Vanishing Scalar Invariant Spacetimes in Higher Dimensions
  We study manifolds with Lorentzian signature and prove that all scalar curvature invariants of all orders vanish in a higher-dimensional Lorentzian spacetime if and only if there exists an aligned non-expanding, non-twisting, geodesic null direction along which the Riemann tensor has negative boost order. 
A. Coley, R. Milson, V. Pravda and A. Pravdova