'Anti-zeolite' Structure

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Optical and Magnetic Properties of #UContaining Borates with 'Anti-Zeolite' Structure Vladimir P. Solntsev, Tatyana B. Bekker, Aleksey V. Davydov, Alexander P. Yelisseyev, Sergey V. Rashchenko, Alexander E. Kokh, Veronika D. Grigorieva, and Sohyun Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00355 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Optical and Magnetic Properties of Сu-containing Borates with 'Anti-zeolite' Structure 1Vladimir

P. Solntsev, *1,2,3 Tatyana B. Bekker, 1 Aleksey V. Davydov, 1Alexander P. Yelisseyev,

1,2 Sergey

V. Rashchenko, 1Alexander E. Kokh, 4Veronika D. Grigorieva, 5So-Hyun Park

1Sobolev

Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of

Science, 630090 Novosibirsk, Russia 2Novosibirsk

State University, 630090 Novosibirsk, Russia

3Novosibirsk

State University of Architecture, Design and Arts (NSUADA), 630099

Novosibirsk, Russia 4Nikolaev

Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of

Science, 630090 Novosibirsk, Russia 5Section

Crystallography, Department of Earth and Environmental Sciences, Ludwig-

Maximilians University, 80333 Munich, Germany *Corresponding

author: [email protected]

RECEIVED DATE

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ABSTRACT: Crystals of copper-doped barium fluoride borate solid solutions Ba3(BO3)2xF3x Cu were grown from high-temperature solution. Using X-ray single crystal diffraction, the structure was solved and refined in Pbam space group. These crystals are colored in brown, revealing linear dichroism. Optical and electron spin resonance spectroscopic investigations concluded that the light absorption of Ba3(BO3)2xF3x:Cu crystals is caused by both the exciton and free charge carriers. This phenomenon is similar to those observed in other colored ‘antizeolites’ built with the positively charged framework of porous [Ba12(BO3)6]6+ building blocks (hence named as ‘anti-zeolites’). The coloring of Ba3(BO3)2xF3x: Cu crystals is associated to the absorption of Cu2+ ions as well. For the first time, constants of the hyperfine interaction of copper ions Cu2+ in octahedral fluorine surroundings [CuF6]4– have been determined.


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INTRODUCTION Borates outstand as the most promising class amongst other inorganic compounds for the creation of new functional optical materials.1 This is first of all for their well-known non-linear optical (NLO) properties and transparency in the UV range.2 A characteristic feature in borates is the orientation and arrangement of [BO3]3- triangle building units, which often eliminate spaceinversion and mirror planes. For this reason, about 35% of all known borates crystallize in noninversion or polar space groups essential to NLO properties. This is a fairly high abundance when considering merely about 11% acentric space groups of inorganic structures.3 Accordingly, numerous studies on crystal growth, structure elucidation, and the first principles quantumchemical calculations have been proceeded to understand the structure-property-relations of NLO borates.4-10 Fluoride borates are of particular interest. KBe2BO3F2 (KBBF) is the only crystal that allows generation of the sixth harmonic of the Nd3+:YVO4 laser with the wavelength of 177.3 nm by a direct second-harmonic generation method.11 Unfortunately, its structure tends to be layered due to the low interaction between building blocks, which prevents to manufacture large single crystals and optical elements. Over the last decade, there have been much effort on finding borates having the functional merit of KBBF but without layering tendency. A relatively new approach is introducing fluorine into the B–O clusters to obtain fluorooxoborates with [BOxF4x](x+1) basic units. 12,13 Fluorooxoborates are supposed to have large optical band gap and high anisotropy.14 A new class of fluoride borates allowing to apply structure-oriented design strategy has been recently discovered.15-17 These fluoride borates are built with positively charged [Ba12(BO3)6]6+ open framework (hence called as ‘anti-zeolitic’) and exhibit the ABAB stacking pattern, as shown in Figure 1a. The A-layer is built of porous [Ba12(BO3)6]6+ framework, hosting (BO3)3– or [2F]2– guest groups. The B layer is located between two A layers, where guest (BO3)3– groups can be situated along with guest anion clusters, e.g. [4F]4–, [(Li, Na)F4]3–, ACS Paragon Plus Environment

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[MnF6]4–. From the structural point of view, the B-layer is geometrically and chemically very flexible because of the large space for anion groups neighboring intra- and interlayer Ba2+ cations. There are channels along the c axis formed by altering anticubes (in layer A) and cubes (in layer B) (Figure 1b).

Figure 1. The framework [Ba12(BO3)6]6+ of the borates with ‘anti-zeolite’ structure (a); anionic filling of the layers A and B (b).

The ‘anti-zeolite’ borates Ba3(BO3)2–xF3x, (Li,Na)Ba12(BO3)7F4 and MnxBa12(BO3)8–2xF8x show linear dichroism and hence can be applied for dichroic polarization.18 According to electron spin resonance and optical spectroscopic investigations, the coloring of ‘anti-zeolitic’ borates is understood by the light absorption by both the exciton (electron of the valence band which is transferred to an excited state but remains bound to the hole) and free charge carriers.19 The light absorption of ‘anti-zeolitic’ borates is tunable by varying the packing degree of Blayers. In fact, the conformation of B-layers in ‘anti-zeolitic’ borates is essential for their optical properties. For instances, with a shift of the absorption edge of colorless (Li,Na)Ba12(BO3)7F4 crystals to vacuum ultraviolet region, their colors strictly depend on the packing degree of Blayers. Furthermore, the site symmetry of ‘guest’ anion groups mainly dictates the presence of non-centrosymmetric symmetry responsible for harmonic generations. Figure 2 shows


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transmission spectra of LiBa12(BO3)7F4 crystals which were grown from high-temperature solution with different lithium concentration, i.e. packing degree in B-layers. The consequent variation with the lowest and the highest Li+ concentration in B-layers resulted in deep purple and colorless crystals, respectively. The shift of the absorption edge might be connected with the exciton mechanism.19

Figure 2. Transmission spectra at 80 K for 1.3 mm plate and photographs of the plates of differently colored LiBa12(BO3)7F4 crystals (in the inset).19 Crystals were grown from hightemperature solutions with different lithium concentration. Li concentration in the initial hightemperature solution was 0.52, 0.80 and 1.16 wt.% for crystals 1, 2 and 3, respectively.

Resembling zeolitic structures, the porous fluoride borates ease with the control of structure-related properties by exchanging the type and amounts of guest anion groups. In this study, we report the structure of the new Cu-doped Ba3(BO3)2xF3x solid solution with containing [CuF6]4 and [CuF4]3 anion groups in the building block B in the context of their investigation by optical and electron-spin resonance spectroscopy.

EXPERIMENTAL PROCEDURES ACS Paragon Plus Environment

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Crystal Growth. Crystals of Ba3(BO3)2xF3x:Cu were grown on platinum wire loop from the same composition as we used for Ba3(BO3)2xF3x crystal growth15: 50 mol. % BaO  33 mol. % BaB2O4  BaF2 but with the addition of 3.10 wt. % of CuO. The batch (40 g) was melted in a platinum crucible (40 mm in diameter). Heating rate was 25 ºC/h; the maximum heating temperature 1110 ºC. At liquidus temperature, which was 1095 ºC, a platinum wire loop was placed into the central part of the melt surface to induce spontaneous crystallization. After the process began, the melt was cooled at a rate of 2 ºC/day for 5 days in order to increase crystal size. Then the platinum loop with the grown crystals was extracted from the melt and cooled to room temperature at a rate of 25 ºC/h. A brown rounded polycrystalline boule about 20 mm in diameter was grown from the specified composition. Structure Solution. Crystals of Ba3(BO3)2xF3x:Cu for single-crystal X-ray diffraction were selected from crushed boule. Examination of several crystals on a STOE IPDS-2T diffractometer using MoKα radiation (graphite monochromator) and an image plate detector showed pseudo-merohedral twinning about the two-fold axis in [110] (Figure 3), which indicates its origin of possible phase transformation from the higher-symmetry high-temperature polymorph. Data reduction and integration were performed using the program package CrysAlisPro.20 The observed pseudo-merohedral twinning caused strong overlap of reflections of two reciprocal lattices, hampering data processing and structure refinement, consequently. With the twinning matrix, reflections closer than 0.2°(2θ) were treated as completely overlapped; those separated at least by 0.25°(2θ) were treated as completely separate. The data set of the first twin component was used for structure solution and refinement with the software Jana2006.21,22 ACS Paragon Plus Environment

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All framework atoms except boron were refined anisotropically; site-specific isotropic atomic displacement parameters (ADPs) were refined for static disorder sites. Details of diffraction data collection and processing are given in Supplementary (CIF-file).

Figure 3. Two reciprocal lattices from twinning of Ba3(BO3)2xF3x:Cu crystals.

Optical Spectroscopy and Electron Spin Resonance (ESR) Study. Transmission spectra were recorded on a UV-2501PC Shimadzu spectrometer in the ranges from UV to near IR. ESR spectra were acquired at frequency 9.3 GHz at 300 K and 80 K on a spectrometer RE 1306 designed by the Institute of Chemical Kinetics and Combustion SB RAS (Novosibirsk, Russia).

RESULTS AND DISCUSSION Crystal Structure of Ba3(BO3)2xF3x: Cu Solid Solution. The topology of Ba3(BO3)2xF3x:Cu structure is identical to that reported for orthorhombic Ba3(BO3)2-xF3x antizeolite solid solution15 and closely related to tetragonal anti-zeolites with guest cations (Li, Na,


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Mn).16,19 The structure was solved and refined in Pbam space group with lattice metrics: a = 13.4355(5) Å; b = 13.8153(6) Å; c = 14.8812(6) Å. A specific feature of Ba3(BO3)2xF3x:Cu is a small a:b ratio of 0.973, which is remarkably smaller than the respective values 0.994-0.996 and 1.000 for Ba3(BO3)2-xF3x and tetragonal ‘anti-zeolites’ solid solutions. Such a phenomenon could be caused by Jahn-Teller distortions of intrachannel copper polyhedral, as discussed below. The presence of distorted copper polyhedra in the layers of B-type could be estimated by maxima of electron density. In the B-layer the copper site most probably has two types of coordination with fluorine atoms: flat rectangles and distorted octahedra strongly elongated in in a direction perpendicular to the c axis (Figure 4).

Figure 4. (a) Electron density maxima localized in intrachannel of the Ba3(BO3)2xF3x:Cu structure, based on Fourier syntheses. Gray spheres correspond to atomic sites for statically disordered oxygen or fluorine, large green to barium, small green to boron, and blue to copper. Colored sections demonstrate the respective occupancy parameters refined; (b) copper atoms in distorted [Cu1F6]4– octahedra and flat [Cu2F4]3– rectangles.

Optical Properties of Ba3(BO3)2xF3x:Cu Crystals. Crystals of Ba3(BO3)2xF3x:Cu are typically in brown tone. All grown crystals similar to other ‘anti-zeolites’ show a linear


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dichroism effect. Figure 5 presents transmission spectra of Ba3(BO3)2xF3x:Cu crystal in variation of the polarization direction of incident radiation with respect to the c axis, compared with those of the undoped compound Ba3(BO3)2xF3x. The linear dichroism effect is observed in the ranges of 350−650 nm and 730−760 nm. The color of the plate cut parallel to the c axis changes from greenish-yellow to dark brown. As the crystals were grown in air in the temperature range of 1140-1095 С, typical for absorption of oxide anions, they could contain O2, O22, O2, OH, as well as electrons (е). These can substitute oxygen and fluorine anions in the structure, e.g. O  F, O + e  O2, O22 2F. It is highly possible for Ba3(BO3)2xF3x crystals, both undoped and activated with copper, to possess the great number of oxygen anions and electrons, causing intense light absorption in the visible region of the spectrum. We believe that similar to other colored ‘anti-zeolite’ crystals, the absorption spectrum of Ba3(BO3)2xF3x:Cu crystals results from the absorption of light by both exciton and free charge carriers.19 The exciton is formed when the electron of the valence band is transferred to an excited state but remains bound to the hole. The energy of formation of this excited state, i.e. exciton, is smaller than the width of the forbidden zone. The channel system of the title compound provides paths for conducting oxide anions and spatial conditions for high vibrational motions of the ‘guest’ anion groups. These vibrational motions would reveal at a wavelength range of 680850 nm. Thus, small increase in the absorption in the area of 400-800 nm might be connected with Cu2+ ions in the Cu-doped crystals.


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Figure 5. (a) Photographs of the plates (5  5  1 mm3) cut parallel to the с axis of Ba3(BO3)2xF3x:Cu and Ba3(BO3)2xF3x crystals; (b) transmission spectra of the same plates of Ba3(BO3)2xF3x:Cu (solid lines) and Ba3(BO3)2xF3x (dashed lines) when the polarization direction of incident radiation was parallel to the c axis (1), at angles of 30° (2) and 90° (3) to the c axis. ESR Investigation of Ba3(BO3)2xF3x:Cu Crystals. The data of X-ray diffraction analysis show that copper ions in the structure of Ba3(BO3)2xF3x:Cu may occupy both flat distorted octahedral [Cu1F6]4 and rectangular [Cu2F4]3 coordination of 19F atoms. The EPR spectrum of Cu2+ in Ba3(BO3)2xF3x:Cu crystal acquired at 300 K under magnetic field (H) || [001] consists of 7 lines with an approximate intensities ratio of 1:6:15:20:15:6:1 (Figure 6). This spectrum was expected for interactions between an unpaired electron and six equivalent

19F

atoms with nuclear spin I = 1/2 (natural isotopic abundance of

100%). Copper with nuclear spin I = 3/2 has two isotopes

63Cu

and

65Cu

in natural isotopic

abundance of 69.09 % and 30.91 %, respectively and close values of magnetic moments. Therefore, the hyperfine structure (HFS) constants are represented by averaged lines. Spectrum parameters at H || [001] were: gc = 2.061 ± 0.001, Ас (19F) = 56 ± 4 G. ESR spectra are described by the following spin-Hamiltonian: Ĥ =  ĝ Ŝ H + Â Ŝ Î, ACS Paragon Plus Environment

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where H is magnetic field, β is Bohr magneton, ĝ is Landé factor, Ŝ is total spin and Î is a nuclear magnetic moment. Â is a constant of hyperfine interaction.23 In our case Ŝ = 1/2, Â = Â1 + Â2, where Â1 is a constant of hyperfine interaction of unpaired electron with the nuclear moment of I = 3/2 (63,65Cu) and Â2  with the nuclear moment of I = 1/2 (19F), respectively.

Figure 6. EPR spectrum of Cu2+ in Ba3(BO3)2xF3x crystal recorded at 300 K with H || [001]. The magnetic field H is given in Gauss. The ESR lines (Н  30 G) are too broad to unambiguously resolve the splitting of isotopes 63Cu and 65Cu. The HFS constant for 63Cu was found to be 63Ас(Cu2+)  20 ± 2 G at gc (gх) = 2.061. In the plane, perpendicular to the c crystal axis, we observed a complex spectrum of 12-15 lines, which can be divided into two sets of lines. One set consisted of 5-7 lines having extremity in this direction (maximum g-factor  gz = 2.411). The other set of lines (gy = 2.123), partly overlapped some of the lines of the first set, was less ordered and quickly disintegrated into a great number of lines at a deviation of 1-2° from this direction. Interpretation of ESR spectra of Cu2+ in octahedral or another surrounding of

19F

is

complicated by the fact that the hyperfine structure constants of Cu2+ ions are comparable in magnitude with the parameters characterizing their interactions with magnetic moments of surrounding fluorine nuclei. Unfortunately, in spite of numerous ESR studies of Cu2+ species in tetrahedral or another surrounding of oxygen atoms, presented in Ref. 24-26, no data of the hyperfine interaction between Cu2+ and fluorine atoms ACS Paragon Plus Environment

19F

are available in literature, so far: 11


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Zaripov et al.27 did not manage to completely decipher the ESR spectrum of Cu2+ in CaF2. ESR spectra of Cu2+ ions in -quartz and beryl were described in our earlier works.28-30 The doping ion Cu2+ in Ba3(BO3)2xF3x:Cu in octahedral coordination in the B-layer have a rhombic symmetry С2 (Wyckoff site 2а) and two magnetically non-equivalent complexes. For this reason, when the lines of the first set had an extremity, gz =2.411 in the ab plane, the second complex was orthogonal to the first and had an intermediate g-factor (gy = 2.123). It is worth noting that the directions of g- and А- tensors of AHFS (additional hyperfine structure) from 19F did not coincide with the crystallographic axis a and b. The angle between the z axis (F6CuF6) of the center and the crystallographic axis b is 29° (Figure 4b). The evaluated values for the lines of the first set (gz = 2.411) are 63А = 79±3, 19А = 40±5 G. Six-seven lines of the second complex (gy = 2.123) with various width and intensity, located at distance of 59±7 Gs, could be caused by interactions of an unpaired electron (hole) of Cu2+ with six nonequivalent 19F nuclear moments (19Ау = 59±7 Gs). Besides, on the extreme lowintensive line (g = 2.077) we observed additional splitting to 4 lines (63Ау = 19±3 Gs) which are due to an unpaired electron interacting with own nuclear moment I = 3/2 of

63Cu.

The Cu2+

species in octahedral fluorine surroundings in Ba3(BO3)2xF3x:Cu gave rise to the ESR parameters evaluated: {(gz =2.411, 63А (Cu2+) х

63А z

= 79 Gs; gy = 2.123,

63А

у

= 19 G; gx (gc) = 2.061

 20 Гс}. These values accord with the parameters for Cu2+ in the distorted oxygen

octahedral crystal fields in TiO231 и GeO232, as given in Table 1. Table 1. Parameters of ESR spectra of Cu2+ in octahedral crystal fields in Ba3(BO3)2-xF3x, TiO2 and GeO2 crystals. gi/Ai [10-4 cm-1] gi/IAiI

i=z

i= y

i=x

Reference

2.344/ 88.0

2.123/ 18.8

2.061/ 19.2

Ba3(BO3)2-xF3x this work

gi/IAiI

2.344/ 88.0

2.106/ 19.0

2.093/ 28.0

TiO231


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gi/IAiI

2.382 83.2

2.047 56.4

2.096 21.6

GeO232

The splitting scheme of 3d9 levels of Cu2+ in a [CuF6]4 octahedron can be summarized as follows: a cubic field splits 3d9 (2D-term) into a lower orbital doublet involving d-orbital functions (х2-у2) and (3z2-r2) and an upper triplet xy, yz, zx (further referred to as dх2-у2, dz2 and dxy, dyz, dzx, respectively). Octahedral complexes of copper have a tetragonal distortion corresponding to the stretching along the F6CuF6 direction in the (a-b) plane, so that the hole (missing electron) is in the dх2-у2 orbit. Neglecting the second order terms, one can show that the main components of the g-tensor will be equal to: g = 2(1+ 4/1), g = 2(1+ 2/2), where 1 is the distance between dх2-у2 and dxy; and 2  between the dх2-у2 and dyz, dzx orbitals. Taking the value gz = g = 2.411 for the z axis of the center and averaging g = (gy + gx) /2 = 2.123 + 2,061 / 2, we obtain 2.092. As the spin-orbital interaction constant for Cu2+  =

830 cm-1, after

introducing small corrections for bond covalence (2 = 0.78), 1 = 11660 cm-1 (790 nm) and 2 = 14390 cm -1 (695nm) are determined. We did not manage to allocate accurately the transitions noted above against the background of intensive own absorption of a crystal (Figure 5b). Data of X-ray spectral analysis suggest that part of copper ions occupy position 2b in the valence state of Cu2+ or Cu+. For Cu2+ in a flat rectangular environment of fluorine atoms, ESR spectrum from five lines with the relation of intensity 1:4:6:4:1 was expected. However, no additional ESR spectra at 300 and 80 K, ruling out Cu2+ in 2a position, were observed. This could be due to the presence of monovalent Cu+ in 2b position. In most alkali halide crystals, the absorption resulting from the 3d10  3d94s transition to Cu+ is in the range of 180–250 nm, and the luminescence is observed in the range of 400–450 nm.33 As one can see from the reported transmission spectra (Figure 5b), no additional absorption bands could be isolated against the background of an intense intrinsic absorption of the crystal. To observe the effect of irradiation on the optical absorption, luminescence and ESR spectra, we need to synthesize crystals that are


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transparent in the visible and UV regions of spectra, the same as for the crystals of (Li,Na)Ba12(BO3)7F4 solid solution (Figure 2), which is the subject of our further research.

Conclusions ‘Anti-zeolitic’ copper-doped Ba3(BO3)2-xF3x crystals were grown and investigated by means of X-ray diffraction combined with optical and ESR spectroscopy. As a result, constants of the hyperfine interaction of Cu2+ with fluorines have been determined. The crystal structurerelated optical properties of ‘anti-zeolite’ borates strongly depend on the degree of filling of their channels. The possibility of stabilization of [CuF4]3– and [CuF6]4– complexes at 300 K in the channels of Ba3(BO3)2-xF3x crystals opens up wide possibilities for using Cu:Ba3(BO3)2-xF3x crystals, e.g. storage phosphors in medical radiography.34 This compound has a density of about 5.9 g/cm3, and a K-edge (K-edge) ~ 10 keV, which is well suited for mammography.34 Monovalent copper single crystals can become a new matrix for effective tissue-equivalent dosimeters.33 The materials in which electrons occupy anion position (electrides)34,35 can be presumably used for thermoionic and electron emission with a cold cathode.36 Combination of electrical conductivity and transparency is possible, which is very useful for flat displays and electronic devices.37,38 Hence, the present system deserves further extensive investigations.

Acknowledgements This work was supported by German Academic Exchange Service (DAAD 57314018 to T.B.) and State Contract №0330-2016-0008.

Supporting Information Ba3(BO3)2xF3x:Cu structure refinement parameters and detailed crystallographic data are given as Supporting Information. Crystallographic information file (CIF) is also enclosed.


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References (1) Chen, C.; Sasaki, T.; Li, R.; Wu, Z.; Lin, Z.; Mori, Y.; Hu, Z.; Wang, J.; Uda, S.; Yoshimura, M.; Kaneda, Y. Nonlinear Optical Borate Crystals, Principles and Applications; Wiley-VCH Verlag GmbH & Co. KGaA. 2012, 387 p. (2) Bubnova, R. S.; Filatov, S. K. High-temperature Crystal Chemistry of borates and borosilicates; SPb.: Nauka. 2008, 760 p. (3) Becker, P. Borate Materials in Nonlinear Optics. Adv. Mater. 1998, 10, 979–992. (4) Wang, Y.; Pan, S. Recent Development of Metal Borate Halides: Crystal Chemistry and Application in Second-order NLO Materials. Coord. Chem. Rev. 2016, 323, 15–35. (5) Jiang, X.; Luo, S.; Kang, L.; Gong, P.; Huang, H.; Wang, S.; Lin, Z.; Chen, C. FirstPrinciples Evaluation of the Alkali and/or Alkaline Earth Beryllium Borates in Deep Ultraviolet Nonlinear Optical Applications. ACS Photon. 2015, 2, 1183–1191. (6) Yeliseyev, A. P.; Solntsev, V. P.; Jiang, X.; Bekker, T. B.; Lin, Z.; Fedorov, P. P. Electronic Structure, Magnetic and Optical properties of the Ba7(BO3)4-xF2+3x Crystals. J. Solid State Chem. 2015, 229, 358–365. (7) Kidyarov, B. I. Comparative Interrelationship of the Structural, Nonlinear-Optical and Other Acentric Properties for Oxide, Borate and Carbonate Crystals. Crystals. 2017, 7, 109. (8) Zou, G.; Huang, L.; Ye, N.; Lin, C.; Cheng, W.; Huang, H. CsPbCO3F: A Strong Second-Harmonic Generation Material Derived from Enhancement via p−π Interaction. J. Am. Chem. Soc. 2013, 135, 18560–18566. (9) Tran, T. T.; Yu, H.; Rondinelli, J. M.; Poeppelmeier, K. R.; Halasyamani, P. S. Deep Ultraviolet Nonlinear Optical Materials. Chem. Mater. 2016, 28, 5238–5258. (10) Bashir, D.; Zhang, B.; Lei, B.-H.; Yang, Z.; Lee, M.-H.; Pan, S. DFT Based Theoretical Study About the Contribution of Fluorine to Nonlinear Optical Properties in Borate Fluoride Crystals. Cryst. Growth Des. 2016, 16, 5067–5073.


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'Anti-zeolite' Structure

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