X-ray diffraction and Raman spectroscopy studies of temperature and composition induced phase transitions in Ba2−xSrxMWO6 (M = Ni, Co and 0 ⩽ x ⩽ 2) double perovskite oxides

Journal of Molecular Structure 1045 (2013) 1–14

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X-ray diffraction and Raman spectroscopy studies of temperature and composition induced phase transitions in Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) double perovskite oxides Bouchaib Manoun a,⇑, A. Ezzahi a, S. Benmokhtar b, L. Bih c, Y. Tamraoui a, R. Haloui a, F. Mirinioui a, S. Addakiri d, J.M. Igartua e, P. Lazor f a

Equipe Ingénierie et environnement, Laboratoire de Chimie Appliquée et Environnement, Université Hassan 1er, Morocco LRCPGM, Laboratoire de Recherche de Chimie-Physique Générale des Matériaux, Département de Chimie, Faculté des Sciences Ben M’Sik Casablanca, Morocco c Laboratoire de Physico-Chimie des Matériaux, Département de Chimie, FST Errachidia, Morocco d Université Hassan I, Laboratoire de Mécanique, FST de Settat, B.P. 577, Settat, Morocco e Fisika Aplikatua II, Zientzia eta Teknologia Fak., UPV/EHU, PB 644, Bilbao 48080, Spain f Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden b

h i g h l i g h t s  Synthesis and characterization of Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) double perovskites.  High temperature studies of these double perovskite using Raman spectroscopy.  Temperature and compositions induced phase transitions in these materials.  Structural determination/refinement of these compounds as a function of composition.

a r t i c l e

i n f o

Article history: Received 11 November 2012 Received in revised form 31 March 2013 Accepted 31 March 2013 Available online 6 April 2013 Keywords: Double perovskite Ba2xSrxMWO6 Phase transition Rietveld refinements Raman spectroscopy

a b s t r a c t X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba2xSrxMWO6 with (M = Ni, Co and 0 6 x 6 2) were investigated. The samples show a transition from cubic to tetragonal phase as a function of composition while increasing strontium amount; both Rietveld refinements and Raman studies showed that this transition occurs between x = 1.2 and 1.4. Furthermore, increasing temperature for the tetragonal compositions (1.4 6 x 6 2), manifest the tetragonal to cubic phase transition. For this series, the transition from tetragonal (I4/m)/to cubic (Fm-3m) shows considerable changes in the composition and temperature dependence of the modes: all the Raman modes show a linear behavior when temperature is increased, then the slope change dramatically indicating the symmetry change from tetragonal to cubic. To better view this phase transition, the full width at half maximum (FWHM) temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached the transition temperature, a drop in the width of this mode was observed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Double perovskites A2MM0 O6 have attracted much interest as they exhibit rich structural and physical properties. For example, the monoclinic Ba2Bi(III)Bi(V)O6 is the parent compound of the superconducting BaBi1xPbxO3 system [1] with a maximum transition temperature (Tc) of about 12 K. It is known that the Pb

⇑ Corresponding author. E-mail address: [email protected] (B. Manoun). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.03.059

substitution rapidly destroys the ordered arrangement of Bi(III)– Bi(V) presented in Ba2Bi(III)Bi(V)O6, resulting in a series of the phase transitions [2]. The superconductivity occurs only in a tetragonal phase region with 0.05 < x < 0.35. On the other hand, some ordered perovskites are interesting candidates for microwave dielectric resonators widely used in today’s telecommunication systems. It has been shown that the dielectric properties of the materials depend strongly on the degree of ordering [3] as well as on the ionic size which determine the symmetry of the structure [4]. Therefore, the detailed crystallographic information is of

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Fig. 1. Principal layout scheme of the Raman experimental setup.

x = 1.8 x = 1.6 x = 1.4 x = 1.2 x=1 x = 0.8 x = 0.4 x=0

Fig. 2a. X-ray powder diffraction patterns for Ba2xSrxNiWO6 (0 6 x < 2).

importance in understanding the material properties and for searching new materials with controllable physical properties. The structural and magnetic properties of double perovskite materials with either A2MM0 O6 or AA0 MM0 O6 stoichiometry are shown to depend on how the MM0 cations are distributed over the octahedral sites, degree of cation inversion as well as on the size and electronic structure of transition metal cations M and M0 [5,6]. Depending on the tolerance factor, t, which is the function of the ionic radii of the cations, the structure of the perovskites distorts from the cubic symmetry. Theoretically, when t = 1, the perovskite adopts a cubic symmetry (space group Pm-3m). For t > 1 or t < 1, the structure is distorted resulting in a symmetry

lower than cubic. Three different types of distortions have been identified [7]: (i) distortions of the MO6 octahedral units, (ii) M-cation displacements within the octahedra, and (iii) the tilting of the MO6 octahedra relative to one another as practically rigid corner-linked units. The third type of distortion, octahedral tilting, is the most common type of distortion. The rotation and tilting of octahedra in the perovskite compounds have been studied in detail earlier by many researchers [8–10]. The octahedral tilting and the presence of cationic ordering produce superlattice reflections. During the past few years, particular attention has been paid to combining tungstate and divalent 3d transition metals M2+. Most investigations on A2MWO6 (A = Ba, Sr, M = Co, Ni) [11–13] have

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x= 1.8 x= 1.6 x= 1.4 x= 1.2 x= 1 x= 0.8 x= 0.4

Fig. 2b. X-ray powder diffraction patterns for Ba2xSrxCoWO6 (0 6 x < 2).

Table 1a Unit cell parameters of Ba2xSrxNiWO6 (0 6 x 6 2). x

a (Å)

0 0.4 0.8 1 1.2

8.070(1) 8.030(9) 7.995(8) 7.971(2) 7.937(9)

1.4 1.6 1.8 2

5.609(1) 5.6065(2) 5.599(1) 5.5613(1)

c (Å)

7.932(3) 7.9288(3) 7.919(1) 7.9185(1)

V (Å3)

Symmetry

525.617 517.840 510.964 506.452 499.923

Cubic

249.517 249.225 248.292 248.253

Tetragonal

Fig. 3a. Variation of the reduced lattice parameters with the average A cation radius for the series Ba2xSrxNiWO6. The vertical lines show the approximate transitions between the three observed structural types. The a and b values in the tetragonal cells have been multiplied by 21/2 for clarity. The transition is between x = 1.2 and x = 1.4.

Table 1b Unit cell parameters of Ba2xSrxCoWO6 (0 6 x 6 2). x

a (Å)

0 0.4 0.8 1 1.2

8.114(1) 8.072(1) 8.047(3) 8.024(1) 7.998(1)

1.4 1.6 1.8 2

5.645(1) 5.616(1) 5.603(1) 5.5854(1)

c (Å)

7.988(3) 7.977(1) 7.982(1) 7.9837(1)

V (Å3)

Symmetry

534.142 525.968 521.097 516.603 511.635

Cubic

254.490 251.621 250.545 249.065

Tetragonal

employed X-ray powder diffraction, neutron powder diffraction, synchrotron X-ray diffraction techniques to probe the global structure of the materials using the Rietveld analysis. Consequently, an uncertainty exists about exact symmetry of these materials. For example, A2CoWO6 (A = Ba, Sr) were investigated by Fresia et al. [11], the authors suggested that Ba2CoWO6 was cubic and Sr2CoWO6 was tetragonal at room temperature, these results have been confirmed by subsequent studies 44 years later [12,13]. However, Zhao et al. [14] described both A2CoWO6 (A = Ba, Sr) as having cubic structures at room temperature. Very recently Gateshki et al. [12] described Sr2CoWO6 as undergoing a continuous phase transition from tetragonal I4/m to cubic Fm-3m near 430 °C, Whilst Viola et al. [13] reported that Sr2CoWO6 be-

Fig. 3b. Variation of the reduced lattice parameters with the average A cation radius for the series Ba2xSrxCoWO6. The vertical lines show the approximate transitions between the three observed structural types. The a and b values in the tetragonal cells have been multiplied by 21/2 for clarity. The transition is between x = 1.2 and x = 1.4.

comes monoclinic upon cooling below 265 K. In all cases there is a rock-salt like ordering of the Co and W cations over the two octahedral sites with the changes in symmetry arising from different

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Fig. 4a. Final Rietveld plots for cubic BaSrNiWO6 (left) and tetragonal Ba0.2Sr1.8NiWO6 (right). The upper symbols illustrate the observed data (circles) and the calculated pattern (solid line). The vertical markers show calculated positions of Bragg reflexions. The lower curve is the difference diagram.

Fig. 4b. Final Rietveld plots for cubic BaSrCoWO6 (left) and tetragonal Ba0.2Sr1.8CoWO6 (right). The upper symbols illustrate the observed data (circles) and the calculated pattern (solid line). The vertical markers show calculated positions of Bragg reflexions. The lower curve is the difference diagram.

Table 2a Details of Rietveld refinement conditions of the cubic compositions for Ba2xSrxNiWO6 (0 6 x 6 1.2). Composition

x = 0.4

x = 0.8

x=1

x = 1.2

Wavelength (Å)

kka1 = 1.5406 kka2 = 1.5444 0.066845 05–90 FULLPROF 0.072(8) g = 0.693(31)

kka1 = 1.5406 kka2 = 1.5444 0.066845 05–90 FULLPROF 0.109(6) g = 0.705(28)

kka1 = 1.5406 kka2 = 1.5444 0.02 15–120 FULLPROF 0.012(2) g = 0.275(6)

kka1 = 1.5406 kka2 = 1.5444 0.066845 10–80 FULLPROF 0.047(8) g = 0.359(23)

U = 0.095(25) V = 0.023(21) W = 0.014(4) 50 15 Fm-3m 8.030(9) 517.84(10) 4 5 0.0456 0.0565 0.0833 0.0851 0.205 0.213

U = 0.057(24) V = 0.016(20) W = 0.009(4) 50 15 Fm-3m 7.995(8) 510.96(8) 4 5 0.0517 0.0704 0.0901 0.121 0.223 0.208

U = 0.099(5) V = 0.012(5) W = 0.033(2) 40 15 Fm-3m 7.971(2) 506.45(2) 4 5 0.0305 0.0323 0.0653 0.0915 0.174 0.164

U = 0.295(50) V = 0.114(40) W = 0.044(8) 44 15 Fm-3m 7.93669 499.9210 4 5 0.0580 0.0876 0.0763 0.126 0.206 0.226

Step scan increment (°2h) 2h range (°) Program Zero point (°2h) Pseudo-Voigt function PV = gL + (1  g)G Caglioti parameters

No. of reflections No. of refined parameter Space group a (Å) V (Å3) Z Atom number RF RB Rp Rwp cRp cRwp

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B. Manoun et al. / Journal of Molecular Structure 1045 (2013) 1–14 Table 2b Details of Rietveld refinement conditions of the tetragonal compositions for Ba2xSrxNiWO6 (1.4 6 x 6 1.8). Composition

x = 1.4

x = 1.6

x = 1.8

Wavelength (Å)

kka1 = 1.5406 kka2 = 1.54439 0.066845

kka1 = 1.5406 kka2 = 1.54439 0.066845

kka1 = 1.5406 kka2 = 1.54439 0.066845

05–90 FULLPROF 0.070(9)

10–80 FULLPROF 0.109(6)

05–90 FULLPROF 0.049(7)

g = 0.582(24)

g = 0.705(28) U = 0.057(24) V = 0.016(20) W = 0.009(4) 118 16 I4/m 5.607(2) 7.929(3) 249.23(2) 2 6 0.0888 0.0983 0.0841 0.155 0.242 0.289

g = 0.651(27) U = 0.041(20) V = 0.012(17) W = 0.009(3) 148 16 I4/m 5.599(1) 7.919(1) 248.29(1) 2 6 0.0401 0.0466 0.0756 0.106 0.162 0.169

Step scan increment (°2h) 2h range (°) Program Zero point (°2h) Pseudo-Voigt function PV = gL + (1  g)G

Caglioti parameters No. of reflections No. of refined parameter Space group a (Å) c (Å) V (Å3) Z Atom number RF RB Rp Rwp cRp cRwp

U = 0.080(27) V = 0.020(22) W = 0.008(4) 148 16 I4/m 5.609(1) 7.932(3) 249.52(1) 2 6 0.0489 0.0474 0.0742 0.108 0.187 0.188

patterns of tilting of the octahedra. Recent structural study of the double perovskites A2MWO6 (A = Ba, Sr, Ca; M = Ni, Co) by Zhou et al. [15,16] demonstrated that progressively increasing the effective size of the A-type cation by chemical substitution of the alkaline earth cation resulted in the sequence of structural phase transitions:

P21 =n ! I4=m ! Fm-3m Because of the important applications of the double perovskites in diverse fields including ferroelectrics, high temperature superconductors, materials exhibiting colossal magnetoresistive effects, ionic conductors, etc., oxides with the perovskite structure in mod-

ern materials continue to grow. To contribute to a better investigation to follow structural phases transition in this type of compounds, we have undertaken the synthesis by solid state method of a large number of oxides Ba2xSrxMWO6 (0 6 x 6 2) using X-ray diffraction and Raman spectroscopy techniques. We report in this work on the effect of Sr substitution at the A-site and on the high temperature induced phase transition in Ba2xSrxNiWO6 and in Ba2xSrxCoWO6 (0 6 x 6 2). Their crystal structure was solved by Rietveld refinements of X-ray powder diffraction patterns. The study with high-temperature Raman spectroscopy showed the phase transition from the tetragonal phase to the cubic phase and thus confirming the X-ray diffraction results. 2. Experimental 2.1. Sample preparation All chemicals were obtained from Aldrich and were dried prior to use. Two series of nine oxides of composition Ba2xSrxNiWO6 and Ba2xSrxCoWO6 (0 6 x 6 2) was prepared by conventional solid-state reaction from stoichiometric amounts of WO3 (99.9%), NiO (99.9%), CoO (99.9%) with the appropriate metal carbonate (BaCO3 99.98%, SrCO3 99.9%). The samples were heated in air at progressively higher temperatures (600 °C/24 h, 800 °C/24 h, and 1100 °C/24 h) with periodic intermediate regrinding. The samples were contained in alumina crucibles. The chemical reaction is:

ð2  xÞBaCO3 þ xSrCO3 þ MO þ WO3 ! Ba2x Srx MWO6 þ 2CO2 "

2.2. XRD measurements The final products of Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) have been controlled by X-ray powder diffraction analysis using Cu Ka radiation. The analysis of powders by X-ray diffraction showed some impurities, with low intensity) such as Ba/SrWO4. The structural refinements were undertaken from the powder data. Diffraction data were collected at room temperature on a phillips D 5000 (h–h) diffractometer: Bragg–Brentano geometry; diffractedbeam graphite monochromator; Cu Ka radiation (40 kV, 40 mA);

Table 2c Details of Rietveld refinement conditions of the cubic compositions for Ba2xSrxCoWO6 (0 6 x 6 1.2). Composition

x = 0.4

x = 0.8

x = 1.0

x = 1.2

Wavelength (Å)

kka1 = 1.5406 kka2 = 1.5444 0.066845 05–90 FULLPROF 0.044(8)

kka1 = 1.5406 kka2 = 1.5444 0.066845 05–90 FULLPROF 0.138(9)

kka1 = 1.5406 kka2 = 1.5406 0.02 15–100 FULLPROF 0.031(6)

kka1 = 1.5406 kka2 = 1.5444 0.066845 10–80 FULLPROF 0.128 8

g = 0.693(31)

g = 0.705(28)

g = 0.275(6)

g = 0.359(23)

U = 0.104(23) V = 0.013(18) W = 0.012(3) 50 20 Fm-3m 8.072(1) 526.0(1) 4 5 0.0425 0.0417 0.0787 0.115 0.200 0.199

U = 0.104(4) V = 0.013(4) W = 0.065(4) 62 20 Fm-3m 8.047(3) 521.1(2) 4 5 0.0517 0.0704 0.0901 0.121 0.223 0.208

U = 0.124(26) V = 0.031(22) W = 0.028(4) 68 20 Fm-3m 8.024(1) 516.6(1) 4 5 0.0799 0.102 0.0291 0.0397 0.473 0.226

U = 0.053(14) V = 0.018(12) W = 0.016(3) 42 20 Fm-3m 7.9981 511.61 4 5 0.0389 0.0512 0.0498 0.0712 0.146 0.133

Step scan increment (°2h) 2h range (°) Program Zero point (°2h) Pseudo-Voigt function PV = gL + (1  g)G Caglioti parameters

No. of reflections No. of refined parameter Space group a (Å) V (Å3) Z Atom number RF RB Rp Rwp cRp cRwp

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B. Manoun et al. / Journal of Molecular Structure 1045 (2013) 1–14

Table 2d Details of Rietveld refinement conditions of the tetragonal compositions for Ba2xSrxCoWO6 (1.4 6 x 6 1.8). Composition

x = 1.4

x = 1.6

x = 1.8

Wavelength (Å)

kka1 = 1.5406 kka2 = 1.54439 0.066845

kka1 = 1.5406 kka2 = 1.54439 0.066845

kka1 = 1.5406 kka2 = 1.54439 0.066845

05–90

10–80

05–90

FULLPROF 0.105(9) g = 0.498(24)

FULLPROF 0.011(7) g = 0.705(28)

FULLPROF 0.087(7) g = 0.651(27)

U = 0.160(38) V = 0.019(30) W = 0.018(5) 152 23 I4/m 5.645(1) 7.9881 254.51 2 6 0.0594 0.0408 0.0876 0.121 0.233 0.219

U = 0.270(37) V = 0.154(29) W = 0.043(5) 116 23 I4m 5.6161 7.9771 251.61 2 6 0.0397 0.0497 0.0544 0.0765 0.169 0.150

U = 0.160(31) V = 0.071(25) W = 0.027(5) 148 23 I4m 5.6021 7.9821 250.61 2 6 0.0619 0.0841 0.0925 0.121 0.231 0.210

Step scan increment (°2h) 2h range (°) Program Zero point (°2h) Pseudo-Voigt function PV = gL + (1  g)G Caglioti parameters

No. of reflections No. of refined parameter Space group a (Å) c (Å) V (Å3) Z Atom number RF RB Rp Rwp cRp cRwp

Table 3b Refined structural parameters for Ba2xSrxCoWO6 (0 6 x 6 1.2). Atom

x

y

z

B (Å2)

Occ.

x=0 W Co Ba O1

0 0.5 0.25 0

0 0.5 0.25 0

0 0.5 0.25 0.2363(1)

0.66 0.99 1.04 2.34

1.0 1.0 2 6.0

x = 0.4 W Co Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2342(2)

0.8 1.2 0.89 0.89 2.0

1.0 1.0 0.4 1.6 6.0

x = 0.8 W Co Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2335(1)

0.82 1.16 0.89 0.89 1.95

1.0 1.0 0.8 1.2 6.0

x=1 W Co Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2326(12)

0.13 0.7 0.64 0.64 1.3

1.0 1.0 1.0 1.0 6.0

x = 1.2 W Co Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2332(4)

0.2 0.4 0.42 0.42 1.5

1.0 1.0 1.2 0.8 6.0

Table 3a Refined structural parameters for Ba2xSrxNiWO6 (0.4 6 x 6 1.2). Atom

x

y

z

B (Å2)

Occ.

x = 0.4 W Ni Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2369(4)

0.290 1.102 0.464 0.464 0.818

1.0 1.0 0.4 1.6 6.0

x = 0.8 W Ni Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2372(6)

0.824 1.139 0.103 0.103 0.757

1.0 1.0 0.8 1.2 6.0

x=1 W Ni Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2374(3)

0.652 0.955 1.047 1.047 0.657

1.0 1.0 1.0 1.0 6.0

x = 1.2 W Ni Sr Ba O1

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0

0 0.5 0.25 0.25 0.2376(7)

0.450 0.351 0.387 0.387 0.661

1.0 1.0 1.2 0.8 6.0

Soller slits of 0.02 rad on incident and diffracted beams; divergence slit of 0.5°; antiscatter slit of 1°; receiving slit of 0.1 mm; with sample spinner. The patterns were scanned through steps of 0.02 (2h), between 10 and 100 (2h) with a fixed-time counting (5–10 s/step). The full pattern refinements were carried out by means of the Rietveld method using the Fullprof program [17] integrated in Winplotr software [18]. The Rietveld refinement of the observed powder XRD data is initiated with scale and background parameters. The background is fitted with a fifth order polynomial. The peak shape is fitted with a pseudo-Voigt profile function. After an

Table 3c Refined structural parameters for Ba2xSrxNiWO6 (1.4 6 x 6 1.8). Atom

x

y

z

B (Å2)

Occ.

x = 1.4 W Ni Sr Ba O1 O2

0 0 0 0 0.2889(6) 0

0 0 0.5 0.5 0.2268(4) 0

0.5 0 0.25 0.25 0 0.2549(6)

0.433 1.525 0.840 0.840 0.870 0.870

1.0 1.0 1.4 0.6 4.0 2.0

x = 1.6 W Ni Sr Ba O1 O2

0 0 0 0 0.2891(7) 0

0 0 0.5 0.5 0.2270(4) 0

0.5 0 0.25 0.25 0 0.2551(6)

0.396 0.063 0.326 0.326 1.033 1.033

1.0 1.0 1.6 0.4 4.0 2.0

x = 1.8 W Ni Sr Ba O1 O2

0 0 0 0 0.2893(5) 0

0 0 0.5 0.5 0.2272(5) 0

0.5 0 0.25 0.25 0 0.2552(4)

0.605 0.615 0.686 0.686 1.338 1.338

1.0 1.0 1.8 0.2 4.0 2.0

appreciable profile matching the atomic position parameters and isotropic atomic displacement parameters of individual atoms were also refined. 2.3. Raman spectroscopy Experiments have been carried out using Raman spectroscopic system designed and built at the Department of Earth Sciences, Uppsala University [19,20]. The key system components include a high-throughput, single stage imaging spectrometer (HoloSpec f/1.8i, Kaiser Optical Systems, Inc.) equipped with a holographic

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Table 4b Selected inter-atomic distances (Å) and O–W–O angles for the tetragonal compositions in Ba2xSrxNiWO6 (1.4 6 x 6 1.8).

Table 3d Refined structural parameters for Ba2xSrxCoWO6 (1.4 6 x 6 1.8). Atom x = 1.4 W Co Sr Ba O1 O2

x

y

0 0 0 0 0.2891 0

B (Å2)

z

0 0 0.5 0.5 0.2269 0

0.5 0 0.25 0.25 0 0.2549

0.92 1.75 0.45 0.45 1.01 1.01

Occ. 1.0 1.0 1.4 0.6 4.0 2.0

x = 1.6 W Co Sr Ba O1 O2

0 0 0 0 0.2890 0

0 0 0.5 0.5 0.2270 0

0.5 0 0.25 0.25 0 0.2550

0.20 1.3 0.17 0.17 2.0 2.0

1.0 1.0 1.6 0.4 4.0 2.0

x = 1.8 W Co Sr Ba O1 O2

0 0 0 0 0.2888 0

0 0 0.5 0.5 0.2272 0

0.5 0 0.25 0.25 0 0.2551

0.19 0.9 0.63 0.63 1.4 1.4

1.0 1.0 1.8 0.2 4.0 2.0

WO6

Tetragonal

x = 1.4

x = 1.6

x = 1.8

4  W–O(1) 2  W–O(2) hW–Oi

1.9374(7) 1.9451(7) 1.9400(7)

1.9322(8) 1.9357(7) 1.9334(8)

1.9223(6) 1.9383(5) 1.9276(6)

4  Ni–O(1) 2  Ni–O(2) hNi–Oi

2.0610(4) 2.0229(3) 2.0483(4)

2.0588(5) 2.0163(4) 2.0446(5)

2.0515(5) 2.0207(5) 2.0412(5)

4  Ba/Sr–O(1) 4  Ba/Sr–O(1) 4  Ba/Sr–O(2) hBa/Sr–O(1)i

2.9857(9) 2.8060(8) 2.6380(9) 2.8099(9)

2.9774(8) 2.8008(7) 2.6299(8) 2.8027(8)

2.9725(7) 2.7887(7) 2.6277(6) 2.7963(7)

4  O1–W–O(1) 2  O1–W–O(1) 8  O1–W–O(2) 1  O2–W–O(2)

90 180 90 180

90 180 90 180

90 180 90 180

Table 4c Selected inter-atomic distances (Å) and O–W–O angles for Ba2xSrxCoWO6 (0.4 6 x 6 1.2). Composition

x = 0.4

x = 0.8

x=1

x = 1.2

12  Ba/Sr–O(1) 6  Co–O(1) 6  W–O(1) 12  O(1)–W–O(1) 3  O(1)–W–O(1)

2.8568(7) 2.1456(1) 1.8905(1) 90 180

2.8545(2) 2.1493(2) 1.8832(2) 90 180

2.8403(5) 2.14560(4) 1.866(10) 90 180

2.8310(6) 2.1339(4) 1.8652(2) 90 180

Ba/Sr Table 4d Selected inter-atomic distances (Å) and O–W–O angles for the tetragonal compositions in Ba2xSrxCoWO6 (1.4 6 x 6 1.8).

MO6

a c

Tetragonal

x = 1.4

x = 1.6

x = 1.8

4  W–O(1) 2  W–O(2) hW–Oi

1.9475(1) 1.9570(2) 1.9507

1.9378(3) 1.9545(4) 1.9461

1.9331(4) 1.9556(4) 1.9444

4  Co–O(1) 2  Co–O(2) hCo–Oi

2.0743(1) 2.0369(2) 2.0618

2.0639(3) 2.0342(4) 2.0491

2.0589(4) 2.0355(5) 2.0472

4  Ba/Sr–O(1) 4  Ba/Sr–O(1) 4  Ba/Sr–O(2) hBa/Sr–O(1)i

3.0038(1) 2.6548(1) 2.8225(2) 2.8270

2.9938(1) 2.6471(2) 2.8084(1) 2.8164

2.9905(2) 2.6452(1) 2.8015(1) 2.8124

4  O(1)–W–O(1) 2  O(1)–W–O(1) 8  O(1)–W–O(2) 1  O(2)–W–O(2)

90 180 90 180

90 180 90 180

90 180 90 180

Fig. 5. Structural views of the tetragonal Fm-3m structure of BaSrMWO6 along 0 1 0 plane.

Table 4a Selected inter-atomic distances (Å) and O–W–O angles for the cubic compositions in Ba2xSrxNiWO6 (0.4 6 x 6 1.2). Composition

x = 0.4

x = 0.8

x=1

x = 1.2

12  Ba/Sr–O(1) 6  Ni–O(1) 6  W–O(1) 12  O1–W–O(1) 3  O1–W–O(1)

2.8412(8) 2.1128(5) 1.9024(6) 90 180

2.8283(7) 2.1010(7) 1.8963(8) 90 180

2.8229(6) 2.0953(5) 1.8943(4) 90 180

2.8086(8) 2.0832(6) 1.8863(4) 90 180

transmission grating and thermoelectrically cooled two-dimensional multichannel CCD detector (Newton, Andor Technology, 1600  400 pixels, thermoelectrically cooled down to – 60 °C), an argon-ion laser (Spectra-Physics, 514.5 nm, 20 mW), and an optical

imaging system (magnification 20, spatial resolution 1 lm). Two holographic notch filters (Kaiser Optical Systems, Inc.) blocked the Rayleigh line. The spectrometer was calibrated by fluorescence lines of the neon lamp. Non-polarized Raman spectra were collected in the back-scattering geometry, in the range 180– 2280 cm1, at a resolution of about 3 cm1. Accuracy and precision of spectral measurements, as estimated from the wavelength calibration procedure and peak fitting results, were 1.5 cm1 and 0.1– 0.4 cm1, respectively. The acquisition time varied from 30 s to 5 min. Heating was accomplished by using a mica insulated band heater (DuraBand, Tempco Electric heater Corporation) mounted around the sample ceramic holder and connected to a variable transformer. Temperature changes during the heating/cooling cycles were induced and controlled by adjusting the transformer’s voltage (0–240 V) and monitored with an accuracy of ±1 °C by

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B. Manoun et al. / Journal of Molecular Structure 1045 (2013) 1–14

Fig. 6. Illustrations of the effect of tilting of the WO6 octahedra in the tetragonal structure.

the K-type thermocouple adjacent to the sample. During the spectral acquisitions, temperatures were stabilized to within 1 and 3 °C, for the low and high temperature measurements, respectively. Principal layout of the experimental setup is shown in Fig. 1.

3. Results and discussions 3.1. Structure determination and description of the structure Fig. 2 shows the X-ray powder diffraction patterns of Ba2xSrxNiWO6 and Ba2xSrxCoWO6 (0 6 x 6 2). Indexing of X-ray powder diffraction patterns for these compositions was performed by means of the computer program Dicvol [21]. The first 15 peak positions, with a maximal absolute error of 0.03° (2h), were used as input data. The X-ray diffraction patterns were assigned to a cubic symmetry with Fm-3m as a space group for the composition range (0 6 x 6 1.2) and to a tetragonal symmetry with I4/m as a space group for the composition range (1.4 6 x 6 2). Table 1 lists the lattice parameters that were refined using the complete powder diffraction data sets. The variation of the unit cell parameters of the cubic and tetragonal phases in the studied composition range in Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) is shown in Fig. 3.

Fig. 8. Observed tolerance factor (calculated from the distances obtained from the Rietveld refinements) as a function of the strontium amount in Ba2xSrxNiWO6 (a) and Ba2xSrxCoWO6 (b). The phase transition from cubic to tetragonal is well illustrated in the figures between the compositions 1.2 and 1.4 for both series.

The X-ray powder patterns were fitted to the calculated ones using a full-profile analysis program [17,18] to minimize the profile discrepancy factor Rp. The refinement of the powder XRD pattern was carried out with cubic (Fm-3m) lattice with starting model taken from Ref. [22]. In this model Ba2+/Sr2+, M2+ and W6+ are placed at 8c (1/4, 1/4, 1/4), 4b (1/2, 1/2, 1/2) and 4a (0, 0, 0) sites, respectively; the oxygen atoms occupy (0, 0, z) sites. As an example we show, in Fig. 4, the typical Rietveld refinement patterns along with the difference plot at ambient temperature for BaSrNiWO6 and Ba0.2Sr1.8NiWO6. For Ba2xSrxMWO6 (M = Ni, Co and 1.4 6 x 6 2), the refinement of the powder XRD pattern was carried out with tetragonal (I4/m) lattice with starting model taken from Ref. [22]. In this model Ba2+/ Sr2+, M2+ and W6+ are placed at 4d (0, 1/2, 1/4), 2a (0, 0, 0) and 2b (0, 0, 1/2) sites, respectively. There are two crystallographically distinct oxygen atoms (O1 (0, 0, z) and O2 (x, y, 0)), present in the unit cell. For all the compounds studied here, the refinements of the occupancies of all the atoms show no significant deviation from

Fig. 7. Coordination environment in the mixed site Ba/Sr.

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B. Manoun et al. / Journal of Molecular Structure 1045 (2013) 1–14

I4/m

the reliability factors were much better, for both Ba0.8Sr1.2NiWO6 and Ba0.8Sr1.2CoWO6 in a cubic symmetry, they favored a tetragonal symmetry for Ba0.6Sr1.4NiWO6 and Ba0.6Sr1.4CoWO6. The analysis of refined crystallographic parameters in Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) indicates that the M2+ and W6+ are octahedrally coordinated with the oxygen atoms. The MO6 and WO6 octahedra are alternatively connected and extended in three dimensions. The O(1) atoms connect the MO6 and WO6 octahedra along the three directions. The typical M–O(1)–W bond angle at all compounds constrained to 180° by space group Fm-3m and O(1) position coordinates (x, 0, 0), indicating no tilt with respect to a, b and c-axes. A (0 0 1) projection of the BaSrNiWO6 unit cell indicating the typical polyhedral arrangement is shown in Fig. 5. The analysis of various inter-atomic distances (Table 4) shows that Ba/Sr atoms form Ba/SrO12 polyhedra with the Ba/Sr–O bond lengths around 2.82 Å. The Ni2+ and W6+ have octahedral coordination with the Ni–O bond lengths around 2.10 Å and the W–O bonds around 1.89 Å. The analysis of refined crystallographic parameters for the tetragonal compositions in Ba2xSrxMWO6 (M = Ni, Co and 1.4 6 x 6 2) indicates that the M2+ and W6+ are octahedrally coordinated with the oxygen atoms. The MO6 and WO6 octahedra are

Fm-3m A 1g

3Ag Eg 3Bg F1g 3Eg 2F2g Fig. 9. Correlations diagrams for the Raman-active vibrations in the tetragonal (I4/ m) and the cubic (Fm-3m) phases of Ba2xSrxMWO6.

their stoichiometric values. Significantly good residuals of the refinements are obtained (Table 2). The refined position coordinates for Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) in the cubic and tetragonal compositions along with other crystallographic data are given in Table 3. Note that for the compositions x = 1.2 and x = 1.4, for both Ni and Co series, the refinements of the powder XRD patterns were carried out with both cubic (Fm-3m) and tetragonal (I4/m). While

x=2 x = 1.8 x = 1.6 x = 1.4 x = 1.2 x=1 x = 0.8 x = 0.4 x=0

x=2 x = 1.8 x = 1.6 x = 1.4 x = 1.2 x = 0.8 x = 0.4 x=0 116

141

166

Fig. 10a. Raman spectra of Ba2xSrxNiWO6 recorded at ambient conditions. The insets show the 130 and 600 cm1 modes as a function of composition.

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x=2 x = 1.8 x = 1.6 x = 1.4 x = 1.2 x = 0.8 x = 0.4 x=0

Fig. 10b. Raman spectra of Ba2xSrxCoWO6 recorded at ambient conditions as a function of composition.

Fig. 11a. Raman modes of Ba2xSrxNiWO6 vs. the composition x, the transition from the cubic phase to the tetragonal phase shows considerable changes in the composition dependence of the modes.

alternatively connected and extended in three dimensions. The O(2) atoms connect the MO6 and WO6 octahedra along the c-axis. In ab-plane MO6 and WO6 octahedra are connected through the O(1) atoms. The typical M–O(2)–W bond angle constrained to 180° by space group I4/m and O(2) position coordinates (0, 0, z), indicating no tilt with respect to c-axis. The appreciable tilt of the octahedra is observed from the M–O(1)–W bond angle (165.8°). The tilt pattern of the octahedral units satisfies the a°a°c tilt system in Glazer’s notation [9,23]. A (0 0 1) projection of the Ba0.4Sr1.6NiWO6 unit cell indicating the typical polyhedral arrangement and the tilt pattern is shown in Fig. 6. In Fig. 7 we illustrate the Ba/Sr environment. The analysis of various inter-atomic distances (Table 4) shows that Ba/Sr atoms form Ba/SrO12 polyhedra with the Ba/Sr–O bond

lengths ranging between 2.63 and 3.00 Å, and the average d value is around 2.80 Å. The M2+ and W6+ have octahedral coordination with the M–O bond lengths ranging between 2.02 and 2.07 Å and the W–O bonds range within 1.922–1.957 Å. Note that the distances of W–O are considerably shorter than expected (2.00 Å) from the Shannon ionic radii of W6+ (0.6 Å) and O2 (1.4 Å). The M–O values are very close to what is expected: 2.09 Å for Ni–O and 2.14 Å for Co–O. It seems that W–O is contracting when Sr2+ substituted Ba2+ cations. What is more important is that when the strontium amount increases in the cubic compositions, the W–O distances decreases until for x = 1.2 (1.886 Å for Ba0.8Sr1.2NiWO6 and 1.865 Å for Ba0.8Sr1.2CoWO6). For both series, for the composition x = 1.4 an important increase of W–O distance is observed (1.94 Å for Ba0.6Sr1.4NiWO6 and 1.95 Å for Ba0.6Sr1.4-

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Fig. 11b. Raman modes of Ba2xSrxCoWO6 vs. the composition x, the transition from the cubic phase to the tetragonal phase shows changes in the composition dependence of the modes.

350 °C 300 °C 250 °C 200 °C 150 °C 100 °C 50 °C

300 °C 250 °C 200 °C 150 °C 100 °C 50 °C

Fig. 12a. The Raman spectra of Ba0.6Sr1.4NiWO6 (bottom) and Ba0.4Sr1.6NiWO6 (up) obtained for selected temperatures, as indicated.

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350 °C 300 °C 250 °C 200 °C 150 °C 100 °C 50 °C

Fig. 12b. The Raman spectra of Ba0.4Sr1.6CoWO6 obtained for selected temperatures, as indicated.

436

Wavenumbers (1/cm)

CoWO6) which is a great sign for the cubic to tetragonal transition. Again once we add more Sr2+ in the compositions a decrease in W– O distances is observed until the barium is completely substituted by strontium. For each series this contraction is not due to Ni2+ or Co2+ cations since for all the compositions the Ni2+ fully occupy M site while W6+ occupy M0 sites in AA0 MM0 WO6. Fig. 8 shows the observed tolerance factor (calculated from the distances obtained from the Rietveld refinements) as a function of

435,5 435 434,5 434 433,5 433 432,5

Wavenumbers (1/cm)

442

100

200

300

400

T (°C)

441,5

Fig. 13b. Raman modes of Ba0.4Sr1.6CoWO6 vs. temperature for x = 1.6, the transition from the tetragonal phase to the cubic phase shows considerable changes in temperature dependence of the modes.

441 440,5 440 439,5 439

0

100

200

300

400

Temperature (°C)

The site symmetry group analysis performed for the face-centered cubic Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) perovskite structure (with Fm-3m space group) leads to the following irreducible representation:

439,4 439,2 439 438,8

C ¼ A1g ðRÞ þ Eg ðRÞ þ 2F2g ðRÞ þ F1g ðsÞ þ 4F1u ðIRÞ þ F2u ðsÞ

438,6

þ F1u ðacÞ

438,4 438,2

the strontium amount in Ba2xSrxNiWO6 (Fig. 8a) and Ba2xSrxCoWO6 (Fig. 8b). The phase transition from cubic to tetragonal is well illustrated in the figures between the compositions 1.2 and 1.4 for both series. 3.2. Group theory analysis of structural Raman-active modes

439,6

Wavenumbers (1/cm)

0

0

50

100

150

200

250

300

Temperature (°C) Fig. 13a. Raman modes of Ba2xSrxNiWO6 vs. temperature for x = 1.4 (bottom) and x = 1.6 (up), the transition from the tetragonal phase to the cubic phase shows considerable changes in temperature dependence of the modes.

where the notation stands for: R—Raman-active modes, IR—infrared active modes, ac—acoustic modes, and s—silent modes. Among all of these modes predicted by the theory, only A1g, Eg and F2g are Raman-active modes. According to the factor group [24] analysis, nine Raman-active modes, represented as M = 3Ag + 3Bg + 3Eg, should be observed for the tetragonal compositions with I4/m space group. Only four Raman-active modes and should be observed for

13

5,6 5,5 5,4 5,3 5,2 5,1 5 4,9 4,8 4,7 4,6

I858/I133 (1/cm)

FWHM (1/cm)

B. Manoun et al. / Journal of Molecular Structure 1045 (2013) 1–14

5,8 5,3 4,8 4,3 3,8 3,3 2,8

0

100

200

300

400

Temperature (°C) 0

100

200

300

400

Temperature (°C)

4,3 3,9

10

3,7

I440/I130

FWHM (1/cm)

4,1 10,5 9,5 9 8,5

3,5 3,3 3,1

8

2,9

7,5

2,7

7

2,5

6,5

0

100

200

300

400

Temperature (°C) 1

Fig. 14. FWHM (cm ) for 133 cm1 (up) and 440 cm1 (bottom) modes of Ba0.4Sr1.6NiWO6 as a function of temperature, the change in the slope shows the phase transition occurrence.

the cubic compositions. Most of the bands are weak; there are only three strong bands and they are observed around 810 cm1, 430 cm1 and 125 cm1 for Ba2MWO6 (M = Ni, Co) and around 855 cm1, 440 cm1 and 135 cm1 for Sr2MWO6 (M = Ni, Co). In Fig. 9 we show the compatibility relations for the space groups present in the known temperature-induced phase-transition sequence: I4/m ? Fm-3m. Fig. 10 shows the Raman spectra of Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) recorded at ambient conditions. For both composition ranges (cubic and tetragonal), the observed Raman modes can be classified into three general families of lattice vibrations [25–29]: Ba2+/Sr2+ translations, as well as translational and rotational modes of the WO6 octahedra, at frequencies below 200 cm1; O–W–O bending vibrations, in the 200–500 cm1 region; and W–O stretching modes, at frequencies over 500 cm1. These frequency families were also observed in double perovskites, such as Ba2MWO6 [25] and in Pb2MgWO6 [26], Sr2MWO6 (M = Co, Ca, Ni, Mg) [27–29]. In Fig. 11 we plot the Raman modes as a function of the amount of strontium in the compositions, clear changes were observed in the curves showing the cubic to tetragonal phase transition between x = 1.2 and 1.4 and thus confirming the Rietveld refinements studies.

0

50

100

150

200

250

300

Temperature (°C) Fig. 15. Intensity ratio variation as a function of temperature, I440/I130 for x = 1.4 (bottom) and I858/I133 for x = 1.6 (up) in Ba2xSrxNiWO6.

3.3. Temperature study of Ba2xSrxNiWO6 (1.4 6 x 6 2) double perovskites Raman spectra of Ba2xSrxNiWO6 (1.4 6 x 6 2) were collected in situ at one atmosphere and elevated temperatures, up to 360 °C. The Raman spectra obtained at several temperatures are presented in Fig. 12. The temperature dependence of the stretching and bending modes are presented in Fig. 13. The strongest temperature changes in wavenumbers we observed are for modes recorded around 130, 440 and 856 cm1. All lattice modes show a monotonous change in wavenumbers while temperature is increased. The transition from the tetragonal phase to the cubic phase shows considerable changes in the temperature dependence of the modes observed around at 130 cm1, 440 cm1 and 856 cm1. The phase transition is observed at 300 °C for Sr2NiWO6 [27], 210 °C for Ba0.4Sr1.6NiWO6 and 120 °C for Ba0.6Sr1.4NiWO6. Since the band shapes are defined by the mode broadening parameters FWHM and in order to investigate the level of band broadening associated with the structural transition, we plot in Fig. 14 the FWHM as a function of temperature in the whole temperature range studied here. By this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes

CUBIC TETRAGONAL

Fig. 16a. Transition temperature as a function of the composition of strontium in Ba2xSrxNiWO6.

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Acknowledgments The authors are grateful to the Swedish Research Council and the Swedish International Development Co-operation Agency (Sida) for the financial grant (MENA) offered in support of this work. We acknowledge also support from STINT IG2010-2 062. This work was done partially under project numbers: UPV 0063.310-13564/2001-2007 and MAT2008-05839/MAT. Finally we would like to acknowledge University Hassan 1er and FSTS, Settat, Morocco, for their support. Fig. 16b. Transition temperature as a function of the composition of strontium in Ba2xSrxCoWO6.

in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached around the transition temperature, a change of the slope was observed. The same behavior was observed when plotting the intensity ratios as a function of temperature (see Fig. 15). The temperature phase transition as a function of the composition is plotted in Fig. 16, the extrapolation of the curve shows that the composition induced phase transition is between x = 1.2 and x = 1.4 and thus confirming our Raman and X-ray diffraction results studied above. The main difference between the high temperature structure and low temperature one is the rotation of the MO6 (M = Co, Ni) and WO6 octahedra around the tetragonal axis in the tetragonal phase. These distortions must occur due to the competing bonding preferences of the Ba/Sr and M site ions. At high temperatures both the expanded cell, and the greater thermal motion of the atoms, allow those to form a cubic cell, however, on cooling the increased bond strain drives the tetragonal distortion. 4. Conclusion In this paper, using X-ray diffraction and Raman spectroscopy techniques, we report on the composition and on the high temperature induced phase transition in Ba2xSrxMWO6 (M = Ni, Co and 0 6 x 6 2) double perovskite oxides. The cubic to tetragonal phase transition was observed as a function of composition while increasing strontium amount; both Rietveld refinements and Raman studies showed that this transition occurs between x = 1.2 and 1.4. Furthermore, increasing temperature for the tetragonal compositions (1.4 6 x 6 2), manifest the tetragonal to cubic phase transition. For this series, the transition from and to tetragonal (I4/ m)/cubic (Fm-3m) shows considerable changes in the composition and temperature dependence of the modes. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum and thus clear transitional effects were observed.

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