Two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2: phase transitions   and AFMorder as seen by 87Rb NMR
  We report the Rb nuclear magnetic resonance (NMR) results in a recently synthesized Na5RbCu4(AsO4)Cl2. This complex novel two-dimensional (2D) cuprate is an unique magnetic material, which contains layers of coupled Cu4O4 tetramers. In zero applied magnetic field, it orders antiferromagnetically via a second-order low-entropy phase transition at TN = 15(1) K. We characterise the ordered state by 87Rb NMR, and suggest for it a noncollinear rather than collinear arrangement of spins. We discuss the properties of Rb nuclear site and point out the new structural phase transition(s) around 74 K and 110 K. 
Raivo Stern, Ivo Heinmaa, Annika Kriisa, Enno Joon, Sirje Vija,   Jeffrey Clayhold, Mutlu Kartin-Ulutagay, Xunhua Mo, Wendy ...