Subsolidus phase relation in the system ZnO–Li2O–MoO3

Journal of Alloys and Compounds 430 (2007) 67–70

Subsolidus phase relation in the system ZnO–Li2O–MoO3 Liping Xue a , Dagui Chen a , Zhang Lin a , Peiwen Lv a , Feng Huang a,∗ , Jingkui Liang a,b,c a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, PR China c International Center for Materials Physics, Academia Sinica, Shengyang 110015, PR China Received 21 November 2005; received in revised form 26 April 2006; accepted 26 April 2006 Available online 2 June 2006

Abstract The subsolidus phase relation of the system ZnO–Li2 O–MoO3 has been investigated by X-ray diffraction (XRD) analyses. The phase diagram has been constructed. There are six binary compounds and one ternary compound in this system. The phase diagram comprises nine three-phase regions. The ternary compound Li2 Zn2 (MoO4 )3 is refined by the Rietveld method. It belongs to an orthorhombic system with space group Pnma ˚ b = 10.4906 A, ˚ c = 17.6172 A. ˚ and lattice constants a = 5.1114 A, © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; Crystal structure; X-ray diffraction; Phase diagram; Rietveld analysis

1. Introduction Wide band gap semiconductor is a kind of promising material that has great advantage over traditional semiconductors. It can be used for fabricating devices and detectors with higher quality and suitable working at severe situation. Single crystal zinc oxide (ZnO) is one of such wide band gap semiconductor with potential applications in substrates, light emitting diodes, UV detectors, laser diodes and high frequency electronic devices [1]. With the increasing need for high quality and large size ZnO crystal, several investigations have already been conducted to grow bulk ZnO crystals using variations of the three primary methods: hydrothermal solution growth [2–4], the vapor phase growth [5], and melt growth [6–8]. Hydrothermal solution method can guarantee the achievement of very good quality crystal, unfortunately the crystals growth speed is very slow and it is not currently commercially feasible from an economic standpoint [2–4]. Good crystal quality can be also achieved by the vapor phase growth, while the growth condition is very difficult to manipulate [5]. The melt growth process employs the use of a modified Bridgman configuration, producing very good quality crystals with low defects in much less time [6–8]. The Czochral-

∗

Corresponding author. E-mail address: [email protected] (F. Huang).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.066

ski method is another alternative for growing large single crystal from a congruent melt. However, it seems does not suitable for growing ZnO single crystal with high melting point (1975 ◦ C) and high volatility. In order to suppress the ZnO evaporation during crystal growth, the crystal must be grown from a solvent with a growth temperature as low as possible. Therefore, in the work, we have studied the phase relation of ZnO–Li2 O–MoO3 system, in order to find suitable fluxes for growing large ZnO single crystal at relative low temperature. 2. Experimental Samples with different compositions were all prepared by solid-state chemistry reaction in air. The purity of the starting materials (ZnO, Li2 CO3 , MoO3 ) is higher than 99.9%. The original samples with a certain chemical compositions were mixed thoroughly, ground in an agate mortar, and pressed into pellets with diameter of 10 mm and thickness of 1–2 mm at a pressure around 108 Pa. Then the pellets were sintered at 500–650 ◦ C in air for about 72 h and slowly cooled in the furnace to room temperature. The temperature of the furnace was measured with a Pt–PtRh thermocouple and was precisely controlled to within ±2 ◦ C up to 1200 ◦ C with an intelligent controller. The above process should repeat several times until the X-ray pattern of the specimen showed no change upon successive heat treatment, which represented the equilibrium was achieved. The compositions of the samples prepared in the system are shown in Table 1. Phase identification of the samples was carried out on a Rigaku D/max 2500 diffractometer with Cu K␣ radiation (50 kV × 250 mA) and a graphite monochromator using continuous mode at a rate of 2θ = 4◦ /min. X-ray powder diffraction (XRD) data used for ternary compound Li2 Zn2 (MoO4 )3 structural

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L. Xue et al. / Journal of Alloys and Compounds 430 (2007) 67–70

Table 1 List of phase identification for samples with different composition in the system ZnO–Li2 O–MoO3 Sample

MoO3 (X) mol.%

ZnO (Y) mol.%

Li2 CO3 (Z) mol.%

Phase identification

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

5 15 18.75 30 20 10 25 40 40 25 50 30 35 43.75 50 47.5 55 70 62.5 70 80 63.6 60 70 61.5 55 60

65 15 43.75 30 50 80 50 20 35 60 25 60 60 50 42.5 50 35 25 25 15 10 18.2 20 5 15.4 15 5

30 70 37.5 40 30 10 25 40 25 15 25 10 5 6.25 7.5 2.5 10 5 12.5 15 10 18.2 20 25 23.1 30 35

ZnO + Li2 CO3 + Li4 MoO5 ZnO + Li2 CO3 + Li4 MoO5 ZnO + Li4 MoO5 ZnO + Li2 MoO4 + Li4 MoO5 ZnO + Li2 MoO4 + Li4 MoO5 ZnO + Li2 MoO4 ZnO + Li2 MoO4 ZnO + Li2 MoO4 ZnO + Li2 MoO4 + Li2 Zn2 (MoO4 )3 ZnO + Li2 MoO4 + Li2 Zn2 (MoO4 )3 Li2 MoO4 + Li2 Zn2 (MoO4 )3 ZnO + Li2 Zn2 (MoO4 )3 ZnO + Li2 Zn2 (MoO4 )3 + Zn3 Mo2 O9 Li2 Zn2 (MoO4 )3 + Zn3 Mo2 O9 Li2 Zn2 (MoO4 )3 + ZnMoO4 Li2 Zn2 (MoO4 )3 + ZnMoO4 + Zn3 Mo2 O9 MoO3 + Li2 Zn2 (MoO4 )3 + ZnMoO4 MoO3 + Li2 Zn2 (MoO4 )3 + ZnMoO4 MoO3 + Li2 Zn2 (MoO4 )3 MoO3 + Li2 Mo4 O13 + Li2 Zn2 (MoO4 )3 MoO3 + Li2 Mo4 O13 + Li2 Zn2 (MoO4 )3 Li2 Mo4 O13 + Li2 Zn2 (MoO4 )3 Li4 Mo5 O17 + Li2 Mo4 O13 + Li2 Zn2 (MoO4 )3 Li4 Mo5 O17 + Li2 Mo4 O13 + Li2 Zn2 (MoO4 )3 Li4 Mo5 O17 + Li2 Zn2 (MoO4 )3 Li4 Mo5 O17 + Li2 MoO4 + Li2 Zn2 (MoO4 )3 Li4 Mo5 O17 + Li2 MoO4 + Li2 Zn2 (MoO4 )3

analyses was collected by step scan mode with a step width of 2θ = 0.02◦ and a sampling time of 2 s. The DTA investigation was conducted by NETZSCH-STA449C (Germany) in platinum crucible. The measurements were performed in the atmosphere of air in the temperature range 30–1200 ◦ C. The heating rate was 10K/min and the reference substance was ␣-Al2 O3 .

3. Result and discussion

and α = 109.24◦ , β = 96.04◦ , γ = 95.95◦ [12]. Besides, in the Li2 O–Li2 MoO4 region, we also obtained Li4 MoO5 compound. It belongs to an orthorhombic system, and the XRD pattern is consistent with the report by Reau et al. [13]. In ZnO–MoO3 binary system, we found two compounds, ZnMoO4 and Zn3 Mo2 O9 . This result is in good agreement with the phase diagram reported by Kohlmuller and Faurie [14]. The compound ZnMoO4 belongs to a triclinic system with space

According to the results of XRD analysis, the subsolidus phase relation of the system ZnO–Li2 O–MoO3 is shown in Fig. 1, which consists of nine three-phase regions. Two-phase regions are joint-line of the two compounds. 3.1. Binary system In Li2 O–MoO3 binary system, the phase diagram of Li2 MoO4 –MoO3 has already been reported by Hoermann [9] and Brown et al. [10]. Three binary compounds, Li2 Mo2 O7 , Li2 Mo3 O10 and Li2 Mo4 O13 , have been found in Hoermann’s work [9]. While two compounds, Li4 Mo5 O17 and Li2 Mo4 O13 , have been found by Brown et al. [10]. Under our experimental conditions, Li4 Mo5 O17 and Li2 Mo4 O13 were identified. This result is in good agreement with Brown’s report. Compound ¯ Its Li4 Mo5 O17 belongs to a triclinic system with space group P 1. ˚ b = 9.461 A, ˚ c = 10.800 A ˚ and lattice parameters are a = 6.777 A, α = 73.16◦ , β = 88.98◦ , γ = 69.76◦ [11]. Compound Li2 Mo4 O13 ¯ Its also belongs to a triclinic system with space group P 1. ˚ b = 11.450 A, ˚ c = 8.225 A ˚ lattice parameters are a = 8.578A,

Fig. 1. Subsolidus phase relations of the system ZnO–Li2 O–MoO3 .

L. Xue et al. / Journal of Alloys and Compounds 430 (2007) 67–70

¯ Its lattice parameters are a = 8.367 A, ˚ b = 9.691 A, ˚ group P 1. ˚ and α = 106.87◦ , β = 101.72◦ , γ = 96.73◦ [15]. Comc = 6.964 A pound Zn3 Mo2 O9 belongs to a monoclinic system with space ˚ b = 7.131 A, ˚ group P21 /m. Its lattice parameters are a = 7.757 A, ˚ and β = 117.39◦ [16]. c = 8.370 A In ZnO–Li2 O binary system, no binary compound has been reported, and under our experimental conditions, no binary compound has been found. The quasi-binary system ZnO–Li2 MoO4 has also been investigated by means of DTA method. The result reveals quasi-binary system ZnO–Li2 MoO4 is eutectic system. But the eutectic temperature is very close to the melting point of Li2 MoO4 . That is to say the solubility of ZnO in melted Li2 MoO4 is very little. 3.2. Ternary system In ZnO–Li2 O–MoO3 ternary system, we only found one compound: Li2 Zn2 (MoO4 )3 , which is consistent with Efremov’s report [17]. Gioquel and Seances [18] mentioned that Li2 Zn2 (MoO4 )3 is orthorhombic system with space group Pnma, its lattice parame˚ b = 10.430 A, ˚ c = 17.540 A. ˚ But the structure ters are a = 5.092 A, parameters have not been reported. We compared the X-ray pattern of Li2 Zn2 (MoO4 )3 to that of known Li2 Co2 (MoO4 )3 and found that these two compounds are isostructural. Both compounds crystallize in an orthorhombic system with space group Pnma and the lattice constants are similar. According to the initial structure model of Li2 Co2 (MoO4 )3 [19], We refined the structure of the Li2 Zn2 (MoO4 )3 compound from the powder XRD data by the Rietveld method [20] using the computer program DBWS-9807 [21]. The XRD data for the 2θ regions between 5◦ and 120◦ was used for the refinement. Good agreement between the experimental and the calculated profile with Rp = 6.69%, Rwp = 9.73%, Rexp = 6.58% was reached. Fig. 2 shows the observed and calculated XRD pattern. The vertical bars at the middle indicate Bragg reflection positions, and the

Fig. 2. Final Rietveld refinement pattern at 300 K of Li2 Zn2 (MoO4 )3 . Small crosses represent the experimental values and solid lines the calculated pattern. The vertical bars at the middle indicate Bragg reflection positions, and the bottom curve is the difference between the observed and calculated values.

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Table 2 Refined structure parameters of Li2 Zn2 (MoO4 )3 in space group Pnma, ˚ b = 10.4906 A, ˚ Z = 4, Rp = 6.69%, Rwp = 9.73%, Rexp = 6.58%, a = 5.1114 A, ˚ c = 17.6172 A Atom

Site

x

y

z

Sof

˚ 2) B (A

Mo1 Mo2 Zn1 Li1 Zn2 Li2 Zn3 Li3 O1 O2 O3 O4 O5 O6 O7

4c 8d 8d 8d 4c 4c 4c 4c 8d 4c 8d 4c 8d 8d 8d

0.2216(5) 0.2788(5) 0.7495(7) 0.7495(7) 0.3916(3) 0.3916(3) 0.7552(5) 0.7552(5) 0.5741(9) 0.1411(2) 0.4216(2) 0.9465(2) 0.3793(9) 0.0710(3) 0.0776(8)

0.2500 0.0270(9) 0.0779(6) 0.0779(6) 0.2500 0.2500 0.2500 0.2500 0.1025(2) 0.2500 0.1155(5) 0.2500 −0.1012(1) −0.0026(3) 0.1180(1)

0.0563(9) 0.8434(7) 0.9710(2) 0.9710(2) 0.2493(7) 0.2493(7) 0.8014(4) 0.8014(4) 0.8660(5) 0.1472(8) 0.0348(9) 0.0116(4) 0.7890(4) 0.9311(9) 0.7847(9)

1.0000 1.0000 0.6600 0.3400 0.4700 0.5300 0.2100 0.7900 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

0.58(7) 0.74(6) 1.37(8) 1.37(8) 1.22(4) 1.22 (4) 1.14 (5) 1.14(5) 1.66(9) 1.62(9) 1.58(8) 1.43(7) 1.52 (6) 1.61(9) 1.69(8)

bottom curve is the difference between the observed and calculated pattern. Table 2 gives the final R factor, the lattice and structure parameters. The bond lengths are listed in Table 3. To examine the validity of the structure model, we used Brown’s bond valence theory [22] to calculate the valence sum of the ions. The results of the calculations are given in Table 4. The crystal structure of Li2 Zn2 (MoO4 )3 is shown in Fig. 3. From Fig. 3 each Li atom could be substituted by Zn atom partially, that indicates solid solution regions exist in Li2 Zn2 (MoO4 )3 compound. Each Li(Zn) is bonded to six oxygen atoms to form an octahedron. The octahedron is distorted due to the different Li(Zn)–O bond lengths.

Table 3 ˚ in Li2 Zn2 (MoO4 )3 Selected bonds distance (A) Bonds

˚ Distance (A)

Zn1(Li1)–O1 × 1 –O3 × 1 –O3 × 1 –O4 × 1 –O6 × 1 –O6 × 1

2.071(4) 2.057(1) 2.213(1) 2.186(9) 1.976(6) 2.105(4)

Zn2(Li2)–O2 × 1 –O2 × 1 –O5 × 2 –O5 × 2

2.207(8) 2.222(9) 2.065(1) 2.200(9)

Zn3(Li3)–O1 × 2 –O7 × 2 –O7 × 2

2.132(1) 2.172(5) 2.247(1)

Mo1–O2 × 1 –O3 × 2 –O4 × 1

1.653(5) 1.782(5) 1.612(1)

Mo2–O1 × 1 –O5 × 1 –O6 × 1 –O7 × 1

1.751(1) 1.729(1) 1.901(4) 1.742(7)

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Table 4 The calculated results of the bond valence sum of the compound Li2 Zn2 (MoO4 )3 Cation

Bond valence sum

Mo1 Mo2 Zn1 Zn2 Zn3 Li1 Li2 Li3

5.93 5.70 2.09 1.78 1.65 0.96 0.93 0.87

Fig. 3. The project of the structure of Li2 Zn2 (MoO4 )3 along the a-axis.

4. Conclusions The phase diagram of the system ZnO–Li2 O–MoO3 comprises nine three-phase regions. There are six binary compounds in the binary system Li2 O–MoO3 , ZnO–MoO3 and ZnO–Li2 O. In the ternary system there is one ternary compound: Li2 Zn2 (MoO4 )3 . Compound Li2 Zn2 (MoO4 )3 is isostructural with Li2 Co2 (MoO4 )3 , and was further refined by the Rietveld method. It belongs to an orthorhombic system with space

˚ b = 10.4906 A, ˚ group Pnma and lattice constants a = 5.1114 A, ˚ Z = 4. c = 17.6172 A, Acknowledgements This research is supported by One Hundred Talent Program of Chinese Academy of Sciences (CAS) and National Natural Science Foundation of China. References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [2] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 47 (1964) 9. [3] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Sakagami, J. Cryst. Growth 214–215 (2000) 72. [4] E. Ohshima, H. Ogino, I. Niikura, K. Maeda, M. Ito, T. Fukuda, Semicond. Sci. Technol. 20 (2005) 49. [5] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jnones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 150 (1998) 399. [6] J.W. Nielsen, E.F. Dearborn, J. Phys. Chem. 64 (1960) 1762. [7] B.M. Wanklyn, J. Cryst. Growth 7 (1970) 107. [8] J. Nause, B. Nemeth, Semicond. Sci. Technol. 20 (2005) 45. [9] F. Hoermann, Z. Anorg. Allg. Chem. 177 (1928) 145. [10] W.S. Brown, H.S. Parker, R.S. Roth, J.L. Waring, J. Cryst. Growth 16 (1972) 115. [11] M. Wiesmann, H. Weitzel, I. Svoboda, H. Fouess, Z. Kristallogr. 212 (1997) 795. [12] B.M. Gatehouse, B.K. Miskin, J. Solid State Chem. 9 (1974) 247. [13] Reau, Fouassier, Hagenmuller, Bull. Soc. Chim. Fr. 10 (1967) 3873. [14] R. Kohlmuller, J.P. Faurie, Bull. Soc. Chim. Fr. 11 (1968) 4381. [15] Natl. Bur. Stand (U.S.) Monogr. 25 (1984) 132. [16] T. Soehnel, Z. Anorg. Allg. Chem. 622 (1996) 1274. [17] V.A. Efremov, Zh. Nerog. Khim. 20 (1975) 2200. [18] Gioquel, C.R. Seances, Acad. Sci.: Ser. C 275 (1972) 265. [19] M. Wiesmann, H. Svoboda, H. Weitzel, H. Fuess, Z. Kristallogr. 210 (1995) 525. [20] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [21] R.A. Young, A. Sakthirel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Crystallogr. 28 (1995) 336. [22] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244.