Raman spectroscopic study of structural transformation in ordered double perovskites La2CoMnO6 bulk and epitaxial film

Solid State Communications 224 (2015) 10–14

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Raman spectroscopic study of structural transformation in ordered double perovskites La2CoMnO6 bulk and epitaxial film Dhirendra Kumar, V.G. Sathe n UGC-DAE Consortium for Scientific Research, D.A. University Campus, Khandwa Road, Indore 452001, India

art ic l e i nf o

a b s t r a c t

Article history: Received 5 May 2015 Received in revised form 1 September 2015 Accepted 27 September 2015 Accepted by D.D. Sarma

Raman spectra of polycrystalline bulk compound and epitaxial thin films of La2CoMnO6 grown on (001) oriented SrTiO3 substrates have been measured between 300 K and 823 K. The polarization dependence of the spectra collected on films provides clear evidence for ordered P21/n structure at room temperature. The high temperature spectra showed appearance of two new modes and simultaneous disappearance of couple of modes above 523 K for bulk and above 583 K in films which is close to previously reported P21/ n to R-3 structural transition temperature. The structural transition is seen to accompany softening and anomalous changes in peak widths of stretching and anti-stretching modes indicating significant changes in the tilts in oxygen octahedra around the cations. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Double perovskites A2BB0 O6 (where A¼rare earth elements and B, B0 ¼transition metal elements) have been extensively studied in the last decade due to its many useful properties near room temperature such as large magnetodielectric effect [1–3], and spin– phonon coupling, [4–10]. In double perovskites the cation ordering and antisite disorder at the B-B0 site causes a drastic difference in the physical properties such as magnetic ordering and crystal structure in comparison of single perovskites. Choudhury et al. have shown explicitly that colossal magnetodielectricity can be obtained by tuning the antisite disorder [3]. It is also shown that [11] the modification in the structure occurs due to the corner linked tilting of the B/B0 O6 octahedral units because of B-site cation ordering. Neutron and X-ray diffraction measurements [1,12–14,2] revealed that for a perfectly B-site ordered La2MnCo(Ni)O6,the structure is monoclinic (P21/n) at low temperatures whereas it is rhombohedral (R-3) at high temperatures. Bull et al. [12] reported first order structural phase transformation from P21/n to R-3 symmetry around 598 K and 623 K in bulk compounds of La2CoMnO6 and La2NiMnO6 respectively using neutron measurements. It is specifically pointed out that the B-site cationic order is retained during this phase transition. It is also observed that the two structures (P21/n and R-3) co-exist in La2MnCoO6 in a wide temperature range of  100 K near transition temperature while phase co-existence was absent in La2MnNiO6 [12]. On the contrary, other n

Corresponding author. Tel.: þ 91 731 2463913x232; fax: þ 91 731 2462294. E-mail address: [email protected] (V.G. Sathe).

http://dx.doi.org/10.1016/j.ssc.2015.09.014 0038-1098/& 2015 Elsevier Ltd. All rights reserved.

groups reported the structural phase coexistence of P21/n and R-3 phases at room temperature in La2MnNiO6 from their neutron diffraction studies [1] and attributed it to the presence of local inhomogeneities and antisite disorder at the B-site. This indicates that the antisite disorder and local inhomogeneities play a crucial role in deciding the structural properties in double perovskites. In our previous study, we have observed spin–phonon coupling and invariance of valance state around magnetic ordering temperature (TC) in bulk La2CoMnO6; however, high temperature studies on bulk and thin film compound was not attempted [10]. Iliev et al. [4] studied the thin films of La2CoMnO6 deposited on the SrTiO3 (001) single crystal substrate using Raman spectroscopy and failed to observe high temperature structural phase transition up to 800 K. Another study [5] on La2NiMnO6 thin films deposited on LaAlO3 (001) substrate reported structural coexistence of P21/n and R-3 phases at room temperature. They observed softening of phonons upto 400 K which was attributed to the presence of short range ferromagnetic order; however, they could not observe sharp structural transition in this system either. Kazan et al. also reported the presence of magnetic ordering in the La2NiMnO6 thin films at room temperature by ferromagnetic resonance measurement [15]. Softening of phonons due to spin–phonon coupling arising from short range magnetic order is not commonly observed. The spin–phonon coupling depends on the spin correlation function oSi.Sj 4 which exponentially decays above TC and thus spin– phonon coupling can have observable value only if there is significant spin correlations above TC. No clear reason is thus known for absence of the high temperature structural transition and softening of phonon mode well above TC in La2MnCo(Ni)O6 thin films. The plausible reasons are

D. Kumar, V.G. Sathe / Solid State Communications 224 (2015) 10–14

3. Results and discussions Fig. 1 shows the X-ray diffraction pattern of La2CoMnO6 (LCMO) thin film deposited on the (001) SrTiO3 substrate. The XRD pattern showed only one peak besides the peaks due to substrate. The position of this peak matched with the position of d040 of bulk monoclinic phase. This shows that the film is highly oriented along (0K0) direction. According to the XRD analysis there is 0.3% tensile strain in film when compared to bulk. The orientation, epitaxy and ordering of Bsite cations was further confirmed by using polarized Raman spectroscopy shown in Fig. 2. The polarized Raman data collected at room temperature (RT) in XX, X0 X0 , XY and X0 Y0 geometry (Fig. 2) follows the intensity rule for the fully B-site cation ordering [4,6], where X||[100], X0 ||[110], Y||[010] and Y0 ||[-110]. The zero field cooled (ZFC) and field cooled (FC) magnetization data as a function of temperature taken at 500 Oe applied magnetic field is shown in Fig. 3(a). It shows a clear

0.1

LCMO (040)

STO (002)

Intensity (Arb.)

1

Film Substrate

0.01

45

46

47 48 2θ (Degree)

49

Fig. 1. X-ray diffraction measurements carried out in θ–2θ scan of La2CoMnO6 thin film deposited on the SrTiO3 [001].

Intensity (Arb.)

The La2CoMnO6 films were grown on single crystal substrate of SrTiO3 (001) using pulse laser deposition technique (Excimer laser, KrF λ ¼248 nm). During deposition, the oxygen partial pressure and the substrate temperature were 600 mT and 700 °C respectively. The thickness of the film was measured using Stylus Profilometer (Model: XP-1, Ambios technology, USA) and found to be 170 nm. The target was prepared from a single phase fully ordered bulk compound synthesized using the conventional solid state reaction method [10]. The crystal structure of the bulk La2CoMnO6 was obtained by Rietveld refinement, it showed monoclinic, P21/n space group symmetry and lattice parameters, a¼ 5.534 Å, b¼7.784 Å, c¼5.495 Å and β ¼90.01°. The Raman measurements were carried out using LABRAM HR-800 single stage spectrometer equipped with a 488 nm excitation, an edge filter, 1800 g/mm grating monochromator and a CCD detector giving a spectral resolution of 1 cm  1. The high temperature Raman data was collected in the THMS 600 stage from Linkam, UK using 50  objective lens. The X-ray diffraction measurements (XRD) with Cu Kα radiation source (Bruker D8 advance) were carried out in the θ–2θ scan mode for characterization of the samples. The magnetization measurement was carried out using the VSM-SQUID magnetometer (SVSM; Quantum Design Inc., USA).

X' X

X'X' XX XY X'Y'

200 300 400 500 600 700 800 Raman Shift (cm-1) Fig. 2. Polarized Raman spectra of La2CoMnO6 thin film deposited on SrTiO3 at room temperature.

Temperature (K) 0

50

3 M(μB/F.U.)

2. Experimental details

Y

Y'

M(μB/F.U.)

strain and deposition conditions and hence detailed study of the structural transition at elevated temperatures in thin films is desired. Therefore, in this paper, we have studied the ordered La2CoMnO6 bulk compound and thin films grown on the (001) SrTiO3 substrate using high temperature Raman spectroscopy.

11

2

FC

100 150 200 250 300 La2CoMnO6/STO 500 Oe

1 0

ZFC

6 4 2 0 -2 -4 -6

5K

-6

-4 -2 0 2 4 Magnetic field (Tesla)

6

Fig. 3. (a) ZFC and FC M-T data of La2CoMnO6/SrTiO3 thin film measured in 500 Oe magnetic field. (b) M-H data of La2CoMnO6/SrTiO3 thin film measured at 5 K.

and sharp transition from paramagnetic to ferromagnetic phase below  220 K. It is now well established that the fully ordered phase shows only one sharp transition whereas the disorder induced due to antisite disorder gives an additional transition at a lower temperature [3,7]. The M–H measurement on thin film is carried out at 5 K that is shown in Fig. 3(b). The observed saturation magnetization is  5.71 mB/F.U. which is comparable to the reported values of the B-site well ordered La2CoMnO6 thin films [6,16]. Therefore, XRD, RT polarized Raman spectra and magnetization data confirm the quality, epitaxy as well full B-site cation ordering in the film. In order to investigate the high temperature structural transition from P21/n to R-3 observed by neutron scattering studies around 600 K [12] in this compound, the Raman spectra of the bulk and the film samples were collected at high temperatures. The unpolarized Raman spectra collected on the bulk sample from RT to 773 K is shown in Fig. 4. The Raman data of the bulk compound at RT matches well with literature for P21/n ordered phase and has been discussed in detail in our previous report [10]. In the inset of Fig. 4, an enlarged view of the low wavenumber data is presented. As the temperature is raised, the intensity of the modes diminishes and modes get broaden due to thermal effects. However, above 523 K three new peaks (shown by up-arrows in inset) start appearing and they gain strength as the temperature is further raised. In general,

D. Kumar, V.G. Sathe / Solid State Communications 224 (2015) 10–14

523 K 573 K

388

236

155

673 K

200 300 Raman Shift (cm-1)

200

300

400

473 K 300K 323K 373K 423K 473K 523K 543K 583K 623K 673K 723K 773K 823K

400

500

600

700

800

543 K 583 K 673 K

200 400 600 800

Raman Shift (cm-1)

-1

Raman Shift (cm )

Fig. 4. Temperature dependent Raman spectra of polycrystalline bulk La2CoMnO6 from 323 K to 773 K. Inset shows enlarge view of Raman spectra at low wave numbers, up-arrows depict appearance of new modes while down arrow depicts disappearance of Raman modes above transition temperature.

493 cm-1

496

773 K

200 300 400 -1

Raman Shift (cm )

Fig. 6. (a) Temperature dependent Raman spectra of La2CoMnO6 thin film deposited on the SrTiO3 from 300 K to 823 K and (b) shows enlarge view of Raman spectra at low wave numbers, up-arrow depict appearance of new modes while down arrow depicts disappearance of Raman modes above transition temperature.

498 LCMO BULK

420

373 K

623 K

100

300 K

388

423 K

260

323 K 473 K 523 K 573 K 623 K 673 K 773 K

Intensity (Arb.)

Intensity (Arb.)

Intensity (Arb.)

LCMO/STO

LCMO Bulk

323 K

100

function of temperature showed a change (in slope) below 600 K indicating softening in phonon mode. The peak width also showed subtle changes around this temperature. The changes in peak width reflect change in lattice correlation length establishing variation in lattice around 600 K. Therefore, these changes can be ascribed to the structural transition occurring in this system around 600 K. The two modes at 493 cm  1 and 645 cm  1 represent anti-stretching, and stretching of the oxygen octahedra around cations and therefore the changes in Raman mode position/width as a function of temperature and occurrence of new modes around 550 K indicates changes in octahedral tilts that bring the structural transition. The high temperature Raman spectroscopic studies carried out on the La2MnCoO6 films deposited on SrTiO3 from RT to 823 K is shown in

236

LCMO Bulk

420

260

with increasing temperature thermal disorder increases and appearance of new peaks are unlikely unless there is a structural rearrangement. The temperature (523 K) where new peaks start appearing is lower than the reported transition temperature [12] for structural transition from P21/n to R-3 ( 600 K). However, it is reported, particularly for La2CoMnO6 compound, that the transition is not sharp but show diffuse nature ( 100 K) [12]. Therefore, the appearance of new peaks can be attributed to the structural transition from P21/n phase to R-3 phase. The Raman modes were fitted using Lorentzian functions and peak position and band width were extracted at all the temperatures. The temperature evolution of peak position and width of the most prominent modes appearing at 493 cm  1 and 645 cm  1 are plotted in Fig. 5. The peak position as a

155

12

493 cm-1

88 84 80

494 72

492

Band width (cm-1)

Raman Shift (cm-1)

76

68 490

60 645 cm-1

648

645 cm-1

55 50

646

45 40

644

35 30

642 300

400

500 600 700 Temperature (K)

800

300

400

500 600 700 Temperature (K)

800

Fig. 5. Temperature evaluation of the peak position of (a) 493 cm  1 mode (b) 645 cm  1 mode and corresponding band width of (c) 493 cm  1 mode (d) 645 cm  1 mode in La2CoMnO6 bulk. Arrow indicates the structural transition temperature.

D. Kumar, V.G. Sathe / Solid State Communications 224 (2015) 10–14

13

85 LCMO/STO thin film

493 cm

498

75

497

70

496

Raman Shift (cm-1)

493 cm

80

65

495

60

494

55

493

50

Band Width (cm-1)

499

60 649

645 cm

648 647

50

646

45

645

40

644

35

643

645 cm

55

30

642 25 300

400

500

600

700

800

Temperature (K)

300

400

500

600

700

800

Temperature (K)

Fig. 7. Temperature evaluation of the peak position of (a) 493 cm  1 mode (b) 645 cm  1 mode and corresponding band width of (c) 493 cm  1 mode (d) 645 cm  1 mode in La2CoMnO6 thin film. Arrow indicates the structural transition temperature.

Fig. 6(a) while the enlarged view of the low wavenumber region is shown in Fig. 6(b). The Raman spectra of film has shown similar trend as bulk compound. Here again three new peaks appear at  155 cm  1, 236 cm  1and  388 cm  1 (shown by up-arrows) and peaks at 172 cm  1, 260 cm  1, and 420 cm  1 (shown by down-arrows) diminish above  583 K (Fig. 6(b)). The generation of new peaks and disappearance of few Raman lines corresponding to room temperature symmetry confirm structural transformation above 583 K. The trend matches with bulk compound with the exception that in bulk new peaks appeared above 523 K. Fig. 7 shows the temperature evolution of mode position and band widths of modes occurring at  493 cm  1 and at 645 cm  1. At first the peak position increases with decreasing temperature but shows a change below  583 K followed by mode softening. The band width also showed a distinct change in slope around this temperature for the 493 cm  1 mode while a subtle change for the 645 cm  1 mode. As discussed before, the changes in band width points toward the changes in the oxygen octahedra as these modes are related with the stretching and anti-stretching of the octahedral bonds. As mentioned in Section 1, earlier studies failed to detect the structural transition in films of La2MnCoO6 and La2MnNiO6 compounds and anomalous softening of stretching mode phonon was observed in La2MnNiO6 that is well above the magnetic transition temperature and was attributed to the possible retention of short range ferromagnetic order [5]. However, our studies now establish the softening as a manifestation of monoclinic P21/n to rhombohedral R-3 phase transition. The new modes observed at high temperature in the R-3 phase are likely to have Eg character. By considering LDC and polarization selection rules provided in Table II of Ref. [4] it can be inferred that in P21/n symmetry Ag and Bg characters are possible while for R-3 symmetry Ag and Eg characters are possible for the Raman modes. There are three peaks for Eg modes according to LDC calculations, at 185 cm  1, 232 cm  1 and 448 cm  1 and Figs. 4 and 6(b) show three new peaks at 155 cm  1, 236 cm  1 and 388 cm  1 at high temperatures representing R-3 symmetry. The Eg modes are allowed in both parallel and cross polarization geometry [4] and hence are expected to show strong appearance in unpolarized geometry.

The stretching mode appearing at 645 cm  1 do not change its character and remains Ag symmetry in both the phases while the 493 cm  1 mode is of Ag/Bg symmetry in P21/n phase while its character changed to Eg symmetry in R-3 phase. Due to this the 493 cm  1 mode showed prominent changes in band position and band width as a function of temperature when compared with the 645 cm  1 mode. It is known that the change in crystal symmetry is always catalyzed by the changes in the B/B0 O6 octahedral tilts in this system. According to the Glazer notation, the observed ordered structure P21/n and R-3 can be obtained from the 3 tilt operations a þ b  b  and a  a  a  respectively on the octahedral cubic aristotype structure Fm-3m (a°a°a°) [11]. Therefore, the transformation of symmetry from the monoclinic to rhombohedral is always expected to accompany a large change in the tilting angle and the bond length of B/B0 -O octahedra, as revealed in the temperature evolution of band position and band width of the Raman modes. It was reported that the two phases i.e. monoclinic and rhombohedral co-exist over a wide temperature range in this compound [12] and the phase purity and transformation depends largely on the amount of ordered phase and inhomogeneity present in the sample. In our present work we carried out studies on perfectly ordered samples. This may be a possible reason for detecting the structural phase transition in the present study.

4. Conclusions Through our observations and studies on ordered double perovskite La2CoMnO6 we confirmed the presence of B–B0 cations order in bulk as well as in the thin film using the polarized Raman spectroscopy technique. Furthermore, the appearance and disappearance of couples of Raman modes at the low wave number in temperature dependent Raman study confirmed the structural transformation from P21/n to R-3 phase around 550 K. The structural transition is marked by anomalous softening of the two most intense Raman modes 493 cm  1 and 645 cm  1.

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Acknowledgments The authors are grateful to Dr. R. J. Choudhary and Dr. Mukul Gupta for magnetization and X-ray diffraction measurements.

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