- Email: [email protected]
Synthesis and structural characterisation of quintupled layered perovskites
~ ~
PHYSICA ©
~
Physica C 282-287 (1997) 751-752
CLSEVIER
Synthesis and Structural Characterisation of Quintupled Layered Perovskites M.J.Pack and M.T.Weller Department of Chemistry, University of Southampton, Highfield, Southampton, Hampshire SO 17 IBJ, U.K. The synthesis and characterisation of three members of the ordered quintupled copper titanium perovskite series Ln2Ba2CaCu2 Ti30 14 , Ln=Nd,Gd,Dy, are reported. Structural characterisation in the space group P4/mmm shows full ordering of the cations for Ln = Gd with the B type cations stacked in the order Cu-Ti-Ti-Ti-Cu. For Ln = Nd partial disorder on both the A and B cation sub lattices occurs and oxygen is incorporated between the CU02 layers; for Ln = Dy the structure distorts slightly to an orthorhombic cell. The in-plane Cu-O distances near 1.94A indicate that control of the copper oxidation state may induce superconductivity in this system and preliminary results of doping studies are reported.
1. INTRODUCTION The quadruple perovskite structure was first 1 reported for La2Ba2Cu2Sn2011 where the B-type cations forming the perovskite blocks are ordered in layers as [Cu-Sn-Sn-Cu]. Replacement of tin with the smaller titanium ion gave a phase of 2 with shorter stoichiometry Gd2Ba2Cu2 Ti 20 11 copper oxygen distances of 1.96A. New layered cuprate oxides containing Ti(IV) and Cu(II) can be prepared by addition of a CaTi0 3 perovskite layer into the Ln2Ba2Cu2 Ti 20 li phase as first reported by W.J.Zhu et aP in Sm 2Ba2CaCu ZTi 30 14 . The structure can be viewed as the intergrowth of two layers formed from copper-oxygen square pyramids and three titanium based perovskite layers. This paper reports the introduction of three different A-type lanthanide cations into the quintupled perovskite structure and the results of structural work which show variations in the structure which occur as a function of lanthanide. Initial doping studies involving partial replacement of titanium by copper in these phases are also reported. 2. EXPERIMENTAL Stoichiometric amounts of Ln203, BaC0 3, CaC03, CuO and TiO z were thoroughly mixed, ground and initially heated at 950°C in order to decompose the carbonates. The resulting mixture of oxides was then pelletised and heated at 1025°C for 48hrs with intermediate regrinding. The phase 0921-4534/97/$17.00 © Elsevier 8cience PH 80921-4534(97)00390-0
s.Y.
All rights reserved.
obtained was further oxygen annealed at 1000°C for 48 hrs and furnace cooled to room temperature to ensure full oxygen occupancy. Phases synthesised and derived lattice parameters from full Rietveld refinement profile are summarised in Table 1. Table 1 Summary of synthesised phases and derived lattice parameters from x-ray data (CuKal radiation) c (A) Stoichiometry a (A) Nd2Ba2CaCu2 Ti 30 l4 GdzBazCaCuz Ti 30 ,4 DyzBazCaCu2Ti3014 NdzBazCaCuz,o Tiz900,4 Nd2BazCaCuz.'6 Tiz.840,4 Nd2Ba2CaCuz.zz Ti z78 0 ,4 Nd2BazCaCuz29 Ti2710,4 Gd2Ba2CaCuz.16 Ti2.84014 Gd2Ba2CaCu2.22 Ti v8 0 14 Gd2Ba2CaCu2.29 Ti v PI4 Gd2Ba2CaCu2.35 Ti2650,4 Gd2BazCaCuZ.42 Tiz.580,4
3.8997(2) 3.8776(1) a = 5.4794(3) b = 5.4596(3) 3.8950(1) 3.8946(1) 3.8945(1) 3.8946(1) 3.8740(4) 3.8759(1) 3.8751(1) 3.8775(8) 3.8784(6)
19.594(1) 19.565(1) 19.543(1) 19.635(1) 19.643(1) 19.637(1) 19.641(1) 19.552(3) 19.557(2) 19.550(1) 19.549(5) 19.560(4)
of NdzBa2CaCu2 Ti 30 14 The structures Gd2Ba2CaCuz Ti 30 ,4 and DY2Ba2CaCuZ Ti 30 l4 have also been refined from powder neutron diffraction data and full structural results will be presented 4 elsewhere . The structures of some of the doped materials were refined from powder x-ray diffraction data in the space group P4/mmm.
752
M.J. Pack, M. T. Weller / Physica C 282-287 (1997) 751-752
3. RESULTS The structure of Gd2Ba2CaCu2 Ti 30 14 is shown in Figure I. The B type cations are perfectly ordered along the c axis with double layers of CU02 separated by triple layers of Ti0 6 octahedra. For all compositions the central Ti0 6 octahedron is distorted and/or tilted, possibly originating from the mismatch in sizes of the Cu-O bond length (~1. 96A in CU02 square planes) and Ti-O distances (typically 2.02A in Ti0 6 octahedra). The degree of ordering of the A cations is observed to increase with decreasing Ln size being perfect for gadolinium. In this phase the distribution of oxygen in consistent with layers formed from CuOs square pyramids. In Nd 2Ba2CaCu2Ti 30 14 extensive disordering occurs on both A and B cations sub lattices and there is evidence for the incorporation of oxygen between the CuOs pyramids. In DY2Ba2CaCu2Ti 30 14 the extent of Ti0 6 distortions is such that the structure crystallises in the orthorhombic space group Cmmm probably allowing better co-ordination between oxygen and the A type cations. For the doped phases results show a decrease in the a and an increase in the c lattice parameters for the neodymium Cu/Ti doped system with small levels of additional copper. However, for higher copper levels the lattice parameters remain unchanged. This suggests only small amounts of additional copper may be incorporated in this phase and this is confirmed by the presence of minor amounts of an additional phase in the products where x>0.15. However, the unchanging lattice parameters in the Gd2Ba2CaCu2+xTi 3_x0 14 system as a function of x implies that copper cannot substitute on to the Ti site in this phase. The differing behaviours in the Nd and Gd systems accords with the structures of the parent phases where the distribution of Cu and Ti is imperfect for Nd phase but for the Gd derivative the two sites are distinct. 4. CONCLUSIONS Nd, Gd and Dy have been introduced into the ordered quintupled perovskite series Ln 2Ba2CaCu2Ti 30 14 and copper/titanium doping studies carried out on the Nd and Gd systems. In the
Figure I. Polyhedral representation of the idealised structure of Ln2Ba2CaCu2 Ti 30 14 showing the regular Ti0 6 octahedra and CU02 square pyramids. undoped phases there is little change in the planarity of the Cu02 structural feature but the Cu-O bond distance is observed to decrease as the lanthanide ionic radii falls. For Ln=Dy the Cu-O distance is 1.936A similar to that found in many cuprate superconductors. However initial attempts to increase the copper oxidation state to that found in superconductors, through doping of copper on to the titanium site, have been unsuccessful except where disorder on these sites already exists (Ln=Nd). ACKNOWLEDGEMENTS We thank the EPSRC for grants in support of this work and a studentship for MJP. REFERENCES 1. M.T.Anderson, K.R.Poeppelmeier, J.P.Zhang, HJ.Fan and L.D.Marks, Chern. Mater., 4 (1992) 1305. 2. A.Gormezano and M.T.Weller, Chern. Mater., 3(7) (1993) 771. 3. WJ.Zhu, Y.Z.Huang, T.S.Ning and Z.X.Zhao, Mat. Res. Bull., 30(2) (1995) 243. 4. MJ. Pack and M.T.Weller, Submitted to Chern. Mater., (1996).
PHYSICA ©
~
Physica C 282-287 (1997) 751-752
CLSEVIER
Synthesis and Structural Characterisation of Quintupled Layered Perovskites M.J.Pack and M.T.Weller Department of Chemistry, University of Southampton, Highfield, Southampton, Hampshire SO 17 IBJ, U.K. The synthesis and characterisation of three members of the ordered quintupled copper titanium perovskite series Ln2Ba2CaCu2 Ti30 14 , Ln=Nd,Gd,Dy, are reported. Structural characterisation in the space group P4/mmm shows full ordering of the cations for Ln = Gd with the B type cations stacked in the order Cu-Ti-Ti-Ti-Cu. For Ln = Nd partial disorder on both the A and B cation sub lattices occurs and oxygen is incorporated between the CU02 layers; for Ln = Dy the structure distorts slightly to an orthorhombic cell. The in-plane Cu-O distances near 1.94A indicate that control of the copper oxidation state may induce superconductivity in this system and preliminary results of doping studies are reported.
1. INTRODUCTION The quadruple perovskite structure was first 1 reported for La2Ba2Cu2Sn2011 where the B-type cations forming the perovskite blocks are ordered in layers as [Cu-Sn-Sn-Cu]. Replacement of tin with the smaller titanium ion gave a phase of 2 with shorter stoichiometry Gd2Ba2Cu2 Ti 20 11 copper oxygen distances of 1.96A. New layered cuprate oxides containing Ti(IV) and Cu(II) can be prepared by addition of a CaTi0 3 perovskite layer into the Ln2Ba2Cu2 Ti 20 li phase as first reported by W.J.Zhu et aP in Sm 2Ba2CaCu ZTi 30 14 . The structure can be viewed as the intergrowth of two layers formed from copper-oxygen square pyramids and three titanium based perovskite layers. This paper reports the introduction of three different A-type lanthanide cations into the quintupled perovskite structure and the results of structural work which show variations in the structure which occur as a function of lanthanide. Initial doping studies involving partial replacement of titanium by copper in these phases are also reported. 2. EXPERIMENTAL Stoichiometric amounts of Ln203, BaC0 3, CaC03, CuO and TiO z were thoroughly mixed, ground and initially heated at 950°C in order to decompose the carbonates. The resulting mixture of oxides was then pelletised and heated at 1025°C for 48hrs with intermediate regrinding. The phase 0921-4534/97/$17.00 © Elsevier 8cience PH 80921-4534(97)00390-0
s.Y.
All rights reserved.
obtained was further oxygen annealed at 1000°C for 48 hrs and furnace cooled to room temperature to ensure full oxygen occupancy. Phases synthesised and derived lattice parameters from full Rietveld refinement profile are summarised in Table 1. Table 1 Summary of synthesised phases and derived lattice parameters from x-ray data (CuKal radiation) c (A) Stoichiometry a (A) Nd2Ba2CaCu2 Ti 30 l4 GdzBazCaCuz Ti 30 ,4 DyzBazCaCu2Ti3014 NdzBazCaCuz,o Tiz900,4 Nd2BazCaCuz.'6 Tiz.840,4 Nd2Ba2CaCuz.zz Ti z78 0 ,4 Nd2BazCaCuz29 Ti2710,4 Gd2Ba2CaCuz.16 Ti2.84014 Gd2Ba2CaCu2.22 Ti v8 0 14 Gd2Ba2CaCu2.29 Ti v PI4 Gd2Ba2CaCu2.35 Ti2650,4 Gd2BazCaCuZ.42 Tiz.580,4
3.8997(2) 3.8776(1) a = 5.4794(3) b = 5.4596(3) 3.8950(1) 3.8946(1) 3.8945(1) 3.8946(1) 3.8740(4) 3.8759(1) 3.8751(1) 3.8775(8) 3.8784(6)
19.594(1) 19.565(1) 19.543(1) 19.635(1) 19.643(1) 19.637(1) 19.641(1) 19.552(3) 19.557(2) 19.550(1) 19.549(5) 19.560(4)
of NdzBa2CaCu2 Ti 30 14 The structures Gd2Ba2CaCuz Ti 30 ,4 and DY2Ba2CaCuZ Ti 30 l4 have also been refined from powder neutron diffraction data and full structural results will be presented 4 elsewhere . The structures of some of the doped materials were refined from powder x-ray diffraction data in the space group P4/mmm.
752
M.J. Pack, M. T. Weller / Physica C 282-287 (1997) 751-752
3. RESULTS The structure of Gd2Ba2CaCu2 Ti 30 14 is shown in Figure I. The B type cations are perfectly ordered along the c axis with double layers of CU02 separated by triple layers of Ti0 6 octahedra. For all compositions the central Ti0 6 octahedron is distorted and/or tilted, possibly originating from the mismatch in sizes of the Cu-O bond length (~1. 96A in CU02 square planes) and Ti-O distances (typically 2.02A in Ti0 6 octahedra). The degree of ordering of the A cations is observed to increase with decreasing Ln size being perfect for gadolinium. In this phase the distribution of oxygen in consistent with layers formed from CuOs square pyramids. In Nd 2Ba2CaCu2Ti 30 14 extensive disordering occurs on both A and B cations sub lattices and there is evidence for the incorporation of oxygen between the CuOs pyramids. In DY2Ba2CaCu2Ti 30 14 the extent of Ti0 6 distortions is such that the structure crystallises in the orthorhombic space group Cmmm probably allowing better co-ordination between oxygen and the A type cations. For the doped phases results show a decrease in the a and an increase in the c lattice parameters for the neodymium Cu/Ti doped system with small levels of additional copper. However, for higher copper levels the lattice parameters remain unchanged. This suggests only small amounts of additional copper may be incorporated in this phase and this is confirmed by the presence of minor amounts of an additional phase in the products where x>0.15. However, the unchanging lattice parameters in the Gd2Ba2CaCu2+xTi 3_x0 14 system as a function of x implies that copper cannot substitute on to the Ti site in this phase. The differing behaviours in the Nd and Gd systems accords with the structures of the parent phases where the distribution of Cu and Ti is imperfect for Nd phase but for the Gd derivative the two sites are distinct. 4. CONCLUSIONS Nd, Gd and Dy have been introduced into the ordered quintupled perovskite series Ln 2Ba2CaCu2Ti 30 14 and copper/titanium doping studies carried out on the Nd and Gd systems. In the
Figure I. Polyhedral representation of the idealised structure of Ln2Ba2CaCu2 Ti 30 14 showing the regular Ti0 6 octahedra and CU02 square pyramids. undoped phases there is little change in the planarity of the Cu02 structural feature but the Cu-O bond distance is observed to decrease as the lanthanide ionic radii falls. For Ln=Dy the Cu-O distance is 1.936A similar to that found in many cuprate superconductors. However initial attempts to increase the copper oxidation state to that found in superconductors, through doping of copper on to the titanium site, have been unsuccessful except where disorder on these sites already exists (Ln=Nd). ACKNOWLEDGEMENTS We thank the EPSRC for grants in support of this work and a studentship for MJP. REFERENCES 1. M.T.Anderson, K.R.Poeppelmeier, J.P.Zhang, HJ.Fan and L.D.Marks, Chern. Mater., 4 (1992) 1305. 2. A.Gormezano and M.T.Weller, Chern. Mater., 3(7) (1993) 771. 3. WJ.Zhu, Y.Z.Huang, T.S.Ning and Z.X.Zhao, Mat. Res. Bull., 30(2) (1995) 243. 4. MJ. Pack and M.T.Weller, Submitted to Chern. Mater., (1996).