The 'RMnO3': R = La, Nd, Pr, phases have been synthesised and characterised by a combination of electron probe microanalysis (EPMA), H2-reduction thermogravimetry (TG), x-ray (XRD) and neutron diffraction (ND). RMnO3 forms, at" 1400C, over the ranges: NdMn0, 95Oz to Nd0 88MnOz PrMn0.97O2 to Pr0 88MnOz LaMno 0.90Oz to La0.97MnOz Oxygen contents vary in air over the range 700 to 1400 C and can be varied further, either by high pressure Oz treatment or by reduction in H2. The structure of 'RMnO3' R = Nd, Pr is based on the GdFe03 structure with a Jahn-Teller distortion associated with the high proportion of Mn3+ ions present. The oxygen deficient 'LaMnOz' compositions also exhibit this structure consistent with earlier reports. By combining EPMA, TG, XRD and ND results various defect models describing the stoichiometry and structure of Mn-rich and R-rich, R = Nd, Pr compositions have been summarised. Both R = Nd and Pr systems exhibit very varied defect structures; depending on composition and heat treatment, vacancies can form on any one or any two of the three sublattices, R, Mn and O and the overall Mn oxidation state can include 2+, 3+ and 4+ contributions. For 'RMn03': R = La, Nd, Pr, data on their compositional ranges and defect crystal structures are presented in the form of novel phase diagram-defect structure maps from which the principal defect structure for a given stoichiometry can be easily obtained. The majority of the Pr-Sr-Mn-O pseudotemary phase diagram has been determined. EPMA was used to follow the progress of reaction and the conditions to achieve complete reaction established. Several solid solutions were evidenced, some previously unreported (3 - 6): 1) Pr1.xSrxMnO3 0[Special character omitted]x[Special character omitted]1.0 2) Pr1+xSr2.xMn2O7 0 [Special character omitted]x [Special character omitted] 0.4 3) SrxPr1-xO2 0[Special character omitted]x[Special character omitted]0.16 4) Sr1-xPrO3 0[Special character omitted]x[Special character omitted]0.15 5) Sr2.xMnxO4 6) Sr2.xPrxMnO4 The perovskite-like Pr1-xSrxMnO3 solid solution extends from PrMnOz to SrMnOz. The unit cell symmetry changes from orthorhombic to rhombohedral to tetragonal to cubic and finally to hexagonal as the Sr content increases. The limits of the Ruddlesden Popper (RP) n=2 Pr1+xSr2_xMn2O7 solid solution were determined: 0 [Special character omitted] x [Special character omitted] 0.4 and a tetragonal unit cell observed consistent with the literature. Synthesis of the RP compositions by solid state methods requires long heating times (up to 36 days) to produce homogeneous samples; qualitative EPMA of younger samples indicated an inhomogeneous distribution of Pr and Sr. Contrary to EPMA results, XRD of younger samples indicated that complete reaction had occurred and single phase compositions produced. It is suggested that the SrxPr1-.xO2 solid solution extends over the range 0 [Special character omitted] x [Special character omitted] 0.16 where similarly to the polymorphism of praseodymium oxides, compositions 0.03 [Special character omitted] x [Special character omitted]0.16 exhibit the cubic fluorite-type structure of Pr6O11 and x [Special character omitted] 0.03 is a mixture of cubic SrxPr1-xO2 and hexagonal SrxPr2.xO3. Perovskite-like SrPrO3 exhibits variable cation ratios; the Pr-rich boundary is Sr0.85PrOz. The lower Sr-rich boundary is yet to be identified. Similarly to 'RMnO3': R = La, Nd, Pr, the oxygen content of 'SrPrOz' is expected to vary. Therefore, various possible defect structures describing vacancies on the three sublattices, Sr, Pr and O could exist and charge compensation would be an interesting example of ionic and electronic mechanisms where Pr adopts the +4 and +3 oxidation states. Four layer hexagonal SrMnO3 exhibits variable Sr:Mn ratios but the solid solution limits are not yet known. The unreported Sr2-xPrxMnO4 solid solution has been observed but the solid solution limits are not yet known. The K2NiF4-type structure of Sr2Mn04 is retained at x = 0.75 and is expected to contain Mn3+ as Mn4+ is reduced to compensate Sr24 substitution by Pr3+.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:287723 |
| Date | January 1999 |
| Creators | Maguire, Elaine T. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU603199 |
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