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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.

A series of Heusler phases Ni47Mn40Sn13−xCux (x = 0, 0.5, 1, 1.25), which are magnetic shape-memory alloys, has been synthesized by mechanical alloying (MA) and characterized. The martensite-austenite phase transition was investigated by X-ray diffraction, differential scanning calorimetry, and DC magnetization measurements. With greater Cu content, the martensitic transition temperature TM increases while the magnetic transition temperature of austenite T c A remains almost unchanged. The maximum inverse magnetic entropy changes Δ S M for the x = 0.5 and x = 1 samples are nearly twice as high as for the undoped sample. Furthermore, the martensitic transformation of the doped samples occurs around room temperature, which is important for applications. Effective refrigerant capacities (RCeff) reduced due to the higher hysteresis losses in the x = 0.5 and x = 1 samples compare to the undoped one. Refrigerant capacities (RC) were examined in T c A . The largest value of RC = 207 J/kg was measured for the undoped sample. Critical properties near the ferromagnetic–paramagnetic (FM–PM) phase transition were analyzed in detailed. The critical parameters β, γ, δ and T c A of the x = 0 and x = 0.5 samples, which were determined through use of modified Arrott plots, the Kouvel-Fisher method, and the Widom scaling relation, differed from the mean-field values. The critical behavior indicates that long-range ferromagnetic order dominates in these Cu-doped samples.

The Cr 5 B 3 -type Ta 5 Si 3 phase was prepared by arc-melting, while the Cr 5 B 3 -type Ta 5 Ge 3 one was synthesized through sintering at 1000 °C. X-ray single crystal diffraction was employed to elucidate their structure. According to the magnetization measurements, both Ta 5 Si 3 and Ta 5 Ge 3 are Pauli paramagnets, with Ta 5 Ge 3 showing a Currie-Weiss-like paramagnetic behavior at low temperatures likely due to presence of paramagnetic impurity. Both Ta 5 Si 3 and Ta 5 Ge 3 display a very low electrical resistivity from 2 to 300 K. The resistivity is constant below 20 K, but displays a positive temperature coefficient above 20 K. Electronic structure calculations with the TB-LMTO-ASA method support the metallic character of the two phases and suggest that the bonding is optimized in both phases.

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).