1. Introduction

Since the first working laser based on a ruby crystal was built in 1960 [1], the development of laser crystal materials has undergone for over half a century. Nowadays, as in the past, the bulk laser crystals play a crucial role in laser technologies and engineering, although the investigation and development of new gain materials, such as fiber, waveguides and transparent ceramics [2], grow very fast. Generally, the laser crystals consist of active ions and host materials. There are hundreds of laser crystals being formed by a combination of the active ions of rare-earth ions or transition metal ions, with oxide and fluoride host crystals [3]. However, as regards practical lasers their number is very limited, and the development of new laser crystals in the manner is to some degree unsustainable.

New laser crystals are highly desirable in novel laser technologies and applications [4]. Several new strategies, including cationic/anionic substitution [5], chemical unit cosubstitution, cation nano-segregation and local structure manipulation were tried and extensively applied on the photoluminescence properties tuning of phosphor materials [6–8]. By manipulating local coordination environment of the active ions, realization of tuning spectral properties was highly expected. The concept of local structure manipulation provides a possible methodology, and has been tentatively tried in tuning spectral properties and laser performances of rare-earth-doped fluorite-structure crystals [9–17].

Due to the charge compensation effects, required to accommodate a trivalent ion in the divalent cationic sites, the doped sites of rare-earth ions become inequivalent, also trivalent ion clusters were easily formed in fluorite crystals [18–19]. The clustering processes were severely rare-earth concentration and host lattice dependent [20–21]. Signals of 18 kinds of centers were observed in the crystal [22]. Therefore, rare-earth doped fluorite-like crystals have very broad absorption and emission band full width at half maximum (FWHM) caused by these centers. Broad emission made the crystals particularly appealing for generation of ultrashort pulses. Additionally, due to the abundant 4f, 5d levels, the energy relaxations will bring benefits for the emissions and laser performances at 1.8 µm of Tm³⁺ and 2.8 µm of Er³⁺ [14–15]. However, luminescence at 1 µm of Nd³⁺ has been deadly quenched by the incoherent dipole-dipole energy transfer process [23]. Kaminskii et al. firstly introduced Y³⁺, La³⁺, Gd³⁺ and Sc³⁺ to break the quenching clusters and ameliorate the quantum yields in 1960s [24–25], and they were called buffer ions [26]. Furthermore, Catlow, Cheetham et al. have devoted themselves on the clusters study by both modeling and experiment methods. Centers of monomers, dimers, trimers, tetramers and hexamers were simulated in CaF₂, SrF₂ and BaF₂, and the stabilities varied with the dopants and matrix crystals [18–19,27]. Local environment of the centers altered with size of the rare-earth ion, and signals of dimer and hexamer clusters were observed by EXAFS [28]. Two kinds of monomers were identified and their relative concentrations in Gd³⁺ doped CaF₂, SrF₂ and BaF₂ were studied by the correlated EPR and ITC technique [29]. The neutron diffraction results show that there are vacancies in normal fluorine lattice sites, and two types of interstitials F_i⁻ were detected. Variations in relative number of these defects intimately connected with local structures and configurations of the clusters [30].

On the base of these achievements, from beginning of 21st century, a lot of important progresses have been obtained in rare-earth doped fluorite crystal. For example, in Yb³⁺-doped calcium fluoride (CaF₂), the spectral properties were modulated by Na⁺ in large scale, and 3-mJ femtosecond pulses were realized [9–10]. The FWHM of the emission band of Nd³⁺ in CaF₂ crystal was manipulated by Y³⁺ to be 31 nm, and mode-locked laser pulses of 103 fs were obtained [11]. The FWHM of the band is even comparable with that of Nd³⁺-doped glasses [31]. By codoping Gd³⁺ in Pr³⁺:CaF₂ the photoluminescence was highly improved and red lasing was firstly achieved [12]. With addition of Na⁺ the lasing wavelength of Tb³⁺:CaF₂ was continuously tuned from 539.5 to 550.0 nm in green range [13]. The laser tuning range of more than 190 nm was firstly demonstrated in Tm³⁺:CaF₂ when codoped with Y³⁺ [14]. Highly efficient dual-wavelength lasing at 2.8 µm was performed in Er³⁺-doped strontium fluoride (SrF₂) [15], and the doping concentration was an order of magnitude lower than that of Er³⁺:YAG crystal [16]. For Nd³⁺:SrF₂, the emission band FWHM was modulated to be 20 nm when codoped with Y³⁺, and pulse of 97 fs was achieved [17]. The spectral properties and laser performances were realized to be tuned in large scope by manipulating local environment of the active ions in fluorite crystals. However, pictures about local structure of the active ions manipulated by Sc³⁺, Y³⁺, Ce³⁺, La³⁺, Gd³⁺, Lu³⁺, O^2-, Na⁺ and K⁺, as well as the other trivalent rare-earth ions, were unknown after nearly six decades.

In this article, local structures of the active Nd³⁺ tailored by Y³⁺ were studied by first principles calculation. It was found that the cubic coordination surroundings of Nd³⁺ were transformed to be distorted lower symmetric square antiprism structures. The more relaxed parity forbiddance made the f-f transition rate and near-infrared emissions be highly enhanced. Moreover, low temperature TRES suggests that a multisite crystal was changed to be a quasi-single center system. From this perspective, the methodology of local structure manipulation could offer new opportunity to tune spectral properties and performances of rare-earth doped fluorite materials.

2. Materials and experiments

Crystals of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂ (x = 0, 1.5, 2, 4.5, 5 and 10 denoted as A, B, C, D, E and F, respectively), y at.% Nd³⁺:SrF₂ (y = 0.09, 0.4, 0.6, 0.85 and 1.65 as A’, B’, D’, E’ and F’, respectively) and y at.% Nd³⁺,5 at.% Y³⁺:SrF₂ (y = 0.09, 0.4, 0.6, 0.85 and 1.65 as A’’, B’’, D’’, E’’ and F’’, respectively) were synthesized by temperature gradient technique method. The concentration of Y³⁺ was 50 times higher to ensure that all Nd³⁺ centers were changed to be [Nd³⁺-Y³⁺] clusters. Raw materials of the fluorites with purity of 99.99% were finely mixed and loaded in the graphite crucible. The melting temperature is about 1693 K, the growth with rate of −1.5 K/hour was completed in the vacuum of 10⁻³ Pa, and finally the crystals were obtained. The part near the seed was cut and prepared in the same dimensions for all the measurement.

In the structure relaxations, the plane-wave basis set and the Perdew-Burke-Ernzerhof exchange correlation potential have been used as implemented in the VASP code [32–36]. The projector augmented wave (PAW) potential was used to describe the electron-ion interactions [37–38]. In addition, the spin polarization was included in all calculations. A 550 eV cut-off in the plane-wave expansion and a 1×1×1 Gamma k-grid were used to ensure that the relaxation has an accuracy of 10⁻⁵ eV. The internal coordinates of the supercell (of size 2×2×2) were optimized until forces on individual atoms reached 0.01 eVÅ⁻¹ to obtain sufficient accuracy. Formation energy of the cluster (ΔE) was obtained based on formula (1).

Room temperature absorption spectra were recorded by a Jasco V-570 UV/VIS/NIR spectro-photometer and the photoluminescence spectra were tested with a FLS980 time-resolved fluorimeter grating blazed at 1200 nm and detected by thermoelectric (TE) cooled InGaAs detector. The excitation was a xenon lamp at 796 ± 5 nm. The lifetime was measured using the fluorimeter and Tektronix TDS 3052 oscilloscope following a flash lamp excitation at 796 ± 5 nm. Time resolved emission spectra (TRES) was realized by testing the decay curves using the same fluorimeter following a flash lamp excitation at 796 ± 5 nm. The detected wavelength ranged from 1020 to 1120 nm with steps of 0.5 nm and integrating time of 20 s. The samples were placed into a Linkam THMS600E (UK) continuous-flow liquid nitrogen cryostat. The temperature was controlled at 78 K by the Linksys32 system.

3. Results

3.1 Rare earth ion clusters simulation

According to the references of [18] and [19], centers of [mNd³⁺] and [nY³⁺] from monomer to hexamer (m = 1∼6, n = 1∼6) were modelled. However, no related content about [Nd³⁺-Y³⁺] cluster was available, and all possible [mNd³⁺-nY³⁺] supercells were tentatively constructed and then relaxed using the VASP code [32–35]. The screening, based on the thermodynamic stability computation, was performed. It can be seen in Figs. 1–2 and Table 1, Nd³⁺ ions, as well as Y³⁺, are easily clustering in SrF₂ crystal. The centers were divided into three groups, the first is monomer centers, and the others are clusters but with different local sublattice, cubic and square antiprism, respectively. In Nd³⁺:SrF₂ crystal, the stability of [1Nd³⁺−1F_i⁻] centers with interstitial F_i⁻ at the nearest and next nearest site is the same. While for Y³⁺:SrF₂ the next nearest site center is more stable, agreed well with the reported results [18]. There are dimer, trimer and tetramer cubic sublattice clusters in Nd³⁺-doped SrF₂, and only dimer centers existed in Y³⁺-doped crystal, as demonstrated in Figs. 1–2. The difference should be ascribed to the competition of Nd³⁺-F_i⁻ or Y³⁺-F_i⁻ attraction with F_i⁻−F_lattice⁻ (matrix lattice) repulsion, which will be discussed further.

 

Fig. 1. Thermodynamic stable Nd³⁺ centers in SrF₂ crystals: (a) first group monomer centers, (b) second group clusters with cubic sublattice local environment and (c) third group clusters and the local is square antiprism. The bracket suffixed with different numbers means the same centers with different configurations.

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Fig. 2. Thermodynamic stable Y³⁺ centers in SrF₂ crystals: (a) first group monomer centers, (b) second group cubic sublattice clusters and (c) third group square antiprism clusters. The bracket suffixed with different numbers means the same centers with different configurations.

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Table 1. Simulated formation energy of Nd³⁺ or Y³⁺ centers in SrF₂ crystals. The bracket suffixed with different numbers means the same centers with different configurations, “ / ” denotes that the center was not stable.

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[Nd³⁺-Y³⁺] clusters were also simulated, as presented in Fig. 3 and Table 2. Nd³⁺ and Y³⁺ also tend to aggregate into clusters of [mNd³⁺-nY³⁺]. Due to the strong repulsion of F_i⁻−F_lattice⁻, nearly all [Nd³⁺] centers are surprisingly changed to be the square antiprism sublattice clusters. It is noted that the cubic and square antiprism sublattice coexisted in [2Nd³⁺−1Y³⁺−4F_i⁻] center, presenting clustering characters of both Nd³⁺ and Y³⁺. When more than one Y³⁺ ions (n ≥ 1) joined in a cluster the cubic sublattice would be transformed to be square antiprism to relieve the inner strain, as shown in Fig. 3(b).

 

Fig. 3. Thermodynamic stable [Nd³⁺-Y³⁺] centers in SrF₂ crystal: (a) second group cubic sublattice clusters, (b) third group square antiprism clusters. The bracket suffixed with different numbers means the same centers with different configurations.

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Table 2. Simulated formation energy of [Nd³⁺-Y³⁺] clusters in SrF₂ crystal. The bracket suffixed with different numbers means the same centers with different configurations.

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Formation energy of the clusters were plotted and presented in Fig. 4(a). The energy of [Nd³⁺-Y³⁺] clusters decreased linearly with increasing number of Y³⁺ ions introduced in the center. The slope of [Nd³⁺-Y³⁺] clusters was nearly identical with that of pure Y³⁺ centers. As shown in Fig. 4(b), formation energy will be reduced by 1.113 eV with one more Nd³⁺ added in the cluster, and it is 1.400 eV for Y³⁺. Furthermore, the formation energy of [mNd³⁺-nY³⁺] clusters with m = 1 and n respectively equals to 1, 4 and 5 in Fig. 4(a), is even lower than that of pure Y³⁺ centers, which means that Y³⁺ is a very potential and efficient regulator to recombine with Nd³⁺ ions. Simultaneously, the local surroundings were modulated to be more distorted, as shown in Fig. 4(c). [3Y³⁺−4F_i⁻] and [4Y³⁺−4F_i⁻] centers are more stable than that of [1Nd³⁺−2Y³⁺−4F_i⁻] and [1Nd³⁺−3Y³⁺−4F_i⁻] clusters, respectively. Some portion of the introduced Y³⁺ would aggregate into pure Y³⁺ centers rather than joined together with Nd³⁺.

 

Fig. 4. (a) Formation energy of Nd³⁺ and/or Y³⁺ clusters versus the number of Y³⁺ ions within a cluster. Hollow symbol with the same solid shape represents the same clusters with different configurations. (b) Number of dopant cations dependent formation energy of Nd³⁺ or Y³⁺ centers. (c) Local distortion of Nd³⁺ in [Nd³⁺−Y³⁺] polyhedron, the arrow with directions to Nd³⁺ and F⁻ means shortened and elongated bond length, respectively, when compared with the pure Nd³⁺ centers.

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Thus clear pictures about local structure of [Nd³⁺] centers tailored by Y³⁺ were obtained. Besides Y³⁺, how about other ions, are they playing the same role. For the answers, it should be known firstly why the local environment of Nd³⁺ could be tailored by Y³⁺. The projected DOS was computed, as demonstrated in Figs. 5(a)–(b) of [1Y³⁺−1F_i⁻] center. The partial bond connectivity between Y³⁺ and interstitial F_i⁻ was established when F_i⁻ at the nearest site, and no such bonding occurred while at the next nearest site which means that only coulomb interaction existed in Fig. 5(b). Due to Y³⁺−F_i⁻ attraction, distance of F_i⁻ and F_lattice⁻ in Fig. 5(a) was shortened. Most of the nonbonding electrons were localized on interstitial F_i⁻ rather than extended to the vacant lattice cation orbitals or the dopant cations. This made the repulsion force of F_i⁻−F_lattice⁻ larger than that of Y³⁺−F_i⁻ attraction when interstitial F_i⁻ at the nearest site. Formation energy of [1Y³⁺−1F_i⁻] center with F_i⁻ at the nearest site was higher about 0.2 eV than that at the next nearest site in virtue of large inner strain. For Y³⁺ clusters with several interstitials F_i⁻ and substitutional impurities, due to the strong repulsion of F_i⁻−F_lattice⁻ and the relatively weak Y³⁺−F_i⁻ interaction, it was sufficient to drive the cubic local sublattice to be square antiprism in Fig. 3(b). As can be seen in Fig. 9(a) (Appendix), p state of F_i⁻ was completely bonded with the dopant cation in square antiprism Y³⁺ clusters.

 

Fig. 5. Projected DOS on local coordination structure of the cluster. The p state of fluorine in [1Y³⁺−1F_i⁻] and [1Nd³⁺−1F_i⁻] centers with interstitial F_i⁻ at the nearest site (a) and (c), and the next nearest site (b) and (d), respectively. The insert represents p, d and f states of the corresponding rare-earth ions.

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However, it is a little different between Nd³⁺ and Y³⁺ centers. As can be seen in Figs. 5(c)–(d) and Fig. 10 (Appendix), p state of interstitial F_i⁻ at the nearest site was hybridized with d, f and p states of Nd³⁺, as well as d state of the nearest strontium atom. The electrons of F_i⁻ would be delocalized to levels on Nd³⁺ and the conduction band. As a result the interaction of Nd³⁺ and F_i⁻ was strengthened. At the same time, the lowered charge density made F_i⁻−F_lattice⁻ repulsion force weakened and then the inner strain relieved. Catlow et al. reported that covalent bonding between the interstitials F_i⁻ could possibly be occurred [45]. The projected density of state results suggest that if electron density of F_i⁻ was largely removed, attraction interaction between F_i⁻−F_i⁻ would make the center more stable. It was thus stabilized [1Nd³⁺−1F_i⁻] center with F_i⁻ at the nearest site, as well as the cubic sublattice trimer clusters. The square antiprism sublattice tetramer, pentamer and hexamer centers also existed, and full bonding was observed in Fig. 9(b) (Appendix).

The local environment tailoring originates from the big difference of bonding characters of Nd³⁺−F_i⁻ and Y³⁺−F_i⁻, in other words, by choosing appropriate ions with desired bonding characters, the target of local environment and spectral properties manipulation could be realized.

3.2 Tailored photoluminescence of the crystal

Based on the results, a strategy of local structure manipulation was proposed and illustrated in Fig. 6. The cubic first coordination shell of Nd³⁺ was tailored by the regulators to be lower symmetric square antiprism sublattice while the matrix keeps in fluorite structure. As shown in Fig. 11 (Appendix), the XRD patterns show that all the crystals with high crystallinity remain in fluorite-like structure, and diffraction of (111) as well as the other planes inclined and shift to higher angles, which indicates that the interplanar distance and the size of the cells were shrinked. The process of local structure transformation from cubic to square antiprism cancelled out the expansion induced by the interstitials F_i⁻ in the lattice, which contributed to the contracted cells. The lowered symmetric square antiprism sublattice tailored by Y³⁺ made the electric dipole transitions of f-f be therefore greatly enhanced, and good laser performance was expected. The demands for new ultrashort and tunable laser materials were highly expected to be realized in the trivalent lanthanide-doped fluorite crystals, rather than changing and trying different matrix crystals tediously as it did before, and the assessment about the dopants induced negative influence that usually diminishes the emission quantum yields should also be considered.

 

Fig. 6. Strategy of local structure manipulation. With no loss of crystal structure integrity, the cubic first coordination shell of active Nd³⁺ (blue core shell) would be manipulated to be distorted lower symmetric square antiprism sublattice when more than one Y³⁺ ion (green core shell) was introduced.

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The peak absorption cross sections calculated by absorption coefficient and real concentration of Nd³⁺ in the crystal measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) of Nd³⁺:SrF₂ [46] increased with increasing concentration of Nd³⁺, as well as product of the cross section and FWHM, as shown in Fig. 7(a) and Fig. 12 (Appendix). The big clusters are more thermal-dynamically stable in Fig. 4(a), which means that the clustering process was dopant concentration dependent. With increasing concentration the high symmetric monomers were transformed to be the lower symmetric clusters in Fig. 1, and the cross sections were thus enhanced gradually. With addition of 5% Y³⁺, the cross section, as well as the product was greatly enhanced, and continuously increased with decreasing concentration of Nd³⁺. The enhanced peak absorption cross section and the area of the absorption band were in well consistent with the calculated results that the local environment was tailored to be the lower symmetric square antiprism structure with addition of Y³⁺. As illustrated in Figs. 4(c) and 6, the seriously distorted local structure of Nd³⁺ contributed to the significantly enhanced emission in samples with 5% Y³⁺. It can be seen in Fig. 7(b), the intensity was enhanced by about 20 times. Taking the doping concentration of Nd³⁺ into consideration, the intensity follows nearly the same pattern as that of absorption cross section in Fig. 7(a).

 

Fig. 7. Nominal molar concentration of Nd³⁺ dependent peak absorption cross section (a) and normalized PL intensity at 1057 nm (b) of y at.% Nd³⁺:SrF₂ and y at.% Nd³⁺,5 at.% Y³⁺:SrF₂. (c) Y³⁺ concentration dependent peak absorption cross section of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂. (d) PL spectra of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂, the intensity of sample A was multiplied by 1.5 for clear view. The insert is Y³⁺ concentration dependent normalized emission intensity at 1057 nm. Instrumental: SBW = 8 cm⁻¹. The samples in same dimensions were measured under the same conditions, and the emission intensity could therefore be compared with each other.

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One more group of Nd³⁺,Y³⁺:SrF₂ crystal was prepared to ensure the repeatability. As can be seen in Figs. 7(c)–(d) and Fig. 13 (Appendix), the absorption cross section and emission were also greatly improved with addition of Y³⁺ in 0.5% Nd³⁺:SrF₂. The emission is very weak in Nd³⁺ alone doped crystal. Notably, when incorporated with Y³⁺, the emission peaks at 1036, 1044, 1075 and 1084 nm of Fig. 7(d), associated with long lifetime component in Fig. 14 (Appendix) [47], were gradually eliminated and intensity boosted up at 1057 nm. The long and short components of lifetimes, respectively connected with Nd³⁺ monomers (Fig. 1(a)) and clusters (Figs. 1(b)–(c)) [48], were finally merged to be one value. At the same time, due to the decreased inhomogeneous broadening effect caused by the decreased number of Nd³⁺ sites, the FWHM of emission decreased exponentially from 28 to 20.5 nm, which is competent for sub-100 fs pulses generation [17], as shown in Fig. 15 (Appendix). That agreed qualitatively with the changing of simulated [Nd³⁺−Y³⁺] clusters in Figs. 3 and 4(a). It is noted when the concentration of Y³⁺ is above 5 at.% the values tend to be constant. It is probably because of the saturated emission of Nd³⁺ in square antiprism lattice of Fig. 3(b).

The absolute quantum yields were also measured and presented in Fig. 16 (Appendix). The efficiency increased exponentially with concentration of Y³⁺, a value of 11.3% of 0.5% Nd³⁺:SrF₂ was improved to be 88.2% due to the combination of [Nd³⁺−Y³⁺] clusters in Fig. 3. The quantum yields (η) and fluorescence lifetime (τ_exp) were utilized to calculate the radiative lifetime (τ_rad) based on η= τ_exp/τ_rad. For the curves of C and D crystal respectively with 2 at.% and 4.5 at.% Y³⁺, the deviation from single exponential profile was relatively small, and the mean lifetime was adopted [49]. Thus, the two samples together with E and F were included in the calculation. As shown in Fig. 17 (Appendix), the radiative lifetime decreased exponentially with addition of Y³⁺. As can be seen in Figs. 3(b), 4(c) and 6, the transformation from cubic to distorted square antiprism sublattice of Nd³⁺ made the parity forbiddance be greatly relaxed. The f-f transition rate was therefore increased exponentially. Similarly, the peak emission cross section, evaluated by Füchtbauer-Ladenburg equation [50], was also tunable, as shown in Fig. 18 (Appendix).

3.3 Low temperature TRES analyses

To better understand the clustering process, A, C and F crystal were chosen for TRES measurement in liquid nitrogen, 3-dimensions (3D) image and 2D TRES were presented in Fig. 8 and Fig. 19 (Appendix), respectively. Due to the inhomogeneous broadening caused by the Nd³⁺ clusters covered from monomers (Fig. 1(a)) to clusters (Figs. 1(b)–(c)), the photoluminescence was observed to be very broad in 0.5% Nd³⁺:SrF₂ crystal. With addition of Y³⁺ from 2 at.% to 10 at.% signals of the monomers were gradually disappeared, and square antiprism [Nd³⁺−Y³⁺] clusters finally dominated in the crystal. The emission peaks were manipulated to be nearly identical with each other, which means a multisite crystal was changed to be a quasi single center system by codoping the lattice regulator ions of Y³⁺. Nd³⁺ centers would be recombined with Y³⁺ and transformed to be the third group of lower symmetric square antiprism clusters. The nearly identical first coordination shell of the active Nd³⁺ was then obtained. However, various configurations of the cluster made the splitting levels slightly different and thus the FWHM inhomogeneously broadened. The transitions were severely overlapped, it is difficult to split them even at liquid helium temperature. The aggregating processes show that the transformation was Y³⁺ concentration dependent. Although high order clusters are more stable in thermodynamics, relatively high concentration doping of Y³⁺ or Nd³⁺ should be essential.

 

Fig. 8. Low temperature TRES on clustering process. 3D image of the TRES of ⁴F_3/2 → ⁴I_11/2 transition of A (a), C (b), and F (c) samples. The insert represents the viewing along z-axis.

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4. Discussion

Based on the aforementioned results, the clusters of [Nd³⁺−Y³⁺] are more stable than that of corresponding [Nd³⁺−Nd³⁺] clusters. It was confirmed that the fluorescence quenching could be eliminated with addition of Y³⁺ ions [24–26]. More interestingly, the cubic local structure was surprisingly driven to be the distorted square antiprism structure and the “phase transition” occurred in the sub-nanometer particles (clusters), and a multisite crystal was manipulated to be a quasi-single center system, which suggested that Y³⁺ contributed as local structure regulator ions. The local structure transformation depends on the coupled interactions induced by the delocalization of charge density of interstitials F_i⁻ in the clusters. For different rare-earth ions and host fluorite materials, the F_i⁻−F_lattice⁻, F_i⁻−F_i⁻ interactions and the bonding of F_i⁻ with the dopant cations are different. The varied local environments are thus expected in rare-earth doped fluorites.

Furthermore, the geometry optimization in the work was all simulated in equilibrium state. It suggests, that local environment of the dopant cations would also be influenced by preparation processes and heat treatment routines [51], as well as the dopants concentrations [20–21]. It is unknown but will be very interesting about the local structures and configurations in the nonequilibrium states, such as the preparation temperatures, pressures, electric and magnetic fields. The local structures of the clusters would be evolved dynamically with such force fields. Direct observation and characterization of the clusters is another interesting topic, and the work is on the way. Nevertheless, designing of laser crystal with tunable spectral properties could be realized by tailoring the rare earth ion clusters in fluorites.

5. Conclusion

In summary, an efficient local structure manipulation strategy was proposed by tailoring local structure distortion of the active ions in fluoride crystal. By virtue of strong repulsion of F_i⁻−F_lattice⁻ interaction, the first shell of Nd³⁺ with cubic sublattice was manipulated to be lower symmetric square antiprism structure in [Nd³⁺−Y³⁺] cluster with no loss of crystal structure. The distorted low symmetric quasi single center system contributed to the greatly enhanced emissions. Our results suggest that local structure manipulation approach could be useful in design of new laser materials with controlled optical spectra properties.

Appendix

Characterization

XRD testing

X-ray diffraction (XRD) was measured by Rigaku Ultima-IV diffractometer (Japan). Cu Kα radiations and scan step of 0.02 ° were used in the angle region of 20 ° to 70 ° on 2θ scale, and signals caused by Kα2 radiation were striped from the data.

Absolute quantum yields measurement

The absolute quantum yields were tested with a FLS980 time-resolved fluorimeter grating blazed at 1200 nm and detected by thermoelectric (TE) cooled InGaAs detector. The excitation was at 796 ± 15 nm. A barium sulfate-coated integrating sphere (Edinburgh) was applied as the sample chamber. The entry and output gates of the sphere located in 90 ° geometry from each other in the plane of the fluorimeter. Both reference and sample emissions were collected in the region of 765 to 1450 nm. The yields were thus obtained.

 

Fig. 9. Projected DOS of p state of interstitial F_i⁻ in the square antiprism lattice of (a) Y³⁺ clusters and (b) Nd³⁺ centers.

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Fig. 10. Projected DOS of d state of Sr atom nearest to [1Nd³⁺−1F_i⁻] center with F_i⁻ at the nearest site.

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Fig. 11. XRD patterns of the crystal.

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Fig. 12. Nominal Nd³⁺ concentration dependent product of peak absorption cross section (σ_abs) and FWHM (λ_abs) of ⁴I_9/2 → ⁴F_5/2 + ²H_9/2 transition.

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Fig. 13. Absorption cross section of the crystal, all peaks were attributed to the absorption of Nd³⁺ ions, the strongest band is around 800 nm corresponding to ⁴I_9/2 → ⁴F_5/2 + ²H_9/2.

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Fig. 14. Logarithmic decaying curves of the sample, for A, B, C and D crystals the lifetime consists of a long and short component, while for E and F the decaying could be well fitted by single exponential expression.

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Fig. 15. Nominal Y³⁺ concentration dependent FWHM of ⁴F_3/2 → ⁴I_11/2 transition.

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Fig. 16. Nominal Y³⁺ concentration dependent absolute quantum yields of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂ crystal.

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Fig. 17. Nominal Y³⁺ concentration dependent fluorescence and radiative lifetime of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂.

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Fig. 18. Nominal Y³⁺ concentration dependent peak emission cross section of 0.5 at.% Nd³⁺, x at.% Y³⁺:SrF₂ crystal.

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Fig. 19. 2D-TRES was ten sliced, integrated and normalized spectra of A (a), C (b), and F (c) crystal.

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Funding

National Natural Science Foundation of China (61635012, 61905289); National Key Research and Development Program of China (2016YFB0402101); Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000).

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