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Publicly Available Published by De Gruyter March 4, 2016

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

  • Matthias Weil EMAIL logo


The diarsenates MM′As2O7 (M = Sr, Ba; M′ = Cd, Hg) were prepared under hydrothermal conditions (~200 °C, autogenous pressure), starting from As2O5 and the corresponding metal oxides or precursor compounds thereof in aqueous solutions. Structure analyses on the basis of single crystal X-ray data revealed the four structures to be isotypic. They are the first diarsenates to crystallize in the triclinic BaZnP2O7 structure type (space group P1̅, Z = 2, a ≈ 5.8 Å, b ≈ 7.3 Å, c ≈ 7.6 Å, α ≈ 101°, β ≈ 91°, γ ≈ 98°). All related MM′As2O7 diarsenates reported so far (M = Sr, Ba, Pb; M′ = Mg, Co, Cu, Zn) crystallize in the monoclinic α-Ca2P2O7 structure type (P21/n, Z = 4). Hence, the size of the divalent M′ cation determines which of the two structure types is adopted.

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–AsV–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 (= HgI2HgII2(AsO4)2·2HgIIO), a basic orthoarsenate [7], Hg3(HAsO4)2 (= HgIIHgI2(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 [1115].

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

2.1 Preparation

As2O5 was prepared by dissolution of As2O3 in a mixture of concentrated nitric acid and 30 % hydrogen peroxide (5:1, v/v), heating of the mixture to dryness and final heating of the obtained solid at 420 °C for 15 h. Phase purity of the colorless product was checked by X-ray powder diffraction.

As-prepared As2O5 was mixed with the corresponding transition metal oxides and alkaline metal hydroxides or carbonates in a Teflon container. The container was filled up to two-thirds of its volume with water and heated mildly (≈ 60 °C) for ca. 2 h to ensure complete dissolution of As2O5, neutralisation of hydroxides and complete reaction of the carbonate precursors (caution CO2 formation!). Subsequently, the container was closed with a Teflon lid, put in a steel autoclave and heated at temperatures of 190 or 220 °C for 4–5 days. The obtained solids were separated by filtration through a glass frit and washed with a water-ethanol mixture (30:70, v/v) and again with ethanol. The dried products were inspected optically and the bulk was subjected to X-ray powder diffraction measurements. Experimental details of representative batches are given in Table 1.

Table 1

Experimental details for crystal growth of MM′As2O7 phases (M = Sr, Ba, Ca; M′ = Cd, Hg).

BatchSample weightSolid phases identified with semi-quantitative phase ratio as revealed by X-ray powder diffractiona
Ba-Hg2.43 g As2O5 (10.57 mmol)

0.22 g HgO (1.02 mmol)

0.95 g Ba(OH)2·8H2O (5.54 mmol)

5 mL water, 190 °C, 5 d
BaHgAs2O7 (light-yellow single crystals) [this work];

Hg3(AsO4)2 (amber-colored crystal aggregates) [1];

Ba2As4O12 (colorless plates) [16];

Ba-Hg1.21 g As2O5 (5.26 mmol)

0.22 g HgO (1.02 mmol)

0.45 g Ba(OH)2·8H2O (2.26 mmol)

3 mL water, 220 °C, 4 d
BaHgAs2O7 (yellowish microcrystalline material) [this work];

Ba2As4O12 (colorless plates) [16];

Sr-Hg2.43 g As2O5 (10.57 mmol)

0.22 g HgO (1.02 mmol)

0.44 g SrCO3 (2.98 mmol)

5.5 mL water, 190 °C, 5 d
SrHgAs2O7 (light-yellow single crystals, partly intergrown) [this work];

Hg3(AsO4)2 (amber-colored crystal aggregates) [1];

Sr-Hg1.21 g As2O5 (5.26 mmol)

0.22 g HgO (1.02 mmol)

0.22 g SrCO3 (1.48 mmol)

3.5 mL water, 220 °C, 4 d
SrHgAs2O7 (very small light-yellow single crystals)

[this work];

Ca-Hg2.43 g As2O5 (10.57 mmol)

0.22 g HgO (1.02 mmol)

0.30 g CaCO3 (3 mmol)

4.5 mL water, 190 °C, 5 d
CaAs2O6 (cementeous grayish mass) [17];

Ca-Hg1.21 g As2O5 (5.26 mmol)

0.22 g HgO (1.02 mmol)

0.3 g CaCO3 (3 mmol)

3.5 mL water
Ca2As2O7 [18];

CaAs2O6 (cementeous grayish mass) [17];

Ba-Cd0.81 g As2O5 (3.52 mmol)

0.04 g CdO (0.34 mmol)

0.32 g Ba(OH)2·8H2O (1.87 mmol)

3 mL water, 190 °C, 5 d
BaCdAs2O7 (colorless microcrystalline material)

[this work];

Ba-Cd1.29 g As2O5 (5.61 mmol)

0.067 g CdO (0.52 mmol)

0.32 g Ba(OH)2.8H2O (1.87 mmol)

3 mL water, 220 °C, 5 d
BaCdAs2O7 (small colorless single crystals) [this work];

Sr-Cd0.81 g As2O5 (3.52 mmol)

0.04 g CdO (0.34 mmol)

0.15 g SrCO3 (1.00 mmol)

3 mL water, 190 °C, 5 d
SrCdAs2O7 (colorless microcrystalline material plus a few small colorless single crystals) [this work];

Ca-Cd0.81 g As2O5 (3.52 mmol)

0.04 g CdO (0.34 mmol)

0.10 g CaCO3 (1 mmol)

3 mL water, 190 °C, 5 d
CaAs2O6 (cementeous grayish mass) [17];


aThe prevalent crystal form observed for the four MM′As2O7 phases is that of a {010} plate.

2.2 Single crystal diffraction and structure analysis

Crystals of the four compounds were isolated manually from the bulk and embedded in perfluorinated polyether to prevent hydrolysis. Crystals of good optical quality showing sharp extinctions when imaged between crossed polarizers in a polarizing microscope were pre-selected and fixed on thin silica glass fibers. Intensity data sets were recorded at room temperature on a Bruker APEX-II diffractometer (MoKα radiation) using φ and ω scans, resulting in data sets of the complete reciprocal spheres up to high diffraction angles with optimized collection strategies under Apex2 [19]. Close inspection of the diffraction pattern of the BaCdAs2O7 crystal with the Rlatt program [19] revealed twinning by non-merohedry, resulting in two twin components. The intensity data of the two twin domains were integrated separately with the program Saint [19], with the following intensity statistics: 4911 data (1557 unique) involved domain 1 only [mean I/σ = 13.9], 4925 data (1549 unique) involved domain 2 only [mean I/σ = 6.3], and 5654 data (2067 unique) involved both domains [mean I/σ = 12.1]. The intensity data were processed in a HKLF-5 file and corrected for absorption effects with the program Twinabs [19]. The diffraction pattern of all other measurements did not indicate signs of twinning. Their intensity data were integrated normally with saint. Absorption corrections for all other measurements were corrected numerically on the basis of the optimized crystal form with the program habitus [20].

The crystal structure of BaHgAs2O7 was solved by Direct Methods and was refined using the shelxtl program package [21]. Based on the similar unit cell parameters of the other three MM′As2O7 structures, isotypism with BaHgAs2O7 was assumed. Atomic coordinates of the latter were used as starting parameters for least-squares refinement of the respective crystal structure which turned out to be straightforward for all three refinements. For the BaCdAs2O7 measurement, the ratio of the two twin domains refined to a value of 4:1.

Numerical details of the data collections and structure refinements are listed in Table 2. Atomic coordinates and equivalent isotropic displacement parameters are summarized in Table 3. Selected bond lengths and angles are gathered in Table 4. Drawings of structural details were produced using the program atoms [22].

Table 2

Details of data collections and structure refinements for MM′As2O7 structures (M = Sr, Ba; M′ = Cd, Hg).

DiffractometerBruker APEX2
Radiation; wavelength λ, ÅMoKα; 0.71073
Crystal dimensions, mm30.024 × 0.036 × 0.1200.012 × 0.012 × 0.2400.080 × 0.150 × 0.4000.012 × 0.036 × 0.120
Crystal descriptioncolorless fragmentcolorless needlelight-yellow fragmentlight-yellow fragment
Space group (no.)P1̅ (2)
Formula units Z2
Lattice parameters
a, Å5.7885(4)5.8109(3)5.8049(5)5.8212(1)
b, Å7.1383(4)7.2955(3)7.2478(7)7.4259(2)
c, Å7.4772(5)7.7197(3)7.5235(7)7.7968(2)
α, deg101.101(1)100.505(2)101.630(1)101.548(1)
β, deg90.424(1)90.725(2)90.206(1)90.978(1)
γ, deg98.868(1)97.371(3)98.895(1)96.942(1)
Volume, Å3299.33(3)318.90(2)306.11(5)327.499(13)
Formula weight431.75511.58550.05599.77
μ(MoKα), cm−1234.2197.5444.7394.0
Absorption correctionHabitusTwinabsHabitusHabitus
X-ray density, g·cm−35.1245.3285.9686.082
Range θminθmax, deg2.78–32.482.69–34.982.77–32.642.67–53.87
h−8 → 8−9 → 9−8 → 8−12 → 12
k−10 → 10−11 → 11−10 → 10−15 → 15
l−11 → 110 → 12−11 → 10−16 → 16
Structure solution and refinementshelxtl
Measured reflections52842657531019058
Independent reflections/Rint2037/0.03092657/–2037/0.04426849/0.0514
Obs. reflections [I > 2 σ(I)]1760226719914827
Coeff. of transm., Tmin/Tmax0.255/0.7280.088/0.7980.014/0.1320.053/0.489
Extinction coef. (shelxl-97)0.0027(2)0.0129(5)0.00637(13)
Number of ref. parameters101101101101
Δρfin (max/min), e Å−30.73/−0.841.85/−2.151.54/−1.482.00/−2.68
R1 [F2 > 2 σ(F2)]/wR2(F2 all)0.0177/0.03490.0348/0.08970.0190/0.04670.0290/0.0394
CSD number430289430290430291430292
Table 3

Atomic coordinates and equivalent isotropic displacement parameters Ueq, (Å2).

Table 4

Selected interatomic distances (Å), and angles (deg) of the four MM′As2O7 structures (M = Sr, Ba; M′ = Cd, Hg)a.

As1 O3 As2130.61(12)As1 O3 As2127.4(2)As1 O3 As2132.19(16)As1 O3 As2129.13(9)

aSymmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) −x + 1, −y, −z + 1; (iii) x, y + 1, z; (iv) −x + 1, −y + 1, −z; (v) −x, −y + 1, −z; (vi) x − 1, y, z; (vii) −x, −y +1, −z +1. Averaged values are given between inequality signs.

Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: , on quoting the depository numbers listed at the end of Table 2.

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 MMX2O7 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 MMX2O7 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 M2O10 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.
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.
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–Obridging distances and shorther As–Oterminal 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.

Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.

Corresponding author: Matthias Weil, Institute for Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria, Fax: +43-1-58801-17199, E-mail:


The author thanks Dr. C. Trinks for assistance with the preparative work. The X-ray center of the Vienna University of Technology is acknowledged for providing access to the powder and single-crystal diffractometers.


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Received: 2015-12-8
Accepted: 2015-12-19
Published Online: 2016-3-4
Published in Print: 2016-5-1

©2016 by De Gruyter

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