High pressure sequence of Ba$_{3}$NiSb$_{2}$O$_{9}$ structural phases: new $S $= 1 quantum spin-liquids based on Ni$^{2+}$*

A quantum spin-liquid (QSL) is a ground-state where strong quantum- mechanical ?uctuations prevents a phase-transition towards conventional magnetic order and makes the spin ensemble to remain in a liquid-like state. Most QSL candidates studied to date are two-dimensional frustrated magnets with either a triangular or a kagome lattice composed of $S$ = 1/2 spins. Here, we report the use of a high pressure, high temperature technique to transform the antiferromagnetically ordered ($T_{N}$ = 13.5 K) 6H-A phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ into two new QSL candidates with larger $S$ = 1 (Ni$^{2+})$ moments: the 6H-B phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ which crystallizes in a triangular lattice and the 3C-phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ which forms a three-dimensional edge-shared tetrahedral lattice. Both compounds show no evidence for magnetic order down to $T$ = 0.35 K despite Curie-Weiss temperatures \textit{$\theta $}$_{CW}$ of -75.5 K (6H-B) and -182.5 K (3C), respectively. Below $\sim $25 K the magnetic susceptibility of the 6H-B phase is found to saturate at a constant value $\chi $ = 0.013 emu/mol which is followed below 7 K, by a linear in temperature dependence for the magnetic contribution to the specificheat ($C_{M})$ which displays a giant coefficient $\gamma $ = 168 mJ/mol-K$^{2}$ comparable to values observed in heavy-fermion metallic systems. Taken together, both observations indicate the development of a Fermi-liquid like ground-state characterized by a Wilson ratio of 5.6 in this otherwise insulating material It also points to the formation at finite temperatures of a well defined Fermi surface of $S$ = 1 spin-excitations which behave as charged quasiparticles. For the 3C phase one observes $C_{M} \quad \propto \quad T ^{2}$ indicating a unique $S$ = 1 three-dimensional QSL ground-state as previously reported for Na$_{3}$Ir$_{4}$O$_{8}$ although this later compound is composed of Ir$^{4+ }$ions having $S$ = 1/2. \\[4pt] Work done in collaboration with J. G. Cheng, G. Li, J. S. Zhou, J. B. Goodenough, C Xu and H. D. Zhou.