Materials Data on HfAs2O9 by Materials Project
HfAs2O9 crystallizes in the monoclinic P2_1/c space group. The structure is two-dimensional and consists of two HfAs2O9 sheets oriented in the (0, 0, 1) direction. Hf is bonded to six O atoms to form HfO6 octahedra that share corners with six AsO4 tetrahedra. There are a spread of Hf–O bond distances ranging from 2.05–2.08 Å. There are two inequivalent As sites. In the first As site, As is bonded to four O atoms to form AsO4 tetrahedra that share corners with three equivalent HfO6 octahedra. The corner-sharing octahedra tilt angles range from 29–39°. There are a spread of As–O bond distances ranging from 1.70–1.76 Å. In the second As site, As is bonded to four O atoms to form AsO4 tetrahedra that share corners with three equivalent HfO6 octahedra. The corner-sharing octahedra tilt angles range from 24–40°. There is three shorter (1.71 Å) and one longer (1.74 Å) As–O bond length. There are nine inequivalent O sites. In the first O site, O is bonded in a distorted single-bond geometry to one O atom. The O–O bond length is 2.08 Å. In the second O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the third O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the fourth O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the fifth O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the sixth O site, O is bonded in a single-bond geometry to one As and one O atom. In the seventh O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the eighth O site, O is bonded in a bent 150 degrees geometry to one Hf and one As atom. In the ninth O site, O is bonded in a single-bond geometry to one As atom.