High‐Pressure Synthesis of Manganese Oxyhydride with Partial Anion Order

Abstract

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.

Chemical Pressure-Induced Anion Order–Disorder Transition in LnHO Enabled by Hydride Size Flexibility

“…So far, La 2 O 3 is known to adopt the largest OLn 4 tetrahedral volume (6.96 Å 3 ) among Ln-containing oxides. 31 Interestingly, the anion-ordered LaHO (BVS = −1.92) exhibits a larger value of 7.49 Å 3 . An even larger value of 7.70 Å 3 (leading to BVS = −1.76) is found in the "hypothetical" anion-disordered LaHO, suggesting that the under-bonded nature of oxygen is a driving force of the transition.…”

“…In the anion-disordered phase, the tetrahedral volume around O 2– /H – at 8 c (Figure a, right) shows a linear evolution, but the presence of two crystallographic anion sites in the anion-ordered phase makes the tetrahedral volume split into a larger site (8 i ) occupied by H – and a smaller site (8 i ) occupied by O 2– (Figure b, right). So far, La 2 O 3 is known to adopt the largest OLn 4 tetrahedral volume (6.96 Å 3 ) among Ln-containing oxides . Interestingly, the anion-ordered LaHO (BVS = −1.92) exhibits a larger value of 7.49 Å 3 .…”

“…Alternatively, the pressurization of disordered LnHO (Ln = Sm–Er) may drive a change to an ordered phase, where contrary to the present study, the hydride anion becomes smaller in size. Given the increasing number of oxyhydride compounds reported recently, ,− we believe that the anion arrangement in oxyhydrides (and other mixed-anion compounds such as hydride-fluoride) can be tuned widely using the hydride’s size flexibility.…”

Ba2ScHO3: H– Conductive Layered Oxyhydride with H– Site Selectivity

“…Oxyhydrides, where both oxide ions (O 2– ) and H – ions form an anion sublattice, allow various compositions and crystal lattices, unlike metal hydrides, e.g., fluorites Ln HO ( Ln = lanthanide), − mayenite [Ca 24 Al 28 O 64 ] 4+ ·H – , ideal perovskites AB O 3– x H x ( A = AE , Eu; B = Sc, Ti, − V, Cr), layered perovskites A 2 B (H, O) 4 ( A = Ln , AE , B = Li, − Ti, V, , Mn, Co, Ni, and Ru) and hexagonal perovskite BaVO 3– x H x . We recently found the first (and so far, only) example of H – conductive oxyhydrides La 2– x – y Sr x + y LiH 1– x + y O 3– y (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) with their function as an H – solid electrolyte for electrochemical devices .…”

High-Pressure Polymorphs of LaHO with Anion Coordination Reversal

“…Since the previously used metathesis synthesis (Ln 2 O 3 + CaH 2 →2LnHO + CaO) , is unsuitable for a high-pressure setup, we employed a direct reaction using a cubic anvil apparatus, as used in the synthesis of perovskite-based oxyhydrides. , Lanthanum sesquioxide and lanthanum hydride in a molar ratio 1:1 were mixed, pressed up to 1, 3, and 5 GPa, and heated at 900 °C for 1 h. The synchrotron X-ray diffraction (SXRD) profile (Figure a) showed that the sample prepared at 1 GPa is the same as the ambient LaHO phase with the anion-ordered fluorite structure (Figure a) . The cell parameters of a = 8.0765(2) Å and c = 5.7257(2) Å are quite close to those of reported.…”