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We present the synthesis and a detailed investigation of structural andmagnetic properties of polycrystallineNH$_4$[(V$_2$O$_3$)$_2$(4,4$^\prime$-$bpy$)$_2$(H$_2$PO$_4$)(PO$_4$)$_2$]$\cdot$0.5H$_2$Oby means of x-ray diffraction, magnetic susceptibility, electron spinresonance, and $^{31}$P nuclear magnetic resonance measurements. Temperaturedependent magnetic susceptibility could be described well using a weaklycoupled spin-$1/2$ dimer model with an excitation gap $\Delta/k_{\rm B}\simeq26.1$ K between the singlet ground state and triplet excited states and a weakinter-dimer exchange coupling $J^\prime/k_{\rm B} \simeq 4.6$ K. A gapped chainmodel also describes the data well with a gap of about 20 K. The ESR intensityas a function of temperature traces the bulk susceptibility nicely. Theisotropic Land$\acute{\rm e}$ $g$-factor is estimated to be about $g \simeq1.97$, at room temperature. We are able to resolve the $^{31}$P NMR signal ascoming from two inequivalent P-sites in the crystal structure. The hyperfinecoupling constant between $^{31}$P nucleus and V$^{4+}$ spins is calculated tobe $A_{\rm hf}(1) \simeq 2963$ Oe/$\mu_{\rm B}$ and $A_{\rm hf}(2) \simeq 1466$Oe/$\mu_{\rm B}$ for the P(1) and P(2) sites, respectively. Our NMR shift andspin-lattice relaxation rate for both the $^{31}$P sites show an activatedbehaviour at low temperatures, further confirming the singlet ground state. Theestimated value of the spin gap from the NMR data measured in an applied fieldof $H = 9.394$ T is consistent with the gap obtained from the magneticsusceptibility analysis using the dimer model. Because of a relatively smallspin gap,NH$_4$[(V$_2$O$_3$)$_2$(4,4$^\prime$-$bpy$)$_2$(H$_2$PO$_4$)(PO$_4$)$_2$]$\cdot$0.5H$_2$Ois a promising compound for further experimental studies under high magneticfields.

physical review. b./physical review. b

Singlet ground state in the spin-$1/2$ weakly coupled dimer compound  NH$_4$[(V$_2$O$_3$)$_2$(4,4$^\prime$-$bpy$)$_2$(H$_2$PO$_4$)(PO$_4$)$_2$]$\cdot$0.5H$_2$O