Investigating the cause of crossover from charge/spin-stripe insulator to correlated metallic phase in layered T′ nickelates R4Ni3O8 (R=La,Pr,orNd)

The infinite layered ($T^{\prime }$) nickelates have recently garnered significant attention due to the discovery of superconductivity in hole-doped ${R\text{NiO}}_2$ ($R=\mathrm{La},\mathrm{Pr},\text{or}\mathrm{Nd}$), which is the $n=\infty$ member of the series $R_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{2n+2}$. Here, we investigate the $n=3$ member, namely $R_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ ($R=\mathrm{La},\mathrm{Pr},\text{or}\mathrm{Nd}$), of this family. The compound ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ 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 ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$. For this purpose, we synthesized solid solutions of the form $(\mathrm{La},{\mathrm{Pr})}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ and $(\mathrm{La},{\mathrm{Nd})}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$, and examined the behavior of $T_N^{*}$ as a function of the average $R$-site ionic radius ($r_{\overline{R}}$). We show that after an initial quasilinear decrease with decreasing $r_{\overline{R}}, T_N^{*}$ suddenly vanishes in the narrow range $1.134\le r_{\overline{R}}\le 1.143\text{Å}$. In the same range, we observed the emergence of a weak anomaly in the specific heat, whose onset temperature ($T^{*}$) increases as $r_{\overline{R}}$ further decreases reaching a maximum of 13 K for ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$. We suggest, therefore, that the sudden vanishing of charge/spin-stripe/MIT ordering upon decreasing $r_{\overline{R}}$ is related to the appearance of this new electronic phase for $r_{\overline{R}}<r_c$. The nature of this phase or the weak anomaly and the point $r_{\overline{R}}\approx r_c$, where $T_N^{*}$ vanishes and $T^{*}$ appears, should be investigated further. In this regard, ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ and Pr-rich samples should be useful due to the weak magnetization response associated with the Pr sublattice, as shown here.