OpenMC/tests/regression_tests/complex_cell/materials.xml

<?xml version="1.0"?>
<materials>

  <material id="1">
    <density value="4.5" units="g/cc" />
    <nuclide name="U235" ao="1.0" />
  </material>
  <material id="2">
    <density value="4.5" units="g/cc" />
    <nuclide name="U238" ao="1.0" />
  </material>
  <material id="3">
    <density value="2.0" units="g/cc" />
    <nuclide name="Zr90" ao="1.0" />
  </material>
  <material id="4">
    <density value="0.1" units="g/cc" />
    <nuclide name="N14" ao="1.0" />
  </material>

</materials>