Local electronic and crystal structure of rare-earth cobalt phosphides RCo2P2 studied by XAFS spectroscopy

The valence state of Ce and Eu was investigated by X-ray absorption near-edge structure (XANES) spectroscopy above L3-Ce and L3-Eu absorption edges in series of RCo2P2 (R = La, Ce, Pr, Eu) magnetic intermetallics. At the same time under similar conditions the rearrangement of Co and Pr local environment vs. temperature was studied by extended X-ray absorption fine structure (EXAFS) spectroscopy above K-Co and L3-Pr absorption edges. Introduction Magnetic ordering in RCo2P2 (R = La, Ce, Pr, Eu) itinerant magnets is dictated by peculiarities of electronic band structure at the Fermi level and can be affected by external perturbation, e.g. pressure [1]. The similar modification of electronic structure can be induced by chemical compression via non-isoelectronic substitution in the rare-earth sublattice. In the series La1−xRxCo2P2, the Ce-substituted phases studied herein behave quite differently from the Prand Nd-substituted ones [2]. Another studied compound, Pr0.8Eu0.2Co2P2, is unexpectedly ferromagnetic (TC = 282K), while the pure PrCo2P2 and EuCo2P2 are antiferromagnets. This might be due to Ce and Eu intermediate valence states which are explored in this work by means of X-ray absorption fine structure (XAFS) spectroscopy along with such parameters of local crystalline structure as interatomic distances. Experimental All the studied samples were synthesized according to the standard reported procedures [2]. The phase purity of bulk products obtained was confrmed by powder X-ray diffraction. Energydispersive X-ray (EDX) analysis and single-crystal X-ray diffraction revealed that the composition of obtained materials is close to the nominal one. The X-ray absorption spectra of Pr0.8Eu0.2Co2P2 and La1−xCexCo2P2 (x = 0.3; 0.6; 0.9) compounds were collected at C beamline of HASYLAB (DESY, Hamburg, Germany) in transmission mode above the L3-Eu (6977 eV), L3-Pr (5964 eV), L3-Ce (5723 eV) and K-Co (7709 eV) absorption edges. Energy resolution of the double-crystal Si (111) monochromator (detuned to reject 50% of the incident signal in order to minimize harmonic contamination) with a 0.3mm slit at 7 keV was about 1.2 eV. Lowtemperature measurements were carried out using a liquid helium continuous flow cryostat with a temperature control of ±1K at 300K and ±0.1K at 5K. The fitting of EXAFS-oscillation functions χ(k) and determination of local crystalline structure parameters were performed with VIPER software pack [3]. 6960 6970 6980 6990 7000 exp fit 8 K 38 K 78 K 151 K 300 K 210 K