Octa­hedral–tetra­hedral framework structures of InAsO4·H2O and PbIn(AsO4)(AsO3OH)

Indium arsenate(V) monohydrate, InAsO₄·H₂O, (I), crystallizes in the structure type of MnMoO₄·H₂O. The structure is built of In₂O₈(H₂O)₂ dimers (mean In—O = 2.150 Å) corner‐linked to slightly distorted AsO₄ tetra­hedra (mean As—O = 1.686 Å). The linkage results in a three‐dimensional framework, with small voids into which the apical water ligand of the InO₅(H₂O) octa­hedron points. The hydrogen bonds in (I) are of medium strength. Lead(II) indium arsenate(V) hydrogen arsenate(V), PbIn(AsO₄)(AsO₃OH), (II), represents the first reported lead indium arsenate. It is characterized by a framework structure of InO₆ octa­hedra corner‐linked to AsO₄ and AsO₃OH tetra­hedra. The resulting voids are occupied by Pb₂O₁₀(OH)₂ dimers built of two edge‐sharing highly distorted PbO₆(OH) polyhedra (mean Pb—O = 2.623 Å). The compound is isotypic with PbFe^III(AsO₄)(AsO₃OH). The average In—O bond length in (II) is 2.157 Å. In both arsenates, all atoms are in general positions.     

CsGa(H1.5AsO4)2(H2AsO4) and isotypic CsCr(H1.5AsO4)2(H2AsO4): decorated kröhnkite‐like chains in two unusual hydrogen arsenates

Hydro­thermally synthesized caesium gallium(III) hydrogen arsenate(V), CsGa(H_1.5AsO₄)₂(H₂AsO₄), (I), and isotypic caesium chromium(III) hydrogen arsenate(V), CsCr(H_1.5AsO₄)₂(H₂AsO₄), (II), represent a new structure type and stoichiometry among M^I–M^III hydrogen arsenates. The crystal structure, determined from single‐crystal X‐ray diffraction data, is based on an infinite octa­hedral–tetra­hedral chain and can be described as a decorated kröhnkite‐like chain. The chains extend parallel to [100] and are separated by ten‐coordinated Cs atoms. The hydrogen‐bonding scheme comprises one very short symmetry‐restricted hydrogen bond, with O⋯O distances of 2.519 (4) and 2.508 (4) Å in (I) and (II), respectively, and two further medium–strong hydrogen bonds, all of which reinforce the connections between adjacent chains. The average Ga—O and Cr—O bond lengths are 1.973 (15) and 1.980 (13) Å, respectively, and the average As—O bond lengths in the two protonated arsenate groups lie within a very narrow range [1.690 (18)–1.69 (3) Å]. The Cs atom is located on a centre of inversion, while the M^III and As2 atoms lie on twofold axes. Relationships to CaBa₂(HPO₄)₂(H₂PO₄)₂ and other compounds containing decorated kröhnkite‐type or kröhnkite‐like chains are discussed.     

M3+(H2AsO4)(H2As2O7) (M3+ = Al,Ga) and In2(H2AsO4)2(H2As2O7)2: a new layer structure type and a new framework structure type containing the rare H2As2O72− group

The crystal structures of hydrothermally synthesized aluminium dihydrogen arsenate(V) dihydrogen diarsenate(V), Al(H₂AsO₄)(H₂As₂O₇), gallium dihydrogen arsenate(V) dihydrogen diarsenate(V), Ga(H₂AsO₄)(H₂As₂O₇), and diindium bis[dihydrogen arsenate(V)] bis[dihydrogen diarsenate(V)], In₂(H₂AsO₄)₂(H₂As₂O₇)₂, were determined from single‐crystal X‐ray diffraction data collected at room temperature. The first two compounds are representatives of a novel sheet structure type, whereas the third compound crystallizes in a novel framework structure. In all three structures, the basic building units are M ³⁺O₆ octahedra (M = Al, Ga, In) that are connected via one H₂AsO₄⁻ and two H₂As₂O₇²⁻ groups into chains, and further via H₂As₂O₇²⁻ groups into layers. In Al/Ga(H₂AsO₄)(H₂As₂O₇), these layers are interconnected by weak‐to‐medium–strong hydrogen bonds. In In₂(H₂AsO₄)₂(H₂As₂O₇)₂, the H₂As₂O₇²⁻ groups link the chains in three dimensions, thus creating a framework topology, which is reinforced by weak‐to‐medium–strong hydrogen bonds. The three title arsenates represent the first compounds containing both H₂AsO₄⁻ and H₂As₂O₇²⁻ groups.     

Mg7(AsO4)2(HAsO4)4: a new magnesium arsenate with a very strong hydrogen bond

A new compound, heptamagnesium bis­(arsenate) tetrakis(hydrogenarsenate), Mg₇(AsO₄)₂(HAsO₄)₄, was synthesized by a hydro­thermal method. The structure is based on a three‐dimensional framework of edge‐ and corner‐sharing MgO₆, MgO₄(OH)₂, MgO₅, AsO₃(OH) and AsO₄ polyhedra. Average Mg—O and As—O bond lengths are in the ranges 2.056–2.154 and 1.680–1.688 Å, respectively. Each of the two non‐equivalent OH groups is bonded to both an Mg and an As atom. One OH group is involved in a very short hydrogen bond [O⋯O = 2.468 (3) Å]. The formula unit is centrosymmetric, with all atoms in general positions except for one Mg atom, which has site symmetry . The compound is isotypic with Mn₇(AsO₄)₂(HAsO₄)₄ and M₇(PO₄)₂(HPO₄)₄, where M is Fe, Co or Mn.     

CsAl(H2AsO4)2(HAsO4): a new monoclinic protonated arsenate with decorated kröhnkite‐like chains

The crystal structure of hydro­thermally synthesized caesium aluminium bis­[dihydrogen arsenate(V)] hydrogen arsen­ate(V), CsAl(H₂AsO₄)₂(HAsO₄), was determined from single‐crystal X‐ray diffraction data collected at room temperature. The compound represents a new structure type that is characterized by decorated kröhnkite‐like [100] chains of corner‐sharing AlO₆ octa­hedra and AsO₄ tetra­hedra. Ten‐coordinated Cs atoms are situated between the chains, which are inter­connected by five different hydrogen bonds [O⋯O = 2.569 (4)–2.978 (4) Å]. All atoms are in general positions. CsAl(H₂AsO₄)₂(HAsO₄) is very closely related to CsGa(H_1.5AsO₄)₂(H₂AsO₄) and isotypic CsCr(H_1.5AsO₄)₂(H₂AsO₄).     

Sodium scandium arsenate,Na3Sc2(AsO4)3

Nasicon-type trisodium discandium tris­(arsenate), Na₃Sc₂(AsO₄)₃, contains a polyhedral network of vertex-sharing octahedral ScO₆ and tetrahedral AsO₄ units [d_av(Sc—O) = 2.089 (2) Å and d_av(As—O) = 1.672 (2) Å] encapsulating two types of Na⁺ species. The sodium site occupancies are similar to those of the equivalent species in β-Na₃Sc₂(PO₄)₃.     

Synthese und Kristallstruktur von K2(HSO4)(H2PO4), K4(HSO4)3(H2PO4) und Na(HSO4)(H3PO4)

Synthesis and Crystal Structure of K₂(HSO₄)(H₂PO₄), K₄(HSO₄)₃(H₂PO₄), and Na(HSO₄)(H₃PO₄) Mixed hydrogen sulfate phosphates K₂(HSO₄)(H₂PO₄), K₄(HSO₄)₃(H₂PO₄) and Na(HSO₄)(H₃PO₄) were synthesized and characterized by X‐ray single crystal analysis. In case of K₂(HSO₄)(H₂PO₄) neutron powder diffraction was used additionally. For this compound an unknown supercell was found. According to X‐ray crystal structure analysis, the compounds have the following crystal data: K₂(HSO₄)(H₂PO₄) (T = 298 K), monoclinic, space group P 2₁/c, a = 11.150(4) Å, b = 7.371(2) Å, c = 9.436(3) Å, β = 92.29(3)°, V = 774.9(4) ų, Z = 4, R₁ = 0.039; K₄(HSO₄)₃(H₂PO₄) (T = 298 K), triclinic, space group P 1, a = 7.217(8) Å, b = 7.521(9) Å, c = 7.574(8) Å, α = 71.52(1)°, β = 88.28(1)°, γ = 86.20(1)°, V = 389.1(8)ų, Z = 1, R₁ = 0.031; Na(HSO₄)(H₃PO₄) (T = 298 K), monoclinic, space group P 2₁, a = 5.449(1) Å, b = 6.832(1) Å, c = 8.718(2) Å, β = 95.88(3)°, V = 322.8(1) ų, Z = 2, R₁ = 0,032. The metal atoms are coordinated by 8 or 9 oxygen atoms. The structure of K₂(HSO₄)(H₂PO₄) is characterized by hydrogen bonded chains of mixed H_nS/PO₄^– tetrahedra. In the structure of K₄(HSO₄)₃(H₂PO₄), there are dimers of H_nS/PO₄^– tetrahedra, which are further connected to chains. Additional HSO₄^– tetrahedra are linked to these chains. In the structure of Na(HSO₄)(H₃PO₄) the HSO₄^– tetrahedra and H₃PO₄ molecules form layers by hydrogen bonds.     

Synthetic mansfieldite,AlAsO4·2H2O

Hydro­thermally prepared mansfieldite, AlAsO₄·2H₂O (aluminium arsenate dihydrate), contains a vertex‐sharing three‐dimensional network of cis‐AlO₄(H₂O)₂ octahedra and AsO₄ tetrahedra [d_av(Al—O) = 1.907 (2) Å, d_av(As—O) = 1.685 (2) Å and θ_av(Al—O—As) = 134.5 (1)°].     

Sol-gel synthesis and structure of MZr2(AsO4)3 arsenates and MZr2(AsO4) x (PO4)3 − x arsenate phosphates (M = K,Rb, Cs)

MZr₂(AsO₄)₃ arsenates and MZr₂(AsO₄) _x (PO₄)_{3 ? x} arsenate phosphates (M = K, Rb, Cs) have been obtained by sol-gel synthesis followed by heat treatment and have been characterized by X-ray diffraction, electron probe microanalysis, and IR spectroscopy. Continuous series of substitutional solid solutions form in the MZr₂(AsO₄) _x (PO₄)_{3 ? x} systems (0 ≤ x ≤ 3). The solid solutions have a kosnarite structure (KZr₂(PO₄)₃, space group \(R\bar 3c\) ). The crystal structures of MZr₂(AsO₄)₃ and MZr₂(AsO₄)_1.5(PO₄)_1.5 have been refined by full-profile analysis. The structural frameworks of these phases are built from ZrO₆ octahedra and AsO₄ tetrahedra or (As,P)O₄ tetrahedra statistically populated by arsenic and phosphorus atoms. The alkali metal atoms occupy extraframework sites. The effect of the crystal chemical properties of alkali metals on the formation of the structures of MZr₂(AsO₄)₃ arsenates (M = Li-Cs) and MZr₂(AsO₄) _x (PO₄)_{3 ? x} solid solutions is discussed.     

Neue Alkalioxoarsenate(V): NaLi2[AsO4] — ein neuer Formeltyp [1]

New Alkali Oxoarsenates(V): NaLi₂[AsO₄] — A New Type of Formula [1] . By heating of well ground mixtures of the binary oxides As₂O₃, Na₂O, and Li₂O₂, molar ratio As:Na:Li = 1.0:1.0:2.0, in a well closed Ni tube (650°C, 21 d) colourless single crystals of NaLi₂[AsO₄] were obtained for the first time. The new orthoarsenate(V) crystallizes orthorhombic (space group P mn2₁-C, No. 31) with Z = 2. The structure determination showed that it is isostructural to β_II-Li₃[VO₄] and that means the Li₃[PO₄]-type. The lattice constants a = 702.9(2) pm, b = 520.5(1) pm, c = 505.4(2) pm were taken from Guinier-Simon powder data. The structure was determined by four-circle diffractometer data [Philips PW 1 100, AgKα , 679 independent out of 2 373 I_o(hkl), R = 3.03%, R_w = 2.29%; parameter see text]. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these calculated via Mean Fictive Ionic Radii, MEFIR, are calculated and discussed.     

Synthesis,structure, and thermal expansion of sodium zirconium arsenate phosphates

Sodium zirconium arsenate phosphates NaZr₂(AsO₄) _x (PO₄)_3?x were synthesized by precipitation technique and studied by X-ray diffraction and IR spectroscopy. In the series of NaZr₂(AsO₄) _x (PO₄)_3?x , continuous substitution solid solutions are formed (0 ≤ x ≤ 3) with the mineral kosnarite structure. The crystal structure of NaZr₂(AsO₄)_1.5(PO₄)_1.5 was refined by full-profile analysis: space group R \(\bar 3\) c, a = 8.9600(4)Å, c = 22.9770(9) Å, V = 1597.5(1) ų, R _wp = 4.55. The thermal expansion of the arsenate-phosphate NaZr₂(AsO₄)_1.5(PO₄)_1.5 and the arsenate NaZr₂(AsO₄)₃ was studied by thermal X-ray diffraction in the temperature range of 20–800°C. The average linear thermal expansion coefficients (α_av = 2.45 × 10^?6 and 3.91 × 10^?6 K^?1, respectively) indicate that these salts are medium expansion compounds.     

Structure Cristalline de l'Arseniate Double de Potassium et de Zirconium: KZr2(AsO4)3

Crystal Structure of KZr₂(AsO₄)₃ KZr₂(AsO₄)₃ crystals have been prepared by the flux method. The compound crystallizes in the rhombohedral system with the parameters: a = 9.028(1), c = 24.399(3) Å. The density is d = 3.694 g/cm³ and Z = 6. The space group is R3 c. The structure has been solved by isotypism with that of NaZr₂(PO₄)₃ to R = 0.041, for 353 independent reflections. It consists in 3-dimensional chains of AsO₄ tetrahedra and ZrO₆ octahedra. K⁺ ions are the center of the antiprism with K—O distances equal to 2.813 Å. The Zr–O and As–O distances have the same values of those usually observed (Zr–O = 2,05 Å and As–O = 1.66 Å).     

Hydrothermal synthesis and structural characterization of two new reduced phosphomolybdates based on an organoamine template and copper ion

Two new compounds, (H₂en)₃(H₂enMe)₄(H₃O){Cu^I[Mo^V ₆O₁₂(OH)₃(HPO₄)(PO₄)₃]₂}?·?6H₂O (1) and (H₂enMe)₄{Cu^ICu^II[Mo^V ₆O₁₂(OH)₃(PO₄)(HPO₄)₂(H₂PO₄)]₂}?·?3H₂O (2), were hydrothermally synthesized and characterized by elemental analysis, IR, TGA, and single-crystal X-ray diffraction analysis. Crystallographic analysis reveals that 1 is constructed from cluster anions {Cu^I[Mo^V ₆O₁₂(OH)₃(HPO₄)(PO₄)₃]₂}^15?, protonated organic amines, and water molecules. Each cluster is bridged through hydrogen bonds to form a 3-D supermolecular structure. For 2, {Cu^I[Mo^V ₆O₁₂(OH)₃(PO₄)(HPO₄)₂(H₂PO₄)]₂}^11? are connected by Cu^II cations to form an infinite chain. The formation of 1 and 2 reveals that organoamines influence the structures of the crystals.     

Thermal analysis and Hot-stage Raman spectroscopy of the basic copper arsenate mineral

The thermal analysis of euchroite shows two mass loss steps in the temperature range 100–105 °C and 185–205 °C. These mass loss steps are attributed to dehydration and dehydroxylation of the mineral. Hot-stage Raman spectroscopy (HSRS) has been used to study the thermal stability of the mineral euchroite, a mineral involved in a complex set of equilibria between the copper hydroxy arsenates: euchroite Cu₂(AsO₄)(OH)·3H₂O → olivenite Cu₂(AsO₄)(OH) → strashimirite Cu₈(AsO₄)₄(OH)₄·5H₂O → arhbarite Cu₂Mg(AsO₄)(OH)₃. HSRS inolves the collection of Raman spectra as a function of the temperature. HSRS shows that the mineral euchroite decomposes between 125 and 175 °C with the loss of water. At 125 °C, Raman bands are observed at 858 cm^?1 assigned to the ν₁ AsO₄ ^3? symmetric stretching vibration and 801, 822, and 871 cm^?1 assigned to the ν₃ AsO₄ ^3? (A₁) antisymmetric stretching vibrations. A distinct band shift is observed upon heating to 275 °C. At 275 °C, the four Raman bands are resolved at 762, 810, 837, and 862 cm^?1. Further heating results in the diminution of the intensity in the Raman spectra, and this is attributed to sublimation of the arsenate mineral. HSRS is the most useful technique for studying the thermal stability of minerals, especially when only very small amounts of mineral are available.     

Über Bleisilberphosphate mit Apatitstruktur

On Lead Silver Phosphates with the Apatite Structure The hitherto unknown leas silver phosphate (Pb₈Ag₂PO₄)₆ has been prepared. It has an apatite structure with unoccupied halide positions like the analogous lead alkali compounds and forms solid solutions with Pb₁₀(PO₄)₆O, Pb₁₀(PO₄)₆(OH)₂, and Pb₁₀(PO₄)₆Cl₂. At 800°C, Pb₈Ag₂(PO₄)₆ decomposes to solid Pb₃(PO₄)₂ and PbAgPO₄. (Pb, Ag) apatites have been precipitated from aqueous solutions. On the side being richer in Ag they can approximately be formulated as solid solutions between Pb₈Ag₂(PO₄)₆ and Pb₁₀(PO₄)₆(OH)₂. However, the i.r. spectrum reveals clear differences compared with thermal and hydrothermal preparations. The distribution of cations shows nonideal behaviour with reduced tendency for fixation of Ag⁺, if the content of Ag in the precipitate is high. The compound PbAgPO₄ decomposes below 520°C to Pb₈Ag₂(PO₄)₆ and Ag₃PO₄. The arsenate apatite Pb₈Ag₂(AsO₄)₆ decomposes below 530°C to Pb₃(AsO₄)₂ and Ag₃AsO₄.     

Layered zincoarsenate templated by N,N,N′,N′‐tetra­methyl­ethyl­ene‐di­amine mol­ecules

The title compound, N,N,N′,N′‐tetra­methyl­ethyl­enedi­ammon­ium di­aqua­(arsenate)­(hydrogen arsenate)­dizinc(II), (C₆H₁₈N₂)_0.5[Zn₂(AsO₄)(HAsO₄)(H₂O)₂], is a new zincoarsenate obtained by hydro­thermal synthesis. The structure consists of infinite two‐dimensional anionic layers alternating with planes containing centrosymmetric organic diprotonated template N,N,N′,N′‐tetra­methyl­ethyl­enedi­ammonium cations, [H₃N­C₆H₁₂NH₃]²⁺. The latter are interconnected to the framework through hydrogen bonds.     

Determination of Formation Regions of Titanium Phosphates; Determination of the Crystal Structure ofβ-Titanium Phosphate, Ti(PO4)(H2PO4), from Neutron Powder Data

The formation region of the various types of layered titanium hydrogen phosphate hydrates was investigated. The materials were prepared by hydrothermal methods, treating amorphous titanium phosphate with phosphoric acid (8 to 16M) in the temperature range 175 to 250°C. The materials obtained were:α-Ti(HPO₄)₂·H₂O,γ-Ti(PO₄)(H₂PO₄)·2H₂O, and its anhydrous formβ-Ti(PO₄)(H₂PO₄). The structure ofβ-Ti(PO₄)(H₂PO₄) has been determined by Rietveld powder refinement of high resolution neutron diffraction data. The structure is refined in the monoclinic space groupP2₁/n(No. 14). The unit cell parameters are:a=18.9503(4) Å,b=6.3127(1) Å,c=5.1391(1) Å,β=105.366(2)°;Z=4. The final agreement factors were:R_p=2.9% andR_wp=3.8%. The structure ofβ-Ti(PO₄)(H₂PO₄) is built from TiO₆octahedra linked together by tertiary phosphate (PO₄) and dihydrogen phosphate ((OH)₂PO₂) tetrahedra. The layers are held together by hydrogen bonds.     

Study of composition and structure of aluminum phosphate binder

In this article, theoretical analysis and different testing techniques were used to study the reaction pathways and synthesized products of phosphoric acid and aluminum hydroxide at different Al/P molar ratios. The results show that: (a) When the molar ratio of phosphoric acid/aluminum hydroxide is 1:3, the reaction will produce stoichiometric aluminum dihydrogen phosphate (Al(H₂PO₄)₃); (b) when Al(OH)₃ is excessive, an intermediate, monohydroxy aluminum dihydrogen phospate (HO-Al-(H₂PO₄)₂), will appear, which is unstable and will continue to react according to two reaction pathways, one is intramolecular dehydration to form phosphoric acid hydrogen-dihydrogen aluminum diphosphate (H₂PO₄)Al(HPO₄); the other is intermolecular dehydration cross-linking to form a polymeric macromolecular aluminum phosphate H-((HPO₄)(H₂PO₄)Al-O-HPO₄-Al(H₂PO₄)-O)- _nH. The ratio of the two pathways is affected by the excess of Al(OH)₃. When the excess of Al(OH)₃ continues to increase, the ratio of the second reaction path begins to increase and the viscosity of the product gradually increases. Adhesion experiments show that the aluminum dihydrogen phosphate has the best bonding performance benefiting from its lower viscosity.     

Pseudo‐Isomerie durch unterschiedliche Jahn‐Teller‐Ordnung: Die Kristallstrukturen des Hemihydrats und des Monohydrats von (pyH)[MnF(H2PO4)(HPO4)]

Pseudo‐Isomerism by Different Jahn‐Teller Ordering: Crystal Structures of the Hemihydrate and the Monohydrate of (pyH)[MnF(H₂PO₄)(HPO₄)] With pyridinium counter cations (pyH⁺) the Mn^III fluoride phosphate anion [MnF(H₂PO₄)(HPO₄)]^— can be stabilized. It forms a chain structure with Mn³⁺ ions bridged by a fluoride ion and two bidentate phosphate groups. Under sleightly differing conditions either the hemihydrate (pyH)[MnF(H₂PO₄)(HPO₄)]·0.5H₂O ( 1 ) or the monohydrate (pyH)[MnF(H₂PO₄)(HPO₄)]·H₂O ( 2 ) is formed. The hemihydrate 1 crystallizes monoclinic in space group P2₁/n, Z = 8, a = 7.295(1), b = 17.052(2), c = 18.512(3) Å, β = 100.78(1)°, R = 0.033, the monohydrate triclinic in space group P1¯, Z = 2, a = 7.374(1), b = 8.628(1), c = 10.329(1) Å, α = 83.658(8)°, β = 77.833(9)°, γ = 68.544(8)°, R = 0.025. Whereas the topology of the chain anions is identical in both structures, the Jahn‐Teller effect is expressed in different ordering patterns: in 1 antiferrodistortive ordering of [MnF₂O₄] octahedra is observed, with alternating elongation of an F—Mn—F‐axis or a O—Mn—O‐axis, respectively. This leads to asymmetrical Mn—F—Mn‐bridges. In 2 ferrodistortive ordering is found, with elongation of all octahedra along the F—Mn—F‐axis. Thus, symmetrical bridges are formed with long Mn—F distances. This unusual pseudo‐isomerism is attributed to the differing influence of inter‐chain hydrogen bonds.     

Sr,Ba and Cd arsenates with the apatite‐type structure

X‐ray diffraction analysis of single crystals of three new arsenates adopting apatite‐type structures yielded formula Sr₅(AsO₄)₃F for strontium arsenate fluoride, (I), (Sr_1.66Ba_0.34)(Ba_2.61Sr_0.39)(AsO₄)₃Cl for strontium barium arsenate chloride, (II), and Cd₅(AsO₄)₃Cl_0.58(OH)_0.42 for cadmium arsenate hydroxide chloride, (III). All three structures are built up of isolated slightly distorted AsO₄ tetrahedra that are bridged by Sr²⁺ in (I), by Sr²⁺/Ba²⁺ in (II) and by Cd²⁺ in (III). Compounds (I) and (II) represent typical fluorapatites and chlorapatites, respectively, with F⁻ at the 2a (0, 0, ) site and Cl⁻ at the 2b (0, 0, 0) site of P6₃/m. In contrast, in (III), due to the requirement that the smaller Cd²⁺ cation is positioned closer to the channel Cl⁻ anion (partially substituted by OH⁻), the anion occupies the unusual 2a (0, 0, ) site. Therefore, Cl⁻ is similar to F⁻ in (I), coordinated by three A2 cations, unlike the octahedrally coordinated Cl⁻ in (II) and other ordinary chlorapatites. Furthermore, in (III), using FT–IR studies, we have inferred the existence of H⁺ outside the channel in oxyhydroxyapatites and provided possible atomic coordinates for a H atom in HAsO₄²⁻, leading to a proposed formulation of the compound as Cd₅(AsO₄)_3−x(HAsO₄)_xCl_0.58(OH)_{0.42−x−(y/2)}O_x+(y/2)□_y/2.