Unusual superconductivity in crystallographically disordered RT 2−xSn2 compounds

The discovery of new superconducting materials represents a complementary and crucial tile to shed light on the mechanism controlling the phenomenon of superconductivity. During the search for new superconducting materials, we discovered a new series of intermetallic compounds with the general composition RZn2−xSn2 (R = La, Ce, Pr, Nd). Their formation and crystal structure have been investigated. The Zn content decreases along the series from ≈ 1.50 in LaZn1.50(1)Sn1.98(2) to ≈ 1.32 in NdZn1.321(8)Sn2.01(2), due to the partial occupation of one of the two Zn sites [the Zn1 (2c) site, which from ≈ 50% in the La compound drops to ≈ 42%, ≈ 37% and ≈ 32% in the Ce, Pr and Nd compounds, respectively]. Both the Zn occupation factor and unit cell volume follow the lanthanide contraction. No such compound has been observed with Sm. AC and DC magnetic susceptibility and transport measurements revealed LaZn1.5Sn2 to undergo a sharp superconducting transition at Tc = 5.5 K. In order to search for further possible superconductive homologues, the lanthanum substituted phases La(Zn,T)2–xSn2 (T = Ti, Mn, Fe, Co, Ni, Cu, Pd, Ag, Pt, Au, Cd; x ≈ 0.5) were also prepared. The crystal structure of the ternary compounds is a disordered defective derivative of the tetragonal CaBe2Ge2-type (tP10, P4/nmm), with a partially occupied Zn Wyckoff site and a disordered Sn position. Three related isostructural series, corresponding to three new prototypes, have been identified; they all contain conventional checkered pyramidal layers characteristic of the archetype. The magnetic and transport properties have been measured for some of these substituted quaternary La compounds and we found that superconductivity is preserved in them but in the orthorhombic compounds or when T was a magnetic transition metal (e.g., Ni or Mn). A slight decrease in Tc down to about 4.0 K was detected as a result of the Zn by T atomic substitution.