Journal of Crystal Growth 362 (2013) 24–28

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Journal of Crystal Growth

0022-02

http://d

n Corr

E-m1 Pr

Institut

journal homepage: www.elsevier.com/locate/jcrysgro

Influence of growth ambience and doping on the structural properties ofmultiferroic DyMnO3

Harikrishnan S. Nair 1, Suja Elizabeth n

Department of Physics, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Available online 17 July 2012

Keywords:

A1. X-ray diffraction

A2. Floating zone technique

B1. Manganites

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.05.043

esponding author. Tel.: þ91 80 2293 3461; f

ail address: liz@physics.iisc.ernet.in (S. Elizab

esent address: Julich Centre for Neutron

e 4, Forschungszentrum Julich GmbH, 52425

a b s t r a c t

We report on the single crystal growth of 50% Sr and Y doped multiferroic DyMnO3 using optical

floating zone technique. A comparison of the effect of growth ambience and of chemical substitution on

the crystal structure of DyMnO3 is attempted. It is observed that DyMnO3 adopts Pm3m cubic structure

with 50% Sr doping whereas with 50% Y doping, the crystal structure is hexagonal P63cm. Orthorhombic

Pnma structure is adopted by DyMnO3 when grown in air, whereas hexagonal P63cm structure is

obtained when grown under the ambience of argon. The structural polymorphism is discussed in terms

of difference in ionic sizes of Sr, Y and Dy, comparable Gibbs free energies and coordination schemes of

surrounding oxygens for hexagonal and orthorhombic structures of DyMnO3.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

In normal conditions, DyMnO3 crystallizes in orthorhombicPnma structure and has attracted scientific interest alongwith related materials like TbMnO3, owing to incommensurate magnetic structure that leads to the development of electricpolarization [1,2]. Orthorhombic DyMnO3 shows three magnetictransitions—sinusoidal spin ordering of Mn3þ below TMn

N � 39 K,lock-in transition at TMn

lock � 18 K and Dy order below 10 K [2].Spontaneous polarization parallel to the c-axis is observed todevelop below TMn

lock which can be flopped to the a-axis with theapplication of a magnetic field [2]. Studies on R MnO3 multi-ferroics (where R¼Tb, Dy, Gd) have initiated the search for newcompounds through chemical manipulation of the parent com-pound [2,3]. In this regard, we must mention that a notablefeature of DyMnO3 is the existence of a crystallographic variant inthe hexagonal structure, P63cm, which is also a multiferroic. Thehexagonal structure of DyMnO3 is stabilized when the crystalgrowth is performed in an argon atmosphere instead of air or byusing a seed crystal of hexagonal YMnO3 [4,5]. The polymorphismof DyMnO3 is due to the comparable values of the Gibbs freeenergy of the orthorhombic and hexagonal structures. Hence, it ismotivating to perform doping experiments on the parent DyMnO3

compound in order to study the structural and magnetic phasetransitions in the newly doped materials.

ll rights reserved.

ax: þ91 80 2360 2602.

eth).

Sciences 2/Peter Grunberg

Julich, Germany.

In the present study, we chose to dope DyMnO3 at the rareearth site with 50% Sr2þ

2Dy0:5Sr0:5MnO3 (DSMO50) in one caseand 50% Y3þ

2Dy0:5Y0:5MnO3 (DYMO50) in another. We per-formed growth experiments using the two chosen compositionsin air as well as in an argon ambience in order to investigate thecrystal structure transformations. Note that in both the composi-tions, the dopants are non-magnetic but in DSMO50, Sr2þ sub-stitution leads to the formation of mixed valence of Mn inMn3þ =Mn4þ states and there by electronic phase separation. Inthe case of DYMO50, Y3þ leads to no valence related changesin Mn, but we observe that the orthorhombic structure of theparent DyMnO3 is transformed to hexagonal.

2. Crystal growth

The crystal growth experiments reported in this paper wereperformed in a four-mirror optical floating zone furnace FZ-T-10000-H-VI-VP procured from Crystal Systems Inc., Japan. Thefurnace is equipped with four halogen lamps with a total outputpower of 1.5 kW�4¼6 kW. In order to perform single crystalgrowth by optical floating zone furnace, polycrystalline ingots ofthe desired composition are prepared first. Dy0:5Sr0:5MnO3 andDy0:5Y0:5MnO3 were prepared via conventional solid state synth-esis route using Dy2O3, SrCO3, Y2O3 and MnO2 all of which wereof 3N purity or above, procured from Sigma Aldrich. The rareearth oxides were fired at 800 1C in air for 12 h prior to reacting.Further, the precursor chemicals were weighed in stoichiometricratios and intimately mixed and ground in a mortar forseveral hours prior to heat treating at 1250 1C for 36 h, groundagain and further heat treated at 1350 1C for 20 h. The procedure

H.S. Nair, S. Elizabeth / Journal of Crystal Growth 362 (2013) 24–28 25

of grinding–and–firing was repeated several times to obtainthe desired compositions. The phase formation of the compoundswas checked by obtaining powder x-ray diffraction (pxd)patterns using a Philips X’pert powder diffractometer working inBragg-Brentano geometry employing Cu Ka radiation ðl¼ 1:54 AÞ.Rietveld refinement [6] using a FULLPROF code [7] was performedon the pxd data in order to obtain the structural parameters. Afterconfirming the phase purity, ingots for float zone growth wereprepared. A hydrostatic press which can apply up to 70 MPa wasused for this purpose. The ingots were then sintered at 1350 1C for12 h. Crystal growth was performed using these ingots. Duringthe initial trials, polycrystalline ingots were used as the seed rod,while in the later growth runs, single crystal from the previousexperiments were used. The growth rate was optimized for boththe compositions after evaluating the quality of crystals grownunder different rates.

2.1. Dy0:5Sr0:5MnO3

For the growth of Dy0:5Sr0:5MnO3, seed and feed ingots wererotated at 35–45 rpm in the counter-clockwise direction. DSMO50was grown at different growth rates in the range of 4–10 mm/h. Itwas observed that crack-free DSMO50 crystals could be grown inthis range of growth rates. However, the best quality crystalswere obtained for a specific growth rate of 4–5 mm/h, as con-firmed by Laue photographs.

2.2. Dy0:5Y0:5MnO3

Interestingly, the Dy0:5Y0:5MnO3 crystals were found to becracked when grown at slower growth rates ð � 223 mm=hÞ butthe cracked parts were shiny and smooth resembling a cleavedsurface. At higher growth speed of 6 mm/h, crack-free crystals

Table 1The optimized growth-parameters for the crystal growth of

method. Also presented are the values for the polymorphs

Parameter Dy0:5Sr0:5MnO3

Seed/feed rotation rate 35

Growth rate (mm/h) 4

Ambience Air, argon, nitrogen

Pressure (atm) 1

Color appearance Black

Appearance Opaque

Fig. 1. (a) A photograph of a single crystal of hexagonal Dy0:5Y0:5MnO3 grown using opt

crystal of hexagonal Dy0:5Y0:5MnO3.

were obtained. At still higher growth rates ð46 mm=hÞ, thegrowth was not possible due to an unstable molten zone. Initially,the growth of Dy0:5Sr0:5MnO3 and Dy0:5Y0:5MnO3 was performedin air. These resulted in crystals that are cubic and hexagonal instructure, respectively. As the next step, air was replaced withargon and later with nitrogen. We observed that the choice of Aror N2 has no effect on the crystal structure of Dy0:5Sr0:5MnO3 orDy0:5Y0:5MnO3. Irrespective of the ambient gas used, they crystal-lized in cubic and hexagonal symmetries, respectively.

2.3. DyMnO3

In the case of parent compound orthorhombic DyMnO3, asignificant phase transformation to hexagonal structure occurredwhen the growth was carried out in argon [4]. DyMnO3 crystals oftwo different crystal modifications were prepared by using differentgaseous environments for crystal growth, viz., argon and air. In thepresence of Ar, the hexagonal (h) structure of DyMnO3 was stabilized,whereas with air, the perovskite orthorhombic (o) DyMnO3 persisted.Both DyMnO3(h) and DyMnO3(o) crystals could be grown withoutcracks using the optimized growth parameters. Both polymorphswere black and opaque. The optimized growth parameters ofDyMnO3 in two crystal modifications as well as in the case ofY- and Sr-doped compounds are summarized in Table 1.

3. Results and discussion

A photograph of the single crystal Dy0:5Y0:5MnO3 is shown inFig. 1(a). The quality of the grown crystals was ascertained fromLaue photographs. Sharp and clear spots were obtained, whichreveal the symmetry of the crystal. The Laue photograph ofDy0:5Y0:5MnO3 taken along the crystallographic c-axis is presented

Dy0:5Sr0:5MnO3 and Dy0:5Y0:5MnO3 by the floating zone

of DyMnO3.

Dy0:5Y0:5MnO3 DyMnO3(o) DyMnO3(h)

42 30 35

6 5 4–7

Air Air, oxygen Argon

1 1–3 1

Light blue Black Black

Shiny Opaque Opaque

ical floating zone technique and (b) The Laue photograph obtained from the single

Fig. 2. The observed x-ray diffraction patterns along with calculated, difference

profiles and Bragg peaks for (a) Dy0:9Sr0:1MnO3 and (b) Dy0:5Sr0:5MnO3.

Fig. 3. The observed x-ray diffraction patterns along with calculated, difference

profiles and Bragg peaks for (a) Dy0:95Y0:05MnO3 and (b) Dy0:5Y0:5MnO3.

Table 2The structural details and refined lattice parameters of Dy0:5Sr0:5M

Dy0:5Sr0:5MnO3 Dy

Space group Pm3m P63

Space group number 221 185

Lattice parameters a¼3.825 A a¼

c¼

Unit cell volume, V x,y,z 55.95 A3 376

Dy/Sr (0,0,0) Dy

Mn ð12 , 12 , 1

2Þ Dy

O ð0, 12 , 1

2ÞMn

O1

O3

O4

Goodness-of-fit, w2 2.85 3.2

H.S. Nair, S. Elizabeth / Journal of Crystal Growth 362 (2013) 24–2826

in Fig. 1(b). The crystal structures of both Dy0:5Sr0:5MnO3 andDy0:5Y0:5MnO3 were refined from the powder x-ray pattern using aRietveld method. For the sake of completeness, we also synthesizedDy1�xSrxMnO3 ½0:1rxr0:5� and Dy1�xYxMnO3 ½0:05rxr0:5�. InDy1�xSrxMnO3, for the parent composition as well as the low dopedregime, the structure was found to be orthorhombic Pnma. Atx¼0.5, the crystal structure is cubic Pm3m. The magnetic propertiesof the Dy1�xSrxMnO3 compounds are crucially dependent on thedopant concentration x [8]. The x-ray diffraction pattern and theresults of Rietveld analysis of Dy0:9Sr0:1MnO3 (DSMO10) is pre-sented along with that of Dy0:5Sr0:5MnO3 (DSMO50) in Fig. 2(a) and(b), respectively. In Fig. 2(a), the analysis of Dy0:9Sr0:1MnO3 iscarried out in Pnma space group whereas in Fig. 2(b), Pm3m isused for Dy0:5Sr0:5MnO3. Dy0:5Sr0:5MnO3 was refined with a latticeparameter of a¼3.825 A. However, the tolerance factor of t� 0:91indicated a high degree of distortion in this manganite. Due to thelarge site disorder because of a smaller Dy ion, the lattice is likely tobe inhomogeneously distorted. The pxd only represents an averagepseudo-cubic structure. At very low Y-doping, DyMnO3 retainsthe orthorhombic structure as evident from Fig. 3(a) whereDy1�xYxMnO3 for x¼0.05 (DYMO05) is presented. With higher Ydoping the orthorhombic structure of DyMnO3 is transformed tohexagonal P63cm. For DYMO50, the refined lattice parameters werea¼6.161 A and b¼11.446 A. The structural details of DSMO50 andDYMO50 and the parent DyMnO3 in orthorhombic and hexagonal

nO3, Dy0:5Y0:5MnO3 and DyMnO3(o) and (h).

0:5Y0:5MnO3 DyMnO3(o) DyMnO3(h)

cm Pnma P63cm

62 185

6.161 A a¼5.832 A a¼6.189 A

11.446 A b¼7.381 A c¼11.461 A

a¼5.280 A

.2 A3 227.36 A3 380.2 A3

1/Y1 (0,0,z) Dy ðx, 14 ,zÞ Dy1/Y1 (0,0,z)

2/Y2 ð13 , 23 ,zÞ Mn ð0,0, 1

2Þ Dy2/Y2 ð13 , 23 ,zÞ

(x,0,0) O1 ðx, 14 ,zÞ Mn (x,0,0)

/O2 (x,0,z) O2 (x,y,z) O1/O2 (x,0,z)

(0,0,z) O3 (0,0,z)

(13, 2

3,z) O4 ð13 , 23 ,zÞ

4 2.48 3.9

Fig. 4. The observed x-ray diffraction patterns along with calculated, difference

profiles and Bragg peaks for (a) DyMnO3(o) and (b) DyMnO3(h).

Fig. 5. Schematic of the crystal structures of (a) Dy0:5Sr0:5MnO3 and (b) Dy0:5Y0:5MnO3. DSMO50 has cubic perovskite structure consisting of MnO6 octahedra, while

DYMO50 has a hexagonal structure that consists of MnO5 bipyramids separated by layers of Dy along the c-axis.

Fig. 7. The observed xps pattern of (a) DyMnO3(o) and (b) Dy0:5Y0:5MnO3 along

with the fitted curves.

Fig. 6. Schematic of the polyhedra: (a) DyO7, (b) MnO5 and (c) MnO6.

H.S. Nair, S. Elizabeth / Journal of Crystal Growth 362 (2013) 24–28 27

structures are collected in Table 2. For comparison, in Fig. 4(a)and (b), respectively, the refined x-ray powder patterns ofDyMnO3(o) and DyMnO3(h) are also presented. A schematic ofthe perovskite crystal structure consisting of oxygen octahedra andthe hexagonal structure of oxygen bipyramids are presented inFig. 5(a) and (b), respectively.

It is interesting to pose the question as to why DyMnO3

transforms from orthorhombic structure to hexagonal when thecrystal growth is carried out with different ambient gases. Severalarguments can be advanced to explain this transformation. Firstof all, the Gibbs free energy of stabilization of the hexagonal andthe orthorhombic structures of DyMnO3 are in close proximity asreported in theoretical calculations [9,10]. The basis of thecoordination of oxygen around R and Mn can be considered asanother perspective. The use of Ar or N2 for crystal growth ofDyMnO3 reduces the number of oxygen atoms around Mn or R tocomplete the coordination polyhedra. This reduction in theoxygen available for completing the polyhedra becomes animportant parameter when the ionic radius of the rare earth ionis small as in the present case of Dy. To illustrate, Fig. 6 presentsthe coordination polyhedra of (a) DyO7, (b) MnO5 (in the case ofhexagonal structure) and (c) MnO6. The small ionic radius of therare earth ion leads to large distortion and rotations of theoctahedra of orthorhombic DyMnO3 (the polycrystalline DyMnO3

stabilizes in orthorhombic structure) and hence results in thestructural transformation. In this respect, it is interesting to notethat the R FeO3 compounds do not display the structural trans-formation for R¼Dy. This might be conjectured as due to the role

played by Jahn–Teller active Mn3þ in manganites, whereas therare earth ferrites do not possess a Jahn–Teller ion. In order tounderstand the valence states of the transition metal ions in theDy compounds under study, we analyzed the x-ray photoelectronspectroscopy data. In Fig. 7(a) and (b) the xps spectra of

H.S. Nair, S. Elizabeth / Journal of Crystal Growth 362 (2013) 24–2828

DyMnO3(o) and Dy0:5Y0:5MnO3, respectively, are presented alongwith the analysis. DyMnO3(o) was fitted by assuming two peakscentered at 641.5 eV and 653.2 eV. They correspond to theMn2p3=2 and Mn2p1=2 peaks of Mn3þ ion, respectively. However,for the Y-doped DyMnO3, a faithful reproduction of the observedspectra was possible only with the inclusion of minor contribu-tion from Mn4þ as is evident from the figure.

4. Conclusions

In conclusion, we have studied the effect of Sr and Y substitu-tion on the structural and magnetic properties of multiferroicDyMnO3. Changing the ambience of growth leads to structuralpolymorphism in DyMnO3. Chemical doping at the Dy-site alsoresults in structural modifications in DyMnO3. It is observed thatDy0:5Sr0:5MnO3 adopts cubic structure whereas Dy0:5Y0:5MnO3 ishexagonal. The magnetic properties are also widely different forboth compounds: Dy0:5Sr0:5MnO3 is a spin glass compound inwhich ferromagnetic as well as antiferromagnetic clusters coexist,whereas Dy0:5Y0:5MnO3 is a frustrated antiferromagnet.

Acknowledgment

The authors wish to express their gratitude to DST, India forthe financial support through project grant.

References

[1] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, Y. Tokura, Nature 426(6962) (2003) 55–58.

[2] T. Kimura, G. Lawes, T. Goto, Y. Tokura, A. Ramirez, Physical Review B 71 (22)(2005) 224425.

[3] T. Arima, A. Tokunaga, T. Goto, H. Kimura, Y. Noda, Y. Tokura, Physical ReviewLetters 96 (2006) 097202.

[4] S. Harikrishnan, S. Roßler, C.M. Naveen Kumar, H.L. Bhat, U.K. Roßler, S. Wirth,F. Steglich, S. Elizabeth, Journal of Physics: Condensed Matter 21 (2009) 096002.

[5] V. Ivanov, A. Mukhin, A. Prokhorov, A. Balbashov, L. Iskhakova, Physics ofSolid State 48 (9) (2006) 1726–1729.

[6] H.M. Rietveld, Journal of Applied Crystallography 2 (1969) 65.[7] Rodriguez-Carvajal, Physica B 192 (1993) 55.[8] S. Roßler, S. Harikrishnan, U.K. Roßler, S. Elizabeth, H.L. Bhat, F. Steglich,

S. Wirth, Journal of Physics: Conference Series 200 (2010) 012168.[9] R. Choithrani, M.N. Rao, S.L. Chaplot, N.K. Gaur, R.K. Singh, New Journal of

Physics 11 (2009) 073041.[10] R. Choithrani, M.N. Rao, S.L. Chaplot, N.K. Gaur, R.K. Singh, Journal of

Magnetism and Magnetic Materials 323 (2011) 1627.