Hamdi Ben Yahia*a,
Alaa Alkhateeba and
Rachid Essehlib
aQatar Environment and Energy Research Institute (QEERI 2.0), Hamad Bin Khalifa University, Qatar Foundation, P. O. Box 34110, Doha, Qatar. E-mail: Hyahia@hbku.edu.qa
bEnergy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
First published on 11th March 2020
The title compounds were prepared via a wet chemistry route and their crystal structures were determined from single crystal X-ray diffraction data. Na2Mn(SO4)2·4H2O crystallizes with a monoclinic symmetry, space group P21/c, with a = 5.5415(2), b = 8.3447(3), c = 11.2281(3) Å, β = 100.172(1)°, V = 511.05(3) Å3 and Z = 2. Na2Ni(SO4)2·10H2O also crystallizes with a monoclinic symmetry, space group P21/c, with a = 12.5050(8), b = 6.4812(4), c = 10.0210(6) Å, β = 106.138(2)°, V = 780.17(8) Å3 and Z = 2. Na2Mn(SO4)2·4H2O is a new member of the blödite family of compounds, whereas Na2Ni(SO4)2·10H2O is isostructural with Na2Mg(SO4)2·10H2O. The structure of Na2Mn(SO4)2·4H2O is built up of [Mn(SO4)2(H2O)4]2− building blocks connected through moderate O–H⋯O hydrogen bonds with the sodium atoms occupying the large tunnels along the a axis and the manganese atom lying on an inversion center, whereas the structure of Na2Ni(SO4)2·10H2O is built up of [Ni(H2O)6]2+ and [Na2(SO4)2(H2O)4]2− layers. These layers which are parallel to the (100) plane are interconnected through moderate O–H⋯O hydrogen bonds. The thermal gravimetric- and the powder X-ray diffraction-analyzes showed that only the nickel phase was almost pure. At a temperature above 300 °C, all the water molecules evaporated and a structural phase transition from P21/c-Na2Ni(SO4)2·10H2O to C2/c-Na2Ni(SO4)2 was observed. C2/c-Na2Ni(SO4)2 is thermally more stable than Na2Fe(SO4)2 and therefore it would be suitable as the positive electrode for sodium ion batteries if a stable electrolyte at high voltage is developed.
Among the Na2M(SO4)2 phases, Na2Fe(SO4)2 showed interesting electrochemical properties in Li- and Na-ion batteries. This phase enables the removal of nearly one sodium at potentials around ∼3.6 V vs. Li+/Li or ∼3.3 V vs. Na+/Na.16 Na2Fe(SO4)2 could also be obtained by intercalating one sodium into the structure of the eldfellite-type NaFe(SO4)2.17 At 0.1C, this material delivers a discharge capacity of 80 mA h g−1 with an operating potential around 3.25 V vs. Na+/Na. Even the hydrated phases such as the blödite-type Na2Fe(SO4)2·4H2O16 or the kröhnkite-type Na2Fe(SO4)2·2H2O18 were active at ∼3.3 V and ∼3.25 V vs. Na+/Na, respectively. On the other hand, the Na2Co(SO4)2 phase did not show any electrochemical activity up to 5 V.16 In the system Na2Mn(SO4)2·nH2O two phases were reported (n = 0 and 2). The thermal decomposition at 500 K of Na2Mn1.167(SO4)2S0.33O1.167·2H2O19 and the kröhnkite-type Na2Mn(SO4)2·2H2O20 led to the formation of two different Na2Mn(SO4)2 phases that crystallize with the glauberite- and alluaudite-type of structures, respectively. The glauberite-type Na2Mn(SO4)2 sample was not tested as positive electrode since it contains few MnS2O7 impurities and the alluaudite-type Na2Mn(SO4)2 has shown to be active in sodium ion batteries (NIBs), however the performance was worse than the iron analogues.21–23 In the system Na2Ni(SO4)2·nH2O three phases are known (n = 0, 4 and 6), however they were not tested as positive electrode materials for NIBs.24–26 For these reasons we prepared recently several Na2M(SO4)2·nH2O phases (M = Mn and Ni) in order to test their electrochemical performances in NIBs.
In this paper we report on the synthesis of the new phases Na2Mn(SO4)2·4H2O and Na2Ni(SO4)2·10H2O by wet chemistry route. The crystal structures of these phases were solved using single crystal X-ray diffraction (XRD) and their compositions were confirmed by the combination of thermal gravimetric analyzes (TGA) and energy-dispersive X-ray spectrometry analyzes (EDX). The fully dehydrated phases were analyzed by ex situ powder XRD. Our results are presented in the following sections.
Na2Ni(SO4)2·10H2O powder was also synthesized via a wet chemistry route (super-saturation method) from a stoichiometric mixture of Na2SO4 (Aldrich, ≥99%) and NiSO4·6H2O (Aldrich, ≥99%). The starting materials with a 1:
1 molar ratio were dissolved in 20 ml of water (solution B). The solution B was stirred for few hours then left drying at room temperature during three weeks. This enabled the growth of large green single crystals of the new phase Na2Ni(SO4)2·10H2O. Powder sample of this material was obtained by grinding few crystals. The solution B was also dried at 80 °C for 12 hours then the resulting powder was grinded and fired at 350 °C for 32 hours. This led to the formation of the green powder of SS-Na2Ni(SO4)2. A similar powder of WC-Na2Ni(SO4)2 was obtained when heating under argon the crystals of Na2Ni(SO4)2·10H2O at 400 °C for 6 hours.
Crystal data | ||
Chemical formula | Na2Mn(SO4)2·4H2O | Na2Ni(SO4)2·10H2O |
Mr | 365.1 | 476.9 |
Crystal system, space group | Monoclinic, P21/c | Monoclinic, P21/c |
Temperature (K) | 293 | 293 |
a, b, c (Å) | 5.5415 (2), 8.3447 (3), 11.2281 (3) | 12.5050 (8), 6.4812 (4), 10.0210 (6) |
β (°) | 100.172 (1) | 106.138 (2) |
V (Å3) | 511.05 (3) | 780.17 (8) |
Z | 2 | 2 |
Radiation type | Mo Kα | Mo Kα |
μ (mm−1) | 1.84 | 1.66 |
Crystal size (mm) | 0.16 × 0.13 × 0.11 | 0.34 × 0.10 × 0.08 |
![]() |
||
Data collection | ||
Diffractometer | Bruker D8 VENTURE | Bruker D8 VENTURE |
Absorption correction | Multi-scan (SADABS) | Multi-scan SADABS |
Tmin, Tmax | 0.88, 0.92 | 0.630, 0.747 |
No. of measured, independent and observed [I > 3σ(I)] reflections | 9618, 1314, 1198 | 22![]() |
Rint | 0.022 | 0.029 |
(sin![]() |
0.676 | 0.853 |
![]() |
||
Refinement | ||
R[F2 > 2σ(F2)], wR(F2), S | 0.018, 0.060, 1.16 | 0.024, 0.079, 1.11 |
No. of reflections | 1314 | 3819 |
No. of parameters | 96 | 147 |
No. of restraints | 6 | 15 |
H-atom treatment | All H-atom parameters refined | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.19, −0.23 | 0.33, −0.33 |
Atom | Wyck | x | y | z | Ueq/iso* |
---|---|---|---|---|---|
Na2Mn(SO4)2·4H2O | |||||
Na1 | 4e | 0.12901(9) | 0.92766(6) | 0.36268(5) | 0.02082(16) |
Mn1 | 2a | 0 | 0.5 | 0.5 | 0.01387(10) |
S1 | 4e | 0.37292(5) | 0.79212(3) | 0.63561(2) | 0.01132(10) |
O1 | 4e | 0.20851(18) | 0.91681(12) | 0.57815(8) | 0.0234(3) |
O2 | 4e | 0.31892(17) | 0.63864(11) | 0.57209(9) | 0.0233(3) |
O3 | 4e | 0.63145(16) | 0.83269(11) | 0.63209(9) | 0.0222(3) |
O4 | 4e | 0.34724(18) | 0.77406(12) | 0.76397(8) | 0.0216(3) |
O5 | 4e | 0.13017(16) | 0.45884(12) | 0.33232(8) | 0.0181(3) |
H5b | 4e | 0.232(3) | 0.3650(13) | 0.3423(18) | 0.040(5)* |
H5a | 4e | 0.235(3) | 0.5421(17) | 0.312(2) | 0.053(6)* |
O6 | 4e | 0.18486(17) | 0.28225(10) | 0.58278(8) | 0.0192(3) |
H6a | 4e | 0.223(3) | 0.210(2) | 0.5222(13) | 0.053(7)* |
H6b | 4e | 0.3331(17) | 0.288(2) | 0.6416(12) | 0.039(5)* |
![]() |
|||||
Na2Ni(SO4)2·10H2O | |||||
Na1 | 4e | 0.48096(4) | 0.25441(7) | 0.39586(5) | 0.02511(14) |
Ni1 | 2c | 1.00000 | 0 | 1/2 | 0.01379(6) |
S1 | 4e | 0.72260(2) | 0.51630(3) | 0.33017(3) | 0.01445(7) |
O1 | 4e | 0.79184(6) | 0.42137(12) | 0.24800(9) | 0.0233(2) |
O2 | 4e | 0.78000(8) | 0.49977(11) | 0.48001(9) | 0.0234(3) |
O3 | 4e | 0.61500(7) | 0.41153(15) | 0.29742(9) | 0.0296(3) |
O4 | 4e | 0.70453(7) | 0.73541(11) | 0.29108(9) | 0.0254(2) |
O5 | 4e | 0.86644(7) | 0.03383(11) | 0.33044(10) | 0.0247(2) |
H5a | 4e | 0.8331(9) | 0.1634(11) | 0.3001(15) | 0.036(4)* |
H5b | 4e | 0.8058(7) | −0.0615(16) | 0.309(2) | 0.057(5)* |
H6a | 4e | 0.9914(11) | 0.294(3) | 0.6835(6) | 0.064(6)* |
H6b | 4e | 0.9025(7) | 0.342(2) | 0.5531(12) | 0.029(4)* |
O6 | 4e | 0.97063(7) | 0.27252(11) | 0.58560(8) | 0.0239(2) |
O7 | 4e | 0.60084(7) | 0.40809(12) | 0.60325(9) | 0.0250(2) |
H7a | 4e | 0.6193(11) | 0.314(2) | 0.6794(13) | 0.059(5)* |
H7b | 4e | 0.6698(7) | 0.435(2) | 0.5838(18) | 0.057(5)* |
O8 | 4e | 0.38581(8) | 0.03475(14) | 0.52844(10) | 0.0287(3) |
H8a | 4e | 0.3566(12) | 0.123(3) | 0.5863(16) | 0.068(6)* |
H8b | 4e | 0.3216(8) | −0.023(2) | 0.4645(15) | 0.050(6)* |
O9 | 4e | 0.90973(6) | −0.16068(11) | 0.61345(8) | 0.0219(2) |
H9a | 4e | 0.8634(10) | −0.085(2) | 0.6567(16) | 0.044(5)* |
H9b | 4e | 0.8662(11) | −0.2770(18) | 0.5734(19) | 0.059(5)* |
![]() | ||
Fig. 1 Images and EDX analyzes of Na2Mn(SO4)2·4H2O (a) and Na2Ni(SO4)2·10H2O (b). These are the single crystals used for the data collections. |
![]() | ||
Fig. 2 Theoretical and experimental powder X-ray diffraction patterns of Na2Ni(SO4)2·10H2O (Cu-Kα radiation). Asterisk (*) corresponds to an unidentified impurity. |
Fig. 3 clearly indicates a 37.7% of weight loss, which corresponds exactly to the evaporation of ten water molecules. This confirms that the prepared sample Na2Ni(SO4)2·10H2O decomposes below 300 °C to form WC-Na2Ni(SO4)2 according to the following scheme:
Na2Ni(SO4)2·10H2O(s) (476.95 g mol−1) → WC-Na2Ni(SO4)2(s) (296.8 g mol−1) + 10H2O(g) (180.15 g mol−1) |
This decomposition mechanism was confirmed by powder XRD, since the powder pattern of the dehydrated Na2Ni(SO4)2·10H2O phase is identical to the theoretical pattern of Na2Ni(SO4)2 and the experimental pattern of SS-Na2Ni(SO4)2 (Fig. 4). Above 700 °C, the sample decomposes by releasing SO2(g). It should be noted that the thermal behavior of the sample containing manganese was also performed (Fig. S2†). The weight loss was only 12.9% which is lower than the weight loss of 19.7% expected for Na2Mn(SO4)2·4H2O, however it is almost identical to the weight loss of 12.4% expected for Na12Mn7(SO4)13·15H2O. This is in good agreement with PXRD data which indicate that the prepared manganese phase contains mainly the Na12Mn7(SO4)13·15H2O phase.
Distances (Å) | Distances (Å) | ||
---|---|---|---|
a Average distances are given in 〈 〉 and coordination numbers are given in [ ].b Bond valence sum, B.V. = e(r0−r)/b with the following parameters: b = 0.37, r0 (NaI–O) = 1.803, r0 (SVI–O) = 1.624, r0 (NiII–O) = 1.654 and r0 (MnII–O) = 1.624. | |||
Na2Mn(SO4)2·4H2O | Na2Ni(SO4)2·10H2O | ||
Na1–O1 | 2.3831(10) | Na1–O3 | 2.3956(11) |
Na1–O3 | 2.3952(11) | Na1–O4 | 2.5474(9) |
Na1–O5 | 2.4111(9) | Na1–O7 | 2.4138(9) |
Na1–O4 | 2.4490(11) | Na1–O7 | 2.4159(9) |
Na1–O1 | 2.4623(12) | Na1–O8 | 2.4671(12) |
Na1–O6 | 2.6162(11) | Na1–O8 | 2.4821(10) |
〈dNa1–O〉 | 〈2.4528〉 | 〈dNa1–O〉 | 〈2.4537〉 |
BVS | b1.06[6] | BVS | b1.04[6] |
Mn1–O2 (×2) | 2.1475(9) | Ni1–O5 (×2) | 2.0365(8) |
Mn1–O5 (×2) | 2.1593(9) | Ni1–O6 (×2) | 2.0409(8) |
Mn1–O6 (×2) | 2.2088(9) | Ni1–O9 (×2) | 2.0892(9) |
〈dMn1–O〉 | 〈2.1718〉 | 〈dNi1–O〉 | 〈2.0555〉 |
BVS | b2.14[6] | BVS | b2.03[6] |
S1–O1 | 1.4565(10) | S1–O1 | 1.4842(10) |
S1–O2 | 1.4707(10) | S1–O2 | 1.4772(9) |
S1–O3 | 1.4794(9) | S1–O3 | 1.4607(9) |
S1–O4 | 1.4807(10) | S1–O4 | 1.4738(7) |
〈dS1–O〉 | 〈1.4718〉 | 〈dS1–O〉 | 〈1.4740〉 |
BVS | b6.03[4] | BVS | b6.00[4] |
The manganese atom is lying on an inversion center and is coordinated to two oxygen atoms and four water molecules forming [MnO2(H2O)4]2− octahedron. This octahedron share corners with two SO4 tetrahedra forming the building block [Mn(SO4)2(H2O)4]2− (Fig. 5e). The [MnO2(H2O)4]2− octahedron share also corners with four [Na2O6(H2O)4]10− dimer units leading to layers in the (010) plane (Fig. 5a). The d(Mn1–O) distances range from 2.1475 to 2.2088 Å with an average distance of 2.1718 Å which is slightly shorter than the sum of the effective ionic radii of the six-coordinated Mn2+ and O2− {IR(Mn2+) + IR(O2−) = 0.83 + 1.4 Å}.38 The BVS value of 2.14 is in good agreement with the oxidation state 2+ expected for the Mn atoms.39
The slightly distorted SO4 tetrahedra share only corners with the MnO2(H2O)4 and NaO4(H2O)2 octahedra (Fig. 5c). The d(S1–O) distances range from 1.4565 to 1.4807 Å with the average value of 1.4718 Å. This value is shorter than the sum of the effective ionic radii of the four-coordinated S6+ and O2−{IR(S6+) + IR(O2−) = 0.12 + 1.38 Å}. The BVS value of 6.03 is in good agreement with the oxidation state 6+ expected for sulfur.
The sodium atom is coordinated to four oxygen atoms and two water molecules forming [NaO4(H2O)2]7− octahedra. These octahedra share edges and form [Na2O6(H2O)4]10− dimer units (Fig. 5f). The average Na1–O distance of 2.4528 Å is consistent with the sum of the effective ionic radii of the six-coordinated Na+ and O2−{IR(Na+) + IR(O2−) = 1.02 + 1.40 Å}. The BVS value of 1.06 is in good agreement with the oxidation state 1+ expected for the Na atoms.
In the crystal structure of Na2Mn(SO4)2·4H2O, the H2O molecules play the role of hydrogen-bond donors whereas the oxygen atoms O3 and O4 are the hydrogen bond acceptors (Table 4 and Fig. 6). The O–H⋯O hydrogen bonds connect the [Mn(SO4)2(H2O)4]2− building blocks (Fig. 5e) forming a tunnel-like structure with large voids along the a axis (Fig. 6a). In these voids, the sodium atoms are located (Fig. 5d and 6a). This feature indicates that the compounds of the blödite-family might be good ionic conductors. This may explain the electrochemical activity of the Na2Fe(SO4)2·4H2O phase in NIBs and LIBs.16 Based on the classification of Jeffrey, all the O–H⋯O hydrogen bonds are moderate (Table 4 and Fig. 6b and c).40,41
Donor | Hydrogen | Acceptor | D–H distance | H⋯A distance | D–A distance | A–H⋯D angle |
---|---|---|---|---|---|---|
Na2Mn(SO4)2·4H2O | ||||||
O5 | H5b | O3 | 0.960(13) | 1.817(12) | 2.7634(13) | 168.3(15) |
O5 | H5a | O4 | 0.960(17) | 1.774(16) | 2.7074(14) | 163.3(14) |
O6 | H6a | O3 | 0.960(17) | 2.067(16) | 2.9400(14) | 150.4(16) |
O6 | H6b | O4 | 0.960(10) | 1.901(10) | 2.8483(12) | 168.8(13) |
![]() |
||||||
Na2Ni(SO4)2·10H2O | ||||||
O5 | H5a | O1 | 0.949(8) | 1.785(8) | 2.7263(11) | 171.2(10) |
O5 | H5b | O4 | 0.955(9) | 1.800(10) | 2.7480(11) | 171.0(13) |
O6 | H6a | O9 | 0.952(6) | 2.088(7) | 3.0022(11) | 160.5(12) |
O6 | H6b | O2 | 0.939(10) | 1.817(10) | 2.7512(11) | 172.9(11) |
O7 | H7a | O3 | 0.955(13) | 1.888(14) | 2.8137(12) | 162.4(12) |
O7 | H7b | O2 | 0.952(12) | 1.988(15) | 2.9045(14) | 160.9(13) |
O8 | H8a | O4 | 0.955(17) | 1.861(17) | 2.8111(14) | 172.9(15) |
O8 | H8b | O1 | 0.952(11) | 2.236(13) | 3.1189(11) | 153.7(13) |
O9 | H9a | O1 | 0.950(15) | 1.795(15) | 2.7389(12) | 172.3(11) |
O9 | H9b | O2 | 0.951(12) | 1.889(13) | 2.8384(10) | 175.4(17) |
It is very interesting to notice that the manganese phases Na2Mn(SO4)2·4H2O [this work], Na2Mn(SO4)2·2H2O,20 Na2+γMn1−γ/2(SO4)3,20 Na12Mn7(SO4)12[S2O7]·12H2O19 and Na2Mn(SO4)219 that crystallize with the blödite-, kröhnkite-, alluaudite-, löweite- and glauberite-type of structures, respectively were prepared via a supersaturation method at different temperatures and using the same precursors (Na2SO4 + MnSO4·H2O + H2O). At 25 and 70 °C, pure kröhnkite- and löweite-phases were formed, respectively whereas at 22 °C a mixture of kröhnkite-, blödite- and löweite-phases was observed. Furthermore, the thermal treatment of the kröhnkite-phase at 227 °C, led to the formation of an alluaudite-phase with the composition Na2+γMn1−γ/2(SO4)3.20 A similar phase could also be prepared via a solid state synthesis route when a mixture of Na2SO4 and MnSO4 was ball milled then annealed at 350 °C for few hours.42 Moreover, at 227 °C the löweite-phase decomposed into the glauberite-phase Na2Mn(SO4)2 besides an impurity that could be MnS2O7.19 Although, the five phases were prepared from the same precursors, their crystal structures are different (Fig. 7). Indeed, the thermal treatments affected significantly the coordination sphere of the Mn atoms. In the blödite-Na2Mn(SO4)2·4H2O, the Mn atom is coordinated to two oxygen atoms and four water molecules forming the [MnO2(H2O)4]2− octahedron. This octahedron share two corners with the SO4 tetrahedra to form the [Mn(SO4)2(H2O)4]2− building block (Fig. 7a). In the kröhnkite-Na2Mn(SO4)2·2H2O, the Mn atom is coordinated to four oxygen atoms and two water molecules. The MnO6 octahedra share corners with four SO4 tetrahedra to form infinite chains along the c axis (Fig. 7b). These chains are condensed at 227 °C to form the 3d-framework of the alluaudite-Na2+γMn1−γ/2(SO4)3. In this structure the MnO6 octahedra share edges and form dimer units that are interconnected through SO4 tetrahedra (Fig. 7c). In the löweite-Na12Mn7(SO4)12[S2O7]·12H2O, two Mn atoms exist. Mn1 is coordinated to two water molecules and four oxygen atoms, whereas Mn2 is coordinated to six oxygen atoms. The Mn1O6 and Mn2O6 octahedra are bridged by the SO4 tetrahedra to form a 3d-framework (Fig. 7d). At 227 °C, the water molecules of the löweite evaporated and a structural transition to the glauberite-Na2Mn(SO4)2 was observed. In this structure the MnO6 octahedra share corners with six SO4 tetrahedra to form a 3d-framework (Fig. 7e).
The nickel cations are coordinated to six water molecules forming isolated distorted [Ni(H2O)6]2+ octahedra. These octahedra are interlinked by the O6–H6a⋯O9 hydrogen bonds forming the layer 2 parallel to the (100) plane (Fig. 8c). The d(Ni1–O) distances range from 2.0365 to 2.0892 Å with an average distance of 2.0555 Å which is slightly shorter than the sum of the effective ionic radii of the six-coordinated Ni2+ and O2− {IR(Ni2+) + IR(O2−) = 0.69 + 1.4 Å}.38 The BVS value of 2.03 is in good agreement with the oxidation state 2+ expected for the Ni atoms.39
![]() | ||
Fig. 8 View along the b axis of the crystal structure of Na2Ni(SO4)2·10H2O (a), view of [Na2(SO4)2(H2O)4]2− layer 1 on the (100) plane (b) and view of [Ni(H2O)6]2+ layer 2 on the (100) plane (c). |
The SO4 tetrahedra are regular. The distances range from 1.4607 to 1.4842 Å with the average value of 1.4740 Å. This value is shorter than the sum of the effective ionic radii of the four-coordinated S6+ and O2−{IR(S6+) + IR(O2−) = 0.12 + 1.38 Å}. The BVS value of 6.00 is in good agreement with the oxidation state 6+ expected for sulfur.
The Na1+ cations are surrounded by six oxygen atoms forming distorted octahedra. These octahedra share edges and form infinite chains running along the b axis (Fig. 8b). These chains share corners with the SO4 tetrahedra forming [Na2(SO4)2(H2O)4]2− layers parallel to the (100) plane (see layer 1 in Fig. 8b). The average Na1–O distance of 2.4537 Å is consistent with the sum of the effective ionic radii of the six-coordinated Na+ and O2− {IR(Na+) + IR(O2−) = 1.02 + 1.40 Å}. The BVS value of 1.04 is in good agreement with the oxidation state 1+ expected for the Na atoms.
In Na2Ni(SO4)2·10H2O the water molecules are the hydrogen-bond donors, whereas the oxygen atoms O1, O3, O3, O4 and O9 play the role of hydrogen bond acceptors (Table 4 and Fig. 9). The hydrogen bonds can be divided into two categories; intra- and inter-layers bonds as depicted on Fig. 9a, c and b, respectively. Based on the classification of Jeffrey all the O–H⋯O hydrogen bonds are moderate (Table 4 and Fig. 9).40,41
![]() | ||
Fig. 9 View of the hydrogen bonds within the layer 1 (a), between layer 1 and layer 2 (b), and within the layer 2 (c). |
It is worth to mention that in the nickel system Na2Ni(SO4)2·nH2O four phases have been reported (n = 0, 2, 4 and 6), however only the crystal structures of Na2Ni(SO4)2,24 Na2Ni(SO4)2·4H2O25 and Na2Ni(SO4)2·6H2O26 were solved (Fig. 10). Theses phases were prepared via solid state-, room temperature- and hydrothermal-synthesis routes, respectively and using different precursors and solvents. These experimental conditions are at the origin of the variation in the degree of hydration n of the Na2Ni(SO4)2·nH2O phases which induced several structural changes. Our careful analyses indicate that the presence of water molecules affects in first place the coordination sphere of the transition metal (nickel) due to its large electrical charge z. Indeed, for n = 0, the NiO6 octahedra share one edge and four corners with five SO4 tetrahedra leading to [Ni(SO4)2]2− 3d-framework (Fig. 10d). Whereas, for n = 4, the Ni atoms are coordinated to four water molecules and two oxygen atoms, from two adjacent SO4 tetrahedra, forming the isolated [Ni(SO4)2(H2O)4]2− building block of the blödite-type of structure (Fig. 10a). For n = 6 and 10, six water molecules are coordinated to the Ni atoms forming the isolated [Ni(H2O)6]2+ octahedra (Fig. 10b and c) which are connected to the SO4 tetrahedra only through hydrogen bonds. For n = 10, the (n − 6) extra water molecules are coordinated to the sodium atoms. Since n ≤ 10, in the four phases the water molecules are coordinated. One would expect to observe interstitial water only for n > 12. Indeed in the case of Na2Mg(SO4)2·16H2O, six water molecules are coordinated to the magnesium atoms, six water molecules are coordinated to the sodium atoms and four water molecules are interstitial.14 Therefore the chemical formula can be written as {[Na2(H2O)6][Mg(H2O)6][(SO4)2]}·4H2O.
Na2Ni(SO4)2·4H2O versus Na2M(SO4)2·4H2O | ||||||||
M | Mg | Zn | Co | Fe | V | Mn | Cd | |
Atom | |u|/Å | |u|/Å | |u|/Å | |u|/Å | |u|/Å | |u|/Å | |u|/Å | |
Ni1/M | 0.0000 | 0.0000 | 0.0000 | 0.0000 | 0.0000 | 0.0000 | 0.0000 | |
Na1 | 0.0153 | 0.0088 | 0.0070 | 0.0091 | 0.0157 | 0.0285 | 0.0467 | |
S1 | 0.0361 | 0.0197 | 0.0142 | 0.0241 | 0.0369 | 0.0523 | 0.0692 | |
O1 | 0.0310 | 0.0216 | 0.0243 | 0.0505 | 0.0298 | 0.0626 | 0.0905 | |
O2 | 0.0602 | 0.0172 | 0.0085 | 0.0226 | 0.0623 | 0.0325 | 0.0628 | |
O3 | 0.0787 | 0.0320 | 0.0212 | 0.0343 | 0.0799 | 0.0797 | 0.1563 | |
O4 | 0.0147 | 0.0183 | 0.0113 | 0.0115 | 0.0122 | 0.0456 | 0.1058 | |
O5 | 0.0174 | 0.0228 | 0.0292 | 0.0580 | 0.0174 | 0.0965 | 0.1814 | |
O6 | 0.0328 | 0.0337 | 0.0400 | 0.0719 | 0.0311 | 0.0971 | 0.1742 | |
Degree of lattice distortion (S) | 0.0034 | 0.0030 | 0.0030 | 0.0046 | 0.0057 | 0.0082 | 0.0138 | |
The maximum distance (dmax)/Å | 0.0787 | 0.0337 | 0.0400 | 0.0719 | 0.0799 | 0.0971 | 0.1814 | |
Arithmetic mean (dav)/Å | 0.0337 | 0.0205 | 0.0183 | 0.0332 | 0.0336 | 0.0582 | 0.1043 | |
Measure of similarity (Δ) | 0.019 | 0.014 | 0.014 | 0.018 | 0.027 | 0.044 | 0.079 | |
Ref. | 7 | 7 | 7 | 34 | 33 | This work | 1 | |
![]() |
||||||||
Na2Mg(SO4)2·10H2O versus Na2Ni(SO4)2·10H2O | ||||||||
Atom | |u|/Å | |||||||
Mg1/Ni1 | 0.0000 | |||||||
Na1 | 0.0117 | |||||||
S1 | 0.0217 | |||||||
O1 | 0.0444 | |||||||
O2 | 0.0220 | |||||||
O3 | 0.0430 | |||||||
O4 | 0.0210 | |||||||
O5 | 0.0132 | |||||||
O6 | 0.0242 | |||||||
O7 | 0.0181 | |||||||
O8 | 0.0094 | |||||||
O9 | 0.0377 | |||||||
Degree of lattice distortion (S) | 0.0017 | |||||||
The maximum distance (dmax)/Å | 0.0444 | |||||||
Arithmetic mean (dav)/Å | 0.0232 | |||||||
Measure of similarity (Δ) | 0.005 |
![]() | ||
Fig. 11 Unit-cell volumes as a function of the ionic radius of the M cation in the blödite-type of compounds Na2M(SO4)2·4H2O (M = Mg, V, Mn, Fe, Co, Ni, Zn, Cd). |
The crystal structure of Na2Ni(SO4)2·10H2O was also compared to the Mg analogue using the compstru program. As indicated in Table 5, the two isotypic compounds are essentially coincident (Δ = 0.005). The largest deviations of 0.0444 and 0.0430 were observed for the atom pairs O1 and O3 of the MgO6 octahedron.
Footnote |
† Electronic supplementary information (ESI) available: The theoretical powder X-ray diffraction patterns (PXRD) of Na12Mn7(SO4)13·15H2O, Na2Mn(SO4)2·2H2O and Na2Mn(SO4)2·4H2O, the experimental PXRD and the TGA of the sample containing manganese and the anisotropic displacement parameters (in Å2) for Na2Mn(SO4)2·4H2O and Na2Ni(SO4)2·10H2O are given in supplementary information. CCDC 1962424 and 1962425. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ra00301h |
This journal is © The Royal Society of Chemistry 2020 |