Substrate-induced anion rearrangement in epitaxial thin films of LaSrCoO_4−xH_x

Abstract

The hydride reduction of a tetragonal layered perovskite LaSrCoO 4 is known to yield orthorhombic LaSrCoO 3 H 0.7 with a complete hydride/oxide order within the ab plane. In this study, epitaxial thin films of LaSrCoO 4 with a-axis and c-axis orientations have been deposited on ( 100) and ( 001) LaSrAlO 4 (LSAO) substrates, respectively, and allowed to react with hydride to convert into oxyhydrides. X-ray diffraction, secondary ion mass spectroscopy and thermal desorption spectroscopy experiments indicate that both films are topochemically reduced and can integrate hydride ions with a chemical composition close to that obtained for the powder. A significant reduction in the a-axis was observed for the a-axis oriented LaSrCoO 3 H 0.7 film, indicating hydride/oxide order, as previously reported. In contrast, the c-axis oriented LaSrCoO 3 H 0.7 film remains tetragonal, suggesting hydride/oxide disorder. These results demonstrate that strain engineering can lead to new materials with designed anion arrangement in mixed anion materials.

A Partial Anion Disorder in SrVO2H Induced by Biaxial Tensile Strain

“…For both films, a high hydrogen signal was observed, with clear contrast at the boundary between the substrate and the film. The hydrogen signal in the KTO substrate is much smaller than the target films, though it is 10-100 times higher than the case of LSAT [19,20], which might reflect some reactivity with CaH 2 . For the film reacted for 24 h (i.e., SrVO 2 H), homogeneous hydrogen distribution is confirmed, with 1 H/ 18 O values at the two areas of 7.2 (2SD 0.5) (Figure 3, blue) and 7.5 (2SD 1.0).…”

“…The intermediate tetragonal phase is less anisotropic with a c/a = 0.96, compared to the 24h-reacted film and the bulk sample of SrVO 2 H with a c/a = 0.93. [19,20], which might reflect some reactivity with CaH2. For the film reacted for 24 h (i.e., SrVO2H), homogeneous hydrogen distribution is confirmed, with 1 H/ 18 O values at the two areas of 7.2 (2SD 0.5) (Figure 3, blue) and 7.5 (2SD 1.0).…”

New chemistry of transition metal oxyhydrides

“…Consequent heat treatments can be conducted at slightly lower temperatures. In this way, films of (Ba, Ca,Sr)TiO 3– x H x [ 31,32 ] and LaSrCoO 4− x H x [ 33 ] have been prepared.…”

Topotactic synthesis of strontium cobalt oxyhydride thin film with perovskite structure

“…11,13 In addition, the epitaxial effect can also affect the local anion arrangement. Recently, Bouilly et al 9 used the topotactic reaction to synthesize the 214-type LaSrCoO 4−x H x thin film and reported that it had a disordered hydride/oxide ion arrangement, in contrast to the bulk phase with completely ordered Co-H − bonds parallel to only one axis. Thus, the topotactic introduction of hydrogen into precursor cobalt oxide thin films on appropriate substrates using CaH 2 is a promising synthetic route for obtaining this type of cobalt oxyhydride.…”

“…[5][6][7][8][9] These cobalt oxyhydrides with low Co valence states were obtained as bulk phases from the corresponding oxides, LaSrCoO 4 and Sr 3 Co 2 O 5.8 , respectively, by topotactic reaction using CaH 2 , which served not only as a strong reducing agent but also a source of hydride ions. In general, topotactic reactions for thin film samples have two advantages: considerably higher reactivity than bulk samples owing to the larger surface area/volume ratio, and stabilization of the perovskite-based framework arising from the epitaxial effect.…”