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High-pressure and high-temperature chemical reaction of arsenic and nitrogen: the discovery of crystalline arsenic nitride

Crystalline arsenic nitride (AsN) has been synthesised for the first time under high-pressure and high-temperature conditions by direct chemical reaction of arsenic and molecular nitrogen. The new compound is made of a covalent three-dimensional network of single-bonded As and N atoms, where non-bonding electron lone pairs play a key role in structure stabilisation.

Nitrogen (N) and arsenic (As) represent the first and third element of group 15 in the periodic table of the elements. Both elements are known to form compounds with phosphorus, the second element in the group, but the observation of an extended crystalline compound entirely made of covalently bonded As and N has never been reported so far. Arsenic and molecular nitrogen do not react spontaneously at ambient conditions. Nevertheless, it is known that under high-pressure (>100 GPa) and high-temperature (~2000 K) conditions, the N2 molecules become unstable and, upon cleavage of the triple bond, polymerise, giving origin to a crystalline form of N, called cubic gauche nitrogen (cg-N), where each N atom is single bonded to three other N atoms. cg-N is considered one of the highest energy density non-nuclear materials and has attracted strong interest for potential applications as green propellant and explosive. However, despite several efforts, this material has never been recovered to ambient conditions so far.

Fig. 12: Selected integrated XRD patterns corresponding to panoramic acquisitions at room T and 30.0 GPa on different points of the sample laser-heated at 25.0 GPa. The patterns were acquired in the points of the grid superimposed on the sample image (a), which shows a shining area at the centre, corresponding to the original piece of As, in the simple cubic (sc) As-II structure, surrounded by N2 in the crystalline ε-N2 structure. The violet full circles on the grid indicate the points of the sample where the reaction product AsN was detected (violet upper trace), whereas the unlabelled points on the outer part of the grid indicate the sample points where only ε-N2 (cyan lower trace) or only ε-N2 and sc-As (blue middle trace) were observed. A portion of a detector image, acquired in one of the violet points of the grid, is shown in (b).

Fig. 13: a,b) Views of the P213 unit cell of AsN (As atoms in violet and N atoms in light blue) along the [100] crystallographic direction. The shaded polyhedra indicate the AsN3 (a) and NAs3 (b) trigonal pyramids mentioned in the text and highlighted on the top of the Figure. c,d,e) Selected regions of the AsN unit cell showing different As4 cage-like tetrahedral structures arranged along one of the C3 axes: (c) As04-As01-As01-As01, (d) As03-As02-As02-As02, (e) As04-As02-As02- As02 and As03-As01-As01-As01. The structures refer to AsN at 35.6 GPa and 293 K.

Benefitting from the experimental approach recently explored to activate chemical reactivity between P and H2 [1], with the formation of PH3 and the discovery of (PH3)2H2, and between P and N2 [2], with the obtainment of the α- and γ-P3N5 crystalline polymorphs from the elements, this work reports inducing a direct chemical reaction between As and N2 under high-pressure and high- temperature conditions (>25 GPa, >1400 K), generated by a laser-heated diamond anvil cell, leading to the first synthesis of crystalline arsenic nitride (AsN).

Utilising the high-brilliance radiation and small beam spot size available at beamlines ID27 and ID15B, the sample was spatially mapped by synchrotron X-ray diffraction (XRD) with micron-size spatial resolution, enabling a full structural characterisation of the reactants and reaction product (Figure 12). The cubic crystalline structure of AsN (P213) was determined by single-crystal XRD at different pressures between 30 and 40 GPa, providing