A non-linear optical crystal, Cd₃Zn₃(BO₃)₄

The title compound, tricadmium trizinc tetraborate, Cd₃Zn₃(BO₃)₄, is a new non-linear optical (NLO) crystal and its structure has been determined by single-crystal X-ray diffraction. This compound is composed of planar [BO₃]^3- groups sharing O atoms with CdO₄ or ZnO₄ tetrahedra. The BO₃ triangles are located on threefold axes and are arranged with nearly the same orientation. The Cd and Zn atoms are disordered on the same site in the proportion 1:1. A strong second harmonic generation of Nd:YAG laser radiation ([lambda] = 1064 nm) has been observed for a crystal of the title compound.

S1. Comment

To date, many promising borate materials have been discovered and studied for second? harmonic generation of laser radiation to produce new laser sources in the visible and UV spectroscopic ranges, such as β-Ba₂B₂O₄ (BBO; Chen et al., 1985), LiB₃O₅ (LBO; Chen et al., 1989), CsLiB₆O₁₀ (CLBO; Mori et al., 1995), KBe₂BO₃F₂ (KBBF; Mei et al., 1995), Sr₂Be₂B₂O₇ (SBBO; Chen et al., 1995) and K₂Al₂B₂O₇ (KAB; Hu et al., 1998). Few borate NLO materials incorporating transition metal elements have been researched, except BaZn₂(BO₃)₂ (Smith & Keszler, 1992), Ba₂Zn(BO₃)₂ (Smith & Keszler, 1994) and ReCa₄O(BO₃)₃ (Re is Y or Gd; Iwai et al., 1997). Whitaker & Channell (1993) investigated the ternary system CdO-ZnO-B₂O₃ at 1123 K and a new phase, Cd₃Zn₃B₄O₁₂, was postulated for the first time. Based on their study, we have grown single crystals of Cd₃Zn₃(BO₃)₄ (abbreviated to CZBO). To the best of our knowledge, CZBO has not previously been grown as a single-crystal.

The crystal structure of trigonal CZBO is characterized by a three-dimensional framework built from corner-sharing BO₃ triangles with CdO₄ or ZnO₄ tetrahedra (Fig. 1). In this configuration, each O atom belongs to two tetrahedra and a triangle. All [BO₃]³⁻ trigonal groups are almost perfectly planar and perpendicular to the c axis. The BO₃ triangles in the different layers are orientated in nearly the same direction and are only slightly rotated around the threefold axis. The B1—O and B2—O bond lengths are nearly equal [1.377 (3) and 1.374 (4) Å, respectively]. The O—B—O bond angles are very close to 120°.

The Cd²⁺ and Zn²⁺ cations in CZBO share the same site in the unit cell and are distributed in a disorderly fashion. The occupancy of the metal position was assigned to be 0.5 C d+0.5Zn, according to the result of an ICP-AES analysis. The Cd(Zn)O₄ moiety is distorted from an ideal tetrahedron and the four O atoms connect to different B atoms (Fig. 2). The Cd/Zn—O distances lie between 2.014 (3) and 2.131 (4) Å, and the O—Cd/Zn—O angles are in the range 92.04 (14)–128.39 (11)°. CdB₄O₇ (Ihara & Krogh-Moe, 1966) and Zn₃B₂O₆ (Baur & Tillmanns, 1970) also contain (CdO₄)⁶⁻ and (ZnO₄)⁶⁻ groups, respectively, with similar bond lengths and angles. The average Cd/Zn—O bond length in CZBO is shorter than the average of the Cd—O bonds in Cd₃B₂O₆ (Laureiro et al., 1991), because the ionic radius of Zn²⁺ is smaller than that of Cd²⁺, and also because Cd is coordinated by six O atoms in the structure of Cd₃B₂O₆.

Chen et al. (1990) classified known NLO borate crystals by their coordinated anionic groups and succeeded in predicting second-order NLO susceptibilities. According to their classification, the NLO-active clusters in CZBO should be the anionic groups, [BO₃]³⁻. It is favourable for the superposition of microscopic second-order NLO susceptibilities if the planar [BO₃]³⁻ groups are arranged with the same orientation in the structure of CZBO. In addition, the presence of distorted Cd(Zn)O₄ tetrahedra sharing vertices with BO₃ triangles may not only enhance the NLO effect, but also make CZBO easier to grow, because of strong binding between the layered structural units. The structural features indicated above show that CZBO is a potential NLO crystal. In fact, a strong second harmonic generation (SHG) of Nd:YAG laser radiation (λ 1064 nm) was observed on the as-grown single crystals.

S2. Experimental

Crystals of the title compound were grown by spontaneous nucleation in a platinum crucible using a vertical cylindrical electric furnace. Starting materials were prepared from a mixture of 43.9 wt% CdO, 27.9 wt% ZnO and 28.2 wt% H₃BO₃. Crystal growth was carried out at 1170 K under an air ambient. A clear colourless single-crystal of CZBO, with a size of 3 × 1.5 × 1 mm, was obtained.

S3. Refinement

The systematic absences of hkil with −h+k+l = 3n, hh2(-h)l with l = 3n, h(-h)0 l with 2 h+l = 3n and l = 2n showed that the space group should be R3c or R3c. However, the nonlinear optical properties of the crystal suggested that the space group must be the noncentrosymmetrical space group, R3c. The metal ions, Cd²⁺ and Zn²⁺, share the same site. According to the results of ICP-AES and occupancy refinement, the occupancy of the metal site was assigned to be 0.5 C d+0.5Zn in the final refinement.

Computing details

Data collection: XSCANS (Bruker, 1997); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL (Bruker, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the molecular packing in CZBO along the [001] direction. Small shaded circles represent B atoms and small open circles represent O atoms, while large hatched circles depict Cd or Zn atoms.
	Fig. 2. A view of the molecule of CZBO with 50% probability displacement ellipsoids, showing the atomic numbering scheme. Cd and Zn share the same site. [Symmetry codes: (a) 1 − y, 1 + x-y, z; (b) 2 − y, 2 + x-y, z; (c) y-x, 1 − x, z; (d) y-x, 2 − x, z; (e) 4/3 − y, 5/3 − x, 1/6 + z; (f) 1/3 + x, 5/3 + x-y, 1/6 + z; (g) 5/3 − y, 4/3 − x, z − 1/6; (h) x − 1/3, 4/3 + x-y, z − 1/6.]

tricadmium trizinc tetraborate

Crystal data

Cd3Zn3(BO3)4	Dx = 5.145 Mg m^−3
Mr = 768.64	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 21 reflections
Hall symbol: R 3 -2"c	θ = 4.8–14.7°
a = 8.364 (6) Å	µ = 13.52 mm^−1
c = 12.286 (10) Å	T = 293 K
V = 744.3 (10) Å^3	Block, colourless
Z = 3	0.12 × 0.10 × 0.10 mm
F(000) = 1050

Data collection

Bruker P4 diffractometer	526 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.094
Graphite monochromator	θmax = 32.5°, θmin = 4.4°
ω scans	h = −12→12
Absorption correction: ψ (North et al., 1968)	k = −12→12
Tmin = 0.195, Tmax = 0.260	l = −18→18
3235 measured reflections	3 standard reflections every 100 reflections
607 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0013P)^2 + 35.8548P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.052	(Δ/σ)max = 0.031
wR(F^2) = 0.121	Δρmax = 1.59 e Å^−3
S = 1.07	Δρmin = −1.38 e Å^−3
607 reflections	Extinction correction: SHELXL97 Ref, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
35 parameters	Extinction coefficient: 0.01004 (19)
1 restraint	Absolute structure: Flack (1983), with x Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.11 (16)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cd1	0.35586 (4)	1.01497 (6)	0.85915 (17)	0.04746 (9)	0.50
Zn1	0.35586 (4)	1.01497 (6)	0.85915 (17)	0.04746 (9)	0.50
B1	0.6667	1.3333	0.7318 (6)	0.0310 (15)
B2	0.3333	0.6667	0.8113 (6)	0.0343 (18)
O1	0.5513 (4)	1.1448 (4)	0.7314 (3)	0.0433 (8)
O2	0.2261 (4)	0.7485 (4)	0.8122 (3)	0.0463 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.02540 (10)	0.03586 (12)	0.07608 (18)	0.01155 (8)	0.00405 (16)	−0.02080 (12)
Zn1	0.02540 (10)	0.03586 (12)	0.07608 (18)	0.01155 (8)	0.00405 (16)	−0.02080 (12)
B1	0.0251 (16)	0.0251 (16)	0.043 (3)	0.0125 (8)	0.000	0.000
B2	0.0286 (18)	0.0286 (18)	0.046 (4)	0.0143 (9)	0.000	0.000
O1	0.0341 (10)	0.0255 (9)	0.0664 (17)	0.0120 (8)	0.0029 (12)	0.0054 (11)
O2	0.0425 (10)	0.0415 (10)	0.0655 (17)	0.0290 (7)	0.0042 (12)	0.0063 (12)

Geometric parameters (Å, º)

Cd1—O2	2.014 (3)	B1—O1^iv	1.377 (3)
Cd1—O1^i	2.016 (4)	B1—O1	1.377 (3)
Cd1—O2^ii	2.095 (4)	B1—O1^v	1.377 (3)
Cd1—O1	2.131 (4)	B2—O2	1.374 (4)
Cd1—Cd1^i	3.3451 (18)	B2—O2^vi	1.374 (4)
Cd1—Cd1^iii	3.3451 (18)	B2—O2^vii	1.374 (4)
Zn1—O2	2.014 (3)	O1—Zn1^iii	2.016 (4)
Zn1—O1^i	2.016 (4)	O1—Cd1^iii	2.016 (4)
Zn1—O2^ii	2.095 (4)	O2—Zn1^viii	2.096 (4)
Zn1—O1	2.131 (4)	O2—Cd1^viii	2.096 (4)
O2—Cd1—O1^i	115.23 (12)	O1^iv—B1—O1^v	119.998 (8)
O2—Cd1—O2^ii	128.39 (11)	O1—B1—O1^v	119.999 (8)
O1^i—Cd1—O2^ii	99.95 (14)	O2—B2—O2^vi	119.994 (12)
O2—Cd1—O1	101.80 (15)	O2—B2—O2^vii	119.994 (12)
O1^i—Cd1—O1	118.55 (17)	O2^vi—B2—O2^vii	119.994 (11)
O2^ii—Cd1—O1	92.04 (14)	B1—O1—Zn1^iii	120.0 (2)
O2—Cd1—Cd1^i	142.72 (10)	B1—O1—Cd1^iii	120.0 (2)
O1^i—Cd1—Cd1^i	37.41 (9)	B1—O1—Zn1	120.0 (3)
O2^ii—Cd1—Cd1^i	62.69 (9)	Zn1^iii—O1—Zn1	107.49 (14)
O1—Cd1—Cd1^i	113.94 (10)	Cd1^iii—O1—Zn1	107.49 (14)
O2—Cd1—Cd1^iii	74.67 (10)	B1—O1—Cd1	120.0 (3)
O1^i—Cd1—Cd1^iii	152.12 (11)	Zn1^iii—O1—Cd1	107.49 (14)
O2^ii—Cd1—Cd1^iii	91.70 (10)	Cd1^iii—O1—Cd1	107.49 (14)
O1—Cd1—Cd1^iii	35.09 (10)	B2—O2—Cd1	115.39 (19)
Cd1^i—Cd1—Cd1^iii	142.32 (3)	B2—O2—Zn1	115.39 (19)
O2—Zn1—O1^i	115.23 (12)	B2—O2—Zn1^viii	118.9 (3)
O2—Zn1—O2^ii	128.39 (11)	Cd1—O2—Zn1^viii	115.94 (18)
O1^i—Zn1—O2^ii	99.95 (14)	Zn1—O2—Zn1^viii	115.94 (18)
O2—Zn1—O1	101.80 (15)	B2—O2—Cd1^viii	118.9 (3)
O1^i—Zn1—O1	118.55 (17)	Cd1—O2—Cd1^viii	115.94 (18)
O2^ii—Zn1—O1	92.04 (14)	Zn1—O2—Cd1^viii	115.94 (18)
O1^iv—B1—O1	119.999 (6)

Symmetry codes: (i) −y+4/3, −x+5/3, z+1/6; (ii) x+1/3, x−y+5/3, z+1/6; (iii) −y+5/3, −x+4/3, z−1/6; (iv) −y+2, x−y+2, z; (v) −x+y, −x+2, z; (vi) −x+y, −x+1, z; (vii) −y+1, x−y+1, z; (viii) x−1/3, x−y+4/3, z−1/6.