Crystal and electronic structures of CaAl2Si2-type rare-earth copper zinc phosphides RECuZnP2 (RE=Pr, Nd, Gd-Tm, Lu)

The quaternary rare-earth phosphides RECuZnP2 (RE=Pr, Nd, Gd-Tm, Lu) have been prepared by reaction of the elements at 900 oC, completing this versatile series which forms for nearly all RE metals. They adopt the trigonal CaAl2Si2-type structure (Pearson symbol hP5, space group P3-bar m1, Z=1), as confirmed by single-crystal X-ray diffraction analysis on ErCuZnP2 and powder X-ray diffraction analysis on the remaining members. The Cu and Zn atoms are assumed to be disordered over the single transition-metal site. Band structure calculations on a hypothetically ordered YCuZnP2 model suggest a semimetal, with a zero band gap between the valence and conduction bands. This electronic structure is supported by XPS valence band spectra for RECuZnP2 (RE=Gd-Er), in which the intensity drops off smoothly at the Fermi edge. The absence of a band gap permits the electron count to deviate from the precise value of 16 e- per formula unit, as demonstrated by the formation of a solid solution in GdCuxZn2-xP2 (1.0≤x≤1.3), while still retaining the CaAl2Si2-type structure. Because the Cu 2p XPS spectra indicate that the Cu atoms are always monovalent, the substitution of Cu for Zn leads to a decrease in electron count and a lowering of the Fermi level in the valence band. The magnetic susceptibility of RECuZnP2 (RE=Gd-Er), which obeys the Curie-Weiss law, confirms the presence of trivalent RE atoms. -- Graphical abstract: The absence of a band gap in the semimetallic quaternary rare-earth phosphides RECuZnP2 permits the formation of a solid solution such as GdCuxZn2-xP2 through hole-doping of the valence band. Display Omitted