Crystal and magnetic structures of Sr 4MMn 2O 9 ( M=Cu or Zn)

Abstract

The crystal and magnetic structures of Sr 4 MMn 2O 9 ( M=Cu, Zn) have been refined from neutron powder diffraction data. These trigonal compounds (space group P321, a=9.5918(1), c=7.8114(1) Å (Cu); a=9.5894(1), c=7.5039(1) Å (Zn)) are n=3 members of the series A 3 n+3 M n B n+3 O 6 n+9 , with each unit cell containing three offset [001] polyhedral chains, each of which ideally contains a 1:1 ratio of B 2O 9 units and MO 6 trigonal prisms. In fact anti-site disorder between Mn and M is observed, and for M=Cu the cations are disordered off the center of the prism towards a rectangular face. Both compositions show 3D anti-ferromagnetic order at 1.6 K, with an ordered magnetic moment of 1.91(6) ( M=Cu) or 1.8(1) ( M=Zn) μ B per Mn. No ordered magnetic moment was detected on the trigonal prismatic site in either compound, consistent with the observed temperature dependence of the magnetic susceptibility.