  We report the discovery of a spin one diamond lattice in NiRh2O4. This spinel
undergoes a cubic to tetragonal phase transition at T = 440 K that leaves all
nearest neighbor interactions equivalent. In the tetragonal phase,
magnetization measurements show a Ni2+ effective moment of p(eff) = 3.3(1) and
dominant antiferromagnetic interactions with {\Theta}(CW) = -11.3(7) K. No
phase transition to a long-range magnetically ordered state is observed by
specific heat measurements down to T = 0.1 K. Inelastic neutron scattering
measurements on sub-stoichiometric NiRh2O4 reveal possible valence-bond
behavior and show no visible signs of magnetic ordering. NiRh2O4 provides a
platform on which to explore the previously unknown and potentially rich
physics of spin one interacting on the diamond lattice, including the
realization of theoretically predicted quantum spin liquid and topological
paramagnet states.
