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The series of quaternary rare-earth phosphides REMnCuP2 (RE = Y, La–Nd, Sm, Gd–Tm, Lu), which had previously been limited to the Yb member, has been prepared by reaction of the elements at 800 °C. These phosphides adopt the CaAl2Si2-type structure (trigonal, space group P 3 ¯ m 1 , Z = 1) with Mn and Cu atoms disordered over the tetrahedral M sites within [M2P2] slabs. However, single-crystal X-ray diffraction studies, carried out on some of these members (RE = Tb, Dy, Er, Tm), provide evidence for an interstitial octahedral site being partially occupied by Cu atoms, resulting in the nonstoichiometric formulation REMnCu1+xP2 (x = 0.10–0.16). A second polymorph of LaMnCuP2 adopting the BaCu2S2-type structure (orthorhombic, space group Pnma, Z = 4) was also identified in the course of these investigations. Electronic structure calculations on a ordered superstructure model of YMnCuP2 indicate the absence of a band gap.

Reactions of the elements at 800 °C with the nominal compositions RE Ag 1 − x Zn x As 2 resulted in quaternary arsenides RE Ag 1 − x Zn y As 2 in which the combined Ag and Zn content deviates increasingly from unity in the Zn-richer phases, reflecting the transition from the fully stoichiometric ternary silver-containing arsenides RE AgAs 2 to the substoichiometric zinc-containing ones RE Zn 0.67 As 2 . Powder X-ray diffraction analysis indicated SrZnBi 2 -type (space group I 4/ mmm , Z =4; RE =La, Ce) and HfCuSi 2 -type structures (space group P 4/ nmm , Z =2; RE =Pr, Nd, Sm, Gd, Tb, Dy). Single-crystal X-ray diffraction analysis performed on LaAg 0.5 Zn 0.5 As 2 , PrAg 0.5 Zn 0.5 As 2 , and NdAg 0.5 Zn 0.5 As 2 indicated that the Ag and Zn atoms are disordered within metal-centred tetrahedra and provided no evidence for distortion of the square As nets. The small electron excess tolerated in these quaternary arsenides and the absence of distortion in the square nets can be traced to the occurrence of As–As states that are only weakly antibonding near the Fermi level. PrAg 0.5 Zn 0.5 As 2 and NdAg 0.5 Zn 0.5 As 2 are paramagnetic with effective magnetic moments consistent with trivalent RE species.

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 P 3 ¯ 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).

The pnictides RE(2)Mn(3)Cu(9)Pn(7) (Pn = P, As) have been prepared by stoichiometric reaction of the elements at 800 °C. They are quaternary ordered variants of the hexagonal Zr(2)Fe(12)P(7)-type structure (Pearson symbol hP21, space group P6, Z = 1; a = 9.6444(3)-9.5970(7) Å, c = 3.9027(1)-3.7761(3) Å for RE(2)Mn(3)Cu(9)P(7) (RE = La-Nd, Sm, Gd-Dy); a = 9.9376(6)-9.9130(3) Å, c = 4.0194(2)-3.9611(1) Å for RE(2)Mn(3)Cu(9)As(7) (RE = La-Nd)). Of the four possible sites available for the transition metal, the square pyramidal site (CN5) is occupied preferentially by Mn atoms, whereas the three tetrahedral sites (CN4) are occupied by Cu atoms. On proceeding to smaller RE members in the RE(2)Mn(3)Cu(9)Pn(7) series, one of the transition-metal-centered polyhedra (Cu1) tends to become less distorted, while the remaining three (Cu2, Cu3, Mn4) become more distorted. Band structure calculations on La(2)Mn(3)Cu(9)P(7) confirm that Mn-P and Cu-P contacts provide the strongest bonding interactions. Electrical resistivity measurements on Ce(2)Mn(3)Cu(9)P(7) reveal metallic behavior with transitions at 165 and 18 K, probably of magnetic origin.

The quaternary rare-earth phosphides REMnCu(4)P(3) (RE = Gd-Ho) were obtained from direct reactions of the elements at 800 °C. They are the first examples in which ordering of two different transition-metal atoms takes place within the orthorhombic YCo(5)P(3)-type structure [Pearson symbol oP36, space group Pnma, Z = 4; a = 12.667(2)-12.6489(4) Å, b = 3.8119(7)-3.7755(1) Å, and c = 10.895(2)-10.8632(4) Å for RE = Gd-Ho]. Columns of trigonal prisms centered by P atoms are connected in propellor-shaped units in zigzag arrangements to generate square-pyramidal (CN5) sites that are occupied by Mn atoms and tetrahedral sites (CN4) that are occupied by Cu atoms. Spin-polarized band-structure calculations predict that the hypothetical compound YMnCu(4)P(3) will exhibit magnetic ordering. Electrical resistivity measurements on TbMnCu(4)P(3) indicate a poor metal.

Three series of quaternary pnictides, LnCuZnAs2 (Ln: Y, La—Nd, Sm, Gd—Lu), LnAgZnP2 (Ln: La—Nd, Sm, Gd—Dy), and LnAgZnAs2 (Ln: La—Nd, Sm, Gd—Dy) are prepared by solid reaction of the elements (evacuated silica tubes, 1.