Structure and Magnetic and Thermal Properties of Polycrystalline RAlGe (R = La, Ce, Pr) Semimetals

Polycrystalline RAlGe (R = La, Ce, Pr) is prepared by arc melting and vacuum solid state reaction technologies. The crystal structures of RAlGe are refined with Rietveld method based on two models of the site-disorderly centrosymmetric I4(1)/amd and the site-orderly noncentrosymmetric I4(1)md. The structural model is suggested to follow the site-orderly noncentrosymmetric I4(1)md, which agrees with the prediction of nontrivial topology. This structure consists of the special alternating stack of A/B layers formed by isosceles triangle Delta in the A layer and reversal isosceles triangle del in the B layer along the a axis, and the lack of space-inversion symmetry with the noncentrosymmetric structure may result in the formation of Weyl points on Fermi faces of RAlGe semimetals. Magnetization measurement confirms an antiferromagnetic transition temperature of 2.9 K for CeAlGe and a melting model is built for checking the consistency of experimental and predicted transition temperatures. The effective moment of polycrystalline CeAlGe is 2.49 mu(B), which is smaller than that of Ce3+ due to the existence of the Ce4+ state in CeAlGe. Polycrystalline PrAlGe exhibits an antiferromagnetic transition at 14.4 K, and its effective moment (3.89 mu(B)) is more than that of Pr3+; the extra moment might be due to the exchanging interaction between itinerant electron in s conducting band and 4f electrons of Pr, i.e., f-s exchanging interaction. The thermal properties and valence electron structures of RAlGe have been studied with empirical electron theory (EET) of solids and molecules. It shows that cohesive energy and melting point of RAlGe are strongly related to their valence electron structures.