Insights into Formation Conditions, Crystal Structures, and Thermal Behavior of Hydrous and Anhydrous Barium Arsenates

Abstract

During systematic phase formation studies in the system BaO/As 2 O 5 /H 2 O, the phases with composition Ba 3 (AsO 4 ) 2Ba 3 (HAs 2 O 7 ) 2 , Ba 3 As 4 O 13 , Ba 2 As 2 O 7 , and Ba 2 As 4 O 12 were isolated and structurally characterized for the first time, using either X-ray powder diffraction data (Ba 3 As 4 O 13 ) or single crystal X-ray diffraction data (all other phases). From the eight phases investigated, three (Ba(H 2 AsO 4 ) 2 , Ba 3 As 4 O 13 , and Ba 2 As 2 O 7 ) crystallize in known structure types and the remaining ones in novel structure types. In the crystal structures, the coordination numbers of the Ba 2+ cations span a range from 8 to 11, and the different arsenate anions are built up from tetrahedral AsO 4 groups, except for Ba 2 As 4 O 12 that contains a novel type of a catenametaarsenate anion consisting of condensed AsO 4 tetrahedra and AsO 6 octahedra. Another remarkable structural feature is the hydrogendiarsenate anion present in Ba 3 (HAs 2 O 7 ) 2 , with the longest As−O bridging distance of a diarsenate group observed so far. Temperature-dependent X-ray diffraction measurements on selected phases show anhydrous Ba 3 (AsO 4 ) 2 to be the only phase stable above 1000 °C.

Crystal structure of BaMn2(AsO4)2containing discrete [Mn4O18]28−units

“…3c). The average Ba-O bond length, 2.83 Å , matches closely with 2.77 Å , the sum of the Shannon radii for eight-coordinate Ba 2+ (1.56 Å ) and two-coordinated O 2À (1.21 Å ) ions, and is in agreement with those of other barium arsenates (Weil, 2016). Fig.…”

Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data

“…Pb(H 2 AsO 4 ) 2 adopts the Ca(H 2 PO 4 ) 2 structure type [31] and crystallizes isotypically with Pb(H 2 PO 4 ) 2 [19,32,33] and Ba(H 2 AsO 4 ) 2 [30] an ordered H atom located on the inversion center are possible in the present case. On basis of difference-Fourier maps, the first possibility was chosen for the hydrogen bond O8-H•••O8 that is located between two layers, and the second possibility for the hydrogen bond O7-H•••O7 that is located within a layer (Figure 4).…”

“…All unit cell parameters (Figure 3) increase more or less in a linear way with temperature, except the γ angle that shows a slight decrease. The evolution of unit cell parameters with temperature resembles that of isotypic Ba 2 As 2 O 7 [30].…”

Crystal structure of sodium dihydrogen arsenate

“…Its contribution of 0.04 valence units to the bond-valence sum might be considered as too low for a significant interaction, and therefore the first coordination sphere of Na2 + is discussed as that of a considerably distorted octahedron. The two dihydrogen arsenate groups show the usual differences (Weil, 2000(Weil, , 2016 In the crystal structure of Na(H 2 AsO 4 ) the AsO 2 (OH) 2 tetrahedra are arranged in layers lying parallel to (010) with the Na + cations approximately on the same level (Fig. 1).…”