5f and 4f States in Layered Zintl Phases: Half- and Semi-Metals

The term Zintl phases denotes compounds of one electropositive element donating electrons to a complex anion with covalent bonding. We focus on layered compounds with the trigonal structure type CaAl₂Si₂, which are known both for actinides (U,Th) and divalent lanthanides (Eu, Yb). The lanthanide-based compounds are narrow-band antiferromagnetic semiconductors with transport properties very sensitive to magnetic fields. UCu₂P₂ is Ising-like ferromagnet. Its <i>T</i>_C = 216 K steeply increases with hydrostatic pressure. Calculations reveal that the spin splitting of the 5<i>f</i> band with only one spin direction at <i>E</i>_F brings to the Fermi level also practically full polarization of light electrons. Hence UCu₂P₂ is a combination of semi metal and half metal. High pressure experiments on EuZn₂P₂ indicate that the band gap is suppressed under hydrostatic pressure of 12 GPa. Interesting aspects is brought by the layered character of the crystal structure, with <i>f</i>-element basal-plane sheets distinctly separated by the anionic slabs. This gives much longer magnetic correlations length in the basal plane. There is a striking contrast between EuZn₂P₂with practically no magnetic anisotropy due to the spin-only magnetism of the <i>f </i>⁷ state and extremely strong (<i>E</i>_a/k_B ≈ 500 K) in-plane 5<i>f</i> bonding induced anisotropy in UCu₂P₂ with the U moments locked into the <i>c</i>-axis.<br/>The results illustrate how the <i>f</i>-systems are affected by the polar bonding. The potential of the band-gap engineering can be used for designing thermoelectrics or materials for spintronics.<br/>This work was supported by the Czech Science Foundation under the grant No. 21-09766S.