- Email: [email protected]
Oxygen order–disorder phase transition in PrBaCo2O5.48 at high temperature
ARTICLE IN PRESS
Physica B 378–380 (2006) 539–540 www.elsevier.com/locate/physb
Oxygen order–disorder phase transition in PrBaCo2O5.48 at high temperature S. Streulea,, A. Podlesnyaka, E. Pomjakushinaa,b, K. Condera,b, D. Sheptyakova, M. Medardea,b, J. Mesota a
Laboratory for Neutron Scattering, ETHZ & PSI, 5232 Villigen PSI, Switzerland Laboratory for Developments and Methods, PSI, 5232 Villigen PSI, Switzerland
b
Abstract We have investigated the PrBaCo2O5.48 compound by means of neutron powder diffraction at temperatures 300 KoTo820 K. We observe a new structural phase transition at T OD ¼ 776 K, which we associate with an oxygen order–disorder transition: the well-known room temperature ordered crystal structure, in which slabs of CoO6 octahedra and CoO5 pyramids interleave (Pmmm symmetry) gets lost at temperatures T4T OD , resulting in a statistical distribution of octahedra and pyramids in the sample. The new phase can be described by the tetragonal P4=mmm space group. The transition is caused by displacement of apical oxygen ions and is an indication that ionic conductivity, which has been observed in 3D cobaltites, may also exist in layered cobaltites. r 2006 Elsevier B.V. All rights reserved. PACS: 75.60.+g; 61.12.q Keywords: Cobaltites; Neutron scattering; Structural phase transition; Ionic conductivity
The oxygen-deficient perovskite-related 112-type RBaCo2 O5þd materials have recently attracted much attention due to their extraordinary magnetic and transport properties. In particular, compounds with d0:5 exhibit a metal–insulator (MI) transition around room temperature, successive para- to ferro/ferri- to antiferromagnetic transitions as well as giant magnetoresistance effect [1–4]. Both the nature of these effects and the spinstate of the cobalt ions are still under debate. Beside these effects, the cobaltites are, at high temperatures, interesting for their large ionic conduction, thus showing potential for applications as gas sensors, oxidation catalysts, or electrode materials for fuel cells [5]. It has been reported that oxygen order–disorder phenomena play an important role for the ionic conductivity in these materials [5,6]. It seems that oxygen ordering restrains oxygen mobility, since too much energy is needed for an oxygen Corresponding author. Tel.: +41 56 310 4192; fax: +41 56 310 2939.
E-mail address: [email protected] (S. Streule). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.244
ion to move from an occupied site to a structural vacancy. It has also been reported that the distance between two vacancies in the ordered state is larger than for the disordered state [7]. This motivated us to investigate the structural properties of PrBaCo2O5.48 at high temperatures. The PrBaCo2 O5þx sample was synthesized by a conventional solid state reaction as previously reported [8]. Phase purity was checked with a conventional x-ray powder diffractometer (CuKa radiation). Differential Scanning Calorimetry (DSC) (NETZSCH DSC 204 F1) was used to detect possible phase transitions. The neutron powder diffraction (NPD) measurements were carried out on the high resolution diffractometer HRPT at SINQ (PSI, Switzerland) in the angular range 4 oYo165 and the temperature range 300 KoTo825 K, using step sizes ˚ between 20 and 25 K. The wavelength was l ¼ 1:886 A. Our NPD data were refined using the program Fullprof [9], resulting in refined oxygen contents which lie within 1.3% of the value obtained by the iodometry method ð5:48 0:01Þ [10].
ARTICLE IN PRESS S. Streule et al. / Physica B 378–380 (2006) 539–540
540
Our DSC measurements revealed a first transition at a temperature of T MI ¼ 343 K, which is commonly associated with a MI transition [2–4]. The analysis of our neutron diffraction measurements shows a jump of the lattice parameters (Fig. 1a) with increasing temperature and an associated sudden increase of the unit cell volume, which cannot be explained by thermal expansion of the
(a)
Lattice Param. [Å]
3.95 b/2 atet = btet
3.90 a 3.85
ctet/2
c/2
3.80 (b)
Occupancy
1.0
O3 0.5
O4
400
600 Temp [K]
800
Fig. 1. Temperature evolution of (a) the lattice parameters and (b) the occupancy of the oxygen ion sites O3 and O4. The vertical lines mark the position of T MI and T OD as determined by DSC.
795K
2000
820K
Neutron Counts [a.u.]
1000
This work was performed at the spallation neutron source SINQ of the Paul Scherrer Institute, Switzerland. The authors thank the Swiss National Science Foundation for financial support through Grants SCOPES, Marie Heim-Voegtlin No. PMPD2–102504 and 200021-100194, as well as NCCR MaNEP project.
770K
1500
750K 500
1000 104
lattice. A plausible explanation for this unusual jump in unit cell volume across a MI transition is given by Frontera et al. [4], for a GdBaCo2O5.5 powder investigated by ultrahigh resolution synchrotron radiation: the observed expansion of the octahedra is caused by a spin-state transition from the low spin to the high spin state of the Co spin, which leads to the metallic behavior of the system. The DSC measurement revealed a second transition at T OD ¼ 776 K. Comparing NPD patterns taken below and above (Fig. 2) T OD one notices a merging of peaks (inset Fig. 2), for which the indices (indexed in the orthorhombic Pmmm space group) have the property 2h ¼ k. As can be seen in Fig. 1a, the values of the lattice parameters a and b=2 converge with increasing temperature, indicating that the symmetry evolves from orthorhombic to tetragonal. The data above T OD is perfectly refined using the tetragonal spacegroup P4=mmm (unit cell ap ap 2ap , with ap the lattice parameter of the cubic unit cell). Additionally, we observe a redistribution of oxygen: the oxygen site O3 loses, while the O4 site acquires oxygen (both sites are located in the Pr ion plane), until the two sites display an equivalent occupancy, see Fig. 1b (site notation according to the Pmmm space group). This redistribution of oxygen is the reason for the lower symmetry of the high temperature structure since the CoO5 pyramids and CoO6 octahedra are no longer ordered along the b direction. No change of the occupation of the planar and the other apical oxygen ions could be observed through T OD . In conclusion, we have performed NPD measurements in the temperature range 300 KpTp820 K and found two transitions: the well known MI transition at T MI ¼ 343 K and a new transition which we associate with a change from an ordered to a disordered state at T OD ¼ 776 K. The transition at T OD is a purely structural one, which leads to a disorder of the interleaving planes of CoO5 pyramids and CoO6 octahedra. The filling of empty oxygen sites most likely leads to ionic conductivity, which still remains to be confirmed.
105
106
500
References 0
0
50
100
150
2Θ [°] Fig. 2. Observed (circles), calculated and difference NPD pattern for PrBaCo2O5.49 at 820 K. Five peaks from the steel container have been excluded, since they are extremely strong and lead to large fluctuations of the difference plot. The inset shows the merging of the (1 6 2) and (3 2 2) peaks (indexed in the orthorhombic notation) on crossing T OD .
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C. Martin, et al., Appl. Phys. Lett. 71 (1997) 1421. A. Maignan, et al., Phys. Rev. Lett. 93 (2004) 026401. F. Fauth, et al., Eur. Phys. J. B 21 (2001) 163. C. Frontera, et al., Phys. Rev. B 65 (2002) R180405. Y. Teraoka, et al., Solid State Ionics 48 (1991) 207. M.V. Patrakeev, et al., Ionics 5 (1999) 444. M. Cherry, et al., J. Solid State Chem. 118 (1995) 125. S. Streule, et al., J. Phys.: Condens. Matter 17 (2005) 3317. J. Rodrı´ guez-Carvajal, Physica B 192 (1993) 55. K. Conder, et al., Mater. Res. Bull. 40 (2005) 257.
Physica B 378–380 (2006) 539–540 www.elsevier.com/locate/physb
Oxygen order–disorder phase transition in PrBaCo2O5.48 at high temperature S. Streulea,, A. Podlesnyaka, E. Pomjakushinaa,b, K. Condera,b, D. Sheptyakova, M. Medardea,b, J. Mesota a
Laboratory for Neutron Scattering, ETHZ & PSI, 5232 Villigen PSI, Switzerland Laboratory for Developments and Methods, PSI, 5232 Villigen PSI, Switzerland
b
Abstract We have investigated the PrBaCo2O5.48 compound by means of neutron powder diffraction at temperatures 300 KoTo820 K. We observe a new structural phase transition at T OD ¼ 776 K, which we associate with an oxygen order–disorder transition: the well-known room temperature ordered crystal structure, in which slabs of CoO6 octahedra and CoO5 pyramids interleave (Pmmm symmetry) gets lost at temperatures T4T OD , resulting in a statistical distribution of octahedra and pyramids in the sample. The new phase can be described by the tetragonal P4=mmm space group. The transition is caused by displacement of apical oxygen ions and is an indication that ionic conductivity, which has been observed in 3D cobaltites, may also exist in layered cobaltites. r 2006 Elsevier B.V. All rights reserved. PACS: 75.60.+g; 61.12.q Keywords: Cobaltites; Neutron scattering; Structural phase transition; Ionic conductivity
The oxygen-deficient perovskite-related 112-type RBaCo2 O5þd materials have recently attracted much attention due to their extraordinary magnetic and transport properties. In particular, compounds with d0:5 exhibit a metal–insulator (MI) transition around room temperature, successive para- to ferro/ferri- to antiferromagnetic transitions as well as giant magnetoresistance effect [1–4]. Both the nature of these effects and the spinstate of the cobalt ions are still under debate. Beside these effects, the cobaltites are, at high temperatures, interesting for their large ionic conduction, thus showing potential for applications as gas sensors, oxidation catalysts, or electrode materials for fuel cells [5]. It has been reported that oxygen order–disorder phenomena play an important role for the ionic conductivity in these materials [5,6]. It seems that oxygen ordering restrains oxygen mobility, since too much energy is needed for an oxygen Corresponding author. Tel.: +41 56 310 4192; fax: +41 56 310 2939.
E-mail address: [email protected] (S. Streule). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.244
ion to move from an occupied site to a structural vacancy. It has also been reported that the distance between two vacancies in the ordered state is larger than for the disordered state [7]. This motivated us to investigate the structural properties of PrBaCo2O5.48 at high temperatures. The PrBaCo2 O5þx sample was synthesized by a conventional solid state reaction as previously reported [8]. Phase purity was checked with a conventional x-ray powder diffractometer (CuKa radiation). Differential Scanning Calorimetry (DSC) (NETZSCH DSC 204 F1) was used to detect possible phase transitions. The neutron powder diffraction (NPD) measurements were carried out on the high resolution diffractometer HRPT at SINQ (PSI, Switzerland) in the angular range 4 oYo165 and the temperature range 300 KoTo825 K, using step sizes ˚ between 20 and 25 K. The wavelength was l ¼ 1:886 A. Our NPD data were refined using the program Fullprof [9], resulting in refined oxygen contents which lie within 1.3% of the value obtained by the iodometry method ð5:48 0:01Þ [10].
ARTICLE IN PRESS S. Streule et al. / Physica B 378–380 (2006) 539–540
540
Our DSC measurements revealed a first transition at a temperature of T MI ¼ 343 K, which is commonly associated with a MI transition [2–4]. The analysis of our neutron diffraction measurements shows a jump of the lattice parameters (Fig. 1a) with increasing temperature and an associated sudden increase of the unit cell volume, which cannot be explained by thermal expansion of the
(a)
Lattice Param. [Å]
3.95 b/2 atet = btet
3.90 a 3.85
ctet/2
c/2
3.80 (b)
Occupancy
1.0
O3 0.5
O4
400
600 Temp [K]
800
Fig. 1. Temperature evolution of (a) the lattice parameters and (b) the occupancy of the oxygen ion sites O3 and O4. The vertical lines mark the position of T MI and T OD as determined by DSC.
795K
2000
820K
Neutron Counts [a.u.]
1000
This work was performed at the spallation neutron source SINQ of the Paul Scherrer Institute, Switzerland. The authors thank the Swiss National Science Foundation for financial support through Grants SCOPES, Marie Heim-Voegtlin No. PMPD2–102504 and 200021-100194, as well as NCCR MaNEP project.
770K
1500
750K 500
1000 104
lattice. A plausible explanation for this unusual jump in unit cell volume across a MI transition is given by Frontera et al. [4], for a GdBaCo2O5.5 powder investigated by ultrahigh resolution synchrotron radiation: the observed expansion of the octahedra is caused by a spin-state transition from the low spin to the high spin state of the Co spin, which leads to the metallic behavior of the system. The DSC measurement revealed a second transition at T OD ¼ 776 K. Comparing NPD patterns taken below and above (Fig. 2) T OD one notices a merging of peaks (inset Fig. 2), for which the indices (indexed in the orthorhombic Pmmm space group) have the property 2h ¼ k. As can be seen in Fig. 1a, the values of the lattice parameters a and b=2 converge with increasing temperature, indicating that the symmetry evolves from orthorhombic to tetragonal. The data above T OD is perfectly refined using the tetragonal spacegroup P4=mmm (unit cell ap ap 2ap , with ap the lattice parameter of the cubic unit cell). Additionally, we observe a redistribution of oxygen: the oxygen site O3 loses, while the O4 site acquires oxygen (both sites are located in the Pr ion plane), until the two sites display an equivalent occupancy, see Fig. 1b (site notation according to the Pmmm space group). This redistribution of oxygen is the reason for the lower symmetry of the high temperature structure since the CoO5 pyramids and CoO6 octahedra are no longer ordered along the b direction. No change of the occupation of the planar and the other apical oxygen ions could be observed through T OD . In conclusion, we have performed NPD measurements in the temperature range 300 KpTp820 K and found two transitions: the well known MI transition at T MI ¼ 343 K and a new transition which we associate with a change from an ordered to a disordered state at T OD ¼ 776 K. The transition at T OD is a purely structural one, which leads to a disorder of the interleaving planes of CoO5 pyramids and CoO6 octahedra. The filling of empty oxygen sites most likely leads to ionic conductivity, which still remains to be confirmed.
105
106
500
References 0
0
50
100
150
2Θ [°] Fig. 2. Observed (circles), calculated and difference NPD pattern for PrBaCo2O5.49 at 820 K. Five peaks from the steel container have been excluded, since they are extremely strong and lead to large fluctuations of the difference plot. The inset shows the merging of the (1 6 2) and (3 2 2) peaks (indexed in the orthorhombic notation) on crossing T OD .
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C. Martin, et al., Appl. Phys. Lett. 71 (1997) 1421. A. Maignan, et al., Phys. Rev. Lett. 93 (2004) 026401. F. Fauth, et al., Eur. Phys. J. B 21 (2001) 163. C. Frontera, et al., Phys. Rev. B 65 (2002) R180405. Y. Teraoka, et al., Solid State Ionics 48 (1991) 207. M.V. Patrakeev, et al., Ionics 5 (1999) 444. M. Cherry, et al., J. Solid State Chem. 118 (1995) 125. S. Streule, et al., J. Phys.: Condens. Matter 17 (2005) 3317. J. Rodrı´ guez-Carvajal, Physica B 192 (1993) 55. K. Conder, et al., Mater. Res. Bull. 40 (2005) 257.