Neutron diffraction study of Ba(N₃)₂

Abstract

The crystal structure of anhydrous barium azide, Ba(Na)2, has been determined bysingle-crystal neutron diffraction methods. The unit cell is monoclinic, a= 9.59, b = 4"39, c= 5.42 A,/~= 99-75 o; space group P2a/m, with two molecules per cell. All atoms were shown by a Patterson map to lie on mirror planes.The trial structure was refined by the method of full-matrix least-squares to a final R index of 0.041. There are two non-equivalent azide groups, a four coordinated apparently symmetric azide and a five coordinated azide with a slight probable asymmetry. Barium atoms are surrounded by nine azides, in the form of a right triangular prism with three azides near the centers of each of the three vertical faces. The structure consists of infinite BaN bond chain layers, packed laterally with long BaN bonds. An inspection of the thermal ellipsoids suggests that the thermal vibrations of the nitrogen atoms can be described largely in terms of rigid-bar angular oscillations of the azide axis with the larger amplitude in the direction of lesser constraint.

Preparation, Crystal Structure, and Properties of Barium Pernitride, BaN2

“…Ba(N 3 ) 2 , purified in such a way, contains no water (<0.01 wt % O) and is stoichiometric according to elemental analysis. The powder X-ray diffraction diagram ( P 2 1 / m with a = 9.617(2), b = 4.395(1), c = 5.438(1) Å, and β = 99.78(3)°) fits the existing literature data …”

Barium–Nitrogen Phases Under Pressure: Emergence of Structural Diversity and Nitrogen-Rich Compounds

“…Stable compounds, which form at the thermodynamic convex hull (Figure 1), were identified at selected pressures, and their stability fields are shown in the pressure−composition phase diagram (Figure 2). It is noteworthy that our EA searches successfully reproduced the experimentally known Ba 3 N, 32 Ba 2 N, 31 BaN 2 , 47 and BaN 6 30,48 structures at atmospheric pressure. When the pressure was increased from 0 to 100 GPa, 13 stable stoichiometries emerged, including 10 N-rich stoichiometries, namely, BaN, Ba 2 N 3 , BaN 2 , BaN 3 , Ba 3 N 8 , BaN 4 , BaN 5 , Ba 2 N 11 , BaN 6 , and BaN 10 , and three Ba-rich compositions, Ba 3 N 2 , Ba 2 N, and Ba 3 N. Several stable Ae−N stoichiometries have been predicted within the same pressure range for Be, 14 Mg, 13,49 and Ca.…”

“…Thus, a stable structure must lie on the convex hull constructed on the plot of the formation enthalpy versus composition N/(Ba + N). If not located on the convex hull, strictly speaking, the phase is thermodynamically unstable and might decompose into other Ba x N y stoichiometries and/or elements, unless the kinetic barrier is sufficiently highas encountered in metastable phases such as the experimental Ba­(N 3 ) 2 azide . We also considered metastable phases with different covalent nitrogen nets and selected the metastable phases with exothermic formation reactions (Δ r H f < 0 meV/atom) that are close to the convex hull (Δ r H f < 150 meV/atom) or which have been experimentally synthesized.…”

“…We predicted the emergence of several (meta)­stable nitrogen networks in 13 different compositions upon compression, namely, Ba:N = 3:1, 2:1, 3:2, 1:1, 2:3, 1:2, 3:8, 1:3, 1:4, 1:5, 2:11, 1:6, and 1:10. To the best of our knowledge, only four of these stoichiometries are experimentally known (3:1, 2:1, 1:2, 1:6). ,− …”

Electrides with Dinitrogen Ligands

“…The corresponding structure details are available in the Supporting Information. Ba 3 N, Ba 2 N, BaN 6 , and BaN 2 have been reported experimentally. − This validated our structure prediction approach used here. Among these known materials, several barium-rich compounds Ba 3 N 2 , Ba 3 N, and Ba 2 N have been proposed to be electrides. , Notably, we identified several new low-energy forms of Ba 2 N, in addition to the well-known R 3̅ m CaCl 2 type.…”