Phase transitions at 462 and 495 K in barium nitrite

摘要

New phase transitions at 462 and 495 K have been found in Ba(NO2)2. The increase in entropy at each transition is about 13 Jthinsp;molminus;1thinsp;Kminus;1. A gain of two orientations per independent NOminus;2ion at each transition corresponds to an entropy increase of 11.5 Jthinsp;molminus;1thinsp;Kminus;1. The crystal melts at 516 K. The highhyphen;temperature phase I is metastably retained at 298 K on rapid solidification, reverting to phase III on reheating above 350 K. Phase III is orthorhombic with lattice constants at 298 K ofa= 7.657,b= 8.355, andc= 6.742 Aring; and four Ba(NO2)2in the unit cell. The linear thermal expansion coefficients are 30.7, 14.0, and minus;5.9times;10minus;6Aring;thinsp;Kminus;1for thea,b, andcaxes, respectively. The powder pattern for phase II has not been indexed. Phase I may be fitted by a monoclinic unit cell witha= 6.692,b= 7.052, andc= 9.638 Aring; and bgr; = 104.61deg; at 507 K with four Ba(NO2)2in the unit cell. A dielectric anomaly is observed at 468 K in the frequency range 0.1ndash;1 MHz: dielectric losses obscure the anomaly at lower frequencies.