Structural studies of ternary copper oxides, inorganic layered compounds and related materials

Lines, Dean Russell (1991) Structural studies of ternary copper oxides, inorganic layered compounds and related materials. University of Southampton, Doctoral Thesis.

Abstract

A wide range of ternary copper oxides of divalent and trivalent copper have been prepared in the solid state. The materials have been thoroughly characterised by powder x-ray diffraction (PXD), powder neutron diffraction (PND) and by thermogravimetric analysis (TGA). This characterisation has enabled the precise descriptions of NaCuO₂ (C2/m), Bi₂CuO₄ (P4/ncc) and In₂Cu₂O₅ (Pna2₁) to be obtained dispelling the contradictions that had previously existed for these phases. The bond valence theory that relates individual bond lengths to associated bond valences has been investigated for copper oxide systems. The validity of these calculations has been confirmed for divalent copper by the application of these calculations to the series of ternary copper oxides prepared in this work. These copper oxides have in addition been used as a basis to modify the coefficients employed in the two expressions used in the theory to improve the reliability observed in the calculations (R_o= 1.691, B= 0.37; R_o= 1.734, N= 6). The relative reliability of bond valence theory for divalent copper in varied coordination goemetries has been investigated by the application of the calculations to a series of greater than 35 well characterised ternary copper oxides drawn from the literature. This has highlighted a discrepancy in the treatment of apical copper oxygen bonds in '4+ 1' and '4 + 2' coordination geometries. Attempts to correct this discrepancy have been made by modifications of the mathematical function employed to relate the bond lengths to bond valences in the theory. Two such modifications have been shown to improve the reliability observed for the theory. The employment of bond valence theory in trivalent copper oxide systems has been assessed. Phases of composition BaCuO₂+ x (x= 0,0.08) have been prepared under differing oxygen partial pressures in the solid state and have been studied using PXD, PND and TGA. The barium and oxygen distributions have been observed to be somewhat different to those previously reported. The differences between the two separate structures have been related to the change in the oxygen content. Bond valence calculations have been employed in an attempt to further rationalise the extremely complex structure. Copper in oxidation states of I, II and III has been postulated to be present in the oxygen rich phase. Alkaline earth metal copper oxide phases of composition M₂CuO₃ and M₂CuO₃+ x have been prepared and studied by PXD, PND and TGA. The ease of formation of solid solutions (Mg_2xCa_xCuO₃, Ca_2-xSr_xCuO₃ and Sr_2-xBa_xCuO₃) and the structural effects of mixed cation occupancy have been analysed. The inability to prepare any materials containing magnesium and the non-stoichiometry of the barium phase have been discussed in relation to the anion deficient K₂NiF₄ type structure assumed by the materials. MNiO₂ (M= Li, Na), LiNbO₃, Li₂CuO₂ and Zn₂M(OH)₆(X^.).xH₂O (M= Al, Cr; X= Cl^.) layered double hydroxides (LDHs) have been prepared and subjected to several ion exchange reactions. The materials obtained from these reactions have been characterised using PXD, TGA and infra-red spectroscopy. The relative ease with which each of the phases undergoes exchange reaction has been monitored and related to structural freedom. In addition the potential use of molten benzoic acid as an ion exchanging reagent has been assessed.

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