Neutron-Diffraction Determination of Antiferromagnetic Structure of Cu Ions in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ with $x=0.0\mathrm{} \mathrm{and} 0.15$

Neutron-diffraction experiments on ceramic powders of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ (nonsuperconducting, with $x=0.0\mathrm{} \mathrm{and} 0.15$ have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a Cu${\mathrm{O}}_2$ plane is (½ ½) and the planes are coupled antiferromagnetically along the $c$ axis. The Néel temperature is 400±10 K for the $x=0.15\mathrm{}\pm 0.05$ sample and ≳500 K for $x=0.0\mathrm{}$.