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Acta Crystallographica Section C: Structural Chemistry logoLink to Acta Crystallographica Section C: Structural Chemistry
. 2021 Feb 22;77(Pt 3):144–151. doi: 10.1107/S2053229621001650

(NH4)Mg(HSO4)(SO4)(H2O)2 and NaSc(CrO4)2(H2O)2, two crystal structures com­prising kröhnkite-type chains, and the temperature-induced phase transition (NH4)Mg(HSO4)(SO4)(H2O)2 Inline graphic (NH4)MgH(SO4)2(H2O)2

Matthias Weil a,*, Uwe Kolitsch b,c
PMCID: PMC7941265  PMID: 33664165

Both dihydrates (NH4)Mg(HSO4)(SO4)(H2O)2 and NaSc(CrO4)2(H2O)2 com­prise kröhnkite-type tetra­hedral–octa­hedral chains made up of {SO4/SO3(OH),[MgO4(OH2)2]} and {CrO4,[ScO4(OH2)2]} building units, respectively. (NH4)Mg(HSO4)(SO4)(H2O)2 shows a reversible phase transition between 100 and 296 K.

Keywords: ammonium magnesium sulfate, strong hydrogen bond, sodium scandium chromate, tetra­hedral-octa­hedral chains, kröhnkite-type chains, phase transition, crystal structure, IR spectroscopy

Abstract

The crystal structure of the mineral kröhnkite, Na2Cu(SO4)2(H2O)2, contains infinite chains com­posed of [CuO4(OH2)2] octa­hedra corner-linked with SO4 tetra­hedra. Such or similar tetra­hedral–octa­hedral ‘kröhnkite-type’ chains are present in the crystal structures of numerous com­pounds with the com­position AnM(XO4)2(H2O)2. The title com­pounds, (NH4)Mg(HSO4)(SO4)(H2O)2, ammonium magnesium hydrogen sulfate sulfate dihydrate, and NaSc(CrO4)2(H2O)2, sodium scandium bis­(chromate) dihydrate, are members of the large family with such kröhnkite-type chains. At 100 K, (NH4)Mg(HSO4)(SO4)(H2O)2 has an unprecedented triclinic crystal structure and contains [MgO4(OH2)2] octa­hedra linked by SO3(OH) and SO4 tetra­hedra into chains extending parallel to [Inline graphic10]. Adjacent chains are linked by very strong hydrogen bonds between SO3(OH) and SO4 tetra­hedra into layers parallel to (111). Ammonium cations and water mol­ecules connect adjacent layers through hydrogen-bonding inter­actions of medium-to-weak strength into a three-dimensional network. (NH4)Mg(HSO4)(SO4)(H2O)2 shows a reversible phase transition and crystallizes at room temperature in structure type E in the classification scheme for structures with kröhnkite-type chains, with half of the unit-cell volume for the resulting triclinic cell, and with disordered H atoms of the ammonium tetra­hedron and the H atom between two symmetry-related sulfate groups. IR spectroscopic room-temperature data for the latter phase are provided. Monoclinic NaSc(CrO4)2(H2O)2 adopts structure type F1 in the classification scheme for structures with kröhnkite-type chains. Here, [ScO4(OH2)2] octa­hedra (point group symmetry Inline graphic) are linked by CrO4 tetra­­hedra into chains parallel to [010]. The Na+ cations (site symmetry 2) have a [6 + 2] coordination and connect adjacent chains into a three-dimensional frame­work that is consolidated by medium–strong hydrogen bonds involving the water mol­ecules. Qu­anti­tative structural com­parisons are made between NaSc(CrO4)2(H2O)2 and its isotypic NaM(CrO4)2(H2O)2 (M = Al and Fe) analogues.

Introduction  

Compounds com­prising tetra­hedral oxoanions (XO4) and two types of cations, viz. a larger cation A and a smaller cation M, often exist as dihydrates with the general formula AnM(XO4)2(H2O)2 (n = 1 or 2) when crystallized from aqueous solutions or under hydro­thermal conditions. Irrespective of the chemical nature of A, M or X, the crystal structures of AnM(XO4)2(H2O)2 com­pounds frequently com­prise infinite chains com­posed of more or less distorted [MO4(OH2)2] octa­hedra corner-linked by XO4 tetra­hedra, a structural motif that is known from the mineral kröhnkite [Na2Cu(SO4)2(H2O)2; Dahlman, 1952 ▸]. An astonishingly large number of natural and synthetic hydrated oxysalts with this formula type is known to contain such ‘kröhnkite-type’ chains in their crystal structures. The widespread occurence of this motif is associated with its flexible nature and assemblies of the corner-sharing octa­hedral–tetra­hedral building units within a chain.

Reviews on natural and synthetic com­pounds with kröhnkite-type chains were given in four subsequent reports (Fleck et al., 2002a ▸; Fleck & Kolitsch, 2003 ▸; Kolitsch & Fleck, 2005 ▸, 2006 ▸). In general, com­pounds with the com­position AnM(XO4)2(H2O)2, where A = NaI, KI, RbI, CsI, NH4, HI, CaII or SrII; M = MgII, CrII, MnII, FeII, CoII, NiII, CuII, ZnII, CdII, AlIII, FeIII, ScIII, InIII or TlIII and X = PV, AsV, SVI, SeVI, CrVI, MoVI or WVI, containing kröhnkite-type chains, can be subdivided into eight major structure types denoted as A–H, for which more than 70 representatives are known up to date. Table 1 ▸ com­piles the most important parameters for these structure types, based on all representatives reported until the end of 2020, including the new type E1 described herein.

Table 1. Comparative com­pilation of structure types with kröhnkite-type chains.

Type Generalized formula(e) Space group Z Generalized unit-cell parameters (Å, °) No. of representatives
A M1II 2 M2II(TO4)2(H2O)2: M1 = Ca, Sr; M II = Mg, Mn, Fe, Co, Ni, Zn; T = P, As. M1I 2 M2II(TO4)2(H2O)2: M1 = Na, K, NH4; M II = Mg, Fe, Co, Ni, Cu, Zn; T = S, Se, Cr, Mo, W P\overline{1} 1 a ≃ 5.7–7.1, b ≃ 6.7–7.9, c ≃ 5.3–6.0, α ≃ 93–105, β ≃ 105–112, γ ≃ 103–111 32
B M1II 2 M2II(TO4)2(H2O)2: M1 = Ca; M2 = Mn, Fe; T = P P\overline{1} 1 a ≃ 5.8–6.0, b ≃ 6.5–6.6, c ≃ 5.5, α ≃ 102–103, β ≃ 108–109, γ ≃ 90–91 2
C M1I 2 M2II(TO4)2(H2O)2: M1 = K, Rb; M2 = Mn, Cd; T = S, Se, Cr P\overline{1} 2 a ≃ 6.6–6.9, b ≃ 7.3–7.7, c ≃ 10.7–11.4, α ≃ 72–73, β ≃ 74–75, γ ≃ 70 4
C1 K2Fe(SO4)2(H2O)2 P1a 2 a ≃ 6.6, b ≃ 7.3, c ≃ 10.7, α ≃ 73, β ≃ 74, γ ≃ 70 1
D M1II 2 M2II(TO4)2(H2O)2: M1 = Ca; M2 = Mg, Mn, Co, Cu, Zn; T = As. M1I 2 M2II(TO4)2(H2O)2: M1 = Na, NH4, Rb; M2 = Cr, Mn, Fe, Cu, Cd; T = S, Se, Cr, Mo P21/c 2 a ≃ 5.8–6.8, b ≃ 12.8–14.3, c ≃ 5.4–5.9, β ≃ 106–111 14
E M1I M2IIH(TO4)2(H2O)2: M1 = K, NH4, Cs; M2 = Mg, Mn, Fe, Co, Zn; T = S, Se P\overline{1} 1 a ≃ 4.6–4.8, b ≃ 5.7–5.9, c ≃ 8.1–8.6, α ≃ 103–104, β ≃ 96–100, γ ≃ 94–97 8
E1 b (NH4)Mg(HSO4)(SO4)(H2O)2 (at 100 K) P\overline{1} 2 a ≃ 7.1, b ≃ 7.7, c ≃ 8.3, α ≃ 84.6, β ≃ 73.3, γ ≃ 77.4 1
F1 c M1I M2III(TO4)2(H2O)2: M1 = Na; M2 = Al, Sc, Fe; T = Cr C2/c 4 a ≃ 14.1–14.5, b ≃ 5.3–5.6, c ≃ 10.7–10.8, β ≃ 109–110 3
F2 c M1I M2III(TO4)2(H2O)2: M1 = Na, K, NH4, Tl; M2 = Al, Fe, In; T = Cr C2/m 2 a ≃ 10.7–11.0, b ≃ 5.4–5.6, c ≃ 7.5–7.6, β ≃ 114 5
G AgSc(CrO4)2(H2O)2 P\overline{1} 1 a ≃ 5.6, b ≃ 6.1, c ≃ 7.4, α ≃ 111, β ≃ 90, γ ≃ 117 1
H d K2Zn(CrO4)2(H2O)2 C2/c 4 a ≃ 15.0, b ≃ 5.7, c ≃ 12.3, β ≃ 117 1
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Notes: (a) Type C1 is represented only by the low-temperature modification of K2Fe(SO4)2(H2O)2 and has an uncertain space group. (b) Type E1 is described for the first time in the present work; it is an ordered variant of type E (see text). (c) Types F1 and F2 are closely related (see text). (d) Type H (Stoilova et al., 2008 ▸) is closely related to type A.

We report here two new representatives of com­pounds with kröhnkite-type chains, viz. (NH4)Mg(HSO4)(SO4)(H2O)2 and NaSc(CrO4)2(H2O)2.

Experimental  

Synthesis and crystallization  

(NH4)Mg(HSO4)(SO4)(H2O)2  

A stoichiometric mixture of MgSO4(H2O)7, TeO2 and KOH (ratio 2:1:2 mmol; all reagents from Merck) was placed in a Teflon container with 6 ml capacity that was filled to approximately two-thirds of its volume with water. The container was closed, placed in a steel autoclave and heated at 480 K under autogenous pressure for 4 d. After slow cooling to room temperature within 1 d, the colourless reaction product was filtered off, washed with water and ethanol, and was dried in air. Inspection under a polarizing microscope revealed a microcrystalline solid with only very few crystals visible (diameter ≃ 0.1 mm). Powder X-ray diffraction (PXRD) of the bulk revealed spiro­ffite-type Mg2Te3O8 (Lin et al., 2013 ▸) as the main product, and MgTe2O5 (Weil, 2005 ▸) as a minor product. The grown crystals correspond to the title com­pound. Structure refinement showed NH4 + cations present in the structure. The source of ammonium remains unclear; most probably, ammonium cations were left in the cracks of the Teflon container from previous reactions in ammonia solution.

For a directed synthesis of (NH4)Mg(HSO4)(SO4)(H2O)2, equimolar aqueous solutions of NH4HSO4 and MgSO4(H2O)7 were mixed at room temperature and stirred for homogeneity. The used NH4HSO4 was freshly prepared by slowly adding concentrated ammonia solution to concentrated sulfuric acid in stoichiometric amounts and recrystallization of the colourless product from water; its purity was checked by PXRD. The mixed NH4HSO4 and MgSO4 solutions were evaporated to dryness at 353 K in a drying oven and also much more slowly at room temperature. Semi-qu­anti­taive phase analysis using the Rietveld method with HighScore Plus (Degen et al., 2014 ▸) revealed a phase mixture of (NH4)MgH(SO4)2(H2O)2 and synthetic boussingaultite [(NH4)2Mg(SO4)2(H2O)6] in a ratio of ≃94:6 wt% for the sample dried at 353 K, and in a ratio of 91:9 wt% for the sample dried at room temperature.

A mid-range IR spectrum was recorded at room temperature for selected crystals of (NH4)MgH(SO4)2(H2O)2 in the attenuated total reflectance (ATR) technique in the range 4000–450 cm−1 on a PerkinElmer Spectrum Two FT–IR spec­trometer with a UATR accessory (diamond detector crystal) attached.

NaSc(CrO4)2(H2O)2  

Small tabular orange crystals with a rhombus-shaped outline crystallized at room tem­pera­ture from an acidic aqueous solution (pH about 4) containing dissolved reagent-grade Na2CO3 (Merck), Sc2O3 (99.99%, alphametall, Germany) and reagent-grade CrO3 (Merck). The crystals were often arranged in radiating clusters. They were associated with pale orange–yellow blade-shaped crystals of Na2Cr2O7·2H2O (Casari et al., 2007 ▸).

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸.

Table 2. Experimental details.

Experiments were carried out with Mo Kα radiation. All H-atom parameters were refined.

  (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K (NH4)MgH(SO4)2(H2O)2 at 296 K NaSc(CrO4)2(H2O)2
Crystal data
Chemical formula (NH4)Mg(HSO4)(SO4)(H2O)2 (NH4)Mg(HSO4)(SO4)(H2O)2 NaSc(CrO4)2(H2O)2
M r 271.51 271.51 335.98
Crystal system, space group Triclinic, P\overline{1} Triclinic, P\overline{1} Monoclinic, C2/c
Temperature (K) 100 296 293
a, b, c (Å) 7.0631 (7), 7.7065 (7), 8.3372 (8) 4.6771 (1), 5.7697 (1), 8.3697 (2) 14.505 (3), 5.563 (1), 10.763 (2)
α, β, γ (°) 84.603 (3), 73.339 (3), 77.387 (3) 104.208 (1), 98.189 (1), 94.508 (1) 90, 109.82 (3), 90
V (Å3) 424.03 (7) 215.20 (1) 817.0 (3)
Z 2 1 4
μ (mm−1) 0.75 0.73 3.51
Crystal size (mm) 0.12 × 0.09 × 0.02 0.12 × 0.09 × 0.01 0.17 × 0.10 × 0.03
 
Data collection
Diffractometer Bruker APEXII CCD Bruker APEXII CCD Nonius KappaCCD
Absorption correction Multi-scan (SADABS; Krause et al., 2015 ▸) Multi-scan (SADABS; Krause et al., 2015 ▸) Multi-scan (SCALEPACK; Otwinowski et al., 2003 ▸)
T min, T max 0.708, 0.747 0.699, 0.747 0.755, 0.949
No. of measured, independent and observed [I > 2σ(I)] reflections 30998, 4431, 3120 11147, 1940, 1843 3418, 1783, 1372
R int 0.051 0.024 0.017
(sin θ/λ)max (Å−1) 0.864 0.812 0.805
 
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S 0.033, 0.081, 1.02 0.019, 0.054, 1.18 0.027, 0.081, 1.08
No. of reflections 4431 1940 1783
No. of parameters 163 95 75
No. of restraints 0 0 2
Δρmax, Δρmin (e Å−3) 0.52, −0.54 0.30, −0.47 1.06, −0.69
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Computer programs: APEX2 (Bruker, 2016 ▸), COLLECT (Nonius, 2003 ▸), DENZO and SCALEPACK (Otwinowski et al., 2003 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018 (Sheldrick, 2015b ▸), ATOMS (Dowty, 2006 ▸) and publCIF (Westrip, 2010 ▸).

For the refinement of (NH4)Mg(HSO4)(SO4)(H2O)2 (100 K data), all H atoms were located from difference Fourier maps and were refined freely. Reflections Inline graphic03, 102, 100 and 230 were obstructed by the beam stop and were therefore omitted from the refinement. For the structure analysis of (NH4)MgH(SO4)2(H2O)2 (296 K data), a different crystal was measured (resulting in a different orienting matrix; see Fig. 1 ▸). For refinement, the starting coordinates and labelling of atoms were adapted from the isotypic Fe com­pound (Heinicke et al., 2004 ▸). In this structure, the H atoms (H1A–H1D) bonded to N atoms are all disordered over two equally occupied sites. The H1O atom located between two symmetry-related SO4 tetra­hedra was clearly discernible from a difference Fourier map; it is disordered across the inversion centre with an occupancy of 0.5 for the two H-atom sites. All H atoms in this structure were refined freely.

Figure 1.

Figure 1

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The hk1 plane of reciprocal space of (NH4)Mg(HSO4)(SO4)(H2O)2 [100 K data in parts (a)/(b)] and of (NH4)MgH(SO4)2(H2O)2 [296 K data in part (c)] reconstructed from CCD data. (a) The small cell with matching reflections is marked with orange circles; no superstructure reflections are visible. (b) The actual supercell with matching reflections is marked with green circles. (c) The actual small cell is marked with orange circles without superstructure reflections.

For refinement of NaSc(CrO4)2(H2O)2, the coordinates and labelling of atoms were taken from isotypic NaFe(CrO4)2(H2O)2 (Hardy & Gravereau, 1970 ▸). The H atoms of the water mol­ecule were located from a difference Fourier map and were refined with a constraint of O—H = 0.90 ± 0.03 Å.

Results and discussion  

(NH4)Mg(HSO4)(SO4)(H2O)2  

Structure analysis  

(NH4)Mg(HSO4)(SO4)(H2O)2 was obtained serendipitously from a hydro­thermal synthesis intended to crystallize a com­pound in the system Mg–SVI–TeIV–O–H (Weil & Shirkhanlou, 2017 ▸). A subsequently performed directed synthesis yielded this material in >90% yield by evaporation of an aqueous solution containing equimolar amounts of NH4HSO4 and MgSO4.

(NH4)Mg(HSO4)(SO4)(H2O)2 is the fourth com­pound in the NH3–MgO–SO3–H2O system. The three other known members are the two minerals boussingaultite, i.e. (NH4)2Mg(SO4)2(H2O)6 (Maslen et al., 1988 ▸), and efremovite, i.e. (NH4)2Mg2(SO4)3 (Shcherbakova & Bazhenova, 1989 ▸), and synthetic (NH4)2Mg3(OH)2(SO4)3(H2O)2 (Marri et al., 2017 ▸). Monoclinic boussingaultite is a representative of the picromerite group and crystallizes isotypically with many other synthetic A I 2 M II(XO4)2(H2O)6 com­pounds (A I = NH4, K, Rb, Cs or Tl; M II = Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd; X = S, Se or Cr), commonly known as Tutton’s salts [crystal structure first determined by Hofmann (1931 ▸)]. Efremovite adopts the cubic langbeinite structure type (Zemann & Zemann, 1957 ▸), and ortho­rhom­bic (NH4)2Mg3(OH)2(SO4)3(H2O)2 is isotypic with its cadmium analogue (NH4)2Cd3(OH)2(SO4)3(H2O)2 (Yin, 2011 ▸).

(NH4)Mg(HSO4)(SO4)(H2O)2 is an unprecedented mem­ber within the family of com­pounds with kröhnkite-type chains and crystallizes in a unique structure type at 100 K, here denoted as E1 in order to conform to the classification of compounds with kröhnkite-type chains (Fleck et al., 2002a ▸; Table 1 ▸). All atoms in the triclinic structure are situated on general positions. [MgO4(OH2)] octa­hedra are corner-linked by SO3(OH) and SO4 tetra­hedra into chains running parallel to [Inline graphic10] (Fig. 2 ▸). Adjacent chains are joined by hydrogen bonds between hydrogen sulfate and sulfate tetra­hedra into sheets extending parallel to (111). Ammonium cations, situated between the sheets, and water mol­ecules are also involved in hydrogen bonding and consolidate the three-dimensional network (Fig. 3 ▸).

Figure 2.

Figure 2

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The kröhnkite-type chain in the crystal structure of (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K. [MgO4(OH2)2] octa­hedra are blue, SO3(OH) tetra­hedra are orange and SO4 tetra­hedra are red. Displacement ellipsoids are drawn at the 74% probability level and H atoms are given as grey spheres of arbitrary radius. [Symmetry codes: (i) −x, −y + 1, −z + 1; (ii) −x + 1, −y, −z + 1.]

Figure 3.

Figure 3

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The crystal structure of (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K in a projection along [Inline graphic10]. [MgO4(OH2)2] octa­hedra are blue, SO4 tetra­hedra are red, SO3(OH) tetra­hedra are orange and ammonium groups are green. The strong hydrogen bond between the SO3(OH) and SO4 tetra­hedra is indicated by green lines, hydrogen bonds involving water mol­ecules by white lines and those involving the ammonium cations by yellow lines. Displacement ellipsoids are drawn at the 74% probability level.

The Mg—O bond lengths in the [MgO4(OH2)2] octa­hedron scatter only slightly [range 2.0382 (9)–2.0715 (9) Å; Table 3 ▸], with the two trans-aligned water mol­ecules (O9 and O19) in the axial positions. The mean Mg—O distance of 2.061 Å fits well into the grand mean value of 2.09 (6) Å for six-coordinate MgII (Gagné & Hawthorne, 2016 ▸). The SO4 tetra­hedron (centred by atom S1) is slightly distorted, with bond lengths and angles in the ranges 1.4659 (9)–1.4901 (9) Å (mean 1.476 Å) and 106.87 (5)–111.52 (5)° (mean 109.5°), respectively. The bond-length values are in very good agreement with those given in a review on the sulfate group, for which the grand mean S—O distance is 1.473 Å, with minimum and maximum S—O distances of 1.430 and 1.501 Å, respectively (Hawthorne et al., 2000 ▸). The longest bond in the S1O4 tetra­hedron is that to atom O4, acting as an acceptor atom for a hydrogen bond involving the OH group of the hydrogen sulfate group. The corresponding S2O3(OH) tetra­hedron shows the typical S—O bond-length distribution where the bond to the OH group (O8) is considerably elongated. The S2—O8 bond of 1.5474 (9) Å is about 0.09 Å longer than the mean bond length (1.456 Å) of the remaining three bonds, in good agreement with other structures com­prising a hydrogen sulfate anion, e.g. Mg(HSO4)2(H2O) (Worzala et al., 1991 ▸) or Th(HSO4)2(SO4) (Betke & Wickleder, 2012 ▸). In the magnesium com­pound, with its two independent SO3(OH) tetra­hedra, mean values of 1.448 Å for the S—O and 1.550 Å for the S—OH bond lengths are found, and for the thorium com­pound, the corresponding mean values are 1.452 and 1.533 Å, respectively, for two independent SO3(OH) tetra­hedra; the SO4 group in the thorium com­pound has a mean S—O bond length of 1.467 Å.

Table 3. Selected bond lengths (Å) for (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K.
Mg1—O2 2.0382 (9) S1—O2 1.4685 (8)
Mg1—O7 2.0601 (9) S1—O3 1.4775 (8)
Mg1—O3i 2.0630 (9) S1—O4 1.4901 (9)
Mg1—O10 2.0645 (9) S2—O5 1.4480 (9)
Mg1—O9 2.0660 (10) S2—O6 1.4583 (8)
Mg1—O6ii 2.0715 (9) S2—O7 1.4611 (8)
S1—O1 1.4659 (9) S2—O8 1.5474 (9)
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Symmetry codes: (i) -x, -y+1, -z+1; (ii) -x+1, -y, -z+1.

In the crystal structure of (NH4)Mg(HSO4)(SO4)(H2O)2, the short hydrogen bond between the S2O3(OH) and the S1O4 tetra­hedra [O8⋯O4iii = 2.5048 (12) Å; Table 4 ▸] is linear [177 (3)°] and considered as strong (Jeffrey, 1997 ▸). In com­parison, the other types of O—H⋯O hydrogen-bonding inter­actions are much weaker and are connected with the two water mol­ecules. One of the water mol­ecules (O9) is involved in a slightly bent hydrogen bond of medium strength to atom O1iv and in a weak trifurcated hydrogen bond to O2i, O5v and O7v; numerical values of these inter­actions, as well as symmetry codes, are collated in Table 4 ▸. The other water mol­ecule (O10) is the donor of one medium–strong and slightly bent hydrogen bond to O5vii, and of a weak bifurcated hydrogen bond to O1vi and O2vi. As expected, the ammonium cation is also engaged in hydrogen bonding. All of its H atoms are accepted in a more or less linear manner [N—H⋯O angles range from 168.2 (15) to 179.2 (18)°] by the O atoms of the sulfate group (O4v, O1i and O3iii) and, inter­estingly, by the OH group of the hydrogen sulfate anion (O8viii). The latter hydrogen bond is much more bent [154.4 (16)°], most probably due to steric reasons to avoid a too close contact with the H atom of the hy­droxy group.

Table 4. Hydrogen-bond geometry (Å, °) for (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K.
D—H⋯A D—H H⋯A D⋯A D—H⋯A
O8—H1O⋯O4iii 0.91 (3) 1.59 (3) 2.5048 (12) 177 (3)
O9—H1W⋯O1iv 0.81 (2) 2.02 (2) 2.7623 (13) 154 (2)
O9—H2W⋯O2i 0.78 (3) 2.56 (3) 3.1278 (12) 132 (2)
O9—H2W⋯O5v 0.78 (3) 2.62 (3) 3.2887 (12) 146 (2)
O9—H2W⋯O7v 0.78 (3) 2.55 (3) 2.9578 (12) 115 (2)
O10—H3W⋯O1vi 0.77 (2) 2.38 (2) 3.0970 (12) 156 (2)
O10—H3W⋯O2vi 0.77 (2) 2.64 (2) 3.0615 (12) 117 (2)
O10—H4W⋯O5vii 0.80 (2) 1.95 (2) 2.7125 (13) 160.5 (19)
N1—H1N⋯O4v 0.896 (19) 2.108 (19) 2.9913 (14) 168.2 (15)
N1—H2N⋯O1i 0.85 (2) 2.04 (2) 2.8920 (14) 179.2 (18)
N1—H3N⋯O3iii 0.868 (19) 1.98 (2) 2.8451 (14) 170.5 (18)
N1—H4N⋯O8viii 0.870 (19) 2.229 (19) 3.0368 (14) 154.4 (16)
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Symmetry codes: (i) -x, -y+1, -z+1; (iii) x+1, y, z-1; (iv) x+1, y, z; (v) -x+1, -y+1, -z+1; (vi) -x, -y, -z+1; (vii) x-1, y, z; (viii) x, y+1, z.

Bond-valence sums (BVSs; Brown, 2002 ▸), calculated with the parameters of Brese & O’Keeffe (1991 ▸), amount to 2.22 valence units (v.u.) for Mg, 5.98 v.u. for S1 and 5.96 v.u. for S2, in good agreement with the formal charges of +II and +VI, respectively.

Phase transition  

As mentioned above, at 100 K, (NH4)Mg(HSO4)(SO4)(H2O)2 crystallizes in an own structure type, denoted as E1. Between 100 K and room temperature, the crystal is transformed into a triclinic structure corresponding to type E (space group P Inline graphic, Z = 1) in the classification of com­pounds with kröhnkite-type chains (Table 1 ▸). Next to the six isotypic sulfates KFeH(SO4)2(H2O)2 (Fleck et al., 2002b ▸), KMgH(SO4)2(H2O)2 (Macíček et al., 1994 ▸), KZnH(SO4)2(H2O)2, KMnH(SO4)2(H2O)2, CsMnH(SO4)2(H2O)2 (Troy­anov et al., 2002 ▸) and NH4FeH(SO4)2(H2O)2 (Heinicke et al., 2004 ▸), and the selenate KMgH(SeO4)2(H2O)2 (Troyanov et al., 2002 ▸), (NH4)Mg(HSO4)(SO4)(H2O)2, or more precisely (NH4)MgH(SO4)2(H2O)2 at this temperature, is the eighth member of this structure type. The [M IIO4(OH2)2] octa­hedron in these structures (Fig. 4 ▸ and Table 5 ▸) is located on an inversion centre, just like the A cation (for A = NH4; the H sites are disordered). A peculiarity of type E pertains to the dynamically disordered H atom between two symmetry-related sulfate groups, defining a short asymmetrical hydrogen bond with O⋯O contacts around 2.5 Å (Table 6 ▸). In com­parison, in the crystal structure of (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K, the H atom is ordered between two sulfate tetra­hedra, defining distinct SO3OH and SO4 groups. This ordering is accom­panied by a doubling of the unit-cell volume of the type E1 relative. The bond lengths of the principal building units in the disordered room-temperature structure (Table 5 ▸; mean values for the Mg—O and S—O bond are 2.065 and 1.474 Å, respectively) are similar to those in the ordered low-temperature structure. Although the S—O(H) bond (O1) in the disordered structure is still the longest in the SO4 tetra­hedron, it is about 0.03 Å shorter than the S—OH bond (O8) in the ordered structure. On the other hand, the O1⋯O1iv distance of the hydrogen bond with the disordered H1O atom [2.4790 (12) Å] is considerably shorter than the corresponding value in the ordered structure [2.5048 (12) Å], indicating a very strong hydrogen bond (Jeffrey, 1997 ▸) for (NH4)MgH(SO4)2(H2O)2.

Figure 4.

Figure 4

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The crystal structure of (NH4)MgH(SO4)2(H2O)2 at 296 K in a projection along [100]. Displacement ellipsoids and colour codes are as in Fig. 2 ▸, except for the SO4 tetra­hedra which are lilac. The disordered ammonium group and the H1O atom disordered between two sulfate tetra­hedra are shown.

Table 5. Selected bond lengths (Å) for (NH4)MgH(SO4)2(H2O)2 at 296 K.
Mg1—O2 2.0509 (6) S1—O2 1.4604 (6)
Mg1—O4i 2.0720 (6) S1—O4 1.4651 (6)
Mg1—O5 2.0731 (6) S1—O1 1.5164 (6)
S1—O3 1.4525 (6)    
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Symmetry code: (i) x+1, y, z.

Table 6. Hydrogen-bond geometry (Å, °) for (NH4)MgH(SO4)2(H2O)2 at 296 K.
D—H⋯A D—H H⋯A D⋯A D—H⋯A
O5—H5B⋯O2ii 0.81 (2) 2.52 (2) 3.0010 (9) 118.9 (19)
O5—H5A⋯O3iii 0.78 (2) 2.05 (2) 2.7684 (9) 154.0 (19)
O5—H5B⋯O3ii 0.81 (2) 2.42 (2) 3.1678 (10) 152 (2)
N1—H1A⋯O3iv 0.90 (5) 2.03 (5) 2.9260 (6) 175 (5)
N1—H1C⋯O1 0.89 (6) 2.28 (6) 3.1536 (7) 164 (4)
N1—H1B⋯O1v 0.83 (5) 2.30 (5) 3.1122 (8) 164 (5)
N1—H1D⋯O4v 0.82 (5) 2.08 (5) 2.9042 (6) 173 (5)
O1—H1O⋯O1iv 0.84 (3) 1.64 (3) 2.4790 (12) 176 (4)
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Symmetry codes: (ii) -x, -y-1, -z; (iii) -x-1, -y-1, -z; (iv) -x-1, -y-1, -z-1; (v) -x-1, -y, -z-1.

The crystal structure of (NH4)Mg(HSO4)(SO4)(H2O)2 at 100 K represents a twofold superstructure with ordered H atoms for the ammonium and hydrogen sulfate groups relative to the crystal structure of (NH4)MgH(SO4)2(H2O)2 at 296 K with a halved unit-cell volume. The subcell of the latter is related to the doubled cell of the (NH4)Mg(HSO4)(SO4)(H2O)2 superstructure by application of the matrix (Inline graphic Inline graphic 0, 1 Inline graphic 0, 0 0 1); the symmetry relationship between the substructure and the superstructure is of isomorphic type with index 2 (i2) (Müller, 2013 ▸). Fig. 1 ▸ shows the hk1 plane of reciprocal space of (NH4)Mg(HSO4)(SO4)(H2O)2 and the relation of the subcell (Fig. 1 ▸ a) and the cell of the actual superstructure (Fig. 1 ▸ b); the missing reflections for the subcell clearly indicate that the doubled cell is correct at this temperature. Fig. 1 ▸(c) shows the hk1 plane of reciprocal space of (NH4)MgH(SO4)2(H2O)2 without noticeable superstructure reflections for the room-temperature data set. Investigations of the exact ordering temperatures for this reversible phase transition upon cooling and heating, as well as a systematic study of other (NH4)M II(HSO4)(SO4)(H2O)2 Inline graphic (NH4)M IIH(SO4)2(H2O)2 (M = first-row transition metals) phases, are underway.

IR spectroscopy  

The IR spectrum of (NH4)MgH(SO4)2(H2O)2 shows similarities to that of synthetic boussingaultite (Jayakumar et al., 1988 ▸) and is displayed in Fig. 5 ▸. Wavenumbers/cm−1: 3547 (w), 3403 (br), 3235 (br), 3100 (w), 2866 (vw), 1753 (vw), 1627 (m), 1429 (m), 1146 (s), 1044 (sh), 918 (m), 884 (m), ≃600 (sh) (br = broad; m = medium; s = strong; sh = shoulder; w = weak; vw = very weak).

Figure 5.

Figure 5

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IR spectrum of (NH4)MgH(SO4)2(H2O)2 at room temperature.

In the wavenumber range 3700–2500 cm−1, bands due to O—H stretching vibrations of the H2O groups overlap with various bands of the NH4 group. The bands at 3547 and 3403 cm−1 are assigned to the O—H stretching vibrations, while the band at 2335 cm−1 is tentatively assigned to the ν3(NH4) stretching vibration, and the shoulder at 3100 cm−1 to the ν1(NH4) stretch, the shoulder possibly also to an additional combination band ν2 + ν4(NH4). The very small band at 2866 cm−1 is probably caused by a combination band 2ν4(NH4). The very strong hydrogen bonding involving the protonated {H(SO4)2} group is reflected by an extremely broad band in the range between roughly 1200 and 1000 cm−1 (Beran & Libowitzky, 1999 ▸; Libowitzky, 1999 ▸), which appears ‘hidden’ in the background. The wavenumber range between 1800 and 1250 cm−1 contains bands due to the ν2(NH4) bending vibration (1627 cm−1, possibly also the very small band at 1753 cm−1) and the ν4(NH4) bending vibration (1429 cm−1). The range 1250–700 cm−1 shows bands due to vibrations of the SO4/HSO4 groups. The band at 1146 cm−1 is due to the ν3(SO4) stretching vibration, while the bands at 1044, 918 and 884 cm−1 are assigned to the ν1(SO4) stretching vibration. The shoulder at ∼600 cm−1 is problably due to the ν4(SO4) vibration. The ν2(SO4) bending vibration will cause bands <500 cm−1, where the spectrum is cut off and where bands due to vibrations of the MgO6 octa­hedron, the librational modes of the NH4 group and lattice modes are expected. Note that the presence of ‘forbidden’ SO4 and NH4 vibrations in the IR spectrum is in agreement with the presence of distorted shapes for these two building units.

NaSc(CrO4)2(H2O)2  

Structure analysis  

NaSc(CrO4)2(H2O)2 adopts type F (subtype F1) of the classification scheme for structures with kröhnkite-type chains (Table 1 ▸). Subtype F1 (space group C2/c, Z = 2) can be considered as a superstructure of subtype F2 (space group C2/m, Z = 1) that has a halved unit-cell volume relative to F1 [transformation matrix F1→F2 is (0 0 1, 0 1 0, Inline graphic 0 Inline graphic)]. The group–subgroup relationship between subtypes F2 and F1 is klassengleich with index 2 (k2) (Müller, 2013 ▸). In the crystal structure of NaSc(CrO4)2(H2O)2, [ScO4(OH2)2] octa­hedra (point-group symmetry Inline graphic) are linked by CrO4 tetra­hedra into chains running parallel to [010] (Fig. 6 ▸). The NaI cations (site symmetry 2) connect adjacent chains into a three-dimensional framework that is stabilized by hydrogen bonds between water mol­ecules and sulfate O atoms (Fig. 7 ▸).

Figure 6.

Figure 6

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The kröhnkite-type chain in the crystal structure of NaSc(CrO4)2(H2O)2. [ScO4(OH2)2] octa­hedra are blue and CrO4 tetra­hedra are yellow. Displacement ellipsoids are drawn at the 74% probability level and H atoms are given as grey spheres of arbitrary radius. [Symmetry codes: (i) x, −y + 1, z + Inline graphic; (ii) x, y + 1, z; (iii) x, −y + 1, z − Inline graphic; (iv) −x + Inline graphic, −y + Inline graphic, −z.]

Figure 7.

Figure 7

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The crystal structure of NaSc(CrO4)2(H2O)2 in a projection along [010]. Colour codes and the probability level of displacement ellipsoids are as in Fig. 3 ▸. Hydrogen bonds are indicated as green lines.

In the [ScO4(OH2)2] octa­hedron, the longest bond [2.1222 (14) Å] is that to the axially bound O5 atom of the water mol­ecule, whereas the equatorial O atoms (O3 and O4), which are also part of a CrO4 tetra­hedron, have shorter Sc—O bonds, with a mean of 2.076 Å (Table 7 ▸). The overall mean value for the Sc—O bond lengths is 2.091 Å, which matches very well the literature values of 2.10 (7) and 2.098 (41) Å given by Serezhkin et al. (2003 ▸) and Gagné & Hawthorne (2020 ▸), respectively. In the CrO4 tetra­hedron, the longest Cr—O bonds (≃1.69 Å) are realized for O1 and O2, which are part of the kröhnkite chains. The other two O atoms (O3 and O4) have considerably shorter Cr—O bonds (≃1.62 Å) and are the acceptor atoms for two nearly linear hydrogen bonds of medium–strong nature involving both water H atoms (Table 8 ▸). Again, the mean Cr—O bond length of 1.651 Å is in very good agreement with the literature value of 1.65 (6) Å (Gagné & Hawthorne, 2020 ▸). The NaI cation shows a [6 + 2] coordination with the six closer O atoms defining a distorted octa­hedron (O1, O2 and their symmetry-related counterparts in equatorial sites, and O3 and its symmetry-related counterpart in axial sites), with the two remote O4 atoms capping two faces of the octa­hedron (Table 7 ▸). Notably, the water mol­ecule is not part of the coordination sphere of Na. The mean Na—O bond length is 2.678 Å, somewhat longer than that of the literature value of 2.60 (19) Å for eightfold-coordinated NaI (Gagné & Hawthorne, 2016 ▸). This elongation is also reflected in the slight underbonding of Na1 in the structure (BVS = 0.83 v.u.), with a deviation of 17% from the expected value of +I. ScIII and CrVI, on the other hand, have BVS values of 3.12 and 5.92 v.u., respectively, and deviate much less (by about 4 and 2%) from the expected values.

Table 7. Selected bond lengths (Å) for NaSc(CrO4)2(H2O)2 .
Na—O3i 2.5201 (15) Sc—O5 2.1222 (14)
Na—O2i 2.5358 (15) Cr—O4 1.6045 (14)
Na—O1ii 2.7207 (17) Cr—O3iv 1.6204 (14)
Na—O4i 2.9360 (18) Cr—O1iv 1.6829 (12)
Sc—O1iii 2.0747 (12) Cr—O2v 1.6960 (11)
Sc—O2 2.0772 (12)    
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Symmetry codes: (i) -x+{\script{1\over 2}}, y+{\script{1\over 2}}, -z+{\script{1\over 2}}; (ii) x-{\script{1\over 2}}, -y+{\script{3\over 2}}, z-{\script{1\over 2}}; (iii) -x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}; (iv) x, -y+1, z-{\script{1\over 2}}; (v) x, y+1, z.

Table 8. Hydrogen-bond geometry (Å, °) for NaSc(CrO4)2(H2O)2 .
D—H⋯A D—H H⋯A D⋯A D—H⋯A
O5—H1⋯O4vi 0.82 (3) 1.86 (3) 2.682 (2) 179 (3)
O5—H2⋯O3 0.82 (2) 1.97 (3) 2.765 (2) 164 (3)
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Symmetry code: (vi) x-{\script{1\over 2}}, y-{\script{1\over 2}}, z.

NaSc(CrO4)2(H2O)2 is isotypic with NaAl(CrO4)2(H2O)2 (Cudennec & Riou, 1977 ▸) and NaFe(CrO4)2(H2O)2 (Hardy & Gravereau, 1970 ▸), the only other members of structure type F1 in the classification of structures with kröhnkite-type chains. The title scandium com­pound is the first of this series for which the H atoms have been localized, thus making an unambiguous assignment of the hydrogen-bonding scheme possible (see above). For a qu­anti­tative structural com­parison of the three isotypic NaM(CrO4)2(H2O)2 (M = Sc, Al or Fe) structures, the program com­pstru (de la Flor et al., 2016 ▸), available at the Bilbao Crystallographic Server (Aroyo et al., 2006 ▸), was used. With NaSc(CrO4)2(H2O)2 as the reference structure, Table 9 ▸ com­piles the absolute distances between paired atoms and numerical values regarding the arithmetic mean of the distance between paired atoms, the degree of lattice distortion (Δ) and the measure of similarity (S). There is no clear trend as to the largest displacement of an atom pair in the three crystal structures. Whereas the water O atom (O5) in the M = Al structure shows the largest displacement, it is O1 in the M = Fe structure. In general, the rather low values of S indicate high similarities between NaSc(CrO4)2(H2O)2 and the two NaM(CrO4)2(H2O)2 (M = Al and Fe) structures, whereby the M = Fe structure has a higher absolute similarity to NaSc(CrO4)2(H2O)2. Most likely, this behaviour is related to the ionic radii (Shannon, 1976 ▸) of the three M III cations. For coordination number 6, the ionic radius of ScIII (0.745 Å) is closer to that of FeIII (0.645 Å, assuming a high-spin state) than to that of AlIII (0.535 Å).

Table 9. Absolute atomic displacements (Å), arithmetic mean (d av, Å), degree of lattice distortion (S) and measure of similarity (Δ) in the isotypic NaM(CrO4)(H2O)2 (M = Al and Fe) structures relative to NaSc(CrO4)2(H2O)2 a .
  M = Alb M = Fec
Na1 0.0595 0.0773
M1 0 0
Cr1 0.0839 0.0536
O1 0.1477 0.1608
O2 0.1227 0.1436
O3 0.0595 0.0395
O4 0.0395 0.0520
O5 0.1916 0.0751
     
d av 0.0964 0.0805
Δ 0.023 0.019
S 0.0186 0.0107
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Notes: (a) H atoms were omitted from com­parison because in the M = Al and Fe structures, H atoms were not localized. (b) Unit-cell parameters a = 14.080 (10), b = 5.338 (3), c = 10.655 (6) Å and β = 110.33 (5)° (Cudennec & Riou, 1977 ▸). (c) Unit-cell parameters a = 14.247 (2), b = 5.425 (5), c = 10.689 (2) Å and β = 109.30 (1)° (Hardy & Gravereau, 1970 ▸).

Supplementary Material

Crystal structure: contains datablock(s) NH4MgHSO4SO4H2O2_100K, NH4MgHSO42H2O_296K, NaScCrO42H2O2, global. DOI: 10.1107/S2053229621001650/ef3013sup1.cif

c-77-00144-sup1.cif (1.3MB, cif)

Structure factors: contains datablock(s) NH4MgHSO4SO4H2O2_100K. DOI: 10.1107/S2053229621001650/ef3013NH4MgHSO4SO4H2O2_100Ksup2.hkl

Structure factors: contains datablock(s) NH4MgHSO42H2O_296K. DOI: 10.1107/S2053229621001650/ef3013NH4MgHSO42H2O_296Ksup3.hkl

Structure factors: contains datablock(s) NaScCrO42H2O2. DOI: 10.1107/S2053229621001650/ef3013NaScCrO42H2O2sup4.hkl

CCDC references: 2062574, 2062573, 2062572

Acknowledgments

The X-ray centre of TU Wien is acknowledged for support and for providing access to the single-crystal and powder X-ray diffractometers. We thank two anonymous reviewers for their construtive criticism that helped to improve the manuscript. Open access funding enabled and organized by Projekt DEAL.

References

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Crystal structure: contains datablock(s) NH4MgHSO4SO4H2O2_100K, NH4MgHSO42H2O_296K, NaScCrO42H2O2, global. DOI: 10.1107/S2053229621001650/ef3013sup1.cif

c-77-00144-sup1.cif (1.3MB, cif)

Structure factors: contains datablock(s) NH4MgHSO4SO4H2O2_100K. DOI: 10.1107/S2053229621001650/ef3013NH4MgHSO4SO4H2O2_100Ksup2.hkl

c-77-00144-NH4MgHSO4SO4H2O2_100Ksup2.hkl (352.9KB, hkl)

Structure factors: contains datablock(s) NH4MgHSO42H2O_296K. DOI: 10.1107/S2053229621001650/ef3013NH4MgHSO42H2O_296Ksup3.hkl

c-77-00144-NH4MgHSO42H2O_296Ksup3.hkl (155.8KB, hkl)

Structure factors: contains datablock(s) NaScCrO42H2O2. DOI: 10.1107/S2053229621001650/ef3013NaScCrO42H2O2sup4.hkl

c-77-00144-NaScCrO42H2O2sup4.hkl (143.9KB, hkl)

CCDC references: 2062574, 2062573, 2062572

Articles from Acta Crystallographica. Section C, Structural Chemistry are provided here courtesy of International Union of Crystallography

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