBN-borate glass, which results in broadening profiles from both coexisting phases [13]. However, the XEOL signal seems to come mainly from those Nd³⁺ ions in the SBN nanocrystals. Two reasons may support this hypothesis: first, quantum efficiencies of the Nd³⁺ emissions in the nanocrystals are much higher and non-radiative lifetimes are longer than in the glass [10]; and second, from luminescence studies [13,14], it has been concluded that around 75% of the Nd³⁺ ions are incorporated in the SBN nanocrystals. In addition, there is another observation: the resemblance of the Nd³⁺ emissions in the glass-ceramic to those found in a pure SBN bulk crystal [4]. They also shows a structured band with two main peaks due to the emissions from the two Stark levels of the ⁴F_3/2 emitting multiplet to the five Stark levels of the ⁴I_9/2 ground state. This result suggests that the short range crystal-field interaction is practically the same in nano and bulk SBN.

The identification of the mechanisms involved in the 17.18 keV hard X-ray photon-in and the 1-1.5 eV NIR photon-out processes is not an easy task. In general terms, the XEOL process can be split into three subsequent steps: 1) core-electron excitation, 2) electron-hole thermalization and diffusion, and 3) radiative recombination. Once the X-ray absorption takes place, Auger and XRF decay processes show up to fill the core holes, producing holes at outer atomic shells and extra energy to excite electrons at shallower levels. Such secondary mechanism continues and generates electron-hole pairs in a cascade fashion till the energy is too low for further electronic excitation. In the glass-ceramic, the carriers undergo diffusion and thermalization until the electron reaches the bottom of the conduction band and the hole reaches the top of the valence band. Then, part of the energy for the electron-hole recombination has to be transferred to the Nd³⁺ ions. Thus there must be efficient non-radiative energy transfer channels to the Nd³⁺ optically active ions from those elements forming the SBN structure (Nb, Sr, Ba and O), whose energy levels are located within the forbidden band gap, or even overlapping the conduction band of the matrix. Such phenomena result into the excitation of Nd³⁺ ions to the high energy levels of the 4f ³ ground configuration in the UV range. The Nd³⁺ ions in the UV excited levels suffer from non-radiative multiphonon and cross-relaxation de-excitation processes down to the ⁴F_3/2 state, from which they relax into the ⁴I_J (J = 9/2, 11/2) states emitting NIR photons.

The use of the nano-XEOL technique is crucial to address the spatial distribution and formation of agglomerations or clusters, or even preferential growth areas of SBN nanocrystals in the glass-ceramic. These features affect critically the optical properties of the Nd³⁺ ions [29,30]. Black, red and green XEOL spectra shown in the upper panel of Fig. 4 represent an spatially averaged (X^ave) and two spatially resolved ⁴F_3/2→⁴I_11/2 curves taken over a representative 3 × 3 µm² sample area and two different spots, respectively (signals normalized for clarity: 0<X<1). The comparison reveals spatially-dependent ⁴F_3/2→⁴I_11/2 emission contrasts at 1050 nm, 1060 nm and 1080 nm wavelengths. There are sample regions where the XEOL presents a clear double peak structure around 1050 nm, whereas in other areas these peaks merge and form a shoulder. Intensity variations at longer wavelengths are also observed with several peaks contributing to the XEOL spectrum tail around 1080 nm. Both effects are spatially correlated since higher emission intensity at the 1050 nm peak structure comes with lower emission at the long wavelength tail. Thus, these structures would play a key role and could be related to the new scheme of energy levels of the Nd³⁺ ions originated after the thermal treatment of the precursor glass with the formation of the SBN nanoparticles (i.e. it does not appear in the emission of those Nd³⁺ ions in the glassy phase).

 

Fig. 4 ⁴F_3/2→⁴I_11/2 NIR XEOL spectra of an SBN ferroelectric glass-ceramic doped with 5 mol% of Nd₂O₃ under non-selective excitation with 17.18 keV hard X-ray energy of a synchrotron radiation. Maps obtained selecting different ranges of emission. They represent the relative variations of the emissions within their spectral ranges.

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A further analysis can be performed by selecting different spectral ranges, Δλ (see vertical semi-transparent bands with different colors). Their corresponding maps have been generated on a pixel-by-pixel basis (i) by normalizing (0<X_i^Δλ<1) and subtracting these signals with respect to the average spectrum (X_i^Δλ− X^ave). Thus, the resulting relative intensity patterns (in %) show a better contrast of the Nd³⁺ luminescence, revealing spatial changes of the different channels in 2D. As a result, a picture of the distribution of Nd³⁺ ions and their site preferences, crystalline or amorphous, can be drawn for the glass-ceramic. Green and blue maps display opposite patterns in defined areas (e.g. A and B). The green pattern shows the luminescence of those Nd³⁺ ions mainly incorporated in the SBN nanocrystals, while the blue one exhibits the emission from Nd³⁺ mainly kept in the glassy phase. These results are confirmed by the red image, which also reveals areas of higher concentration of Nd³⁺ ions in the SBN nanocrystals associated with the existence of agglomerations or clusters of these nanoparticles in the glass-ceramic. Such spatial heterogeneity in the formation of the SBN nanocrystals, with cluster and other areas of uniform distribution well separated from the glassy phase, is confirmed by further XEOL scanning maps (not shown here). The origin is directly related to the kinetics of the nucleation and growth of SBN crystals in the borate glasses. Using laser induced crystallization techniques, the growth of crystals would be controlled by the diffusion of constituent Sr²⁺, Ba²⁺, Nb⁵⁺ and RE³⁺ ions in order to form the SBN crystals, especially the Nb⁵⁺ and RE³⁺ ions, which show larger diffusion coefficients [11]. Temperature gradient induced by the laser appears to be the leading driving force for the elemental redistribution of the ions in the precursor glass. For the glass, it seems reasonable to assume that almost all the Nb⁵⁺ ions will participate in the formation of the Nb₂O₆ network (basis of the SBN nanocrystals), competing with the B₂O₃ network for the Sr²⁺, Ba²⁺ and Nd³⁺ ions to build their final short range order. During heating, and once the crystallization temperature is reached, the nucleation and growth of the SBN crystals are triggered. Its basic structure depends basically on the Nb⁵⁺ ions forming NbO₆ octahedral, with a degree of disorder imposed by the concentration of Sr²⁺, Ba²⁺ and Nd³⁺ ions [26], reaching a volume content of about 40% [31]. This heating-induced reorder is generated by the diffusion of ions. They could be favored since the nominal composition of the Nb₂O₅−SrO−BaO compounds, easily dissolved in the B₂O₃ glass network, is the same in the precursor glass and in the SBN crystal. In fact, this ‘compositional memory’ seems to play a key role in the formation of the SBN nanocrystals, although the presence of B₂O₃ affects the arrangements and diffusions of Sr²⁺, Ba²⁺ and Nb⁵⁺ in such a way that induces a clearly non-uniform spatial distribution of SBN crystals. In addition, due to the large diffusion coefficients of Nb⁵⁺ and Nd³⁺ ions, a high concentration of the optically active Nd³⁺ ions might form part of the Nb₂O₆ tetrahedron structure, probably acting as heterogeneous seeds for the nucleation and growth of the SBN nanocrystals.

4. Summary

The formation and spatial distribution of Sr_0.5Ba_0.5Nb₂O₆ nanocrystals in a borate-based glass-ceramic has been successfully described based on nano-XEOL imaging. Nd³⁺ rich areas of agglomerations and/or clusters of nanocrystals were revealed and ascribed to the diffusion coefficients of their constituent elements. The Nd³⁺ ions seem to act as heterogeneous agents for the nucleation and growth of these nanocrystals.