Short-range-order stacking in superconducting composite crystal: simulation of diffuse scattering in neutron powder diffraction pattern of deuterated sodium cobaltate

Figures & data

Figure 1. Possible configurations of fundamental structure model of the guest sheet, based on a cell a₂ = b₂ = 3.26 Å, c = 9.8 Å. The cell is based on the axes a₂ = (2/3)a₁ + (4/3)b₁, b₂ = −(4/3)a₁–(2/3)b₁ and c = c_2H/2 = c_3R/3, whereas the basic axes a₁ and b₁ are used for the CoO₂ host layer. Assumed local symmetry operations in a layer unit are threefold rotation axis and inversion centre. Layer unit is illustrated using circles with dark rims for a half part (0.0 ≤ z < 0.5) and with light rims for the other half part (−0.5 ≤ z < 0.0). Occupancies of Na, O and D are estimated as 0.47, 0.89 and 0.61, respectively. Four configurations, Q1, Q2, Q3 and Q4, with respective shift vectors 0, (1/2)a₂, (1/2)b₂ and (1/2)a₂ + (1/2)b₂, are considered.

Table 1. Probability table (P) for an interlayer short-range order model.

l  l + 1	Q1	Q2	Q3	Q4
Q1	0	1/3	1/3	1/3
Q2	1/3	0	1/3	1/3
Q3	1/3	1/3	0	1/3
Q4	1/3	1/3	1/3	0

Figure 2. (a) The powder neutron diffraction pattern simulated using the layer units shown in figure 1 and the probability table shown in table 1 and (b) the observed pattern of 3R and 2H phases.

Figure 3. A probable model of intralayer ordering in the guest part. Bounded projections along [00] (left) and along [110] (right). Occupancies of Na sites are assumed to be 1.0 (small solid circle) and 0.2 (hatched circle), and those of O and D are estimated as 0.89 and 0.96.

Figure 4. (a) The simulated powder neutron diffraction pattern of 3R phase based on a commensurate composite crystal model (see text) and (b) the observed pattern.