The crystal structures of La2ZnxMg1-xIrO6(x=0.0, 0.125, 1.0) have been
refined from neutron powder diffraction data. The compound adopts a m
onoclinic (space group P2(1)/n) form of the perovskite structure with
a 1:1 ordered arrangement of Ir and Zn(Mg) cations for all values of x
. The magnetic properties of the system are reviewed in the light of t
he structural data, and possible explanations for the contrasting beha
viour of the compositions x=0 (antiferromagnetic) and x=1 (ferromagnet
ic) are discussed.