Raman scattering and optical birefringence were used in to investigate a low-temp. phase transition in a single crystal of the 2D Na5RbCu4(AsO4)4Cl2. Phonon anomalies point to a 1st-order nature of the transition. The obsd. transition is most probably related to an order-disorder transition of the Rb ion positions along the z axis within the ionic framework of mixed alkali metal chloride lattices. (c) 2007 American Institute of Physics.