  In contrast to the simultaneous structural and magnetic first order phase
transition $T_{0}$ previously reported, our detailed investigation on an
underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously
revealed that the transitions are not concomitant. The tetragonal ($\tau$:
I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at
$T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition
at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $\tau$ and
$\vartheta$ phases over a finite temperature range observed in our NMR
experiments confirm the first order character of the structural transition and
provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our
data also show that superconductivity (SC) develops in the $\vartheta$ phase
below $T_{c}$ = 20 K and coexists with long range AFM. This new observation,
$T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped
BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide
superconductors.
