Interconfiguration fluctuation of cerium in CeN as a function of temperature and pressure

The lattice parameters of the cubic (NaCl-type) compounds LaN, CeN, NdN and ErN were determined in the 77 to 800 K temperature range at normal pressure, and between 1 atm and 80 kbar at room temperature. From the analysis of the crystallographic data it is concluded that La, Nd and Er are trivalent in their nitrides, whereas Ce in CeN shows, under every examined condition, an ionic radius lower than that expected for Ce3+, indicating the possible occurrence of interconfiguration fluctuations between the Ce3+ and Ce4+ states. The average valency of Ce in CeN appears to be 3.85 under standard conditions and its dependence upon the thermodynamic parameters (T,P) was determined; the results indicate that the valency value increases continuously when the temperature decreases or the pressure increases, rising to 3.89 at 100 K, 1 atm and to 3.94 at 298 K, 50 kbar. By extrapolation of the thermal data it appears that Ce should be trivalent in CeN for temperatures higher than 1100 K at normal pressure. To confirm the structural data approach, the thermal behaviour of the magnetic susceptibilities was determined for the four phases in the 77 to 800 K temperature range. A revision of the literature data on these compounds was also carried out. (author)