Fluoride Borates with [(BO3)F]4â â [F4]4â Anionic Isomorphism and X

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Fluoride borates with [(BO3)F]4–# [F4]4– anionic isomorphism and X-ray sensitivity T. B. Bekker, V. P. Solntsev, A. P. Yelisseyev, and S. V. Rashchenko Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00615 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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Crystal Growth & Design

Fluoride borates with [(BO3)F]4–↔ [F4]4– anionic isomorphism and X-ray sensitivity *1,2,3

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T.B. Bekker, 1V. P. Solntsev, 1A.P. Yelisseyev, 1,2 S.V. Rashchenko

Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Science, 630090 Novosibirsk, Russia 2

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Novosibirsk State University, 630090 Novosibirsk, Russia

Novosibirsk State Academy of Architecture and Arts, 630099 Novosibirsk, Russia *Corresponding author: [email protected]

RECEIVED DATE ABSTRACT: Crystals of Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y solid solutions display a phenomenon that has never been described for borates before – X-ray irradiation induces strong absorption in the visible range and, as the result, coloring of the crystals up to dark purple. The induced absorption can be removed by illumination with a wavelength of 300−400 nm. Owing to this phenomenon these crystals are promising sensor applicable to fixing X-ray images. With the use of electron spin resonance and optical spectroscopy it has been found that the formation of color centers is connected with the [(BO3)F]4– and [F4]4– tetrahedral groups, which are the distinguishing feature of Ba7(BO3)4–yF2+3y and Ba4– xSr3+x(BO3)4–yF2+3y yF2+3y

structure. Partial replacement of Ba2+ to Sr2+ in the crystal lattice of Ba4–xSr3+x(BO3)4–

leads to the formation of the stable electron-hole pairs under X-ray irradiation. The opportunity to

vary the cationic and anionic sublattice composition within the same crystal structure becomes a prospective tool to vary physical and chemical properties of the phosphors.

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INTRODUCTION Inorganic borates are a prospective class of optical materials. They are quite well known for their nonlinear optical properties and transparency in the UV range of the electromagnetic spectrum. Most widely used non-linear optical borates are the low temperature modification of barium borate β-BaB2O4 [1-4], lithium triborate LiB3O5 [5-8] and lithium-cesium borate CsLiB6O10 [9-11]. Fluoride borates stand out amongst other borates, as they possess much broader area of transparency with the cutoff edge shifted further in the UV-range [12,13,16]. Borates are characterized by a unique combination of properties (high laser damage threshold, high birefringence, wide optical transparency windows, acceptable values of nonlinear optical coefficients) and by the exceptional variety of structural motifs. Currently borate systems are considered as the most promising for the creation of the new optical materials in the UV range in the form of single crystals, glasses, nanocomposites. The BaO−BaF2−B2O3 ternary system has received a lot of attention recently. It is interesting for both its low temperature modification of barium borate β-BaB2O4 [14] and fluoride borates crystal growth [1520]. Five ternary phases have been described in this system (Figure 1, Table 1). Alekel and Keszler [15] claimed the existence of pyrofluoroborate Ba5(B2O5)2F2 (C2/c). A novel deep UV nonlinear optical crystal Ba3B6O11F2 (P21) was described in 2012 by two research groups [16, 17]. In the structure, a new fundamental building block [B6O14]10−, composed of four [BO4]5− tetrahedra and two [BO3]3− triangles, was observed. According to the calculations, the short-wavelength absorption edge is below 190 nm, nonlinear-optical coefficient is three times higher than that of KDP [16]. Another new fluoride borate Ba4B11O20F (Cmc2) with promising properties was described in [18]. Short-wave absorption edge of the compound is 175 nm, nonlinear-optical coefficient, estimated by the Kurtz-Perry method, is ten times higher than that of KDP.


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Figure 1. Concentration triangle of the BaO−BaF2−B2O3 ternary system. Table 1. Ternary phases in the BaO–B2O3–BaF2 system. Chemical formula

Composition, mol. %

Space

Reference

group, Z Ba5(B2O5)2F2

57.1BaO–28.6B2O3–14.3BaF2

C2/c, 4

[15]

Ba3B6O11F2

33.3BaO–0.5B2O3–16.7 BaF2

P21, 2

[16, 17]

Ba4B11O20F

36.8BaO–57.9B2O3–5.3 BaF2

Cmc21, 4

[18]

Ba7(BO3)4–yF2+3y (y = 0.49)

60BaO–20B2O3–20 BaF2

P63, 2

[19]

Ba5(BO3)3F

69.2BaO–23.1B2O3–7.7 BaF2

Pnma, 4

[20]

Our recent study of the BaF2−Ba3(BO3)2 subsystem led to the discovery of two intermediate phases: Ba7(BO3)4–yF2+3y (P63) and Ba5(BO3)3F (Pnma) (Figure 1) [19-22]. Discovered solid-solution Ba7(BO3)4– yF2+3y

and its strontium analogue Ba4–xSr3+x(BO3)4–yF2+3y (P63mc) [23] exhibit previously unknown type of


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heterovalent anionic isomorphism.

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A comparison of Ba7(BO3)4–yF2+3y, Ba4–xSr3+x(BO3)4–yF2+3y and

Ba5(BO3)3F structures allows for the disclosure of the mechanism of (BO3)3– ↔ 3F– heterovalent anionic substitution in fluoride borates via [(BO3)F]4– tetrahedral groups being replaced by four fluoride anions. The mechanism suggests that investigated fluoride borates do not demonstrate direct (BO3)3– ↔ 3F– anionic isomorphism; rather, there is a different substitution of the type [BO3F]4–↔ [F4]4– taking place, and it is a fourth fluoride anion in these fragments that makes the [BO3F]4–↔ [F4]4– isomorphism possible [20]. The most appropriate representation of the structure of Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y phases is cation sublattice with anion-filled cavities as per approach suggested for alkali-earth borates by A. Vegas [24]. The following three types of cation-coordinated anions are present in the structure: (1) octahedrally coordinated F– anions, (2) (BO3)3– triangles in three-capped trigonal prisms and (3) tetrahedral [X4]4– anionic groups. The tetrahedral [X4]4– groups, located in the large 11 vertex cavities of the cationic sublattice, include either four fluoride anions or a combination of a (BO3)3– triangles and a fluoride anion (Figure 2). The main goal of the present paper is to look into the connection between the structural features and the unique physical properties of the Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y solid solutions, and investigate their potential as X-ray storage phosphors which are important materials for X-ray imaging in medicine, industry and science. An X-ray exposure of the storage phosphor causes appearance of optical centers, recording an X-ray image in the material. For readout of the stored image, an effect of photostimulated luminescence is used [25,26]. After the readout procedure, the storage phosphor is bleached by bright halogen source to be ready for the next exposure. Despite of intense development of digital X-ray imaging on the basis of such technologies as charge-coupled device, thin-film transistor, and complementary metal-oxide-semiconductor, the image plate detectors based on X-ray storage phosphors remain the cheapest, robust and well-established equipment for X-ray imaging.


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Figure 2. The coordination of anions in Ba7(BO3)4–yF2+3y (a-d) and Ba4–xSr3+x(BO3)4–yF2+3y (a, e-g) solid solutions: (1) F1 site in Ba6 octahedron (a); (2) (BO3)3– group in three-capped trigonal prism(b,e); (3) [X4]4– group, represented by [F4]4– (c, f) and [BO3F]4– (d,g) tetrahedral group in 11 vertex cavities. Yellow spheres represent F– anions; (BO3)3– anions are given as a combination of red oxygen and black boron atoms; Ba2+ and Sr2+ cations are shown in dark and light blue, respectively. EXPERIMENTAL PROCEDURES Crystal growth. The single crystals of Ba7(BO3)4–yF2+3y were grown on a platinum wire loop in air in the BaO–BaF2–B2O3 system; crystals of Ba4–xSr3+x(BO3)4–yF2+3y phase – in the Ba4–xSr3+x(BO3)4–yF2+3y– NaF system. The crystal of Ba7(BO3)4-yF2+3y was grown from the same composition as we used in Ref. [19] (BaB2O4 : BaF2 : BaO = 1 : 1.4 : 1.6), wherein the crystal composition was determined to be Ba7(BO3)3.51F3.47 by means of single crystal structure analysis. The intermediate member of Ba4xSr3+x(BO3)4-yF2+3y

was grown from the same composition as we used in Ref. [23] (BaB2O4 : BaF2 : SrO :

NaF = 1 : 1 : 2 : 2), wherein the crystal composition was determined to be Ba3.12Sr3.88(BO3)3.65F3.05 by means of single crystal structure analysis. End members, Ba4Sr3(BO3)4F2 and Ba3Sr4(BO3)4F2, were also ACS Paragon Plus Environment

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grown with the use of NaF solvent. The X-ray diffraction patterns of grown crystals of end-members coincided with the X-ray diffraction patterns of the synthesized ones [23]. High purity (99.99 wt. %) BaF2, SrF2, BaCO3, SrCO3, H3BO3 and NaF were used as starting materials. The batch (40 g) was melted in a platinum crucible (40 mm in diameter) through the stages of solid-state synthesis. The heating rate was 25 ºC/hour; the maximum heating temperature − 1120 ºC. The liquidus temperature was determined by visual polythermal analysis. At liquidus temperature platinum wire loop was placed into the central part of the melt surface to induce spontaneous crystallization. After the latter process began, the melt was cooled at a rate of 0.2 °C/day for 10 days. Then the platinum loop with grown crystals was extracted from the melt and cooled to room temperature at a rate of 15 °C/hour. These allowed us to grow crystals up to 20 mm in size. X-ray diffraction analyzes of the grown crystals were carried out with the use of ARL X’TRA powder X-ray diffractometer (CuKα, Bragg-Brentano geometry). Optical spectroscopy and electron spin resonance (ESR) study. Transmission spectra were recorded with a UV-2501PC Shimadzu spectrometer in the UV to near IR, whereas in the mid-IR we used a Fourier-Transform spectrometer Infralum FT-801. The photoluminescence (PL) spectra were measured using a SDL1 diffraction luminescence spectrometer with excitation from 1 kW Xe lamp through MDR2 diffraction monochromator at 80 and 300 K. Necessary excitation wavelength was separated using a diffraction MDR2 monochromator combined with an appropriate color glass filter. X-ray excited luminescence (RL) was recorded with a MDR2 diffraction monochromator with a FEU100 photomultipliers as emission detector. A table 1 kW X-ray set-up with a W-anticathode tube, at 40 kV voltage and 20 mA current was used in experiments on RL and X-ray induced absorption. RESULTS AND DISCUSSION The area of homogeneity of Ba7(BO3)4–yF2+3y solid solution spans between Ba7(BO3)3.79F2.63 and Ba7(BO3)3.35F3.95 (0.21 < y < 0.65) compositions [19]. Crystals of optical quality of Ba7(BO3)3.51F3.47 ACS Paragon Plus Environment

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compound with structural formula Ba3Sr3(BO3)3F×[(BO3F)0.51(4F)0.49] were grown (Figure 3a). The ‘Ba3Sr3(BO3)3F’ describes the non-isomorphic part of the structure, the square brackets represent anionic [4F]4– ↔ [BO3F]4– isomorphism. All intermediate members of the Ba4–xSr3+x(BO3)4–yF2+3y solid solution may be visualized in the isomorphism square, where x and y correspond to the horizontal and vertical axes, respectively (Fig. 3b). The structural formula of grown crystals of end-members Ba4Sr3(BO3)4F2, Ba3Sr4(BO3)4F2 and intermediate

member

Ba3.12Sr3.88(BO3)3.65F3.05

may be written

as

Ba3Sr3(BO3)3F×[Ba][BO3F],

Ba3Sr3(BO3)3F×[Sr][BO3F] and Ba3Sr3(BO3)3F×[Ba0.12Sr0.88][(BO3F)0.65(4F)0.35], correspondingly, where ‘Ba3Sr3(BO3)3F’ describes the non-isomorphic part of the structure, the square brackets − cationic Ba2+ ↔ Sr2+ and anionic [4F]4– ↔ [BO3F]4– isomorphism.

a

b

Figure 3. A 10×10×3 mm3 polished plate of Ba7(BO3)3.51F3.47 (y = 0.49) (a). The isomorphism square of Ba4–xSr3+x(BO3)4–yF2+3y solid solution. The Ba2+ ↔ Sr2+ substitution in M position is shown along the horizontal axis; the [4F]4– ↔ [BO3F]4– anion isomorphism in [X4]4– group is shown along the vertical axis.


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Filled circles represent the experimentally obtained compounds; open circles represent phases whose existence has not been confirmed. Photographs of as-grown Ba4Sr3(BO3)4F2, Ba3.12Sr3.88(BO3)3.65F3.05 Ba3Sr4(BO3)4F2 crystals and samples for investigations, made of them (b). Optical centers in Ba7(BO3)4–yF2+3y solid solution. Some properties of Ba7(BO3)3.51F3.47 crystals were described in [22]. Figure 4a shows transmission spectra for 0.5 and 3 mm thick plates. The crystal is transparent from 215 nm to 2.5 µm (for 0.5 mm plate), and there are some windows of transparency at 3-3.7, 56.5 µm. X-ray irradiation induces strong absorption in the visible range and, as the result, coloring of the crystal up to dark purple. The most intense induced bands are 518 nm (2.39 eV) and 587 nm (2.11 eV). The induced absorption can be effectively removed by illumination with a wavelength of 300-400 nm. The combined X-ray irradiation and optical bleaching may provide an effective method to control the color centre distribution and create different images (Figure 4b). In order to understand the structure of the optical centers both optical spectroscopy and ESR technique were used. No ESR signals were observed in the as-grown crystals. After X-ray irradiation signals from two electron-type (with g ≤ gе = 2.0023) and hole-type (with g ≥ gе) centers were observed at room temperature. At 80 K the ESR spectra from four different centers in irradiated crystals were clearly exhibited when the magnetic field Н is parallel to the [001] axis or [010] axis of the crystal (Figure 5). ESR spectra are described by the following spin-Hamiltonian [30]: Ĥ = β ĝ Ŝ H + Â Ŝ Î, where H is magnetic field, β is Bohr magneton, ĝ is Lande factor, Ŝ is total spin and Î is a nuclear magnetic moment. Â is a constant of hyperfine interaction. In our case I = 0, 1/2 and 3/2 for oxygen 16O (natural isotopic abundance 99.96 %), fluorine

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F (natural isotopic abundance 100 %) and boron

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B

atoms (natural isotopic abundance 81.17 %), correspondingly; S = 1/2 for these atoms. Analysis of the angular dependence for the ESR lines showed that the electron-type centers е6- (g-factor gmax=1.959, gmin=1.953) and е4- (gmax=1.980, gmin=1.969) are an electrons captured on a fluorine vacancy located in F1 position inside the Ba6 octahedron (see Figure 2a) and in the vertex X2 of [BO3F]4– tetrahedral group, ACS Paragon Plus Environment

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correspondingly (see Figure 2d). The structure of the centers coincides well with an expected fluorine positions in the Ba7(BO3)3.51F3.47 lattice. Such defects are well-known as F-centers in literature [27]. Two hole-type centers О1- (gс = 2.0088) and Ох1- (gс = 2.0052) in Figure 5 are oxygen ions which have captured a hole (or lost an electron). Centers О1- and е4- have six magnetically nonequivalent complexes in the structure, Ох1- and е6- − two ones. The center О1- is due to a hole on the closest to F1 oxygen О12– (F1−О1 = 3.478 Å), interacting with F1. The other hole-type center Ох1- at H||[001] demonstrates four lines with the ratio between components intensities as 1:3:3:1 (Figure 5a) and can appear as the result of the interaction with three equivalent

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F atoms. The only possible position for such center is X1 position in

[F4]4– tetrahedron (see Figure 2c), occupied by oxygen atom which captures the hole: [Ox1F3]5– + X-ray irradiation → е– + [Ox1F3]4–. The electrons knocked from O2– are captured by fluorine vacancies inside the Ba6 octahedron (е6-) and in the vertex of [BO3F]4– tetrahedral group (е4-). According to the estimations of the p-levels splitting due to spin-orbital interactions [28-29], the optical transitions for Ох1- center correspond to E = 2.459–2.118 eV or 504–585 nm and E = 4.57–4.13 eV or 271–310 nm, which is in accordance with the induced absorption bands in the transmission spectrum (Figure 4a). Similar estimations for О1- center give energy of 5.84 eV or 212 nm and 0.43 eV or 2868 nm, falling into UV- and IR-region, correspondingly. Thus, optical transitions between p-sublevels in Ох1- position of [Ox1F3]4– tetrahedron are responsible for strong X-ray induced absorption in the visible region.

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Figure 4. Transmission spectra of Ba7(BO3)3.51F3.47 (y = 0.49). Spectra were measured at 300 K for the sample 0.5 mm (1) and 3 mm thick, before (1,2) and after the X-ray irradiation for 30 min (3). In the inset: a 10×10×3 mm3 polished plate of Ba7(BO3)3.51F3.47 after irradiation (a); the same plate after X-ray irradiation followed by 10 s photobleaching using an expanded beam of a 405 nm, 50 mW diode laser, through a butterfly image printed on the offset film (b).

Figure 5. ESR spectra for X-ray irradiated Ba7(BO3)3.51F3.47 crystal, recorded at 80 K with H||[001] (a) and H||[010] (b). Magnetic field is given in Gauss. Optical centers in Ba4–xSr3+x(BO3)4–yF2+3y solid solution. Fig. 6a shows transmission spectra for the end member Ba4Sr3(BO3)4F2 0.5 mm thick plate at 300 K. One can see the shoulder near the fundamental absorption edge at about 260 nm which can be connected with native defects. The crystal is transparent from 212 nm to 2.7 µm, and there are some windows of transparency at 3-3.7, 5-6.3 µm. Grown crystals of another end-member Ba3Sr4(BO3)4F2 have a light pink color; in its spectrum additional broad absorption band at about 500-520 nm appears. Analysis of absorption spectrum shape for Ba4Sr3(BO3)4F2, Ba3.12Sr3.88(BO3)3.65F3.05 and Ba3Sr4(BO3)4F2 0.5 mm thick plates showed that the spectrum can be approximated by a straight line when represented in (α×hν)2 and (hν) coordinates, where α is absorption coefficient and hν is proton energy. It means that direct allowed electronic transitions are responsible for the fundamental absorption edge. The


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band gap values for Ba4Sr3(BO3)4F2, Ba3.12Sr3.88(BO3)3.65F3.05 and Ba3Sr4(BO3)4F2 crystals are 5.517, 5.88 and 6.10 eV at 300 K, correspondingly. We believe that the observed shift of the absorption edge in the UV range is due to the increased content of strontium.

Figure 6. Transmission spectra for 0.5 mm thick Ba4Sr3(BO3)4F2 plate before (1) and after X-ray irradiation for 30 min (2). In the inset: the Ba4Sr3(BO3)4F2 fundamental absorption edge, represented in coordinates (α×hν)2 = f (hν) at T = 300 K (3) and at T = 80 K (4) for a plate 0.5 mm thick (a); Ba4Sr3(BO3)4F2 photoluminescence excitation spectra for emissions at 450 nm (1), 500 nm (2), 550 nm (3), 600 nm (4), 650 nm (5) at 300 K. X-ray irradiation of Ba4Sr3(BO3)4F2 crystal induces weak absorption bands and pink-purple coloring of the crystal (Figure 6a). In order to clarify the maximum of the absorption bands in the irradiated crystal, the photoluminescence excitation spectra were studied (Figure 6b). According to the spectra obtained there are two main absorption bands at about 400 nm and 520 nm in the irradiated crystal. Similar to Ba7(BO3)4–yF2+3y (y = 0.49) crystals, ESR spectra in Ba4–xSr3+x(BO3)4–yF2+3y ones were observed only after X-ray irradiation. In the ESR spectra of the Ba4Sr3(BO3)4F2 crystals, where all [X4]4– groups are presented by [BO3F]4– ones, three centers were observed: two electron-type centers е4(gc=1.978) and е6- (gc = 1.954), which are electrons captured on a fluorine vacancy located in the X2 vertex of [BO3F]4– tetrahedral group (see Figure 2g) and in F1 position inside Ba6 octahedron (see Figure 2a), correspondingly, and one hole-type center О2х- (Figure 7a). Estimated concentration of electron-hole pairs ACS Paragon Plus Environment

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formed is about 1014 per 1 cm3. Centers е4- and О2х– have six magnetically nonequivalent complexes in the crystal structure, center е6- − two complexes. Hole-type center О2х– is a hole on oxygen interacting with 11

B atom in X2 position of [BO3F]4– tetrahedral group (see Figure 2g): [BO3Ox2]5– + X-ray irradiation →

е– + [BO3Ox2]4–. The electrons knocked from O2– are captured by е6- or е4- centers. The parameters of the О2х– center at Н||с are gc= 2.0097, Ac = 7.3 G. It is established that axes of g-tensor of О2х– center do not coincide with the axes of А-tensor and crystallographic axes. The parameters of the A-tensor changed from Аmin = 6.5 to Аmax = 8.9 Gauss. Estimations of p-level splitting from the deviation of gx = 2.0168 and gy = 2.0121 from g-factor of free electron [28-29] have shown that the optical transitions for О2х– center correspond to E = 2.21 eV or 562 nm and E = 3.30 eV or 376 nm. This is in a good agreement with the absorption bands at 520 and 400 nm in the transmission and luminescence spectra. The same types of optical centers were observed in the irradiated Ba3Sr4(BO3)4F2 crystals. In Ba3.12Sr3.88(BO3)3.65F3.05 crystal in addition to О2х–, е4-, е6- centers, displayed in the Ba3Sr4(BO3)4F2 and Ba3Sr4(BO3)4F2 ones, hole-type center Ох1-, interacting with three equivalent

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F atoms in [Ox1F3]4–

tetrahedron, appears (Figure 7b). This confirms the fact that both [BO3F]4– and [F4] 4– groups are present in the structure.

Figure 7. ESR spectra for X-ray irradiated Ba4Sr3(BO3)4F2 (a) and Ba3.12Sr3.88(BO3)3.65F3.05 (b) crystals, recorded at 80 K with H||[001] and H||[010], correspondingly. Center е6- is not shown in Figure 7a. Magnetic field is given in Gauss.


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Stability of X-ray induced optical centers and luminescence of the fluoride borates. It is well known that X-ray generates electrons and holes that can move inside the luminophor lattice. If a considerable quantity of these carriers is trapped by lattice defects and they are stable at room temperature, then this luminophor is suitable for storing X-ray images. A storage luminophor, therefore, must be capable of absorbing X-ray and part of electron-hole pairs must not recombine [31, 32]. It has been found that the color in the X-ray irradiated Ba7(BO3)3.51F3.47 samples can decay spontaneously at T = 300 K in the darkness (about 25 % in the first hour after X-ray exposure) and the intensity of ESR spectra decreases [22]. Optical centers in Ba3Sr4(BO3)4F2, Ba3Sr4(BO3)4F2 and Ba3.12Sr3.88(BO3)3.65F3.05 crystals turned out to be much more stable: the intensity of optical centers in the irradiated crystals does not change after the storage in darkness for three months. We believe that partial replacement of Ba2+ to Sr2+ in the crystal lattice of Ba4–xSr3+x(BO3)4–yF2+3y solid solution changes the size of the F-centers and the nature of hole-type centers − leads to the formation of stable [BO3Ox2]4– centers, which are absent in Ba7(BO3)4–yF2+3y one. These results are in agreement with the results obtained for the Ba0.82Sr0.18F1+xBr1−x:Eu2+ compound by Batentschuk with co-authors [33, 34]. They showed that the presence of Sr2+ leads to the increasing formation of F-centers and stabilizes the hole centers at room temperature [34].


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Figure 8. X-ray excited luminescence of Ba7(BO3)3.51F3.47 (1) and Ba4Sr3(BO3)4F2 (2), T = 300 K. In the inset: photo of the Ba7(BO3)3.51F3.47 crystal under X-ray excitation (a); PL spectra of Ba4Sr3(BO3)4F2 and Ba3.12Sr3.88(BO3)3.65F3.05 crystals (1,2) and of Ba4Sr3(BO3)4F2 crystal after X-ray exposure for 30 min (3), 365 nm excitation, T = 300 K (b). X-ray excited luminescence and photoluminescence spectra have been investigated. In both Ba7(BO3)3.51F3.47 and Ba4Sr3(BO3)4F2 phases X-ray irradiation produces luminescence emission in broad band at 420 nm (Figure 8a). Under the X-ray excitation free carrier of both signs are generated which makes the exiton processes possible. The photon energies at recombination of free excitons is typically close to the values of band gap energy which are 4.97 eV for Ba7(BO3)3.51F3.47 and 5.17 eV for Ba4Sr3(BO3)4F2 at 300 K. We believe that 420 nm (hν ~ 3.0 eV) band is due to the recombination of selflocalized excitons. Figure 8b shows PL spectra of Ba4Sr3(BO3)4F2 and Ba3.12Sr3.88(BO3)3.65F3.05 crystals (lines 1,2) and of Ba4Sr3(BO3)4F2 crystal after X-ray exposure for 30 min (line 3). PL before the X-ray exposure produces broad band at about 400−550 nm with maximum at about 450 nm. After X-ray exposure Ba4Sr3(BO3)4F2 crystal produces two intensive bands at about 455 and 610 nm. These emission bands are in accordance with the observed absorption bands at 400 nm and 520 nm and with the results of estimations of p-level splitting for the hole-type О2х– center. A study of REE-doped Ba4–xSr3+x(BO3)4–yF2+3y polycrystalline samples by Sun et al. [35] revealed its potential as a single-component blue-green-emitting phosphor. We believe that combination of these properties with the reported X-ray sensitivity and formation of stable electron-hole pairs makes the creation of X-ray storage phosphor possible. CONCLUSIONS The nature of color centers in the solid solutions Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y has been investigated. The X-ray sensibility and coloring of the irradiated crystals is due to the presence of ACS Paragon Plus Environment

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[(BO3)F]4– and [F4]4– tetrahedral groups in their structure. Cation Sr2+ stabilizes the electron-hole pairs in the crystal lattice at room temperature and leads to the formation of [BO3Ox2]4– groups in Ba4– xSr3+x(BO3)4–yF2+3y

phase, which are absent in Ba7(BO3)4–xF2+3x one. We believe that the opportunity to

vary composition within the same crystal structure becomes a prospective tool to vary physical and chemical properties of the Ba4–xSr3+x(BO3)4–yF2+3y phosphor, in particular, the value of K-edge, which is 37.4 keV for Ba2+ and 16.1 keV for Sr2+. Also it should be mentioned that investigated compounds can be easily synthesized in air in contrast with traditional X-ray storage phosphors requiring oxygen-free atmosphere. ACKNOWLEDGEMENTS The authors are grateful to A. Davydov for his help in preparation of the manuscript. This work was supported by the Russian Foundation for Basic Research (grant №16-08-00477 to TB).


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(17) McMillen, C. D.; Stritzinger, J.T.; Kolis, J.W. Inorg. Chem. 2012, 51 (7), 3953–3955. (18) Wu, H.; Yu, H.; Yang, Z.; Hou, X.; Su, X.; Pan, K.; Poeppelmeier, K. R.; Rondinelli J. M. J. Am. Chem. Soc. 2013, 135, 4215–4218. (19) Bekker, T. B.; Rashchenko, S. V.; Bakakin, V. V.; Seryotkin, Yu. V.; Fedorov, P. P.; Kokh, A. E.; Stonoga S. Yu. CrystEngComm 2012, 14, 6910–6915. (20) Rashchenko, S.V.; Bekker, T. B.; Bakakin, V. V.; Seryotkin, Yu. V.; Kokh, A. E.; Gille, P.; Popov, A. I.; Fedorov P. P. J. Appl. Cryst. 2013, 46, 1081–1084. (21) Yelisseyev, A. P.; Jiang, X.; Solntsev, V. P.; Bekker, T. B.; Lin Z. Phys. Chem. Chem. Phys. 2014, 16, 24884–24891. (22) Yelisseyev, A. P.; Solntsev, V. P.; Jiang, X.; Bekker, T. B.; Lin, Z.; Fedorov, P. P. J. Solid State Chem. 2015, 229, 358–365. (23) Rashchenko, S.V.; Bekker, T. B.; Bakakin, V. V.; Seryotkin, Yu. V.; Shevchenko, V. S.; Kokh, A. E.; Stonoga S. Yu. Cryst. Growth Des. 2012, 12 (6), 2955–2960. (24) Vegas, A. Acta Cryst. C. 1985, 64, 112–122. (25) Takahashi, K.; Miyahara, J.; Shibahara, Y. J. Electrochem. Soc. 1985, 132, 1492–1494. (26) Takahashi, K.; Kohda, K.; Miyahara, J. J. Luminesc. 1984, 31-32, 266–268. (27) Fowler, W. B. Physics of Color Centers. Academic Press, New York. 1968. Chapter 2 and 4. (28) Schirmer, O.F. J. Phys. Chem. Solids 1968, 29, 1407–1429. (29) Schirmer, O.F. J. Phys. Chem. Solids 1971, 32, 499–509. (30) O’Brien, M.C.V The structure of the colour centers in smoky quartz. Proc. Roy. Soc. London, Ser. A. V. 231. 1955. P. 135–153. (31) Leblans, P.; Vandenbroucke, D.; Willems, P. Materials 2011, 4, 1034–1086. (32) Lakshmanan, A. R. Radiation induced defects and photostimulated luminescence process in BaFBr:Eu2+. Phys. Stat. Sol. 1996, 153, 3–27. (33) Batentschuk, M.; Hackenschmied, P.; Winnacker, A. Mat. Res. Soc. Symp. Proc. 1999, 560, 27–32. ACS Paragon Plus Environment

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For Table of Contents Use Only Title: Fluoride borates with [(BO3)F]4–↔ [F4]4– anionic isomorphism and X-ray sensitivity Author list: T.B. Bekker, V. P. Solntsev, A.P. Yelisseyev, S.V. Rashchenko TOC graphic (1.375 inches high x 3.5 inches wide):

Synopsis: Crystals of Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y fluoride borates are perspective sensors applicable to fixing X-ray images. The formation of color centers under X-ray irradiation is connected with the [(BO3)F]4– and [F4]4– tetrahedral groups, which are the distinguishing feature of their structure. Cation Sr2+ stabilizes the electron-hole pairs in the crystal lattice at room temperature.


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Fluoride Borates with [(BO3)F]4â â [F4]4â Anionic Isomorphism and X

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