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

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In situ high-pressure X-ray powder diffraction experiments on LaN up to 60.1 GPa at ambient temperature in a diamond-anvil cell revealed a reversible, first-order structural phase transition starting at $22.8 GPa and completed at $26.5 GPa from the ambient cubic phase (Fm 3m, no. 225) to a tetragonal high-pressure phase (P4/nmm, no. 19, a ¼ 4.1060(6), c ¼ 3.0446(6) Å , Z ¼ 2, wR p ¼ 0.011), which has not been claimed in theoretical predictions. HP-LaN is isotypic with a high-pressure polymorph of BaO, which crystallizes in a tetragonally distorted CsCl-type structure. The phase transition is accompanied by a volume collapse of about 11% which corresponds well with the reported data on HP-BaO. A linear extrapolation of the c/a ratio of the tetragonally distorted CsCl-type sub-cell reaches a value c/a ¼ 1 of cubic CsCl-type HP-LaN at 91(12) GPa. In addition, the compressibility of LaN was investigated and resulted in a bulk modulus for the ambient pressure phase of

Dinitrogen (N2) ligation is a common and well-characterized structural motif in bioinorganic synthesis. In solid-state chemistry, on the other hand, homonuclear dinitrogen entities as structural building units proved existence only very recently. High-pressure/high-temperature (HP/HT) syntheses have afforded a number of binary diazenides and pernitrides with [N2](2–) and [N2](4–) ions, respectively. Here, we report on the HP/HT synthesis of the first ternary diazenide. Li2Ca3[N2]3 (space group Pmma, no. 51, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) Å, Z = 4, wRp = 0.08109) was synthesized by controlled thermal decomposition of a stoichiometric mixture of lithium azide and calcium azide in a multianvil device under a pressure of 9 GPa at 1023 K. Powder X-ray diffraction analysis reveals strongly elongated N–N bond lengths of dNN = 1.34(2)–1.35(3) Å exceeding those of previously known, binary diazenides. In fact, the refined N–N distances in Li2Ca3[N2]3 would rather suggest the presence of [N2](3·–) radical ions. Also, characteristic features of the N–N stretching vibration occur at lower wavenumbers (1260–1020 cm(–1)) than in the binary phases, and these assignments are supported by first-principles phonon calculations. Ultimately, the true character of the N2 entity in Li2Ca3[N2]3 is probed by a variety of complementary techniques, including electron diffraction, electron spin resonance spectroscopy (ESR), magnetic and electric conductivity measurements, as well as density-functional theory calculations (DFT). Unequivocally, the title compound is shown to be metallic containing diazenide [N2](2–) units according to the formula (Li(+))2(Ca(2+))3([N2](2–))3·(e(–))2.

ABSTRACT:In situ high-pressure X-ray powder diffraction measurements on Re 2 P up to 37.0 GPa at ambient temperature in diamond-anvil cells were carried out at two different synchrotron facilities (ESRF and DESY). The compressibility of Re 2 P (Pnma, no. 62, a = 5.5464(17), b = 2.9421(8), c = 10.0483(35) Å, V = 163.97(9) Å 3 , Z = 4, R p = 0.1008, wR p = 0.1341 at ambient conditions) was investigated and resulted in a bulk modulus of B 0 = 320(10) GPa after fitting the experimental p-V data to a second-and third-order Birch-Murnaghan equation of state. In addition, the determined bulk modulus is compared to values obtained from an Eulerian strain versus normalized stress plot with values ranging form 315(7) to 321(15) GPa. These experimental findings are confirmed by DFT-calculations ranking Re 2 P amongst ultra-incompressible materials. However, the Vickers hardness of a highpressure sintered Re 2 P-Re x C y composite material in the asymptotic hardness region was found to be of only 13(2) GPa. Electrical conductivity measurements indicate that metallic Re 2 P exhibits Pauli-paramagnetism. Analysis of temperature-dependent in situ Xray diffractometry reveals an approximately isotropic expansion of the lattice parameters with a thermal expansion coefficient of (α(V) = 28.5-32.8(2)·10 -6 K -1 ).

Black powder of Li2N2 is prepared by decomposition of LiN3 in a Walker‐type multianvil assembly (9 GPa, 750 K, 50 min).

Überraschung beim Diazenid: Beim ersten bekannten Alkalimetalldiazenid, Li2N2, wurde ein für Verbindungen der Zusammensetzung A2B2 neuartiger Strukturtyp gefunden (siehe Bild). Spektroskopische Untersuchungen zeigen bei 1328 cm−1 eine signifikante Schwingung, die der Diazenideinheit zugeordnet wird. Die Berechnung der elektronischen Struktur unterstreicht den metallischen Charakter und bestätigt die Besetzung der antibindenden π‐Zustände in [N2]2− zu 50 %.

Electrical conductivity measurements of alkaline earth diazenides SrN2 and BaN2 revealed temperature-dependent metal-like behavior. As CaN2 is isotypic with SrN2 its electronic properties are supposed to show similar characteristics. For the alkali diazenide Li2N2, the corresponding measurement shows not only the typical characteristics of metallic materials but also an unexpected rise in electrical conductivity above 250 K, which is consistent with an ionic contribution. This interpretation is further corroborated by static 6Li and 7Li nuclear magnetic resonance measurements (NMR) of the spin-lattice relaxation time (T 1) over an extended temperature range from 50 to 425 K. We observe a constant Heitler-Teller product (T 1 T) as expected for metals at low temperatures and a maximum in the temperature-dependent relaxation rates, which reflects the suggested ionic conductivity. A topological structural analysis indicates possible 3D ion migration pathways between two of the three crystallographic independent Li positions. A crude estimate of temperature-dependent self-diffusion coefficients D(T) of the lithium motion classifies Li2N2 as a mixed electronic/ionic conductor.