The high-pressure synthesis of a manganese oxyhydride LaSrMnO3.3 H0.7 is reported. Neutron and X-ray Rietveld analyses showed that this compound adopts the K2 NiF4 structure with hydride ions positioned exclusively at the equatorial site. This result makes a striking contrast to topochemical reductions of LaSrMnO4 that result in only oxygen-deficient phases down to LaSrMnO3.5 . This suggests that high H2 pressure plays a key role in stabilizing the oxyhydride phase, offering an opportunity to synthesize other transition-metal oxyhydrides. Magnetic susceptibility revealed a spin-glass transition at 24 K that is due to competing ferromagnetic (Mn(2+) -Mn(3+) ) and antiferromagnetic (Mn(2+) -Mn(2) , Mn(3+) -Mn(3+) ) interactions.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaON that crystallizes in a centrosymmetric (R3̅c) high-temperature LiNbO-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order-disorder mechanism of the phase transition. It is found that the closed d-shell of Zn, as well as the high-valent Ta ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnON coordination. Interestingly, only 3% Zn substitution for MnTaON induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

The high‐pressure synthesis of a manganese oxyhydride LaSrMnO3.3H0.7 is reported. Neutron and X‐ray Rietveld analyses showed that this compound adopts the K2NiF4 structure with hydride ions positioned exclusively at the equatorial site. This result makes a striking contrast to topochemical reductions of LaSrMnO4 that result in only oxygen‐deficient phases down to LaSrMnO3.5. This suggests that high H2 pressure plays a key role in stabilizing the oxyhydride phase, offering an opportunity to synthesize other transition‐metal oxyhydrides. Magnetic susceptibility revealed a spin‐glass transition at 24 K that is due to competing ferromagnetic (Mn2+–Mn3+) and antiferromagnetic (Mn2+–Mn2, Mn3+–Mn3+) interactions.

The B-site sublattice in the double perovskite oxides A 2 BB′O 6 (B: magnetic cation; B′: nonmagnetic cation) causes spin frustration, but the relationship between the structure and spin frustration remains unclear although a number of compounds have been studied. The present study systematically investigated A 2 Mn II B′O 6 (S = 5/2) and found that the frustration factor, defined by f = |θ W |/T N (θ W : Weiss temperature; T N : Neél temperature), scales linearly with the tolerance factor t, i.e., octahedral rotation. Unexpectedly, La 2 MnTaO 5 N (space group: P2 1 /n) synthesized under high pressure is more frustrated (f = 6) than oxides with similar t values, despite the large octahedral rotation due to the small t value of 0.914. Structural analysis suggests that the enhanced frustration can be attributed to the site preference of nitride anions at the equatorial positions, which reduces the variance of neighboring Mn−Mn distances. Our findings provide a new guide to control and improve spin frustration in double perovskites with multiple anions.