Effect of H on the crystalline and magnetic structures of the Y-Co3 H(D) system. II YCo-H(D) from X-ray and neutron powder diffraction

The crystalline and magnetic structures of the YCo3 -H(D) system have been investigated by means of x-ray and neutron diffraction with the objective of understanding the complex magnetic changes that are observed in this system as hydrogen is added. Synchrotron x-ray diffraction (XRD) patterns were first refined to yield the lattice parameters and coordination of Y and Co atoms in the metal and two B-hydride phases while XRD was used for the y phase. In situ neutron powder diffraction measurements of YCo3Dx were then made in all four phases to determine the deuterium site occupancies and magnetic structures. The site occupancies were also rationalized using the Westlake geometric model. The highest hydrogen concentration measured was YCo3H4.6 . Using the Westlake model, we conclude that the saturated hydrogen content would be YCo3H5 . Our results reported here and in Part I (Phys. Rev. B 76, 184443 (2007)) have enabled us to rationalize the changes in the magnetic structures in terms of changes in the cobalt-cobalt distance caused by the addition of hydrogen. In particular, in the antiferromagnetic y phase, we observe Co atomic displacements that enable the structure to adopt a particular antiferromagnetic structure in a manner that is reminiscent of a Peierls distortion as observed in transitions from the conducting to nonconducting hydrides on addition of hydrogen in YH3.