Detection of zinc blende phase by the pulsed laser photoacoustic technique in ZnO thin films deposited via pulsed laser deposition

Materials Science in Semiconductor Processing 34 (2015) 93–98

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Review

Detection of zinc blende phase by the pulsed laser photoacoustic technique in ZnO thin films deposited via pulsed laser deposition Aldebarán Rosales a,n, Rosalba Castañeda-Guzmán b, Antonio de Ita a, C. Sánchez-Aké b, S.J. Pérez-Ruiz b a Área de Ciencia de Materiales, Universidad Autónoma Metropolitana, Av. San Pablo No. 180, Col. Reynosa Tamaulipas, Delegación Azcapotzalco, C.P. 02220, Mexico b Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior S/N, Ciudad Universitaria, A.P. 70-186, C.P. 04510, México D.F., Mexico

a r t i c l e i n f o

abstract

Available online 27 February 2015

Pulsed laser deposition (PLD) was used to grow ZnO thin films on corning glass and silicon substrates at different oxygen pressures (1 y 10 mTorr). The structural analysis of the films was performed by X-ray diffraction and pulsed laser photoacoustic (PLPA) techniques. Both methods were employed to identify the minority zinc blende phase in the films. The relative difference between the structural changes detected in the films with the temperature increases was statistically analyzed. It was found that regardless of the substrate and the oxygen pressure used for the growth, the films exhibit a phase transition at 310 1C, which corresponds to the transformation of zinc blende structure to hexagonal wurtzite. The results demonstrate that the zinc blende phase in the films is present not only on cubic substrates but also on glass, and confirm that PLPA technique is a very sensitive method for the detection of minority phase changes. & 2015 Elsevier Ltd. All rights reserved.

Keywords: PLD Zinc blende PLPA technique X-ray diffraction Statistical analyses

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . Materials and methods Results and discussion. Conclusion . . . . . . . . . . Aknowledgements . . . . References . . . . . . . . . .

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93 94 95 97 98 98

1. Introduction

n Correspondence to: Cuautitlan Izcalli, C.P. 54740, Mexico. Tel:þ 52 44 5518053186. E-mail address: [email protected] (A. Rosales).

http://dx.doi.org/10.1016/j.mssp.2015.02.017 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

ZnO is a promising semiconductor material for various technological applications as: piezoelectric transducers, chemical sensors, varistors, catalysis, optical coatings, and photovoltaics materials [1,2]. Moreover, ZnO combined with different elements is one of the most promising candidates for spintronic devices due to their magnetic properties at

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room-temperature. For example, there have been recent reports about high magnetization with giant coercivity of ZnO films co-doped with Co and Eu [3] and the presence of magnetic order at the interfaces between ZnO nanowires core and Al2O3 shell [4]. ZnO is an oxide material that belongs to the group II–VI and has four structural phases: a) wurtzite (B4), which is the most thermodynamically stable phase at ambient conditions due to its ionicity, that resides exactly at the borderline between the covalent and the ionic materials [5]; b) zinc blende (B3), which can be stabilized only using cubic substrates [6,7]; c) rocksalt (B1), that has been obtained at pressures about  9 GPa [7–9]; and d) cesium chloride (B2), which has been observed under high pressure range ( 260 GPa) [7]. The atomic arrangement of the zinc blende structure is quite similar to that of the wurtzite structure, but the angle of adjacent tetrahedral units is different, having values of 601 for zinc blende and 01 for wurtzite phase. Numerous studies have been performed about thin films of zinc oxide deposited by pulsed laser deposition (PLD) on glass and silicon substrates [10–14]. In most of them, the hexagonal wurtzite phase has been detected and no evidence of zinc blende phase has been found. This could be due to two possible reasons: a) the zinc blende phase is present in a tiny amount or b) it is masked by the wurtzite phase [15]. One of the advantages of materials with zinc-blende structure over those with wurzite structure is their lower ionicity, what becomes absolutely covalent, and with this, which makes them viable for applications in semiconductor technology [16,17]. One technique that has proven to be useful for analyzing the structural changes in different kinds of materials is pulsed laser photoacoustic (PLPA), whose main characteristic is that it provides of high sensitivity to detect minor phases [18–21]. The photoacoustic effect consists on the generation of acoustic waves in a material produced by the absorption of low-energy laser pulses [22,23]. The absorption of pulsed laser light generates stresses within the material, causing volume expansions and compressions in the illuminated area, which in turn produce pressure waves and therefore acoustic waves. This is result of the dissipation of energy from the non-radiative decays as heat. In this way, a photoacoustic (PA) signal can be detected due to the pressure wave traveling in the material, induced by the incidence of the pulsed laser beam. The characteristics of the PA signal produced in a sample depend on several factors. In the direct coupling method, the acoustic wave is related to the thermal expansion ΔV th of the irradiated volume V 0 that in the isotropic case can be expressed as V th ¼

β

ð1Þ

H Cpρ

where β is the volume expansion coefficient, C p is the specific heat at constant pressure, ρ is the density, and H is the heat deposited in the volume V 0 . This expansion creates a pressure wave which travels outwards at the velocity of sound in the material. Since the electric signal generated in the transducer is proportional to the pressure [24] PAsignal ¼ K

β

Cpρ

ð1  10  A ÞE0

ð2Þ

where A is the optical absorbance of the sample, E0 is the laser pulse energy, and K is an instrumental constant. When pulsed laser sources are used to produce the PA effect, the absorption of each pulse in the medium leads the generation of a broadband thermoelastic pressure wave that propagates through the medium with its respective speed of sound. Moreover, by using pulsed lasers with temporal width in the range of nanoseconds as light sources, and piezoelectric ceramics as sensors, an improvement in the signal–noise relation has been achieved (among the acoustic signal measured with piezoelectric and the acoustic aerial and electrical sound that can be introduced into the measurement) [18,25] The main characteristics of the acoustic waves propagating through a material depend on the crystalline structure of the medium. Thereby, it is possible to detect structural changes in the studied material under controlled temperature increase. The PLPA technique using laser pulses for excitation and piezoelectric detection has been successfully used for the analysis of both structural and state phase transitions [26,27]. It is noteworthy that there are only few researches on thin films characterization by the PLPA technique [26,27] and none of them is about ZnO thin films and its characteristic phase (blende). The aim of this work is detection of zinc blende phase by the pulsed laser photoacoustic technique in ZnO thin films grown by PLD. Statistical analyzes were used to determine the influence of the oxygen pressure and the substrates on the crystalline phases detected in the films. 2. Materials and methods Four sets of ZnO thin films were deposited by PLD. Two of them were grown with an excimer laser on corning glass and silicon (100) substrates using 1 mTorr of O2 pressure. The other two sets were deposited with a Nd:YAG laser on corning glass employing 1 and 10 mTorr pressure of O2. Depending on the laser used to produce the film, a high purity ZnO target was ablated by: i) a 248 nm KrF excimer laser (COMPex 102, Lambda Physik) with a pulse duration of 30 ns (FWHM) [14,28–30], ii) a Nd:YAG (Surelite I, Continuum) emitting at 1064 nm and delivering pulses of 8 ns (FWHM). The dimensions of the substrates were: 5  5 mm2 for the case of silicon (100) and 10  10 mm2 for corning glass. All films were deposited at 400 1C in a commercial stainless steel vacuum chamber (Kurt J. Lesker), evacuated by a turbomolecular pump until a base pressure of 1  10  6 Torr was achieved. During the film growth, the pressure was kept constant at 1 or 10 mTorr of O2 respectively. This was achieved by allowing a constant flow of O2 into the chamber. The election of this pressures was due to the fact of having obtained with them the best composition and structure of ZnO films in previous works [31]. The laser pulses were focused using a plane-convex lens with focal length of 50 cm, leading to fluences of 3 and 8 J/cm2. All films were grown using 30,000 laser shots, and their thickness was about 200 nm estimated by a Dekat IIA profilometer. The crystalline structure of the films was determined by a X-ray powder diffraction (XRD) technique using two Bruker D8 discover diffractometers with Cu Kα radiation. The photoacoustic analysis of our thin films was performed using as laser source the second harmonic emission (532 nm)

A. Rosales et al. / Materials Science in Semiconductor Processing 34 (2015) 93–98

of a Nd:YAG laser (Continuum Surelite) with a pulse duration of 7 ns and repetition rate of 10 Hz. As it is shown in Fig. 1 [32], the laser light passed through a beam splitter. One part of the beam was directed to a photodiode detector (Thorlabs Inc., model 201/579/7227, with a rise timeo1 ns), in order to trigger the oscilloscope and to monitor variations of the laser energy that can affect the acoustic signals. The other fraction of the beam was focused on the surface of the film at a fluence of 0.03 J/cm2, generating an acoustic signal, which was detected by a piezoelectric sensor (PZT, with a resonance frequency of 240 kHz) and visualized through a digital oscilloscope (Tektronix TDS 5054B). Photoacoustic signal generation is based on the model of Tam [33]. All films were analyzed from room temperature to 495 1C, with an increase in temperature of 5 1C/min. The PA signals were acquired by the oscilloscope averaging 200 pulses for each temperature, e. g., T1, T2, T3, T4, T5, etc. The signals were analyzed on a PC

95

using a commercial software (MatLabs), where a correlation analysis between the signals—T1 vs T2, T2 vs T3, T3 vs T4, etc. —was performed by plotting the maximum of correlation for each pair of the analyzed distributions. We employed a statistical analysis to determine the influence of the substrate and the O2 pressure during the films growth, on the structural changes detected by the PLPA technique. In first instance, the normality of data was analyzed using two graphs (Q–Q plot and boxplot) and a normality test (Shapiro) with a significance level (α)¼0.05. Finally, a non-parametric hypothesis test known as Wilcoxon rank sum with α ¼0.05 was used. [34]. 3. Results and discussion Fig. 2 shows the XRD patterns of the ZnO thin films grown using different oxygen pressures and substrates. It can be seen

Fig. 1. Experimental setup used for the PLPA technique.

Fig. 2. XRD patterns of ZnO films grown on: (a) silicon substrate with an excimer laser and 1 mTorr pressure of O2, (b) corning glass with an excimer laser and 1 mTorr pressure of O2, (c) corning glass with a Nd:YAG and 10 mTorr pressure of O2, (d) corning glass with a Nd:YAG and 1 mTorr pressure of O2. Inset: Enlarged view of the (110) diffraction peak of blende phase.

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that regardless of the oxygen pressure and the substrate used for deposition, the films are textured with preferential orientation along the c-axis. The films show the (002) and the (004) basal plane reflections of wurtzite structure, in concordance with previous reports [13,35,36]. According to several authors [1,12,35], thin films of ZnO deposited by PLD and characterized by XRD, only show the most stable phase at room temperature (wurtzite), and there are no evidence of other characteristic phases. However, the (110) peak shown in the diffractograms of Fig. 1 does not belong to the wurtzite phase, it belongs to the cubic blende structure of ZnO [15,37,38]. This fact was confirmed with the obtained lattice parameter, which is reported in Table 1 [39,40]. For the zinc blende phase, the calculated lattice parameter as well as some previous results and simulations are summarized in Table 1. As it can be seen, there are very few experimental data available related to the lattice parameter for B3 structure, and nearly all information about this structure are simulations as exposed also the works of Kuang [1] and Boufatah [41]. To identify clearly the minority phase associated to the (110) peak illustrated in the inset of Fig. 2, PLPA technique was performed. The correlation analysis between the averages of the photoacoustic signals acquired at different temperatures are shown in Fig. 3. This correlation exhibits the structural changes detected in the ZnO thin films deposited on different substrates by subjecting the films to a controlled temperature increase (5 1C/min). The phase transition of the ZnO film deposited on Si observed at  310 1C corresponds to the transformation of cubic zinc blende to hexagonal wurtzite structure (the most stable phase at this temperature). This result agrees with previous reports about the appearance of the blende phase at 310 1C [47] in films grown on cubic substrates [5]. For the case of the films deposited on glass with different lasers and oxygen pressures, it is of great interest to note the appearance of the same phase transition at  310 1C even though the substrate is amorphous. Also, it can be seen that at temperatures below 2801C and above 350 1C a number of variations are seen before and after the main transition, which are due to rearrangements in the structure of the film and are found in very low amounts. These changes can be detected by the PLPA technique because of its high sensitivity, but in order to determine with certainty what these rearrangements are, specialized techniques would be needed. Although it is hypothesized that the change that occurs at 145 1C in the ZnO thin film deposited on silicon is due to the presence of a tiny amount of rocksalt structure since rocksalt–wurtzite phase transition occurs at  130 1C [48,49].

A statistical analysis was performed in order to determine if there is a significant difference between the observed variations—temperature, width, and breadth—in the transition of interest (310 1C) for the different films. Fig. 4 shows the graphs Q–Q and boxplot of the thin films grown under different conditions. Q–Q and boxplot visually demonstrate the absence of normality in data, where the quantiles samples did not follow a normal distribution and negative biases and variability are observed respectively. Shapiro test confirmed the lack of normality in the calculated data for the four films, obtaining: W¼0.7914, α o0.05 (p¼5.02e10) for the film (a), W¼ 0.5729, α o0.05 (p¼9.39e16) for the film (b), W¼0.9038, α o0.05 (p¼5.994e 06) for the film (c) and W¼0.8045, α o0.05 (p¼1.901e 09) for the film (d). The results obtained in Wilcoxon rank sum were W¼5218, α 40.05 (p¼0.1367) for the films deposited by excimer laser on different substrates, indicating that there is no significant

Fig. 3. PLPA technique of ZnO films grown by PLD with different lasers, oxygen pressures and substrates from room temperature to 450 1C.

Table 1 Lattice parameter (a) of zinc blende. Phase

Lattice parameter

Present work

Experiment Simulation

Zinc blende (B3)

a(Å)

4.10

4.18 [39]

4.51 [42], 4.63 [37], 4.58 [43], 4.52 [44], 4.65 [44], 4.62 [41], 4.53 [45], 4.63 [46], 4.62 [1]

A. Rosales et al. / Materials Science in Semiconductor Processing 34 (2015) 93–98

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Fig. 4. Q–Q plot of ZnO films grown on: (a) silicon with an excimer laser and 1 mTorr of O2 pressure, (b) corning glass with an excimer laser and 1 mTorr of O2 pressure, (c) corning glass with a Nd:YAG and 10 mTorr of O2 pressure and (d) corning glass with a Nd:YAG and 1 mTorr of O2 pressure. Insets: Boxplot for each distribution.

difference in the structural change detected by the PLPA technique by using a silicon or glass substrate. For the films deposited with Nd:YAG laser on corning glass and different pressures of oxygen, we obtained W¼1721, α 40.05 (p¼ 7.383e 11), suggesting that the partial pressure of oxygen affects the structural change detected at  310 1C. This can be seen in Fig. 3, where the peak widens in the film deposited at 10 mTorr indicating that the transition occurs at a lower temperature than that in the other films. It is clear that in the correlation graphs shown in Fig. 3, an important structural change is observed around 310 1C regardless of the condition used for the films growth (oxygen pressure, kind of substrate and laser), in accordance with the detection of the minority phase blende of ZnO at that temperature previously reported [47]. In addition, these results were confirmed by the diffraction patterns at 2θ ¼311 for all films. 4. Conclusion Zinc oxide thin films were successfully deposited on corning glass and silicon substrates by the PLD method. The

calculus of the lattice parameter and the structural analysis performed with XRD demonstrate that the films have two structures: a minority zinc blende located at 2θ ¼311 and the wurtzite mainly oriented in the c-axis direction. These results suggest that employing PLD under certain specified conditions is possible to generate the characteristic blende phase of zinc oxide. The photoacoustic correlation analysis exhibited a phase transition at  310 1C in the thin films deposited by PLD. A statistical analysis was performed to determine the influence of growth conditions on the phase transition. The hypothesis tests showed that there is no influence of the substrate, either glass or silicon, on the structural changes detected by the PLPA technique, indicating that the phase transition from cubic blende to hexagonal wurtzite occurs in both films at 310 1C and confirming that not only zinc blende phase is generated on a cubic substrate. However, it was found significant influence of the oxygen pressure employed for the growth of films, even so, the phase transition occurs at 310 1C. This indicates the existence of a direct relationship between the oxygen pressure and the percentage of present phase.

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These results demonstrate the high sensitivity of the PLPA technique to detect minority phases in thin films.

Aknowledgements This work is supported by National Council of Science and Technology (Conacyt) (Grant no. 329037/233996), and the projects IG100314 and IN110612 of Technological Innovation (PAPIIT). We thank also Ing. Georgina Flores from the Metropolitan University for her help in X-ray analysis. References [1] F.G. Kuang, X.Y. Kuang, S.Y. Kang, M.M. Zhong, Mat. Sci. Semicond. Process. 23 (2014) 63–71. [2] C.F. Klingshirm, B.K. Meyer, A. Waag, A. Hoffman, J. Geurts, Zinc Oxide: From Fundamental Properties Towards Novel Applications, Electronic Reproduction, Springer, New York, 2011. [3] J.J. Lee, G.Z. Xing, J.B. Yi, T. Chen, M. Ionescu, S. Li, Appl. Phys. Lett. 104 (2014) 012405. [4] G.Z. Xing, D.D. Wang, C.-J. Cheng, M. He, S. Li, T. Wu, Appl. Phys. Lett. 103 (2013) 022402. [5] R.J. Guerrero, N. Takeuchi, Phys. Rev. B. 66 (2002) 205205. [6] H. Morkoc, Ü. Özgür, Zinc Oxide: Fundamentals, Materials and Device Technology, WILEY-VCH Verlag GmbH & Co. KGaA, Winheim, 2009. [7] D. Maouche, F.S. Saoud, L. Louail, Mater. Chem. Phys. 106 (2007) 11–15. [8] Ü. Özgür, I. Alivoy, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkov, J. Appl. Phys. 98 (2005) 041301. [9] A.B.M. Ashrafi, A. Ueta, A. Avramescu, H. Kumano, I. Suemune, Appl. Phys. Lett. 76 (2000) 550. [10] Z.G. Zhang, F. Zhou, X.Q. Wei, M. Liu, G. Sun, C.S. Chen, C.S. Xue, H.Z. Zhuang, B.Y. Man, Physica E 39 (2007) 253–257. [11] M. Suchea, S. Christoulakis, M. Katharakis, N. Vidakis, E. Koudoumas, Thin Solid Films 517 (2009) 4303–4306. [12] Q. Li, Y. Wang, J. Liu, W. Kong, B. Ye, Appl. Surf. Sci. 289 (2014) 42–46. [13] S. Christoulakis, M. Suchea, E. Koudoumas, M. Katharakis, N. Katsarakis, G. Kiriakidis, Appl. Surf. Sci. 252 (2006) 5351–5354. [14] J.N. Zeng, J.K. Low, Z.M. Ren, T. Liew, Y.F. Lu, Appl. Surf. Sci. 197-198 (2002) 362–367. [15] S.W. Hwang, J. Korean Phys. Soc. 51 (2007) 862–865. [16] M. Murayama, T. Nakayama, Phys. Rev. B 49 (1994) 4710. [17] L. Zhang, H. Huang, Appl. Phys. Lett. 90 (2007) 023115. [18] A.P. Pacheco, R.C. Guzmán, C.O. Montes de Oca, A.E. García, S.J. Pérez, Appl. Phys. A 102 (2011) 699–704.

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