Syntheses and luminescence of complete solid solutions Gd_1−xEu_x[B₆O₉(OH)₃] and α-Gd_1−xEu_xB₅O₉

Abstract

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H₂O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1) and their dehydrated products, α-Gd_1−xEu_xB₅O₉. Efficient energy transfer from Gd³⁺ to Eu³⁺ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B₆O₉(OH)₃], the re-crystallization to α-EuB₅O₉, and further decomposition to α-EuB₃O₆. During those processes, the Eu³⁺ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu³⁺. It is observed that the symmetry of the Eu³⁺ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB₅O₉ shows the highest luminescence efficiency when excited by Eu³⁺ CT transition. For the first time, complete solid solutions α-Gd_1−xEu_xB₅O₉ were synthesized directly by the sol–gel method, and their luminescence properties were also studied.

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Introduction

Rare earth borates are good hosts for luminescent materials due to their high transparency, good thermal stability, exceptional optical damage threshold and high luminescence efficiency.^1–3 During the past decades, numerous hydrated rare earth borates were prepared by the boric acid flux and hydrothermal methods; their luminescence properties, however, were usually overlooked.^4–12 A possible reason is that the vibrations of OH⁻ or H₂O may lead to emission quenching through non-radioactive pathways. On the contrary, the rare earth borates prepared under mild conditions usually have complex and large polyborate anions,^6–12 which can sufficiently separate the rare earth cations to prevent the concentration quench effect.^6,7,12 As a typical example, hydrothermally prepared Gd₂B₆O₁₀(OH)₄·H₂O:Eu³⁺ exhibits very highly efficient Eu³⁺ f–f transitions and obvious energy transfer from Gd³⁺ to Eu³⁺ ions, which indicates that this series of borates may have applications in VUV- and UV-LED phosphors.¹² There is no clear evidence indicating any effective energy transfer from Eu³⁺ to H₂O molecules or hydroxyl groups.¹²

In addition, the calcination of hydrated polyborates at appropriate temperatures can give rise to new anhydrous borates.^6,7 For example, α-LnB₅O₉ (Ln = Pr-Eu) was first prepared by a long-time calcination of H₃LnB₆O₁₂ at 650–700 °C for 5 days.⁶ A preliminary study of the luminescence properties of a partially solid solution of α-Gd_1−xEu_xB₅O₉ (x ≤ 0.16) was reported by Lin et al.,⁶ which focused on the luminescence under VUV excitation (125 nm). The appearance of strong Gd³⁺ absorptions implied effective energy transfer from Gd³⁺ to Eu³⁺ in this series of phosphors. However, the luminescence properties of α-GdB₅O₉:Eu³⁺ under UV excitation have not been reported.

In this work, complete solid solutions of α-Gd_1−xEu_xB₅O₉ have been prepared, not only by thermal decomposition from Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1), but also directly by the sol–gel method, using Ln₂O₃ and H₃BO₃ as the starting materials. The UV-excited luminescence properties of Gd_1−xEu_x[B₆O₉(OH)₃] and α-Gd_1−xEu_xB₅O₉ (prepared by both methods) were systematically investigated. We also studied the structural evolution by the luminescence characteristics of Eu³⁺ during the dehydration of Eu[B₆O₉(OH)₃] and the re-crystallization process to α-EuB₅O₉. There exists an intermediate amorphous state, the structure of which can not be studied by regular powder X-ray diffraction (XRD). Meanwhile, by analysing the Eu³⁺ luminescence, we noticed that the symmetry of the Eu³⁺ coordination environment is the lowest in the amorphous state.

Experimental section

Syntheses

The syntheses of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1) were carried out in closed Teflon autoclaves using the boric acid flux method. Typically, 0.22 mmol of Gd₂O₃ and 20 mmol of H₃BO₃ (Gd/B = 1 : 45) were first mixed and placed into a 25 mL Teflon autoclave. Then 1.0 mL of deionized water was added. The autoclave was sealed and heated at 240 °C in an oven for 7 days. The products were washed with water (25 °C) until the excess boric acid was completely removed, and then dried at 80 °C for further characterization. By the same procedure, a series of Eu-doped samples, Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1), were prepared. Block single crystals were obtained, and there was a gradual color change from white (for Gd[B₆O₉(OH)₃]) to light pink (for Eu[B₆O₉(OH)₃]). Eu[B₆O₉(OH)₃] is isostructural with Gd[B₆O₉(OH)₃],⁶ so complete solid solutions of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1) can be easily obtained by the boric acid flux method. The substitution contents of the final products may not be exactly the same as the starting ratios, so elemental analyses were performed on the as-synthesized products, and the real doping contents of Eu³⁺ (the values of x) were determined accordingly. The x values obtained from the inductively coupled plasma-atomic emission spectrometry (ICP-AES) method on a Leeman Profile-Spec were close to the starting ratios of Eu/(Gd + Eu), and therefore were used in the following sections.

α-Gd_1−xEu_xB₅O₉ (x = 0–1) were prepared by heating the as-synthesized Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1) compounds at 670 °C for 10 hours. The powder products of α-Gd_1−xEu_xB₅O₉ (x = 0–1) also showed a gradient color change from white to light pink.

The sol–gel method was employed to synthesize α-Gd_1−xEu_xB₅O₉ (x = 0–1) directly. Stoichiometric Ln₂O₃ (Ln = Gd/Eu) was first dissolved in warm concentrated nitric acid. Then, H₃BO₃ (with a 10 mol% excess, in order to compensate for the volatilization of boron oxide at high temperature), an appropriate amount of citric acid, and deionized water were charged into the solution. After stirring and heating, a clear and homogeneous sol was formed, which was further loaded into an oven and kept at 80 °C, 120 °C and 180 °C for 5 hours, respectively. A light brown precursor was obtained, indicating the existence of carbon. The precursor was then heated in a muffle furnace at 250 °C and 550 °C for 15 hours respectively to decompose the nitrates, and it changed to a dark brown amorphous powder. Finally, the target material α-Gd_1−xEu_xB₅O₉ was obtained by grinding and re-calcination at 670 °C for 10 hours. The colour of the obtained sample turned to white. Moreover, the product was washed by deionized water in order to remove the possible excess of B₂O₃. All of the abovementioned samples were checked to be pure by powder XRD and ready for further characterization.

Characterization

Powder XRD data were collected at room temperature on a PANalytical Empyrean instrument equipped with a PIXcel 3D detector (Cu Kα radiation). The operation voltage and current were 40 kV and 40 mA, respectively. Le Bail refinements were performed to obtain the cell parameters using the TOPAS software package.¹³ Combined thermogravimetric analyses (TG) and differential scanning calorimeter (DSC) analyses for Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0, 0.31, 0.72, 1.00) were performed on a Mettler-Toledo TGA/DSC1 instrument under a N₂ flow. Photoluminescence spectra were measured on a Hitachi F4600 fluorescence spectrometer. The voltage of the Xe lamp was fixed at 700 V. The emission intensities were calculated from the integrals of the corresponding peaks.

Results and discussion

Synthesis of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1)

For either Gd³⁺ or Eu³⁺, three structures, Ln[B₆O₉(OH)₃], Ln[B₉O₁₃(OH)₄]·H₂O and Ln₂B₆O₁₀(OH)₄·H₂O, can be synthesized by using the boric acid flux or hydrothermal methods.^6,7,12 Differences in the synthesis conditions should be clarified first. In the literature,^6,7 the only difference for the formation of Ln[B₆O₉(OH)₃] and Ln[B₉O₁₃(OH)₄]·H₂O is the Ln/B ratio of the starting materials, which is Ln/B = 1 : 30 and 1 : 15, respectively. In order to improve the crystallinity of the sample in our work, a small amount of water was added; therefore the resultant Ln/B ratios were set to 1 : 45 and 1 : 30 with additional 1 mL and 0.25 mL quantities of water, respectively. The final products of Ln[B₆O₉(OH)₃] are indeed well-qualified single crystals. More boric acid or water was used for the synthesis of Ln[B₆O₉(OH)₃] than Ln[B₉O₁₃(OH)₄]·H₂O. The possible reason is that the formation of Ln[B₆O₉(OH)₃] may require a higher inner pressure, which can be achieved by adding more boric acid or water in a closed system. Ln₂B₆O₁₀(OH)₄·H₂O was synthesized under hydrothermal conditions, which means a larger amount of water was necessary.¹²

XRD study for Gd_1−xEu_x[B₆O₉(OH)₃] and α-Gd_1−xEu_xB₅O₉

10 single-crystal samples of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1) were finely ground and characterized by powder XRD without any observable impurity peaks (see Fig. 1a and Fig. S1, ESI†), indicating the Eu³⁺ is successfully doped into the structure. The XRD patterns do not show any obvious peak shifting with the increase of x, which is understandable due to the similar cationic radii for Gd³⁺ and Eu³⁺. The difference in the cell lattice parameters, including a, c and V (summarized in Table 1), can be only verified by Le Bail refinement of the whole XRD patterns. The resultant cell volumes expand slightly and linearly (see Fig. 1b).

Fig. 1 (a) X-ray diffraction patterns of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1). (b) Lattice parameters from the Le Bail fitting.

Table 1 Lattice parameters of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1), obtained by Le Bail refinements in the space group R3c. x is determined by ICP-AES

x	a (Å)	c (Å)	V (Å^3)
0	8.409	20.749	1270.6
0.10	8.411	20.753	1271.3
0.18	8.412	20.755	1271.9
0.31	8.413	20.759	1272.5
0.39	8.414	20.763	1273.1
0.50	8.415	20.761	1273.1
0.60	8.417	20.770	1274.3
0.72	8.419	20.773	1275.2
0.90	8.421	20.778	1275.9
1	8.421	20.781	1276.2

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The powder XRD patterns of the anhydrous pentaborates α-Gd_1−xEu_xB₅O₉, prepared by both the thermal decomposition and sol–gel methods, are shown in Fig. 2. The unit cell parameters of these compounds are also obtained by Le Bail fitting using TOPAS,¹³ and the refined results are listed in Table 2. Fig. 3 shows the unit cell volumes against the substitution level (x) for α-Gd_1−xEu_xB₅O₉. There is a clear expanding tendency with the increasing substitution. Moreover, the cell parameters of the samples prepared by the different processes are generally consistent with each other.

Fig. 2 X-ray diffraction patterns of α-Gd_1−xEu_xB₅O₉ synthesized by the (a) thermal decomposition and (b) sol–gel methods.

Table 2 (a) Unit cell parameters of α-Gd_1−xEu_xB₅O₉ obtained by thermal decomposition, in the space group I4₁/acd. The Sm/Eu ratios did not change during the thermal decomposition, therefore the x values are the same as for Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1). (b) Unit cell parameters of α-Gd_1−xEu_xB₅O₉ obtained by the sol–gel method, in the space group I4₁/acd

x	a (Å)	c (Å)	V (Å^3)
(a)
0	8.238	33.645	2283.1
0.10	8.239	33.652	2284.1
0.18	8.241	33.658	2285.7
0.31	8.244	33.651	2287.0
0.39	8.247	33.654	2288.9
0.50	8.248	33.658	2289.9
0.60	8.248	33.663	2290.1
0.72	8.253	33.666	2293.2
0.90	8.256	33.672	2295.1
1	8.259	33.675	2297.0
(b)
0.0	8.235	33.657	2282.2
0.10	8.239	33.662	2285.0
0.20	8.241	33.666	2286.6
0.30	8.243	33.668	2287.7
0.40	8.245	33.675	2289.2
0.50	8.246	33.670	2289.2
0.60	8.248	33.669	2290.5
0.70	8.250	33.673	2291.6
0.80	8.253	33.678	2294.0
0.90	8.255	33.683	2295.3
1	8.256	33.678	2295.6

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Fig. 3 Changes in the unit cell volume for α-Gd_1−xEu_xB₅O₉ along with the doping content (x).

Thermal behaviour of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0, 0.31, 0.72, 1)

TG-DSC analyses for Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0, 0.31, 0.72, 1) were performed (see Fig. 4a). The thermal behavior of the as-synthesized Gd_1−xEu_x[B₆O₉(OH)₃] are quite similar, all of which show one sharp weight loss of dehydration from about 500 to 700 °C. There is a small weight loss (<1 wt%) from room temperature to 500 °C, which may relate to the loss of absorbed water molecules on the sample surface. From the DSC curve (Fig. 4b), this large weight loss actually comprises two endothermic processes, corresponding to two peaks at ∼578 and 630 °C for the Eu compound (see Fig. 4b). The total observed weight losses from Gd_1−xEu_x[B₆O₉(OH)₃] to α-Gd_1−xEu_xB₅O₉ are 6.5–6.7 wt%, which agree well with the calculated value of ∼6.6 wt%. Moreover, a small and broad exothermic band and a sharp exothermic peak are observed at about ∼733 °C and ∼797 °C, respectively. To better understand these endothermic and exothermic processes, the products obtained by heating the as-synthesized Eu[B₆O₉(OH)₃] at different temperatures in a muffle furnace (10 hours for each step) were characterized by powder XRD (see Fig. 5).

Fig. 4 TG-DSC curves of the as-synthesized Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0, 0.31, 0.72, 1).

Fig. 5 X-ray diffraction patterns of Eu[B₆O₉(OH)₃] and its calcined products at different temperatures in the furnace.

The structure of Eu[B₆O₉(OH)₃] is retained until 500 °C. At 600 °C, it is completely converted to the amorphous phase by the removal of the hydroxyl groups. Therefore, the two endothermic peaks at ∼578 and 630 °C correspond to two continuous dehydration processes. It should be noted that the structure of the parent compound apparently possesses very strong hydrogen bonds, which can only be dehydrated at a very high temperature (>550 °C). At a temperature of about 670 °C, the amorphous phase undergoes a re-crystallization to α-EuB₅O₉, which corresponds to the first broad exothermic band at about 733 °C on the DSC curve. The temperature difference between the DSC and heating experiments can not be neglected. We speculate that the difference comes from the different heating periods.

In the literature, the pentaborate α-LnB₅O₉ (Ln = Sm-Er) was first obtained by a long annealing period (650–700 °C, 5 days) using H₃LnB₆O₁₂ as the precursor.⁶ In our case, the crystallization process of α-Gd_1−xEu_xB₅O₉ using Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0–1) as the precursors was completed within 10 hours. Additionally, the α-Gd_1−xEu_xB₅O₉ samples remain stable below ∼780 °C and then decompose into amorphous B₂O₃ and α-LnB₃O₆, which relates to the sharp exothermic peak at 797 °C.

Luminescence properties of hydrated Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1)

The excitation spectra of Gd_1−xEu_x[B₆O₉(OH)₃] (Fig. 6a) were obtained with the strongest emission at 596 nm (both of the input and output slits were set to be 2.5 nm). The broad absorption band at 220–250 nm is typical for charge transfer (CT) of O²⁻ → Eu³⁺.¹² In most Eu³⁺-doped phosphors, the CT absorption is much stronger than the f–f transitions, as the latter are parity-forbidden.^14,15 However, this is the opposite in Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1). It is known that the position of the CT band usually shifts along with the change of the local structure of Eu³⁺, which tends to lower the wavelength with the increasing coordination number.^16,17 The positions of the CT band in the excitation spectra of the Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1) samples are all located at relatively low wavelengths (the peak wavelength is around 225 nm, while the common CT band for other Eu³⁺-phosphors usually appears at ∼250 nm^18,19), which is consistent with the short Gd–O distances (2.39–2.47 Å) and the high coordination number (9-coordinated).⁶ Moreover, the exact peak positions of the CT band show a slight red shift with the increase in Eu³⁺ content (as shown in Fig. 6a), indicating that the average Ln–O distance is slightly elongated.

Fig. 6 (a) Excitation and (b) emission spectra of the Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1) samples. (c) Dependence of the emission intensity, and (d) R/O values excited at 396 nm with the doping concentration x.

The sharp peaks at about 273 and 311 nm are from the ⁸S_7/2 → ⁶I_J and ⁸S_7/2 → ⁶P_7/2 transitions of Gd³⁺, respectively.^20,21 As a consequence, the appearance of the Gd³⁺ absorption indicates an energy transfer from Gd³⁺ to Eu³⁺.²¹ With an increase of the Eu³⁺ concentration, the intensities of the f–f excitation peaks of Eu³⁺ increase accordingly. Meanwhile, the absorptions of Gd³⁺ become weaker, and the intensity of the CT band does not change significantly. Gd³⁺ usually exhibits strong absorption in the VUV region,^6,22 so it is expected that Gd_1−xEu_x[B₆O₉(OH)₃] samples are potential VUV-excited red phosphors (not reported in the present work).

The emission spectra of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1) are typical for Eu³⁺ (as shown in Fig. 6b). The emission spectra of the whole series of compounds with different Eu³⁺ concentrations generally show similar characteristic. The five emissions, at 582, 585–605, 610–640, 650–680 and 680–720 nm, are attributed to the ⁵D₀ → ⁷F_J (J = 0–4) transitions.²³ The ⁵D₀ → ⁷F₀ transition (at 582 nm) is exhibited as a single peak, because the Eu³⁺ ions are only present in one crystallographic site in the structure.⁶⁵D₀ → ⁷F₁ (magnetic dipole–dipole transition) is stronger than ⁵D₀ → ⁷F₂ (electric dipole–dipole transition). The intensity ratio of R/O = I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) is a probe of the site symmetry of Eu³⁺. Generally, a lower symmetry of the crystal field around Eu³⁺ leads to a larger R/O.²³ This result agrees with the coordination environment of Gd³⁺, which is close to centrosymmetric (as shown in Fig. S2, ESI,† all of the Gd–O distances are almost the same).⁶

When excited at 396 nm (f–f transition of Eu³⁺), the intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions and the total emissions increase with x, and no quenching or saturation effects are observed (Fig. 6c). This is reasonable because the Eu³⁺ ions are well separated and therefore diluted by borate groups (the nearest Eu–Eu distance is 6.0 Å),⁶ which hampers cross-relaxation or energy transfer between excited and non-excited Eu³⁺ ions. The values of R/O with different doping contents (x) are shown in Fig. 6d, which shows a monotonously increasing tendency from 0.27 to 0.34. Since the ionic radii of Gd³⁺ (1.11 Å, 9-coordinated) and Eu³⁺ (1.12 Å, 9-coordinated) are very similar, the two end members, Eu[B₆O₉(OH)₃] and Gd[B₆O₉(OH)₃], have almost the same local coordination environment for Eu³⁺ or Gd³⁺, and as a consequence, R/O shows only a small change in the Gd_1−xEu_x[B₆O₉(OH)₃] series.

Luminescence properties of anhydrous α-Gd_1−xEu_xB₅O₉ (x = 0.10–1)

We first discuss the luminescence properties of α-Gd_1−xEu_xB₅O₉ (x = 0.10–1) prepared by thermal decomposition. The input and output slits were selected to be 1.0 nm. As shown in Fig. 7a and b, the CT absorption is predominant. The appearance of Gd³⁺ absorptions at 273, 305 and 311 nm indicates that there is also energy transfer in α-Gd_1−xEu_xB₅O₉. When x = 0.60, the Gd³⁺ absorptions start to diminish. As shown in Fig. 7c, the ⁵D₀–⁷F₀ (579 nm) transition is a single peak, the ⁵D₀–⁷F₁ transitions (582–605 nm) are three separate sharp peaks, the ⁵D₀–⁷F₂ transitions (610–630 nm) also contain three peaks, with the total intensity comparable with that of the ⁵D₀–⁷F₁ transitions, and the ⁵D₀–⁷F₄ transitions (685–710 nm) contain four main peaks.

Fig. 7 Excitation spectra of α-Gd_1−xEu_xB₅O₉, obtained by the thermal decomposition method, by monitoring at (a) 586 nm and (b) 620 nm. Emission spectra under (c) 254 nm and (e) 392 nm excitations. The insets are the calculated emission intensities. The R/O ratios are shown in (d) and (f).

The influence of the Eu³⁺ concentration on the integrated intensity of the emissions of ⁵D₀–⁷F₁, ⁵D₀–⁷F₂ excited by 254 nm (CT of O²⁻ → Eu³⁺) is shown in the inset of Fig. 7c. It is obvious that the emission first increases slowly in the range of x = 0.10–0.50, and then decreases in the subsequent range of x = 0.50–1. Usually, the saturation or quenching effect by increasing the Eu³⁺ concentration is mainly due to the interaction of the activated centers by the cross-relaxation or energy transfer between excited and unexcited Eu³⁺ ions. In α-Ln_1−xEu_xB₅O₉, the concentration quenching effect does not appear to be strong, because the Eu³⁺ ions are efficiently separated by large borate groups. We speculate that the decrease in the emission is due to the decreasing energy transfer from Gd³⁺ and Eu³⁺ when x increases. The values of R/O for α-Gd_1−xEu_xB₅O₉ under 254 nm excitation (see Fig. 7d) show a monotonously increasing tendency from 1.11 to 1.18.

When excited at 393 nm, the emission patterns are generally the same, as shown in Fig. 7e. The emission intensities of ⁵D₀–⁷F₁ and ⁵D₀–⁷F₂ reach maximum values, which are approximately maintained in the range of x ≥ 0.72 (inset of Fig. 7e). The calculated R/O values show a monotonously increasing tendency with x. Our study is a good comparison to the previous research of α-Gd_1−xEu_xB₅O₉ under VUV excitation, which showed a saturation at x = 0.10.⁶

Different synthesis methods have an impact on the luminescence intensities due to the differences in crystallinity.^24–26 The excitation and emission spectra of α-Gd_1−xEu_xB₅O₉ (x = 0.10–1) prepared by the sol–gel method are shown in Fig. 8. The influence of the Eu³⁺ concentration on the integrated intensities of the emissions of the ⁵D₀–⁷F₁ and ⁵D₀–⁷F₂ transitions, excited at 254 nm and 392 nm, are shown in the insets of Fig. 8c and d, respectively. The CT band is much stronger and the maximum emission occurs at x = 0.40 when excited by 254 nm; meanwhile, no saturation was observed under 392 nm excitation. The emission intensities under 254 nm excitation for the samples synthesized by the sol–gel method are stronger than those prepared by thermal decomposition.

Fig. 8 Excitation spectra of α-Gd_1−xEu_xB₅O₉ (x = 0.10–1), obtained by the sol–gel method, by monitoring at (a) 586 nm and (b) 620 nm. Emission spectra under (c) 254 nm or (d) 392 nm excitations. The insets are the calculated emission intensities.

Structural evolution detected by the luminescence of Eu³⁺

Hydrous borates usually show rich phase transitions, including dehydration and amorphization, re-crystallization and further decomposition, as in the case of Ln₂B₆O₁₀(OH)₄·H₂O.¹² During the phase transitions, the luminescence of Eu³⁺ is a good medium to gain more structural information about the coordination environment of rare-earth cations, even for the intermediate amorphous phase, which can not be studied by regular XRD. Careful investigation of the thermal behavior of Eu[B₆O₉(OH)₃] by TG-DSC and powder XRD experiments suggest that there is a dehydration/amorphization step which starts at ∼500 °C, and the re-crystallization of α-EuB₅O₉ at 670 °C in the furnace, which further decomposes into α-EuB₃O₆ at 820 °C. Therefore, it is of interest to study the luminescence properties of Eu[B₆O₉(OH)₃] and its annealed products during these phase transitions (see Fig. 9). The similar excitation and emission spectra of Eu[B₆O₉(OH)₃] (calcined at 300 °C) and the as-synthesized Eu[B₆O₉(OH)₃] (25 °C) indicate that the coordination environment of Eu³⁺ remains unchanged. The R/O value also does not change, as shown in Fig. 9e and f, confirming that the local structure around Eu³⁺ is maintained.

Fig. 9 For the as-synthesized Eu[B₆O₉(OH)₃] and its annealed products at different temperatures: (a) excitation spectra (λ_em ∼ 590 nm); (b) excitation spectra (λ_em ∼ 620 nm); (c) emission spectra (λ_ex ∼ 254 nm); (d) emission spectra (λ_ex ∼ 396 nm); (e) and (f) are the corresponding R/O values deduced from (c) and (d).

At 500 °C, the stepwise dehydration of Eu[B₆O₉(OH)₃] starts. As shown in Fig. 9a and b, the excitation spectra monitoring the strongest emissions of the ⁵D₀–⁷F₁ and ⁵D₀–⁷F₂ transitions are slightly different. When monitoring at 596 nm, the excitation spectrum is similar to that of the sample calcined at 300 °C, however, if monitoring at 621 nm, the intensity of the CT band is significantly enhanced by the calcination at 500 °C. As a result, the emission spectra of the sample calcined at 500 °C, which excited by CT and at 393 nm are therefore different, as shown in Fig. 9c and d. When excited by a CT transition, the emission of the ⁵D₀–⁷F₂ transition becomes stronger than that of the ⁵D₀–⁷F₁ transition, and moreover, the R/O value increases at the same time. If excited at 396 nm, the emission spectrum remains unchanged, and the R/O value is also maintained. According to its interesting and mixed luminescence behavior, we speculate that the sample calcined at 500 °C partially decomposes and contains a substantial amorphous component, which can not be clearly detected by XRD.

At 600 °C, the crystal structure collapses completely and an amorphous phase is obtained. Interestingly, the emission intensity was enhanced after amorphization when excited by CT or at 393 nm (Fig. 9c and d), which is also confirmed by the increased intensities of both CT and Eu³⁺ f–f transitions in the excitation spectrum after amorphization. Moreover, the emission spectrum shows an intense red emission at 616 nm and the R/O reaches a maximum value, indicating the lowest symmetry of the local structure around the Eu³⁺ ions. The high efficiency of the f–f transitions and the large R/O value (3.7 under 254 nm and 4.0 under 393 nm) implies that this amorphous phase may be a good red-emitting UV-LED phosphor.

At 670 °C, the amorphous phase undergoes re-crystallization to anhydrous α-EuB₅O₉. The inset of Fig. 9c shows the CT-excited emission spectra of the amorphous phase, α-EuB₅O₉ and α-EuB₃O₆. A significant increase in the emission intensity and a red-shift of the strongest emission peak are found during the re-crystallization. However, when excited by the Eu³⁺ f–f transition, the luminescence intensity of α-EuB₅O₉ is close to the amorphous phase. These results are consistent with the significantly increased intensity of the CT absorption and the almost unchanged Eu³⁺ f–f transitions in the excitation spectrum. The R/O value decreases after re-crystallization. In addition, α-EuB₅O₉ has a low temperature stable phase, which begins to decompose at about 780 °C and completely decomposes to α-EuB₃O₆ at 820 °C. Thus, we also collected the emission spectrum for α-EuB₃O₆ calcined at 820 °C, and found that the luminescence intensity decreases (Fig. 9c and d). Moreover, the R/O values are even smaller than that of α-EuB₅O₉. All of these results confirm the abundant structural evolutions during the thermal annealing.

Conclusions

In conclusion, complete solid solutions of Gd_1−xEu_x[B₆O₉(OH)₃] were prepared by the boric acid flux method. Careful investigations of the thermal behavior of Eu[B₆O₉(OH)₃] by TG-DSC and powder XRD experiments reveal that there is a dehydration/amorphization step which starts at ∼500 °C, re-crystallization of α-EuB₅O₉ at 670 °C in the furnace, which further decomposes into α-EuB₃O₆ at 820 °C. Accordingly, complete solid solutions of anhydrous α-Gd_1−xEu_xB₅O₉ were prepared by thermal decomposition. In addition, the sol–gel method was also applied to obtain pure powder samples of α-Gd_1−xEu_xB₅O₉ (x = 0–1). The sol–gel method is favored for enhancing the luminescence intensity. Efficient energy transfer from Gd³⁺ to Eu³⁺ was observed for all of the compounds, as expected. Interestingly, by studying the luminescence of Eu³⁺, we found that the coordination symmetry of Eu³⁺ is the lowest in the amorphous state during the annealing treatments of Gd_1−xEu_x[B₆O₉(OH)₃], as indicated by the highest R/O value of 4.0. The high efficiency of the f–f excitations and the large R/O value imply that this amorphous phase may be a good red-emitting UV-LED phosphor. Our study of the luminescence properties of the hydrated borates and dehydrated products complements the traditional research of anhydrous borates. In most cases, the hydrate borates have large borate groups, which separate the luminescent activator very well and therefore avoid the concentration quenching. Many hydrated polyborates with crystalline water molecules remain unexplored, and our study shows their potential as good phosphors.

Acknowledgements

This work was supported by the Nature Science Foundation of China (NSFC 21101175, 21171178, 91222106). Financial support from the Natural Science Foundation Project of Chongqing (CSTC 2012jjA0438) is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Le Bail fitting of the powder XRD patterns for Gd[B₆O₉(OH)₃] and α-GdB₅O₉, crystal structure view of Gd[B₆O₉(OH)₃]. See DOI: 10.1039/c3nj00925d

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