Effect of the isoelectronic substitution of Sb for As on the magnetic and structural properties of LaFe(As<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>Sb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mi>x</mml:mi></mml:msub></mml:math>)O

The antiferromagnetic (AFM) order and structural distortion in the LaFe(As${}_{1\ensuremath{-}\mathrm{x}}$Sb${}_{\mathrm{x}}$)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As${}_{1\ensuremath{-}\mathrm{x}}$Sb${}_{\mathrm{x}}$)O ($x$ 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed.