5. Transition metal nitridometalates containing infinite linear chains
5.2. Synthesis and characterisation of SrLi 2 {Li[CoN 2 ]}
5.2.3. Characterisation
5.2.3.1. Structure Determination
Reflections from single crystal X-ray diffraction experiments were initially assigned to a body centred tetragonal crystal lattice and refined in the space group I41/amd (No. 141), with the substituted Co atoms being located exclusively on the infinite chains, which would be expected.
The unit cell parameters were in agreement with other known substitutions of the isostructural SrLi2[(Li1−xMx)N]2 (M = Fe, Ni and Cu),[100,234,235] as well as the unsubstituted Li4SrN2
(a = 3.822(2) Å, c = 27.042(9) Å)[225] (Table 5.1.). All substituted phases of Li4SrN2 show a decrease of the a unit cell parameter. This is a result of the, in general, shorter interatomic distances between the transition metal and N, d(Fe−N) = 1.896 Å,[233] d(Ni−N) = 1.912 Å[234] and d(Cu−N) = 1.885 Å,[235] than those seen between Li and N in the non-substituted Li4SrN2
(1.913 Å),[225] which causes an overall shortening of the Li – N interatomic distances that build up the pentagonal bipyramids. An increase in the c unit cell parameter and volume is observed for the substituted phases, compared to Li4SrN2, however the cause for this has proven difficult to identify, with no discernible structural reason being identified. Structural refinements (Table 5.2.) agreed well for the Li4SrN2 structure type, however the refinement produced a high Rint = 0.2109, suggesting that the chosen symmetry could be incorrect.
Table 5.1. Unit cell parameters of SrLi2{Li[CoN2]}, Li4SrN2[225] and SrLi2[(Li1−xMx)N]2 (M = Fe, Ni and Cu).[225,233,234]
A closer inspection of the diffraction data, through reconstructed diffraction patterns, showed diffuse streaks superimposed with reflections, except for hkl where l = 2n (Figure 5.2.).
Overlaying the allowed reflections for a tetragonal lattice onto the reconstructed diffraction patterns showed that all reflections were described, except for the diffuse reflections (Figure 5.2e.). Also in the images, splitting of reflections at higher angles could be observed. These two factors, along with the high Rint value, gave significant evidence that the assumed tetragonal symmetry was incorrect and a monoclinic lattice would be the correct lattice.
Composition SrLi2{Li[CoN2]} Li4SrN2 SrLi2[(Li0.54Fe0.46)N2] SrLi2[(Li1.9Ni0.1)N2] SrLi2[(Li1.22Cu0.78)N2]
Z 4 4 4 4 4
a/Å 3.7414(2) 3.822(2) 3.7909(2) 3.822(2) 3.770(1)
c/Å 27.931(2) 27.0472(2) 27.719(3) 27.042(2) 27.368(6)
c/a 7.465 7.077 7.312 7.075 7.259
ρcalc/gcm−3 3.319 2.41 3.14 2.50 3.20
Volume V/Å3 390.98 395.02 398.35 395.02 388.98
69 Figure 5.2. Reconstructed diffraction patterns of SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]} (a) 0kl; (b) Magnified section from Figure 5.1a.; (c) h0l; (d) hk0; (e) 0kl with superimposed reflections for a tetragonal lattice (red); (f) 0kl with superimposed reflections for a monoclinic lattice (green).
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A tetragonal space group was deemed insufficient to describe the crystal structure properly.
Therefore, a monoclinic subcell, with the space group P21/c (No. 14), was derived from the original tetragonal space group, I41/amd (No. 141), through a Bärnighausen symmetry tree diagram (Figure 5.3.). The symmetry was first reduced to a face-centred orthorhombic unit cell (Figure 5.4.), which then allows the reduction in symmetry needed to free the β angle of the unit cell to create a C-centred monoclinic unit cell (Figure 5.4.). Finally, a loss of the C-centring allows the mixed Li/Co position along the infinite chains to be split into two crystallographically independent positions in a primitive monoclinic unit cell.
The previously mentioned splitting of reflections at high angles was a characteristic indication of the presence of crystal twinning. To determine the presence of twinning, the program PLATON[242] was used and the following twinning matrix was proposed.
(
−1 0 1
0 −1 0
0 0 1
)
This twinning matrix would mean that the crystal twins are related to each other through a two-fold rotation axis. This axis, in fact, belongs to the c-axis of the orthorhombic unit cell and lies parallel to [102] (Figure 5.5.)in the monoclinic setting. This can explain why the structure was at first misinterpreted as a tetragonal unit cell, due to the pseudo-orthorhombic reflections mimicking higher symmetry, resulting from the crystal twinning. Since this unit cell translates very well into the orthorhombic, as well as the tetragonal system, the twin shows merohedral characteristics however, due to the symmetry reduction to the monoclinic unit cell, the twinning present is pseudo-merohedral.
This pseudo-merohedral twinned monoclinic lattice correctly describes the splitting of reflections at high angles (Figure 5.2f.). The splitting of the crystallographic unique mixed occupied Li/Co position in the tetragonal system produced two crystallographically independent positions in the monoclinic crystal system. One position occupied with mainly Li (Li(1)/Co(1)) and the other occupied with mainly Co (Li(2)/Co(2)). These positions show a strong indication of ordering however, there are no definitive positions on these chains which are solely occupied by either Li or Co. This produces an overall disorder, which appears on the reconstructed diffraction patterns as diffuse scattering. The monoclinic indexing of the reflections led to the space group of P21/c (No. 14), with the new unit cell (a = 5.2958(3) Å, b = 5.2898(4) Å, c = 14.2164(5) Å,
71 β = 100.728(1) °). The new monoclinic unit cell can be derived from the tetragonal unit cell by multiplying the lengths a and b by √2 and a near halving of the length of the c axis. The crystallographic relationship is shown in Figures 5.2 and 5.3.
A refinement of the monoclinic unit cell reduced the Rint and wR2 values from 0.2109 and 0.1611, to 0.0985 and 0.1470 respectively, indicating that this monoclinic space group describes the crystal structure better and produces a more accurate refinement, when compared with the refinement of the tetragonal unit cell. The splitting of the mixed Li/Co position on the
[(Li,Co)N2/22
∞
1 ] chains, allowed independent refinement of the occupation along these chains, resulting in different amounts for Li and Co on both positions (Figure 5.6.), leading to a composition of SrLi2{Li0.753(7)Co0.246(7)[Co0.748(6)Li0.252(6)N2]}. This showed a clear preference for ordering along the chains. With this occupation, the atomic displacements were refined isotropically. Attempts to refine anisotropically proved problematic for the mixed valence Li/Co and N positions. An attempt to fix the occupation and refine anisotropically still proved difficult.
A different approach was to take the refined occupation from the tetragonal unit cell and apply it to the twinned monoclinic unit cell. After applying this fixed occupation, the anisotropic displacements parameters for each atomic position were able to be refined successfully. This gave the most accurate refinement (Tables 5.2., 5.3. and 5.4.), and described the diffuse reflections (Figure 5.2f.), producing a final composition of SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]}.
72
Figure 5.3. Bärnighausen symmetry tree diagram[243] describing the relation from I41/amd to P21/c for SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]}.
73 Figure 5.4. Symmetry reduction from a body-centred tetragonal unit cell to a face-centred orthorhombic
unit cell, which is then reduced to a C-centred monoclinic unit cell. Black spheres are in plane and grey spheres are half the length of the projection direction above the plane. Red lines indicate the unit cell of lower symmetry.
74
Figure 5.5. Twinning of the primitive monoclinic unit cell, viewed along [01̅0], with the twinning axis [102] (red) and the resulting twinned unit cell (blue).
75 Table 5.2. Single crystal structural refinement of SrLi2{Li[CoN2]} in the space groups
I41/amd (subcell) and P21/c (supercell).
Composition SrLi2{Li0.55(1)Co0.45(1)[Co0.55(1)Li0.45(1)N2]} SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]}
Crystal System Tetragonal Monoclinic
Space Group I41/amd (No. 141) P21/c (No. 14)
Z 4 4
a/Å 3.741(2) 5.2958(3)
b/Å 5.2898(4)
c/Å 27.93(2) 14.2164(5)
β/° 90 100.728(1)
ρcalc/gcm−3 3.319 3.317
Volume V/Å3 390.98 391.29
Measurement
temperature/K 293(2) 293(2)
Index range
−4 ≤ h ≤ 4
−4 ≤ k ≤ 4
−36 ≤ l ≤ 36
−6 ≤ h ≤ 6
−6 ≤ k ≤ 6
−18 ≤ l ≤ 18
Max. 2θ /deg 54.85 54.98
F(000) 360.8 352.0
µ/mm−1 18.03 17.64
Observed reflections 4373 7168
Unique reflections 148 878
Refined parameters 17 56
Rint/Rσ 0.2109/0.0515 0.0985/0.0473
R1/wR2 0.0636/0.1611 0.0640/0.1470
GooF 1.136 1.133
Remaining electron
density (max/min)/Å−3 1.55/–3.63 1.60/–1.20
BASF 0.49(1)
Twin matrix –1 0 0 0 –1 0 1 0 1
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Table 5.3. Atomic positions and occupations for the refinement of SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]} in the space group P21/c.
Wyckoff Position
x/a y/b z/c Occupation Ueq
Sr(1) 4e 0.7499(3) 0.1269(4) 0.25006(6) 1 0.0221(5)
Li(1)/Co(1) 4e 0.177(3) 0.120(1) 0.1108(3) 0.65/0.35 0.041(1) Li(2)/Co(2) 4e 0.686(1) 0.6307(5) 0.1130(1) 0.35/0.65 0.0145(5)
Li(3) 4e 0.164(8) 0.605(5) 0.044(1) 1 0.032(5)1
Li(4) 4e 0.634(9) 0.132(7) 0.044(2) 1 0.037(5)1
N(1) 4e 0.926(4) 0.372(3) 0.1111(6) 1 0.023(2)
N(2) 4e 0.436(4) 0.864(2) 0.1113(6) 1 0.020(2)
1 Value was treated isotropically during refinement.
77 Table 5.4.Anisotropic displacement parameters from the refinement of SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]} in the space groupP21/c.
U11U22U33U23U13U12Ueq
Sr(1) 0.0246(9)0.0221(6) 0.0198(6) −0.0001(4)0.005(2) 0.000(1) 0.0221(5)
Li(1)/Co(1) 0.059(4) 0.045(3) 0.032(2) 0.003(3) 0.038(4) 0.017(4) 0.041(1)
Li(2)/Co(2) 0.017(1) 0.0099(9) 0.020(1) −0.001(1) 0.014(2) 0.001(1) 0.0145(5)
N(1) 0.023(6) 0.028(5) 0.019(4) 0.004(6) 0.011(9) 0.011(7) 0.023(2)
N(2) 0.019(5) 0.019(4) 0.025(4) −0.007(6) 0.012(8) −0.004(6) 0.020(2)
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5.2.3.2. Crystal Structure
SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]} crystallises in a structure very similar to Li4SrN2 (Figure 5.6.).
The splitting of the mixed valence Li/Co position along the infinite chains, shows that there is a strong indication of ordering, which results in Li and Co almost alternating along the chains. Since Co is larger than Li, this alternating probably allows structural stability and prevents the distortion of the surroundings of the chains.
The influence of Co in the structure can be seen in the interatomic distances along the [(Li,Co)N2/22
∞
1 ] chains (Table 5.5.). When comparing the interatomic distances from this compound with the Li4SrN2 structure, the Li(1)/Co(1)−N(1)/(2) interatomic distances (1.89(2) Å/1.92(2) Å), which are occupied with mainly Li, are in clear agreement with the distances found in Li4SrN2 (1.913 Å).[225] In comparison, the position occupied with mainly Co, Li(2)/Co(2)−N(1)/(2) interatomic distances (1.87(2) Å/1.81(2) Å), exhibit shorter interatomic distances that tend more towards a typical Co−N interatomic distance, similar to those found in Li2[Li0.57Co0.43N] (1.824 Å).[211] This interatomic distance is in fact shorter than the distances already known for M−N in the series SrLi2[(Li1−xMx)N]2, d(Fe−N) = 1.896 Å,[233]
d(Ni−N) = 1.912 Å[234] and d(Cu−N) = 1.885 Å[235].
Table 5.5. Interatomic distances (Å) and angles (°) of selected interatomic distances for SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]}.
Sr(1) Li(1)/Co(1) Li(2)/Co(2) Li(3) Li(4) M−N(1) 2.67(1) 1.89(2) 1.87(2) 2.11(4) 2.09(4) M−N(2) 2.71(1) 1.92(2) 1.81(2) 2.09(4) 2.09(4)
N(1)−M−N(1) 121.1(3) — — 117(1) —
N(1)−M−N(2) 87.8(3) 179.4(9) 175.8(7) 126(2) 115(1)
N(2)−M−N(2) 121.1(3) — — — 117(1)
79 Figure 5.6. Crystal structure comparisons of: a) Li4SrN2;[225] b) SrLi2{Li[CoN2]}; c) Li4SrN2 viewed
along the c-axis;[225] d) SrLi2{Li[CoN2]} viewed along the c-axis. Sr−orange, N−green, Li−silver and Co−blue. Unit cells edges are shown by black lines.
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