RbH₂AsO₄

RbH₂AsO₄, rubidium di­hydrogenarsenate (RDA), was synthesized by partial neutralization of an aqueous H₃AsO₄ solution with aqueous Rb₂CO₃. Its paraelectric room-temperature phase is composed of virtually regular tetra­hedral [AsO₂(OH)₂]⁻ anions and Rb⁺ cations, both located on -4 positions. The [AsO₂(OH)₂] units are connected via O—H⋯O hydrogen bonds into a three-dimensional network, whereby the H atoms are equally disordered between the O atoms. The Rb⁺ cations are located in channels running along the <100> directions and coordinated by eight O atoms located at the vertices of a snub disphenoid.

Related literature

For isotypic phases, see: Al-Karaghouli et al. (1978); Delain (1958); Ferrari et al. (1956); Helmholtz & Levine (1942); Novotny & Szekely (1952); West (1930); Tenzer et al. (1958). For related phases, see: Stöger et al. (2012). For isoformular phases crystallizing in a different structure type, viz.LiH₂PO₄, see: Catti & Ivaldi (1977); Catti & Ferraris (1974); Nelmes & Choudhary (1978); Fanchon et al. (1987). For phase transition, see: Fairall & Reese (1974). For physical properties of RDA and isotypic analogs, see: Ichikawa et al. (2001); Shen (1984); Negres et al. (2005). For crystal growth, see: Rashkovich (1991). For bond-valence analyses, see: Brown & Altermatt (1985). The extinction correction is described by Becker & Coppens (1974).

Experimental

Crystal data

• RbH₂AsO₄

• M_r = 226.4

• Tetragonal,

• a = 7.7865 (9) Å

• c = 7.466 (2) Å

• V = 452.64 (14) ų

• Z = 4

• Mo Kα radiation

• μ = 18.07 mm⁻¹

• T = 295 K

• 0.50 × 0.29 × 0.27 mm

Data collection

• Bruker Kappa APEXII CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2013) T_min = 0.004, T_max = 0.009

• 9202 measured reflections

• 955 independent reflections

• 567 reflections with I > 3σ(I)

• R_int = 0.076

Refinement

• R[F² > 2σ(F²)] = 0.025

• wR(F²) = 0.032

• S = 1.25

• 955 reflections

• 20 parameters

• 1 restraint

• All H-atom parameters refined

• Δρ_max = 1.02 e Å⁻³

• Δρ_min = −0.53 e Å⁻³

• Absolute structure: Flack (1983), 409 Friedel pairs

• Absolute structure parameter: −0.010 (13)

Table 1
Selected geometric parameters (Å, °)

Rb—O	3.0890 (17)
Rb—O^i	2.9304 (12)
As—O	1.6828 (11)

O—As—O^ii	109.80 (5)
O—As—O^iii	109.31 (5)
Symmetry codes: (i) ; (ii) -x, -y, z; (iii) y, -x, -z.

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT-Plus (Bruker, 2013); data reduction: SAINT-Plus; program(s) used to refine structure: JANA2006 (Petříček et al., 2006); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

During formation studies and subsequent structure analysis of compounds in the system K₂O–As₂O₅–H₂O (Stöger et al., 2012), related alkali phosphates and arsenates with formula type MH₂XO₄ (M = K, Rb, Cs, NH₄; X = P, As) came into attention. With the exception of CsH₂PO₄, dihydrogenphosphates and -arsenates MH₂XO₄ (M = K, Rb, Cs, NH₄; X = P, As) are isotypic at room temperature and said to belong to the KH₂PO₄ (KDP) family. Members of the KDP family are ferroelectrics with low T_c (Ichikawa et al., 2001) and feature non-linear optical (NLO) properties (Shen, 1984). They have been intensely studied for their physical properties paired with a simple crystal-chemistry. Moreover, they are of technical importance in optical applications due to their favourable transparency, high damage threshold (Negres et al., 2005) and ready access to large single crystals (Rashkovich, 1991). Notably, KDP is used as a standard NLO active compound to evaluate the performance of novel NLO materials.

Structural data was published for all members of the KDP family with the exception of RbH₂AsO₄ (RDA): KH₂PO₄ (West, 1930), RbH₂PO₄ (Al-Karaghouli et al., 1978), (NH₄)H₂PO₄ (Tenzer et al., 1958), KH₂AsO₄ (Helmholtz & Levine, 1942), CsH₂AsO₄ (Ferrari et al., 1956) and (NH₄)H₂AsO₄ (Delain, 1958). The germanate SrH₂GeO₄ (Novotny & Szekely, 1952) crystallizes likewise in the KDP structure type. The dihydrogenphosphates and arsenates with larger or smaller alkali metals crystallize in different structure types: LiH₂PO₄ (Catti & Ivaldi, 1977), NaH₂PO₄ (Catti & Ferraris, 1974), CsH₂PO₄ (Nelmes & Choudhary, 1978) and LiH₂AsO₄ (Fanchon et al., 1987).

At room temperature RDA, like all members of the KDP family, exists in the tetragonal paraelectric phase. Below T_c = 110 K it transforms into the orthogonal ferroelectric phase (Fairall & Reese, 1974). The room temperature phase of RDA crystallizes in I42d symmetry. The crystal structure is made up of one [AsO₂(OH₂)]^- anion and one Rb⁺ cation, both located on 4 positions. The [AsO₂(OH₂)] tetrahedra are virtually regular (As—O bond lengths 1.6828 (11) Å; O—As—O angles 109.80 (5)° and 109.31 (5)°). They are connected via hydrogen bonding in the <100> directions, forming a three dimensional network (Figs. 1 and 2). Thus, every O atom is either donor or acceptor of an O—H···O hydrogen bond, whereby the proton is equally disordered between both oxygen atoms.

The total bond valence sum (BVS) of the unique O atom calculated using Σexp((r₀-r)/b) and the parameters of Brown and Altermatt (1985) for Rb^I—O (r₀=2.263 Å, b=0.37) and As^V—O (r₀=1.767 Å, b=0.37) is 1.527 (4) valence units (v.u.). This value is in good agreement with the observed disorder, as it lies halfway between the ideal values of O^2- and O^- (2 and 1 v.u., respectively).

The Rb⁺ cation is located in channels running along the <100> directions (Fig. 1). It is coordinated by eight O atoms located at the vertices of a snub disphenoid (Fig. 3). The total BVS of Rb⁺ calculates as 1.0878 (15) v.u. using the parameters above, which is in excellent agreement with the expected value (1 v.u.). More remote O atoms are located at 4.3005 (12) Å from the Rb⁺ ion and can therefore not be considered part of the coordination sphere (contribution of 0.004 v.u.).

Related literature

For isotypic phases, see: Al-Karaghouli et al. (1978); Delain (1958); Ferrari et al. (1956); Helmholtz & Levine (1942); Novotny & Szekely (1952); West (1930); Tenzer et al. (1958). For related phases, see: Stöger et al. (2012). For isoformular phases crystallizing in a different structure type, viz.LiH₂PO₄, see: Catti & Ivaldi (1977); Catti & Ferraris (1974); Nelmes & Choudhary (1978); Fanchon et al. (1987). For phase transition, see: Fairall & Reese (1974). For physical properties of RDA and isotypic analogs, see: Ichikawa et al. (2001); Shen (1984); Negres et al. (2005). For crystal growth, see: Rashkovich (1991). For bond-valence analyses, see: Brown & Altermatt (1985). The extinction correction is described by Becker & Coppens (1974).

S2. Experimental

Rb₂CO₃ and H₃AsO₄ were obtained commercially and used without purification. 1 g 80% aq. H₃AsO₄ was dissolved in 10 ml water and titrated against aq. Rb₂CO₃ using one drop of methyl red in EtOH as indicator. The water was evaporated and the residue recrystallized from a small amount of water and washed with acetone to obtain large single crystals of RbH₂AsO₄.

S3. Refinement

An initial model was generated by using the published coordinates of the non-H atoms of the isotypic room temperature phase of RbH₂PO₄ (Al-Karaghouli et al., 1978).

The structure was refined against F values using the Jana2006 software package (Petříček et al., 2006). The disordered H atom was located in a difference Fourier map and was refined with an occupancy of 0.5. The O—H distance was restrained to 0.850 (1) Å. All non-H atoms were refined with anisotropic displacement parameters.

Computing details

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT-Plus (Bruker, 2013); data reduction: SAINT-Plus (Bruker, 2013); program(s) used to solve structure: coordinates of Al-Karaghouli et al. (1978); program(s) used to refine structure: JANA2006 (Petříček et al., 2006); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. The crystal structure of the paraelectric room temperature phase of RDA viewed approximately down [100]. [AsO4] tetrahedra are drawn in yellow; Rb, O and H atoms are represented by purple, red and white spheres of arbitrary radii.
	Fig. 2. Crystal structure of the paraelectric room temperature phase of RDA viewed down the tetragonal axis [001]. Atom colour codes as in Fig. 1.
	Fig. 3. Coordination polyhedron of Rb in the paraelectric room temperature phase of RDA. Rb and O are represented by purple and red ellipsoids drawn at 75% probability level. Symmetry codes: (i) -x, -y, z; (ii) y, -x, -z + 1; (iii) -y, x, -z + 1; (iv) -x + 1/2, y, -z + 3/4; (v) x - 1/2, -y, -z + 3/4; (vi) -y, -x + 1/2, z + 1/4; (vii) y, x - 1/2, z + 1/4

Rubidium dihydrogenarsenate

Crystal data

RbH2AsO4	Dx = 3.321 Mg m^−3
Mr = 226.4	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I42d	Cell parameters from 2250 reflections
Hall symbol: I -4 2bw	θ = 3.7–44.1°
a = 7.7865 (9) Å	µ = 18.07 mm^−1
c = 7.466 (2) Å	T = 295 K
V = 452.64 (14) Å^3	Block, clear colourless
Z = 4	0.50 × 0.29 × 0.27 mm
F(000) = 416

Data collection

Bruker Kappa APEXII CCD diffractometer	955 independent reflections
Radiation source: X-ray tube	567 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.076
ω and ϕ scans	θmax = 45.3°, θmin = 3.8°
Absorption correction: multi-scan (SADABS; Bruker, 2013)	h = −15→15
Tmin = 0.004, Tmax = 0.009	k = −15→14
9202 measured reflections	l = −14→14

Refinement

Refinement on F	Hydrogen site location: difference Fourier map
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.025	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 0.032	(Δ/σ)max = 0.021
S = 1.25	Δρmax = 1.02 e Å^−3
955 reflections	Δρmin = −0.53 e Å^−3
20 parameters	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
1 restraint	Extinction coefficient: 4440 (110)
0 constraints	Absolute structure: Flack (1983), 409 Friedel pairs
Primary atom site location: isomorphous structure methods	Absolute structure parameter: −0.010 (13)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Rb	0	0	0.5	0.01896 (5)
As	0	0	0	0.01382 (5)
O	0.15295 (14)	0.08872 (11)	0.12961 (14)	0.0203 (2)
H	0.147 (6)	0.1975 (7)	0.122 (10)	0.048 (12)*	0.5

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb	0.02023 (8)	0.02023 (8)	0.01640 (11)	0	0	0
As	0.01201 (7)	0.01201 (7)	0.01744 (11)	0	0	0
O	0.0173 (3)	0.0172 (3)	0.0265 (4)	0.0032 (3)	−0.0093 (3)	−0.0063 (4)

Geometric parameters (Å, º)

Rb—O	3.0890 (17)	Rb—O^vii	2.9304 (12)
Rb—O^i	3.0890 (17)	As—O	1.6828 (11)
Rb—O^ii	3.0890 (17)	As—O^i	1.6828 (11)
Rb—O^iii	3.0890 (17)	As—O^viii	1.6828 (11)
Rb—O^iv	2.9304 (12)	As—O^ix	1.6828 (11)
Rb—O^v	2.9304 (12)	O—H	0.850 (8)
Rb—O^vi	2.9304 (12)
O—Rb—O^i	52.94 (3)	O—As—O^i	109.80 (5)
O—Rb—O^ii	143.26 (3)	O—As—O^viii	109.31 (5)
O—Rb—O^iii	143.26 (3)	O—As—O^ix	109.31 (5)
O—Rb—O^iv	82.32 (3)	O^i—As—O^viii	109.31 (5)
O—Rb—O^v	133.06 (3)	O^i—As—O^ix	109.31 (5)
O—Rb—O^vi	67.05 (3)	O^viii—As—O^ix	109.80 (5)
O—Rb—O^vii	80.84 (3)

Symmetry codes: (i) −x, −y, z; (ii) y, −x, −z+1; (iii) −y, x, −z+1; (iv) −x+1/2, y, −z+3/4; (v) x−1/2, −y, −z+3/4; (vi) −y, −x+1/2, z+1/4; (vii) y, x−1/2, z+1/4; (viii) y, −x, −z; (ix) −y, x, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O—H···O^x	0.850 (8)	1.665 (6)	2.5125 (13)	175 (6)

Symmetry code: (x) x, −y+1/2, −z+1/4.