High-Pressure Sequence of ${\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$ Structural Phases: New $S=1$ Quantum Spin Liquids Based on ${\mathrm{Ni}}^{2+}$

Two new gapless quantum spin-liquid candidates with $S=1$ (${\mathrm{Ni}}^{2+}$) moments: the $6H\mathrm{\text{−}}\mathrm{B}$ phase of ${\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$ with a ${\mathrm{Ni}}^{2+}$-triangular lattice and the 3C phase with a ${\mathrm{Ni}}_{2/3}{\mathrm{Sb}}_{1/3}$-three-dimensional edge-shared tetrahedral lattice were obtained under high pressure. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures ${\theta }_{\mathrm{CW}}$ of $-75.5$ ($6H\mathrm{\text{−}}\mathrm{B}$) and $-182.5\mathrm{K}$ (3C), respectively. Below $\sim 25\mathrm{K}$, the magnetic susceptibility of the $6H\mathrm{\text{−}}\mathrm{B}$ phase saturates to a constant value ${\chi }_0=0.013\mathrm{emu}/\mathrm{mol}$, which is followed below 7 K by a linear-temperature-dependent magnetic specific heat ($C_M$) displaying a giant coefficient $\gamma =168\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$. Both observations suggest the development of a Fermi-liquid-like ground state. For the 3C phase, the $C_M\propto T^2$ behavior indicates a unique $S=1$, 3D quantum spin-liquid ground state.