Hydrogenation Reaction Pathways and Crystal Structures of La 2 H 2 Se, La 2 H 3 Se and La 2 H 4 Se

The crystal structures and formation reactions of lanthanum hydride selenides were studied by neutron diffraction on the deuterides. The reaction of La 2 D 2− x Se with deuterium gas (298 K≤ T ≤567 K, 0.1 MPa≤ p (D 2 )≤9.0 MPa) was followed in situ by neutron diffraction in a sapphire single‐crystal gas‐pressure cell with a time resolution of two minutes. The crystal structures and phase fractions of La 2 D 2− x Se, La 2 D 3 Se and La 2 D 4 Se were determined by Rietveld refinement. With stepwise increasing deuterium pressure at 498 K up to 3.0 MPa La 2 D 2− x Se is deuterated to La 2 D 4 Se. La 2 D 3 Se remains present, even when further increasing the pressure up to 9.0 MPa and heating up to 567 K. The previously suggested structural models were confirmed by Rietveld refinement (La 2 D 1.6 Se: Ce 2 O 2 S type, P 3 ‾m 1, a =4.1268(1) Å, c =6.8733(3) Å at 298 K; La 2 D 3 Se: Ba 2 H 3 Cl type, P 3 ‾m 1, a =4.139(2) Å, c =6.960(5) Å at 298 K; La 2 D 4 Se: Ce 2 F 4 Se type, R 3 ‾m , a =4.0542(2) Å, c =22.609(2) Å at 310 K). La 2 D 2− x Se exhibits a considerable hydrogen gas pressure dependent phase width (0≤ x <0.40).