Hydrothermal synthesis, characterization and nonlinear optical effect of orthorhombic phase Ca2BsOll H20

GUO Fan ', FU Peizhen *, WANG Junxin ', LIU Feng 2, YANG hi ping * & WU Yicheng I . Department of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 2. Department of Chemistry, University of Science and Technology of China, Hefei 230026, China Correspondence should be addressed to Guo Fan (e-mail: gof@ustc.edu.cn)

Abstract Hydrothermal treatment of calcium oxide and boric acid mixtures at temperatures between 234°C and 300°C has produced a colorless, transparent, orthorhombic compound Ca2B,O,, H20. Of the seven known members of the series of hydrated dicalcium hexaborate containing boron-oxygen six-membered ring anionic group (B~O~)'., only the title compound has been found to have the nonlinear optical effect. The second harmanic generation (SHG) effect of its crystal is larger than that of KH2P0, (KDP). The reflection spectrum has shown that this compound has no absorption in the experimental wavelength range (800-240 nm). Its crystal structure is favorable for generating the nonlinear optical effect.

Keywords: nonlinear optical crystal, hydrothermal synthesis, Cad36011 H10, calcium borate, (~308)'-.

Over the past years, many anhydrous borate nonlinear optical (NLO) crystals with excellent properties have been discovered, including P-BaB204 (BBO), LiB305 (LBO), KBe2B03F2 (KBBF), Sr2Be2B207 (SBBO) and others"]. However, there are many hydrated borates. Some of them possess the end oxygen atoms in boron-oxygen group combining with H+ ions, forming (OH) group. The harmful effect of (OH) to the second harmanic generation (SHG) coefficients is negligible. Taking this into consideration, it will be valuable in both theory and application to search for hydrated borate NLO crystals by means of hydrothermal synthesisi2'.

It is well known that dicalcium hexaborate hydrate (formula: Ca2BaOll H20, or 2Ca0 3B203 H20, or Ca[B3O5(0H)I ) has two different structures: monoclinic phase (point group C2h. space group P2&) and orthorhombic phase (point group C2,,, space group Pbn2'). The former occurs in nature, as the mineral fabianite, whereas the latter is synthetic. Their anionic groups are nonplanar (~308)'-.

The orthorhombic Ca2B6011 H20 was prepared for the first time in the Clark laboratory when inyoite (2Ca0 3B203 13H20) with water were heated at 400°C and under 2 OOOX lo5 Pa Hz0 pressure[3s41 . ~arkerson '~ ' synthesized this compound by reaction of boric acid aqueous solution with the minerals meyerhofferite 2Ca0 3B203 7H20, gowerite CaO 3B203 5H20, and ginorite 2Ca0 7B203 8H20, respectively. Nevertheless, there are more phases of calcium borate obtained by the above methods. In this work, calcium oxide and boric acid were used as the starting materials, and pure dicalcium hexaborate hydrate products were formed by hydrothermal treatment between 234°C k 2°C and 300°C k 2°C under a corresponding pressure of (25-70)x lo5 pac6'.

A small autoclave with a polytetrafluoroethylene seal ring was used as the container. First, the analytical grade CaC03 was decomposed into CaO under the temperature of above 900°C, and then the analytical grade H3B03 and the newly-born CaO were mixed with the mole ratio of B203:CaO ranging from 1.5 : 1 to 2 : 1. The mixtures were put in the autoclave, which was then filled with distilled water to the degree of filling of 30%-70% and heated at 234-300°C 2°C for 48-96 h.

At the end of the experiment, the autoclave was cooled to room temperature, and opened. The resulting products were moved into a flask, boiled and washed successively with distilled water in order to exclude the excessive boric acid. Finally, the products were filtered and dried at 40°C.

The powder SHG experiment was conducted on a Q-switched N ~ ~ + : Y A G laser at room tempera- ture. KDP crystal powder severed as the standard. The intensity of the green light produced by the

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orthorhombic Ca2B601 H20 crystal powder irradiated with IR ray was 2-3 times that produced by KDP powder, which indicated that orthorhombic Ca2B6011 H20 has a powder SHG effect larger than that of KDP.

2 Results and discussio.i

( i ) Synthesis temperature and process. schaferf6] pointed out that the lowest temperature for synthesizing orthorhombic Ca2B6Ol1 H20 was (213 f 5)"C in the Ca0-B203-H20 system under hydrothermal conditions, the corresponding pressure was about 18 X 10' Pa. However, in our experiment, the pure orthorhombic dicalcium hexaborate hydrate could not be obtained until the temperature was above (234 + 2)"C. When the temperature was lower than (234 f 2)"C, the products consisted of 2Ca0 5B203 5Hz0 and 4Ca0 5Bz03 7H20.

The anionic group of 2Ca0 5B203 5H20 and 4Ca0 5B203 7H20 is a twined boron-oxy en six-membered ring of five boron atoms, its chemical formula is expressed as [B506+a(OH)b] (2a+%-3)-

(0 2 a 5 8, 0 5 b 2 8 and 0 5 a+b 5 8), at above (234 f 2)"C, [ B ~ o ~ + ~ ( o H ) ~ ] ( ~ ~ + ~ - ~ ) was firstly dissolved and destroyed by hot boric acid aqueous solution. The dissolving process can be described as follows:

[ B S O ~ + ~ ( O H ) ~ ] ( ~ ~ + ~ - ~ ) - + (a+6)H2O + (8-2a-b)B(OH)3 + (2a+b-3)(B(OH)J (1) According to Clark polymerization rulesf4], the polymerization of anionic groups can be presented

by the following equations:

B(OH)3+2B(OH)4 + [ B ~ o ~ ( o H ) ~ ] ~ - + 3H20 (2)

( ii ) Characterization. The orthorhombic phase Ca2B601 H20 is colorless and transparent. No obvous cleavage or fracture has been observed; and in air for several months, this crystal has not been found to have deliquescence properties.

The typical samples were synthesized at 285" in the B2O3 : CaO mole ratio of 2 : 1 and with the degree of filling of 40%. X-ray powder diffraction pattern (XRD), thermogravimetry (TG), differential thermal analysis (DTA), and infrared spectrum (IR) are shown in figs 1, 2 and 3, respectively.

Fig. 1 shows that the product is the pure orthorhombic phase Ca2B6OI1 H20, and there are no other phases. The indexed X-ray powder diffraction data agreed well with the JCPDS card. If the samples for X-ray powder diffraction are not ground to enough fine and homogeneous grains, the diffi-action peaks of (200) and (040) are abnormaly intensive.

2 8/(" ) Fig. 1. X-ray powder diffraction patterns of orthorhombic Ca2BhOll Hz0 (CuKa, 1.540 56 X 0.1 nm).

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From fig. 2(a), the weight loss of water is about 5.32%-5.33% at 686-741 "C, which is in good agreement with theoretical calculation value 5.3 1 % of the water content in orthorhombic Ca2B601 H 2 0 . The dehydration endothermic peak appears on the DTA curve (see fig. 2(b)) at about 657- 686"C, which agrees basically with the temperature at which colemanite (2Ca0 3B203 5H20 or Ca(B304(0H)3) H20) and inyoite (2Ca0 3B3 O3 13H20 or Ca(B203(0H)~) 5H20) split out of the last water molecule[71. The exothermic peak is at about 774°C.

The infrared spectrum of the orthor- hombic Ca2B6OI1 H20 (fig. 3) is very similar to that of fabianiteCs1. First , the absorption peaks of 3-fold coordination boron-oxygen group (B03) in both com- pounds appear at 1 400 cm-I f , but smoother single peak of fabianite and continuous double peaks of orthorhombic Ca2B6011 H20 and the intense

Fig. 2. Curve of the thermogravimetry (a) and differential thermal analysis (b) (20eC/min) of orthorhombic CazB601 I HzO.

4000 3500 3000 2500 2000 1750 1500 1250 lo00 750 500 Wave numberlcm-'

Fig. 3. Infrared spectrum of orthorhombic CazB6011 HzO.

absorption peak of orthorhombic Ca2B601 H20 appear at about 1 28 1 cm-I. Second, the absorption peaks of 4-fold coordination boron-oxygen group (B04) are observed at about 830-1 180 cm-I. At about 1 640 cm-' HOH bending vibration peaks are not shown for both compounds. At about 3 400 cm-I, the (OH) stretching vibration peak of both compounds is obvious and sharp.

(iii) Nonlinear optical effect and structure. The basic structural unit of the orthorhombic Ca2B6OI1 Hz0 is the ( ~ ~ 0 ~ ) ' - (AlT2) anionic group. It consists of a nonplanar boron-oxygen six-mem-

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bered ring with one of the boron atoms three-fold coordinated by oxygen atoms, another boron atom four-fold coordinated by oxygen atoms, and the third boron atom four-fold coordinated by three oxygen atoms and one hydroxyl.

Chen calculated the microscopic second-order susceptibilities and SHG coefficients of boron-oxygen anionic groups with or without hydroxyl groups. The results showed that there is no obvious difference between both kinds of anionic groups'91. The power effect of SHG of the orthorhombic Ca2B601 H20 crystal is larger than that of KDP. This demonstrated that the existence of (OH) group may have a negligible effect on the overall SHG effect of the crystal.

The seven known members of series of dicalcium hexaborate hydrates, which have the general oxide formula 2Ca0 3B2O3 xH20 (x=l, 3,5,7,9, 13, respectively ) are composed of ( ~ 3 0 8 ) ~ - (AlT2) group, the basic structural unit. According to the number of (OH) groups in anionic group, they can be divided into three kinds of groups: (B~o~(oH))~- with one (OH) group, ( B ~ o ~ ( o H ) ~ ) ~ - with three (OH) groups, and ( B ~ o ~ ( o H ) ~ ) ~ - with five (OH) groups.

The (OH) groups in (B~o~(oH)~)'- are made up of five end oxygen atoms and five hydrogen atoms (eq. ( 2 ) ) , ( B ~ o ~ ( o H ) ~ ) ~ - groups are linked through ca2' ion and hydrogen bonds, which belongs to isolated ( ~ 3 0 ~ ) ~ - structural type. Eq. (3) shows that (B~o~(oH)~)~ - groups are polymerized into (B~o~(oH)~)~ - , whose boron-oxygen six-membered rings are linked into chains ([B~o~(oH)~],)~"- through oxygen bridge B(II1)-0-B(1V). ( B ~ O ~ ( O H ) ~ ) ~ - is a chain- structure type of ( B ~ O ~ ) ~ - .

( B ~ o ~ ( o H ) ~ ) ~ groups were polyme rized further into layer structure type [B~o~(oH)]~- (eq. (4)), existing in orthorhombic Ca2B6 011 *H20, the only compound with SHG effect in the series of the seven known hydrated dicalcium hexaborates. From fig. 4, it can be seen that the only (OH) group of Ca2BsOll H20 lies in B(lV)(2, (the subscript figures represent marks of B atoms or 0 atoms on the boron-oxygen six-membered ring, the same below). The rings are linked into chains through oxygen bridge B(III)( I )-0(40r 7)-B(N)(3). Additionally, by means of oxygen bridge B(III)(2)-0(5 or *)-B(IV)(3) these chains are further Fig. 4. Projection on (010) of orthorhombic Ca~BsOll HzO. 0, linked into layer ([B~o~(oH)],)~"- in which the oxygen; 0, hydroxyl group; a, boron; Ca atoms are not shown.

end of its (B30817- anionic group has no isolated oxygen atoms (fig. 4). ([B~O~(OH)],,)~- layers are tightly joined together through the calcium-oxygen bond. Each calcium ion is surrounded by an irregular octahedral array of five oxygen atoms and one hydroxyl. Although the microscopic second-order susceptibility of ( ~ ~ 0 ~ ) ' - is smaller than that of (B30613- or ( ~ ~ 0 7 ) ~ - [ " ~ ' , the suitable spatial arrangement of in the orthorhombic Ca2B6011 Hz0 crystal is beneficial to the geometric sum of the microscopic second-order susceptibilities of the basic structural units. On the other hand, the number of anionic groups per unit volume of this crystal is 0.97X (x10-~ nm3), which is about 1.5 times as large as that of BBO or 5 LBO crystal1'", thus enhancing the SHG effects of this crystal.

In general, the planar ( ~ 0 ~ ) ~ - groups are more 'g ::;I 5 favorable to producing larger SHG coefficients, g whereas tetrahedral (~04)'- groups tend to shift the "240 400 600 800

absorption edge to a shorter wavelength. As has been WaveIengtNnm stated, the boron-oxygen six-membered ring in the Fig, 5. Reflection spectrum of orthor~ombic

orthorhombic Ca2B60 I H20 crystal has two B(W) CazB601 I HzO (air medium).

Chinese Science Bulletin Vol. 45 No. 19 October 2000 1759

atoms. It can be inferred that the absorption edge is in the UV region. The reflection spectra of this crystal showed that this compound has not resulted in absorption in the experimental wavelength range of 80@-240 nm (fig. 5). At the same time, four of the five end oxygen atoms in ( ~ ~ 0 ~ ) ' - group of this crystal link with B atoms, and the last end oxygen atom forms (OH) group, thus there is no isolated oxygen atom in (~308)'- group of this crystal. As a result, it is favorable for the shift of the absorption edge to a shorter wavelength.

Acknowledgements The authors gratefully acknowledge Wang Guofu, Guo Weifu, Zhang Guochun, Liu Hongjun, et al. for their contribution to this work. This work was supported by the National Natural Science Foundation of China (Grant No. 69678026).

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(Received March 2,2000)

Chinese Science Bulletin Vol. 45 No. 19 October 2000