Na2La4(NH2)14·NH3, a lanthanum-rich intermediate in the ammonothermal synthesis of LaN and the effect of ammonia loss on the crystal structure

Abstract

Abstract Single crystals of Na2La4(NH2)14·NH3 were obtained from supercritical ammonia under ammonobasic conditions at a temperature of 573 K and 120 MPa pressure. It represents a lanthanum-rich intermediate in the ammonothermal synthesis of LaN. Upon aging, the title compound loses the crystal ammonia, resulting in pale crystals of Na2La4(NH2)14, the original space group P212121 being retained in a very similar unit cell. However, the crystal structure reacts to subtle changes in the composition as well as to the modified coordination of particularly the sodium cations interconnecting lanthanum amide layers within a third dimension. Results of Raman spectroscopic studies are reported. The observations of thermal analysis measurements indicating the formation of lanthanum nitride, in combination with the observed retrograde solubility in liquid ammonia, contribute to the knowledge of the ammonothermal crystal growth of lanthanum nitride.