The first model-independent measurements of H diffusion in LaHx, where x was between 2 and 3, were carried out by means of pulsed-field gradient nuclear magnetic resonance techniques. At x-values of up to 2.92, the diffusivity exhibited an Arrhenius behavior over wide temperature ranges. The activation enthalpy decreased with increasing x-value; from 0.55eV at x = 2.00 to 0.17eV at x = 2.92 (table 128), thus leading to a sharp increase in diffusivity (table 129). The formation of a H superstructure, that was associated with the semiconductor-to-metal transition, resulted in a marked reduction in the diffusivity. It was noted for the first time that, in the stoichiometric limit, the diffusivity depended upon the thermal history of the sample.

G.Majer, U.Kaess, R.G.Barnes: Physical Review Letters, 1999, 83[2], 340-3

 

Table 128

Diffusion Parameters for H in LaHx

 

x

Do (m2/s)

E (eV)

Temperature (K)

2.00

1.1 x 10-7

0.55

500-800

2.26

1.6 x 10-7

0.44

410-800

2.35

1.4 x 10-7

0.40

390-800

2.50

4.7 x 10-8

0.29

330-680

2.70

2.3 x 10-8

0.21

330-500

2.92

1.3 x 10-8

0.17

280-550

Table 129

Diffusivity of H in LaHx at 500K

 

x

D (m2/s)

2.00

3.0 x 10-13

2.26

5.9 x 10-12

2.35

1.3 x 10-11

2.50

5.6 x 10-11

2.70

1.76 x 10-10

2.92

2.50 x 10-10