  Guided by experiment and band structure, we introduce and study a
phenomenological Landau theory for the unusual charge and spin ordering
associated with the Mott transition in the perovskite nickelates, with chemical
formula RNiO$_3$, where R=Pr, Nd, Sm, Eu, Ho, Y, and Lu. While the Landau
theory has general applicability, we show that for the most conducting
materials, R=Pr, Nd, both types of order can be understood in terms of a
nearly-nested spin density wave. Furthermore, we argue that in this regime, the
charge ordering is reliant upon the orthorhombic symmetry of the sample, and
therefore proportional to the magnitude of the orthorhombic distortion. The
first order nature of the phase transitions is also explained. We briefly show
by example how the theory is readily adapted to modified geometries such as
nickelate films.
