Mixed rare earth oxides derived from monazite sand as an inexpensive precursor material for room temperature magnetic refrigeration applications

Accepted Manuscript Title: Mixed rare earth oxides derived from monazite sand as an inexpensive precursor material for room temperature magnetic refrigeration applications Authors: B. Arun, V.R. Akshay, Geeta R. Mutta, Ch. Venkatesh, M. Vasundhara PII: DOI: Reference:

S0025-5408(17)30821-8 http://dx.doi.org/doi:10.1016/j.materresbull.2017.07.006 MRB 9435

To appear in:

MRB

Received date: Revised date: Accepted date:

1-3-2017 5-7-2017 7-7-2017

Please cite this article as: B.Arun, V.R.Akshay, Geeta R.Mutta, Ch.Venkatesh, M.Vasundhara, Mixed rare earth oxides derived from monazite sand as an inexpensive precursor material for room temperature magnetic refrigeration applications, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mixed rare earth oxides derived from monazite sand as an inexpensive precursor material for room temperature magnetic refrigeration applications B. Arun1,2, V.R. Akshay1,2, Geeta R. Mutta3, Ch. Venkatesh4, M. Vasundhara1,2* [email protected] [email protected] 1

Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, India. 2 Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST, Trivandrum, India. 3 Nano-Materials Laboratory, School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh EH14 4AS, United Kingdom. 4 Department of Physics, Indian Institute of Technology, Kharagpur. India. *

Corresponding author: Tel.: +91 4712515491

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Graphical abstract

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Highlights 

Compound is synthesized using mixed rare earth oxide derived from monazite sand.



Rietveld refinement confirms the compound is a mixture of rare earth manganites and CeO2.



The compound exhibits a magnetic entropy change of 3.28 J kg-1 K-1 under 50 kOe magnetic field at 310 K.



The compound exhibit a RCP of 120 J kg-1 and ΔTad of 2.11 K at 310 K under 50 kOe field.

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Abstract An inexpensive perovskite type (REMIX)0.67Sr0.33MnO3 compound is synthesized via solid state method using mixed rare earth oxide precursor derived from monazite sand. Rietveld refinement of X-ray powder diffraction patterns confirms the compound is a mixture of rare earth manganites which is the major phase and CeO2 as the secondary phase. The compound shows a second order ferromagnetic to paramagnetic transition near room temperature and exhibits a magnetic entropy change (-∆SM) of 3.28 J kg-1 K-1 with a relative cooling power (RCP) of 120 J kg-1 and an adiabatic temperature change (ΔTad) of 2.11 K at 310 K under 50 kOe magnetic field. The Debye temperature of the compound is found to be 543 K and room temperature thermal conductivity is 4.26 W m-1 K-1. The temperature variation of electrical resistivity shows a metal to insulator transition around 180 K, which gets shifted towards higher temperature upon the application of magnetic field. The compound shows a large value of -∆SM and this work makes an endeavor to develop low–cost materials for the magnetic refrigeration applications near the room temperature.

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Introduction Conventional gas compression based refrigeration technology is a major consumer of energy and is an important cause of greenhouse gas emissions in modern society. Thus it is important to conduct research on efficient and affordable cooling technologies that would results in reduction in energy consumption. Magnetic refrigeration based on the magnetocaloric effect (MCE) is an upcoming technology alternative to the conventional gas-compression technology. MCE is a magneto- thermodynamic phenomenon that results from the warming and cooling response of a magnetic material to the applied magnetic field, characterized by the isothermal entropy change (∆SM) and the adiabatic temperature change (∆Tad), which is the working principle of the magnetic refrigeration [1, 2]. A giant MCE was found in Gd5Si2Ge2 a pseudo-binary alloy [3], which was considered as a milestone in the field of magnetic refrigeration technology. Since then the researchers in this area have been focusing theoretically and experimentally on identifying the materials with large MCE values near room temperature for domestic applications. Large MCE values are also observed in compounds such as MnAs1-xSbx [4], La (Fe1-xSix) and substituted compounds [5, 6], MnFe (P1-xSix) [7], Gd-based amorphous alloys [8], ErCo2 [9] and Ni-Mn-X (X=Ga, In, Sn) Heusler alloys [10] respectively. However, most of these materials show high electrical conductivity, large thermal and field hysteresis, which are the factors considered to be detrimental for an active magnetic refrigerator. Moreover, most of the known MCE materials contain either very expensive elements or involves a very complex and high priced synthesis routes.

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In recent years, mixed valence manganites with general formula A1-xA’xMnO3 (A = trivalent rare earth (RE), elements such as La3+, Nd3+, Pr3+ etc, A’= divalent alkaline earth elements such as Ca2+, Sr2+, Ba2+ etc.) are perceived as interesting materials not only due to rich physics involving charge, spin and orbital degrees of freedom [11-14], but also they are potential candidates for the magnetic refrigeration applications due to its low cost, simple fabrication, higher chemical stability, small magnetic hysteresis and low eddy current heating [15]. The mixed valence manganites can be regarded as solid solutions between end members such as LaMnO3 and SrMnO3 with formal valence states La3+Mn3+O32- and Sr2+Mn4+O32, leading to mixed valence compounds such as (La1-x3+Srx2+)(Mn1-x3+Mnx4+)O3. Usually the end members are antiferromagnetic and insulating while the solid solutions with x≈0.33 are ferromagnetic and conducting in nature [16], thus making the particular composition, A0.67A’0.33MnO3 an interesting material. Double exchange (DE) interaction, Jahn–Teller effect (JT) and phase separation play a key role in these mixed valence manganites [17-19]. The magnetic behavior of these materials strongly depends on strength of DE interactions between Mn3+/Mn4+ ions which results from the motion of the eg electron between the two partially filled d-orbitals [20]. La1-xSrxMnO3 is one of the most studied families among the manganites having the highest TC of 370 K and a -∆SM of 1.55 J kg-1 K-1 at x = 0.33 under 10 kOe magnetic field [21]. Magnetic entropy change of 2.06 J kg-1 K-1 (at 252 K) and 1.48 J kg-1 K-1 (at 292 K) was reported in La0.67Ca0.33MnO3 and La0.67Ba0.33MnO3 respectively under a magnetic field of 50 kOe [22]. Guo et al. [23] found the -∆SM of La0.8Ca0.2MnO3 as 5.5 J kg-1 K-1 at 230 K under a field change of 15 kOe, which is higher than that of Gd (4.2 J kg-1 K-1) under the same magnetic field. A commendable work in this direction was done by Phan et al. [24] who reported a large -∆SM value of 2.12 J kg-1 K-1 in La0.65Sr0.35MnO3 at 305 K under a magnetic field of 10 kOe. Recently, La0.5Na0.5MnO3 was reported to be a potential candidate for room

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temperature magnetic refrigeration applications with a magnetic entropy change of 1.5 J kg-1 K-1 at 290 K under field 50 kOe [25]. A large value of -∆SM of 7.5 J kg-1 K-1 was reported for Nd0.5Sr0.5MnO3at 183 K for a field of 1 T [26]. Tang et al. reported a -∆SM of 3.45 J kg-1 K-1 in La0.8Ag0.2MnO3 278 K for a field of 1 T [27]. La0.7Ca0.25K0.05MnO3 and La0.7Ca0.25Sr0.05MnO3 were reported to be potential candidates that exhibits -∆SM of 3.95 J kg-1 K-1 (at 270 K) and 10.5 J kg-1 K-1 (at 275 K) for 2 T and 5T field respectively [28, 29]. Pr0.68Ca0.32MnO3 and Pr0.63Sr0.37MnO3 compounds show large -∆SM of 24 J kg-1 K-1 (at 21.5 K) and 8.52 J kg-1 K-1 (at 300 K) for a field of 5 T [30, 31]. Nd1-xA’xMnO3 and Pr1-xA’xMnO3 families show different magnetic phenomena from that of La1−xA’xMnO3 series due to the weakening of the DE interaction caused by smaller Pr and Nd ions [32, 33]. However, it is understood that the magnetic and electrical properties of manganite materials can be tuned by substituent elements, their ionic radii, doping concentration, synthetic routes and annealing conditions [14]. In the present study, we have used the raw material RE2O3 which is a mixture of various rare earth oxides obtained after the removal of radioactive thorium from Indian monazite sand. Indian monazite, a beach placers sand contain up to 60–66 % RE2O3, 8–10 % ThO2 and 20–25 % P2O5 as major constituents. RE2O3 contain CeO2 (47.5%), La2O3 (22.0%) as the major components and Pr6O11, Nd2O3 and Sm2O3 in appreciable amount [34]. It is well understood that the separation of RE elements is difficult due to their similar chemical nature, electronic structure and oxidation state and thereby the subsequent separation and purification techniques are rather complex and expensive. We are the pioneers in attempting to make the direct use of mixed rare earth oxides (without separation of individual rare earth oxides) as cost effective and efficient raw materials for the development of Perovskite manganite materials for the magnetic refrigeration applications.

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Therefore, in this article, we synthesize and report the structural, magnetic and magnetocaloric properties of (REMIX)0.67Sr0.33MnO3 compound.

Experimental Polycrystalline (REMIX)0.67Sr0.33MnO3 compound was prepared through solid state reaction method by taking RE2O3 (Indian Rare Earth Ltd. Kerala, India), SrCO3 (Sigma-Aldrich, 98 %), MnCO3 (Sigma-Aldrich, 99.9+ %) as raw materials. The stoichiometric amounts of the same were mixed in an agate mortar for several hours until the mixture became homogenous. It was then calcined at 1000 °C for 12 hrs followed by crushing and recalcination at 1200 °C for 12 hrs. After multiple calcinations, the powder was pulverized and pelletized in the form of uniform and compact pellets, which were sintered at 1350 °C for 12 hrs in air. The crystal structure and phase purity of the powdered compound were analyzed using x-ray diffractometer (PANalytical X’Pert Pro Diffractometer having Ni filtered Cu Kα radiation, Netherlands). Rietveld refinement of the diffraction patterns was carried out using the FullProf software. Micro structural analysis was conducted using scanning electron microscope (JEOL-SEM 5601v, Tokyo, Japan). The room temperature thermal conductivity was measured using a thermal property analyzer (Flash line 2000, Anter Corporation, Pittsburgh, USA) using alumina as the reference material. Magnetic, electrical resistivity and specific heat measurements of the compound were made as a function of temperature and applied field using a physical property measurement system supplied by Quantum Design Inc., USA.

Results and discussion

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The Rietveld refined XRD patterns of (REMIX)0.67Sr0.33MnO3 compound is shown in Fig.1. The compound is crystallized into a mixed phase with a major phase having Perovskite structure of orthorhombic crystal symmetry with Pbnm space group (pattern no.89-8474), while minor phase has CeO2 of cubic symmetry with Fm3m space group (pattern no. 01-081-0792) [35]. The refinement results are shown in Table 1. However, the compound doesn’t show the presence of Mn3O4 phase in the XRD pattern, since the amount of impurity phase may be below the detection limit of the XRD instrument. The surface microstructure of well densified (REMIX)0.67Sr0.33MnO3 compound was studied and shown with two different scales in Fig. 2 (a) and (b). The micrographs show grains with two different types of morphology: one is bigger grains with size ranging from 3 m to 10 m (area 1) which are in polygonal shape and the other one is smaller grains of spherelike shape with size varying from 1 m to 3 m (area 2). There is a noticeable contrast difference between the bigger and smaller grains. The polygonal kind of grains are well packed and the grain boundaries are clearly visible. Further, the analysis of chemical composition using EDAX (shown in Fig. 2) on several areas, confirmed that the bigger grains correspond to mixed rare earth manganites and smaller grains correspond to CeO2. It is to be noted that Ce has higher thermal stability and smaller value of enthalpy of fusion compared to other RE ions, and thereby, rare earth manganite phase does not accept Ce, thus forming the secondary phase of CeO2 [35]. However, Ce being the most abundant cation in the raw material, some amount of Ce was substituted at the RE site of (REMIX)0.67Sr0.33MnO3 which is confirmed via EDAX. Table. 2 shows the EDAX analysis of the (REMIX)0.67Sr0.33MnO3 compound taken during the SEM analysis. From the table it can be seen that area 1 corresponds to rare earth manganite phase and area 2 corresponds to Ce rich phase which further corroborates with the XRD results.

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Fig. 3 shows the temperature variation of magnetization, M-T, carried out in zero-field cooled and field cooled mode from 2 K-380 K, under an external field of 50 Oe. The compound undergoes a sharp paramagnetic (PM) to ferromagnetic (FM) transition, TC, around 310 K and it remains ferromagnetic down to 42 K and then magnetization decreases. The TC of the compound was determined from the derivative of magnetization (dM/dT) which is around 310 K. Further, the temperature dependence of the inverse susceptibility of the compound has been plotted and it is shown in Fig. 3. From the figure, it is clear that the compound obey the Curie- Weiss law above TC which indicates positive value of paramagnetic Curie temperature θP. Thus the compound shows a decrease in disorder and low magnetic frustration. The drop in magnetization observed at lower temperatures indicate the existence of an antiferromagnetic (AFM) coupling between RE: 4f and Mn: 3d sub lattices in this compound [36]. The bifurcation in the M-T curve may be due to the magnetic frustration between the competing magnetic exchange interactions [37]. The irreversibility can also arise as a result of magnetic hysteresis contributed by the domain structure. However, variation of magnetization with the applied magnetic field is essential to confirm the existence of competing magnetic states. Thus, we have carried out the isothermal hysteresis loop at 2 K and is shown in Fig. 4(a). It shows a typical hysteresis with small coercivity of 266 Oe, indicates the compound having soft FM nature. Again, the magnetization values increase rapidly at lower magnetic fields but do not saturate even at applied magnetic fields as high as 90 kOe. This unusual behavior could be due to the presence of two different magnetic components, a FM component that gets easily saturated at lower fields and an AFM component that doesn’t saturate even at very high fields. The competing nature of these two components results in such a nonsaturating behavior. The presence of FM component can be understood due to the DE interaction

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between Mn3+-O- Mn4+, whereas AFM component is due to the superexchange interaction, and these results are in agreement with the earlier reports in conventional manganites [38]. In order to determine the magnetic phase transitions precisely, we have measured a series of isothermal magnetization curves (M-H) at different temperatures around TC with an interval of 4 K measured in a range of 0-90 kOe and the same is shown in Fig. 4(b). It is noticed that the M-H curves below TC, show a non-linear behavior with a sharp rise at lower fields and it is not saturated even at a field of 90 kOe, reflecting the FM nature of the compound. However, for temperatures above TC, the magnetization decreases drastically and the curves show almost linear behavior indicating the PM nature. We have performed Arrott plots of M2 vs. H/M in the vicinity of TC (shown in Fig. 4(c)) and according to Banerjee’s criterion [39], the positive slope in the Arrott plot indicates that the phase transition is of second order in nature. The variation of magnetization near the TC results in a significant effect in ΔSM [40]. The value of ΔSM is measured using the Maxwell relation as [41], 𝐻 𝜕𝑀(𝑇,𝐻) ) 𝑑𝐻 𝜕𝑇 𝐻

∆𝑆𝑀 (𝑇, 𝐻) = ∫0 (

(1)

We have plotted the temperature variation of -ΔSM under the magnetic fields of 10 kOe and 50 kOe is shown in Fig. 4(d). We obtained a -ΔSM value of 1.01 J kg-1 K-1 and 1.71 J kg-1 K-1 under 10 kOe and 20 kOe magnetic fields respectively. It can be seen that the -ΔSM reaches a maximum value of 3.28 J kg-1 K-1 at 310 K for 50 kOe field. The maximum -ΔSM obtained in the present case is compared with previous reports for several magnetocaloric materials and is given in Table. 3. From the table it is clear that (REMIX)0.67Sr0.33MnO3 possess larger value of -ΔSM at room temperature, in comparison with other manganites prepared using individual RE oxides, i.e, RE0.67Sr0.33MnO3 (RE = La, Pr, Nd) manganites reported so far. Also the relative cooling power (RCP) of the compound was determined using the width of half the maximum value of magnetic 11

entropy change [41], and it was found to be around 120 J kg-1. Further, we have carried out the temperature variation of specific heat in a temperature range of 200 K to 380 K under 10 kOe and 50 kOe magnetic field in order to measure the adiabatic temperature change (ΔTad), which can be calculated from the relation given below 𝐻 𝑇

∆𝑇𝑎𝑑 = −𝜇0 ∫0

𝐶𝑝

(

𝜕𝑀

) 𝑑𝐻

𝜕𝑇 𝐻

(2)

where CP is the specific heat capacity, which is independent of the external magnetic field variations. ΔTad for different temperature is estimated using an indirect method by relating temperature, specific heat and magnetic entropy change as

∆𝑇𝑎𝑑 =

𝑇 𝐶𝑝

∆𝑆𝑀

(3)

Specific heat as a function of temperature from 200 K to 380 K is measured and is depicted in Fig. 5 (a). From the figure, it can be seen that the specific heat capacity approached a highest value of 538 J kg-1 K-1 at 300 K for zero magnetic field. It is also observed that specific heat capacity decreases with increase in magnetic field. The adiabatic temperature change is calculated from the CP value, which is plotted as a function of temperature under 10 kOe and 50 kOe magnetic fields and shown in Fig. 5(b). Further, ΔTad increases with increase in temperature and shows a linear variation which reaches a value of 0.62 K under 10 kOe and 2.11 K under 50 kOe at 310 K as evident from Fig. 5 (b). The linear behavior of ΔTad clearly indicate that the compound is a good candidate for room temperature magnetic refrigeration applications. The basic magnetic refrigeration cycles are magnetic Carnot cycle, magnetic Stirling cycle, magnetic Ericsson cycle and magnetic Brayton cycle. Among these, magnetic Ericsson and magnetic Brayton cycles are applicable for room temperature magnetic refrigeration [42]. In

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(REMIX)0.67Sr0.33MnO3 compound, the magnetic entropy change is large enough, but adiabatic temperature change is not so large, of the order of few kelvin only, i.e., 0.62 K and 2.11 K under 10 kOe and 50 kOe magnetic fields respectively. Here the adiabatic temperature change is limited and hence a regenerative cycle is needed for the practical magnetic refrigeration operation. The best way to execute the regenerative cycle is through an active magnetic regeneration (AMR) process. The AMR cycle was first introduced by Barclay in order to overcome the limited temperature span of magnetocaloric materials [43]. In the AMR cycle, the magnetic material matrix works both as a refrigerating medium and as a heat regenerating medium, while the fluid flowing in the porous matrix works as a heat transfer medium. Regeneration can be accomplished by blowing a heat transfer fluid in a reciprocating fashion through the regenerator made of magnetocaloric material that is alternately magnetized and demagnetized. The regenerator composed of magnetic material utilizes the reversibility nature to achieve large temperature change. The AMR cycle is divided in to four steps. Magnetization, hot blowing, demagnetization and cold blowing. The magnetocaloric regenerator is periodically magnetized and demagnetized by moving in and out of the magnetic field. Thus temperature increases / decreases due to the magnetocaloric effect of the material. A heat transfer fluid is used to transfer heat from the “cold end” to the “hot end” of the regenerator, and this cycle is repeated until cyclic steady state is achieved. The Debye temperature of the compound is calculated using the formula [44, 45] = 12R5𝑇𝐷3

(4)

where R is the universal gas constant and is the number of atoms in formula unit. The parameter is obtained from the fitting of low temperature specific heat data using the equation

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reported in literature elsewhere [44, 45].We obtained a Debye temperature of 543 K and it is in good agreement with the literature for Perovskite manganites [46, 47]. The room temperature thermal conductivity was calculated using the equation [34, 48]  CP

(5)

where,  is the thermal diffusivity,  is the density and CP is specific heat capacity of the material. (REMIX)0.67Sr0.33MnO3 compound shows a thermal conductivity of 4.26 W m-1 K-1 at room temperature. The low value of thermal conductivity may be due to the disorder effect arising from the strong Jahn-Teller effect and the value lies in the range of manganites reported so far [49-51]. In order to understand the transport behavior of the compound, we have carried out the variation of electrical resistivity as a function of temperature, ρ (T), in the range of 2 K - 300 K with and without the application of magnetic fields and is shown in Fig. 6(a). ρ (T) measured under zero magnetic field increases gradually while lowering the temperature and then decreases by making a broad maxima around 180 K, indicating a metal to insulator transition. The resistivity is found to decrease upon the application of magnetic field (9T), and the metal to insulator transition is shifted to higher temperatures. It is interesting to notice that the absolute value of ρ (T) and the behavior of ρ (T) are similar to those RE0.67Sr0.33MnO3 (RE = La, Pr, Nd) manganites as reported in the literature [52-54]. Fig. 6 (b) shows the variation of MR of the compound as a function of applied magnetic field at different temperatures. The MR value was calculated using the formula [55] MR % = {[ρ(0) - ρ(H)]/ ρ(0)} x100 ,

(6)

where ρ(0) is the resistivity without magnetic field and ρ(H) is the resistivity with magnetic field.

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The MR was found to increase with the application of magnetic field. Here the magnetic spin scattering is suppressed due to the local ordering of magnetic spins with the application of magnetic fields. The compound shows 54 % of MR at 90 kOe at 30 K. The compound exhibits an improvement in -ΔSM and ΔTad values at the room temperature without affecting its electrical transport behavior, which is a challenging move towards the development of cost-effective room temperature magnetic refrigeration materials.

Conclusion In summary, we have synthesized an inexpensive (REMIX)0.67Sr0.33MnO3 compound using mixed rare earth oxides precursors derived from monazite sand using solid state technique. Rietveld refinement of X-ray powder diffraction patterns confirms the compound is a mixture of rare earth manganite which is the major phase and CeO2 as the secondary phase. The compound is found to be ferromagnetic at room temperature with a TC of 310 K, and the phase transition is of second order in nature. The compound exhibits a -ΔSM of 3.28 J kg-1 K-1 along with a RCP of 120 J kg-1 and ΔTad of 2.11 K at 310 K under 50 kOe field. The Debye temperature of the compound is found to be 543 K and room temperature thermal conductivity is 4.26 W m-1 K-1. The electrical resistivity value and its temperature dependence behavior is found to be similar to that of conventional RE0.67Sr0.33MnO3 (RE = La, Pr and Nd) manganites. The present compound shows larger values of -ΔSM and ΔTad at 310 K which makes it a potential candidate for room temperature magnetic refrigeration application.

Acknowledgments

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The authors would like to acknowledge the financial support received from Council of Scientific and Industrial Research (CSIR), Govt. Of India, sponsored project no. CSC0132. B. Arun and V. R. Akshay are thankful to Council of Scientific and Industrial Research (CSIR), Government of India for granting the Senior Research Fellowship and also thankful to Academy of Scientific and Innovative Research (AcSIR), CSIR. The authors would also like to thank Board of Research in Nuclear Sciences, sponsored project no. GAP 218939 for partially supporting this work.

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Figures Legend Fig. 1: Rietveld refined XRD patterns of (REMIX)0.67Sr0.33MnO3 compound at room temperature. Dotted circles (black) correspond to the XRD data and the lines (red) are theoretical fits to the observed XRD data. The difference pattern between the observed data and the theoretical fit is shown at the bottom (green). Fig. 2: SEM images of (REMIX)0.67Sr0.33MnO3 compound Fig. 3: Temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) dc magnetization of (REMIX)0.67Sr0.33MnO3 compound measured under the magnetic field of 50 Oe and their inverse susceptibility along with linear fit to Curie-Weiss law. Inset figure shows the derivative of magnetization (dM/dT) versus Temperature curve.

Fig. 4: (REMIX)0.67Sr0.33MnO3 compound: (a) Hysteresis loops, Inset: Closure picture of the hysteresis loop, (b) Isothermal field dependence of magnetization of the compound, (c) Arrott plots (M2 Vs H/M), (d) The magnetic entropy change at 10 kOe and 50 kOe magnetic fields.

Fig. 5: (a) Specific heat of the compound as a function of temperature, (b) Adiabatic temperature change of the compound for a field of 10 kOe and 50 kOe.

Fig. 6: (a) Temperature variation of electrical resistivity of (REMIX)0.67Sr0.33MnO3 compound, (b) Magnetoresistance vs. magnetic field at different temperature.

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Tables Table 1. (a) Rietveld refinement results obtained from the refinement of raw XRD data as shown in Fig. 1 details space group a (Å) b (Å) c (Å) V% RP RWP Rexp

rare earth phase Pbnm 5.4465(10) 5.4880(08) 7.6771(13) 88.59 9.36 11.7 7.62

CeO2 Fm-3m 5.4130(5) 5.4130(5) 5.4130(5) 11.41

Table 1 (b)Refinement parameters obtained for CeO2 phase

atom Ce O

site 4a 8c

x 0 0.25

y 0 0.25

z 0 0.25

occupation 1.0 1.0

Table 1 (c)Refinement parameters obtained for rare earth manganite phase atom site x y z occupation Mn 4a 0 0 0 1.0 Nd* 4c 0.4965 0.0010 0.25 0.24 La* 4c 0.4965 0.0010 0.25 0.30 Pr* 4c 0.4965 0.0010 0.25 0.11 Sr 4c 0.4965 0.0010 0.25 0.33 O1 4c 0.5270 0.5401 0.25 1.0 O2 8d 0.2475 0.2559 0.0102 1.0 * the occupations for rare earth atoms are adjusted accordingly from EDAX data. However, we do not find much difference in χ2 values for the refinement of XRD data with single rare earth occupation (χ2= 2.09) and with multiple rare earth occupations (χ2= 1.98) for a given site 4c.

23

Table 2. EDAX analysis of (REMIX)0.67Sr0.33MnO3 compound element O K Mn K Sr L La L Ce L Pr L Nd L Sm L Eu L Gd L O K Ce L

wt. % area 1 25.73 22.11 14.55 10.42 13.56 2.55 8.67 1.23 0.24 0.94 area 2 26.40 73.60

At. % 65.84 16.48 6.80 3.07 3.96 0.74 2.46 0.34 0.07 0.24 75.85 24.15

24

Table 3. Summary of magnetocaloric properties of (REMIX)0.67Sr0.33MnO3 and several magnetocaloric materials

TC

ΔH

ΔSM

RCP

Tad

Compound

(K)

(kOe)

(J kg-1 K-1)

(J kg-1)

(K)

Gd La0.67Sr0.33MnO3 La0.67Sr0.33MnO3 La0.6Bar0.33MnO3 La0.67Ca0.33MnO3 La0.87Sr0.13MnO3 La0.67Ca0.25K0.05MnO3 Nd0.67Sr0.33MnO3 La0.84Sr0.16MnO3 La0.67Ba0.33MnO3 La0.67Sr0.33MnO3 (REMIX)0.67Sr0.33MnO3 (REMIX)0.67Sr0.33MnO3

299 370 377 350 278 196.5 270 257.5 243.5 337 361 310 310

20 10 20 20 20 15 20 10 15 10 10 10 20

4.20 1.50 2.02 1.72 4.33 2.90 3.95 3.25 2.70 2.70 0.69 1.01 1.71

25

87.87 68 40 34 63

0.42 0.62

Reference 56 21 56 56 56 57 28 58 57 59 60 present work present work