硼酸盐体系荧光粉

Syntheses, Crystal and Electronic Structures, and Linear Optics of LiMBO3 (M ) Sr, Ba) Orthoborates W.-D. Cheng,* H. Zhang, Q.-S. Lin, F.-K. Zheng, and J.-T. Chen Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002, People’s Republic of China Received October 10, 2000. Revised Manuscript Received March 7, 2001 The syntheses, crystal and electronic structures, and linear optical properties of the new orthoborates LiMBO3 (M ) Sr, Ba) are reported here. These compounds, which crystallize in the monoclinic space group P21/n with cell dimensions a ) 6.476(2), b ) 6.684(3), c ) 6.843(3) Å, â ) 109.41(3)°, and Z ) 4 for M ) Sr, a ) 6.372(1), b ) 7.022(3), c ) 7.058(1) Å, â ) 113.89(1)°, and Z ) 4 for M ) Ba, are modeled in terms of the cluster units (LiMBO3)2. The calculated electronic structures show that the top of the valence band consists of mostly the O 2p orbitals and the bottom of the conduction band consists of cationic orbitals. The dynamic refractive indices of these orthoborates are obtained in the framework of the INDO/ SCI approximation together with the “sum-over-states” method. It is found that the refractive index is larger for LiSrBO3 than for LiBaBO3 and the charge transfer from O2- anionic orbitals to cationic orbitals appears to provide significant contribution to the linear polarizability of these compounds. 1. Introduction Research efforts have been directed at solid-state borates due to a variety of physical and chemical features exhibited by these compounds, such as lumi- nescence of doped borates SrB4O71,2 and X2Z(BO3)2 (X ) Ba, Sr, Ca; Z ) Ca, Mg),3 nonlinear optical properties4 of borate compounds including the (B3O6)3- group, and catalytic activity of the zinc-copper borate ZnCu2(BO3)2.5 Among nonlinear optical crystals, solid- state borates have emerged as the pre-eminent materi- als for high-power applications, and now, they are being utilized in the manufacture of components in essentially all complex microprocessor-based devices from PCs to cellular phones. An understanding of the relationship between microscopic structure and macroscopic proper- ties has been recognized as an important aid in the design and improvement of materials.6 Cheng et al. have calculated the electronic structures and nonlinear opti- cal coefficients of MB6O10 (M ) Cs2, Li2, CsLi) with an aim to understand the electronic origin of their optical susceptibility.7 Keszler and co-workers have systemati- cally investigated the syntheses, structures, and proper- ties of alkali-metal borates searching for a trend in the structure-property relationship.8,9 In this study, we first report the syntheses and single- crystal structural determinations for mixed alkaline and alkaline-earth metal orthoborates LiMBO3 (M ) Sr, Ba), and we find that the structural types of our obtained compounds are different from those of the LiABO3 orthoborates (A ) Mg, Mn, Co, Zn, and Cd) reported in references.10-13 Belkebir and co-workers have calculated the crystallographic cell of the phases of LiZnBO3 obtained by solid-state reaction without melting by indexing their X-ray power diffraction patterns, and the two structures found are monoclinic probably with the same Li-Zn cationic disorder as evidenced by vibra- tional behavior.13 Piffard and co-workers have deter- mined the structure of LiCoBO3 to be of the space group C2/c and indicated that the Co and Zn cations have occupied similar positions within the trigonal bipyr- amids above and below the center plane in LiCoBO3 and LiZnBO3, respectively.10 Then, we use a cluster unit representing the crystalline orthoborate lattice to cal- culate the electronic structure and refractive indices of mixed alkaline-earth metal and lithium orthoborates LiMBO3 (M ) Sr, Ba). In this way, we obtain a means to examine structural and compositional contributions to linear optical properties. The calculated results show a larger refractive index for LiSrBO3 than for LiBaBO3 and indicate that the charge transfers from O 2p orbitals to cation valence orbitals have major contributions to the refractive indices of LiMBO3 (M ) Sr, Ba). * To whom correspondence should be addressed. (1) Blasse, G.; Dirksen, G. J.; Meijerink, A. Chem. Phys. Lett. 1990, 167, 41-44. (2) Meijerink, A.; Nuyten, J.; Blasse, G. J. Lumin. 1989, 44, 19- 31. (3) Verstegen, J. M. P. J. J. Electrochem. Soc. Solid-State Sci. Technol. 1974, 121, 1631-1633. (4) Chen, C.-Z.; Gao, D.-S.; Chen, C.-T. Acad. Thesis Conf. Cryst. Growth Mater. (China) 1979, B44, 107-111. (5) Zletz, A. U.S. Patent Application, 709, 790, March 11, 1985, Amoco Corp. (6) Munowitz, M.; Jarman, R. H.; Harrison, J. F. Chem. Mater. 1993, 5, 661-671; 1993, 5, 1257-1267. (7) Cheng, W.-D.; Chen, J.-T.; Lin, Q.-S.; Zhang, Q.-E.; Lu, J.-X. Phys. Rev. B 1999, 60, 11747-11754. (8) Akella, A.; Keszler, D. A. J. Solid State Chem. 1995, 120, 74- 79. (9) Smith, R. W.; Keszler, D. A. J. Solid State Chem. 1997, 129, 184-188. (10) Piffard, Y.; Rangan, K. K.; An, Y.; Guyomard, D.; Tournoux, M. Acta Crystallogr. 1998, C54, 1561-1563. (11) Norrestam, R. Z. Kristallogr. 1989, 187, 103-110. (12) Sokolova, E. V.; Simonov, M. A.; Belov, N. V. Z. Kristallogr. 1980, 25, 1285-1286. (13) Belkebir, A.; Tarte, P.; Rulmont, A.; Gilbert, B. New J. Chem. 1996, 20, 311-316. 1841Chem. Mater. 2001, 13, 1841-1847 10.1021/cm000808i CCC: $20.00 © 2001 American Chemical Society Published on Web 04/27/2001 2. Experimental and Computational Procedures 2.1. Syntheses and Crystal Growths. 2.1.1. LiSrBO3. A mixture containing appropriate amounts of LiCO3 (Chemical pure), SrCO3 (Chemical pure), and H3BO3 (Analytical reagent) was ground into fine powder in a mortar of agate. This mixture was heated to 450 °C in a platinum crucible and kept at this temperature for 4 h, followed by heating at 840 °C for 10 h and at 910 °C for 24 h. The mixture, then, was cooled from 910 to 600 °C at a rate of 2.7 °C h-1. It finally was quenched to room temperature. A few colorless pillar crystals were found from the melt of the mixture. 2.1.2. LiBaBO3. A stoichiometric mixture of LiCO3 (0.74 g, chemical pure), BaF2 (1. 71 g, analytical reagent), SrCO3 (1.48 g, chemical pure), and H3BO3 (1.24 g, analytical reagent) was ground into fine powder in a mortar of agate, then heated to 450 °C in a platinum crucible, and kept at this temperature for 4 h, followed by heating at 840 °C for 24 h and at 910 °C for 24 h, then cooled from 910 to 800 °C at a rate of 1.0 °C h-1 and from 800 to 600 °C at a rate of 4.0 °C h-1, and finally air-quenched to room temperature. A few colorless pillar crystals were found from the melt. 2.2. X-ray Determination. A single crystal of LiSrBO3 and LiBaBO3 with approximate dimensions 0.18 � 0.08 � 0.06 and 0.30 � 0.20 � 0.10 mm3 was selected for single-crystal X-ray diffraction, respectively. The diffraction data were collected on an Enraf-Nonius CAD4 diffractometer with graphite mono- chromator Mo KR radiation for these two crystals. Cell constants were obtained from least-squares refinement, using the setting angles of 25 reflections in the range 26° < 2ı < 48° for LiSrBO3 and 25 reflections in the range 22° < 2ı < 46° for LiBaBO3. The crystallographic parameters of these two crystals are listed in Table 1. The intensity data were collected at 273 K in the range -11 e h e 11, 0 e k e 12, and 0 e l e 12 for LiSrBO3 and the range 0 e h e 12, 0 e k e 13, and -13 e l e 13 for LiBaBO3, using the ö/2ı scan technique with a scan speed of 5°/min and a scan width of ¢ö ) (0.8 + 0.35 tan ı)°, respectively. The intensities of three standard reflec- tions were measured every 60 min, and the intensity decay was 0.6 and 4.7% for crystals LiSrBO3 and LiBaBO3, respec- tively. Lorentz and polarization corrections were applied to the data. The linear absorption coefficients are 14.3 and 135.3 mm-1 for these two crystals, respectively. An empirical absorp- tion correction based on a ª-scan was applied and the relative transmission coefficients ranged from 0.324 to 1.00 with an average value of 0.662 and from 0.484 to 0.996 with an average value of 0.740, respectively. The 1824 and 2301 reflections were used to measured with 2ımax ) 80°; 1223 and 1490 reflections with I > 3ó(I) were used in structural determination and refinement for LiSrBO3 and LiBaBO3 crystals, respec- tively. The structures of LiSrBO3 and LiBaBO3 were separately determined by the Shelx/PC and MolEN/PC programs. From the systematic absence of h0l: l ) 2n; 0k0: k ) 2n and from subsequent least-squares refinement, the space group of these two crystals was determined to be P21/n. Note that this space group is not a standard space group. However, it is often convenient and the angle of â will not become much larger than 90° when we prefer the space group P21/n in determining the crystal structures. The Sr and Ba atoms were located from the direct method; the remaining atoms were located in succeeding difference Fourier synthesis. The final full-matrix least-squares refinement for 56 and 55 variable parameters converged to R ) 4.43%, Rw ) 8.98% [in which w ) 1/(ó2(Fo2) + (aP)2 + bP); P ) (2Fc2 + Max(Fo2,0))/3], S ) 0.982, and (¢/ ó)max ) 0.0001 for LiSrBO3 and converged to R ) 5.51%, Rw ) 6.26% [in which w ) 1/(ó2(F) + (0.020F)2 + 1.0)], S ) 0.93, and (¢/ó)max ) 0.0001 for LiBaBO3. Neutral atomic scattering factors were taken from Cromer and Waber.14 The maximum and minimum peaks on the final different Fourier map are 1.43 and -1.00 e/Å3 and 5.57 and -1.72 e/Å3 for SrLiBO3 and BaLiBO3 crystals, respectively. The atom coordinates and thermal parameters are listed in Table 2. Here, it is noted that the BaF2 attempted as a flux was added into the mixture of SrCO3, LiCO3, and H3BO3, and the variation of temperature was controlled to try and obtain a new phase of LiSrBO3. However, the crystal of LiBaBO3 was found in the X-ray structural determination and the atom of Sr was not present in this crystal. We made a trial structure including the Sr atoms in the LiBaBO3 during the solution of crystal structure by the Shelxtl/PC program. The result showed a large value of R and unreasonable coordinations of metal atoms. Accord- ingly, we believe no Sr atoms are in this crystal structure. 2.3. Computational Details. The wave functions and energies obtained from electronic structural calculations were employed to compute the polarizabilities of clusters, and the electronic structural calculations of the clusters were based on an all-valence-electron, semiempirical INDO self-consistent field (SCF) molecular orbital (MO) procedure with configura- tion interaction (CI) modified by Zerner and co-workers.15-18 There are the one-center core integral Uíí, resonance integral âíî, two-electron integral çíî, overlap integral Síî, and density matrix element Píî in the matrix element of the Fock operator under the INDO approximation. The INDO model as employed herein included all one-center two-electron integrals and two- center two-electron integrals çíî. The one-center two-electron integrals çíí were chosen from the Pariser approximation, çíí ) F0(íí) ) IPí - EAí, and the two-center two electron integrals were calculated using the Mataga-Nishimoto formula, çíî ) 1.2/[RAB + 2.4/(çíí + çîî)] in the spectroscopic version of the INDO method. The Slater orbital exponents œ and the other calculating parameters are listed in Table 3. The molecular orbital calculations were performed by the restricted Hartree- Fock method. The ground state was constructed as a single (14) Cromer, D. T.; Waber, J. T. In International Table for X-ray Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV, Table 2.2A, p 71. (15) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21- 54. (16) Zerner, M. C.; Lovw, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T. J. Am. Chem. Soc. 1980, 102, 589-599. (17) Anderson, W. P.; Edwards, E. D.; Zerner, M. C. Inorg. Chem. 1986, 25, 2728-2732. (18) Anderson, W. P.; Cundari, T. R.; Zerner, M. C. Int. J. Quantum Chem. 1991, 39, 31-45. Table 1. Crystal Parameters formula LiSrBO3 LiBaBO3 fw 153.37 203.09 space group P2(1)/n P2(1)/n a (Å) 6.4800(13) 6.372(1) b (Å) 6.6800(13) 7.022(3) c (Å) 6.8400(14) 7.058(1) â (deg) 109.41(3) 113.89(1) V (Å3) 279.25(10) 288.7(2) Z 4 4 Dcalc (mg/m3) 3.648 4.67 R 0.0443 0.0551 Rw 0.0898 0.0626 S 0.982 0.93 Table 2. Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters (Å) atom x y Z Ueqa LiSrBO3 Sr 0.20197(5) 0.12378(5) 0.86760(5) 0.00811(9) O(1) 0.1560(5) 0.1105(4) 1.2178(4) 0.0130(5) O(2) 0.1073(5) 0.2993(4) 0.4940(4) 0.0109(4) O(3) 0.1963(5) 0.4897(4) 0.9367(4) 0.0109(5) B 0.1886(6) 0.1326(6) 0.4270(6) 0.0084(5) Li 0.1002(13) 0.4086(10) 1.1930(11) 0.0136(13) LiBaBO3 Ba 0.33502(8) 0.13980(7) 0.15907(6) 0.01093(6) O(1) 0.3793(9) 0.3180(7) 0.5164(8) 0.0085(9) O(2) 0.291(1) 0.1598(9) 0.7740(8) 0.017(1) O(3) 0.2199(9) 0.0011(7) 0.4569(7) 0.0074(9) B 0.296(1) 0.160(1) 0.579(1) 0.007(1) Li 0.896(2) 0.071(2) 0.298(2) 0.009(3) a Ueq ) 1/3∑i∑jUijRi*Rj*aiaj. 1842 Chem. Mater., Vol. 13, No. 5, 2001 Cheng et al. determinant from the Hartree-Fock SCF calculated results. Only single-substituted determinants relative to the ground state configuration were considered and only singlet spin- adapted configurations needed to be included in the CI calculations. The ground state and all excited states had the multiplicity of 1. The electron was promoted from the 13 highest occupied orbitals to the 13 lowest unoccupied orbitals and the configuration space was constructed by these 26 active orbitals. The wave functions and energy eigenvalues of the excited states were determined by solving the secular equation relating to configuration coefficients. The dipole and transition moment matrix elements were expressed as a sum of one- electron integrals. The cluster units (LiSrBO3)2 and (LiBaBO3)2 representing extended oxide crystals LiSrBO3 and LiBaBO3 were selected for calculations, as shown in Figure 1. The coordinate systems are defined in the calculations. The coordinate center is located at the center of the ring [M1M2O3O5]. The x axis is defined to be parallel to the connecting line between the O3 and O5 atoms, and the y axis is lain down the plane constructed by three atoms, O3-O5-M1 in Figure 1. The z axis is defined as the right-hand rule of the coordinate system for these two clusters, respectively. Theoretical calculations of electronic structures and polarizabilities are based on the crystallo- graphic structural data for these two clusters. The tensor components of the polarizability R(ö) with fre- quency dependence for the clusters (LiSrBO3)2 and (LiBaBO3)2 were calculated by the sum-over-states (SOS) method as follows: 3. Results and Discussions 3.1. Crystal Structures. Drawings of the contents of the unit cells of compounds LiMBO3 (M ) Sr, Ba) are shown in Figure 2. It is found that LiMBO3 structures can be constructed from a stack of [MO] and [LiO] layers along the [101h] direction and B atoms localized in adjacent layers as a bridging role. In the [LiO] layers, the adjacent polyhedrons constructed from the LiO5 form dimers by sharing edges, and each dimer connects with the four adjacent dimers by the four O atoms to form two-dimensional sheets along the ac diagonal plane, as shown in Figure 3. The Li atoms are coordinated by five O atoms and the LiO5 polyhedron is a distorted trigonal bipyramid. For the LiSrBO3 crystal, the Li-O distances vary from 1.955(7) to 2.169(8) Å with an average value of 2.056 Å, where the long bond lengths of Li-O are in the axial direction of the trigonal bipyramid. An average bond length of the Li-O in the LiBaBO3 crystal is close to that of the LiSrBO3 one. The B-O distances vary from 1.369(4) to 1.385(6) Å with an average value of 1.377 Å and the O-B-O angles are between 118.3(7)° and 122.6(7)°. These values are normal in a [BO3] plane of LiMBO3 (M ) Sr, Ba). Table 4 lists the interatomic distances and angles of LiMBO3. There are different coordinations Table 3. INDO/S Model Parameters parameter B O Li Sr Ba œns,np (Å-1) 1.300 2.275 0.650 1.214 1.263 œ(n-1)d (Å-1) 2.058 2.658 -Ins (eV) 14.05 32.90 5.41 5.84 5.21 -Inp (eV) 8.70 17.28 3.61 3.76 3.43 -I(n-1)d (eV) 3.66 3.22 -âns,np (eV) 17.00 34.20 9.00 1.88 2.66 -â(n-1)d (eV) 10.05 12.50 çns,np (eV) 8.68 13.00 4.57 3.75 4.68 ç(n-1)dd (eV) 5.31 5.19 ç(n-1)dns,np 4.53 3.79 Figure 1. Selected cluster unit model of (LiMBO3). Rij(ö) ) 1/p“mígm iímg j[(ömg - öp) -1 + (ömg + öp) -1] (1) Figure 2. Crystal structure stacked from the [MO] and [101] direction in [LiMBO3]. The M-O bonds are omitted for clarity. Figure 3. Structure of the [LiO] layer along the ac diagonal plane in [LiMBO3]. Properties of LiMBO3 (M ) Sr, Ba) Orthoborates Chem. Mater., Vol. 13, No. 5, 2001 1843 between the Sr and Ba atoms in LiMBO3 (M ) Sr, Ba) crystals. The Sr atoms are coordinated by seven O atoms, and the SrO7 polyhedron may be described as a mono-capped distorted trigonal prism. The Sr-O dis- tances vary from 2.495(3) to 2.692(3) Å with an average value of 2.594 Å, which compared very well to the expected value 2.590 Å calculated from the crystal radii for the seven-coordinate Sr2+ ion.19 In the [SrO] layers, the Sr atoms through sharing O-O edges extend along the b direction to form chains, and the adjacent chains link together by O atoms to form puckering sheets parallel to the ac diagonal plane, as shown in Figure 4. These sheets are connected along the [101h] direction to form the three-dimensional framework by the bridging O atoms of the [LiO] layers. For the LiBaBO3 crystal, however, the Ba atoms are coordinated by nine O atoms and the BaO9 polyhedron is described as a mono-capped distorted square antiprism. The Ba-O distances vary from 2.622(6) to 3.185(5) Å with an average value of 2.813 Å, which compared well to the expected value 2.850 Å calculated from the crystal radii for the nine- coordinate Ba2+ ion.19 In the [BaO] layers, the Ba atoms through sharing the planes of three oxygen atoms extend along the ac diagonal direction to form chains, and the adjacent chains link together by the two oxygen atoms to form puckering sheets in the b direction, as shown in Figure 5. These sheets are connected along the [101h] direction to form the three-dimensional frame- work by the bridging O atoms of the [LiO] layers. 3.2. Electronic Structures. The energy bands for LiSrBO3 and LiBaBO3 are calculated in terms of MO procedures. Note that the top of the valence band of LiSrBO3 and LiBaBO3 is at -7.07 and -5.65 eV, respectively, and is taken to be zero and regarded as a reference in the following discussion. The dominant contribution to the lower valence band (i.e., -31.5 to -23.5 eV for LiSrBO3, -28.5 to -24.5 eV for LiBaBO3) comes from O 2s orbitals along with a relatively small (<8%) contribution from B 2s orbitals. It is therefore assigned as an s valence band. The upper(19) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. Table 4. Interatomic Distances (Å) and Bond Angles (deg) Interatomic Distances (Å) Bond Angles (deg) Interatomic Distances (Å) Bond Angles (deg) a. LiSrBO3 b. LiBaBO3 Sr-O(1) 2.6928(7) O(1)-Sr-O(1) 79.57(2) Ba-O(1) 2.726(6) O(1)-Ba-O(1) 131.2(2) Sr-O(1) 2.513(1) O(1)-Sr-O(2) 98.01(2) Ba-O(1) 2.670(6) O(1)-Ba-O(1) 89.3(2) Sr-O(2) 2.689(1) O(1)-Sr-O(2) 82.02(3) Ba-O(1) 2.677(5) O(1)-Ba-O(2) 149.6(2) Sr-O(2) 2.5171(9) O(1)-Sr-O(2) 155.856(9) Ba-O(2) 2.622(6) O(1)-Ba-O(2) 112.9(1) Sr-O(2) 2.5297(8) O(1)-Sr-O(3) 123.03(2) Ba-O(2) 3.067(8) O(1)-Ba-O(2) 79.1(2) Sr-O(3) 2.495(1) O(1)-Sr-O(3) 93.19(2) Ba-O(2) 3.030(8) O(1)-Ba-O(3) 53.0(2) Sr-O(3) 2.5469(9) O(1)-Sr-O(2) 149.26(1) Ba-O(3) 2.675(6) O(1)-Ba-O(3) 78.3(2) Li-O(1) 2.0219(9) O(1)-Sr-O(2) 79.05(1) Ba-O(3) 2.646(5) O(1)-Ba-O(3) 68.4(2) Li-O(1) 2.0131(5) O(1)-Sr-O(2) 96.83(2) Ba-O(3) 3.185(5) O(1)-Ba-O(1) 69.3(2) Li-O(2) 2.1664(9) O(1)-Sr-O(3) 80