Phase relations in the ternary system SrO–TiO2–B2O3

Phase relations in the ternary system SrO–TiO2–B2O3

Journal of Alloys and Compounds 327 (2001) L10–L13 L www.elsevier.com / locate / jallcom Letter Phase relations in the ternary system SrO–TiO 2 –B...

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Journal of Alloys and Compounds 327 (2001) L10–L13

L

www.elsevier.com / locate / jallcom

Letter

Phase relations in the ternary system SrO–TiO 2 –B 2 O 3 a,b a, b b a a Z.F. Wei , X.L. Chen *, F.M. Wang , W.C. Li , M. He , Y. Zhang a

Centre for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing 100080, PR China b Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China Received 20 April 2001; accepted 7 May 2001

Abstract Phase relations of the ternary system SrO–TiO 2 –B 2 O 3 have been investigated by X-ray powder diffraction (XRD). This system can be divided into eight three-phase regions. Seven binary compounds are observed in this system, which are SrTiO 4 , Sr 3 Ti 2 O 7 , SrTiO 3 , Sr 3 B 2 O 6 , Sr 2 B 2 O 5 , SrB 2 O 4 , SrB 4 O 7 , but no ternary compound is found. The Rietveld refinement method was used to determine the structure of the compound Sr 3 B 2 O 6 and found it to be calcium-orthoborate structure. The cell parameters are determined to be ˚ c512.5665(2) A, ˚ and Sr, B, and O occupy the 18e, 12c, 36f positions, respectively.  2001 Elsevier Science B.V. a5b59.0405(2) A, All rights reserved. Keywords: Oxide materials; Optical materials; X-Ray diffraction; Phase diagram

1. Introduction Considerable attention has been focused on borates for decades. Excellent nonlinear optical (NLO) crystals have been found in borates. The first is K[B 5 O 6 (OH) 4 ]? 2H 2 O(KB 5 ), which is reported to be a useful ultraviolet NLO material [1]. After this, borates such as b-BaB 2 O 4 (BBO) [2], LiB 3 O 5 (LBO) [3], Sr 2 B 2 Be 2 O 7 (SBBO) [4] and the latest Ca 4 LnO(BO 3 ) 3 (CLnOB, where Ln5Gd, Y) [5] have been studied as promising NLO materials [6]. Recently, with the development of optical communications, there is a demand for birefringent crystals, which can be used as isolators and beam displacers. YVO 4 is presently one of the most widely used crystals for this purpose, but large and qualitative YVO 4 crystals are hard to grow. It has been known that B can coordinate to O in a variety of ways to form different B–O atom groups, such as [BO 3 ] 32 planar, and [B 3 O 7 ] 52 benzene-like rings. Polarizations perpendicular to and parallel to the [BO 3 ] 2 plane atoms are expected to be very different, just like the [CO 3 ] 22 in CaCO 3 , to produce large birefringence. TiO 2 is included to further enhance the optical anisotropy. The *Corresponding author. Tel.: 186-10-826-49039; fax: 186-10-82649531. E-mail address: [email protected] (X.L. Chen).

binary system SrO–B 2 O 3 has been investigated for years, and some compounds have been synthesized by various methods [7–10]. The ternary system SrO–TiO 2 –B 2 O 3 has rarely been studied before, nor has any ternary compound (Sr–Ti–B) been reported. In this study, we systematically investigate the phase relations in the ternary system SrO– TiO 2 –B 2 O 3 to search for new compounds with large birefringence.

2. Experimental procedure

2.1. Preparation of samples A series of samples with different compositions were prepared by standard solid state reaction techniques, heating mixtures of high purity SrCO 3 , TiO 2 and H 3 BO 3 which act as the starting materials. Raw materials were weighed accurately, then thoroughly mixed in an agate mortar. The specimens were put into Al 2 O 3 crucibles, and different heat treatments in air were carried out in a SiC globar muffle furnace according to the various compositions of the specimens. The firing temperatures were all below 1300 K and were measured with a Pt–PtRh thermocouple.

0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01564-X

Z.F. Wei et al. / Journal of Alloys and Compounds 327 (2001) L10 –L13

2.2. X-Ray powder diffraction analysis Phase identification of the samples was carried out with Rigaku D/ Max-2400 diffractometer with Cu Ka radiation at room temperature. Equilibrium was considered to have been reached when the X-ray pattern of the specimen showed no change with successive heat treatments or the X-ray powder data were consistent with results predicted from previous experiments.

2.3. Determination of the Sr3 B2 O6 structure In the binary system SrO–B 2 O 3 , there exists a binary compound Sr 3 B 2 O 6 , whose structure has not been accurately determined before. In this study we set out to obtain the value of the atomic positions by means of X-ray powder diffraction. High purity SrCO 3 and H 3 BO 3 were mixed thoroughly in a proportion 3:2 as the starting material. According to Richter [11], the firing temperature should be set at 1000–1500 K for 4 h. In our experiment, we found that when heated at 1300 K for at least 24 h, the corresponding XRD pattern can be best indexed. The data for the Sr 3 B 2 O 6 powder diffraction were collected in a step scanning mode with steps of 0.028 (2u ) at 291 K. An angular range from 2u 515 to 1258 and a measuring time of 1 s per step are applied. The divergence, scattering and receiving slits were set at 18, 18 and 0.3 mm, respectively. A total of 5751 points with a maximum count of 22 967 and 50 independent reflections were observed. The diffraction pattern is shown in Fig. 1. The Rietveld

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refinement was carried out using the computer program DBW9411 [12].

3. Results and discussion

3.1. The three binary systems In the system SrO–TiO 2 , there are the compounds Sr 2 TiO 4 [13], Sr 3 Ti 2 O 7 [14], Sr 4 Ti 3 O 10 [15], SrTiO 3 [16], Sr 2 Ti 6 O 13 [17], SrTi 11 O 20 [18], and SrTi 12 O 19 [15]. In this study under the present synthetic conditions, we only found three of these compounds, Sr 2 TiO4, Sr 3 Ti 2 O 7 , and SrTiO 3 , whose XRD patterns were in good agreement with those already reported. In the system SrO–B 2 O 3 , five compounds have been reported before, which are Sr 3 B 2 O 6 [9,11], Sr 2 B 2 O 5 [19], SrB 2 O 4 [20], SrB 4 O 7 [21] and SrB 6 O 10 [7]. Except SrB 6 O 10 , the other compounds were all observed in our experiment, and their diffraction data are consistent with those reported. In the system TiO 2 –B 2 O 3 , only one compound TiBO 3 is reported by Huber [22]. However, in this study this compound has not been observed after the heat-treatment. Some compounds reported before were, however, not observed in our experiment. We consider that this is due to the experimental difference between our study and those of others. In the present experiment, the samples were all sintered in air below 1300 K, while in the studies reported, high temperature and high pressure were often used.

Fig. 1. Final Rietveld refinement results for Sr 3 B 2 O 6 structure. 1 is the experimental pattern, the solid line shows the calculated pattern, the vertical bar indicates the Bragg positions for the diffraction line, and the solid line at the bottom is the difference between the experimental and calculated patterns.

Z.F. Wei et al. / Journal of Alloys and Compounds 327 (2001) L10 –L13

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Fig. 2. Subsolid phase relations in the system SrO–TiO 2 –B 2 O 3 below 1300 K.

3.2. The ternary system In the ternary system SrO–TiO 2 –B 2 O 3 , no ternary compound has been reported, and in our XRD analysis, no new compound was observed other than the reported binary compounds. Fig. 2 shows the phase diagram which is constructed from the XRD data of 21 specimens. The phase diagram contains eight three-phase regions. The phase identifications for the samples with different compositions are listed in Table 1.

hence we set the values of the atom positions of Ca 3 B 2 O 6 as the initial values of those of Sr 3 B 2 O 6 . The unit cell contains six Ca 3 B 2 O 6 formula units. The positions of the atoms Ca, B, O in Ca 3 B 2 O 6 are 18e (0.35814, 0, 0.25), 12c (0, 0, 0.11761) and 36f (0.16624, 0.01297, 0.11496), respectively [23]. For Sr 3 B 2 O 6 the parameters to be refined are x, y, z for Sr, z for B, and x for O. According to Hata [9], we set the initial structural parameters as a5 ˚ and c512.566 A. ˚ The temperature factors of B, 9.046 A, O, Sr were initially set as 1.0. The Rietveld method is a method to refine a crystal structure and it is based on a whole pattern point fitting method whereby thermal diffuse scattering, incoherent scattering and air scattering can be corrected. In the program DBW94, we use the Newton–Raphson algorithm to obtain the refined structural parameters including lattice parameters, positional parameters, the temperature factor and so on. The pseudo-Voigt function is chosen in the present study. After successive refinement the agreement factors R wp at last reached R wp 57.03%, and R p 55.03%. The result of the final Rietveld refinement is plotted in Fig. 1. Table 2 gives the corrected structural parameters at the end of the refinement.

4. Conclusions

3.3. The Sr3 B2 O6 structure refinement

From the research of the system SrO–TiO 2 –B 2 O 3 , we have attained the following results:

According to Richter [11], the compound Sr 3 B 2 O 6 is of ] the Ca 3 B 2 O 6 type structure and the space group R3c (167),

1. In the three binary systems, seven compounds have been observed in the XRD patterns. Eight three-phase

Table 1 List of phase identifications for various composition specimens in the system SrO–TiO 2 –B 2 O 3 No.

SrO (at.%)

TiO 2 (at.%)

BO 1.5 (at.%)

Phase identification

1 2 3 4 5 6 7 8a 9 10 11 12 13 14 15 16 17 18 19 20 21

60 20 20 40 40 80 48 6 10 62 56 56 50 50 25 60 62 56 46 56 60

20 60 20 40 20 10 12 34 60 20 10 30 28 40 75 40 32 20 26 4 0

20 20 60 20 40 10 40 60 30 18 34 14 22 10 0 0 6 24 28 40 40

Sr 3 Ti 2 O 7 1Sr 3 B 2 O 6 SrTiO 3 1TiO 2 1SrB 2 O 4 TiO 2 1SrB 2 O 4 1SrB 4 O 7 SrTiO 3 1TiO 2 1SrB 2 O 4 SrTiO 3 1SrB 2 O 4 SrO1Sr 2 TiO 4 1Sr 3 B 2 O 6 SrTiO 3 1Sr 2 B 2 O 5 1SrB 2 O 4 TiO 2 1SrB 4 O 7 TiO 2 1SrB 2 O 4 1SrB 4 O 7 Sr 2 TiO 4 1Sr 3 Ti 2 O 7 1Sr 3 B 2 O 6 SrTiO 3 1Sr 3 B 2 O 6 1Sr 2 B 2 O 5 Sr 3 Ti 2 O 7 1SrTiO 3 1Sr 3 B 2 O 6 SrTiO 3 1Sr 3 B 2 O 6 1Sr 2 B 2 O 5 SrTiO 3 1Sr 3 B 2 O 6 SrTiO 3 1TiO 2 Sr 3 Ti 2 O 7 1SrTiO 3 Sr 2 TiO 4 1Sr 3 Ti 2 O 7 1Sr 3 B 2 O 6 SrTiO 3 1Sr 3 B 2 O 6 SrTiO 3 1Sr 2 B 2 O 5 1SrB 2 O 4 SrTiO 3 1Sr 3 B 2 O 6 1Sr 2 B 2 O 5 Sr 3 B 2 O 6

a

The diffraction pattern of B 2 O 3 could not be observed because of its weak X-ray diffraction intensity.

Z.F. Wei et al. / Journal of Alloys and Compounds 327 (2001) L10 –L13

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Table 2 ] Final refined parameters from powder X-ray diffraction data for Sr 3 B 2 O 6 , R3c Atom

Sr B O

Wyck

18e 12c 36f

Atom positions x

y

z

0.3551(1) 0 0.1588(5)

0 0 0.0107(7)

0.25 0.1146(10) 0.1148(3)

Temperature ˚ 2) factor (A

Cell parameters

0.57(3) 0.66(32) 0.20(10)

˚ a5b59.0429(1) A ˚ c512.5664(1) A ˚3 v5889.834(30) A

R wp 57.03%, R p 55.03%, R exp 53.39%

regions exist in the phase diagram. No ternary compound has been observed. 2. The compound Sr 3 B 2 O 6 is of calcium-orthoborate ] structure, space group R3c, with cell parameters a5b5 ˚ c512.5664(1) A, ˚ v5889.834(30) A ˚ 3 , and 9.0429(1) A, Z56. The atom positions are (0.3551, 0, 0) for the atom Sr, (0, 0, 0.1145) for the atom B and (0.1587, 0.0105, 0.1148) for O.

Acknowledgements This work was financially supported by the Chinese Academy of Science and the National Natural Science Foundation of China. We would also thank T.S. Ning for his help in making the X-ray powder diffraction measurements.

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