Abstract
Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.
Highlights
The hydride ion H− has a unique chemical character due to its medium electronegativity, high polarizability, and small size, and is a promising candidate in the design of functional materials
The 1:1:1 compounds REHO form a CaF2 superstructure with tetragonal symmetry and space group P4/nmm, which was later confirmed by neutron diffraction analysis on LaHO and NdHO (Figure 1, bottom center) [7,31,36,37]
The temperature stability of the REHO compounds increases with decreasing cationic radius, which is opposed to the trends predicted by Density functional theory (DFT) methods
Summary
The hydride ion H− has a unique chemical character due to its medium electronegativity, high polarizability, and small size, and is a promising candidate in the design of functional materials. A rising number of so-called heteroanionic or mixed anionic hydrides is being investigated that contain additional anions and expand the accessible chemical space of ionic hydrides [1,2,3,4] These compounds show several interesting properties such as hydride ion conductivity [5,6,7,8,9,10,11], catalytic activity [12,13,14,15,16], superconductivity [17,18], and luminescence [19,20,21,22,23,24,25] or photochromism [26,27,28,29,30]. The structure of LaH1+2xO1−x and its compositional range with respect to H and O were further investigated, and a tetragonal superstructure was assigned It shows a broad phase width with increasing occupation of the octahedral interstice by hydrogen [6,34], similar to LaH2+x [35], and full mixing of anions. The composition had a great influence on the transport properties: while LaHO showed low and LaH1+2xO1−x high ionic conductivity depending on x, LaH1+xO1−y was metallically conducting [6,31]
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