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Print version ISSN 1516-1439
Mat. Res. vol.15 no.4 São Carlos July/Aug. 2012 Epub June 26, 2012
Sundararaman SankarrajanII; Kathiresan SakthipandiI; Venkatachalam RajendranI, *
ICentre for Nanoscience and Technology, K. S. Rangasamy College of Technology - KSRCT, Tiruchengode, 637 215, Tamil Nadu, India
IIDepartment of Physics, National Engineering College - NEC, Kovilpatti, 628 503, Tamil Nadu, India
On-line measurements of ultrasonic longitudinal velocity, shear velocity and longitudinal attenuation were carried out on R1-xSrxMnO3 perovskites (R = La, Pr, Nd and Sm) for different compositions of Sr, at a fundamental frequency of 5 MHz over wide range of temperatures using the through-transmission method. The observed maxima/minima in velocities and attenuation have been discussed with decrease in ionic radii and composition. As a decrease in the ionic radii of rare earth elements leads to a decrease in transition temperature (Tc), the results that are observed show that measurement is one of the best tools to explore the structural/phase transition on-line velocity in perovskite manganese materials as a function of the ionic radii of rare earth elements.
Keywords: perovskite, magnetic materials
R1-xSrxMnO3 (where R is a rare earth element such as La, Nd, Sm, Pd) perovskite manganites have been a subject of interest due to their exotic properties and phenomena like colossal magnetoresistance (CMR)1-3. It exhibits a very rich phase diagram due to the subtle competition among the interactions involving the spin, lattice and charge degrees of freedom1. The modest variation in the dopant concentration, preparation methods, cation deficiency etc., can cause profound changes in their magnetic states i.e., ferromagnetic (FM) and antiferromagnetic (AFM)4. The transport properties like dirty metal, insulator, semiconductor, polaron hopping and the structural characteristics such as Jahn-Teller induced strains and orthorhombic to rhombohedral transformation depends on dopant and its concentrations5-6.
The phase transition temperature (TC) which separates FM metallic state from paramagnetic (PM)/AFM insulating states depends on the concentration of rare earth and alkaline elements6. Further, the substitution on dopant results in the CMR effect several times larger than pristine manganite. It is evident that the absence of unpaired electrons on the doped Mn sites which break the hole propagation in the manganese oxygen lattice as the doping content increases, and consequently leads to a decrease in TC7-8. The small difference between the different substitutions may be due to the combined effects of size and valence of the dopant. The substitution of trivalent by divalent element leads to the replacement of Mn3+ ions by Mn4+ sites9. The doping of rare earth ions on Mn sites with unchangeable tetravalent element inevitably weakens the double exchange (DE) interactions in Mn3+-O-Mn4+ co-valent structures. Hence, it is expected that the electrical/magnetic properties and the CMR effect of the perovskite manganite material are more sensitive due to the substitution of the dopant element10. The interactions between Mn3+ and Mn4+ ions play a vital role in the determination of the phase transition of manganite perovskites. The DE interactions between Mn3+ and Mn4+ ions are also affected by the influence of ionic radius of trivalent elements (rare earth elements) and tetravalent elements (Mn sites)11.
Rare earth oxides such as La, Sm, Pr and Nd are one of the most important additives in SrMnO3 based perovskites for sensor applications12-13. The various attempts14-17 have been focused on perovskites doped with various rare earth elements. Even though, it is necessary to investigate the correlation between the phase transition temperature and with ionic radius of the rare earth elements. The structural changes in the substitution of rare earth element in SrMnO3 perovskite manganite materials are well reflected in the density/modulus and hence, these changes are well reflected in the measured ultrasonic velocities. The excellent property of the ultrasonic waves makes it to gain knowledge about the origin of the metal-insulator transition, structural changes, phase transitions and magnetostriction effects. The importance of ionic radii in addition to the change in composition of Sr has been studied extensively employing resistivity17, conductivity18, Mossbauer19 and magnetic20 studies.
Even though, different techniques are known in exist to explore structural/phase transition temperatures in perovskites manganite materials, on-line ultrasonic measurement is important tool due to their precise on-line evaluation of structural/phase transitions, non-destructive method of measurement, accurate and reproducibility of results than any other experimental methods21. It is inferred from the literature that no attempts have been made to compare the Tc using ultrasonic studies by changing the R and x values.
In the present investigation, a systematic study has been made on ultrasonic longitudinal velocity (UL), shear velocity (Us) and longitudinal attenuation (αL) in R1-xSrxMnO3 perovskites manganite materials for different Sr contents over wide range of temperatures. The observed anomalous behaviour in ultrasonic velocities and attenuation are discussed with the phenomenon such as phase transition, CMR and GMR effects with the change in ionic radius of rare earth elements and Sr content.
2.1. Sample preparation
La1-xSrx MnO3 (LSMO; x = 0.28, 0.31 and 0.36), Pr1-xSrxMnO3 (PSMO; x = 0.28, 0.33, 0.35, 0.38 and 0.41), Nd1-xSrxMnO3 (NSMO; x = 0.31, 0.35, 0.37, 0.39 and 0.41) and Sm1-xSrxMnO3 (SSMO; x = 0.25, 0.30, 0.37, 0.40 and 0.44) perovskite samples were prepared using a solid-state reaction technique22-25]. The measurement of UL, Us and αL was made over a wide range of temperatures as described elsewhere22-25.
2.2. Ultrasonic velocity measurements
Ultrasonic velocities were measured using a high-power ultrasonic process control system (FUI1050; Fallon Ultrasonics Inc., Canada), a 100 MHz DSO (54600B; HP, USA) and a PC as discussed in our earlier studies22.
2.3. Attenuation measurements
The attenuation coefficient was calculated by measuring the amplitude of received pulses with waveguides (Aw(f)) and waveguides with sample (As(f)), using the following relation22:
3. Results and Discussion
The Tc values of each composition in all the perovskites were obtained from the observed maxima/minima in UL, Us and αL. The composition- and ionic radii-dependent Tc values are shown in Figure 1. The observed prominent peaks in UL and Us for all perovskites with different compositions have been obtained from the measured velocities22-25. The Tc values obtained from the magnitude of UL and Us in all perovskites and compositions are shown in Figures 2 and 3 respectively. The maxima/minima values in velocities that are observed are mainly because of the occurrence of lattice softening, that is, the contributed effects of lattice softening above Tc and lattice hardening below Tc23,26. Similarly, the attenuation maxima/minima in all perovskites at the respective Tc value that are observed have been identified. The maxima/minima αL as a function of perovskites and their composition are shown in Figure 4. The Tc in all perovskites that is observed shows the existence of competition among the PM, FM and AFM phases21,24,26. The values of ionic radii for rare earth elements, La, Pr, Nd and Sm, are 1.172, 1.13, 1.123 and 1.098 Å respectively.
Figure 1 shows that Tc values strongly depend on the ionic radii of rare earth elements. For example, at a fixed composition of Sr, say x = 0.28 in case of La1-xSrxMnO3 and Pr1-xSrxMnO3 perovskites, the replacement of the rare earth element, La, by Pr reduces the Tc value from 380 to 261 K. Similarly, for x = 0.35, the replacement of Pr by Nd (Nd1-xSrxMnO3) leads to a reduction in Tc from 298 to 254 K as discussed above. It is further evident that if one considers the replacement of Nd by Sm (Sm1-xSrxMnO3) for the composition, x = 0.37, the value of Tc decreases from 273 to 138 K. A detailed discussion of the basic properties of rare earth oxides supports the above observation27. It is clear that the basic strength of rare earth oxides is attributed to lanthanide contraction, that is, the strength of the basic sites decreases with a decrease in the ionic radii of rare earth elements. It is inferred from above observations that the transition temperature TC decreases with decrease in ionic radii of rare earth elements.
The ultrasonic parameters cannot directly correlate with ionic radius of the rare earth element; rather it depends on the structural compactness of sample which is influenced by various factors like preparation method28, sintering temperature29 and particle size30. However, attempts have been made to reveal the ultrasonic behaviour of rare earth element31-32 and found that a decrease in ultrasonic velocities with an increase in ionic radii.
It is known that the perovskite structure is distorted with a decrease in the ionic radii of rare earth elements. The distortion of the perovskite-type structure causes a decrease in electrical and thermal conductivities33. The variation of Tc as a function of the average size of A-site cations (Ln and Sr) has been observed in Ln0.5Sr0.5MnO3 (Ln = La, Pr, Y, Sm and Gd)33. It is interesting to note that a decrease in Tc value from 310 to 85 K corresponds to a decrease in ionic radius from 1.263 to 1.221 Å. Further, the ionic radii of the R ions are used to know the state of perovskites, that is, PM or AFM states34. On the basis of the above studies, it is concluded that a decrease in the Tc value, which is obtained from the maxima/minima that are observed in the ultrasonic parameters, strongly supports the results obtained from resistivity, electrical, Mossbauer and magnetic property studies16,17,19,22-25.
On-line ultrasonic velocity measurement as a function of temperature determines the Tc in perovskites. Tc that is observed with a decrease in the ionic radii of the rare earth elements decreases as observed by other studies such as conductivity. Thus, it is concluded that an on-line ultrasonic velocity/attenuation measurement is one of the best tools to explore the structural/phase transition in perovskites as a function of the ionic radii of rare earth elements.
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Received: August 5, 2011
Revised: March 19, 2012