Synthesis and phase stability of Ln(2)MnTa(1+x)O(7+delta) (Ln equals rare earth and yttrium) compounds

The complex oxides Ln(2)MnTa(1+x)O(7+delta) containing pentavalent tantalum with a pyrochlore-related structure have been obtained at 1523 K in air for Ln = Eu to Er, Yb and Y as a single phase with the Ta/Mn composition ratio being 1+x (0.16 less than or equal to x less than or equal to 0.35). The excess amount of tantalum (x) varies for Ln(2)MnTa(1+x)O(7+delta). phases with different rare earth elements and was found to increase with increasing atomic weight of the rare earth elements. The results of the investigation of phase stability of these phases under various oxygen partial pressures indicated that the Ln(2)MnTa(1+x)O(7+delta). phases with Ln = Gd to Er, Yb and Y are stable over the range p(O-2)=1-10(-21) atm, but Eu2MnTa1+xO7+delta decomposes when p(O-2) is below 10(-19) atm. The oxidation states of the Ln, Mn and Ta ions in Ln(2)MnTa(1+x)O(7+delta) were established by measurements of the magnetic susceptibility.