  Low-valent nickelates R$_{n+1}$Ni$_n$O$_{2n+2}$ (R = rare earth) containing
Ni$^{1+}$ (d$^{9}$) with a quasi-two-dimensional (quasi-2D) square planar
coordination geometry possess structural and electronic properties that are
similar to those of high-Tc cuprates, including superconductivity itself in the
doped infinite layer ($n = \infty$) RNiO$_2$ system. Within this
R$_{n+1}$Ni$_n$O$_{2n+2}$ nickelate family, the crystallographic isomorphs
Pr$_4$Ni$_3$O$_8$ and La$_4$Ni$_3$O$_8$ exhibit singularly different ground
states: Pr$_4$Ni$_3$O$_8$ is metallic and La$_4$Ni$_3$O$_8$ is a charge- and
spin-stripe ordered insulator. To explore and understand the ground state
evolution from metallic Pr$_4$Ni$_3$O$_8$ to stripe-ordered La$_4$Ni$_3$O$_8$
in the R$_4$Ni$_3$O$_8$ family, we have grown a series of isovalent-substituted
single crystals (Pr$_{1-x}$La$_x$)$_4$Ni$_3$O$_8$. Combining thermodynamic,
transport, magnetic, and synchrotron X-ray single crystal diffraction
measurements, we reveal a transition between metallic and stripe-insulator
phase regions, with a putative quantum phase transition at x = 0.4. We propose
two possible models for (Pr$_{1-x}$La$_x$)$_4$Ni$_3$O$_8$: an electronically
inhomogeneous system that could serve as a candidate for exploring quantum
Griffiths phase physics and a homogeneous system with a putative quantum
critical point at the phase boundary.
