A combined approach using the Rietveld technique of structure refinement and DIFFaX simulations of the powder patterns enables us to not only arrive at the complete structure of the layered double hydroxides (LDHs), but also classify and quantify the nature of structural disorder. Hydrolysis of urea dissolved in mixed-metal salt solutions containing a divalent metal (Mg2+, Co2+) with Al3+ results in the homogeneous precipitation of the corresponding LDH. The products obtained are highly crystalline enabling a complete structure determination including subsequent refinement by the Rietveld method. In contrast, the LDH of Ni2+ with Al3+ crystallizes with the incorporation of stacking faults. A combined Rietveld-DIFFaX approach shows that even `crystalline' samples of this LDH incorporate up to 9% of stacking faults, which are not eliminated even at elevated temperatures (473 K). These studies have implications for the order, disorder and `crystallinity' of layered phases in general and metal hydroxides in particular.
Supporting information
Solid urea was added to 40 ml of a 0.5 M mixed metal [M(II) +Al(III)] nitrate solution maintaining the urea/[M(II) + Al(III)] molar ratio at 3.3. The clear solution obtained was heated at different temperatures between 353 K and 473 K for 24 h. Heating above the boiling point of water was carried out in a teflon lined autoclave under autogenous pressure. The solids were recovered by filtration, copiously washed to constant pH and then dried at 353 K.
The Rietveld refinement technique was used for structure determination (FULLPROF. 2k code, version 3.3 June 2005-LLB JRC). In all the refinements, the modified pseudo-voigt line shape function with five variables (U, V, W, X and ?) is used to fit the experimental profiles. Refinements were also carried out with profiles incorporating the Pearson VII correction for comparing the effect of profile function on the quality of the fit and the difference profile. The background adjustment in the calculated patterns was made using a twelve polynomial function. The crystal structure of mineral hydrotalcite, was chosen as the model (ICSD No. 6296, Space group: R-3 m, a = 3.054 Å; c = 22.81 Å) for the Rietveld refinement of the structures of the M—Al—CO32- LDHs (M = Ni2+, Co2+). For M = Mg2+, a more recent model structure (CC-81963, Space group: R-3 m, a = 3.046 Å; c = 22.772 Å) was used (Bellotto et al., 1996).
For both compounds, program(s) used to refine structure: FULLPROF.
(Mg0.667Al0.33OH2CO30.1666.0.56H2O)
top
Crystal data top
? | γ = 120° |
Mr = ? | V = 181.68 (4) Å3 |
Rhombohedral, R3M | Z = ? |
Hall symbol: -R 3 2" | Cu Kα radiation, λ = 1.540530 Å |
a = 3.04245 (2) Å | T = 298 K |
c = 22.664143 (1) Å | ?, ? × ? × ? mm |
α = 90° | |
Data collection top
X'pert Pro Philips diffractometer | Data collection mode: reflection |
Radiation source: rotating-anode X-ray tube | Scan method: continuous |
Graphite monochromator | 2θmin = 5.043°, 2θmax = 70.023°, 2θstep = 0.020° |
Refinement top
Rp = 22.558 | 3250 data points |
Rwp = 32.313 | 19 parameters |
Rexp = 16.708 | 0 restraints |
RBragg = 10.291 | |
χ2 = NOT FOUND | |
Crystal data top
? | γ = 120° |
Mr = ? | V = 181.68 (4) Å3 |
Rhombohedral, R3M | Z = ? |
a = 3.04245 (2) Å | Cu Kα radiation, λ = 1.540530 Å |
c = 22.664143 (1) Å | T = 298 K |
α = 90° | ?, ? × ? × ? mm |
Data collection top
X'pert Pro Philips diffractometer | Scan method: continuous |
Data collection mode: reflection | 2θmin = 5.043°, 2θmax = 70.023°, 2θstep = 0.020° |
Refinement top
Rp = 22.558 | χ2 = NOT FOUND |
Rwp = 32.313 | 3250 data points |
Rexp = 16.708 | 19 parameters |
RBragg = 10.291 | 0 restraints |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
Mg | 0.00000 | 0.00000 | 0.00000 | 0.00000* | 0.66667 |
Al | 0.00000 | 0.00000 | 0.00000 | 0.00000* | 0.33333 |
O1 | 0.00000 | 0.00000 | 0.3747 (4) | 0.00000* | |
O2 | 0.125 (3) | −0.125 (3) | 0.50000 | 0.00000* | 0.176 (4) |
C | 0.33333 | 0.66667 | 0.50000 | 0.00000* | 0.08329 |
(Co.681Al0.311OH2CO30.1557.0.53H2O)
top
Crystal data top
? | γ = 120° |
Mr = ? | V = 183.93 (2) Å3 |
Rhombohedral, R3M | Z = ? |
Hall symbol: -R 3 2" | Cu Kα radiation, λ = 1.540530 Å |
a = 3.066045 (19) Å | T = 298 K |
c = 22.593113 (1) Å | ?, ? × ? × ? mm |
α = 90° | |
Data collection top
Radiation source: rotating-anode X-ray tube | 2θmin = 4.988°, 2θmax = 69.968°, 2θstep = 0.020° |
Refinement top
Rp = 19.866 | 3250 data points |
Rwp = 27.538 | 16 parameters |
Rexp = 22.252 | 0 restraints |
RBragg = 9.063 | |
χ2 = NOT FOUND | |
Crystal data top
? | γ = 120° |
Mr = ? | V = 183.93 (2) Å3 |
Rhombohedral, R3M | Z = ? |
a = 3.066045 (19) Å | Cu Kα radiation, λ = 1.540530 Å |
c = 22.593113 (1) Å | T = 298 K |
α = 90° | ?, ? × ? × ? mm |
Data collection top
2θmin = 4.988°, 2θmax = 69.968°, 2θstep = 0.020° | |
Refinement top
Rp = 19.866 | χ2 = NOT FOUND |
Rwp = 27.538 | 3250 data points |
Rexp = 22.252 | 16 parameters |
RBragg = 9.063 | 0 restraints |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
Co | 0.00000 | 0.00000 | 0.00000 | 0.00921* | 0.68157 |
Al | 0.00000 | 0.00000 | 0.00000 | 0.00921* | 0.31147 |
O1 | 0.00000 | 0.00000 | 0.3747 (4) | 0.00000* | |
O2 | 0.11386 | −0.11386 | 0.50000 | 0.00000* | 0.16627 |
C | 0.00000 | 0.00000 | 0.16700 | 0.00000* | 0.07787 |
Experimental details
| (Mg0.667Al0.33OH2CO30.1666.0.56H2O) | (Co.681Al0.311OH2CO30.1557.0.53H2O) |
Crystal data |
Chemical formula | ? | ? |
Mr | ? | ? |
Crystal system, space group | Rhombohedral, R3M | Rhombohedral, R3M |
Temperature (K) | 298 | 298 |
a, c (Å) | 3.04245 (2), 22.664143 (1) | 3.066045 (19), 22.593113 (1) |
V (Å3) | 181.68 (4) | 183.93 (2) |
Z | ? | ? |
Radiation type | Cu Kα, λ = 1.540530 Å | Cu Kα, λ = 1.540530 Å |
Specimen shape, size (mm) | ?, ? × ? × ? | ?, ? × ? × ? |
|
Data collection |
Diffractometer | X'pert Pro Philips diffractometer | ? |
Specimen mounting | ? | ? |
Data collection mode | Reflection | ? |
Scan method | Continuous | ? |
2θ values (°) | 2θmin = 5.043 2θmax = 70.023 2θstep = 0.020 | 2θmin = 4.988 2θmax = 69.968 2θstep = 0.020 |
|
Refinement |
R factors and goodness of fit | Rp = 22.558, Rwp = 32.313, Rexp = 16.708, RBragg = 10.291, χ2 = NOT FOUND | Rp = 19.866, Rwp = 27.538, Rexp = 22.252, RBragg = 9.063, χ2 = NOT FOUND |
No. of data points | 3250 | 3250 |
No. of parameters | 19 | 16 |