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金滕滕, 张志军, 张辉, 赵景泰. v-PrBO3的晶体结构、相变和光学性质 . 无机材料学报, 2013, 28(10): 1153-1157
JIN Teng-Teng, ZHANG Zhi-Jun, ZHANG Hui, ZHAO Jing-Tai. Crystal Structure, Phase Transition and Optical Properties of ν-PrBO3 . Journal of Inorganic Materials, 2013, 28(10): 1153-1157  
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v-PrBO3的晶体结构、相变和光学性质
金滕滕1,2, 张志军1, 张辉1, 赵景泰1
1.中国科学院 上海硅酸盐研究所,中国科学院透明光功能无机材料重点实验室,上海200050
2.中国科学院大学,北京100039
摘要

采用高温固相合成法分别在1200和1500℃合成了λ-PrBO3和ν-PrBO3样品, 并通过单晶X射线衍射确定了ν-PrBO3的晶体结构。结果表明该结构为三斜晶系, 空间群为P]]>a = 0.6302 (4) nm,b = 0.6521 (4) nm,c = 0.6525 (4) nm,α = 94.312(7) º,β = 107.335(7) º,γ = 106.455(7) º,V = 0.2417(2) nm3,Z = 4。同时研究了从λ-PrBO3到ν-PrBO3的相变过程, 并从漫反射光谱得出λ-PrBO3和ν-PrBO3的光学带隙均为4.96 eV。在X射线和紫外激发下,均未观测到λ-PrBO3和ν-PrBO3样品在紫外可见波长范围内的Pr3+特征发光。

关键词: ν-PrBO3晶体结构; 相变; 光学性质
中图分类号:O78   文献标志码:A    文章编号:1000-324X(2013)10-1153-05
Crystal Structure, Phase Transition and Optical Properties of ν-PrBO3
JIN Teng-Teng1,2, ZHANG Zhi-Jun1, ZHANG Hui1, ZHAO Jing-Tai1
1. Key Laboratory of Transparent Opto-Functional Inorganic Materials of Chinese Academy of Sciences, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
2. University of Chinese Academy of Sciences, Beijing 100039, China
Corresponding author: ZHAO Jing-Tai, professor. E-mail:jtzhao@mail.sic.ac.cn
Fund:Foundation item: National Natural Science Foundation of China(11104298)
Abstract

The praseodymium orthoborates λ-PrBO3 and ν-PrBO3 were synthesized from Pr6O11 and H3BO3 by solid state reaction method at 1200 and 1500℃, respectively. The crystal structure of ν-PrBO3 was refined on the basis of single-crystal X-ray diffraction data. It crystallizes in the triclinic system belonging to space group P]]>a = 0.6302 (4) nm,b = 0.6521 (4) nm andc = 0.6525 (4) nm, andα = 94.312(7)º,β = 107.335(7)º,γ = 106.455(7)º,V = 0.2417(2) nm3,Z = 4. The phase transition process from λ-PrBO3 to ν-PrBO3 was also investigated. The optical band gaps of λ-PrBO3 and ν-PrBO3 are both determined to be about 4.96 eV according to the diffuse reflection spectra. There is no emission of Pr3+ in the visible range for both of λ-PrBO3 and ν-PrBO3 under X-ray and UV excitation.

Keyword: ν-PrBO3 crystal structure; phase transition; optical properties

Over the last several decades, the rare-earth orthoborates REBO3 have attracted a lot of attention owing to their extraordinary optical properties such as vacuum ultraviolet transparency and exceptional optical damage threshold[ 1, 2]. It is well known that REBO3 exhibits the related structure types as the three forms of CaCO3, i.e., aragonite, vaterite, and calcite[ 3]. RE3+ adopt different coordination features depending on the size of the rare earth cations[ 4], but not with the above three forms at the same time. Most types of REBO3 are designated with Greek letters in accordance with the nomenclature of Meyer[ 5, 6]. In general, compounds containing larger ions (La-Nd, Sm, Eu), exhibit the aragonite-type structure (λ-REBO3) at low temperature[ 3, 5], then transform to H-REBO3 (RE=La, Ce, Nd)[ 7] and ν-REBO3 (RE=Ce-Nd, Sm-Dy)[ 4, 5, 8] at high temperature. However, compounds containing smaller ions exist as a low-temperature modification π-REBO3 (RE =Y, Nd, Sm-Lu)[ 1, 3, 9, 10] and a high-temperature modification µ-REBO3 (RE =Y, Sm-Gd, Dy-Lu)[ 1, 10]. Although the high-temperature phases are different, they all exhibit low symmetry. For the smallest ion, ScBO3 forms the calcite-type structure (β-REBO3)[ 3, 11]. There are some other types of REBO3 (RE =Er-Lu) which also adopt the same structure at low temperature. Additionally, the orthoborate phases χ-REBO3 (RE = Dy and Er), which contain the new non-cyclic [B3O9]9- anion, were synthesized under high- pressure condition by Huppertz, et al[ 12], considered to be intermediates between the low-temperature ( π) and high-temperature ( µ) polymorphs.

Due to the possible usage for fast scintillators, the Pr3+-doped scintillators have attracted the attention of researchers recently, such as Pr3+-doped single crystal hosts (Y3Al5O12, YAlO3 and Y2SiO5)[ 13, 14]. This is because that the Pr3+ ion can show even faster 5d-4f luminescence shifted by about 1.5 eV towards higher energies with respect to 5d-4f emission of Ce3+[ 15].

In 1961, Levin, et al[ 3] reported the phase transitions of REBO3 (RE=La, Sm, Nd), which occured at 1488, 1090 and 1285℃, respectively. The high temperature forms NdBO3 and SmBO3 are similar but different from LaBO3. For the case of PrBO3, there are few reports on phase transition process. According to previous studies[ 2, 6, 16, 17], PrBO3 mainly has two phases: the aragonite phase λ-PrBO3 (low temperature, LT) and the triclinic phase ν-PrBO3 (high temperature, HT). The two phases exhibit different structures and densities, and the crystal structure determination of λ-PrBO3 from single crystals obtained through high-pressure and high-temperature synthesis method was reported in 2010[ 16]. However, the detailed crystal structure of ν-PrBO3is still unknown besides the cell parameters of ν-PrBO3 reported by Meyer[ 6]. What’s more, there is no report about the phase transition and optical properties of PrBO3. In the present work, we focus on the investigation of the crystal structure of ν-PrBO3, phase transformation from λ-PrBO3 (LT) to ν-PrBO3 (HT), as well as its optical properties.

1 Experiment

The λ-PrBO3 (LT) and ν-PrBO3 (HT) samples were both prepared by solid-state reaction in the muffle furnace. The starting materials were Pr6O11 (99.99%) and H3BO3 (99.99%). A stoichiometric mixture of the starting materials (with 10mol% excess of boric acid to compensate for its evaporation loss during heating) was ground in agate mortar. Then the mixed powders were loaded into platinum crucibles, and calcined in the furnace within air atmosphere at different temperatures. The λ-PrBO3 (LT) sample was obtained at 1200℃ for 12 h, and the ν-PrBO3 (HT) sample was obtained at 1500℃ for 5 h. Both powder samples were green. But there are some single crystals in powder ν-PrBO3 (HT) sample.

The powder X-ray diffraction data for λ-PrBO3 (LT) were collected at ambient temperature in air with a HUBER Imaging Plate Guinier camera G670 [S] (CuKα1 radiation, λ= 0.154056 nm, Ge monochromator). The 2 θ range of all the data sets is from 5° to 100° with a step of 0.005°.

Single crystal X-ray diffraction data for ν-PrBO3 (HT) were collected with CCD area detector (Mo Kα radiation, λ= 0.071073 nm) at 293(2) K in the range of 3.31° < θ < 24.98° using ω scan. Absorption correction was performed with a multi-scan procedure. The crystal structure refinement was performed by a full-matrix least-squares procedure within the SHELXS97 (Sheldrick, 1990)[ 18]. For crystal structure presentation the program Diamond 3.0 was used[ 19].

The Fourier transform infrared (FT-IR) spectra of ν-PrBO3 were recorded at room temperature in the range of 400-1600 cm-1 with a Spectrum 100 Optical (Perkin- Elmer).

The diffuse reflectance spectra of the samples were measured at room temperature by a Perkin Elmer LS 50B spectrophotometer equipped with a Xe flash lamp. The reflection spectra were calibrated with the reflection of black felt (reflection 3%) and white barium sulfate (BaSO4, reflection~100%) in the wavelength region of 200-900 nm.

2 Results and Discussion
2.1 Phase transformation and the structure of the ν-PrBO3

Crystal data for ν-PrBO3: Triclinic, space group P (no. 2), a = 0.6302 (4) nm, b = 0.6521 (4) nm and c = 0.6525 (4) nm, and α = 94.312(7)º, β = 107.335(7)º, γ = 106.455(7)º, V = 0.2417(2) nm3, Z = 4, Mr = 199.72, Dcalcd = 5.489 g/cm3. A total of 1137 data were collected and merged to give 783 unique reflections ( Rint = 0.041) of which 624 were considered to be observed [ F2 > 2 σ( F2)]. Final R1 = 0.046, wR2 = 0.121, S (goodness of fit) = 0.98 were obtained for all the data. It confirms that the lattice parameters decrease regularly from Pr, Sm to Dy, along with the size of the cations[ 4, 6]. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (×10-2nm2) of ν-PrBO3 are shown in Table 1. For the light atoms of O and B isotropic displacement parameters are used. Important distances (nm) and angles (°) of ν-PrBO3 are given in Table S1. In the structure of ν-PrBO3, the Pr atoms are coordinated by eight O atoms, and the B atoms are in a threefold planar coordination by three O atoms. Figure 1 is the perspective view of the structure of ν-PrBO3 with distorted PrO8 dodecahedron with Pr-O distances in the range of 0.2297 (11) to 0.2654 (12) nm. Each PrO8 shares edges with four neighbors to build infinite double chains along the [110] direction. The BO3 groups are regular, with average B-O distances in the range of 0.135(2) to 0.139(2) nm and O-B-O angles in the range of 116.3 (15) -126.0 (16)°.

Table 1 Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (×10-2, nm2) of single crystal ν-PrBO3

Table 1 1 Anisotropic atomic displacement parameters (×10-4,nm2)

Table S1 Important distances (×10-1, nm) and angles (°) of single crystal ν-PrBO3

Fig. 1 Crystal structure of ν-PrBO3 Grey and black polyhedra contain atoms Pr1 and Pr2, respectively

The powder X-ray diffraction pattern of λ-PrBO3 (Fig. S1) confirms the previous study since all diffractions peaks can be readily indexed to the pure orthorhombic-aragonite phase of PrBO3 (ICSD# 421745) with the cell parameters a= 0.81419 nm, b= 0.57776 nm, c= 0.50692 nm, V= 0.2385 nm3. As described by Haberer, et al[ 16], the aragonite PrBO3 crystallizes in the orthorhombic Pnma space group with the structure composing of PrO9 polyhedra and trigonal BO3 groups. Each PrO9 polyhedron shares edges with six other PrO9 polyhedra and three BO3 triangles. In addition, each BO3 triangle shares edges with three PrO9 polyhedra at the same time. By comparison of the two modifications, it is obvious that the Pr atoms in the high temperature phase has less coordination number of O than the low temperature phase, and each polyhedron share only 4 edges while 6 edges shared in the low temperature modification. This leads to the obvious changes of the lattice parameters.

Fig. S1 The powder X-ray diffraction pattern of λ-PrBO3

According to the discussion above, it is possible that PrBO3 compound exists in two different phases, i.e., aragonite and triclinic phases. Since there are few reports on the structure phase transition process, it is more worth of notice. In order to study the relationship between the two phases, the sample λ-PrBO3synthesized at 1200℃ were annealed at 1200℃, 1300℃, 1400℃ and 1450℃ in air for 5 h, respectively. The results (Fig. 2) turn out that the phase transition point occurs around 1450℃. Since the transition between the two structure modifications did not follow group-subgroup relationship, it may be concluded that the phase transition is first order with reconstructive manner. The detailed mechanism of phase transition of PrBO3 will be further investigated and published elsewhere.

Fig. 2 XRD patterns of λ-PrBO3 annealed in air at different temperatures

2.2 FT-IR spectroscopy of ν-PrBO3

To ascertain the structure of samples, corresponding FT-IR spectroscopy of samples were investigated. The FT-IR spectrum of ν-PrBO3 in the range of 400-1600 cm-1 is shown in Fig. 3.

Fig. 3 FT-IR spectrum of ν-PrBO3 in the range of 400-1600 cm-1

According to the previous literatures, for an isolated planar trigonal BO3 group, there are usually six fundamental modes of vibrations, which are symmetric stretch, ν1 (non degenerate, 950 cm-1), out of plane bending, ν2 (non degenerate, 740 cm-1), antisymmetric stretch, ν3 (doubly degenerate, 1250 cm-1), plane bending, ν4 (doubly degenerate, 600 cm-1)[ 20]. The observed vibration frequencies of ν-PrBO3 are listed in Table S2 with their assignments. The modes of vibrations of ν-PrBO3 are those typical for the isolated planar ion BO3 as observed in NdBO3 and SmBO3[ 7]. Therefore, the result is consistent with its corresponding powder XRD data.

Table S2 Observed frequencies of the different vibration modes in ν-PrBO3

2.3 Diffuse reflection spectra of ν-PrBO3 and λ-PrBO3

The diffuse reflection spectrum of ν-PrBO3 is recorded in the range of 230-700 nm (Fig. 4). The figureexhibits a drastic drop in the reflection in the UV range around 250 nm (4.96 eV), which defines the optical band gap of ν-PrBO3 host lattice. It shows several absorption lines at long wavelength ( i.e. low energy) in the range of 448-484 nm as well as 586-598 nm, which are ascribed to the typical f-f transitions from3H4 →3P2 (~448 nm),3H4 →3P1 (~474 nm),3H4 →3P0 (~485 nm) and3H4 →1D2 (~598 nm) of Pr3+ ions, respectively. The absence of charge transfer band of O2--Pr4+ rules out the possibility of Pr4+ in ν-PrBO3, indicating that all the Pr ions in ν-PrBO3exist in the trivalent state. The daylight color of ν-PrBO3shows light green as a result of the strong absorption by Pr3+ in the visible range of 420-650 nm.

Fig. 4 Diffuse reflection spectrum of ν-PrBO3Inset shows the absorption spectrum of ν-PrBO3 obtained from the reflection spectrum by using the Kubelka-Munk function

In order to better localize the thresholds for host lattice absorption and the absorption by Pr3+, the absorption spectrum of ν-PrBO3 was obtained from the reflection spectrum by using the Kubelka-Munk function[ 21]: F(R)=(1-R)2/2R=K/S

Where R, K, S are the reflection, absorption coefficient, and scattering coefficients, respectively. The absorption ( K/ S) spectrum of ν-PrBO3 derived with the Kubelka- Munk function is shown in the inset of Fig. 4. The value of the optical band gap of ν-PrBO3 is calculated to be about 4.96 eV ( i.e. 250 nm) by extrapolating the Kubelka-Munk function to K/ S = 0.

Figure 5 shows diffuse reflection spectrum of λ-PrBO3 under the same conditions as ν-PrBO3. It shows no obvious change compared to ν-PrBO3. The value of optical band gap of λ-PrBO3 is also around 250 nm (4.96 eV), which is calculated from the absorption spectrum (the inset of Fig. 5).

Fig. 5 Diffuse reflection spectrum of λ-PrBO3Inset shows the absorption spectrum of λ-PrBO3 obtained from the reflection spectrum by using the Kubelka-Munk function

The X-ray excited luminescence spectrum and photoluminescence spectra of λ-PrBO3 and ν-PrBO3were measured. However, no emission of Pr3+ in the UV-Vis range were observed in both of the two host lattices, probably due to the compositional quenching caused by high Pr content.

3 Conclusions

λ-PrBO3 and ν-PrBO3were successfully synthesized by solid state method. The XRD results confirm that the high-temperature phase ν-PrBO3 is isostructural with ν-SmBO3, crystallizes in the triclinic system belonging to space group P . According to the XRD results, the phase transition temperature from λ-PrBO3 to ν-PrBO3occurs at about 1450℃. The diffuse reflection spectrum of ν-PrBO3 is also studied and its optical band gap is determined to be about 4.96 eV, which has no obvious change compared to λ-PrBO3. PL and XEL studies of both phases show no emission at visible region.

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