ZrBe2H0.6: an investigation into the two-dimensional diffusion of hydrogen using neutrons

Zema Chowdhuri, Ronald Cappelletti, Terry Udovic

NCNR, Division 856, Material Science and Engineering Laboratory

The intermetallic compound ZrBe2 forms a layered structure with triangular nets of zirconium atoms separated by honeycomb nets of beryllium, resulting in a unit cell containing one Zr and two Be atoms. At room temperature and atmospheric pressure, this material takes up hydrogen, up to 1.5 atoms per unit cell. The hydrogen atoms reside in the same layer as the zirconium atoms, directly above and below the beryllium atoms. Full occupation of hydrogen corresponds to 2 atoms per unit cell, and the partially filled net allows mobility of the hydrogen atoms. The lattice parameters of the structure depend on the hydrogen content and the temperature as determined by diffraction measurements using the BT1 powder-diffractometer. The hydrogen component in the compound was determined by prompt gamma activation analysis.

We have found that the hydrogen atoms order on a sub-lattice below a temperature of 500 K. Above this ordering temperature, the hydrogen atoms diffuse and the motion is expected to be constrained to the plane of the hydrogen atoms. We have measured the quasielastic scattering arising from the diffusion at three widely different energy resolutions: (i) around 1 micro-eV (corresponding to nanosecond time scale) on the backscattering instrument (HFBS), (ii) 23 micro-eV on the disk-chopper-spectrometer (DCS), and (iii) 64 micro-eV, also on the DCS. Suitable analysis at all measured resolutions give consistent results for the linewidths corresponding to the diffusive motions. Comparison is made to model calculations for two-dimensional diffusion on the honeycomb net.

Presenting author:

Zema Chowdhuri
Division: 856
Laboratory: MSEL
Room: E126, Building: 235
Mailstop: 8562
Telephone: 4404
Fax: 301 921 9847
Email: zema@nist.gov
Sigma Xi member: yes