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Assignment of Analytical Chemistry

Review of International Journal

Study of Phase Transition on Nanocrystalline (La, Sr)(Mn, Fe)O System Using

High-pressure Mossbauer Spectroscopy and High-pressure X-ray Diffraction

Lecturer: Sulistyo Saputro, M.Si., Ph.D

By:Riezky Dwi Anggoro

K3310073

Chemistry Education

DEPARTMENT OF MATHEMATICS AND NATURAL SCIENCES

FACULTY OF TEACHER TRAINING AND EDUCATION

UNIVERSITY OF SEBELAS MARET

2012

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The aim of this research was to study of phase transition on nanocrystalline(La, Sr) (Mn,Fe) O system using high-pressure Mossbauer spectroscopy and high-pressureX-ray Diffraction. Pressure is an important parameter to study and understand the unknownknowledge about manganities. The properties of manganities was changed by external

pressure by changing Mn-O bond length. Under pressure transition from orthorhombic tomonoclinic was observed in La0.5Ca0.5MnO3 and type of antiferromagnetic state was stateseen in La0.5Ca0.5MnO3 and Pr0.7Ca0.3MnO3. Some of the relevant studies were done byRamos et al stated stability of Jahn-Teller distortion in LaMnO3 under pressure, Hwang etall stated Increase in metal insulator transition temperature TM-1 and Murata et allinvestigated that La2-2xSr1-2xMn2O7 have a planar ferromagnetic structure with easy axis inthe ab-plane, which changes to uniaxial ferromagnetic structure whose easy axis is parallelto c-axis at a pressure between 0.5 to 1.0 GPa. The background of this research was fewstudies have been reported on high-pressure behavior of the manganites perovskites, butwith nanocrystalline structured reported was rare. So, this research reported the pressuredependence on the magnetic and structural properties of the nanocrystallineLa0.8Sr0.2Mn0.8Fe0.2O3 system using diamond anvil cell, through High Pressure MossbourSpectroscopy and High-Pressure X-ray Diffraction.

The nanocrystalline La0.8Sr0.2Mn0.8Fe0.2O3 synthesized by sol gel nitrate methodwas characterized for the particle size distribution by X-ray diffraction (XRD) and Atomicforce Microscopy (AFM) techniques. The particle size estimated for the sample is 13 nm.Mssbauer measurements at ambient conditions were made using Co 57(Rh) source atconstant acceleration mode using krypton filled proportional counter as detector. For high-pressure Mssbauer measurements, a Si PIN solid-state detector with a resolution of 250

eV was used. The sample was loaded along with ruby crystal in a Merrill Bassett diamondanvil cell (DAC) using tantalum gasket and methanol : ethanol (4:1) mixture as pressuretransmitting medium. High-pressure X-ray diffraction (HPXRD) was carried out with aMao-Bell type of diamond anvil cell (DAC) in angle depressive mode using the Guiniergeometry. The detailed description of the high-pressure Guinier diffractometer set-up isgiven elsewhere. The incident Mo X-ray is obtained from a Rigaku 18 kW rotating anodeX-ray generator. A 50 mm long position sensitive detector is used to detect the diffractedX-rays. The overall resolution is found to be d/d= 0.015. The finely powdered sample andruby crystals were loaded into a stainless steel gasket hole, together with a mixture ofmethanol, ethanol and water (MEW) in volume ratio of 16:3:1 as the pressure transmitting

medium. The sample pressure in the DAC was determined by the ruby fluorescencemethod.

The pressure dependence of Isomer Shift (IS) and Quodrupole (QS) at roomtemperature is shown in Fig. 1. Two different values of Isomer shift indicate differentenvironment of iron, low spin Fe3+in octahedral site (site a) and Fe4+ in cubic symmetry(site b). Kopcewicz et al assumed that the low symmetry with large QS corresponds to the

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position surrounded by six Mn3+ions (site a) while the higher symmetry site with smallerQS, have at least on of the Mn in Mn4+state (site b). a sudden increase in isomer shift (IS)of Fe4+configuration (site b) at 0.52 GPa represents first order phase transition to highspin Fe3+while the IS for site a remains almost constant up to 10 GPa. The ferromagnetic

interactions of the electrons might decrease the s electron density at the Fe nucleussimultanesously increasing the IS. The increase in QS indicated the distortion of Fe3+lattice. On further increasing the pressure to 3.7 GPa IS of site b drops to low spin Fe3+, tocoincide with site a indicating convergence of the two different configurations of Fe intoone. The prominence of electronic contribution of other atoms (La, Sr) increases the selectron density at the Fe nucleus thereby decreasing the isomer shift. Only Fe in site bresponds to the variations with pressure, whereas the other site (site a) remainsunchanged, indicates the directional nature of the stress generated with the application ofpressure. Quadrupole splitting show a continuous increase with pressure up to 9.2 GPa,representing an anisotropic distortion of lattice. High-pressure diffraction experiments onthe stoichiometric sample La0.8Sr0.2Mn0.8Fe0.2O3 were carried up to 4.3GPa. Fig. 2 comparesthe XRD of pattern at ambient pressure, 0,3 GPa and 0.6 GPa. At the 0.6 GPa, there arenew peak where the # (112) and (123). This may be explained as reorientation of certainplanes with the application of pressure. And then we could see the full width at halfmaxima of the peaks of XRD pattern in the Fig. 3a and Fig. 3b. From the figure either Fig.3a or Fig. 3b, we could see the peak widths of (220) and (022) show unusual decrease at 0.6and 0.9 respectively, while (020) and (200) show general increasing trend with pressuregiving the hint reorientation of grain/grain boundaries in a articular direction in thispressure range. Gradual but small variation are observed up to 1,1 GPa. At 3.6 GPa (Fig. 4)there was a drastic change or a discontinuity in the values clearly indicating a phasetransition at that point. Another interesting feature seen was the unusual expansion of (220)d-spacing at 0.6 GPa (Fig. 3a), while other planes show normal decreasing trend (Fig. 3b)confirming the effect of unixaxial stress acting in ac-plane. At the 3.6 GPa the plane (202)breaks up into (201) and (211) planes indicating structural transition from orthorhombicwith cell parameters as a = 3.896 , b = 5.4886 and c = 7.795 to monoclinic with cellparameters as a = 3.872 , b = 5.4286 and c = 7.8015 and = 88.69 (Fig. 4), showingagreement with the results of HPMssbauer spectroscopy at same pressure. All the abovepoints suggest that pressure generates the uniaxial stress which reorients the grain/grainboundaries in the pressure range of 0.60.9 GPa in a particular direction parallel to ac-plane(easy axis of pressurization). Further increment of pressure gives a structural transition at3.6 GPa from orthorhombic to monoclinic.

Nano-crystalline perovskites La0.8Sr0.2Mn0.8Fe0.2O3 as a function of pressure up to10 GPa show sudden increase in isomer shift at 0.52 GPa indicating the transformation

from Fe4+

to Fe3+

due to transfer of eg electrons from ligand to iron 3 d level underpressure. Second transition is seen at 3.7 GPa where only Fe3+ is distributed with singlekind of environment. These phase-transition were accompanied by structural transition seenin HPXRD at 0.6 and 3.6GPa respectively. Transition at 0.6GPa were explained by thereorientation of grain/grain boundaries in a particular direction, while at 3.6 GPa thestructure transforms from orthorhombic to monoclinic.