Octahedral and trigonal-prismatic coordination preferences in Nb-, Mo-, Ta-, and W-based ABX2 layered oxides, oxynitrides, and nitrides

Abstract

Crystallographic and electronic structures of Nb-, Mo-, Ta-, and W-based layered oxides, oxynitrides, and nitrides were analyzed to elucidate the structural relationship between layered oxides and nitrides consisting of octahedral and trigonal-prismatic layers. The electron density, as derived by synchrotron X-ray analysis of LiNbO2 and Ta5−x(O,N)6, showed orbital overlaps between Nb-Nb and TaTa metals in the trigonal layers. Computational calculations based on DFT exhibited that these overlaps stabilized these structures by lowering the hybridization states composed of the , 2 − 2 , and 2 orbitals below the Fermi level. The crystal structures and formation energies suggest that tuning the Fermi level through the substitutions and vacancies of the cation/anion sites determines the structural preferences of the coordination. The properties and syntheses of these compounds are briefly described. This study enhances the understanding of layered oxides, oxynitrides, and nitrides to further the development of new synthetic approaches, compounds, and applications.

Selective metathesis synthesis of MgCr2S4 by control of thermodynamic driving forces

“…For example, the reaction of chlorides/oxides with H2S or NH3 gas could also be used to synthesize sulfides, oxysulfides, nitrides and oxynitrides by generating HCl/H2O gas as a byproduct. [26][27][28] Inclusion of these extra species into the phase diagram changes the relevant chemical reactions and drastically modifies the thermodynamic landscape. The additional degrees of freedom afforded by including these extra elements into the phase diagram opens up a vast and promising design space for clever metathesis reactions.…”

Selective Metathesis Synthesis of MgCr2S4 by Control of Thermodynamic Driving Forces

“…For example, the reaction of chlorides/oxides with H2S or NH3 gas could also be used to synthesize sulfides, oxysulfides, nitrides and oxynitrides by generating HCl/H2O gas as a byproduct. [26][27][28] Inclusion of these extra species into the phase diagram changes the relevant chemical reactions and drastically modifies the thermodynamic landscape. The additional degrees of freedom afforded by including these extra elements into the phase diagram opens up a vast and promising design space for clever metathesis reactions.…”

Low-temperature synthesis and rational design of nitrides and oxynitrides for novel functional material development

“…Figure 9 shows the energy differences between trigonal prismatic and octahedral coordination of layered Nb and Mo oxides/ oxynitrides/nitrides in terms of d-electron counts based on density functional theory (DFT) calculations, revealing that electron counts of 12.4 favor trigonal prismatic coordination, with other counts (up to a maximum of 3) favoring octahedral coordination. 81) Trigonal prismatic coordination increases the hybridization of d-orbitals, stabilizing the corresponding structures formed by non-closed-packed anions, which can be experimentally visualized using XRD-derived electron density data. 81) The experimentally obtained structures of Nb-, Mo-, Ta-, and W-based layered oxides/oxynitrides/nitrides agree with the electron count results.…”

“…81) Trigonal prismatic coordination increases the hybridization of d-orbitals, stabilizing the corresponding structures formed by non-closed-packed anions, which can be experimentally visualized using XRD-derived electron density data. 81) The experimentally obtained structures of Nb-, Mo-, Ta-, and W-based layered oxides/oxynitrides/nitrides agree with the electron count results. 81) Furthermore, structures containing 3dand 4d-metals, e.g., Fe x WN 2 , 82)86) MnMoN 2 , 82) AgTaN 2 , 59),60) CuNbN 2 , 87), 88) and CuTaN 2 , 89) can be rationalized in the same manner, assuming that Cu + and Ag + are monovalent and Fe 2+ and Mn 2+ are divalent.…”