Synthesis of Alkaline Earth Diazenides M_AEN₂(M_AE= Ca, Sr, Ba) by Controlled Thermal Decomposition of Azides under High Pressure

Abstract

The alkaline earth diazenides M AE N 2 with M AE = Ca, Sr and Ba were synthesized by a novel synthetic approach, namely, a controlled decomposition of the corresponding azides in a multianvil press at highpressure/high-temperature conditions. The crystal structure of hitherto unknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6) Å, c = 5.9844(9) Å, Z = 2, wR p = 0.078) was solved and refined on the basis of powder X-ray diffraction data as well as that of SrN 2 and BaN 2 . Accordingly, CaN 2 is isotypic with SrN 2 (space group I4/mmm (no. 139), a = 3.8054(2) Å, c = 6.8961(4) Å, Z = 2, wR p = 0.057) and the corresponding alkaline earth acetylenides (M AE C 2 ) crystallizing in a tetragonally distorted NaCl structure type. In accordance with literature data, BaN 2 adopts a more distorted structure in space group C2/c (no. 15) with a = 7.1608(4) Å, b and their remarkable properties (e.g., superconductivity, photoluminescence, magnetism and low compressibility comparable to that of c-BN) 15−24 justify the investigation of the crystalline structure, stability, elasticity and electronic structures of the diazenides. However, except for M = Sr, Ba, Os, Ir, Pd and Pt, no other metal diazenides or pernitrides of formula type MN 2 have been synthesized in crystalline form as yet, but have been predicted by density-functional calculations to form under HP/HT conditions. 24−29 In order to extend the class of nitrogen rich metal diazenides or pernitrides, we have targeted new synthetic approaches for these compounds, and we were successful using controlled decomposition of highly reactive precursors like the corresponding azides. In this contribution, we present our novel synthesis route for the alkaline earth diazenides SrN 2 and BaN 2 . In addition, we report on the synthesis, structural, spectroscopic and electronic characterization of the novel alkaline earth diazenide CaN 2 and compare its structure to the predicted model. 26,29

Thermodynamic Routes to Novel Metastable Nitrogen-Rich Nitrides

“…The metastable pernitrides are in contrast to the stable alkali earth diazenides, CaN 2 , SrN 2 , and BaN 2 , of bond order 2 (isoelectronic to OO) which can be synthesized by decomposition of the corresponding azides under high pressure. 80 The discovery of PtN 2 and IrN 2 under high-pressure synthesis, 81−84 coupled with their high bulk moduli (∼400 GPa), spurred an extensive effort to synthesize such ultraincompressible materials. In the past decade, all of the precious metal pernitrides have been synthesized, spanning IrN 2 , PtN 2 , PdN 2 , RuN 2 , RhN 2 , and OsN 2 .…”

Synthesis of Arsenopyrite‐Type Rhodium Pernitride RhN2 from a Single‐Source Azide Precursor

“…Schnick et al successfully used controlled decomposition of azides to obtain diazenides BaN 2 , SrN 2 and CaN 2 , as well as Li 2 N 2 in a large-volume press. [23,24] Transition metals azides and azide-coordinated transition metals salts, such as trans-[Pt(NH 3 ) 2 (N 3 ) 2 ] (N/Pt = 8) [25] or (NH 4 ) 2 [Pt(N 3 ) 6 ] (N/Pt = 20) [26] might be considered as ideal candidates for nitrogen-rich precursors. However, such compounds are difficult in preparation, extremely unstable and can be dangerous.…”

High‐Pressure Synthesis and Characterization of the Alkali Diazenide Li2N2

“…[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] We have been able to extend the range of diazenide composition through a novel synthetic approach employing controlled thermal decomposition of ionic azides in a multianvil device under high-pressure/high-temperature (HP/HT) conditions. [37] We thereby obtained calcium diazenide CaN 2 , which is isotypic with SrN 2 and alkaline earth acetylenides crystallizing in a tetragonally distorted NaCl-type structure. To further extend the compositional range of simple metal diazenides, we turned to the investigation of the thermal decomposition of alkali azides under extreme conditions.…”

“…[41,42] According to Shannon [43] , to compare the obtained LiÀN distances with the sum of the ionic radii, the radius of the diazenide ion has to be evaluated first, as the ionic radii of Li + depends on its coordination. Starting from the structural data of the alkaline earth diazenides CaN 2 , SrN 2 , and BaN 2 [37] we estimated the average radius of one nitrogen atom in the diazenide ion to be 1.27 . Based on this empirical value, the LiÀN distances obtained correspond well with the sum of the ionic radii: r(Li [44] Infrared spectroscopy revealed a significant feature at 1326.8 cm À1 , which was assigned to the NÀN stretching vibration of the diazenide ion, in accordance with previous vibrational spectroscopic measurements on CaN 2 , SrN (a nitride-diazenide), SrN 2 , and BaN 2 .…”

“…Based on this empirical value, the LiÀN distances obtained correspond well with the sum of the ionic radii: r(Li [44] Infrared spectroscopy revealed a significant feature at 1326.8 cm À1 , which was assigned to the NÀN stretching vibration of the diazenide ion, in accordance with previous vibrational spectroscopic measurements on CaN 2 , SrN (a nitride-diazenide), SrN 2 , and BaN 2 . [37,45] To better understand the electronic character of Li 2 N 2 and its bonding situation, first-principles calculations on the electronic structure were performed using the LMTO method. [46] As expected, the electronic band structure calculations suggest that the compound is metallic, which is in good agreement with the black color of the sample.…”