Nuclear magnetic resonance line shape of 139La in non-stoichiometric lanthanum hydrides at low temperatures

Abstract

The 139La nuclear magnetic resonance of lanthanum hydrides in LaH 2+ x ( x = 0.32, 0.38, 0.40, 0.74) was measured over the temperature range 2.3 to 303 K. A single, temperature-dependent line width was found in LaH 2.74 above 250 K, whereas a complex spectrum was observed at lower temperatures. The low temperature line shape can be described as the superposition of two resonances with the same intensity but different electric field gradients and Knight shift tensors. They were correlated with two lanthanum sites in the LaH 2.75 superlattice proposed on the basis of neutron diffraction and specific heat measurements. For the LaH 2.32-LaH 2.40 samples, the spectrum can also be decomposed into two resonances. However, in this case, one line can be attributed to the LaH 2.5 superlattice and the origin of the second line is briefly discussed.