Excitation Spectrum and Superexchange Pathways in the Spin DimerVODPO4̇12D2O

Magnetic excitations have been investigated in the spin dimer material ${\mathrm{VODPO}}_4̇\frac{1}{2}{\mathrm{D}}_2\mathrm{O}$ using inelastic neutron scattering. A dispersionless magnetic mode was observed at an energy of 7.81(4) meV. The wave vector dependence of the scattering intensity from this mode is consistent with the excitation of isolated ${\mathrm{V}}^{4+}$ spin dimers with a V-V separation of 4.43(7) Å. This result is unexpected since the V-V pair previously thought to constitute the magnetic dimer has a separation of 3.09 Å. We identify an alternative V-V pair as the likely magnetic dimer, which involves superexchange pathways through a covalently bonded ${\mathrm{PO}}_4$ group. This surprising result casts doubt on the interpretation of $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$ as a spin ladder.