Direct visualization of cubic to tetragonal phase transition in La0.2Sr0.8MnO3−δ using transmission electron microscopy

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Direct visualization of cubic to tetragonal phase transition in La0.2Sr0.8MnO3
δ using transmission electron microscopy Aga Shahee, Niranjan Prasad Lalla n UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore-452001, India

art ic l e i nf o

Keywords: Manganite Coupled phase-transition Low temperature transmission electron microscopy Low temperature x-ray diffraction

a b s t r a c t Structural phase transition studies employing low-temperature transmission electron microscopy (LT-TEM) and low-temperature x-ray diffraction (LT-XRD) have been carried out on wide-band La0.2Sr0.8MnO3
δ (δ ¼ 0.01) manganite. These studies reveal clear evidence of cubic to tetragonal phase transition occurring below 260 K in La0.2Sr0.8MnO3
δ (δ ¼0.01). TEM observations depict that cubic to tetragonal phase-transition results in formation of nanotwinned microstructure of the tetragonal phase. Formation of nanotwins has been attributed to orbital-orientation phase-separation leading to minimization of strain. The magnetic and electron transport measurements reveal that the structural phase transition is coupled with a canted C-type antiferromagnetic and semi-conductor to insulating phase transition. & 2014 Published by Elsevier B.V.

1. Introduction Studies on strong electron-correlation effect in transition metal oxides have enriched the understanding of two technologically important materials: the high-Tc superconducting cuperates [1] and the colossal magnetoresistive [2–4] manganites. Colossal magnetoresistance (CMR) is the ability to suddenly boost resistance to electrical conductivity by orders of magnitude when a magnetic field is applied. Manganites are also promising candidates for spintronic devices [5] that can manipulate both the parameters of electrons i.e. their spin as well as charge for efficient storage and transmission of information. It is well established that these physical properties of manganites severely depend on its crystal structures [6–8]. In this regard study on structural phasetransition of wide-band manganite is particularly important to understand the behavior of associated charge and orbital ordering. Most of the previous experimental and theoretical studies in wide one-electron bandwidth LaxSr1
xMnO3 manganite have been focused on its hole-doped regime (xo 0.5) [9–15]. It was due to the observation of strong CMR phenomena for compositions around x E0.3. Recent electron doped region of LaxSr1
xMnO3, and CeySr1
yMnO3 (x 40.5 and y40.25) has received significant attention, as the well-known coupled first-order structural, spinstate phase transitions [16,17] have been correlated to the chargeorbital disorder to charge-orbital order phase transition [18] as

n

Corresponding author. Tel.: þ 91 731 2463913; fax: þ91 731 2462294. E-mail address: [email protected] (N.P. Lalla).

well as a metallic G-type antiferromagnetic phase has been reported just by 1–2% Ce doping in cubic SrMnO3 [19]. A general phase-diagram [20,21] of manganite indicates that La1
xSrxMnO3 manganite has the widest one-electron band-width (W) [22]. La1
xSrxMnO3 for x 40.5 being cubic or tetragonal with Mn–O–Mn bond angle equal to 1801, and the hopping of eg-electrons will be the maximum due to strong Mn (3d) and O (2p) hybridization. Hence the electron band-width will be the widest. Therefore, unlike narrow band manganites like La1
xCaxMnO3 and Pr1
xCaxMnO3, where hybridization between Mn (3d) and O (2p) is weak and charge ordering (CO) is mostly reported [23–28], no CO is expected for La1
xSrxMnO3. Structural, resistivity, and magnetization results for La1
xSrxMnO3 (0.4ox o0.85) have been reported by Hemberger et al. [16]. They report a paramagnetic and metallic cubic phase at room temperature for La0.2Sr0.8MnO3, which transforms to C-type AFM and insulating tetragonal phase with c/a 4 1 at around 260 K. These results were further supported by the results of Chmaissem et al. [17]. These results are also in accordance with those of Goodenough [6] and Fang et al. [29] who modelled the magnetic structure of over doped systems, which describes the stability of a magnetic structure in terms of c/a ratio. Indeed, the theory given by Fang et al. [29] and Goodenough [6,7] completely described the observed C-type antiferromagnetic tetragonal phase with c/a ¼1.02 for x ¼0.8 as reported by Hemberger et al. [16] and Chmaissem et al. [17]. The experimental results of Konishi et al. [30] have shown that the c/a ratios and the magnetic state of La1
xSrxMnO3 thin film can be controlled by using substrates of different lattice mismatches and thus have presented strong

http://dx.doi.org/10.1016/j.physb.2014.02.055 0921-4526 & 2014 Published by Elsevier B.V.

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2. Experiments Polycrystalline La0.2Sr0.8MnO3
δ was prepared using a conventional solid-state reaction route using 99.99% pure La2O3, SrCO3 and MnO2. The stoichiometric mixture of the ingredients was thoroughly mixed using mortar pastel for effectively about 24 h and then calcined in air at 1100 1C for 24 h. The calcined powder was reground and fired at 1400 1C for 24 h. The fired powder was reground and pelletized in the form of 14 mm  1 mm disks and then finally sintered at 1400 1C for 48 h in air. After sintering the furnace was cooled slowly with a rate of 2 1C/min. An earlier study [32] has shown that SrMnO3 based manganites lose oxygen above 1000 oC and become oxygen deficient at around 1400 1C. Oxygen off-stoichiometry as large as δ ¼0.11 is reported for SrMnO3
δ [32]. Therefore to achieve oxygen stoichiometry slow cooling was performed during the current synthesis process. The phase-purity, compositional homogeneity and oxygen-stoichiometry characterizations of polycrystalline La0.2Sr0.8MnO3
δ were done at roomtemperature (RT) using the powder x-ray diffraction (XRD), EDAX and idiometric titration respectively. The XRD was carried out on a Rigaku made diffractometer (D-max) configured in symmetric Bragg–Brentano geometry with angular resolution (Δθ) of 0.0471 (8.33  10
4 rad) and equipped with a graphite (0 0 2) monochromator. For temperature dependent XRD a LN2 based cryostat was used. It was mounted on a rotating anode source operating at 11 kW and producing Cu Kα x-rays. The room temperature and low temperature XRD data was Rietveld refined using space-group Pm-3m, and I4/mcm respectively; see Fig. 1. All room temperature XRD data peaks were found to be accounted by Rietveld refinement with space-group Pm-3m. The absence of any unaccounted peak confirms that the samples are pure cubic phase. Compositional homogeneity analysis was carried out using the EDAX. For this purpose the EDAX was carried out at various close by (  50 nm) points in a single grain using a  20 nm probe. The variation in the ratio of the elements La, Sr and Mn at different points of the grain was found to match within 71% to the expected ration of 10%, 40% and 50% respectively, which is well within the typical error limit of the EDAX technique. Idiometric titration confirmed the nearly oxygen stoichiometric δ E0.01 composition. The samples were then subjected to lowtemperature (LT) structural phase-transition studies, employing the LT-XRD (down to 80 K) and transmission electron microscopy (LT-TEM) (down to 100 K), and electrical-transport, magnetization and calorimetric measurements down to  40 K. For TEM studies thin samples were prepared using the conventional method of ion-beam polishing [33]. For the LT-TEM studies a Gatan made liquid-nitrogen based sample holder 636MA was used. For resistance-vs-temperature (R–T) measurements the Keithley made

25000

Intensity (counts)

evidence towards structural and magnetic correlation. But defying all these experimental results and theories Bindu et al. [18,31] have shown the occurrence of intermediate charge-ordering in La0.2Sr0.8MnO3
δ manganite and low temperature tetragonal phase with c/a o 1. They have shown that a phase transition from cubic to CO phase occurs at around 265 K and then this CO phase slowly transforms to C-type antiferromagnetic (AFM) tetragonal phase; they compete with each other at low temperature. At 100 K the major phase (  90%) transforms to C-type AFM tetragonal phase and only a small fraction of CO phase (  10%) coexists with it. Keeping in view the above contradictory results on this material, we have very carefully prepared La0.2Sr0.8MnO3
δ samples with proper stoichiometry and have presented here the results of our investigations comprising low-temperature structural phase-transition, electrical-transport, calorimetric and magnetization studies.

Iobs

XRD at 320K space group =Pm-3m a=3.8262(1)Å.

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0

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30

40

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70

80

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2θ Fig. 1. Rietveld refined XRD patterns of La0.2Sr0.8MnO2.99 sample at 300 K and 80 K respectively. The 300 K and 80 K data were fitted with Pm-3m and I4/mcm space groups respectively.

nano-voltmeter model-182, constant-current source 2400 and Lakeshore made temperature controller model (DRC-93CA) were used. Zero-field cool (ZFC) and field-cool (FC) magnetizationvs-temperature (M–T) at 100 Oe and 50,000 Oe fields and magnetization-vs-field (M–H) measurements up to 70,000 Oe were done using the SQUID-VSM (Quantum Design).

3. Results The resistivity-vs-temperature (R–T) data of La0.2Sr0.8MnO2.99 sample is shown in Fig. 2. A step like feature in R–T at temperature (Ts) of 260 K followed by a monotonous increase can be seen. The step feature shows a narrow but clear hysteresis (as shown in the inset) indicating that it corresponds to a first order phasetransition. It will be shown in the following that this corresponds to a cubic to tetragonal phase transition. The data of LT-TEM observations are shown in Figs. 3 and 4. Fig. 3a and b shows selected area diffraction (SAD) patterns taken along [1 0 0] zone at 300 K (i.e. above Ts) and at 100 K (i.e. below Ts) respectively. Fig. 3c and d shows electron micrographs taken at room-temperature (300 K) and 100 K respectively. It can be seen that the smooth and contrast less microstructure at RT changes the nearly periodically arranged nano-band (10–20 nm) contrast at 100 K. The SAD in Fig. 3a corresponds to the cubic-phase, whereas the SAD in Fig. 3b is analyzed to be due to nanotwins of tetragonal phase appearing due to a cubic to tetragonal transformation. It corresponds to a composite pattern arising from two groups of tetragonal variants, which occur alternately having mirror relation parallel to {0 1 1} planes. Due to tetragonality (c/a41) the cubic reflections appear to split in the composite SAD pattern along [1 0 0] zone. Here we would like to emphasize that unlike the report by Bindu et al. [18], we could not observe any intermediate CO phase in the SAD pattern along the expected zone during lowtemperature TEM explorations at different temperatures. To further confirm the absence of CO phase we also applied the technique of convergent beam electron diffraction (CBED). We could not find any

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‘higher order Laue zone’ (HOLZ) ring smaller radius than (1 0 0) HOLZ, which irrespective of the zone direction should have been present if CO modulation would have been present along any crystallographic direction. The microstructural changes due to temperature dependent evolution of tetragonal twin phase out of cubic phase are shown in Fig. 4. It clearly shows that the twine structure, which corresponds to a tetragonal phase, starts evolving at around 260 K and completes transformation of whole grain at around  190 K. This indicates that the cubic to tetragonal transformation starts at 260 K and finishes at  190 K, thus showing distinct phase coexistences of cubic and tetragonal phases at nano-scale level between 260 K and 190 K. The occurrence of the nano-scale

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200

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Temperature (K) Fig. 2. (a) Semi-log plots of resistivity-vs-temperature data of La0.2Sr0.8MnO2.99 sample. The step-like feature in ρ–T corresponds to cubic to tetragonal and C-type AFM phase-transitions.

3

phase coexistence region indicates that the phase transition is of first-order nature. Details of nanotwin formation will be discussed in terms of orbital-ordering and its orientation phase-separation. The bulk occurrence of cubic to tetragonal phase transformation can be clearly seen in the XRD data in Fig. 5. It shows emergence of (0 0 4)/(2 2 0) tetragonal splitting below Ts ¼260 K during cooling. Fig. 6 shows the temperature dependence of the lattice parameters of the cubic and tetragonal phases. The lattice parameters were obtained through Rietveld refinement [34] of the XRD data taken at various temperatures. Region between dashed lines represents the region of phase-coexistence of Pm-3m and P4/mmm phases. The tetragonal phase with c/a ¼1.02 indicates the occurrence of ferro-ordering of 3d2-r 3z orbitals, which resulted due to cooperative JT-distortion. The observed broadening of (0 0 4) peak is a direct evidence of bulk occurrence of (0 1 1) mirror related nanotwins of the tetragonal phase below Ts. No evidence of any superlattice peaks in low temperature XRD scan in a wide 2θ range (10–1201) with a rate of 0.21/min could be seen. This confirms charge disorder nature of La0.2Sr0.8MnO2.99, tetragonal phase with c/a ¼1.02. The data of magnetic measurements are summarized in Fig. 7. Bifurcation of the zero-field cool (ZFC) and field-cool (FC) M–T data taken under 100 Oe field and a small kink was found around 260 K as shown in Fig. 7a. This indicates the presence of both ferromagnetic and antiferromagnetic interaction in the material. Details of the occurrences of weak ferromagnetism below antiferromagnetic transition temperature (TN E 260 K) and its origin will be discussed in terms of frustrated bonds. The M–H data taken at 150 K further supports occurrences of antiferromagnetic state with weakly ferromagnetism see Fig. 7b. The sharp drop in magnetization at  260 K, corresponding to 50,000 Oe M–T data as shown in Fig. 7a, is identical to that in Ref. [17] and hence approves the good quality of our sample La0.2Sr0.8MnO3
δ and therefore it also approves that the cubic to C-type AFM tetragonal

Fig. 3. (a) and (b) SAD patterns from the same region taken (a) at 300 K and (b) at 100 K corresponding to cubic and tetragonal states respectively. Electron micrographs of the same area taken (c) at 300 K and (d) at 100 K. (d) Nanotwinned (10–20 nm) microstructure of the tetragonal phase appearing after cubic to tetragonal transformation below 260 K indicates (0 1 1) mirror related twins.

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4

Fig. 4. Electron micrographs showing the temperature dependent growth of nanotwinned tetragonal phase appearing out of cubic-tetragonal phase transition.

phase-transition being reported here is the true behavior of a stoichiometric La0.2Sr0.8MnO3
δ.

4. Discussion La0.2Sr0.8MnO3 is a typical transition metal oxide displaying strongly correlated electron states. Upon cooling, this undergoes cubic to tetragonal phase transition at Ts E257 K giving rise to splitting of degenerate (2 0 0) reflection into (0 0 4) and (2 2 0) with 1:2 intensity ratio, as shown in Fig. 5, with tetragonality (c/a)¼ 1.02. It is supported by diffraction and microstructural results of transmission electron microscopy; see Figs. 3 and 4. This phase-transition is associated with long range C-type antiferromagnetic spin ordering, as shown in Fig. 7 and supported by neutron scattering experiments of Chmaissem et al. [17], and semiconducting to insulating phase transition with abrupt increase in resistivity, as shown in Fig. 2. These coupled phase transition for a wide composition range was reported by Hemberger et al. [16] and Chmaissem et al. [17]. c/a ratio strongly depend on the orbital structure, i.e., whether the orbitals are aligned as 3dx2
y2 or as 3d3z2
r2. They report different

magnetic and orbital occupied states on the basis of c/a ratio. For La0.2Sr0.8MnO3 they report C-type antiferromagnetic and 3d3z2
r2 occupied tetragonal phase at low temperatures as shown in Fig. 8, on the basis of its c/a ratio which is around 1.021. No signature of CO in neutron/x-ray diffraction measurements was observed for La0.2Sr0.8MnO3. Thus for x¼0.8, purely C-type AFM and insulating state evolves with a tetragonally elongated structure without any CO. But defying all these experimental results and theoretical prediction of Goodenough [6] and Fang et al. [29] Bindu et al. [18,31] have shown the occurrence of intermediate charge-ordering state in La0.2Sr0.8MnO3 manganite and the low temperature tetragonal phase with c/ao1 i.e. a tetragonally compressed and 3dx2
y2 orbital occupied state. The absence (occurrence) of CO by Hemberger et al. [16] and Chmaissem et al. [17] (Bindu et al., [18]) may be due to strong scattering power of electron as compared to x-ray or neutron. But our detail temperature dependent diffraction and microstructural analysis support the above discussed theoretical predictions and neutron/x-ray diffraction results and completely discord the presence of CO in La0.2Sr0.8MnO3. Our temperature dependent x-ray diffraction result also support the tetragonally elongated structure with c/a¼1.021 i.e. ferro-orbital ordered state of 3d3z2
r2 orbital as shown in Fig. 8.

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2θ Fig. 5. Temperature dependent XRD profiles of peaks around 47.51, i.e. (2 0 0) of cubic and (220/004) of tetragonal phases, of La0.2Sr0.8MnO2.99. The occurrence of splitting of (2 0 0) reflection of cubic (Pm-3m) into (0 0 4) and (2 2 0) reflections of tetragonal (I4/mcm) phase at 260 K during cooling indicates structural transformation with Ts E260 K.

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Temperature (K) Fig. 6. Temperature variation of lattice-parameters determined from Rietveld refinement of the low temperature XRD data taken from 20–1201 2θ. The middle portion around 240 K represents phase coexistence region of cubic and tetragonal phases.

According to the Goodenough model [6,7] for manganites þ 2þ 3.75 þ 2
A31
O3 where A3 þ ¼La3 þ , Pr3 þ etc. and B2 þ ¼ Ca2 þ , xBx Mn þ 2þ Sr2 þ , Ba2 þ etc., A30.25 B0.75Mn3.75 þ O2
compounds are C-type 3 antiferromagnets where the coupling is ferromagnetic along one-dimensional chain which are ordered antiferromagnetically in two dimensions as shown in Fig. 8. This C-type spin state will be adopted by a JT-distorted tetragonal perovskite structure with c/a41. Goodenough also predicted the same C-type antiferromagnetic phase for x¼0.8 simply based on its lattice distortion (i.e. c/a41) which was later supported by neutron diffraction result of Wollan [35] on La0.2Ca0.8MnO3. Radaelli et al. [24,26] did detailed x-ray and neutron measurements of the structural distortions and

antiferromagnetic structures for La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3. Radaelli et al. [24,26] used the Wigner crystal model and the Goodenough–Kanamori rules [7] to predict structure of La0.33Ca0.67MnO3 by refining the antiferromagnetic structure, which was not investigated by Goodenough. The rules lead to inherent contradictions for La0.33Ca0.67MnO3. These contradictions necessitate the introduction of ‘frustrated bonds’ where a ferromagnetic coupling is introduced that does not agree with the Goodenough–Kanamori rules [7]. Radaelli et al.'s experimental results and its final analysis thus produced the canted structure [24] for La0.33Ca0.67MnO3, which is taken to be the way the structure minimizes the frustration and predicted similar frustrated bonds for composition other than x¼0.5, 0.75 and 1.0. Our temperature dependent 100 Oe ZFC and FC magnetization results clearly show bifurcation between TN ¼260 K and a weak magnetic momentum, which indicates the presence of weak ferromagnetic state. This was further supported by magnetization-vs-field measurements. On the basis of composition of La0.2Sr0.8MnO2.99, which is quite away from La0.25Sr0.75MnO3 (i.e. perfect C-type antiferromagnet), the presence of ‘frustrated bonds’ will favor ferromagnetic interaction and on the basis of Radaelli et al. [24] a cantered magnetic state will be energetically favorable. Thus weak ferromagnetism in our case is explained on the basis of a canted C-type antiferromagnetic state. The above R–T, TEM, XRD, M–T and M–H data collected on well characterized sample clearly and independently approve that La0.2Sr0.8MnO2.99 undergoes a first-order cubic to tetragonal phase-transition, with c/a ¼1.021 without any evidence of intermediate COO phase. The measured tetragonality of c/a 41 is due to ferro-ordering of the 3d23z
r orbitals resulting into coherent JT-distortion [36]. This is also accompanied by a C-type antiferromagnetic ordering [37]. This is in contradiction with the results reported by Bindu et al.[18,31]. The occurrence of CO phase as an intermediate phase during cubic to tetragonal transformation in La0.2Sr0.8MnO3 sample, and a low temperature tetragonal phase with c/a o1 as reported by Bindu et al. [18,31] may be a case of

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5. Conclusions Single phase nearly stoichiometric polycrystalline perovskite sample of La0.2Sr0.8MnO3
δ (δ ¼0.01) was synthesized and studied. The structural, micro-structural, electron transport and magnetic properties as a function of temperature were investigated. Based on the above results and discussion it is concluded that wide band La0.2Sr0.8MnO2.99 manganite shows a direct cubic to JT-distorted tetragonal phase transition with c/a ¼ 1.021 at around 260 K. It is interesting to note that the structural phase transition is coupled with magnetic and electronic phase transitions. The structural phase transition from cubic to tetragonal, with elongated octahedron indicated ferro-type of orbital ordering of 3d3z2
r2 orbitals as shown in Fig. 8, which has been theoretical predicted by Goodenough [6,7] and Fang et al. [29] and experimentally supported by Hemberger et al. [16] and Chmaissem et al. [17]. It is against Bindu et al. [31] report of occurrence of compressed octahedron and orbital ordering of 3dx2
y2 type orbitals. Also unlike the report of Bindu et al. [18] we did not observe any intermediate charge-ordered phase. Based on the type of lattice distortion and magnetization behavior a canted C-type AFM ordering with ferro-orbital-ordering of 3d23z
r orbital is concluded.

-2 Acknowledgment

-4 -70000

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0

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H (Oe) Fig. 7. (a) 50,000 Oe field-cool (FC) M–T and 100 Oe Zero-field cool (ZFC) and FC M–T data of La0.2Sr0.8MnO2.99 sample. The sharp drop at  260 K corresponds to a canted C-type AFM phase transition. (b) M–H taken at150 K, the unsaturated trend of M–H even up to 70,000 Oe of applied field indicates an antiferromagnetic nature of sample at 150 K.

Fig. 8. The schematic diagram showing ferro-type orbital and C-type spin ordering in a C-type antiferromagnetic tetragonal phase.

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