Anomalous chemical pressure evolution in ThFePnN (Pn for pnictogens)

The layered compound ThFeAsN is the only member in the 1111-type iron-arsenides which exhibits superconductivity without chemical substitution or physical pressure. The unique properties of ThFeAsN were attributed to built-in chemical pressure. In this paper, we investigate the effects of iso-valent substitution of P/Sb for As in ThFePnN systems to modulate the chemical pressure. The physical properties are characterized by measuring magnetic susceptibility, electric resistivity, and specific heat. Our findings indicate that in the P-doped ThFeAs1−xPxN system, superconductivity disappears at x = 0.3, and we did not observe a second superconducting dome in the phase diagram. Instead, a possible low-temperature magnetic/charge ordering state emerges around the end member ThFePN. In the case of the Sb-doped ThFeAs1−ySbyN samples, superconductivity is gradually suppressed at y = 0.25, and the expected long-range antiferromagnetic transition does not occur thereafter. Thus, the physical properties of ThFePnN samples significantly differ from those of LaFePnO, regardless of P or Sb doping. Structural analysis reveals that the key parameters, such as the height of the pnictogens and the two-fold Fe-Pn-Fe bond angle, vary non-monotonically with doping. The inconsistency between ThFePnN and LaFePnO is discussed based on the crystal structure evolution.