Structure-property relationships in a series of mixed layer pnictide oxide compounds: A₂Mn₃Pn₂O₂ (A = Sr, Ba; Pn = P, As, Sb)

Results of magnetic property measurements for the compounds A₂Mn³Pn₂O₂ (A = Sr, Ba; Pn = P, As, Sb) are reported and discussed in the light of structural trends in the series. The structure consists of a 1:1 intergrowth of metal oxide layers in which the metal coordination is square planar and metal pnictide layers in which the coordination is tetrahedral. With the exception of Sr₂Mn₃As₂O₂, magnetic susceptibility data reveal the presence of broad maxima that have been assigned to coupling of the MnO^2- ₂ layers. The data have been fitted assuming a 2D Heisenberg antiferromagnet model and the intraplanar coupling constants obtained are observed to increase as the a parameter or metal oxygen bond length decreases. The same trend is observed for the relationship of transition temperatures, obtained from dχT/dT plots, to the a axial parameters. The exception to the rule is Sr₂Mn₃As₂O₂, which has an anomalously low transition temperature and different magnetic behavior for field-cooled versus zero-field-cooled samples. The field-cooled data show a sharp antiferromagnetic transition whereas the zero-field-cooled data show no transition to 3D order between 5 and 300 K.