Effect of synthesis temperature on the structural behavior of   Ca0.95Sr0.05FeO2.5 compound

a Mohammed Sadok MAHBOUB *, a Soria ZEROUAL& b Ali BOUDJADA

a Laboratoire d'exploitation et valorisation des ressources sahariennes, Département des Sciences de la matière, Faculté des Sciences et Technologie, Université d’El-Oued, B.P. 789 El-Oued RP, El-Oued 39000, ALGERIE.

b Laboratoire de Cristallographie, Département de Physique, Faculté des Sciences Exactes, Université Constantine 1, 25000, ALGERIE.

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ABSTRACT: Ca1-xSrxFeO2.5 (x=0, 0.05 and 0.1) samples are synthesized by both solid solution and mirror furnace methods, and the effect of synthesis temperature on the behaviour structure is investigated. The phase structures are comparatively characterized and studied by means of X-ray powder diffraction. Experimental results reveal that the synthesis temperature has a strong influence on the structure of the synthesized compounds. Samples obtained by solid solution method at 1473 K that they crystallizes in the orthorhombic lattice undergoes transition from primitive (Pnma space group) to body-centered (Imma or I2mb space group) up to x=0.1, whereas that obtained by mirror furnace method about 1900 K using melting zone technique show the transition at x=0.05. Both of synthesis methods were giving same lattice parameters values within the estimated standard deviation. Very high temperature applied during preparation in the ‘’mirror furnace method’’ has influencedon the arrangement of the FeO4 tetrahedra, and therefore on the Ca0.95Sr0.05FeO2.5 compound structure.

KEYWORDS:Ca1-xSrxFeO2.5; Chemical synthesis ; Crystal symmetry ; Mirror furnace ; X-ray Powder diffraction.

 ملخص: لقد تم تصنيع العيناتCa1-xSrxFeO2.5 (x=0، 0.05 و 0.1) بواسطة طريقتي المحلولالصلب و الفرنذو المرآة، حيث تم دراسة تأثير درجة الحرارةعلىسلوك بنياتها.لقد قمنا بدراسة البنيات البلورية عن طريق انعراجالأشعة السينيةعلى المساحيق. لقد كشفت النتائج التجريبيةعلى أندرجة حرارة التصنيع لها تأثيرقوي علىالهيكلالداخلي للمركباتالمصنعة، حيث وجدنا أن العيناتالتي تم الحصول عليهابواسطة طريقة المحلولالصلب عند درجة الحرارة1473 K تتبلورفي الشبكة المعينية المستقيمة و تخضعللانتقال منالبسيطة (Pnma) إلى الممركزة ( I2mb أو Imma) ابتداء من x=0.1، في حينأنالحصول عليها بواسطة طريقة الفرن ذو المرآة التي تستخدم تقنيةمنطقةالذوبان عند حوالي 1900 K تظهرالتحول فيها عند x = 0.05. و لقد أعطت طريقتي التصنيع على حد سواءنفس ثوابت الشبكة في حدود الانحراف المعياريالمقدر. فقد أثرتطبيق درجات الحرارة المرتفعةجدافي'' طريقة الفرن ذوالمرآةخلالعملية التصنيععلىترتيب و توضُع رباعيات الأسطح FeO4،وبالتالي علىالبنية البلورية للمركب Ca0.95Sr0.05FeO2.5.

كلمات دالة:Ca1-xSrxFeO2.5 ؛ التصنيعالكيميائي ؛التناظر البلوري ؛الفرنذو المرآة ؛ انعراجالأشعة السينيةعلى المساحيق.

 RÉSUMÉ: Les échantillons Ca1-xSrxFeO2.5 (x=0, 0.05 et 0.1) sont synthétisés par la méthode de solution solide et celle du four a image, et l'effet de la température de synthèse sur le comportement de la structure est étudié. Les structures de phase sont relativement caractérisées et étudiées au moyen de la diffraction des rayons X sur poudre. Les résultats expérimentaux révèlent que la température de synthèse a une forte influence sur la structure interne des composés synthétisés. Les échantillons obtenus par la méthode de solution solide à 1473 K qu'ils se cristallisent dans le réseau orthorhombique subit une transition d'un réseau primitif (groupe d'espace Pnma) au réseau centré (groupe d'espace Imma ou I2mb) jusqu'à x = 0.1, alors que celle obtenue par la méthode du four à image utilisant la technique de la fusion de zone autour de 1900 K montrent la transition à x = 0.05. Les deux méthodes de synthèse donnaient même valeurs des paramètres de maille au sein de l'écart-type estimé. Très haute température appliquée lors de la préparation dans la méthode du "four à image" a influencé sur l'arrangement des tétraèdres FeO4, et donc sur la structure cristalline du composé Ca0.95Sr0.05FeO2.5.

MOTS-CLÉS: Ca1-xSrxFeO2.5 ; Synthèse chimique ; Symétrie cristalline ; Four à image ; Diffraction des rayons X sur poudre.

1. Introduction:

Among candidate for the ionic conductor materials we can quote the isomorphic compounds SrFeO2.5 and CaFeO2.5 [1-9], which became two interesting areas of research, such as: ceramic membranes for oxygen separation and electrodes of solid oxide fuel cells (SOFCs), electrocatalysis, battery electrodes and sensor materials [10-17].To date, the information about the structure of SrFeO2.5 is still contradictory at room temperature. It was reported that SrFeO2.5 has either the disordered Imma structure (S.G. N° 74) [5, 6, 18, 19]or ordered I2mb structure (S.G. N° 46) [20, 21]. On the other hand, there is no disputethat CaFeO2.5 crystallizes in orthorhombic Pnma space group (S.G. N° 62) [6, 8, 9, 22, 23]. While FeO6 octahedra in these structures can not show any preferred orientation, the FeO4 tetrahedra do. For instance, SrFeO2.5 unlike CaFeO2.5 compound shows the possibility of the electrochemical intercalation of oxygens [2, 24-27]. It seems that the order of the FeO4 tetrahedra plays an important role in explaining the chemical reactivity inBrownmillerite compounds according to Paulus et al [28].Deepened study by investigating in the   Ca1-xSrxFeO2.5 compounds is therefore more requires, considering theirimportant function as a key to better understanding of theionic conduction mechanism in perovskite-related oxides. In their works, Nemudry & al. [25] were found that the structure of Ca1-xSrxFeO2.5 compounds obtained by solid solution (SS) method change from Pnma to body centered space group take place from x=0.1 at room temperature.The effect of synthesis conditions on properties of these compounds is essentially unexplored. Recently, we have reported the new synthesis method using the mirror furnace (MF) where we are success to synthesize a pure homogenous Ca0.5Sr0.5FeO2.5+d compound for the first time [29]. Based on the relationship between structure and physical properties, we will compare in this paperbetween Ca1-xSrxFeO2.5 (x=0, 0.05 and 0.1) brownmillerite compounds obtained by this new method (mirror furnace) with the results of the conventional solid solution method, and assess the effect of applied temperature in different synthesized method on structural behavior of these compounds.

2. Experimental

 2.1. Synthesis Method

The Ca1-xSrxFeO2.5(x=0, 0.05 and 0.1) samples have been prepared by the solid solution and mirror furnace using melting zone technique synthesis methods.

2.1.1. Solid solution method

Ca1-xSrxFeO2.5 (x=0, 0.05 and 0.1) compounds oxide was prepared in air by solid-state reaction of stoichiometric amounts of commercial CaCO3 (ALDRICH, 98%), SrCO3 (ALDRICH, 99.9+%) and Fe2O3(ALDRICH, 99+%) oxides were well mixed with acetone in an agate mortar for few minutes. The mixture was annealed at 1273 K for 12 hours, and then the powder was compacted in pellets, each of 1 g and 13 mm in diameter. The pellets were heated in air in conventional furnace at 1473 K for 24 hours. Then, the samples were quenched in liquid nitrogen. This operation is repeated several times. The pellets were ground to fine powder for phase characterization. A portion of the obtained powder was used as a starting point of the second method so-called “mirror furnace method” based on melting zone technique.

2.1.2. Mirror furnace method

Each amount of the powder of Ca1-xSrxFeO2.5 samples was put in a latex tube in order to prepare the feed rod. A hydraulic pressure of 10 bars was applied to obtain a solid bar, and it was calcined in air for 12 h at 1223 K. Afterwards, a high temperature around 1900 K is applied using the mirror

furnace concentrated on a relatively small spot size at the bottom of the rod until melting. The bottom falls down as a molten drop directly in liquid nitrogen (see Fig. 1).

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While, in the XRD pattern collected from the Ca1-xSrxFeO2.5 [x=0.1 (SS) and x= 0.05, 0.1 (MF)] samples, the (111), (131) and (151) reflections and all representative P lattice (h+k+l=2n+1) are absent, indicating transition into a body-centered lattice (Fig. 5a, 5b and 5c). Also, it was noted previously that the I2mb space group gave the same results as Imma within the experimental error. In these types of structures, the general problem lies in the fact that only slight differences exist in the arrangement of the FeO4 tetrahedral chains for these 3 space groups: ordered Pnma, I2mb or disordered Imma.

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We can explain this as thehigh temperatureinduces a change in displacementsof the oxygen ions in the tetrahedra that build up the structure in alternatinglayers, and therefore transformation from Pnma to Imma or I2mb. This phase transformation was also observed about CaFeO2.5 in the literature but in the range 950–1000 K [6, 9, 33]. Then, we can say that depending on the adopted synthesis method, the Ca0.95Sr0.05FeO2.5 compound would show various structures under different formation mechanisms.

On the other hand, the average crystallite sizes of Ca0.95Sr0.05FeO2.5 samples extracted from the (020), (200) and (002) reflections are varied from 83 nm to 120 nm for the sample obtained by SS method, and varied from 129 to 160 nm for the one obtained by MF method (Table 2).

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Also, we can see the increase in peak intensities in the XRPD pattern of the second method which is due to the enhancement of the crystallinity and particle size during the synthesis process. In the general case, the growth of the grains supported by the thermal energy contribution is generally associated with the increasing of the temperature, which allows the interpretation of the increasing of crystallite sizes.

It is worth mentioning that Ca0.95Sr0.05FeO2.5 sample prepared with the MF method can have a smaller value of full-width at half-maximum (FWHM) and higher peak intensities than the sample prepared with the SS method indicated the improvement of the crystallinity and particle size.

4. Conclusion

In summary, the influences of synthesis method on the structural behaviors of Ca1-xSrxFeO2.5 [x=0, 0.05 and 1] compounds were investigated. The two synthesis methods studied lead to differences in the compounds, namely a different structure, which is a result of the higher temperature during preparation via the “mirror furnace method” based on melting zone technique. Thehigh temperature during sample synthesizeinduces a change in displacementsof the oxygen ions in the FeO4 tetrahedra that build up the structure with the FeO6 octahedra in alternatinglayers, and therefore from Pnma to body centered Imma or I2mb space group. We can also say that the critical value of x in the Ca1-xSrxFeO2.5 compounds governing the change of the P to I lattice is not stable but varies with the temperature (x=0.1 at T=1473 K and x=0.05 at T~1900 K). Also, examination of the FWHM has shown that raising the synthesis temperature accompanies the increase of the crystallite size, in agreement with that the growth of the grains supported by the thermal energy contribution is generally associated with the increasing of the temperature. As a result, we can conclude that the preparation method for Ca0.95Sr0.05FeO2.5 influences on the compound structure to a considerable extent and thereby in its physical properties.

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