Neutron-diffraction experiments on ceramic powders of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (nonsuperconducting, with $x=0.0 \mathrm{and} 0.15$ 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 (\textonehalf{} \textonehalf{}) and the planes are coupled antiferromagnetically along the $c$ axis. The N\'eel temperature is 400\ifmmode\pm\else\textpm\fi{}10 K for the $x=0.15\ifmmode\pm\else\textpm\fi{}0.05$ sample and \ensuremath{\gtrsim}500 K for $x=0.0$.

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Neutron-Diffraction Determination of Antiferromagnetic Structure of Cu Ions inYBa2Cu3<mml...