Temperature Dependent Elastic and Ultrasonic Properties of La_0.8Na_0.2MnO₃perovskite Manganite Material

Temperature Dependent Elastic and Ultrasonic Properties of La_0.8Na_0.2MnO₃perovskite Manganite Material

M. Arunachalam1,2, P. Thamilmaran1,2, S. Sankarrajan3and K. Sakthipandi4*

1Department of Physics, Sri SRNM College, Sattur 626 203, Tamil Nadu, India

2Research scholars in Physics, ManonmaniumSundaranar University, Tirunelveli- 627 012. Tamil Nadu, India.

3Department of Physics, Unnamalai Institute of Technology,Kovilpatti 628 503, Tamil Nadu, India

4Department of Physics, Sethu Institute of Technology,Kariapatti 626 115, Tamil Nadu, India

Abstract Ultrasonicvelocity and attenuation measurement is a versatile tool to explore the mechanical properties as well as the phase transition in the perovskite solids. La0.8Na0.2MnO3perovskite manganite sample was prepared employing solid state reaction technique. The rhombohedral structure with R3c space group existing in the prepared samples was confirmed from the obtained X-ray diffractionspectra. In-situ ultrasonic velocity and attenuation measurements were carried out in the range of temperature from 300 to 400K. The observed anomalous behaviour in the ultrasonic parameters and elastic constants were used to explore ferromagnetic (FM) to paramagnetic (PM) phase transition temperature.

Keywords Manganite, Ultrasonic velocity, phase transition

1. INTRODUCTION

   Perovskite manganites, with a general formula R1- xAxMnO3 where R and Aare trivalent rare earth and alkaline elements respectively, have been recently become a subject of intense investigation in view of their unusual electrical, mechanical and magnetic properties. They show rich phase illustrationdue to the competition among interactions involving the spin, lattice and charge degrees of freedom [1-2]. Because of their special properties such as colossal magneto resistance (CMR), metal-insulator (MI) transition, paramagnetic (PM) to ferromagnetic (FM), antiferromagnetic (AFM) phase transition, they find a wide range of applications in frequency switching devices, magneto storage devices, sensor technology etc which are useful in bio-medical and electronic industries [1]. Measurement of elastic constants using ultrasonic waves is a very sensitive tool to study different types of phase transition like ferromagnetic and structural ones, as they are lattice related properties. These techniques are found to be successful in studying electron-phonon coupling and spin- phonon coupling in manganites [2-3]. The transition temperature (TC)that separates FM metallic state and paramagnetic PM/AFM insulating state depends on the concentration of rare earth and alkaline earth elements [4].

   Survey of the literature reveals that most of the studies carried out in divalent alkaline elements such as calcium and strontium as doping material [5]. In fact the donor property is much pronounced in monovalent alkali elements than divalent alkaline elements like calcium,

   strontium etc. So one can expect perovskite manganite materials doped with monovalent elements to show distinguishing feature in their structural, electronic and magnetic properties from the ones doped with divalent alkaline elements like sodium, potassium etc [6]. But only a small quantum ofwork has been done with monovalent alkaline earth elements like sodium, lithium, potassium etc [6]. Substitution of lanthanum ions by monovalent potassium ions increases the ionic radius of the La site which produces crystallographic distortion and an increase in the Mn-O-Mn bond angle. As this increase in bond angle is associated with the shortening of Mn-O distance it leads to a stronger double exchange (DE) interaction and an increase the Curie temperature TC[6].As no work has been reported employing ultrasonic measurements on sodium doped perovskite manganites an attempt has been made to investigate the temperature variation of elastic behavior of La0.8Na0.2MnO3perovskite manganite samples by means of ultrasonic studies employing through transmission technique.

2. MATERIALS AND METHODS

1. Sample preparation La0.8Na0.2MnO3perovskitemanganite material sample

   was prepared employing solid state reaction technique. Pure grade manganese carbonate (99.99%; Sigma Aldrich), lanthanum nitrate (99.99%; Sigma Aldrich) and sodium nitrate (99.0%; Himedia GR, India) powders were taken in stoichiometric quantity and weighed in a digital balance. Then, the powders mixed in an agate mortar. The mixture was calcinated two times at 873K in air for 2h and further ground to obtain a homogeneous mixture. Once again the mixture was calcinated at 873K in air for 2h and then the obtained powder was pressed into pellets of diameter 12 mm and thickness 4 mm with a stainless steel dye. The pellets were sintered at 1373K for 12h in the atmospheric air. Thus the sample was prepared in the form of pellets. These pellets were used for further characterization.

2. XRay Diffractometry

   The X-Ray diffraction ( XRD ) patterns of the prepared La0.8Na0.2MnO3sample was obtained using powder X- Ray diffractometer (XPertPro, Analytical, Netherland) at a scan range of 10 80o with an increment

   of 0.020 using Cu K as the radiation source with an operating voltage of 45 kV and a current rating of 35 mA. The crystalline size (D) of the sample was determined from full width at half maximum () and angle of diffraction (2) of the XRD peaks using Scherrer equation:

   D = 0.94 (1)

   Cos

   transit time t1just with the buffer rods (without the sample) and transit time t2 by placing the sample between the two buffer rods (with the sample). The overall accuracy in the measurement of velocity was + 2 m s-1. The amplitudes of the first and second back wall echoes in the time domain trace were measured to obtain attenuation coefficient as follows

   1 Aw ( f )

   Where is the wavelength of the source used in x ray radiation.

   ( f )

   1nT 1n

   d A

   ( f )

   (4)

   s

3. Density measurement

   The density of the sample was measured employing Archimedes principle using CCl4 as a buoyant. The weight

   of the sample in air and buoyant, Waand Wb respectively

   where As(f) is the amplitude of the received signal with sample andAw(f) the amplitude of the received signal without sample.

   were measured using a digital balance (Sartorius, Germany). The density of the sample was obtained using the formula,

   T 4Z w Z s

   2

   (Z w Z s )

   (5)

   b

   = Wa

   Wa Wb

   (2)

   where Tis the transmission coefficients of the sample-wave guide interface, Zw and Zsare the acoustic impedance of the wave guide and the sample respectively. The

   where b is the density of the buoyant.The experiment was

   repeated for five times and then the average value was taken as the density of the sample. The percentage error in the measurement of density was + 0.05.

4. Microscopy

   A scanning electron microscope (SEM) integrated with the energy dispersive analysis of X- rays (EDAX; JEOL,

   percentage error in the attenuation measurement is

   ±2.Elastic constants such as bulk (K) and Youngs (E) modulus were obtained using following relations.

   K = L – (4/3) G (6)

   Poissons ratio

   L 2G

   JED5300, Japan) spectrometer was used to obtain the images of the La0.8Na0.2MnO3sample to explore the morphology, microstructure and the composition of the perovskite sample. The spectrometer was operated at an

   and

   2(L G)

   (7)

   accelerating voltage of 0kV.

5. Ultrasonic measurement

A high-power ultrasonic pulse receiver (Olympus NDT, 5900PR, USA) was used for the transmission and reception of the ultrasonic signals. For recording the digital RF ultrasonic signals, a computer-in-built 1GHz digital storage oscilloscope (Lecroy, Wave Runner 104Mxi, USA) was used. To produce longitudinal and shear ultrasonic waves X and Y cut transducers operating at a fundamental frequency of 5MHz were used respectively. An indigenously designed experimental setup was used to measure the ultrasonic velocity andattenuation over a wide range of temperature starting from 300 to 400K with the help of a programmable temperature controller (Eurotherm, 2604, USA).The temperature required has been kept under the dynamic mode of operation. The error in temperature measurement was + 1K. The ultrasonic velocities (UL and US) of the perovskite samples were obtained using the equation

E = (1 + ) 2G (8)

III Result and discussion

The XRD pattern (shown in Fig. 1) of the La0.8Na0.2MnO3 perovskite manganite sample confirms the crystalline nature of the sample. The observed sharp peaks in the XRD pattern indicate that they are in close agreement with the samples of similar composition with JCPDS files 89-8122 and 89-8028. From this it is observed that the samples have rhombohedralstructure with R3c space group.

d

U = (3)

t

where d is the thickness of the sample in micron resolution and transit time t = (t2-t1) in of the time taken for the propagation of ultrasonic waves into the specimen which innano second resolution. It was obtained by measuring

Fig. 1 XRD spectrum of La0.8Na0.2MnO3 sample

The crystallite size of the sample is obtained usingScherrer formula from the obtained diffraction angle (2) and full width half maximum(FWHM) of the peaks. The crystallite size of the sample is 29.85 nm.The elemental composition of the constituents of the prepared perovskite manganite La0.8Na0.2MnO3sample was confirmed by the obtained energy dispersive X-ray analysis (EDX) pattern.The SEM image of the prepared sample is shown in Fig. 2. It is inferred that the prepared sample has spherical like particle morphology. The particle size from the SEM image of the sample is found to be 120 nm.

Fig. 2 SEM image of La0.8Na0.2MnO3 sample

The longitudinal velocity (UL),shear velocity (US), longitudinal attenuation (L) and shear attenuation (S)of the ultrasonic waves were measured over the range of temperature from 300 to 400 K. These measurements were used as an effective tool to obtain the temperature dependent mechanical property of the solids.Using the measured density ( = 5457 kgm-3) and ultrasonic velocities(both UL and US), the bulk modulus (K) the Youngs modulus (E) were obtained. The bulk modulus and Youngs modulus for the sample are 107.15 and 117.99GPa respectively at the room temperature (300 K).

Fig. 3 Variation of longitudinal (UL) and shear (US) velocities with temperature in La0.8Na0.2MnO3 sample

Fig. 4Variation of Bulk (K) and Youngs (E) moduli with temperature in La0.8Na0.2MnO3 sample

The temperature dependence of UL and US are shown in Fig. 3 and that of elastic constants are depicted in Fig.4.From Fig. 3and 4, it is observed in the temperature range 300 to 328 K (region I) and from 342 to 400 K (region III) that the ultrasonic velocity and the elastic constants of the sample are monotonically decreases with an increase in temperature. As like in any other solid material,these parameters (UL, US,K and E) decreasemonotonically with an increase in temperature. However, an anomalous behaviourin ultrasonic parameters is observed over atemperature region from 328 to 342 K (region II).There is a sharp decline in both velocity (UL andUS)andelastic constants(K and E) that reaches a minimum at the temperature 334 K followed by a sharp rise up to 342 K. Hence, in this regionof the Figs 2-3 areconsiderable importance and it reveals the physical significance of the prepared perovskite manganite sample.

As observed in the measurements using super conducting quantum interference device magnetometer in the study carried out by Xuebin, Yuping Sun et.al, Curie temperature (Tc), the temperature at which a transition takes place from FM to PM state is 333 K for the perovskite material La0.8Na0.2MnO3[7]. Similar observations were also made for the perovskite materials doped with monovalent element potassium [8]. Correlating the above results with the ones observed in this investigation, it is confirmed that the temperature at which UL, US, K and Eare minimum in the anomalous region (334 K)is the Curie temperature (TC) of the prepared perovskites sample. Thus, the anomalous temperature (334 K) of the ultrasonic parameter is identified as a TC of the prepared La0.8Na0.2MnO3 perovskite manganite.

Fig. 5Variation of longitudinal (L) and shear (S) attenuations with temperature in La0.8Na0.2MnO3 sample

The longitudinal and shear attenuation of ultrasonic waves as a function of temperature is shown in Fig. 5. The attenuation increases as the temperature increases in region I and region III. As like in Fig. 3 and 4, the sample shows an anomalous behavior in region II. At the temperature 328 K the value of L and S increase steeply and reach a maximum of 1.589and 9.537 dBcm-1respectively at 334 K then falls sharply up to 342 K. The occurrence of magneton, electron and lattice ordering features implies strong spin-phonon coupling and electron-phonon coupling known to exist for the rhombohedral coordinated d4 ion originating in the Jahn-Teller (JT) effect is the reason for such a peak [9]. The Curie temperature of the undoped LaMnO3 material is only 130 K [10] whereas the doping of 20% of sodium makes it as large as 334 K. This fact is attributed to the increase in DE interaction caused by the reduction in Mn-O distance that takes place. This is due to the increase in average ionic radius of La3+ (rion= 1.36A° for CN=12) site produced by the doping of monovalent Na1+ (rion= 1.39A° for CN=12) in the perovskite manganite [11]. Further the coexistence of anomalous behavior in both longitudinal and shear mode in velocity and attenuation confirms the existed electron-phonon interaction in the sample. Also the increase in velocity and the decrease in attenuation of the ultrasonic waves in the sample below TCreveal the occurrence of lattice hardening below the Curie temperature

IV. Conclusion

Perovskite manganite La0.8Na0.2MnO3sample was prepared employing solid state technique. Crystalline nature of the sample was confirmed by the obtained XRD. In- situ ultrasonic measurements of velocity and attenuation were carried out on the prepared sample in the temperature range from 300 to 400 K. Bulk and Youngs modulus of the sample were determined using the standard formula. The temperature dependence of the ultrasonic parameters as well as the elastic constantswas used to explore the phase transition in the sample. The results of the investigation show that the doping of sodium causes the increase in the phase transition temperature TC.The electron-phonon interaction in the sample is ascertained by the anomalous behavior obtained in both longitudinal and

shear modes. Further the lattice hardening below TC is confirmed by the increase in velocity and the decrease in attenuation.As the phase transition temperature TC is neighboring the room temperature La0.8Na0.2MnO3samples may be used for potential applications.

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