Structural studies of layered transition metal oxides

A number of transition metal oxide materials, both new and previously reported, have been<br/>synthesised, their structures based on the n = 2 Ruddlesden-Popper layered perovskites. Their<br/>structures were determined through Rietveld refinement against X-ray and neutron powder<br/>diffraction, supported by vibrating sample magnetometry and thermal analysis.<br/><br/>The structure of the double layered phase Gd2SrCo2O7 was determined in the temperature<br/>range 300 – 973 K by refinement of powder neutron diffraction data. The room temperature<br/>structure is tetragonal, and displays small tilts of the oxygen sublattice that are described by a<br/>2a0 2a0 x c supercell model in the space group P42/mnm rather than the idealised<br/>Sr3Ti2O7 I4/mmm structure as previously reported. A structural transition at ~ 580 K is<br/>characterised by reduction in the unit cell symmetry and distortion of CoO6 octahedra; the<br/>behaviour is concurrent with a spin crossover in the Co (III) ions from intermediate to high<br/>spin.<br/><br/>The structure of the new phase Nd2BaCo2O7 was determined in the temperature range 298 –<br/>873 K by refinement of powder neutron diffraction data. At all temperatures, the structure is<br/>metrically tetragonal, however the octahedral rotations are best described in the orthorhombic<br/>space group Bbcb, with a = b = 2a0.<br/><br/>The structures of the phases Ln2SrFe2O7 (Ln = La, Nd, Gd) have been determined by<br/>refinement of variable temperature powder neutron diffraction data. At room temperature all<br/>the materials crystallise in the tetragonal space group P42/mnm, isostructural with the phase<br/>Tb2BaFe2O7. On heating, the gadolinium and neodymium phases undergo transitions to lower<br/>symmetry; the octahedral tilts of the high temperature phases are best described in the space<br/>group Bbmm.<br/><br/>The new phases Nd2-xSr1+xFe2-xO7 have been prepared and their structures determined by<br/>Rietveld refinement of room temperature powder neutron diffraction. All the materials<br/>crystallise in the tetragonal space group P42/mnm. The degree of octahedral tilting decreases<br/>as x increases, indicating a reduction of compression of the perovskite lattice as the average<br/>A-site cationic radius increases.<br/><br/>The phase Sr3CoNbO7 has been prepared and its structure determined by refinement of<br/>powder neutron diffraction data. At room temperature and 3.4 K there is no evidence of<br/>magnetic ordering, B-site cationic ordering or octahedral tilting. The material is isostructural<br/>with the undistorted Sr3Ti2O7, which implies that the spin state of Co (III) in the material is<br/>low spin.<br/><br/>The material Sr3Ti2O7 was prepared and its structure determined in the temperature range<br/>298 – 1073 K. At all temperatures, the structure is tetragonal, with no rotations of octahedra.<br/>Unit cell parameters and bond lengths expand smoothly over the temperature range.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Hickey, Peter James

Advisor dc:contributor.advisor

• Weller, Mark T.