Analysis Crystal Structure of La0.7(Ba1-xSrx)0.3MnO3 by Sol-Gel Method

In this research, La0.7(Ba1-xSrx)0.3MnO3 compound (x = 0; 0.2; 0.3; and 0.5) by sol-gel method has been investigated. The compound used is mixed on a hot plate until reached a pH 7 when dropped ammonia solution, then let stand until turn into a gel. Dehydrated gel at 120°C, pra-calcination at 650°C for 6 hours, calcination t 1000°C for 12 hours, and sintering at 1200°C for 12 hours. The result of refinement XRD pattern shown that samples are single phase with rhombohedral crystal structure with R-3c space group. The intensity decrease and peak list shift to larger angle when Sr-substitution increased, it’s caused ionic radii of Sr is smaller than Ba .


INTRODUCTION
perovskite is one of the most interested engineering materials for researchers because unusual their electrical and magnetic properties. This structure can be modified through doping of divalent ion at La-site and transition ion at Mn-site. Substituted La-site with divalent ion such as Ba, Sr, and Ca can change Mn content from Mn 3+ to Mn 4+ , this phenomena can be explain with double exchange (DE) effect [1][2], a system by which electron mobility between the nearest Mn ions, preferably, those with aligned spins is preferred [3].
In general, crystal structure of LaMnO3 compound is cubic. Their crystal structure can be change or distortion caused of divalent ions substituted at La-site. Distortion on perovskite structure occurs of different ionic radii or size mismatch at La-site and Jahn-Teller effect. The average size of the A-site cation that modified the Mn-O-Mn bond angle and Mn-O distances can control this distortion [4]. Goldschmidt tolerance factor (t) of perovskite compounds ABO3 is commonly used to determine the stability of the geometry and crystal structure distortions. The t is defined by ratios of constituent ionic radii of A, B, and O which expressed by [5]: where are the average ionic radii on site A, and are the average ionic radii of the manganese and oxygen ions. The ideal perovskite compounds take on a cubic structure with t = 1. When the ratio of the ionic radii deviates from the ideal value t ≠ 1, a geometric strain and distortion of crystal occur [5].
In the previous work of Ref. [6], substitutes Ba 2+ ions to the LaMnO3 compound (La0.7Ba0.3MnO3) has a rhombohedral structure with R-3c space group. The structure of La1-xSrxMnO3 switch from orthorhombic to rhombohedral when Sr 2+ concentration is rising [7]. Mcbride et.all [1] suggested that La0.6Ba0.4MnO3 and La0.6Sr0.4MnO3 have a synthesized LaMnO3 rhombohedral structure using the same process. There are several methods have been reported can be used to synthesis of LaMnO3 with substitution divalent ions such as sol-gel method, solid state reaction, solution combustion method, molten salt reaction, and hydrothermal method. Among these methods, the sol-gel method makes it easier to obtain highly crystalline nanoparticles with the smaller size and stoichiometry desired [8][9][10].
Analysis crystal structure and crystallographic characterization generally used X-ray diffractometer (XRD), this is non-destructive analytical technique [11]. The properties of polycrystalline materials depend on several things, such as the crystalline size. Using Scherrer equation, we can calculated the crystalline size expressed by [12][13].
where D is crystalline size, k have value 0.9, is wavelength of X-ray ( = 1.5406 Å), B is FWHM, and is bragg angle. In this work, La 0.7 (Ba 1-x Sr x ) 0.3 MnO 3 where (x = 0; 0.2; 0.3; and 0.5) have been synthesized by sol-gel method. Using Rietveld refinement of X-ray diffraction data the structural parameters (unit cell volume, crystalline size, bond length, bond angle, and tolerance factor (t)) were refined. 37 for 12 h. The black powder obtained was pressed at 10 tons and sintered at 1200°C for 12 h. XRD Panalitycal X'pert Pro MPD identified the crystal structure and phase information of the samples with Fast Detector X'celerator using CuK⍺ ( = 1.5406 Å) radiation in the range 10°-90° with step size of 0.02°. The X-ray diffraction data were analyzed by Rietveld refinement using HighScore Plus software.

RESULT AND DISCUSSION
XRD character trends in Fig. 1 shown the single phase of LBSMO where all samples have rhombohedral structure with R-3c space group. XRD results also indicated that the intensity decreased when Sr 2+ substitution increased. The sample's unit cell parameters, other fitting parameter and Goldscmidth tolerance factor are shown in Table 1 using the refined crystallography data. Fig. 2 shown the peak LBSMO shifted to the right when substitution Sr 2+ increased. The higher value of 2-theta indicates that LBMO (x = 0) have larger d-spacing compared to substituted with Sr 2+ so d-spacing decreased, which is caused by the ionic radii of Sr 2+ (1.44 Å) is smaller to replaces of Ba 2+ (1.61 Å).  Beside the result of refinement in table 1, substituted Sr 2+ also caused a decrease of volume cell crystal shown fig. 3, it's occurred that ionic radii of Sr 2+ smaller than ionic radii of Ba 2+ so the d-spacing is decreased. The different ionic radii also affected Goldscmidth tolerance factor (t) as a result distortion of LaMnO3 structure caused mismatch between ionic radii which is substituted with the ionic radii of La. The ionic radii of Ba 2+ (1.61 Å) and Sr 2+ (1.44 Å) too large to occupied site La (1.172 Å). Substitution increased causes the value of Goldscmidth tolerance factor decreased. This occurred when substituted increased, the value of ionic radii at site A from La0.7(Ba1-xSrx)0.3MnO3 decreased from 1.435 Å to 1.4025 Å. Fig. 4 shown the crystalline size of La0.7(Ba1-xSrx)0.3MnO3 calculated using the Eq.(2).

CONCLUSION
Analysis of La0.7(Ba1-xSrx)0.3MnO3 crystal structure was studied using sol-gel method. All samples have rhombohedral structure with R-3c space group. Volume cell, crystalline size, and tolerance factor (t) are decreased with increasing Sr 2+ substituted. On the other, the increased substitution of Sr 2+ caused bond length dMn-O increased and bond angle θMn-OMn decreased.