2014 Nous avons mesuré la chaleur spécifique de monocristaux de nitrite de baryum et potassium K2Ba(NO2)4 entre 130 K et 435 K. Il apparait deux transitions, respectivement à 208,6 K et 428 K. La transition basse-température présente les caractéristiques d'une transition du second ordre et l'excès de chaleur spécifique est caractérisé par les exposants critiques 03B1 = 03B1' = 0,60 ± 0,03. La transition haute-température apparait à quelques degrés de la décomposition de K2Ba(NO2)4 et ne peut

more »

... e complètement étudiée. Abstract. -The specific heat of single crystals of potassium baryum nitrite K2Ba(NO2)4 has been measured between 130 K and 435 K. There are two transitions at 208.6 K and 428 K, respectively. The low temperature transition exhibits second order features and the excess specific heat is characterized by critical exponents 03B1 = 03B1' = 0.60 ± 0.03. The high temperature transition occurs a few degrees below the decomposition temperature and so cannot be studied in detail. J. Physique 43 (1982) 337-339 FTVRIER 1982, Classification Physics Abstracts 77.80B -65.40 1. Introduction. -Little [1-6] has been published on the properties of potassium baryum nitrite K2Ba(N02)4. Their crystallographic properties at room temperature were first described a long time ago by Fock [1]. More recently, IvanQv [4] reported optical, dielectric and ferroelastic properties of K2Ba(N02)4 and pointed out that this crystal undergoes a phase transition in the vicinity of 146 ~C between a high temperature hexagonal phase 6/mmm (a phase) and an orthorhombic mmm phase (j8 phase) stable below 146 ~C. Moreover, from birefringence measurements, he suggests that a second phase transition takes place in the vicinity of -70 ~C between the orthorhombic ~i phase and a third phase (y phase). It is suggested that the high temperature transition ( T ~ 146 ~C) is second order [4] whereas there is no information relating to the character of the low temperature phase. We have investigated the thermal properties of K2Ba(N02)4 over a large range of temperature, in order to clarify the existence and type of the low temperature phase transition suggested by Ivanov [4].