Nuclear Magnetic Resonance of Hydrogen in CeHx for x>2
Experimental evidence reviewed by Libowitz (1) indicates that at room temperature, the CeHx system undergoes a metal-to-onmetal (MNM) transition with increasing hydrogen content at about x=2.75. For x>2 CelHx is cubic, and hydrogen fills the octahedral (or nearby off-center sites) of the CeH2 fluorite structure. The existence of an MNM transition is consistent with band structure calculations in the literature (2).
To further the study of the MNM transition, we have measured 1H (proton) and 2H (deuteron) NMR spin-lattice relaxation times T1 for 2 < x < 2.9 in CeHx. NMR parameters are sensitive to changes in electronic structure through hyperfine interactions. At room temperature the proton T1 is weakly dependent upon concentration up to x≃2.5, but for x>2.5, T1 increases rapidly with x. This increase in T1 is consistent with the compositional MNM transition. From temperatures of about 100K to 300K, T1 increases linearly with temperature for samples with x>2.5 as reported (3) for x=2.0 and x=2.53. T1 data for deuterons show that the relaxation mechanism is principally magnetic so that quadrupolar contributions to deuteron relaxation can be neglected.
The strong magnetic relaxation mechanism is attributed mostly to indirect (RKKY) and direct (dipolar) coupling between hydrogen nuclear spins and the local cerium 4f electron magnetic dipole moment. It is expected that the MNM transition affects the RKKY coupling through the change in electronic structure.