Topochemical Nitridation with Anion Vacancy-Assisted N^3–/O^2– Exchange

Abstract

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.

New chemistry of transition metal oxyhydrides

“…However, even at higher temperatures where the hydride has already left, the remaining anion-deficient structure can be useful for further anion-exchange reactions. This has been demonstrated by the formation of EuTiO 2 N by Mikita et al [ 41 ]. Here, EuTiO 2.82 H 0.18 was first prepared.…”

“…For Eu [ 30,41 ], or other highly absorbing elements (e.g. Sm, Gd, Dy) NPD should be nonetheless avoided, though not impossible with a special cell arrangement [ 41 ]. High-resolution time-of-flight (TOF) NPD instruments are often more suited than constant-wavelength diffractometers to determine subtle structural features, for instance coexistence of O 2– and H – on the same site, as shown in Ln SrCo 3+ α H β oxyhydrides ( Ln = Pr, Nd) [ 9 ].…”

Defect Engineering for Photocatalysis: From Ternary to Perovskite Oxynitrides

“…This reduced evolution was attributed to the lower exciton lifetime arising from the larger amount of intrinsic anion vacancies in the PrTaON 2 lattice, along with the presence of a more reduced Ta cation. Mikita et al . recently demonstrated that synthesis of EuTiO 2 N in a nitriding atmosphere led to a metastable EuTiO 2.82 N 0.12 □ 0.06 intermediate phase (where □ denotes an anion vacancy defect) in which anion vacancies facilitated N 3− ↔O 2− exchange.…”

“…recently demonstrated that synthesis of EuTiO 2 N in a nitriding atmosphere led to a metastable EuTiO 2.82 N 0.12 □ 0.06 intermediate phase (where □ denotes an anion vacancy defect) in which anion vacancies facilitated N 3− ↔O 2− exchange. Using a EuTiO 3 precursor this exchange resulted in a maximum N stoichiometry of 0.75, whereas a Eu 2 Ti 2 O 7 precursor decomposed into Eu 2 O 3 and uncharacterized phases . Oka et al .…”

Ammonothermal Synthesis of the Mixed‐Valence Nitrogen‐Rich Europium Tantalum Ruddlesden‐Popper Phase EuIIEuIII2Ta2N4O3

“…Europium can occur in oxidation states Eu II and Eu III and therefore the N/O distribution is variable. The Eu valence state can be probed with different analytical techniques like Mößbauer spectroscopy and susceptibility measurements …”