- Email: [email protected]
Syntheses and characterization of two alkali-metal zinc borates, α-LiZnBO3 and Li0.48Na0.52ZnBO3
Solid State Sciences 11 (2009) 2086–2092
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Syntheses and characterization of two alkali-metal zinc borates, a-LiZnBO3 and Li0.48Na0.52ZnBO3 Xuean Chen a, *, Chunyan Yang a, Xinan Chang a, Hegui Zang a, Weiqiang Xiao b a b
College of Materials Science and Engineering, Beijing University of Technology, Ping Le Yuan 100, Beijing 100124, P.R. China Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2009 Received in revised form 3 August 2009 Accepted 30 August 2009 Available online 6 September 2009
Two alkali-metal zinc borates, a-LiZnBO3 and Li0.48Na0.52ZnBO3, have been prepared by solid-state reactions below 850 C. Single-crystal XRD analyses showed that the former crystallizes in the C2/c group with a ¼ 8.746(2) Å, b ¼ 5.091(1) Å, c ¼ 6.129(1) Å, b ¼ 118.75(3) , Z ¼ 4 and the latter in the group P1 with a ¼ 5.054(1) Å, b ¼ 6.113(1) Å, c ¼ 8.045(2) Å, a ¼ 75.73(2) , b ¼ 89.87(3) , g ¼ 89.86(3) , Z ¼ 4. The crystal structure of a-LiZnBO3 is composed of tetrahedral ZnO4 and triangular BO3 groups that are arranged into a three-dimensional (3D) network by sharing O vertices. Li0.48Na0.52ZnBO3 is also characterized by a 3D framework, but built up from corner-sharing ZnO4 tetratahedra, ZnO5 trigonal bipyramids, and BO3 triangles. Both structures afford open channels that are occupied by alkali-metal cations. The IR spectra further confirmed the presence of BO3 groups and UV-vis diffuse reflectance spectra showed band gaps of about 3.10 and 2.95 eV for the Li and Li/Na compounds, respectively. Band structure calculations indicated that both compounds are direct semiconductors with the calculated band gaps close to the observed ones. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: LiZnBO3 Li0.48Na0.52ZnBO3 Borate Synthesis Crystal Structure
1. Introduction Borate materials are of considerable interest because they show a great variety of physical properties ranging from nonlinear optical (NLO), ferroelectric, piezoelectric to semiconducting behaviors and in addition, a boron atom may adopt triangular or tetrahedral oxygen coordination, the BO3 and BO4 groups may be further linked via common oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets and networks, leading to the rich structural chemistry [1,2]. Zinc-containing borates have been studied due to their potential value as catalysis and optical materials [3–5]. In the ternary system of Li2O–ZnO–B2O3, two LiZnBO3 polymorphs have been previously proposed: one prepared by solidstate reaction (we called a-LiZnBO3 here) and another obtained from hydrothermal synthesis (we called b-LiZnBO3 in this work). 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 they found evidence that the structure of LiZnBO3 depends on the preparation method, and the two LiZnBO3 structures found are monoclinic probably with the same Li–Zn cationic disorder as
* Corresponding author. Tel.: þ86 10 62546928. E-mail address: [email protected] (X. Chen). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.08.024
evidenced by vibrational behavior [6]. However, no atomic coordinates were given in that work. Bodareva et al. have determined the crystal structure of b-LiZnBO3 and found this phase to really have monoclinic symmetry [7], while Ki-Seog Chang has refined the a-LiZnBO3 structure in the triclinic space group P1, with Li and Zn atoms disordered over two very close tetrahedral sites [8]. In an attempt to synthesize non-centrosymmetric compounds that are potentially applicable as NLO materials, we have obtained single crystals of a-LiZnBO3. Our X-ray structural analysis established that this phase actually crystallizes in a monoclinic group C2/c instead of the triclinic group P1. Moreover, we have also obtained a new phase, Li0.48Na0.52ZnBO3, which is the first quaternary borate to be discovered in the Li2O–Na2O–ZnO–B2O3 system and crystallizes in a new unique structure type never observed for any of the known borates. Herein we report their syntheses, crystal structures, IR and UV-vis diffuse reflectance spectra as well as electronic structure calculations for the first time. 2. Experimental 2.1. Sample preparation and general characterization The title compounds were synthesized by employing conventional solid-state reaction methods. All reagents were of analytical grade. For the preparation of a-LiZnBO3 crystals, a powder mixture
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092 Table 1 Crystallographic data for a-LiZnBO3 and Li0.48Na0.52ZnBO3. Formula
LiZnBO3
Li0.48Na0.52ZnBO3
Space group
C2/c (No.15)
P1 (No.2)
a (Å) b (Å) c (Å)
8.746(2) 5.091(1) 6.129(1) 90 118.75(3) 90 239.26(11), 4 3.640 9.975 64.98 433 406 33 1.190 0.0483/0.1273 0.0517/0.1302
5.054(1) 6.113(1) 8.045(2) 75.73(2) 89.87(3) 89.86(3) 240.88(9), 4 3.847 10.006 65 1746 1672 112 1.120 0.0646/0.1950 0.0658/0.1964
a b g V (Å3), Z dcalc (g/cm3) m (mm1) 2qmax ( ) Unique reflection Observed [I 2s(I)] No. of variables GOF on F2o R1/wR2 [I 2s(I)] R1/wR2 (all data)
of 1.407 g Li2CO3, 3.103 g ZnO, 2.358 g H3BO3, and 2.435 g Li2B4O7 (the Li2CO3/ZnO/H3BO3/Li2B4O7 molar ratio ¼ 4:8:8:3) was transferred to a Pt crucible. The sample was gradually heated to 830 C, where it was kept for three days, then cooled down to 820 C at a rate of 0.5 C/h, and further to 320 C at 5 C/h, followed by cooling to room temperature at a rate of 20 C/h. The colorless, plate-like crystals of a-LiZnBO3 with dimensions of up to 1.2 1.0 0.3 mm3 were embedded in a lithium borate matrix. Several small crystals were recovered and mechanically separated from the reaction product. Li0.48Na0.52ZnBO3 crystals were synthesized from a reaction containing 2.401 g Li2CO3, 1.765 g ZnO, 4.023 g H3BO3, and 4.965 g Na2B4O7$10H2O (the Li2CO3/ZnO/H3BO3/Na2B4O7 molar ratio ¼ 15: 10:30:6). The sample was placed in a Pt crucible and gradually heated to 800 C, where it was kept for one week, then cooled down to 700 C at a rate of 1 C/h. Finally, the sample was cooled to room temperature at a rate of 20 C/h and the furnace was switched off. The colorless, plate-like crystals of Li0.48Na0.52ZnBO3 were found in the solidified melt. The single-phase polycrystalline sample of a-LiZnBO3 was prepared by heating a stoichiometric mixture of LiBO2$8H2O and ZnO at 600 C for one month with several intermediate re-mixings, while the single-phase polycrystalline sample of Li0.48Na0.52ZnBO3 was prepared in the same way, except that the starting material is a stoichiometric mixture of Li2CO3, Na2CO3, ZnO, and H3BO3. X-ray powder diffraction data were collected by using the monochromatized Cu Ka radiation of a Bruker D8 ADVANCE diffractometer. Infrared spectra were recorded from 4000 to 400 cm1 on a Perkin Elmer 1730 FT-IR spectrometer from KBr pellets. Optical diffuse reflectance spectra were measured at room temperature with a Shimadzu UV-3101PC double-beam, doublemonochromator spectrophotometer. Data were collected in the wavelength range 200–800 nm. BaSO4 powder was used as a standard (100% reflectance). A similar procedure as previously described [9,10] was used to collect and convert the data using the Kubelka– Munk function F (R) ¼ (1R)2/2R, where R is the reflectance. The minima in the second-derivative curves of the Kubelka–Munk function are taken as the position of the absorption bands. 2.2. Structure determination Single-crystal X-ray intensity data were collected at room temperature (298 K) on an automated Rigaku AFC7R four-circle diffractometer using monochromatized Mo Ka radiation (l ¼ 0.71073 Å). The data were corrected for Lorentz and polarization
2087
effects, and for absorption by empirical method based on j-scan data. The crystal structures were solved by direct methods and refined in SHELX-97 system [11] by full-matrix least-squares methods on Fo2. For a-LiZnBO3, the refinement of 33 parameters with 406 observed reflections [I 2s(I)] resulted in the residuals of R1/ wR2 ¼ 0.0483/0.1273. The reliability factors for Li0.48Na0.52ZnBO3 converged to R1/wR2 ¼ 0.0646/0.1950 for 1672 observed reflections and 112 variables. The final difference electron density maps were featureless in both cases, with the highest electron density less than 1.54 eÅ3. Details of crystal parameters, data collection and structure refinements are given in Table 1 and the atomic coordinates and the equivalent isotropic displacement parameters are summarized in Table 2. Structural refinements of a-LiZnBO3 revealed that lithium and zinc atoms occupy two very close positions [Li1–Zn1 ¼ 0.631(12) Å] with occupation factors of 0.5 (see Table 2). The Liþ/Zn2þ disorder is not surprising due to their very similar coordination geometries as well as similar Li–O and Zn–O bond lengths. A similar situation has also been previously observed, e. g., in b-LiZnBO3 [7], KLiZn3O4 [12], and NaLiZnO2 [13]. All of our attempts to refine the structure in lower-symmetry space groups with an ordered distribution of Li and Zn atoms have been unsuccessful. An examination of the a-LiZnBO3 crystal with a slower scan rate and lower intensity limitation on a Rigaku AFC7R four-circle diffractometer did not indicate symmetry lower than monoclinic or a larger unit cell that would allow Li/Zn ordering. Therefore, the Liþ/Zn2þ disorder model was finally assumed. This model has also been confirmed by the powder diffraction results (Fig. 1). Note that in the a-LiZnBO3 structure reported by Ki-Seog Chang [8], although the low symmetry space group P1 was used, Li and Zn atoms are still disordered. Positional parameters in that work were checked by us using the program MISSYM [14], potential additional symmetry was found and the space group C2/c was suggested, indicating that the previous space group assignment is wrong. Our present refinements give the correct structural data for this compound. During the structural refinements of Li0.48Na0.52ZnBO3, Zn, B, and O atoms are well resolved, while Li and Na atoms were found to occupy two remaining atomic sites. Refinements of atomic occupancy parameters for these two positions gave the compositions
Table 2 Atomic coordinates and equivalent isotropic displacement parameters (Å2) for a-LiZnBO3 and Li0.48Na0.52ZnBO3. Atoms
X
Y
Z
Ueq
0.3561(14) 0.31550(10) 0 0.1513(3) 0
0.592(2) 0.56910(15) 0.4199(8) 0.5553(4) 0.1518(7)
0.816(2) 0.7010(2) 1/4 0.3054(4) 1/4
0.010(2) 0.0203(4) 0.0116(8) 0.0136(5) 0.0343(12)
Li0.48Na0.52ZnBO3 M1 0.1672(8) M2 0.1624(10) Zn1 0.65797(11) Zn2 0.32732(14) B1 0.1711(10) B2 0.6640(10) O1 0.3036(7) O2 0.8999(7) O3 0.7007(7) O4 0.7995(7) O5 0.2055(7) O6 0.5983(7)
0.7216(8) 1.0048(12) 0.46336(9) 0.83148(17) 0.3364(8) 0.8364(8) 0.5084(6) 0.3201(6) 0.8270(7) 0.7303(7) 0.9997(6) 0.2126(6)
0.4018(6) 0.2479(11) 0.28454(7) 0.08311(10) 0.0871(6) 0.5782(6) 0.1969(5) 0.1005(5) 0.0369(5) 0.4330(5) 0.3016(5) 0.4006(5)
0.0185(14) 0.025(2) 0.0091(2) 0.0193(3) 0.0058(8) 0.0057(8) 0.0084(6) 0.0099(8) 0.0121(9) 0.0133(9) 0.0083(8) 0.0107(8)
a-LiZnBO3 Li1 Zn1 B1 O1 O2
Note: For a-LiZnBO3, occupation factors of Li1 and Zn1 are 1/2; For Li0.48Na0.52ZnBO3, M1 and M2 have the compositions Li0.53(2)Na0.47(2) and Li0.42(3)Na0.58(3), respectively. Ueq is defined as one third of the trace of the orthogonalized U tensor.
2088
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
Fig. 1. XRD patterns of a-LiZnBO3 and Li0.48Na0.52ZnBO3 observed from powder polycrystalline sample (b, d) and calculated from the single-crystal data (a, c).
Li0.53(2)Na0.47(2) and Li0.42(3)Na0.58(3), respectively, which leads to the final formula Li0.48Na0.52ZnBO3. 2.3. Electronic structure calculations Density of states (DOS) and band calculations were performed using a first principle plane-wave pseudopotential technique based on density functional theory (DFT) with CASTEP code [15] distributed inside a computational commercial pack [16]. The ion–electron interaction was modeled by an ultrasoft nonlocal pseudopotential [17]. The density functional was treated by the generalized gradient approximation (GGA) with the exchangecorrelation potential parametrization (PBE) of Perdew–Burke– Ernzerhof type [18]. Pseudoatomic calculations were performed for B 2s2 2 p1, O 2s2 2p4, Zn 3d10 4s2, Li 1s2 2s1, and Na 2s2 2p6 3s1. The number of plane-waves included into the basis was determined by a cutoff energy Ec of 330 eV. Twelve empty bands were included to reach the convergence of calculations. The calculating parameters and convergent criterions were set by the default values of CASTEP code [15].
Fig. 1 shows the observed powder XRD patterns of a-LiZnBO3 and Li0.48Na0.52ZnBO3, together with those calculated from the single crystal data for comparison. It is clear that the observed XRD patterns are in agreement with the corresponding theoretical ones, further confirming our structural models. The slight intensity difference between the two patterns is believed to be due to the existence of a small amount of unreacted ZnO impurities or caused by preferred orientation of the powder sample during collection of the experimental XRD data because both compounds show a platelike crystal growth habit. Moreover, the observed patterns are generally very broad, which may be associated with the Li/Zn and Li/Na cationic disorder in these two compounds. It is known that four normal vibrations with frequencies ns (symmetric stretching) at 850–960 cm1, g (out-of plane bending) at 650–800 cm1, nas (asymmetrical stretching) at 1100–1450 cm1, and d (in-plane bending) at 500–600 cm1 correspond to the [BO3]3 group, of which ns is the infrared non-active vibration [20]. In order to further confirm the coordination surroundings of B atoms in both structures, the infrared spectra were measured and shown in Fig. 2. It is clear that three sets of bands characteristic of the planar triangular BO3 group were indeed observed. They are the out-of plane bending modes (g) occurring in the range 676.9–720.2 (670.2–728.8) cm1, the antisymmetric stretch (nas) at about 1253.0 (1192.4–1273.4) cm1, and the in-plane mode (d) at about 509.3 (558.0) cm1 for Li0.48Na0.52ZnBO3 (a-LiZnBO3), respectively. The optical diffuse reflectance spectra of two compounds are shown in Fig. 3. It is observed that a-LiZnBO3 has no absorption above 400 nm, while below 260 nm this compound has strong absorption. Li0.48Na0.52ZnBO3 has no absorption above 420 nm. From the absorption edges of UV-vis diffuse reflectance spectra, the optical band gaps are estimated to be roughly 3.10 and 2.95 eV for the Li and Li/Na compounds, respectively. 3.2. Description of the structure
a-LiZnBO3 contains a three-dimensional (3D) framework of vertex-sharing ZnO4 tetrahedra and BO3 triangles. In this structure, two inversion-center related ZnO4 tetrahedra are linked together by
3. Results and discussion 3.1. Synthesis and general characterization We have succeeded in obtaining single crystals of a-LiZnBO3 and Li0.48Na0.52ZnBO3. It is natural to be think if it is possible to prepare the corresponding Na analog. In the ternary Na2O–ZnO–B2O3 system, a hypothetical compound, ’’NaZnBO3’’, has been tried by us via solid-state reactions of a stoichiometric mixture of Na2CO3, ZnO, and H3BO3 powders at 600 C for three weeks with several intermediate re-mixings. Unfortunately, no new phase except for NaBO2 [19] along with the unreacted ZnO has been obtained. Our attempts to prepare ‘‘Li0.50K0.50ZnBO3’’ with a synthesis procedure similar to that was used for obtaining Li0.48Na0.52ZnBO3 crystals have also been unsuccessful. The reaction again resulted in a-LiZnBO3 crystals, judged from cell dimensions measured on a Rigaku AFC7R four-circle diffractometer.
Fig. 2. Infrared spectra of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
2089
Table 3 Selected bond lengths (Å) and angles ( ) for a-LiZnBO3 and Li0.48Na0.52ZnBO3.
a-LiZnBO3
Fig. 3. Optical absorption spectra of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
sharing one edge (O1.O1) to form a Zn2O6 dimer. Each Zn2O6 dimer is linked to six others through sharing O vertices to form a 3D Zn–O framework. Boron atoms are incorporated into the triangular hollows of oxygen atoms within this network to strengthen the structure. The 3D framework affords six-edge channels that are occupied by Li atoms, as displayed in Fig. 4. Each Liþ ion is strongly bonded to three O atoms at distances of 1.912(11)–1.944(11) Å and also weakly bonded to two more O atoms at distances of 2.350(13)–2.767(13) Å (Table 3). Taking all these five bonds into account, the concept of bond valence [21,22] allows us to calculate a bond valence sum (BVS) equal to 0.98 for Li1 which proves that the long bonds indeed participate in the metal coordination scheme. The five-fold coordination around Li1 can be described as a distorted trigonal bipyramid. The similar distorted Li coordination geometries have also been observed in Li3In(BO3)2 [23] and Sr2LiInB4O10 [24]. Each Zn atom is coordinated to four O atoms forming a distorted tetrahedral configuration with Zn–O distances of 1.869(2)–2.144(3) Å, which are in the similar range as reported for the four-coordinated Zn2þ in Na3ZnB5O10 [1.935(2)– 1.982(2) Å] and Ba2Zn(BO3)2 [1.95(3)–2.03(4) Å] [25,26]. Bond valence analysis produced a reasonable value of 2.01 for Zn in this environment.
Fig. 4. The crystal structure of a-LiZnBO3 projected along the b-axis. Li atoms: black circles; B atoms: circles with parallel lines; ZnO4 groups: tetrahedra filled by parallel dashed lines.
Li1–O1 Li1–O1 Li1–O2 Li1–O2 Li1–O1 Average Zn1–O2 Zn1–O1 Zn1–O1 Zn1–O1 Average B1–O2 B1–O1 2 Average
1.912(11) 1.935(12) 1.944(11) 2.350(13) 2.767(13) 2.182 1.869(2) 1.937(2) 1.938(2) 2.144(3) 1.972 1.365(6) 1.382(3) 1.376
Li1–O1 Zn1–O2
3.173(11) 2.9798(15)
Li0.48Na0.52ZnBO3 M1–O4 M1–O6 M1–O1 M1–O5 Average M2–O3 M2–O5 M2–O6 M2–O4 M2–O2 Average Zn1–O4 Zn1–O2 Zn1–O1 Zn1–O6 Average Zn2–O5 Zn2–O2 Zn2–O3 Zn2–O1 Zn2–O3 Average B1–O1 B1–O2 B1–O3 Average B2–O4 B2–O5 B2–O6 Average
1.875(5) 1.942(6) 1.957(5) 2.063(6) 1.959 1.905(7) 1.910(6) 1.954(6) 2.512(12) 2.520(12) 2.160 1.908(4) 1.954(4) 1.967(4) 2.005(4) 1.958 1.907(4) 1.919(4) 1.923(4) 2.376(4) 2.511(5) 2.127 1.370(6) 1.381(6) 1.386(6) 1.379 1.372(6) 1.377(6) 1.380(6) 1.376
M1–O5 M2–O5 Zn1–O6 Zn2–O3
3.185(6) 3.226(6) 3.090(4) 3.187(4)
O1–B1–O2
119.6(4)
O2–B1–O1 2 O1–B1–O1 O1–Li1–O1 O1–Li1–O2 O1–Li1–O2 O1–Li1–O2 O1–Li1–O2 O2–Li1–O2 O1–Li1–O1 O1–Li1–O1 O2–Li1–O1 O2–Li1–O1 O2–Zn1–O1 O2–Zn1–O1 O1–Zn1–O1 O2–Zn1–O1 O1–Zn1–O1 O1–Zn1–O1
119.92(18) 120.2(4) 119.5(6) 115.6(6) 114.6(6) 94.2(5) 102.5(5) 105.7(5) 81.3(4) 75.1(4) 81.5(4) 172.7(5) 118.10(12) 118.02(11) 118.12(9) 102.64(8) 91.99(9) 99.39(10)
O1–B1–O3 O2–B1–O3 O4–B2–O5 O4–B2–O6 O5–B2–O6 O4–M1–O6 O4–M1–O1 O6–M1–O1 O4–M1–O5 O6–M1–O5 O1–M1–O5 O3–M2–O5 O3–M2–O6 O5–M2–O6 O3–M2–O4 O5–M2–O4 O6–M2–O4 O3–M2–O2 O5–M2–O2 O6–M2–O2 O4–M2–O2 O4–Zn1–O2 O4–Zn1–O1 O2–Zn1–O1 O4–Zn1–O6 O2–Zn1–O6 O1–Zn1–O6 O5–Zn2–O2 O5–Zn2–O3 O2–Zn2–O3 O5–Zn2–O1 O2–Zn2–O1 O3–Zn2–O1 O5–Zn2–O3 O2–Zn2–O3 O3–Zn2–O3 O1–Zn2–O3
122.5(4) 117.8(4) 117.9(4) 120.9(4) 121.1(4) 120.0(3) 117.0(3) 115.8(3) 98.6(3) 103.7(3) 94.5(2) 121.1(3) 118.4(3) 120.3(3) 100.5(4) 83.8(4) 90.2(4) 87.8(4) 92.2(4) 85.5(3) 171.7(3) 113.18(16) 113.09(16) 111.98(16) 113.56(17) 101.40(16) 102.60(16) 124.35(15) 118.33(17) 117.21(16) 86.45(15) 88.58(15) 98.81(16) 91.53(15) 87.79(15) 87.24(16) 173.87(12)
There is one crystallographically independent B atom, which is located on a crystallographic 2-fold axis, resulting in two sets of B–O bond lengths [1 1.365(6), and 2 1.382(3) Å] and two sets of O–B–O angles [2 119.92(18) and 1 120.2(4) ]. These geometric parameters compare well with those observed in the structures of Li3In(BO3)2 [23] and Pb2Zn(BO3)2 [27], where BO3 groups are also observed. Of the two unique O atoms, O2 lies on a crystallographically 2-fold axis, while O1 occupies a general position. Bond valence calculations gave a value of 2.96 for the B atom, consistent with its expected formal valence. As mentioned above, the a-LiZnBO3 structure has been previously refined by Ki-Seog Chang in the space group P1 [8], where the asymmetric unit contains eight independent atoms, i.e., 2Li,
2090
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
2Zn, 1B, and 3O. All these atoms occupy crystallographic general positions, and Li and Zn atoms are disordered with occupation factors of 0.5. This is in contrast to the situation in our refinements where the space group C2/c has been assigned and the asymmetric unit consists of five unique atoms including 1Li, 1Zn, 1B, and 2O, of which the B atom and one O atom lie on crystallographic 2-fold axes, and Li and Zn atoms are again disordered. In the previous refinements, Li–O, Zn–O, and B–O bond lengths are 1.90(1)–1.95(1), 1.863(3)–2.044(3), and 1.366(1)–1.391(5) Å, and O–Li–O, O–Zn–O, and O–B–O angles in the range 94.8(5)– 119.8(6), 92.6(1)–118.4(2), and 119.3(3)–120.6(4) , respectively, which are comparable to those in the present refinements (Table 3). The significant difference between our and previous refinements is the equivalent isotropic displacement parameter for the Li atom, which is reasonable with beq ¼ 0.8(2) Å2 in our case [equivalent to Ueq ¼ 0.010(2) Å2, Table 2], but a negative value or very small in the previous refinements [beq ¼ 0.4(8) Å2 for Li1 and 0.1(9) Å2 for Li2, respectively, Ref. [8]]. A comparison of a-LiZnBO3 with Li0.48Na0.52ZnBO3 indicates that the partial replacement of Li by Na leads to a volume per formula unit slightly different, that are 60.22(2) and 59.82(3) Å3 for Li0.48Na0.52ZnBO3 and a-LiZnBO3, respectively, associated to density values of 3.847 and 3.640 g/cm3, therefore, Li0.48Na0.52ZnBO3 exhibits a denser structure than a-LiZnBO3. Moreover, the substitution also results in a lowering of the space group symmetry from C2/c to P1. The other borate that is closely related to the title compounds in stoichiometry is b-LiZnBO3 [7], which was reported to crystallize in the monoclinic C2/c group, with the cell volume being about two times of that of the a-form [a ¼ 5.094(1) Å, b ¼ 8.806(3) Å, c ¼ 10.374(4) Å, b ¼ 91.09(3) , V ¼ 465.3(3) Å3, Z ¼ 8 in b-LiZnBO3 vs a ¼ 8.746(2) Å, b ¼ 5.091(1) Å, c ¼ 6.129(1) Å, b ¼ 118.75(3) , V ¼ 239.26(11) Å3, Z ¼ 4 in a-LiZnBO3]. The crystal structure of b-LiZnBO3 consists of a 3D ½ZnBO3 1 framework built up from ZnO5 trigonal bipyramids and BO3 groups. Each ZnO5 bipyramid shares two edges with adjacent bipyramids to form single chains running along the [101] direction. These chains are bridged by planar BO3 groups via corner-sharing to form the 3D ½ZnBO3 1 framework. Within this framework, the Li atoms occupy statistically two tetrahedral sites [Li1–Li2 ¼ 1.09(6) Å] sharing a face. Such pairs of tetrahedra share edges to form chains running along the [001] direction. The Zn cation occupies statistically two close positions [Zn1–Zn2 ¼ 0.242(4) Å] within the ZnO5 bipyramid, above and below the central plane. A comparison of a-LiZnBO3 with its b-form indicates that the latter has a denser structure, as reflected from density values (3.744 g/cm3 in b-LiZnBO3 vs 3.640 g/ cm3 in a-LiZnBO3). This is reasonable because b-LiZnBO3 was prepared by hydrothermal synthesis under autogenous pressure. Li0.48Na0.52ZnBO3 represents a new structure type (Pearson symbol aP24) unknown for the borates and the crystal structure has 3D character, which can be considered as being built of onedimensional (1D) chains in the following way (see Fig. 5): two trigonal bipyramids are linked inversion-center related ZnO8 5 dimer. together by sharing one edge (O3/O3) to form a Zn2O12 8 dimers are double bridged by tetrahedral Zn2þ centers The Zn2O12 8 through sharing two O vertices of each [ZnO4]6 tetrahedron to generate a 1D infinite ½ZnBO3 4 chain extending along the [010] direction. These ½ZnBO3 4 chains are arranged in a parallel way and further bridged by B atoms of BO3 groups, resulting in the final 3D ½ZnBO3 1 framework. The 3D framework affords 1D open channels running along the [010] direction. Four rows of Liþ/Naþ cations reside in the channels to balance charge. In this structure, lithium and sodium atoms are statistically distributed over the two crystallographic sites, M1 and M2.
Fig. 5. The crystal structure of Li0.48Na0.52ZnBO3 projected along the b-axis (a) as well as the single chain of ½ZnBO3 4 (b). Li/Na atoms: black circles; B atoms: circles with parallel lines; ZnO4 groups: tetrahedra filled by parallel dashed lines; ZnO5 groups: trigonal bipyramids filled by grid lines.
Among them, M1 is lithium-rich and coordinated with four oxygen atoms forming a distorted tetrahedral geometry, while M2 is sodium-rich and surrounded by five O atoms in an approximately trigonal bipyramidal configuration. M–O distances (Table 3) are reasonable when compared with the corresponding ones observed in the other known lithium sodium borates [7,19,23]. The zinc atoms also occupy two crystallographically distinct sites, one (Zn1) adopting tetrahedral oxygen coordination geometry with Zn–O distances of 1.908(4)–2.005(4) Å, another (Zn2) having trigonal bipyramidal coordination configuration with Zn–O bond lengths 1.907(4)–2.511(5) Å. The calculated BVS values are 2.02 for Zn1 and 1.97 for Zn2, which supports the choice of four-fold and five-fold coordination for these two atoms, respectively. Note that tetrahedral coordination geometries are preferred by Zn atoms, as found in a number of compounds, e. g., Zn3(BO3)2 [28], Ba2Zn(BO3)2 [26], and Pb2Zn(BO3)2 [27], while edge-sharing ZnO5 trigonal bipyramids are rather rare, but have been first described by Bondareva and coauthors in the b-LiZnBO3 modification prepared by hydrothermal synthesis [7]. Zn–O distances and O– Zn–O angles in Li0.48Na0.52ZnBO3 (Table 3) are close to the literature values [7,26,27]. The boron atoms reside in two distinct triangular planar sites. B–O distances and O–B–O angles are also normal, and in good agreement with those found in a-LiZnBO3 discussed above. The BVS values for B atoms are also reasonable, lying in the range 2.94–2.96.
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
Several other orthoborates with similar formula of LiMIIBO3 (M ¼ Mg, Mn, Fe, Co, Zn, and Cd) have been previously reported [7,29–34] and the overall structural information can be summarized as follows: (i) LiMgBO3 [29], LiMnBO3 [7], LiFeBO3 [30], LiCoBO3 [31], and b-LiZnBO3 [7] are all isostructural and crystallize in the space group C2/c, with cell dimensions different from those of a-LiZnBO3 reported in this work. (ii) LiCdBO3 crystallizes in three forms, two of which have been structurally characterized, i.e. LiCdBO3-I with a triclinic P1 space group [32] and LiCdBO3-II with a hexagonal P6 group [33]. A third form, with a monoclinic cell, has been proposed [34], but the structure has not yet been established. The crystal structures of all these LiMIIBO3 compounds are different from that of Li0.48Na0.52ZnBO3 presented here, which is probably related to the difference in the coordination of alkali-metal cations, i. e., trigonal bipyramidal coordination configuration is observed for the Liþ ion in a-LiZnBO3; both tetrahedral and trigonal bipyramidal coordination geometries are observed for Liþ/Naþ cations in Li0.48Na0.52ZnBO3; while only tetrahedral coordination geometries are found for Liþ ions in other LiMIIBO3 compounds.
2091
Fig. 6. It is observed that the valence band maximum (VBM) and conduction band minimum (CBM) are located at the G point resulting in direct energy gaps for both compounds. The values of the calculated energy gaps are 3.6 and 3.3 eV for the Li and Li/Na compounds, respectively, which are comparable with the experimental ones (3.10 and 2.95 eV, respectively). The bands can be assigned according to the total and partial densities of states (DOS) as plotted in Fig. 7. For a-LiZnBO3, the band structure can be divided into five regions. The lowest VB region with energy located at around 69 eV is composed of Li 1s states not plotted in the figure. The second region ranging from 19.5 to 16.5 eV mostly originates from mixed states of O 2s, B 2s/2p, and Li 1s. The third region with energy located at around 12 eV consists of Zn 3d states. The fourth region of VBs between 9.0 eV and the Fermi level is dominated by the O 2p, mixing with small amount of Zn 3d, B 2s/2p, and Li 1s/2s states. The CBs in the range of 3.6–10 eV are mostly contributions from Zn 4s/4p
3.3. Band electronic structures The calculated band structures of a-LiZnBO3 and Li0.48Na0.52ZnBO3 along high-symmetry points of the first Brillouin zone are shown in
Fig. 6. The band structures of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
Fig. 7. Total and partial densities of states (TDOS, PDOS) of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
2092
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
and B 2p states with small contributions from Li 2s and O 2p states and the CBM is controlled by Zn 4s states. For Li0.48Na0.52ZnBO3, the band structure can be divided into six principal groups. The lowest and the second group are located around 48 and 43 eV, respectively, which correspond with Li/ Na s electronic states and are not shown in the Fig. 7. The third group around 20.0 eV has significant contributions from Li/Na p and very small admixture of O 2s states. The fourth group which is located at the energy range from 20 to 16.5 eV mainly originates from O 2s states with some admixtures of B 2s/2p states. The group from 8.0 eV up to Fermi energy (EF) is mainly of O 2p, Zn 3d, and B 2s/2p origins, of which the bands near EF mostly originate from the O 2p states, while the bands localized about 5.0 eV are the main contributions from the Zn 3d states. The group from the CBM up to 9.0 eV is due to Zn 4s/4p and B 2p states with small contributions from O 2p states. Again the CBM is controlled by Zn 4s states. Accordingly, the edges of UV-vis diffuse reflectance spectra of a-LiZnBO3 and Li0.48Na0.52ZnBO3 that are observed at 400 nm (3.10 eV) and 420 nm (2.95 eV), as shown in Fig. 3, are assigned as the charge transfers from the O 2p to Zn2þ 4s states. From the PDOS, we note a strong hybridization between B 2s/2p and O 2s/2p states as well as between Zn 3d and O 2p states. This is also reflected by the population analysis. For a-LiZnBO3, the calculated bond orders of B–O, Zn–O, and Li–O bonds in a unit cell are 0.87–0.95e, 0.13–0.59e, and 0.09–0.01e, while the calculated bond orders of B–O, Zn–O, and Li/Na–O bonds are 0.85–0.91e, 0.06– 0.55e, and 0.06–0.03e for Li0.48Na0.52ZnBO3. Therefore, we can say that the covalent character of the B–O bond is larger than that of the Zn–O bond, and the ionic character of the Li–O or Li/Na–O bond is larger than that of the Zn–O bond.
4. Conclusions
a-LiZnBO3 and Two alkali-metal zinc borates, Li0.48Na0.52ZnBO3, have been synthesized and their crystal structures, IR and UV-vis diffuse reflectance spectra have been studied. a-LiZnBO3 was previously refined in a triclinic space group P1, and the present works indicate that this compound actually crystallizes in a monoclinic group C2/c and the corresponding correct structural data are presented. Li0.48Na0.52ZnBO3 represents a new structure type which is unknown for the borates. For band structure calculations we have applied the PBE–GGA exchangecorrelation potential within the DFT with CASTEP code. Our calculations show that both compounds have direct energy gaps. The values of the calculated energy gaps are 3.6 and 3.3 eV, respectively, comparable with our measured ones (3.10 and 2.95 eV, respectively).
Acknowledgments This work was supported by the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality and the National Natural Science Foundation of China (grant No. 20871012). Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.solidsatesciences.2009.08.024. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
P. Becker, Adv. Mater. 10 (1998) 979. J.D. Grice, P.C. Burns, F.C. Hawthorne, Can. Mineral. 37 (1999) 731. A. Zletz (Amoco Corp.), U.S. patent application 709, 790, 11; March 1985. R.W. Smith, D.A. Keszler, J. Solid State Chem. 100 (1992) 325. D.-G. Chen, W.-D. Cheng, D.-S. Wu, H. Zhang, Y.-C. Zhang, Y.-J. Gong, Z.-G. Kan, Solid State Sci. 7 (2005) 179. A. Belkebir, P. Tarte, A. Rulmont, B. Gilbert, New J. Chem. 20 (1996) 311. O.S. Bondareva, M.A. Simonov, K. Yu, Egorov- Tismenko, N.V. Belov, Kristallografiya 23 (1978) 487. K.-S. Chang, J. Korean Chem. Soc. 45 (2001) 251. J. Li, Z. Chen, X.-X. Wang, D.M. Proserpio, J. Alloys Compd. 262-263 (1997) 28. W.W.M. Wendlandt, H.G. Hecht, Reflectance Spectroscopy. Interscience: A Division of John Wiley & Sons, New York, 1966. G.M. Sheldrick, SHELX-97: Program for Structure Refinement. University of Goettingen, Germany, 1997. R. Hoppe, E. Seipp, R. Baier, J. Solid State Chem. 72 (1988) 52. R. Hoppe, Rev. Chim. Miner. 20 (1983) 549. Y. Le Page, J. Appl. Crystallogr. 20 (1987) 264. M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. Materials Studio, Version 4.1. Accelrys Inc., San Diego, 2006. D. Vanderbilt, Phys. Rev. B 41 (1990) 7892. J.P. Perdew, K. Burke, W. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. M. Marezio, H.A. Plettinger, W.H. Zachariasen, Acta. Crystallogr. 16 (1963) 594. S. Filatov, Y. Shepelev, R. Bubnova, N. Sennova, A.V. Egorysheva, Y.F. Kargin, J. Solid State Chem. 177 (2004) 515. I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244. N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 192. N. Penin, M. Touboul, G. Nowogrocki, Solid State Sci. 3 (2001) 461. K.L. Schaffers, D.A. Keszler, Acta Crystallogr. C 49 (1993) 211. X. Chen, M. Li, X. Chang, H. Zang, W. Xiao, J. Solid State Chem. 180 (2007) 1658. R.W. Smith, L.J. Koliha, Mater. Res. Bull. 29 (1994) 1203. X.-A. Chen, Y.-H. Zhao, X.-A. Chang, L. Zhang, H.-P. Xue, Acta Crystallogr. C 62 (2006) i11. X. Chen, H. Xue, X. Chang, L. Zhang, Y. Zhao, J. Zuo, et al., J. Alloys Compd. 425 (2006) 96. R. Norrestam, Z. Kristallogr. 187 (1989) 103. V. Legagneur, Y. An, A. Mosbah, R. Portal, A. Le Gal La Salle, A. Verbaere, D. Guyomard, Y. Piffard, Solid State Ionics 139 (2001) 37. Y. Piffard, K.K. Rangan, Y. An, D. Guyomard, M. Tournoux, Acta Crystallogr. C 54 (1998) 1561. E.V. Sokolova, V.A. Boronikhin, M.A. Simonov, N.V. Belov, Dokl. Akad. Nauk SSSR 246 (1979) 1126. E.V. Sokolova, M.A. Simonov, N.V. Belov, Kristallografiya 25 (1980) 1285. L. Wei, Q. Huang, Z. Zhou, X. Yin, G. Dai, J. Liang, J. Solid State Chem. 89 (1990) 16.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Syntheses and characterization of two alkali-metal zinc borates, a-LiZnBO3 and Li0.48Na0.52ZnBO3 Xuean Chen a, *, Chunyan Yang a, Xinan Chang a, Hegui Zang a, Weiqiang Xiao b a b
College of Materials Science and Engineering, Beijing University of Technology, Ping Le Yuan 100, Beijing 100124, P.R. China Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2009 Received in revised form 3 August 2009 Accepted 30 August 2009 Available online 6 September 2009
Two alkali-metal zinc borates, a-LiZnBO3 and Li0.48Na0.52ZnBO3, have been prepared by solid-state reactions below 850 C. Single-crystal XRD analyses showed that the former crystallizes in the C2/c group with a ¼ 8.746(2) Å, b ¼ 5.091(1) Å, c ¼ 6.129(1) Å, b ¼ 118.75(3) , Z ¼ 4 and the latter in the group P1 with a ¼ 5.054(1) Å, b ¼ 6.113(1) Å, c ¼ 8.045(2) Å, a ¼ 75.73(2) , b ¼ 89.87(3) , g ¼ 89.86(3) , Z ¼ 4. The crystal structure of a-LiZnBO3 is composed of tetrahedral ZnO4 and triangular BO3 groups that are arranged into a three-dimensional (3D) network by sharing O vertices. Li0.48Na0.52ZnBO3 is also characterized by a 3D framework, but built up from corner-sharing ZnO4 tetratahedra, ZnO5 trigonal bipyramids, and BO3 triangles. Both structures afford open channels that are occupied by alkali-metal cations. The IR spectra further confirmed the presence of BO3 groups and UV-vis diffuse reflectance spectra showed band gaps of about 3.10 and 2.95 eV for the Li and Li/Na compounds, respectively. Band structure calculations indicated that both compounds are direct semiconductors with the calculated band gaps close to the observed ones. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: LiZnBO3 Li0.48Na0.52ZnBO3 Borate Synthesis Crystal Structure
1. Introduction Borate materials are of considerable interest because they show a great variety of physical properties ranging from nonlinear optical (NLO), ferroelectric, piezoelectric to semiconducting behaviors and in addition, a boron atom may adopt triangular or tetrahedral oxygen coordination, the BO3 and BO4 groups may be further linked via common oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets and networks, leading to the rich structural chemistry [1,2]. Zinc-containing borates have been studied due to their potential value as catalysis and optical materials [3–5]. In the ternary system of Li2O–ZnO–B2O3, two LiZnBO3 polymorphs have been previously proposed: one prepared by solidstate reaction (we called a-LiZnBO3 here) and another obtained from hydrothermal synthesis (we called b-LiZnBO3 in this work). 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 they found evidence that the structure of LiZnBO3 depends on the preparation method, and the two LiZnBO3 structures found are monoclinic probably with the same Li–Zn cationic disorder as
* Corresponding author. Tel.: þ86 10 62546928. E-mail address: [email protected] (X. Chen). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.08.024
evidenced by vibrational behavior [6]. However, no atomic coordinates were given in that work. Bodareva et al. have determined the crystal structure of b-LiZnBO3 and found this phase to really have monoclinic symmetry [7], while Ki-Seog Chang has refined the a-LiZnBO3 structure in the triclinic space group P1, with Li and Zn atoms disordered over two very close tetrahedral sites [8]. In an attempt to synthesize non-centrosymmetric compounds that are potentially applicable as NLO materials, we have obtained single crystals of a-LiZnBO3. Our X-ray structural analysis established that this phase actually crystallizes in a monoclinic group C2/c instead of the triclinic group P1. Moreover, we have also obtained a new phase, Li0.48Na0.52ZnBO3, which is the first quaternary borate to be discovered in the Li2O–Na2O–ZnO–B2O3 system and crystallizes in a new unique structure type never observed for any of the known borates. Herein we report their syntheses, crystal structures, IR and UV-vis diffuse reflectance spectra as well as electronic structure calculations for the first time. 2. Experimental 2.1. Sample preparation and general characterization The title compounds were synthesized by employing conventional solid-state reaction methods. All reagents were of analytical grade. For the preparation of a-LiZnBO3 crystals, a powder mixture
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092 Table 1 Crystallographic data for a-LiZnBO3 and Li0.48Na0.52ZnBO3. Formula
LiZnBO3
Li0.48Na0.52ZnBO3
Space group
C2/c (No.15)
P1 (No.2)
a (Å) b (Å) c (Å)
8.746(2) 5.091(1) 6.129(1) 90 118.75(3) 90 239.26(11), 4 3.640 9.975 64.98 433 406 33 1.190 0.0483/0.1273 0.0517/0.1302
5.054(1) 6.113(1) 8.045(2) 75.73(2) 89.87(3) 89.86(3) 240.88(9), 4 3.847 10.006 65 1746 1672 112 1.120 0.0646/0.1950 0.0658/0.1964
a b g V (Å3), Z dcalc (g/cm3) m (mm1) 2qmax ( ) Unique reflection Observed [I 2s(I)] No. of variables GOF on F2o R1/wR2 [I 2s(I)] R1/wR2 (all data)
of 1.407 g Li2CO3, 3.103 g ZnO, 2.358 g H3BO3, and 2.435 g Li2B4O7 (the Li2CO3/ZnO/H3BO3/Li2B4O7 molar ratio ¼ 4:8:8:3) was transferred to a Pt crucible. The sample was gradually heated to 830 C, where it was kept for three days, then cooled down to 820 C at a rate of 0.5 C/h, and further to 320 C at 5 C/h, followed by cooling to room temperature at a rate of 20 C/h. The colorless, plate-like crystals of a-LiZnBO3 with dimensions of up to 1.2 1.0 0.3 mm3 were embedded in a lithium borate matrix. Several small crystals were recovered and mechanically separated from the reaction product. Li0.48Na0.52ZnBO3 crystals were synthesized from a reaction containing 2.401 g Li2CO3, 1.765 g ZnO, 4.023 g H3BO3, and 4.965 g Na2B4O7$10H2O (the Li2CO3/ZnO/H3BO3/Na2B4O7 molar ratio ¼ 15: 10:30:6). The sample was placed in a Pt crucible and gradually heated to 800 C, where it was kept for one week, then cooled down to 700 C at a rate of 1 C/h. Finally, the sample was cooled to room temperature at a rate of 20 C/h and the furnace was switched off. The colorless, plate-like crystals of Li0.48Na0.52ZnBO3 were found in the solidified melt. The single-phase polycrystalline sample of a-LiZnBO3 was prepared by heating a stoichiometric mixture of LiBO2$8H2O and ZnO at 600 C for one month with several intermediate re-mixings, while the single-phase polycrystalline sample of Li0.48Na0.52ZnBO3 was prepared in the same way, except that the starting material is a stoichiometric mixture of Li2CO3, Na2CO3, ZnO, and H3BO3. X-ray powder diffraction data were collected by using the monochromatized Cu Ka radiation of a Bruker D8 ADVANCE diffractometer. Infrared spectra were recorded from 4000 to 400 cm1 on a Perkin Elmer 1730 FT-IR spectrometer from KBr pellets. Optical diffuse reflectance spectra were measured at room temperature with a Shimadzu UV-3101PC double-beam, doublemonochromator spectrophotometer. Data were collected in the wavelength range 200–800 nm. BaSO4 powder was used as a standard (100% reflectance). A similar procedure as previously described [9,10] was used to collect and convert the data using the Kubelka– Munk function F (R) ¼ (1R)2/2R, where R is the reflectance. The minima in the second-derivative curves of the Kubelka–Munk function are taken as the position of the absorption bands. 2.2. Structure determination Single-crystal X-ray intensity data were collected at room temperature (298 K) on an automated Rigaku AFC7R four-circle diffractometer using monochromatized Mo Ka radiation (l ¼ 0.71073 Å). The data were corrected for Lorentz and polarization
2087
effects, and for absorption by empirical method based on j-scan data. The crystal structures were solved by direct methods and refined in SHELX-97 system [11] by full-matrix least-squares methods on Fo2. For a-LiZnBO3, the refinement of 33 parameters with 406 observed reflections [I 2s(I)] resulted in the residuals of R1/ wR2 ¼ 0.0483/0.1273. The reliability factors for Li0.48Na0.52ZnBO3 converged to R1/wR2 ¼ 0.0646/0.1950 for 1672 observed reflections and 112 variables. The final difference electron density maps were featureless in both cases, with the highest electron density less than 1.54 eÅ3. Details of crystal parameters, data collection and structure refinements are given in Table 1 and the atomic coordinates and the equivalent isotropic displacement parameters are summarized in Table 2. Structural refinements of a-LiZnBO3 revealed that lithium and zinc atoms occupy two very close positions [Li1–Zn1 ¼ 0.631(12) Å] with occupation factors of 0.5 (see Table 2). The Liþ/Zn2þ disorder is not surprising due to their very similar coordination geometries as well as similar Li–O and Zn–O bond lengths. A similar situation has also been previously observed, e. g., in b-LiZnBO3 [7], KLiZn3O4 [12], and NaLiZnO2 [13]. All of our attempts to refine the structure in lower-symmetry space groups with an ordered distribution of Li and Zn atoms have been unsuccessful. An examination of the a-LiZnBO3 crystal with a slower scan rate and lower intensity limitation on a Rigaku AFC7R four-circle diffractometer did not indicate symmetry lower than monoclinic or a larger unit cell that would allow Li/Zn ordering. Therefore, the Liþ/Zn2þ disorder model was finally assumed. This model has also been confirmed by the powder diffraction results (Fig. 1). Note that in the a-LiZnBO3 structure reported by Ki-Seog Chang [8], although the low symmetry space group P1 was used, Li and Zn atoms are still disordered. Positional parameters in that work were checked by us using the program MISSYM [14], potential additional symmetry was found and the space group C2/c was suggested, indicating that the previous space group assignment is wrong. Our present refinements give the correct structural data for this compound. During the structural refinements of Li0.48Na0.52ZnBO3, Zn, B, and O atoms are well resolved, while Li and Na atoms were found to occupy two remaining atomic sites. Refinements of atomic occupancy parameters for these two positions gave the compositions
Table 2 Atomic coordinates and equivalent isotropic displacement parameters (Å2) for a-LiZnBO3 and Li0.48Na0.52ZnBO3. Atoms
X
Y
Z
Ueq
0.3561(14) 0.31550(10) 0 0.1513(3) 0
0.592(2) 0.56910(15) 0.4199(8) 0.5553(4) 0.1518(7)
0.816(2) 0.7010(2) 1/4 0.3054(4) 1/4
0.010(2) 0.0203(4) 0.0116(8) 0.0136(5) 0.0343(12)
Li0.48Na0.52ZnBO3 M1 0.1672(8) M2 0.1624(10) Zn1 0.65797(11) Zn2 0.32732(14) B1 0.1711(10) B2 0.6640(10) O1 0.3036(7) O2 0.8999(7) O3 0.7007(7) O4 0.7995(7) O5 0.2055(7) O6 0.5983(7)
0.7216(8) 1.0048(12) 0.46336(9) 0.83148(17) 0.3364(8) 0.8364(8) 0.5084(6) 0.3201(6) 0.8270(7) 0.7303(7) 0.9997(6) 0.2126(6)
0.4018(6) 0.2479(11) 0.28454(7) 0.08311(10) 0.0871(6) 0.5782(6) 0.1969(5) 0.1005(5) 0.0369(5) 0.4330(5) 0.3016(5) 0.4006(5)
0.0185(14) 0.025(2) 0.0091(2) 0.0193(3) 0.0058(8) 0.0057(8) 0.0084(6) 0.0099(8) 0.0121(9) 0.0133(9) 0.0083(8) 0.0107(8)
a-LiZnBO3 Li1 Zn1 B1 O1 O2
Note: For a-LiZnBO3, occupation factors of Li1 and Zn1 are 1/2; For Li0.48Na0.52ZnBO3, M1 and M2 have the compositions Li0.53(2)Na0.47(2) and Li0.42(3)Na0.58(3), respectively. Ueq is defined as one third of the trace of the orthogonalized U tensor.
2088
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
Fig. 1. XRD patterns of a-LiZnBO3 and Li0.48Na0.52ZnBO3 observed from powder polycrystalline sample (b, d) and calculated from the single-crystal data (a, c).
Li0.53(2)Na0.47(2) and Li0.42(3)Na0.58(3), respectively, which leads to the final formula Li0.48Na0.52ZnBO3. 2.3. Electronic structure calculations Density of states (DOS) and band calculations were performed using a first principle plane-wave pseudopotential technique based on density functional theory (DFT) with CASTEP code [15] distributed inside a computational commercial pack [16]. The ion–electron interaction was modeled by an ultrasoft nonlocal pseudopotential [17]. The density functional was treated by the generalized gradient approximation (GGA) with the exchangecorrelation potential parametrization (PBE) of Perdew–Burke– Ernzerhof type [18]. Pseudoatomic calculations were performed for B 2s2 2 p1, O 2s2 2p4, Zn 3d10 4s2, Li 1s2 2s1, and Na 2s2 2p6 3s1. The number of plane-waves included into the basis was determined by a cutoff energy Ec of 330 eV. Twelve empty bands were included to reach the convergence of calculations. The calculating parameters and convergent criterions were set by the default values of CASTEP code [15].
Fig. 1 shows the observed powder XRD patterns of a-LiZnBO3 and Li0.48Na0.52ZnBO3, together with those calculated from the single crystal data for comparison. It is clear that the observed XRD patterns are in agreement with the corresponding theoretical ones, further confirming our structural models. The slight intensity difference between the two patterns is believed to be due to the existence of a small amount of unreacted ZnO impurities or caused by preferred orientation of the powder sample during collection of the experimental XRD data because both compounds show a platelike crystal growth habit. Moreover, the observed patterns are generally very broad, which may be associated with the Li/Zn and Li/Na cationic disorder in these two compounds. It is known that four normal vibrations with frequencies ns (symmetric stretching) at 850–960 cm1, g (out-of plane bending) at 650–800 cm1, nas (asymmetrical stretching) at 1100–1450 cm1, and d (in-plane bending) at 500–600 cm1 correspond to the [BO3]3 group, of which ns is the infrared non-active vibration [20]. In order to further confirm the coordination surroundings of B atoms in both structures, the infrared spectra were measured and shown in Fig. 2. It is clear that three sets of bands characteristic of the planar triangular BO3 group were indeed observed. They are the out-of plane bending modes (g) occurring in the range 676.9–720.2 (670.2–728.8) cm1, the antisymmetric stretch (nas) at about 1253.0 (1192.4–1273.4) cm1, and the in-plane mode (d) at about 509.3 (558.0) cm1 for Li0.48Na0.52ZnBO3 (a-LiZnBO3), respectively. The optical diffuse reflectance spectra of two compounds are shown in Fig. 3. It is observed that a-LiZnBO3 has no absorption above 400 nm, while below 260 nm this compound has strong absorption. Li0.48Na0.52ZnBO3 has no absorption above 420 nm. From the absorption edges of UV-vis diffuse reflectance spectra, the optical band gaps are estimated to be roughly 3.10 and 2.95 eV for the Li and Li/Na compounds, respectively. 3.2. Description of the structure
a-LiZnBO3 contains a three-dimensional (3D) framework of vertex-sharing ZnO4 tetrahedra and BO3 triangles. In this structure, two inversion-center related ZnO4 tetrahedra are linked together by
3. Results and discussion 3.1. Synthesis and general characterization We have succeeded in obtaining single crystals of a-LiZnBO3 and Li0.48Na0.52ZnBO3. It is natural to be think if it is possible to prepare the corresponding Na analog. In the ternary Na2O–ZnO–B2O3 system, a hypothetical compound, ’’NaZnBO3’’, has been tried by us via solid-state reactions of a stoichiometric mixture of Na2CO3, ZnO, and H3BO3 powders at 600 C for three weeks with several intermediate re-mixings. Unfortunately, no new phase except for NaBO2 [19] along with the unreacted ZnO has been obtained. Our attempts to prepare ‘‘Li0.50K0.50ZnBO3’’ with a synthesis procedure similar to that was used for obtaining Li0.48Na0.52ZnBO3 crystals have also been unsuccessful. The reaction again resulted in a-LiZnBO3 crystals, judged from cell dimensions measured on a Rigaku AFC7R four-circle diffractometer.
Fig. 2. Infrared spectra of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
2089
Table 3 Selected bond lengths (Å) and angles ( ) for a-LiZnBO3 and Li0.48Na0.52ZnBO3.
a-LiZnBO3
Fig. 3. Optical absorption spectra of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
sharing one edge (O1.O1) to form a Zn2O6 dimer. Each Zn2O6 dimer is linked to six others through sharing O vertices to form a 3D Zn–O framework. Boron atoms are incorporated into the triangular hollows of oxygen atoms within this network to strengthen the structure. The 3D framework affords six-edge channels that are occupied by Li atoms, as displayed in Fig. 4. Each Liþ ion is strongly bonded to three O atoms at distances of 1.912(11)–1.944(11) Å and also weakly bonded to two more O atoms at distances of 2.350(13)–2.767(13) Å (Table 3). Taking all these five bonds into account, the concept of bond valence [21,22] allows us to calculate a bond valence sum (BVS) equal to 0.98 for Li1 which proves that the long bonds indeed participate in the metal coordination scheme. The five-fold coordination around Li1 can be described as a distorted trigonal bipyramid. The similar distorted Li coordination geometries have also been observed in Li3In(BO3)2 [23] and Sr2LiInB4O10 [24]. Each Zn atom is coordinated to four O atoms forming a distorted tetrahedral configuration with Zn–O distances of 1.869(2)–2.144(3) Å, which are in the similar range as reported for the four-coordinated Zn2þ in Na3ZnB5O10 [1.935(2)– 1.982(2) Å] and Ba2Zn(BO3)2 [1.95(3)–2.03(4) Å] [25,26]. Bond valence analysis produced a reasonable value of 2.01 for Zn in this environment.
Fig. 4. The crystal structure of a-LiZnBO3 projected along the b-axis. Li atoms: black circles; B atoms: circles with parallel lines; ZnO4 groups: tetrahedra filled by parallel dashed lines.
Li1–O1 Li1–O1 Li1–O2 Li1–O2 Li1–O1 Average Zn1–O2 Zn1–O1 Zn1–O1 Zn1–O1 Average B1–O2 B1–O1 2 Average
1.912(11) 1.935(12) 1.944(11) 2.350(13) 2.767(13) 2.182 1.869(2) 1.937(2) 1.938(2) 2.144(3) 1.972 1.365(6) 1.382(3) 1.376
Li1–O1 Zn1–O2
3.173(11) 2.9798(15)
Li0.48Na0.52ZnBO3 M1–O4 M1–O6 M1–O1 M1–O5 Average M2–O3 M2–O5 M2–O6 M2–O4 M2–O2 Average Zn1–O4 Zn1–O2 Zn1–O1 Zn1–O6 Average Zn2–O5 Zn2–O2 Zn2–O3 Zn2–O1 Zn2–O3 Average B1–O1 B1–O2 B1–O3 Average B2–O4 B2–O5 B2–O6 Average
1.875(5) 1.942(6) 1.957(5) 2.063(6) 1.959 1.905(7) 1.910(6) 1.954(6) 2.512(12) 2.520(12) 2.160 1.908(4) 1.954(4) 1.967(4) 2.005(4) 1.958 1.907(4) 1.919(4) 1.923(4) 2.376(4) 2.511(5) 2.127 1.370(6) 1.381(6) 1.386(6) 1.379 1.372(6) 1.377(6) 1.380(6) 1.376
M1–O5 M2–O5 Zn1–O6 Zn2–O3
3.185(6) 3.226(6) 3.090(4) 3.187(4)
O1–B1–O2
119.6(4)
O2–B1–O1 2 O1–B1–O1 O1–Li1–O1 O1–Li1–O2 O1–Li1–O2 O1–Li1–O2 O1–Li1–O2 O2–Li1–O2 O1–Li1–O1 O1–Li1–O1 O2–Li1–O1 O2–Li1–O1 O2–Zn1–O1 O2–Zn1–O1 O1–Zn1–O1 O2–Zn1–O1 O1–Zn1–O1 O1–Zn1–O1
119.92(18) 120.2(4) 119.5(6) 115.6(6) 114.6(6) 94.2(5) 102.5(5) 105.7(5) 81.3(4) 75.1(4) 81.5(4) 172.7(5) 118.10(12) 118.02(11) 118.12(9) 102.64(8) 91.99(9) 99.39(10)
O1–B1–O3 O2–B1–O3 O4–B2–O5 O4–B2–O6 O5–B2–O6 O4–M1–O6 O4–M1–O1 O6–M1–O1 O4–M1–O5 O6–M1–O5 O1–M1–O5 O3–M2–O5 O3–M2–O6 O5–M2–O6 O3–M2–O4 O5–M2–O4 O6–M2–O4 O3–M2–O2 O5–M2–O2 O6–M2–O2 O4–M2–O2 O4–Zn1–O2 O4–Zn1–O1 O2–Zn1–O1 O4–Zn1–O6 O2–Zn1–O6 O1–Zn1–O6 O5–Zn2–O2 O5–Zn2–O3 O2–Zn2–O3 O5–Zn2–O1 O2–Zn2–O1 O3–Zn2–O1 O5–Zn2–O3 O2–Zn2–O3 O3–Zn2–O3 O1–Zn2–O3
122.5(4) 117.8(4) 117.9(4) 120.9(4) 121.1(4) 120.0(3) 117.0(3) 115.8(3) 98.6(3) 103.7(3) 94.5(2) 121.1(3) 118.4(3) 120.3(3) 100.5(4) 83.8(4) 90.2(4) 87.8(4) 92.2(4) 85.5(3) 171.7(3) 113.18(16) 113.09(16) 111.98(16) 113.56(17) 101.40(16) 102.60(16) 124.35(15) 118.33(17) 117.21(16) 86.45(15) 88.58(15) 98.81(16) 91.53(15) 87.79(15) 87.24(16) 173.87(12)
There is one crystallographically independent B atom, which is located on a crystallographic 2-fold axis, resulting in two sets of B–O bond lengths [1 1.365(6), and 2 1.382(3) Å] and two sets of O–B–O angles [2 119.92(18) and 1 120.2(4) ]. These geometric parameters compare well with those observed in the structures of Li3In(BO3)2 [23] and Pb2Zn(BO3)2 [27], where BO3 groups are also observed. Of the two unique O atoms, O2 lies on a crystallographically 2-fold axis, while O1 occupies a general position. Bond valence calculations gave a value of 2.96 for the B atom, consistent with its expected formal valence. As mentioned above, the a-LiZnBO3 structure has been previously refined by Ki-Seog Chang in the space group P1 [8], where the asymmetric unit contains eight independent atoms, i.e., 2Li,
2090
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
2Zn, 1B, and 3O. All these atoms occupy crystallographic general positions, and Li and Zn atoms are disordered with occupation factors of 0.5. This is in contrast to the situation in our refinements where the space group C2/c has been assigned and the asymmetric unit consists of five unique atoms including 1Li, 1Zn, 1B, and 2O, of which the B atom and one O atom lie on crystallographic 2-fold axes, and Li and Zn atoms are again disordered. In the previous refinements, Li–O, Zn–O, and B–O bond lengths are 1.90(1)–1.95(1), 1.863(3)–2.044(3), and 1.366(1)–1.391(5) Å, and O–Li–O, O–Zn–O, and O–B–O angles in the range 94.8(5)– 119.8(6), 92.6(1)–118.4(2), and 119.3(3)–120.6(4) , respectively, which are comparable to those in the present refinements (Table 3). The significant difference between our and previous refinements is the equivalent isotropic displacement parameter for the Li atom, which is reasonable with beq ¼ 0.8(2) Å2 in our case [equivalent to Ueq ¼ 0.010(2) Å2, Table 2], but a negative value or very small in the previous refinements [beq ¼ 0.4(8) Å2 for Li1 and 0.1(9) Å2 for Li2, respectively, Ref. [8]]. A comparison of a-LiZnBO3 with Li0.48Na0.52ZnBO3 indicates that the partial replacement of Li by Na leads to a volume per formula unit slightly different, that are 60.22(2) and 59.82(3) Å3 for Li0.48Na0.52ZnBO3 and a-LiZnBO3, respectively, associated to density values of 3.847 and 3.640 g/cm3, therefore, Li0.48Na0.52ZnBO3 exhibits a denser structure than a-LiZnBO3. Moreover, the substitution also results in a lowering of the space group symmetry from C2/c to P1. The other borate that is closely related to the title compounds in stoichiometry is b-LiZnBO3 [7], which was reported to crystallize in the monoclinic C2/c group, with the cell volume being about two times of that of the a-form [a ¼ 5.094(1) Å, b ¼ 8.806(3) Å, c ¼ 10.374(4) Å, b ¼ 91.09(3) , V ¼ 465.3(3) Å3, Z ¼ 8 in b-LiZnBO3 vs a ¼ 8.746(2) Å, b ¼ 5.091(1) Å, c ¼ 6.129(1) Å, b ¼ 118.75(3) , V ¼ 239.26(11) Å3, Z ¼ 4 in a-LiZnBO3]. The crystal structure of b-LiZnBO3 consists of a 3D ½ZnBO3 1 framework built up from ZnO5 trigonal bipyramids and BO3 groups. Each ZnO5 bipyramid shares two edges with adjacent bipyramids to form single chains running along the [101] direction. These chains are bridged by planar BO3 groups via corner-sharing to form the 3D ½ZnBO3 1 framework. Within this framework, the Li atoms occupy statistically two tetrahedral sites [Li1–Li2 ¼ 1.09(6) Å] sharing a face. Such pairs of tetrahedra share edges to form chains running along the [001] direction. The Zn cation occupies statistically two close positions [Zn1–Zn2 ¼ 0.242(4) Å] within the ZnO5 bipyramid, above and below the central plane. A comparison of a-LiZnBO3 with its b-form indicates that the latter has a denser structure, as reflected from density values (3.744 g/cm3 in b-LiZnBO3 vs 3.640 g/ cm3 in a-LiZnBO3). This is reasonable because b-LiZnBO3 was prepared by hydrothermal synthesis under autogenous pressure. Li0.48Na0.52ZnBO3 represents a new structure type (Pearson symbol aP24) unknown for the borates and the crystal structure has 3D character, which can be considered as being built of onedimensional (1D) chains in the following way (see Fig. 5): two trigonal bipyramids are linked inversion-center related ZnO8 5 dimer. together by sharing one edge (O3/O3) to form a Zn2O12 8 dimers are double bridged by tetrahedral Zn2þ centers The Zn2O12 8 through sharing two O vertices of each [ZnO4]6 tetrahedron to generate a 1D infinite ½ZnBO3 4 chain extending along the [010] direction. These ½ZnBO3 4 chains are arranged in a parallel way and further bridged by B atoms of BO3 groups, resulting in the final 3D ½ZnBO3 1 framework. The 3D framework affords 1D open channels running along the [010] direction. Four rows of Liþ/Naþ cations reside in the channels to balance charge. In this structure, lithium and sodium atoms are statistically distributed over the two crystallographic sites, M1 and M2.
Fig. 5. The crystal structure of Li0.48Na0.52ZnBO3 projected along the b-axis (a) as well as the single chain of ½ZnBO3 4 (b). Li/Na atoms: black circles; B atoms: circles with parallel lines; ZnO4 groups: tetrahedra filled by parallel dashed lines; ZnO5 groups: trigonal bipyramids filled by grid lines.
Among them, M1 is lithium-rich and coordinated with four oxygen atoms forming a distorted tetrahedral geometry, while M2 is sodium-rich and surrounded by five O atoms in an approximately trigonal bipyramidal configuration. M–O distances (Table 3) are reasonable when compared with the corresponding ones observed in the other known lithium sodium borates [7,19,23]. The zinc atoms also occupy two crystallographically distinct sites, one (Zn1) adopting tetrahedral oxygen coordination geometry with Zn–O distances of 1.908(4)–2.005(4) Å, another (Zn2) having trigonal bipyramidal coordination configuration with Zn–O bond lengths 1.907(4)–2.511(5) Å. The calculated BVS values are 2.02 for Zn1 and 1.97 for Zn2, which supports the choice of four-fold and five-fold coordination for these two atoms, respectively. Note that tetrahedral coordination geometries are preferred by Zn atoms, as found in a number of compounds, e. g., Zn3(BO3)2 [28], Ba2Zn(BO3)2 [26], and Pb2Zn(BO3)2 [27], while edge-sharing ZnO5 trigonal bipyramids are rather rare, but have been first described by Bondareva and coauthors in the b-LiZnBO3 modification prepared by hydrothermal synthesis [7]. Zn–O distances and O– Zn–O angles in Li0.48Na0.52ZnBO3 (Table 3) are close to the literature values [7,26,27]. The boron atoms reside in two distinct triangular planar sites. B–O distances and O–B–O angles are also normal, and in good agreement with those found in a-LiZnBO3 discussed above. The BVS values for B atoms are also reasonable, lying in the range 2.94–2.96.
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
Several other orthoborates with similar formula of LiMIIBO3 (M ¼ Mg, Mn, Fe, Co, Zn, and Cd) have been previously reported [7,29–34] and the overall structural information can be summarized as follows: (i) LiMgBO3 [29], LiMnBO3 [7], LiFeBO3 [30], LiCoBO3 [31], and b-LiZnBO3 [7] are all isostructural and crystallize in the space group C2/c, with cell dimensions different from those of a-LiZnBO3 reported in this work. (ii) LiCdBO3 crystallizes in three forms, two of which have been structurally characterized, i.e. LiCdBO3-I with a triclinic P1 space group [32] and LiCdBO3-II with a hexagonal P6 group [33]. A third form, with a monoclinic cell, has been proposed [34], but the structure has not yet been established. The crystal structures of all these LiMIIBO3 compounds are different from that of Li0.48Na0.52ZnBO3 presented here, which is probably related to the difference in the coordination of alkali-metal cations, i. e., trigonal bipyramidal coordination configuration is observed for the Liþ ion in a-LiZnBO3; both tetrahedral and trigonal bipyramidal coordination geometries are observed for Liþ/Naþ cations in Li0.48Na0.52ZnBO3; while only tetrahedral coordination geometries are found for Liþ ions in other LiMIIBO3 compounds.
2091
Fig. 6. It is observed that the valence band maximum (VBM) and conduction band minimum (CBM) are located at the G point resulting in direct energy gaps for both compounds. The values of the calculated energy gaps are 3.6 and 3.3 eV for the Li and Li/Na compounds, respectively, which are comparable with the experimental ones (3.10 and 2.95 eV, respectively). The bands can be assigned according to the total and partial densities of states (DOS) as plotted in Fig. 7. For a-LiZnBO3, the band structure can be divided into five regions. The lowest VB region with energy located at around 69 eV is composed of Li 1s states not plotted in the figure. The second region ranging from 19.5 to 16.5 eV mostly originates from mixed states of O 2s, B 2s/2p, and Li 1s. The third region with energy located at around 12 eV consists of Zn 3d states. The fourth region of VBs between 9.0 eV and the Fermi level is dominated by the O 2p, mixing with small amount of Zn 3d, B 2s/2p, and Li 1s/2s states. The CBs in the range of 3.6–10 eV are mostly contributions from Zn 4s/4p
3.3. Band electronic structures The calculated band structures of a-LiZnBO3 and Li0.48Na0.52ZnBO3 along high-symmetry points of the first Brillouin zone are shown in
Fig. 6. The band structures of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
Fig. 7. Total and partial densities of states (TDOS, PDOS) of a-LiZnBO3 (a) and Li0.48Na0.52ZnBO3 (b).
2092
X. Chen et al. / Solid State Sciences 11 (2009) 2086–2092
and B 2p states with small contributions from Li 2s and O 2p states and the CBM is controlled by Zn 4s states. For Li0.48Na0.52ZnBO3, the band structure can be divided into six principal groups. The lowest and the second group are located around 48 and 43 eV, respectively, which correspond with Li/ Na s electronic states and are not shown in the Fig. 7. The third group around 20.0 eV has significant contributions from Li/Na p and very small admixture of O 2s states. The fourth group which is located at the energy range from 20 to 16.5 eV mainly originates from O 2s states with some admixtures of B 2s/2p states. The group from 8.0 eV up to Fermi energy (EF) is mainly of O 2p, Zn 3d, and B 2s/2p origins, of which the bands near EF mostly originate from the O 2p states, while the bands localized about 5.0 eV are the main contributions from the Zn 3d states. The group from the CBM up to 9.0 eV is due to Zn 4s/4p and B 2p states with small contributions from O 2p states. Again the CBM is controlled by Zn 4s states. Accordingly, the edges of UV-vis diffuse reflectance spectra of a-LiZnBO3 and Li0.48Na0.52ZnBO3 that are observed at 400 nm (3.10 eV) and 420 nm (2.95 eV), as shown in Fig. 3, are assigned as the charge transfers from the O 2p to Zn2þ 4s states. From the PDOS, we note a strong hybridization between B 2s/2p and O 2s/2p states as well as between Zn 3d and O 2p states. This is also reflected by the population analysis. For a-LiZnBO3, the calculated bond orders of B–O, Zn–O, and Li–O bonds in a unit cell are 0.87–0.95e, 0.13–0.59e, and 0.09–0.01e, while the calculated bond orders of B–O, Zn–O, and Li/Na–O bonds are 0.85–0.91e, 0.06– 0.55e, and 0.06–0.03e for Li0.48Na0.52ZnBO3. Therefore, we can say that the covalent character of the B–O bond is larger than that of the Zn–O bond, and the ionic character of the Li–O or Li/Na–O bond is larger than that of the Zn–O bond.
4. Conclusions
a-LiZnBO3 and Two alkali-metal zinc borates, Li0.48Na0.52ZnBO3, have been synthesized and their crystal structures, IR and UV-vis diffuse reflectance spectra have been studied. a-LiZnBO3 was previously refined in a triclinic space group P1, and the present works indicate that this compound actually crystallizes in a monoclinic group C2/c and the corresponding correct structural data are presented. Li0.48Na0.52ZnBO3 represents a new structure type which is unknown for the borates. For band structure calculations we have applied the PBE–GGA exchangecorrelation potential within the DFT with CASTEP code. Our calculations show that both compounds have direct energy gaps. The values of the calculated energy gaps are 3.6 and 3.3 eV, respectively, comparable with our measured ones (3.10 and 2.95 eV, respectively).
Acknowledgments This work was supported by the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality and the National Natural Science Foundation of China (grant No. 20871012). Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.solidsatesciences.2009.08.024. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
P. Becker, Adv. Mater. 10 (1998) 979. J.D. Grice, P.C. Burns, F.C. Hawthorne, Can. Mineral. 37 (1999) 731. A. Zletz (Amoco Corp.), U.S. patent application 709, 790, 11; March 1985. R.W. Smith, D.A. Keszler, J. Solid State Chem. 100 (1992) 325. D.-G. Chen, W.-D. Cheng, D.-S. Wu, H. Zhang, Y.-C. Zhang, Y.-J. Gong, Z.-G. Kan, Solid State Sci. 7 (2005) 179. A. Belkebir, P. Tarte, A. Rulmont, B. Gilbert, New J. Chem. 20 (1996) 311. O.S. Bondareva, M.A. Simonov, K. Yu, Egorov- Tismenko, N.V. Belov, Kristallografiya 23 (1978) 487. K.-S. Chang, J. Korean Chem. Soc. 45 (2001) 251. J. Li, Z. Chen, X.-X. Wang, D.M. Proserpio, J. Alloys Compd. 262-263 (1997) 28. W.W.M. Wendlandt, H.G. Hecht, Reflectance Spectroscopy. Interscience: A Division of John Wiley & Sons, New York, 1966. G.M. Sheldrick, SHELX-97: Program for Structure Refinement. University of Goettingen, Germany, 1997. R. Hoppe, E. Seipp, R. Baier, J. Solid State Chem. 72 (1988) 52. R. Hoppe, Rev. Chim. Miner. 20 (1983) 549. Y. Le Page, J. Appl. Crystallogr. 20 (1987) 264. M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. Materials Studio, Version 4.1. Accelrys Inc., San Diego, 2006. D. Vanderbilt, Phys. Rev. B 41 (1990) 7892. J.P. Perdew, K. Burke, W. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. M. Marezio, H.A. Plettinger, W.H. Zachariasen, Acta. Crystallogr. 16 (1963) 594. S. Filatov, Y. Shepelev, R. Bubnova, N. Sennova, A.V. Egorysheva, Y.F. Kargin, J. Solid State Chem. 177 (2004) 515. I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244. N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 192. N. Penin, M. Touboul, G. Nowogrocki, Solid State Sci. 3 (2001) 461. K.L. Schaffers, D.A. Keszler, Acta Crystallogr. C 49 (1993) 211. X. Chen, M. Li, X. Chang, H. Zang, W. Xiao, J. Solid State Chem. 180 (2007) 1658. R.W. Smith, L.J. Koliha, Mater. Res. Bull. 29 (1994) 1203. X.-A. Chen, Y.-H. Zhao, X.-A. Chang, L. Zhang, H.-P. Xue, Acta Crystallogr. C 62 (2006) i11. X. Chen, H. Xue, X. Chang, L. Zhang, Y. Zhao, J. Zuo, et al., J. Alloys Compd. 425 (2006) 96. R. Norrestam, Z. Kristallogr. 187 (1989) 103. V. Legagneur, Y. An, A. Mosbah, R. Portal, A. Le Gal La Salle, A. Verbaere, D. Guyomard, Y. Piffard, Solid State Ionics 139 (2001) 37. Y. Piffard, K.K. Rangan, Y. An, D. Guyomard, M. Tournoux, Acta Crystallogr. C 54 (1998) 1561. E.V. Sokolova, V.A. Boronikhin, M.A. Simonov, N.V. Belov, Dokl. Akad. Nauk SSSR 246 (1979) 1126. E.V. Sokolova, M.A. Simonov, N.V. Belov, Kristallografiya 25 (1980) 1285. L. Wei, Q. Huang, Z. Zhou, X. Yin, G. Dai, J. Liang, J. Solid State Chem. 89 (1990) 16.