Uranium Hydrides—From Polar Bonds to Magnetism

Ferromagnetism of uranium binary hydrides arising from weakly paramagnetic U metal has been traditionally associated with the volume expansion, reaching about 60%. Such expansion surely reduces the 5<i>f</i> band width. The enhanced density of states at the Fermi level <i>E</i>_F may lead to fulfillment of the Stoner criterion, guaranteeing splitting of spin-up and spin-down sub-bands. Is that all or are there still other factors in the game? There is extended evidence that UH₂ and two known forms of UH₃ are NOT simple band ferromagnets. Similar magnetic properties, namely the spontaneous moments of ≈ 1 μ_B/U and practically identical Curie temperatures <i>T</i>_C of 165 K for α- and β-UH₃ are contrasting with very different crystal structure features, such as U-U spacings. The situation gives a clear impression that the U-H interaction is more important than the U-U spacing and that the basic building blocks (practically identical in all 3 species), the U tetrahedra filled with H atom, are determining all magnetic properties [1]. Analysis of the U-H bonding (Bader charges) reveals that there is a substantial electron transfer from U towards H in real space, and analysis of reoccupation of individual partial density of states indicates that electrons in the H-1<i>s</i> states are topped up (increasing significantly the size of H^- ions) mainly by a transfer from the U-6<i>d</i> states [2,3]. Remaining 6<i>d</i> states are shifted above <i>E</i>_F, so the hybridization with the 5<i>f</i> states is limited. This gives account of the high <i>T</i>_C values, while the modest U moments are (as ab-initio GGA+<i>U</i> calculations show) due to cancellation of much larger spin and orbital moments, aligned mutually antiparallel. The ab-initio study also showed that β-UH₃ with two U sublattices is likely a non-collinear ferromagnet. As moments of one of the two U sites are not fixed by symmetry, they tend to incline (by ≈ 15^o) from the other U sites, the moments of which point along [1 1 1]. Calculated phonons (in quasiharmonic approximation) well describe the lattice specific heat, which allows to establish that the Sommerfeld coefficient in the paramagnetic state is 60 mJ/mol K², i.e. about twice as much as in the <i>T</i> → 0 limit. This means that at the end some of the band character of magnetism is present, as part of the 5<i>f</i> band splitting happens at or below <i>T</i>_C (as indicated by a large spontaneous magnetostriction [2]). Last calculations attempted to explain the surprizing fact, that substituting part of U by weakly magnetic metal, such as Mo, Ti, V, enhances <i>T</i>_C [1]. The H atoms remain stable inside e.g. U₃Mo tetrahedra, which increases the H/U ratio, while Mo stays rather inert.<br/>As a conclusion we demostrate that the U/H ratio is in the condition of polar U-H bonding an important tuning parameter, which gives guidelines how to achieve higher temperatures of magnetic ordering in U systems.<br/>This work was supported by the Czech Science Foundation under the grant No. 21-09766S and by P2E project No. CZ.02.1.01/0.0/0.0/16_013/0001791) within OPRDE and by the project e-INFRA CZ (ID:90140) by the Ministry of Education, Youth and Sports of the Czech Republic.<br/>[1] O. Koloskova et al., J. Alloys Comp. 856, 157406 (2021).<br/>[2] I. Tkach et al., Phys. Rev. B 91, 115116 (2015).<br/>[3] L. Kyvala et al., sumbitted to J.Nucl.Mater.