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Large-size and high-quality Zn2TiO4 single crystal grown by the optical floating zone method
Journal of Crystal Growth 312 (2010) 3561–3563
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Large-size and high-quality Zn2TiO4 single crystal grown by the optical floating zone method Liang Li a,c, Hang Yang a,c, Dejun Wang b, Guanlin Feng c, Benxian Li d, Zhongmin Gao d, Dapeng Xu a,c,n, Zhanhui Ding d, Xiaoyang Liu d a
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Adult Education College of Changchun University, Changchun 130000, China c College of Physics, Jilin University, Changchun 130012, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e in fo
abstract
Article history: Received 4 February 2010 Received in revised form 9 September 2010 Accepted 16 September 2010 Communicated by R.S. Feigelson Available online 22 September 2010
Crack-free and transparent Zn2TiO4 single crystals of 4–6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The powder X-ray diffraction (XRD) results show that the as-grown crystals have the spinel-type Zn2TiO4 structure. XRD2 measurements on Zn2TiO4 wafers cut perpendicular to the growth direction display only one peak at 42.71, which indicates that the Zn2TiO4 single crystals grow along the /4 0 0S direction (a-axis). The formation of bubble inclusions was effectively suppressed by lowering rotation rate. Transmission polarized-light microscopy results showed that as-grown crystals were free of low angle grain boundaries. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. X-ray diffraction A2. Floating zone method A2. Single crystal growth B1. Zn2TiO4
1. Introduction Spinel structures, including normal (AB2O4) and inverse (A2BO4) spinels, have awakened great interest due to their potential use in important technological applications such as magnetic [1] or superhard materials [2], high-temperature ceramics [3] and highpressure sensors (with Cr3 + doping) [4]. Zinc titanate (Zn2TiO4) spinel is a high-effect regenerable catalytic sorbent for H2S, As, Se and other contaminants needing removal from hot coal gases [5–10]. Due to its excellent dielectric properties, Zn2TiO4 has also been used in the microwave devices. Numerous studies have been focused on its electrical and optical properties under different conditions to investigate its potential applications [11–15]. The phase diagram of the ZnO–TiO2 system was established by Dulin and Rase [16] and indicated that two compound forms: a spinel-type Zn2TiO4 and an illuminate-type ZnTiO3. The spinel-type Zn2TiO4 has a face-centered cubic unit cell, belonging to space group Fd-3m with a¼0.84608 nm [17], and is stable from room temperature to its melting point.
n
Corresponding author at: National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. Tel.: + 86 43188499048; fax: + 86 43188513216 E-mail address: [email protected] (D. Xu). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.09.045
Single-crystal samples with well-defined crystallographic orientation can help in understanding the materials properties. In 1979, Damen et al. [18] reported on the growth of Zn2TiO4 crystals using the Bridgman method, but these as-grown crystals had high-dislocation densities and some impurities introduced from the Pt crucible. There was little characterization of these Zn2TiO4 crystals because these as-grown Zn2TiO4 crystals were only of interest as a substrate. Since 1979, there have been no other reports on the growth of Zn2TiO4 crystals. Thus, all the properties of Zn2TiO4 measured to date have only been made on polycrystalline samples. To investigate its intrinsic characteristics, large Zn2TiO4 single crystals are preferable. The optical floating zone method is crucible-free, the atmosphere is controllable and steep temperature gradients can be maintained along the growth direction, which allows for faster and more stable crystal growth [19]. Therefore, this method is a suitable technique for growing high-quality single crystals. In this work, high-quality Zn2TiO4 single crystals have been successfully grown by this method for the first time.
2. Experimental procedures The starting Zn2TiO4 powders were prepared by calcining stoichiometric amounts of ZnO (Alfa Aesar 99.99%) and TiO2
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L. Li et al. / Journal of Crystal Growth 312 (2010) 3561–3563
(Alfa Aesar 99.99%) at 1473 K for 20 h with intermediate grinding. The Zn2TiO4 powders were packed into a cylindrical shape rubber tube and then pressed hydrostatically up to 70 MPa in a cold isostatic press to form a cylindrical rod of 4–6 mm in diameter and 50 mm in length. The rods were sintered in air at 1600 K for 10 h for use as the feed and support rods. The crystals were grown in an optical floating zone furnace (CSI FZ-T-10000-H-VI-VP, Crystal systems, Inc., Japan) equipped with four 1000 W halogen lamps as the heat source. The growth conditions were as follows: the seed and feed rods were rotated in opposite directions at the rates of 30, 10 and 5 rpm. The growth rate was 10.0 mm/h. The air flow of 0.1 L/min and the pressure of 0.1 MPa were applied. Macroscopic defects such as low-angle grain boundaries and inclusions were studied with an Olympus Model BX-51 polarizing microscope in transmission configuration. The crystal structure was characterized using a Rigaku D/max-r A 12 kW X-ray diffractometer (XRD) with Cu Ka radiation. A Bruker AXS D8 Discover with GADDS X-ray diffractometer (XRD2) was used to measure the orientation of the as-grown boules.
3. Results and discussion The power needed to melt the rods was about 55% of the total output power of the lamps. When a pure oxygen atmosphere was applied, bubbles were observed in the melt. This resulted in an unstable molten zone. In an air atmosphere, the molten zone was very stable during the entire growth procedure. When the crystals were grown at a high rotation rate and a growth rate of 10 mm/h in an air atmosphere, a large number of bubble inclusions were found in the as-grown crystals. Fig. 1a shows the microscopic photo of a crystal wafer grown with a rotation rate of 30 rpm. This wafer was cut perpendicular to the growth direction and polished. It can be seen that the inclusions are mainly located at the central region of the wafer. When the growth rate was varied between 3 and 20 mm/h, the bubble concentration was unaffected. In the growth of Tm: GdVO4 and TiO2 crystals, similar situations were also observed. Higuchi et al. [20] used a higher rotation rate to suppress the bubble inclusions in the growth of Tm: GdVO4 crystals. While Koohpayeh et al. [21] turned the rotation rate down to 0 rpm to suppress the bubble inclusions during the growth of TiO2 crystals. To reduce the formation of bubble inclusions in Zn2TiO4 crystals, lower rotation rates of 5 and 10 rpm were used. Polarizing microscopy showed that there were few bubble inclusions in the crystal grown at a rotation rate of 10 rpm, and at 5 rpm the crystals were bubble-free, as shown in Fig. 1b and Fig. 1c. Two possible origins of the bubbles include the
voids in the sintered feed rods, which were not 100% dense [21,22] or oxygen from the air atmosphere dissolved in the molten zone and rejected at the growth interface, which was suggested by the effect of O2 on the molten zone stability. When a high rotation rate was applied, the gas bubbles moved towards the center axis due to the centrifugal force effect, coalesced and became trapped within the crystal. When a lower rotation rate was applied, the gas bubbles moved to the upper part of the molten zone, away from the growth interface, hence grew inclusion-free crystals. In addition, low-angle grain boundaries were not observed in the grown crystal from Fig. 1b and Fig. 1c, which were taken in the transmission configuration of the polarizing microscope. Fig. 2 shows a photograph of a crystal grown at a rotation rate of 5 rpm and a rate of 10 mm/h under an air flow, and the insert shows a wafer. It can be seen that the crystal is crack-free and transparent, with 4–6 mm in diameter and 30 mm in length. The powder XRD pattern of the crushed crystal is shown in Fig. 3. It was found that all peaks could be indexed in the cubic system and corresponded to the diffraction peaks for Zn2TiO4 spinel structure (Ref: ICSD Code: 080851). To determine the orientation of the as-grown Zn2TiO4 single crystal, the wafers cut perpendicular to the growth direction were studied by XRD2. Fig. 4 gives the XRD2 pattern integrated from the
Fig. 2. Photographs of an as-grown Zn2TiO4 crystal and the crystal wafer cut perpendicular to the growth direction.
Fig. 1. (a) A wafer with a rotation rate of 30 rpm under optical microscope, (b) a wafer with a rotation rate of 10 rpm and (c) a wafer with a rotation rate of 5 rpm under polarizing microscope in transmission configuration.
L. Li et al. / Journal of Crystal Growth 312 (2010) 3561–3563
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4. Conclusions High-quality Zn2TiO4 single crystals with 4–6 mm in diameter and 30 mm in length have been grown for the first time by the optical floating zone technique at a growth speed of 10 mm/h and a rotation rate of 5 rpm under an air atmosphere. The change of growth speed has no obvious influence on the formation of bubble inclusions, but the change of rotation rate does. A lower rotation rate is effective in suppressing bubble inclusions. The as-grown crystals were free of low angle grain boundary and grew uniformly along the /4 0 0S direction (a-axis). While under an oxygen atmosphere, it is very difficult to grow Zn2TiO4 crystal because the melting zone is very unstable.
Acknowledgements Financial support from Project 20101048 supported by Graduate Innovation Fund of Jilin University and the Scientific and Technical development Foundation of Jilin Province of China (Grant no. 20080513) are greatly appreciated.
Fig. 3. Powder XRD pattern of a crushed crystal.
Fig. 4. XRD2 pattern of the Zn2TiO4 wafer.
XRD2 image and only one peak at 42.71 is observed, which can be indexed to the (4 0 0) plane. This means that the Zn2TiO4 crystals grow along the a-axis direction.
References [1] H. Martinho, N.O. Moreno, J.A. Sanjurjo, C. Rettori, A.J. Garcı´a-Adeva, D.L. Huber, S.B. Oseroff, W. Ratcliff, S.W. Cheong, P.G. Pagliuso, J.L. Sarrao, G.B. Martins, Phys Rev B 64 (2001) 024408. [2] A. Zerr, G. Miehe, G. Serghiou, M. Schwarz, E. Kroke, R. Riedel, H. Fuesz, P. Kroll, R. Boehler, Nature 400 (1999) 340–342. [3] B.N. Kim, K. Hiraga, K. Morita, Y. Sakka, Nature 413 (2001) 288–291. [4] A.H. Jahren, M.B. Kruger, R. Jeanloz, J. Appl. Phys. 71 (1992) 1579–1582. [5] L. Alonso, J.M. Palacios, R. Moliner, Energy Fuel 15 (2001) 1396–1402. [6] K. Jothimurugesan, S.K. Gangwal, Ind. Eng. Chem. Res. 37 (1998) 1929–1933. [7] M. Pineda, J.G. Fierro, J. Palacios, C. Cilleruelo, E. Garcı´a, J. Ibarra, Appl. Surf. Sci. 119 (1997) 1–10. [8] N.N. Lysova, L.P. Shapovalova, V.P. Luk’yanenko, Inorg. Mater 27 (1991) 542–544. [9] K.S. Walton, R.K. Datta, Ceram. Trans. 66 (1995) 637–644. [10] M.M. Mikhailov, M.I. Dvoretskii, Inorg. Mater 27 (1991) 2021–2025. [11] Z.W. Wang, S.K. Saxena, C.S. Zha, Phys. Rev. B. 66 (2002) 024103–024106. [12] A.C. Chaves, S.J.G. Lima, R.C.M.U. Arau´jo, M.A.M.A. Maurera, E. Longo, ~ P.S. Pizani, L.G.P. Simoes, L.E.B. Soledade, A.G. Souza, I.M.G.d. Santos, J. Solid State Chem. 179 (2006) 985–992. [13] Y. Yang, X.W. Sun, B.K. Tay, J.X. Wang, Z.L. Dong, H.M. Fan, Adv. Mater 19 (2007) 1839. [14] Y. Yang, R. Scholz, H.J. Fan, D. Hesse, U. Gosele, M. Zacharias, Acs. Nano 3 (2009) 555–562. [15] M.V. Nikolic´, N. Obradovic´, K.M. Paraskevopoulos, T.T. Zorba, S.M. Savic´, M.M. Ristic´, J. Mater. Sci. 43 (2008) 5564–5568. [16] F.H. Dulin, D.E. Rase, J. Am. Ceram. Soc. 43 (1960) 125–131. [17] R.L. Miliard, R.C. Peterson, B.K. Hunter, Am. Mineral. 80 (1995) 885–896. [18] J.P.M. Damen, J.M. Robertson, M.A.H. Huyberts, J. Cryst. Growth 47 (1979) 486–492. [19] J.D. Pless, N. Erdman, D. Ko, L.D. Marks, P.C. Stair, K.R. Poeppelmeier, Cryst. Growth Des. 3 (2003) 615–619. [20] M. Higuchi, K. Kodaira, Y. Urata, S. Wada, H. Machida, J. Cryst. Growth 265 (2004) 487–493. [21] S.M. Koohpayeh, D. Fort, J.S. Abell, J. Cryst. Growth 282 (2005) 190–198. [22] S.G. Yoon, S.W. Kim, M. Hirano, D.H. Yoon, H. Hosono, Cryst. Growth Des. 8 (2008) 1271–1275.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Large-size and high-quality Zn2TiO4 single crystal grown by the optical floating zone method Liang Li a,c, Hang Yang a,c, Dejun Wang b, Guanlin Feng c, Benxian Li d, Zhongmin Gao d, Dapeng Xu a,c,n, Zhanhui Ding d, Xiaoyang Liu d a
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Adult Education College of Changchun University, Changchun 130000, China c College of Physics, Jilin University, Changchun 130012, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e in fo
abstract
Article history: Received 4 February 2010 Received in revised form 9 September 2010 Accepted 16 September 2010 Communicated by R.S. Feigelson Available online 22 September 2010
Crack-free and transparent Zn2TiO4 single crystals of 4–6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The powder X-ray diffraction (XRD) results show that the as-grown crystals have the spinel-type Zn2TiO4 structure. XRD2 measurements on Zn2TiO4 wafers cut perpendicular to the growth direction display only one peak at 42.71, which indicates that the Zn2TiO4 single crystals grow along the /4 0 0S direction (a-axis). The formation of bubble inclusions was effectively suppressed by lowering rotation rate. Transmission polarized-light microscopy results showed that as-grown crystals were free of low angle grain boundaries. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. X-ray diffraction A2. Floating zone method A2. Single crystal growth B1. Zn2TiO4
1. Introduction Spinel structures, including normal (AB2O4) and inverse (A2BO4) spinels, have awakened great interest due to their potential use in important technological applications such as magnetic [1] or superhard materials [2], high-temperature ceramics [3] and highpressure sensors (with Cr3 + doping) [4]. Zinc titanate (Zn2TiO4) spinel is a high-effect regenerable catalytic sorbent for H2S, As, Se and other contaminants needing removal from hot coal gases [5–10]. Due to its excellent dielectric properties, Zn2TiO4 has also been used in the microwave devices. Numerous studies have been focused on its electrical and optical properties under different conditions to investigate its potential applications [11–15]. The phase diagram of the ZnO–TiO2 system was established by Dulin and Rase [16] and indicated that two compound forms: a spinel-type Zn2TiO4 and an illuminate-type ZnTiO3. The spinel-type Zn2TiO4 has a face-centered cubic unit cell, belonging to space group Fd-3m with a¼0.84608 nm [17], and is stable from room temperature to its melting point.
n
Corresponding author at: National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. Tel.: + 86 43188499048; fax: + 86 43188513216 E-mail address: [email protected] (D. Xu). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.09.045
Single-crystal samples with well-defined crystallographic orientation can help in understanding the materials properties. In 1979, Damen et al. [18] reported on the growth of Zn2TiO4 crystals using the Bridgman method, but these as-grown crystals had high-dislocation densities and some impurities introduced from the Pt crucible. There was little characterization of these Zn2TiO4 crystals because these as-grown Zn2TiO4 crystals were only of interest as a substrate. Since 1979, there have been no other reports on the growth of Zn2TiO4 crystals. Thus, all the properties of Zn2TiO4 measured to date have only been made on polycrystalline samples. To investigate its intrinsic characteristics, large Zn2TiO4 single crystals are preferable. The optical floating zone method is crucible-free, the atmosphere is controllable and steep temperature gradients can be maintained along the growth direction, which allows for faster and more stable crystal growth [19]. Therefore, this method is a suitable technique for growing high-quality single crystals. In this work, high-quality Zn2TiO4 single crystals have been successfully grown by this method for the first time.
2. Experimental procedures The starting Zn2TiO4 powders were prepared by calcining stoichiometric amounts of ZnO (Alfa Aesar 99.99%) and TiO2
3562
L. Li et al. / Journal of Crystal Growth 312 (2010) 3561–3563
(Alfa Aesar 99.99%) at 1473 K for 20 h with intermediate grinding. The Zn2TiO4 powders were packed into a cylindrical shape rubber tube and then pressed hydrostatically up to 70 MPa in a cold isostatic press to form a cylindrical rod of 4–6 mm in diameter and 50 mm in length. The rods were sintered in air at 1600 K for 10 h for use as the feed and support rods. The crystals were grown in an optical floating zone furnace (CSI FZ-T-10000-H-VI-VP, Crystal systems, Inc., Japan) equipped with four 1000 W halogen lamps as the heat source. The growth conditions were as follows: the seed and feed rods were rotated in opposite directions at the rates of 30, 10 and 5 rpm. The growth rate was 10.0 mm/h. The air flow of 0.1 L/min and the pressure of 0.1 MPa were applied. Macroscopic defects such as low-angle grain boundaries and inclusions were studied with an Olympus Model BX-51 polarizing microscope in transmission configuration. The crystal structure was characterized using a Rigaku D/max-r A 12 kW X-ray diffractometer (XRD) with Cu Ka radiation. A Bruker AXS D8 Discover with GADDS X-ray diffractometer (XRD2) was used to measure the orientation of the as-grown boules.
3. Results and discussion The power needed to melt the rods was about 55% of the total output power of the lamps. When a pure oxygen atmosphere was applied, bubbles were observed in the melt. This resulted in an unstable molten zone. In an air atmosphere, the molten zone was very stable during the entire growth procedure. When the crystals were grown at a high rotation rate and a growth rate of 10 mm/h in an air atmosphere, a large number of bubble inclusions were found in the as-grown crystals. Fig. 1a shows the microscopic photo of a crystal wafer grown with a rotation rate of 30 rpm. This wafer was cut perpendicular to the growth direction and polished. It can be seen that the inclusions are mainly located at the central region of the wafer. When the growth rate was varied between 3 and 20 mm/h, the bubble concentration was unaffected. In the growth of Tm: GdVO4 and TiO2 crystals, similar situations were also observed. Higuchi et al. [20] used a higher rotation rate to suppress the bubble inclusions in the growth of Tm: GdVO4 crystals. While Koohpayeh et al. [21] turned the rotation rate down to 0 rpm to suppress the bubble inclusions during the growth of TiO2 crystals. To reduce the formation of bubble inclusions in Zn2TiO4 crystals, lower rotation rates of 5 and 10 rpm were used. Polarizing microscopy showed that there were few bubble inclusions in the crystal grown at a rotation rate of 10 rpm, and at 5 rpm the crystals were bubble-free, as shown in Fig. 1b and Fig. 1c. Two possible origins of the bubbles include the
voids in the sintered feed rods, which were not 100% dense [21,22] or oxygen from the air atmosphere dissolved in the molten zone and rejected at the growth interface, which was suggested by the effect of O2 on the molten zone stability. When a high rotation rate was applied, the gas bubbles moved towards the center axis due to the centrifugal force effect, coalesced and became trapped within the crystal. When a lower rotation rate was applied, the gas bubbles moved to the upper part of the molten zone, away from the growth interface, hence grew inclusion-free crystals. In addition, low-angle grain boundaries were not observed in the grown crystal from Fig. 1b and Fig. 1c, which were taken in the transmission configuration of the polarizing microscope. Fig. 2 shows a photograph of a crystal grown at a rotation rate of 5 rpm and a rate of 10 mm/h under an air flow, and the insert shows a wafer. It can be seen that the crystal is crack-free and transparent, with 4–6 mm in diameter and 30 mm in length. The powder XRD pattern of the crushed crystal is shown in Fig. 3. It was found that all peaks could be indexed in the cubic system and corresponded to the diffraction peaks for Zn2TiO4 spinel structure (Ref: ICSD Code: 080851). To determine the orientation of the as-grown Zn2TiO4 single crystal, the wafers cut perpendicular to the growth direction were studied by XRD2. Fig. 4 gives the XRD2 pattern integrated from the
Fig. 2. Photographs of an as-grown Zn2TiO4 crystal and the crystal wafer cut perpendicular to the growth direction.
Fig. 1. (a) A wafer with a rotation rate of 30 rpm under optical microscope, (b) a wafer with a rotation rate of 10 rpm and (c) a wafer with a rotation rate of 5 rpm under polarizing microscope in transmission configuration.
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4. Conclusions High-quality Zn2TiO4 single crystals with 4–6 mm in diameter and 30 mm in length have been grown for the first time by the optical floating zone technique at a growth speed of 10 mm/h and a rotation rate of 5 rpm under an air atmosphere. The change of growth speed has no obvious influence on the formation of bubble inclusions, but the change of rotation rate does. A lower rotation rate is effective in suppressing bubble inclusions. The as-grown crystals were free of low angle grain boundary and grew uniformly along the /4 0 0S direction (a-axis). While under an oxygen atmosphere, it is very difficult to grow Zn2TiO4 crystal because the melting zone is very unstable.
Acknowledgements Financial support from Project 20101048 supported by Graduate Innovation Fund of Jilin University and the Scientific and Technical development Foundation of Jilin Province of China (Grant no. 20080513) are greatly appreciated.
Fig. 3. Powder XRD pattern of a crushed crystal.
Fig. 4. XRD2 pattern of the Zn2TiO4 wafer.
XRD2 image and only one peak at 42.71 is observed, which can be indexed to the (4 0 0) plane. This means that the Zn2TiO4 crystals grow along the a-axis direction.
References [1] H. Martinho, N.O. Moreno, J.A. Sanjurjo, C. Rettori, A.J. Garcı´a-Adeva, D.L. Huber, S.B. Oseroff, W. Ratcliff, S.W. Cheong, P.G. Pagliuso, J.L. Sarrao, G.B. Martins, Phys Rev B 64 (2001) 024408. [2] A. Zerr, G. Miehe, G. Serghiou, M. Schwarz, E. Kroke, R. Riedel, H. Fuesz, P. Kroll, R. Boehler, Nature 400 (1999) 340–342. [3] B.N. Kim, K. Hiraga, K. Morita, Y. Sakka, Nature 413 (2001) 288–291. [4] A.H. Jahren, M.B. Kruger, R. Jeanloz, J. Appl. Phys. 71 (1992) 1579–1582. [5] L. Alonso, J.M. Palacios, R. Moliner, Energy Fuel 15 (2001) 1396–1402. [6] K. Jothimurugesan, S.K. Gangwal, Ind. Eng. Chem. Res. 37 (1998) 1929–1933. [7] M. Pineda, J.G. Fierro, J. Palacios, C. Cilleruelo, E. Garcı´a, J. Ibarra, Appl. Surf. Sci. 119 (1997) 1–10. [8] N.N. Lysova, L.P. Shapovalova, V.P. Luk’yanenko, Inorg. Mater 27 (1991) 542–544. [9] K.S. Walton, R.K. Datta, Ceram. Trans. 66 (1995) 637–644. [10] M.M. Mikhailov, M.I. Dvoretskii, Inorg. Mater 27 (1991) 2021–2025. [11] Z.W. Wang, S.K. Saxena, C.S. Zha, Phys. Rev. B. 66 (2002) 024103–024106. [12] A.C. Chaves, S.J.G. Lima, R.C.M.U. Arau´jo, M.A.M.A. Maurera, E. Longo, ~ P.S. Pizani, L.G.P. Simoes, L.E.B. Soledade, A.G. Souza, I.M.G.d. Santos, J. Solid State Chem. 179 (2006) 985–992. [13] Y. Yang, X.W. Sun, B.K. Tay, J.X. Wang, Z.L. Dong, H.M. Fan, Adv. Mater 19 (2007) 1839. [14] Y. Yang, R. Scholz, H.J. Fan, D. Hesse, U. Gosele, M. Zacharias, Acs. Nano 3 (2009) 555–562. [15] M.V. Nikolic´, N. Obradovic´, K.M. Paraskevopoulos, T.T. Zorba, S.M. Savic´, M.M. Ristic´, J. Mater. Sci. 43 (2008) 5564–5568. [16] F.H. Dulin, D.E. Rase, J. Am. Ceram. Soc. 43 (1960) 125–131. [17] R.L. Miliard, R.C. Peterson, B.K. Hunter, Am. Mineral. 80 (1995) 885–896. [18] J.P.M. Damen, J.M. Robertson, M.A.H. Huyberts, J. Cryst. Growth 47 (1979) 486–492. [19] J.D. Pless, N. Erdman, D. Ko, L.D. Marks, P.C. Stair, K.R. Poeppelmeier, Cryst. Growth Des. 3 (2003) 615–619. [20] M. Higuchi, K. Kodaira, Y. Urata, S. Wada, H. Machida, J. Cryst. Growth 265 (2004) 487–493. [21] S.M. Koohpayeh, D. Fort, J.S. Abell, J. Cryst. Growth 282 (2005) 190–198. [22] S.G. Yoon, S.W. Kim, M. Hirano, D.H. Yoon, H. Hosono, Cryst. Growth Des. 8 (2008) 1271–1275.