The isotypic family of the diarsenates MM′As₂O₇ (M = Sr, Ba; M′ = Cd, Hg)

1 Introduction

Mercury oxoarsenates(V) have been investigated thoroughly during the last two decades. The tribasic character of arsenic acid, its conversion into different condensed anions and the possibility to stabilize mercury in different and/or mixed-valent oxidation states constitutes this family of compounds as compositionally and structurally very rich. So far, in the system Hg–As^V–O–(H) the following phases have been reported: The mercuric phases Hg₃(AsO₄)_2, an orthoarsenate with a graphtonite-type structure [1], and HgAs₂O₆, a metaarsenate crystallizing in the PbSb₂O₆ structure type [2, 3]; the mercurous phases (Hg₂)₃(AsO₄)₂, an orthoarsenate with two polymorphic forms [4, 5], Hg₂(H₂AsO₄)₂, a dihydrogenarsenate [3], (Hg₂)₂As₂O₇, a diarsenate [6], and Hg₂As₂O₆, a metaarsenate [3]; the mixed-valent mercury phases Hg₆As₂O₁₀ (= Hg^I₂Hg^II₂(AsO₄)₂·2Hg^IIO), a basic orthoarsenate [7], Hg₃(HAsO₄)₂ (= Hg^IIHg^I₂(HAsO₄)₂), a hydrogenarsenate [7], and the orthoarsenate (Hg₃)₃(AsO₄)₄ with discrete Hg₃⁴⁺ clusters [8]. Apart from the unique structural features of the triangular Hg₃⁴⁺ cluster cation [9], the crystal chemistry of mercury is dominated by a more or less linear arrangement of ligands around mercury cations in their different oxidation states [10].

The aim of the current project was to investigate how an incorporation of heavier alkaline earth cations (Ca, Sr, Ba) into mercuric arsenates influences the structural behavior of the Hg²⁺ cation. For this purpose, reactions under comparatively mild hydrothermal conditions (ca. 200 °C, autogeneous pressure) were preferred over solid state reactions in closed systems. The latter typically require much higher reaction temperatures which - in some cases - are counterproductive with regard to the weak thermal stability of most mercury oxo compounds. Moreover, hydrothermal reaction conditions have already been successfully applied for a number of related arsenate systems [11–15].

In fact, we succeeded in the preparation and structural characterisation of the two isotypic mercuric diarsenate phases MHgAs₂O₇ (M = Sr, Ba). In a further step it was investigated whether Hg²⁺ could be replaced by homologous Cd²⁺ in these structures under retention of the adopted structure type. The results of these studies are presented in this article.

2 Experimental section

3 Results and discussion

From investigations intended on formation conditions of pyroarsenic acids and its salts, Rosenheim & Antelmann have already reported on the existence of phases with composition SrHgAs₂O₇ and BaHgAs₂O₇(H₂O). Both phases were prepared in closed glass ampoules (Carius tubes) from arsenic acid, HgO and alkaline earth hydroxides [11], hence the given reaction conditions can be considered as similar to that of the current study. Whereas the existence of anhydrous SrHgAs₂O₇ was confirmed by single crystal structure analysis, a hydrous phase BaHgAs₂O₇(H₂O) was not obtained, and anhydrous BaHgAs₂O₇ had formed as the only pyroarsenate phase in this system during the current study. In some of these batches, crystals of Ba₂As₄O₁₂ with a new type of meta-arsenate anion [16] had formed as an additional minor product. It seems most likely that anhydrous BaHgAs₂O₇ was also obtained by Rosenheim & Antelmann during the original study [11] because the water content of the claimed hydrate phase was not determined directly at that time but estimated on the basis of the gravimetrically determined BaO, HgO and As₂O₅ contents. In addition, the existence of the reported calcium mercury arsenate phase with empirical composition 4CaO·2HgO·5As₂O₅·H₂O obtained under similar conditions by the original authors [11] could not be confirmed during our study. Under the reaction conditions described above, no mercury-containing solid product was found in the system Ca-Hg(Cd)-As-O-H₂O, and only the diarsenate and metaarsenate phases Ca₂As₂O₇ [18] and CaAs₂O₆ [17], respectively, were obtained. Finally, in contrast to the original study, where attempts to produce the cadmium-containing diarsenate phases MCdAs₂O₇ (M = Sr, Ba) proved to be unsuccessful [11], crystals of SrCdAs₂O₇ and BaCdAs₂O₇ could be grown by us during the present study. From batches of the Ca-Cd system, CaAs₂O₆ was the only phase obtained (Table 1).

The four MM′As₂O₇ title compounds (M = Sr, Ba; M′ = Cd, Hg) are isotypic and adopt the BaZnP₂O₇ structure type (P1̅, Z = 2). This is interesting since neither Sr-containing phases nor diarsenate phases have been reported up to date to crystallize in this structure type. Known representatives of the BaZnP₂O₇ structure type (source: ICSD, version 2015-1 [23]) were restricted to BaM′P₂O₇ with M′ = Zn [24], Cd [24, 25], Cu [26], Mn [27], Co [28], Cr [29]. On the other hand, a variety of MM′As₂O₇ pyroarsenate phases with M = Sr, Pb, Ba, M′ = Cu, Co, Zn, Mg adopts the α-Ca₂P₂O₇ structure type (P2₁/n, Z = 4) [30] which is also known for numerous MM′P₂O₇ diphosphate phases. Crystal-chemical details of the monoclinic MM′X₂O₇ family, including M = Ba, Sr, Pb, Ca; M′ = Mg, Cr, Co, Ni, Cu, Zn; X = As, P, have been recently compiled and reviewed [15]. BaMnP₂O₇ is the only compound reported so far to exist in both structure types [27], with the high-temperature modification in the α-Ca₂P₂O₇ structure type and the low-temperature modification in the BaZnP₂O₇ structure type.

Which of the two structure types is adopted by MM′X₂O₇ phases, seems to depend on the sizes of the three structure components M, M′ and X₂O₇. Representatives with Mg²⁺ and smaller divalent first-row transition metals for M′ (ionic radii << 1 Å for coordination number six [31]) crystallize in the monoclinic α-Ca₂P₂O₇ structure type, regardless which of the bigger-sized M cations (Ba, Sr, Pb) and the type of X₂O₇⁴⁻ anion (X = As, P) is present. On the other hand, representatives with the bigger M′ cations Cd²⁺ and Hg²⁺ (ionic radii ≈ 0.95 and 1.02 Å [30]) crystallize in the triclinic BaZnP₂O₇ structure type only if M = Ba, Sr and X = As, whereas SrCdP₂O₇ [25] with the smaller P₂O₇⁴⁻ anion again adopts the α-Ca₂P₂O₇ structure type. Since phases of compositions BaHgP₂O₇ and SrHgP₂O₇ have not been reported so far, a classification in terms of their structure types cannot be made. Studies on preparation, structure analysis and possible phase transitions of these two and other related phases are planned for the future.

In the crystal structure of the four BaZnP₂O₇-type MM′As₂O₇ title compounds (Fig. 1), M′O₆ octahedra form centrosymmetric M′₂O₁₀ dimers by edge-sharing (O5···O5′). These dimers are arranged in layers parallel to (001) and are bridged parallel to this direction by diarsenate groups. Each diarsenate group chelates two M′O₆ octahedra of adjacent dimers and bridges a third dimer through corner-sharing, whereby each dimer is surrounded by four diarsenate groups. The so formed ∞2[M′(As2O7)]2− layers are linked along [001] through nine-coordinated M cations. The α-Ca₂P₂O₇-type MM′As₂O₇ crystal structures [15] show a different set-up. The M′ cations are in a square-pyramidal coordination environment and are linked to five As₂O₇⁴⁻ anions through corner-sharing, thereby forming a framework structure with two types of channels where the bigger M cations (range of coordination numbers 7–9) are located.

Fig. 1:

Representative for all MM′As₂O₇ compounds, the crystal structure of SrHgAs₂O₇ in a projection along [010] is given here. M′O₆ octahedra (M′ = Hg) are blue, AsO₄ tetrahedra of As₂O₇ groups are red, and M atoms (M = Sr) are yellow. Displacement ellipsoids are drawn at the 90 % probability level.

All M′O₆ octahedra in the four title structures have a distorted octahedral coordination environment (Fig. 2a). The trans O–M′–O angles between axially bound O atoms deviate significantly from linearity, with values of 151.25(6)° for the SrCd, 156.13(14)° for the BaCd, 147.22(9)° for the SrHg and 152.31(6)° for the BaHg phase. The cis O–M′–O angles between equatorially bound O atoms range from 82.71(7) to 103.55(7)°, 84.32(16) to 100.17(17)°, 80.97(10) to 104.89(10)°, and 83.46(6) to 101.38(6)° in the four structures. The mean M′–O bond lengths (Table 3) are slightly longer for the two Hg-containing compounds, as expected for this ion with its larger ionic radius [30]. The characteristic feature of Hg²⁺ in an oxidic environment, viz. a (more or less) linear coordination by two tightly bound O atoms and additional bonds to remote O atoms ([2 + x] coordination; x = 2–8, with x = 4 being the most frequently observed case), is not found in the BaHgAs₂O₇ and SrHgAs₂O₇ structures. Here a rather rare [4 + 2] coordination with four shorter equatorial and two longer axial Hg–O bonds is realized and resembles the situation in one of the two HgO₆ octahedra in the dichromate-type crystal structure of Hg₂P₂O₇ [32]. A much less pronounced [4 + 2] coordination is also seen for the two CdO₆ octahedra in the BaCdAs₂O₇ and SrCdAs₂O₇ structures.

Fig. 2:

The principal building units in the structures of MM′As₂O₇ compounds (data from SrHgAs₂O₇). a) The axially elongated M′O₆ octahedron, b) the eclipsed As₂O₇ anion, c) the distorted coordination sphere of the M cation. Color code and displacement ellipsoids as in Fig. 1; symmetry operators refer to Table 4.

The diarsenate groups in the triclinic MM′As₂O₇ series have a nearly perfectly eclipsed conformation (Fig. 2b) as evidenced by the dihedral angles between planes defined by the backbone atoms O2–As1–O3 and O6–As2–O3 with values of 6.6(4)° for the BaCd, 6.28(12)° for the BaHg, 6.15(14)° for the SrCd, and 5.4(2)° for the SrHg structures. In the related MM′As₂O₇ compounds adopting the monoclinic α-Ca₂P₂O₇ structure type, the conformation of the diarsenate group is staggered. The characteristic As–O bond lengths distribution for the diarsenate anion [33, 34] with longer As–O_bridging distances and shorther As–O_terminal distances is also found in the MM′As₂O₇ title series, with mean values of 1.759 and 1.669 Å, respectively. The anions are bent with a mean As1–O3–As2 bridging angle of 129.8° for the four structures. It should be noted that the bridging atom O3 is neither part of the coordination spheres of the M cations nor of the M′ cations. Atoms O2 show the shortest As–O distances for all four diarsenate groups in the MM′As₂O₇ series and are not bound to the transition metal M′. They occupy the outer corners of the As₂O₇ anions within the ∞2[M′(As2O7)]2− layers and are bound to three M cations that are situated between the layers. The coordination number of all M cations is nine. The corresponding MO₉ coordination polyhedra (Fig. 2c) are considerably distorted and difficult to derive from simple geometric figures. Their bond lengths ranges reflect the different radii of the two alkaline earth cations, with 2.50–3.15 Å (average 2.73 Å) for the two Sr and 2.65–3.20 Å (average 2.86 Å) for the two Ba structures.