From H₁₂C₄N₂CdI₄ to H₁₁C₄N₂CdI₃: a highly polarizable CdNI₃ tetrahedron induced a sharp enhancement of second harmonic generation response and birefrin

The replacement of CdI₄ with a CdNI₃ unit results in H₁₁C₄N₂CdI₃ featuring an overwhelming NLO effect (Cc, 6 × KDP, Δn = 0.074 @1064 nm, phase matchable) compared to those of H₁₂C₄N₂CdI₄ (P2₁2₁2₁, 0.5 × KDP, 0.047 @1064 nm, non phase matchable).