  We investigate the magnetic properties of LiYbO$_2$, containing a
three-dimensionally frustrated, diamond-like lattice via neutron scattering,
magnetization, and heat capacity measurements. The stretched diamond network of
Yb$^{3+}$ ions in LiYbO$_2$ enters a long-range incommensurate, helical state
with an ordering wave vector ${\bf{k}} = (0.384, \pm 0.384, 0)$ that "locks-in"
to a commensurate ${\bf{k}} = (1/3, \pm 1/3, 0)$ phase under the application of
a magnetic field. The spiral magnetic ground state of LiYbO$_2$ can be
understood in the framework of a Heisenberg $J_1-J_2$ Hamiltonian on a
stretched diamond lattice, where the propagation vector of the spiral is
uniquely determined by the ratio of $J_2/|J_1|$. The pure Heisenberg model,
however, fails to account for the relative phasing between the Yb moments on
the two sites of the bipartite lattice, and this detail as well as the presence
of an intermediate, partially disordered, magnetic state below 1 K suggests
interactions beyond the classical Heisenberg description of this material.
