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

Pressure-driven valence change in ternary Eu pnictides

We observed a structural phase transition with extremely anisotropic changes of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered structure type. On the basis of the results of pressure-dependent x-ray diffraction experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in EuPdP is accompanied by a valence change of the Eu. Strong but continuous changes of the lattice parameters with increasing pressure, which are due to increase of the Eu valence, were observed in EuNiP, EuPtP and EuPdAs, too. An estimation of the average Eu valence in these compounds leads to preferred values of the order of 2 n/6

First-order phase transitions in EuCo2P2\mathrm{EuCo_2P_2} and SrNi2P2\mathrm{SrNi_2P_2}

First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr2Si2 structure-type compounds EuCo2P2 and SrNi2P2. At room temperature, with increasing pressure the phase transition occurs in SrNi2P2 at 4 kbar and in EuCo2P2 at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of ACo2P2 (A = Ca, Sr, La, Ce, Pr, Nd, Eu) at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds

First- and second-order phase transitions in ternary europium phosphides with ThCr2Si2\mathrm{ThCr_2Si_2}-type structure

The ThCr2Si2- type compounds EuFe2P2 and EuRu2P2 exhibit as a function of pressure continuous phase transitions which are accompanied by extremely strong changes of the lattice parameters and particularly of the P–P distance dP−P along the tetragonal c-axis. In the isostructural integralvalent LaT2P2 compounds similar phase transitions of second (T=Fe) and first order (T=Co) occur as a function of pressure indicating that the T element essentially determines the nature of the phase transition. The different types of phase transitions are explained phenomenologically