Abstract
In this study, a new cobalt arsenate belonging to the alluaudite supergroup compounds with the general formula of Co3(AsO4)0.5+x(HAsO4)2−x(H2AsO4)0.5+x[(H,□)0.5(H2O,H3O)0.5]2x+ (denoted as CoAsAllu) was synthesized under hydrothermal conditions. Its crystal structure was determined by a room-temperature single-crystal X-ray diffraction analysis: space group C2/c, a = 11.6978(8), b = 12.5713(7), c = 6.7705(5) Å, β = 113.255(5)°, V = 914.76(11) Å3, Z = 2 for As6H8Co6O25. It represents a new member of alluaudite-like protonated arsenates and the first alluaudite-like phase showing both protonation of the tetrahedral site and presence of the H2O molecules in the channels. In the asymmetric unit of CoAsAllu, one of the two Co, one of the two As and one of the seven O atoms lie at 4e special positions (site symmetry 2). The crystal structure consists of the infinite edge-shared CoO6 octahedra chains, running parallel to the [101¯] direction. The curved chains are interconnected by [(As1O4)0.5(H2As1O4)0.5]2− and [HAs2O4]2− tetrahedra forming a heteropolyhedral 3D open framework with two types of parallel channels. Both channels run along the c-axis and are located at the positions (1/2, 0, z) and (0, 0, z), respectively. The H2 and H4 hydrogen atoms of O2H2 and O4H4 hydroxyl groups are situated in channel 1, while the uncoordinated water molecule H2O7 at half-occupied 4e special positions and hydrogen atoms of O6H6 hydroxyl group were found in channel 2. The results of the magnetic investigations confirm the quasi one-dimensional structure of divalent cobalt ions. They are antiferromagnetically coupled with the intrachain interaction parameter of J ≈ −8 cm−1 and interchain parameter of J’ ≈ −2 cm−1 that become effective below the Néel temperature of 3.4 K.
Highlights
In the past two decades, structural crystallography of the synthetic analogues of the minerals adopting alluaudite-type structure have attracted great scientific attention because of their open-framework structure and their unique physical properties [1,2], which raise interest in their potential applications e.g., corrosion inhibition, metal surfaces passivation, energy storage and catalysis [3,4,5,6,7]
In this study, a new cobalt arsenate belonging to the alluaudite supergroup compounds with the general formula of Co3(AsO4)0.5+x(HAsO4)2−x(H2AsO4)0.5+x[(H, )0.5(H2O,H3O)0.5]2x+ was synthesized under hydrothermal conditions
Its crystal structure was determined by a room-temperature single-crystal X-ray diffraction analysis: space group C2/c, a = 11.6978(8), b = 12.5713(7), c = 6.7705(5) Å, β = 113.255(5)◦, V = 914.76(11) Å3, Z = 2 for As6H8Co6O25
Summary
In the past two decades, structural crystallography of the synthetic analogues of the minerals adopting alluaudite-type structure have attracted great scientific attention because of their open-framework structure and their unique physical properties [1,2], which raise interest in their potential applications e.g., corrosion inhibition, metal surfaces passivation, energy storage and catalysis [3,4,5,6,7]. The alluaudite supergroup minerals are further divided into the following four groups: alluaudite-, wyllieite-, bobfergusonite- and manitobaitegroups [10] The minerals of these groups share the same topology but are differentiated by diverse cation-ordering schemes over the octahedrally coordinated M-sites. Synthetic members of the group exhibit much larger chemical variability In these compounds, distinctly different cation sites for A1 and A2 are found, i.e., the monovalent cations may be replaced by protons leading to the compounds AM3(XO4)(XO3OH), where A = H+, Na+, K+, Ag+ and Cd2+ [13,14,15,16]. We report on the hydrothermal synthesis, crystal structure and on selected magnetic properties of Co3(AsO4)0.5+x(HAsO4)2−x(H2AsO4)0.5+x[(H, )0.5(H2O, H3O)0.5]2x+ (denoted as CoAsAllu) with alluaudite-type crystal structure, the first alluauditetype compound showing both protonation of the tetrahedral site and presence of the water molecules in the channels
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