Singlet ground state in the alternating spin- 12 chain compound NaVOAsO4

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline ${\mathrm{NaVOAsO}}₄$ by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and $⁷⁵\mathrm{As}$ 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 $J/{k}_{\mathrm{B}}\ensuremath{\simeq}52$ K and an alternation parameter $\ensuremath{\alpha}\ensuremath{\simeq}0.65$. From the high-field magnetization measured at $T=1.5$ K, the critical field of the gap closing is found to be ${H}_{\mathrm{c}}\ensuremath{\simeq}16$ T, which corresponds to the zero-field spin gap of ${\mathrm{\ensuremath{\Delta}}}₀/{k}_{\mathrm{B}}\ensuremath{\simeq}21.4$ 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, ${\mathrm{NaVOAsO}}₄$ is a promising compound for further experimental studies under high magnetic fields.