Studies on the valence state of Ce and Eu in some new boron and silicon containing rare earth intermetallic compounds

Abstract

Compounds of the formula RPd₃B _x (R=rare earth with 0⩽x⩽1) and RPd₃Si _x (R=La, Ce, Eu with 0⩽x⩽0.3) can be prepared by alloying boron or silicon with parent RPd₃ compounds. Addition of boron (silicon) does not change the structure but results in lattice expansion. The valence state of Ce in CePd₃ and that of Eu in EuPd₃ is strongly influenced by boron and silicon. Ce is known to be in a valence fluctuating state in CePd₃ while Eu is trivalent (J=0) in EuPd₃. The increase in the lattice parameter as a function of boron concentration is observed to be larger in CePd₃B _x and EuPd₃B _x compared to that in other RPd₃B _x alloys giving the first indication of the change in the valence state of Ce and Eu. This is confirmed from susceptibility measurements. With the addition of boron, susceptibility increases and the effective paramagnetic moments approach the values corresponding to Ce³⁺ (J=5/2, μ_eff=2.54 μ _B ) and Eu²⁺ (J=7/2, μ_eff=7.94 μ _B ) in the two alloy systems CePd₃B _x and EuPd₃B _x respectively. In the case of europium alloys,¹⁵¹Eu Mössbauer studies point out the importance of near-neighbour environment effects. Further, in EuPd₃B, where all the europium ions are crystallographically equivalent, a single Mössbauer line, with an isomer shift characteristic of europium ions in valence-fluctuating state, is observed at 300 K. However, at 88 K the Mössbauer absorption splits into two lines corresponding to europium ions in two valence states,e.g. divalent- and trivalent-like. Such a behaviour indicates thermally-induced charge ordering of europium ions. Addition of silicon to CePd₃, like boron, results in unusual lattice expansion and changes the valency of cerium towards 3⁺. the valence change is further corroborated by susceptibility measurements. In EuPd₃Si _x alloys, susceptibility and Mössbauer studies indicate that in the limiting single phase alloy EuPd₃Si_0.25 the europium ions are on the verge of valence instability. Susceptibility results on CeRh₃B _x alloys are also presented.