Synthesis and characterisation of some layered and perovskite

Gurusinghe, Nicola Narmada Muthumanike (2013). Synthesis and characterisation of some layered and perovskite. University of Birmingham. Ph.D.

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Abstract

The perovskites LaSr $_2$ Fe $_2$ SbO $_9$ and La $_2$ Sr $_2$ Fe $_3$ SbO $_1$ $_2$ crystallise in P2 $_1$ /n space group with Fe $^3$ $^+$ occupying the two crystallographically distinct six-coordinate sites in a ratio of approximately 1:2. Neutron diffraction data suggest a G-type long-range antiferromagnetism in both materials at 2 K and 300 K.

The A site cation ordering in Ln $_2$ SrFe $_2$ O $_7$ (Ln = La, Nd, Eu) and La $_2$ BaFe $_2$ O $_7$ increases as the mismatch in A site cation size increases. These phases exhibit long-range antiferromagnetic ordering at 300 K and 2 K; the magnetic moments in La $_2$ BaFe $_2$ O $_7$ and Nd $_2$ SrFe $_2$ O $_7$ gradually rotate in the temperature ranges 210 K to 190 K and 17 K to 9 K, respectively. The fluorine incorporation into these phases happens via anion insertion and anion substitution. The partial cation ordering or large overall size of the A site cations allows accommodation of more F $^-$ ions causing no change to the Fe oxidation state.

Both CaMnO $_3$ $_-$ $_\delta$ and SrFeO $_3$ $_-$ $_\delta$ are amenable to phosphate and sulphate anion incorporation. The unit cell sizes increase with the increase of dopant level. The oxy-anion insertion stabilises the cubic structure in all doped phases of SrFeO $_3$ $_-$ $_\delta$ , but only the higher phosphate levels of CaMnO $_3$ $_-$ $_\delta$ phases. The anion incorporation induced some mixed ion valencies but failed to introduce extra oxygen vacancies.