Rhombohedral distortions

The C0O2 layer is formed by the edge-shared CoC>6 octahedra which are compressed along c-axis (Fig. 1). The rhombohedral distortion of the CoC>6 octahedra is measured by the deviation of the O-Co-O bond angle from 90°, 95° 99° [2, 24-27], The distortion leads to the crystal-field splitting in t2g states of 3[Pg.202]

In the conventional view of the structure of zeolite A each Si atom is surrounded by four A1 atoms and vice-versa, but recent m.a.s.n.m.r. data56 was interpreted to indicate that the structure was actually one in which each Si atom was surrounded by one Si and three A1 atoms and vice-versa. In support of this work a powder neutron diffraction experiment found a rhombohedral distortion in a sample of Na zeolite A.57... 

These revolutionary ideas lead to further neutron measurements of the structure of zeolite A, which have confirmed the correctness of the traditional 4 0 ordering scheme.58 59 Neutron diffraction traces for several samples of a dehydrated Na zeolite A with Si to A1 ratios of 1.03, 1.09 and 1.12 failed to show any rhombohedral distortions similar to those reported in ref. 57, and in each case the data was consistent with a cubic structure.58 Neutron diffraction experiments on a T1 exchanged sample of the same Na zeolite that had shown the rhombohedral distortion in ref. 57 showed that the crystals now had cubic symmetry59 60 and therefore the distortion that had been measured for the Na zeolite A must be very sensitive to the identity of the exchangeable cations. Profile refinement of this neutron data56 57 also showed a pronounced bimodal distribution of the bond lengths as would be predicted by the 4 0 model. In conclusion it appears that the chemical shifts observed in the n.m.r. experiments can be influenced by factors such as local strain, as well as by the local environment of each Si atom. 

The thallium-A sample was prepared by ion exchange (15) from a rhombohedral, sodium sample with Si/Al = 1.10. The neutron data show no hint of a rhombohedral distortion, and the structure refines to a final R-factor (based upon profile intensities) of 11.3 percent using the model of Pluth and Smith (9) ... 

Rhombohedral Sodium Zeolite-A Our neutron data on three samples of sodium zeolite-A show clear evidence for a rhombohedral distortion of the structure that is most apparent with Si/Al = 1.0. The lattice parameters are set out below (12) ... 

Rhombohedral distortion of Na-A zeolite as a function of temperature expressed as the deviation Aa from 90 of the face-centered cell. The solid line is drawn as a guide to the eye. 

The transition from the bco structure to the )3-Po structure was observed at 162 5 GPa and characterized as very gradually and subtly [200]. In the P-To structure the S atoms are sixfold coordinated with nearest neighbors at a distance of 258.4 pm (Fig. 24). The rhombohedral unit cell, which can be derived from a body-centered cubic structure by a rhombohedral distortion, contains one atom. The transition from the bco to the j8-Po structure was interpreted as of second order due to the absence of any volume change at the transition (both structures have the same atomic volume of... 

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