N Nuclear Quadrupole Resonance Study of the Phase Transitions of Ba[Fe(CN) 5 NO]·3H 2 O

The 14 N nuclear quadrupole resonance (NQR) of the cyanides in Ba[Fe(CN) s NO] · 3H 2 O (BNP) was observed between 77 and 293 K. Phase transitions exist at 118,131 (both weakly first order) and 233 K (second order). Most of the cyanides were perturbed by the structural changes except for one of the trans cyanides that was only affected by the transition at 131 K. The mechanism of the phase transitions is interpreted in terms of soft modes resulting from the ion rearrangement produced by the slowing down of the water molecules upon cooling. A comparison of the NQR spectrum of BNP with that of Na 2 [Fe(CN) 5 NO] · 3H 2 O showed that the substitution of Na + by Ba 2+ produced only minor changes in the charge distribution of the nitroprusside ion.