  The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground
state. We have investigated the nature of ground state of Sr3NiPtO6 using
magnetic susceptibility $\chi(T)$, heat capacity $C_{\rm p}(T)$, muon spin
relaxation ($\mu$SR) and inelastic neutron scattering (INS) measurements. The
$\chi(T)$ and $C_{\rm p}(T)$ do not exhibit any pronounced anomaly that can be
associated with a phase transition to a magnetically ordered state. Our $\mu$SR
data confirm the absence of long-range magnetic ordering down to 0.04 K.
Furthermore, the muon spin relaxation rate increases below 20 K and exhibits
temperature independent behavior at low temperature, very similar to that
observed in a quantum spin-liquid system. The INS data show a large excitation
near 8~meV, and the analysis of the INS data reveals a singlet CEF ground state
with a first excited CEF doublet state at $\Delta_{\rm CEF}$ = 7.7 meV. The
estimated CEF parameters reveal a strong planar anisotropy in the calculated
$\chi(T)$, consistent with the reported behavior of the $\chi(T)$ of single
crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a
large $\Delta_{\rm CEF}$ (much larger than the exchange interaction
$\mathcal{J}_{\rm ex}$) are responsible for the absence of long-range magnetic
ordering and can mimic a classical spin-liquid behavior in this quasi-1D spin
chain system Sr3NiPtO6. The classical spin-liquid ground state observed in
Sr3NiPtO6 is due to the single-ion property, which is different from the
quantum spin-liquid ground state observed in geometrically frustrated systems,
where two-ion exchanges play an important role.
