Properties of rare earth dihydrides as reflected in the magnetic behavior of TmH2, a van Vleck compound

It is shown that TmH 2 , which is a van Vleck compound with a Γ 2 singlet ground state and a Γ 5 (2) triplet first excited state, can be used to detect variations in crystal fields of thulium atoms. These variations were deduced from a comparison of measurements of TmH 2 using susceptibility nuclear magnetic resonance of thulium, neutron diffraction, resistivity and electron spin resonance (for which gadolinium and erbium impurities were added). Analysis of the results showed a spread from 170 to 186 K in the energy ΔE , of the first excited state for different thulium atoms in TmH 2 , in addition to a drastic change in ΔE at 100 K and 80 K for thulium atoms neighboring gadolinium and erbium impurities respectively. The origin of the spread in δE is attributed to defects which induce changes in the crystal field around the defect. These changes have a range of influence which decreases as a function of distance from the defect. Since all the rare earth and rare-earth-like dihydrides have inherent defects owing to the deficiency of hydrogens in TmH 2 and to the small occupation of octahedral sites by hydrogens, we can deduce that the spread in crystal fields may be common to all these dihydrides.