- Email: [email protected]
Structural variants in ABO3 type perovskite oxides. On the structure of BaPbO3
PERGAMON
Solid State Communications 119 (2001) 549±552
www.elsevier.com/locate/ssc
Structural variants in ABO3 type perovskite oxides. On the structure of BaPbO3 Sandra M. Moussa a, Brendan J. Kennedy a,*, Tom Vogt b b
a School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Department of Physics, Brookhaven National Laboratory, Building 510B, Upton, NY 11973-5000, USA
Received 12 December 2000; accepted 26 April 2001 by R.G. Clark; received in ®nal form by the Publisher 13 June 2001
Abstract The structure of BaPbO3 has been re®ned from powder neutron and synchrotron X-ray diffraction data at both room temperature and 15 K. At room temperature the structure is orthorhombic, space group Imma a 6:0299 1; b 8:5094 1; Ê . Rp 5.26, Rwp 6.48, x 2 2.45%. On cooling the structure transforms to a monoclinic form, space group C2/m, c 6:6094 1 A Ê b 134:806 1: Rp 5.30, Rwp 6.50, x 2 2.64%. q 2001 Elsevier Science Ltd. All a 8:5548 1 b 8:4808 1 c 6:0118 1 A rights reserved. PACS: 61.10; 61.12; 64.70 Keywords: A. Superconductors; D. Phase transitions
1. Introduction The structure and properties of stoichiometric and nonstoichiometric BaPbO3 have long been of interest as a consequence of the presence of superconductivity in phases in the Ba±Pb±Bi±O system. Like many ABO3-type oxides BaPbO3 adopts a distorted perovskite structure [1]. The earliest reports of the structure of BaPbO3 used X-ray diffraction methods and described it in the orthorhombic Pnma space group [2,3]. Later, using medium-resolution powder neutron diffraction data, Thornton and Jacobson con®rmed the structure at 4.2 K to be orthorhombic but placed it in the higher symmetry space group Imma [4]. Subsequently Marx et al. [5], Ritter et al. [6] and Ivanov et al. [7] contended that BaPbO3 is actually monoclinic at room temperature with space group I2/m. Ivanov et al. reported that the orthorhombic Imma phase exists at elevated temperatures [7]. The three proposed space groups differ in the nature of the tilting of the PbO6 octahedra and distinguishing these is often far from trivial when using powder diffraction data. As part of a wider study of the structural variants and * Corresponding author. Tel.: 161-2-9351-2742; fax: 161-29351-3329. E-mail address: [email protected] (B.J. Kennedy).
phase transitions in perovskite oxides [8±12] we have investigated the structure of BaPbO3 at both room temperature and low temperature using a combination of high resolution powder neutron and synchrotron X-ray diffraction data. At room temperature the structure is orthorhombic while upon cooling the symmetry is lowered to monoclinic. 2. Experimental A polycrystalline sample of BaPbO3 was prepared by the solid state reaction of BaCO3 (Aldrich) and PbO2 (Aldrich) at 7508C for one day and then at 9508C for ®ve days. X-ray powder measurements con®rmed the presence of the single phase perovskite. High-resolution synchrotron X-ray diffraction patterns were collected with a step size of 0.018 at X7A at the National Synchrotron Light Source, at Brookhaven National Laboratory, USA, making use of a linear position-sensitive detector [14]. The sample was housed in a 0.3 mm diameter capillary that was mounted in a closedcycle He cryostat for the low temperature measurements. The neutron powder diffraction patterns were recorded Ê in the range 10 , using neutrons of wavelength 1.494 A 2u , 1508 with a step size of 0.058 on the high resolution powder diffractometer (HRPD) [15] on high ¯ux Australian reactor (HIFAR) operated by the Australian
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00265-4
550
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
Table 1 Ê 2) for Atomic coordinates and atomic displacement parameters (A BaPbO3 at room temperature. Space group Imma a 6:0299 1; Ê . Rp 5.26, Rwp 6.48, x 2 2.45%. b 8:5094 1; c 6:0694 1 A Here and elsewhere the ®gures in parenthesis refer to the esd in the last ®gure Atom
X
Y
Z
Biso
Ba Pb O(1) O(2)
0 0 0 0.25
0.25 0 0.25 2 0.0269(2)
0.5012(6) 0 0.0497(3) 0.25
0.78(3) 0.43(2) 1.26(5) 1.46(3)
Table 2 Ê 2) for Atomic coordinates and atomic displacement parameters (A BaPbO3 at 15 K. Space group C2/m a 8:5548 1; b 8:4808 1; Ê , b 134:806 18: Rp 5.30, Rwp 6.50, x 2 2.64% c 6:0118 1 A Atom
X
Y
Z
Biso
Ba Pb O(1) O(2) O(3)
0.2532(5) 0.25 0 0 0.1926(3)
0 0.25 0.2850(5) 0.7235(5) 0
0.5022(12) 0 0.5 0 2 0.0624(8)
0.26(2) 0.26(2) 0.78(6) 0.58(5) 0.66(4)
Fig. 1. Observed, calculated and difference neutron powder diffraction pro®les for BaPbO3 at (a) rt, and (b) 15 K. The observed data are indicated by crosses and the calculated pro®le by the solid line. The short vertical lines below the pro®les mark the position of all possible Bragg re¯ections.
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
551
3. Discussion
Fig. 2. Portions of the synchrotron X-ray diffraction patterns for BaPbO3 at rt and 15 K. The format is the same as for Fig. 1.
Nuclear Science and Technology Organisation (ANSTO). For all the powder neutron diffraction measurements the sample was rotated about the vertical axis to reduce the effects of preferred orientation. The structure re®nement was undertaken with the computer program LHPM [9], which uses a full pro®le Rietveld analysis. The background was obtained by linear interpolation between 25 points. A Voigt peak shape function was employed, where the Gaussian component has widths given by the function FWHM2 U tan2 Q 1 V tanQ 1 W with re®nable parameters U, V and W. The widths of the Lorentzian component varied as h sec Q to model particle size effects and a peak asymmetry parameter was included. The re®nement was considered to have converged when all parameter shifts were less than 10% of their associated esd's in the last cycle. Final parameters are given in Table 1. Table 3 Ê ) for BaPbO3 in the orthorhombic rt Selected atomic distances (A structure and monoclinic low temperature structure 15 K
rt
Pb±O(1) Pb±O(2) Pb±O(3) Ba±O(1)
2.153(3) £ 2 2.1504(5) £ 2 2.1488(4) £ 2 2.796(5) £ 2 3.240(5) £ 2
2.1488(3) £ 2 2.1511(2) £ 4
Ba±O(2)
3.176(5) £ 2 2.846(5) £ 2 3.051(8) 3.005(8) 2.707(5) 3.363(6)
Ba±O(3)
2.740(4) 3.329(4) 3.0307(5) £ 2 3.186(2) £ 4 2.856(2) £ 4
Final atomic coordinates are given in Tables 1 and 2 and a plot of the observed, calculated and difference pro®les for the Rietveld re®nement from the neutron diffraction study is shown in Fig. 1. As originally reported BaPbO3 has an orthorhombic structure at room temperature and the observed synchrotron X-ray and neutron patterns are consistent with space group Imma. This space group is relatively rare for ABO3 perovskites, most orthorhombic perovskite structures, including CaTiO3 itself, are described in Pnma [1]. These two structures differing in the tilting of the BO6 octahedra, Imma having two equal out-of-phase tilts (a 0b 1b 1) whereas Pnma has both in-phase and out-of-phase tilts (a 2b 1b 1). While the ®t in Imma was better than that obtained in Pnma; (Rp 5.26, Rwp 6.48%, x 2 2.45 in Imma vs. 7.67, 10.30%, 6.20 in Pnma), the vital evidence for determining the correct space group came from examination of the room temperature neutron pro®le for BaPbO3. The neutron pro®le showed no evidence of re¯ections indicative of in-phase tilting, demonstrating Imma to be the correct space group. While it was possible to ®t the room temperature synchrotron and neutron diffraction data in the lower symmetry monoclinic space group none of the orthorhombic degenerate re¯ection pairs showed evidence for splitting and the re®ned b angle showed only marginal deviation from the orthorhombic value (134.85(1)8). The measures of ®t did not show any signi®cant improvement so we believe it is appropriate to use the higher symmetry space group. Although Marx et al. described the room temperature structure as monoclinic they also observed a monoclinic angle of close to 908 and stated that the space group may not be correct [5]. No additional re¯ections were observed in either the synchrotron X-ray or neutron patterns upon cooling the sample to 15 K, showing that only out-of-phase tilts were present. Nevertheless the quality of the ®t of the 15 K neutron diffraction pattern in Imma was noticeably lower than that obtained at room temperature, Rp 6.99, Rwp 8.91, x 2 5.00%, demonstrating the model to be incorrect. Examination of the, higher-resolution, synchrotron X-ray data revealed increased complexity in the peak shapes for a number of the stronger re¯ections. More importantly a number of the observed re¯ections that would not be split in orthorhombic symmetry were broader than expected suggesting they contained more than one re¯ection, Fig. 2. The synchrotron X-ray pattern clearly demonstrated that the sample has monoclinic symmetry. Considering possible lower symmetry space groups, that only have out-of-phase tilting of the BO6 groups, the most likely candidate is C2/m (an alternate setting of I2/m) [13]. The neutron data was then analysed in C2/m and the pro®les are shown in Fig. 1. This model also provides a satisfactory description of the synchrotron X-ray diffraction data. We note that although the re®ned monoclinic angle, b 134:806 18; represents only a small deviation from the orthorhombic structure
552
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
this results in signi®cant changes in the diffraction patterns. A continuous transition from Imma to C2/m upon cooling is also observed in PrAlO3, although in that case a Jahn±Teller transition involving the Pr(III) ion is believed to be important [16,17]. Despite the metallic nature of BaPbO3 the transition is probably related to the relative size of the cations rather than electronic factors. Since the Imma to C2/ m transition can be continuous and involves only very small changes in both the PbO6 and BaO12 polyhedra (Table 3) it would be very dif®cult to identify the precise transition temperature using diffraction methods. A structural study of the higher temperature transition to the tetragonal phase is likely to be of more interest [7]. The tetragonal I4/mcm structure is observed with increasing Bi content in BaPb12xBixO3 [5]. This is obtained from the Imma structure via a ®rst order transition, partially explaining the problems associated with obtaining single phase samples in this series [5,18]. It would be worthwhile to establish if the tetragonal and orthorhombic structures co-exist in BaPbO3 at elevated temperatures. Acknowledgements This work has been supported by the Australian Research Council, The Australian Institute of Nuclear Science and Engineering and at the National Synchrotron Light Source, Brookhaven National Laboratory, by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. The assistance of Dr B.A. Hunter (ANSTO) with collecting the neutron diffraction data and numerous discussions with Dr C.J. Howard (ANSTO) on perovskites is gratefully acknowledged.
References [1] P.M. Woodward, Acta Crystallogr. B53 (1997) 44. [2] R.D. Shannon, B.E. Bierstedt, J. Am. Ceram. Soc. 53 (1970) 635. [3] E.T. Shuvaeva, E.G. Fesenko, Kristallogra®ya 15 (1970) 379. [4] G. Thornton, A.J. Jacobson, Mater. Res. Bull. 11 (1976) 837. [5] D.T. Marx, P.G. Radaelli, J.D. Jorgensen, R.L. Hitterman, D.G. Hinks, S. Pei, B. Dabrowski, Phys. Rev. B 46 (1992) 11444. [6] J. Ritter, K.J. Ihringery, W. Maichle, A. Prandly, A. Hoser, Z. Hewat, Physik B 75 (1989) 297. [7] S.A. Ivanov, S.G. Eriksson, R. Tellgren, H. Rundlof, Abstract PA59ÐSeventh European Powder Diffraction Conference, J. Rius, X. Torrelles (Ed.), 2000. [8] B.J. Kennedy, B.A. Hunter, Phys. Rev. B 58 (1997) 653. [9] B.J. Kennedy, A.K. Prodjosantoso, C.J. Howard, J. Phys. C.: Condens. Matter 11 (1999) 6319. [10] C.J. Howard, B.J. Kennedy, B.C. Chakoumakos, J. Phys. C.: Condens. Matter 12 (2000) 349. [11] B.J. Kennedy, C.J. Howard, B.C. Chakoumakos, Phys. Rev. B 60 (1999) 2972. [12] C.J. Howard, K.S. Knight, E.H. Kisi, B.J. Kennedy, J. Phys.: Condens. Matter 12 (2000) L677. [13] C.J. Howard, H.T. Stokes, Acta Crystallogr. B54 (1998) 782. [14] G.C. Smith, Synchr. Rad. News 4 (1991) 24. [15] C.J. Howard, C.J. Ball, R.L. Davis, M.M. Elcombe, Aust. J. Phys. 36 (1983) 507. [16] R.J. Birgeneau, J.K. Kjems, G. Shirane, L.G. Van Uitert, Phys. Rev. B 10 (1974) 2512. [17] S. Moussa, B.J. Kennedy, B.A. Hunter, C.J. Howard, T. Vogt, J. Phys.: Condens. Matter 13 (2001) L203. [18] A.W. Sleight, D.E. Cox, Solid State Commun. 58 (1986) 347.
Solid State Communications 119 (2001) 549±552
www.elsevier.com/locate/ssc
Structural variants in ABO3 type perovskite oxides. On the structure of BaPbO3 Sandra M. Moussa a, Brendan J. Kennedy a,*, Tom Vogt b b
a School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Department of Physics, Brookhaven National Laboratory, Building 510B, Upton, NY 11973-5000, USA
Received 12 December 2000; accepted 26 April 2001 by R.G. Clark; received in ®nal form by the Publisher 13 June 2001
Abstract The structure of BaPbO3 has been re®ned from powder neutron and synchrotron X-ray diffraction data at both room temperature and 15 K. At room temperature the structure is orthorhombic, space group Imma a 6:0299 1; b 8:5094 1; Ê . Rp 5.26, Rwp 6.48, x 2 2.45%. On cooling the structure transforms to a monoclinic form, space group C2/m, c 6:6094 1 A Ê b 134:806 1: Rp 5.30, Rwp 6.50, x 2 2.64%. q 2001 Elsevier Science Ltd. All a 8:5548 1 b 8:4808 1 c 6:0118 1 A rights reserved. PACS: 61.10; 61.12; 64.70 Keywords: A. Superconductors; D. Phase transitions
1. Introduction The structure and properties of stoichiometric and nonstoichiometric BaPbO3 have long been of interest as a consequence of the presence of superconductivity in phases in the Ba±Pb±Bi±O system. Like many ABO3-type oxides BaPbO3 adopts a distorted perovskite structure [1]. The earliest reports of the structure of BaPbO3 used X-ray diffraction methods and described it in the orthorhombic Pnma space group [2,3]. Later, using medium-resolution powder neutron diffraction data, Thornton and Jacobson con®rmed the structure at 4.2 K to be orthorhombic but placed it in the higher symmetry space group Imma [4]. Subsequently Marx et al. [5], Ritter et al. [6] and Ivanov et al. [7] contended that BaPbO3 is actually monoclinic at room temperature with space group I2/m. Ivanov et al. reported that the orthorhombic Imma phase exists at elevated temperatures [7]. The three proposed space groups differ in the nature of the tilting of the PbO6 octahedra and distinguishing these is often far from trivial when using powder diffraction data. As part of a wider study of the structural variants and * Corresponding author. Tel.: 161-2-9351-2742; fax: 161-29351-3329. E-mail address: [email protected] (B.J. Kennedy).
phase transitions in perovskite oxides [8±12] we have investigated the structure of BaPbO3 at both room temperature and low temperature using a combination of high resolution powder neutron and synchrotron X-ray diffraction data. At room temperature the structure is orthorhombic while upon cooling the symmetry is lowered to monoclinic. 2. Experimental A polycrystalline sample of BaPbO3 was prepared by the solid state reaction of BaCO3 (Aldrich) and PbO2 (Aldrich) at 7508C for one day and then at 9508C for ®ve days. X-ray powder measurements con®rmed the presence of the single phase perovskite. High-resolution synchrotron X-ray diffraction patterns were collected with a step size of 0.018 at X7A at the National Synchrotron Light Source, at Brookhaven National Laboratory, USA, making use of a linear position-sensitive detector [14]. The sample was housed in a 0.3 mm diameter capillary that was mounted in a closedcycle He cryostat for the low temperature measurements. The neutron powder diffraction patterns were recorded Ê in the range 10 , using neutrons of wavelength 1.494 A 2u , 1508 with a step size of 0.058 on the high resolution powder diffractometer (HRPD) [15] on high ¯ux Australian reactor (HIFAR) operated by the Australian
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00265-4
550
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
Table 1 Ê 2) for Atomic coordinates and atomic displacement parameters (A BaPbO3 at room temperature. Space group Imma a 6:0299 1; Ê . Rp 5.26, Rwp 6.48, x 2 2.45%. b 8:5094 1; c 6:0694 1 A Here and elsewhere the ®gures in parenthesis refer to the esd in the last ®gure Atom
X
Y
Z
Biso
Ba Pb O(1) O(2)
0 0 0 0.25
0.25 0 0.25 2 0.0269(2)
0.5012(6) 0 0.0497(3) 0.25
0.78(3) 0.43(2) 1.26(5) 1.46(3)
Table 2 Ê 2) for Atomic coordinates and atomic displacement parameters (A BaPbO3 at 15 K. Space group C2/m a 8:5548 1; b 8:4808 1; Ê , b 134:806 18: Rp 5.30, Rwp 6.50, x 2 2.64% c 6:0118 1 A Atom
X
Y
Z
Biso
Ba Pb O(1) O(2) O(3)
0.2532(5) 0.25 0 0 0.1926(3)
0 0.25 0.2850(5) 0.7235(5) 0
0.5022(12) 0 0.5 0 2 0.0624(8)
0.26(2) 0.26(2) 0.78(6) 0.58(5) 0.66(4)
Fig. 1. Observed, calculated and difference neutron powder diffraction pro®les for BaPbO3 at (a) rt, and (b) 15 K. The observed data are indicated by crosses and the calculated pro®le by the solid line. The short vertical lines below the pro®les mark the position of all possible Bragg re¯ections.
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
551
3. Discussion
Fig. 2. Portions of the synchrotron X-ray diffraction patterns for BaPbO3 at rt and 15 K. The format is the same as for Fig. 1.
Nuclear Science and Technology Organisation (ANSTO). For all the powder neutron diffraction measurements the sample was rotated about the vertical axis to reduce the effects of preferred orientation. The structure re®nement was undertaken with the computer program LHPM [9], which uses a full pro®le Rietveld analysis. The background was obtained by linear interpolation between 25 points. A Voigt peak shape function was employed, where the Gaussian component has widths given by the function FWHM2 U tan2 Q 1 V tanQ 1 W with re®nable parameters U, V and W. The widths of the Lorentzian component varied as h sec Q to model particle size effects and a peak asymmetry parameter was included. The re®nement was considered to have converged when all parameter shifts were less than 10% of their associated esd's in the last cycle. Final parameters are given in Table 1. Table 3 Ê ) for BaPbO3 in the orthorhombic rt Selected atomic distances (A structure and monoclinic low temperature structure 15 K
rt
Pb±O(1) Pb±O(2) Pb±O(3) Ba±O(1)
2.153(3) £ 2 2.1504(5) £ 2 2.1488(4) £ 2 2.796(5) £ 2 3.240(5) £ 2
2.1488(3) £ 2 2.1511(2) £ 4
Ba±O(2)
3.176(5) £ 2 2.846(5) £ 2 3.051(8) 3.005(8) 2.707(5) 3.363(6)
Ba±O(3)
2.740(4) 3.329(4) 3.0307(5) £ 2 3.186(2) £ 4 2.856(2) £ 4
Final atomic coordinates are given in Tables 1 and 2 and a plot of the observed, calculated and difference pro®les for the Rietveld re®nement from the neutron diffraction study is shown in Fig. 1. As originally reported BaPbO3 has an orthorhombic structure at room temperature and the observed synchrotron X-ray and neutron patterns are consistent with space group Imma. This space group is relatively rare for ABO3 perovskites, most orthorhombic perovskite structures, including CaTiO3 itself, are described in Pnma [1]. These two structures differing in the tilting of the BO6 octahedra, Imma having two equal out-of-phase tilts (a 0b 1b 1) whereas Pnma has both in-phase and out-of-phase tilts (a 2b 1b 1). While the ®t in Imma was better than that obtained in Pnma; (Rp 5.26, Rwp 6.48%, x 2 2.45 in Imma vs. 7.67, 10.30%, 6.20 in Pnma), the vital evidence for determining the correct space group came from examination of the room temperature neutron pro®le for BaPbO3. The neutron pro®le showed no evidence of re¯ections indicative of in-phase tilting, demonstrating Imma to be the correct space group. While it was possible to ®t the room temperature synchrotron and neutron diffraction data in the lower symmetry monoclinic space group none of the orthorhombic degenerate re¯ection pairs showed evidence for splitting and the re®ned b angle showed only marginal deviation from the orthorhombic value (134.85(1)8). The measures of ®t did not show any signi®cant improvement so we believe it is appropriate to use the higher symmetry space group. Although Marx et al. described the room temperature structure as monoclinic they also observed a monoclinic angle of close to 908 and stated that the space group may not be correct [5]. No additional re¯ections were observed in either the synchrotron X-ray or neutron patterns upon cooling the sample to 15 K, showing that only out-of-phase tilts were present. Nevertheless the quality of the ®t of the 15 K neutron diffraction pattern in Imma was noticeably lower than that obtained at room temperature, Rp 6.99, Rwp 8.91, x 2 5.00%, demonstrating the model to be incorrect. Examination of the, higher-resolution, synchrotron X-ray data revealed increased complexity in the peak shapes for a number of the stronger re¯ections. More importantly a number of the observed re¯ections that would not be split in orthorhombic symmetry were broader than expected suggesting they contained more than one re¯ection, Fig. 2. The synchrotron X-ray pattern clearly demonstrated that the sample has monoclinic symmetry. Considering possible lower symmetry space groups, that only have out-of-phase tilting of the BO6 groups, the most likely candidate is C2/m (an alternate setting of I2/m) [13]. The neutron data was then analysed in C2/m and the pro®les are shown in Fig. 1. This model also provides a satisfactory description of the synchrotron X-ray diffraction data. We note that although the re®ned monoclinic angle, b 134:806 18; represents only a small deviation from the orthorhombic structure
552
S.M. Moussa et al. / Solid State Communications 119 (2001) 549±552
this results in signi®cant changes in the diffraction patterns. A continuous transition from Imma to C2/m upon cooling is also observed in PrAlO3, although in that case a Jahn±Teller transition involving the Pr(III) ion is believed to be important [16,17]. Despite the metallic nature of BaPbO3 the transition is probably related to the relative size of the cations rather than electronic factors. Since the Imma to C2/ m transition can be continuous and involves only very small changes in both the PbO6 and BaO12 polyhedra (Table 3) it would be very dif®cult to identify the precise transition temperature using diffraction methods. A structural study of the higher temperature transition to the tetragonal phase is likely to be of more interest [7]. The tetragonal I4/mcm structure is observed with increasing Bi content in BaPb12xBixO3 [5]. This is obtained from the Imma structure via a ®rst order transition, partially explaining the problems associated with obtaining single phase samples in this series [5,18]. It would be worthwhile to establish if the tetragonal and orthorhombic structures co-exist in BaPbO3 at elevated temperatures. Acknowledgements This work has been supported by the Australian Research Council, The Australian Institute of Nuclear Science and Engineering and at the National Synchrotron Light Source, Brookhaven National Laboratory, by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. The assistance of Dr B.A. Hunter (ANSTO) with collecting the neutron diffraction data and numerous discussions with Dr C.J. Howard (ANSTO) on perovskites is gratefully acknowledged.
References [1] P.M. Woodward, Acta Crystallogr. B53 (1997) 44. [2] R.D. Shannon, B.E. Bierstedt, J. Am. Ceram. Soc. 53 (1970) 635. [3] E.T. Shuvaeva, E.G. Fesenko, Kristallogra®ya 15 (1970) 379. [4] G. Thornton, A.J. Jacobson, Mater. Res. Bull. 11 (1976) 837. [5] D.T. Marx, P.G. Radaelli, J.D. Jorgensen, R.L. Hitterman, D.G. Hinks, S. Pei, B. Dabrowski, Phys. Rev. B 46 (1992) 11444. [6] J. Ritter, K.J. Ihringery, W. Maichle, A. Prandly, A. Hoser, Z. Hewat, Physik B 75 (1989) 297. [7] S.A. Ivanov, S.G. Eriksson, R. Tellgren, H. Rundlof, Abstract PA59ÐSeventh European Powder Diffraction Conference, J. Rius, X. Torrelles (Ed.), 2000. [8] B.J. Kennedy, B.A. Hunter, Phys. Rev. B 58 (1997) 653. [9] B.J. Kennedy, A.K. Prodjosantoso, C.J. Howard, J. Phys. C.: Condens. Matter 11 (1999) 6319. [10] C.J. Howard, B.J. Kennedy, B.C. Chakoumakos, J. Phys. C.: Condens. Matter 12 (2000) 349. [11] B.J. Kennedy, C.J. Howard, B.C. Chakoumakos, Phys. Rev. B 60 (1999) 2972. [12] C.J. Howard, K.S. Knight, E.H. Kisi, B.J. Kennedy, J. Phys.: Condens. Matter 12 (2000) L677. [13] C.J. Howard, H.T. Stokes, Acta Crystallogr. B54 (1998) 782. [14] G.C. Smith, Synchr. Rad. News 4 (1991) 24. [15] C.J. Howard, C.J. Ball, R.L. Davis, M.M. Elcombe, Aust. J. Phys. 36 (1983) 507. [16] R.J. Birgeneau, J.K. Kjems, G. Shirane, L.G. Van Uitert, Phys. Rev. B 10 (1974) 2512. [17] S. Moussa, B.J. Kennedy, B.A. Hunter, C.J. Howard, T. Vogt, J. Phys.: Condens. Matter 13 (2001) L203. [18] A.W. Sleight, D.E. Cox, Solid State Commun. 58 (1986) 347.