Covalency effects on the magnetism of EuRh2P2

In experiments, the ternary Eu pnictide EuRh2P2 shows an unusual coexistence of a non-integral Eu valence of about 2.2 and a rather high Neel temperature of 50 K. In this paper, we present a model which explains the non-integral Eu valence via covalent bonding of the Eu 4f-orbitals to P2 molecular orbitals. In contrast to intermediate valence models where the hybridization with delocalized conduction band electrons is known to suppress magnetic ordering temperatures to at most a few Kelvin, covalent hybridization to the localized P2 orbitals avoids this suppression. Using perturbation theory we calculate the valence, the high temperature susceptibility, the Eu single-ion anisotropy and the superexchange couplings of nearest and next-nearest neighbouring Eu ions. The model predicts a tetragonal anisotropy of the Curie constants. We suggest an experimental investigation of this anisotropy using single crystals. From experimental values of the valence and the two Curie constants, the three free parameters of our model can be determined.Comment: 9 pages, 5 figures, submitted to J. Phys.: Condens. Matte

The anomalous valence state of Eu and magnetic order in EuRh2P2\mathrm{EuRh_2P_2}

According to x-ray absorption, Mössbauer effect measurements, lattice constants and static susceptibility, Eu in the novel compound EuRh$_2$P$_2$ is non-integral valent with a valence of the order of 2.2. On the other hand, the compound orders antiferromagnetically below 50 K, the $^{151}$Eu Mössbauer isomer shift is rather independent of temperature, and the resistivity shows no anomalies in the paramagnetic region. Hence, according to existing theories the Eu 4f level cannot be strongly hybridized with the conduction band as in usual intermediate-valent compounds. We therefore propose a considerable participation of the 4f electrons in a covalent Eu - P bond. However, the absence of the ESR signal of EuRh$_2$P$_2$ as well as the small but finite temperature dependence of the L$_{111}$ valence clearly demonstrate that the hybridization of the 4f electrons with the conduction band is not negligible in this compound