Three series of quaternary rare-earth transition-metal pnictides with CaAl2Si2-type structures: RECuZnAs2, REAgZnP2, and REAgZnAs2

Three series of quaternary rare-earth transition-metal pnictides REMM′Pn2 (M=Cu, Ag; M′=Zn; Pn=P, As) have been prepared by reaction of the elements at 800 °C, with crystal growth promoted through the addition of iodine. The extent of RE substitution is broad in these series: RECuZnAs2 (RE=Y, La-Nd, Sm, Gd–Lu), REAgZnP2 (RE=La–Nd, Sm, Gd–Dy), and REAgZnAs2 (RE=La-Nd, Sm, Gd-Dy). Powder and single-crystal X-ray diffraction analysis revealed that they adopt the trigonal CaAl2Si2-type structure (space group P3¯m1, Z=1), in which Cu or Ag atoms are disordered with Zn atoms over the unique tetrahedrally coordinated transition-metal site. Magnetic measurements indicated Curie–Weiss behavior for several members of the RECuZnAs2 and REAgZnP2 series. Core-line X-ray photoelectron spectra (XPS) collected on some RECuZnAs2 members corroborate the charge assignment deduced by the Zintl concept for these compounds, (RE3+)(M1+)(Zn2+)(Pn3−)2. Optical diffuse reflectance spectra and valence band XPS spectra established that these compounds are small band-gap semiconductors (up to ∼0.8 eV in REAgZnP2) or semimetals (RECuZnAs2). Band structure calculations also support this electronic structure and indicate that the band gap can be narrowed through appropriate chemical substitution (RE=smaller atoms, M=Cu, and Pn=As). - Graphical abstract: Cu or Ag atoms are disordered with Zn atoms over the tetrahedral site within relatively rigid [M2Pn2] slabs in three series of quaternary pnictides adopting the CaAl2Si2-type structure. - Highlights: • Three series (comprising 25 compounds) of pnictides REMM'Pn2 were prepared. • Cu or Ag atoms are disordered with Zn atoms within relatively rigid [M2Pn2] slabs. • They are semimetals or small band-gap semiconductors. • RECuZnAs2 and REAgZnP2 are generally paramagnetic