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(namely a, b and c of the elementary cubic perovskite cell) of the TiO6 octahedron. Each
tilt about a given axis forces tilts of opposite sense about the other two tetrad axes leading
to a doubling of the unit cell parameters in the two perpendicular directions. In addition, the
octahedra along the tilt axis in the neighbouring unit cells may be tilted ‘in phase’ (+ve tilt)
or ‘antiphase’ (−ve tilt). For antiphase tilting, the unit cell will be doubled along the axis of
the tilt also. The ‘in-phase’ tilt gives rise [26] to superlattice reflections with one even and
two odd indices with respect to the doubled pseudocubic cell. The antiphase tilt, on the other
hand, leads to superlattice reflections with all odd indices. The various superlattice reflections
observed in SCT50 (see figure 3(d)) have indices which are either all odd or one even and
two odd. The observation of superlattice reflections with both types of index suggests the
presence of ‘in-phase’ (+) as well as ‘antiphase’ tilts (−) in the SCT50 structure. A similar set
of reflections has been reported [26] for pure CT also which belongs to the a−a−c+ tilt system
in the Glazer’s classification scheme. It therefore appears likely that SCT is inheriting the tilt
system of pure CT.
4. Rietveld refinement
Rietveld refinement of the powder neutron diffraction data was carried out for SCT (x = 0.04,
0.12, 0.25 and 0.50) as well as CT. The programme DBWS 9411 [24] was used for the profile
refinements. In all the refinements, scale factor, zero correction, background parameters and
half widths in addition to the lattice parameters (A0, B0, C0), positional coordinates (X, Y,
Z), occupancy (N) and isotropic thermal parameters (B) were varied. Refining the occupancy
parameters of all the atoms together led to very high standard deviations. In order to overcome
this problem, the occupancy parameters for the oxygen atoms had to be refined separately from
the occupancy parameters for Sr, Ca and Ti atoms. The observed, calculated and difference
profiles for CT, SCT50, SCT25, SCT12 and SCT4 are shown in figure 4.
The presence of odd–odd–odd (ooo) and odd–odd–even (ooe) types of reflections in the
experimentally observed neutron diffraction pattern of CT confirms the presence of − and +
tilts respectively in agreement with the earlier findings of Glazer [26]. The tilt system proposed
by Glazer for this structure is a−a−c+ and corresponds to the space group Pbnm proposed by
Kay and Bailey [27] in their XRD studies. We therefore refined the structure of CT using the
P bnm space group.
Some of the superlattice reflections such as 320, 410 and 330 (marked with arrows in
figure 4(a)), which were present in the neutron diffraction pattern for CT, are absent in the
pattern for SCT50. However, as explained in the previous section, the remaining superlattice
peaks still indicate the presence of − and + tilts similar to that for CT. Accordingly, the
refinement for SCT50 was also carried out assuming the Pbnm space group.
A comparison of the experimentally observed powder neutron diffraction patterns for
SCT25 and SCT12 with that of SCT50 shows that some of the superlattice reflections like 310
and 312 (marked with arrows in figure 4(b)) are absent in the diffractograms for SCT25 and
SCT12. This may be due to two different reasons. For the a−a−c+ tilt system, it is possible
that with decreasing calcium content, the tilt angles have become so small that the intensities
of these superlattice peaks have become nearly zero. The absence of ‘odd–odd–even’ type
reflection, on the other hand, may also be attributed to the absence of + tilt. In this situation,
the tilt system for SCT25 and SCT12 can as well be a−a−c0 belonging to the space group
Ibmm. In order to make a choice between these two possibilities, we refined the structure of
SCT25 using both the models. Refinement corresponding to the a−a−c0 tilt system invariably
led to slightly higher Rwp as compared to that for the a−a−c+ tilt system. It was therefore
concluded that the correct tilt system for SCT25 is a−a−c+. The structure of SCT12 was also