Synthesis and characterisation of Mn $^3$ $^+$ condensed phosphate phases and Fe $^3$ $^+$ substituted analogues

Begum, Yasmin (2013). Synthesis and characterisation of Mn $^3$ $^+$ condensed phosphate phases and Fe $^3$ $^+$ substituted analogues. University of Birmingham. Ph.D.

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Abstract

This thesis describes the synthesis, properties and structural characterisation of a series of Mn $_3$ $_+$ condensed phosphate phases and Fe $_3$ $_+$ substituted analogues. To date, the synthesis of such mixed cation compositions has been unexplored, but this work has highlighted that they present an interesting array of properties.

α-NH $_4$ MnP $_2$ O $_7$ was successfully synthesised and structurally characterised. A second polymorph β-NH $_4$ MnP $_2$ O $_7$ was also identified, with both found to be key components of a commerical manganese violet pigment. The α-NH $_4$ Mn $_x$ Fe $_1$ $_-$ $_x$ P $_2$ O $_7$ solid solution was also characterised, with the introduction of Fe $_3$ $_+$ resulting in a distinct colour change, an improvement in thermal stability and a change in the magnetic properties of these materials. Similar changes were seen in the RbMn $_x$ Fe $_1$ $_-$ $_x$ P $_2$ O $_7$ system but a marked improvement in thermal stability, with an unusual decomposition mechanism involving manganese expulsion observed for the intermediate members. Analogous studies were performed on compounds of composition RbMn $_x$ Fe $_1$ $_-$ $_x$ HP $_3$ O $_1$ $_0$ , Mn $_x$ Fe $_1$ $_-$ $_x$ H $_2$ P $_3$ O $_1$ $_0$ (x ≤ 0.5), Mn $_x$ Fe $_1$ $_-$ $_x$ P $_3$ O $9$ (0 ≤ x ≤ 0.4 and 0.75 ≤ x ≤ 1) and β-HMnP $_2$ O $_7$ .

All phases showed interesting colourations which has been
attributed to the Jahn-Teller active Mn $_3$ $_+$ being able to exhibit a “plastic” coordination sphere. The magnetic structures of a number of compounds have also been solved in order to supplement the existing literature on condensed phosphate systems.

The layered compound MnH $_2$ P $_3$ O $_1$ $_0$ ·2H $_2$ O was also investigated and found to readily undergo monoamine intercalation. Attempts to incorporate additional transition metal ions into the interlayer region of this compound proved inconclusive.