Portal de Periódicos da CAPES

Structure and Properties of Hydrogenated Ca, Sr, Ba, and Eu Silicides

2009; Wiley; Volume: 635; Issue: 12 Linguagem: Inglês

10.1002/zaac.200900220

1521-3749

Markus Armbruster, Michael Wörle, Frank Krumeich, Reinhard Nesper,

Boron and Carbon Nanomaterials Research

Abstract The binary metal silicides CaSi, BaSi, SrSi, and EuSi that crystallize in the CrB type absorb reversibly hydrogen. Hydrogen contents of the products were measured by the carrier gas hot extraction method (CaSiH x : 1.56 wt %, x ≈︁ 1.06; SrSiH x : 1.35 wt %, x ≈︁ 1.6; BaSiH x : 2.08 wt %, x ≈︁ 3.4; EuSiH x : 0.99 wt %, x ≈︁ 1.8). According to simultaneous thermogravimetric, DTA and mass‐spectroscopic analyses, the dehydrogenation for the different hydrides starts at 610 K for BaSiH x , at 675 K for CaSiH x and SrSiH x , and at 640 K for EuSiH x . BaSiH x releases further hydrogen at 765 K and 1000 K, respectively, CaSiH x at 760 K. Fundamental morphological and structural changes caused by the hydrogenation of the silicides were detected and comprehensively characterized by SEM and X‐ray diffraction. According to XRD results, the crystal structures of the phases M SiH x ( M = Ca, Ba, Sr, Eu) are all orthorhombic (space group Pnma ). The lattice parameters change from a = 4.559(1) Å, b = 10.731(1) Å, c = 3.890(1) Å (space group Cmcm, Z = 1) in CaSi to a = 14.4884(9) Å, b = 3.8247(1) Å, c = 11.2509(5) Å ( Z = 3) in CaSiH x caused by the hydrogen uptake that induces the formation of a threefold superstructure along the a axis. In CaSiH x , the planes of the [Si n ] 2– zigzag chains are tilted and inter‐chain Si–Si contacts are formed formally corresponding to an oxidation to Si – . The hydrogen desorption process was investigated by in situ X‐ray diffraction studies which revealed the presence of two different hydrogen species in CaSiH x . The new phase SrSiH x crystallizes in a related structure comprising tilted [Si n ] 2– chains ( a = 15.1051(5) Å, b = 3.95400(8) Å, c = 12.3431(3) Å, space group Pnma ). The lattice parameters of BaSiH x are a = 15.7(2) Å, b = 4.13(3) Å, c = 13.43(8); that of EuSiH x a = 15.17 Å, b = 3.95 Å, c = 11.56 Å ( Z = 3).