Investigating the cause of crossover from charge/spin-stripe insulator to correlated metallic phase in layered 𝑇�′ nickelates 𝑅�4⁢Ni3⁢O8 (𝑅�=La,Pr,orNd)

Abstract

The infinite layered (𝑇′) nickelates have recently garnered significant attention due to the discovery of superconductivity in hole-doped 𝑅⁢NiO2 (𝑅=La,Pr,orNd), which is the 𝑛=∞ member of the series 𝑅𝑛+1⁢Ni𝑛⁢O2⁢𝑛+2. Here, we investigate the 𝑛=3 member, namely 𝑅4⁢Ni3⁢O8 (𝑅=La,Pr,orNd), of this family. The compound La4⁢Ni3⁢O8 exhibits simultaneous charge/spin-stripe ordering at 𝑇*𝑁 = 105 K, which occurs concomitantly with the onset of the metal-to-insulator (MIT) transition below 𝑇*𝑁. We investigate the conspicuous absence of this transition in the Pr and Nd analogs of La4⁢Ni3⁢O8. For this purpose, we synthesized solid solutions of the form (La,Pr)4⁢Ni3⁢O8 and (La,Nd)4⁢Ni3⁢O8, and examined the behavior of 𝑇*𝑁 as a function of the average 𝑅-site ionic radius (𝑟‾𝑅). We show that after an initial quasilinear decrease with decreasing 𝑟‾𝑅, 𝑇*𝑁 suddenly vanishes in the narrow range 1.134≤𝑟‾𝑅≤1.143Å. In the same range, we observed the emergence of a weak anomaly in the specific heat, whose onset temperature (𝑇*) increases as 𝑟‾𝑅 further decreases reaching a maximum of 13 K for Nd4⁢Ni3⁢O8. We suggest, therefore, that the sudden vanishing of charge/spin-stripe/MIT ordering upon decreasing 𝑟‾𝑅 is related to the appearance of this new electronic phase for 𝑟‾𝑅<𝑟𝑐. The nature of this phase or the weak anomaly and the point 𝑟‾𝑅≈𝑟𝑐, where 𝑇*𝑁 vanishes and 𝑇* appears, should be investigated further. In this regard, Pr4⁡Ni3⁢O8 and Pr-rich samples should be useful due to the weak magnetization response associated with the Pr sublattice, as shown here.