Five-coordinate Cd in the crystal structure of triploidite-type Cd₂(AsO₄)(OH)

Author(s)

Tamara Đorđević, Uwe Kolitsch

Abstract

This paper reports on hydrothermal synthesis and crystal structure refinement of dicadmium arsenate hydroxide, Cd2(AsO4)(OH), obtained at 220 °C and autogenous pressure. Its crystal structure is monoclinic, space group P21/a, with a = 13.097(3), b = 14.089(3), c = 10.566(2) Å, ß = 108.38(3)°, V = 1850.2(6) Å3 (Z = 16). It is isotypic with the members of the triploidite group of minerals and synthetic compounds, and thus shows a close topological relationship with the triplite group. The complex framework contains edge- and corner-sharing CdO4(OH) and CdO4(OH)2 polyhedra, linked via corner-sharing to AsO4 tetrahedra (average As-O distances range between 1.682 and 1.688 Å). Four five-coordinated Cd sites are at the centers of distorted trigonal bipyramids (average Cd-O distances are between 2.225 and 2.251 Å), whereas the remaining four Cd sites have a distorted octahedral coordination environment (average Cd-O distances are between 2.297 and 2.320 Å). The positions of all the hydrogen atoms were located in a difference-Fourier map and refined with an isotropic displacement parameter. The hydrogen-bonds are weak to very weak. The unusual five-coordination of Cd is briefly discussed in relation to comparable minerals and compounds. Among triploidite-type compounds, Cd2(AsO4)(OH) is the member with the largest unit cell reported so far, and the second known arsenate member.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Naturhistorisches Museum Wien (NHM)

Journal

Mineralogy and Petrology

Volume

107

Pages

243-251

No. of pages

9

ISSN

0930-0708

DOI

https://doi.org/10.1007/s00710-012-0235-5

Publication date

2013

Peer reviewed

Yes

Austrian Fields of Science 2012

105113 Crystallography

Portal url

https://ucrisportal.univie.ac.at/en/publications/2845dc53-185d-481a-af6f-5772cbfad9c4