Monoclinic distorted site

Figures 8 and 9, plate 3, are lattice images of two compounds (n = 2 and n = 5 respectively) showing evidence of 180° domain walls. The c axes, normal to the dark Bi202 sheets in the plane of the image, are nearly, but not exactly, parallel in the two domains the two component c axes, C and C, are inclined at about 3°. These observations suggest that the domain walls are of 180° type and that the polar axes of both the n = 2 and n = 5 compounds deviate slightly from the Ct or c0 axes. As in Bi4Ti3012, there may be some small monoclinic distortion. Selected area diifraction patterns, nominally taken from both sides of the domain boundary, were identical, but the small size of the domain, about 50 nm across, makes the evidence from selected area diffraction inconclusive. As expected for 180° domains, the domain walls are quite thin since only a small adjustment or relaxation of the [B08] octahedra would be necessary, the structure of the wall is undoubtedly simple. As a consequence, the lines of contrast due to the A site cations are clearly visible in the neighbourhood of the wall (figure 9), with only minor distortions around the boundary (dashed line).

Pyroxenes have the general formula X(Y)(Si03)2. They crystallize in a monochnic (clinopyroxenes) or orthorhombic (orthopyroxenes) structure. The Y cations are located in the Ml sites which are moderately distorted octahedra whereas X represents cations in highly distorted sites with six-, seven- or eight-fold coordination according to the kind of mineral. Generally, the structure is orthorhombic when X is Mg or Fe " and is monoclinic when X is Ca or Na. 

The structural relationships in Bi203 are more complex. At room temperature the stable fonn is monoclinic o -Bi203 which has a polymeric layer structure featuring distorted, 5-coordinate Bi in pseudo-octahedral iBiOs units. Above 717°C this transforms to the cubic -form which has a defect fluorite structure (Cap2, p. 118) with randomly distributed oxygen vacancies, i.e. [Bi203D]. The )3-form and several oxygen-rich forms (in which some of the vacant sites are filled... 

Subsequently the transition has been called the Verwey transition and the transition temperature the Verwey temperature Ty. Verwey also guessed that, below Ty, the mobile electrons order as Fe on [110] rows and Fe on [llO] rows of B-site cations to produce a distortion to orthorhombic symmetry with lattice parameters approximately y/2 + 6)ao X (V2 - 6)ao x ao, where 5 is a small fraction and ao is the cubic lattice parameter. Although Bickford was able to confirm that Fc304 is magnetically orthorhombic at temperatures T < Ty, it is now known that the low-temperature structure is in fact monoclinic with lattice parameters V2ao x y/lz x 2ao and that the electronic ordering is more complex than originally proposed by Verwey. 

The first high temperature phase transition from the Cap2 is displacive, and is therefore a classic cooperative J-T distortion in which d-state degeneracies are lifted when the basal plane distortions are taken into account [7,8]. The second transition represents a further and more significant reduction in local site symmetry. This symmetry is sufficiently low so that all d-state degeneracies are lifted, and in addition the local coordination of Hf is reduced from eightfold to sevenfold [9]. The definition of a cooperative J-T effect in [7] clearly includes the monoclinic phases of Hf02 and... 

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