Theoretical Investigation of the Role of the Nitride Ion in the Magnetism of Oxynitride MnTaO₂N

Abstract

Oxynitride MnTaO2N exhibits a helical spin order in contrast to the isostructural oxide MnTiO3 with a G-type antiferromagnetism. To understand the role of the nitride ions on the magnetism, in this study, we theoretically investigated the structural and magnetic properties of MnTaO2N. Band calculations based on the density functional theory revealed that besides the most stable anion coordinations of cis-MO4N2 octahedra (M = Mn and Ta), the other coordinations such as trans-MO4N2, MO3N3, and MO5N are also considered to coexist due to the small total energy difference. This results in random existence of the nitride ions in MnTaO2N. The magnetic properties were investigated using the Heisenberg model to quantify the coupling energy (J) between Mn 3d5 local moments. Interestingly, the average J of the Mn–N–Mn bonds (J 1N) was four times larger than that of the Mn–O–Mn bonds (J 1O), mainly due to the covalent nature of Mn–N bonds. In comparison to MnTiO3, which contains only one type of magnetic interaction, J 1O, MnTaO2N has various kinds of magnetic interactions due to the random existence of the nitride ions. In addition, the J 1O value became comparable to the next-nearest-neighbor interaction (J 2). These results caused spin frustration, which is a prerequisite to the emergence of the helical spin order in MnTaO2N.

Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling

“…In the ideally disordered case, that is to say, Fe 3+ /M 5+ are distributed in a fully random manner in Zn 2 FeMO 6 and equivalent to a (Fe, M) 4+ ion, the periodic chemical potential wave (Coulomb repulsion toward the face-shared Zn partner) renders similar displacement of Zn toward the opposite direction, and thus statistically no Zn-splitting is observed as in ZnMO 3 (M = Pb, Sn, Ti). ,, However, the short-range ordering of Fe 3+ /M 5+ disturbs the periodicity of Coulomb repulsion and yields different displacement of the paired Zn atoms, which is manifested as local Zn-splitting and can be structurally taken as intergrowth of two LN-type subcells (Figure b). Recent theoretical calculations show that, in MnTaO 2 N, cis -MO 4 N 2 octahedra (M = Mn and Ta, the most stable anion coordination) coexist with the other coordination such as trans -MO 4 N 2 , MO 3 N 3 , and MO 5 N, which suggest local anion ordering of O and N . This local anion ordering around Zn and Ta affirmatively drives complete Zn-splitting in HTLN-ZnTaO 2 N .…”

“…8,17,34 However, the short-range ordering of Fe 3+ /M 5+ disturbs the periodicity of Coulomb repulsion and yields different displacement of the paired Zn atoms, which is manifested as local Zn-splitting and can be structurally taken as intergrowth of two LN-type subcells (Figure 2b). Recent theoretical calculations show that, in MnTaO 2 N, cis-MO 4 N 2 octahedra (M = Mn and Ta, the most stable anion coordination) coexist with the other coordination such as trans-MO 4 N 2 , MO 3 N 3 , and MO 5 N, which suggest local anion ordering of O and N. 48 This local anion ordering around Zn and Ta affirmatively drives complete Zn-splitting in HTLN-ZnTaO 2 N. 10 A similar phenomenon is also expected in the predicted ZnNbO 2 N as listed in Table 1.…”

Above-Room-Temperature LiNbO3-Type Polar Magnet Stabilized by Chemical and Physical Pressure

“…Accordingly, one may note Pbnm (CaTiO 3 , x < 0.4)– P 4 2 mc (CaMnTi 2 O 6 )– R 3 c (MnTiO 3 , 1 < x < 2.0) phase transitions in Ca 2– x Mn x Ti 2 O 6 upon heating at 7 GPa. MnTaO 2 N is, to the best of our knowledge, the first and the only LN-type polar magnet in the oxynitride family, , which can be synthesized under 7.5 GPa with impurity phases even reacted at temperature as high as 1973 K. Recently, Inaguma et al have intensively studied the (MnTaO 2 N) 1– x –(Mn 4 Ta 2 O 9 ) x ( x = 0.00–1.00) system, where the corundum-related Mn 4 Ta 2 O 9 ( P 3̅ c 1) can be prepared at AP and preserves the similar atomic arrangement as in MnTaO 2 N . It was found that the impurity phases can be largely suppressed at 7.5 GPa and 1573 K, and the LN-type pure phase was achieved for x = 0.25 (Mn­(Mn 1/6 Ta 5/6 )­O 2.5 N 0.5 ) .…”