Chemical Twinning of Salt and Metal in the Subnitridometalates Ba₂₃Na₁₁(MN₄)₄ with M=V, Nb, Ta

Abstract

The subnitridometalates Ba23 Na11 (MN4 )4 (M=V, Nb, Ta) crystallize in a new structure type, which shows ionic ortho-nitridometalate anions and motifs from simple (inter)metallic packings: Na-centered [Na8 ] cubes as cutouts of the bcc structure of elemental Na and Na-centered [Ba10 Na2 ] icosahedra as found in Laves phases, for example. Single-crystal and powder X-ray diffraction studies in combination with quantum-chemical calculations of the electronic structure and Raman spectroscopy support the characterization of the subnitridometalates as "chemical twins". They consist of independent building units with locally prevalent ionic or metallic bonding in an overall metallic compound.

The Triple Salt Sr14[Ta4N13][TaN4]O–A Nitridotantalate Oxide with 19-fold Rock Salt Superstructure

“…Bond angles (see Table S4 in the Supporting Information) range from 99.7(2) to 127.3(3)°. The interatomic Ta–N distances vary between 1.961(5) and 2.006(6) Å (see Table S2 in the Supporting Information and Figure ) and are very well in agreement with those found in comparable crystal structures. ,, The strong distortion of the [TaN 4 ] 7– tetrahedron may be attributed to the coordination needs of the N atoms versus Ta and Sr cations. All N atoms have a sixfold coordination by one Ta and five Sr atoms (see Table and Figure b), arranged in distorted octahedra.…”

“…Our synthetic approach based on the application of reducing conditions with surplus elemental alkali or earth alkaline metal combined with relatively low temperatures can yield metal-rich compounds in two ways. As we have shown previously, subvalent compounds, for example, subnitridometalates, can be prepared . Here we could show that the formation of ionic structures with unusually small, highly charged anions can also be observed.…”

“…Literature data for known ortho- and ino- nitridotantalate compounds is given for comparison. t = terminal; b = bridging. − , …”

“…Because of the unexpected and very unusual condensation degree in Sr 14 [TaN 4 ]­[Ta 4 N 13 ]O the question of thermodynamic stability arises with regard to decomposition of the triple salt into the three quasi-binary phases and also the “comproportionation” of the tetracatena- and the ortho- nitridotantalate anion into one with intermediate chain length. Taking this into account it is important to emphasize that the reaction of Sr 2 N and Sr with elemental tantalum in a sodium flux actually was aimed at the synthesis of a different metastable compound class, the so-called subnitridometalates, as has been done in previous work …”

Festkörperchemie 2016

“…Hoch und Mitarbeiter stellten die Subnitridometallate Ba 23 Na 11 -(MN 4 ) 4 (M = V, Nb, Ta) dar, drei Verbindungen, die sich wie chemische Zwillinge verhalten. 6) Die Kristallstruktur enthält neben diskreten tetraedrischen ortho-Nitridometallat-[MN 4 ] 7--Ionen auch Baueinheiten, die in einfachen (inter)metallischen Phasen vorkommen: Neben verzerrten natriumzentrierten Na@Na 8 -Würfeln treten Na@Ba 10 Na 2 -Ikosaeder auf, wie sie etwa in Laves-Phasen zu finden sind. Die Nitridometallat-Tetraeder sind von sieben Bariumatomen in Form einfach überkappter trigonaler Prismen umgeben.…”