  We present the synthesis and a detailed investigation of structural and
magnetic properties of polycrystalline NaVOAsO$_4$ by means of x-ray
diffraction, magnetization, electron spin resonance (ESR), and $^{75}$As
nuclear magnetic resonance (NMR) measurements as well as density-functional
band structure calculations. Temperature-dependent magnetic susceptibility, ESR
intensity, and NMR line shift could be described well using an alternating
spin-$1/2$ chain model with the exchange coupling $J/k_{\rm B}\simeq 52$ K and
an alternation parameter $\alpha \simeq 0.65$. From the high-field magnetic
isotherm measured at $T=1.5 $ K, the critical field of the gap closing is found
to be $ H_{\rm c}\simeq 16$ T, which corresponds to the zero-field spin gap of
$\Delta_0/k_{\rm B}\simeq 21.4$ K. Both NMR shift and spin-lattice relaxation
rate show an activated behavior at low temperatures, further confirming the
singlet ground state. The spin chains do not coincide with the structural
chains, whereas the couplings between the spin chains are frustrated. Because
of a relatively small spin gap, NaVOAsO$_4$ is a promising compound for further
experimental studies under high magnetic fields.
