Nowotny phases, chimney-ladder structures

1 Nowotny phases, chimney-ladder structures. The Nowotny chimney-ladder phases are an example in alloy field chemistry of composite structures. They are a series of intermetallic T X , compounds formed by transition metals T from the 4th to 9th groups with p-block elements X from the 13th to the 15th groups. 

In these phases, the unit cell (superstructure cell, super-cell) contains along the oaxis n pseudo-cells of T atoms and m interpenetrating pseudo-cells of X atoms. These phases (Nowotny phases) have been called chimney-ladder phases because they contain rows of atoms X (the ladder ), with variable interatomic spacing from one compound to another, which are inserted into channels ( chimneys ) in the T array. The T metals in all of the superstructures form a (3-Sn-like array (see Chapter 7) with the number of T metal atoms in the formula of the compound corresponding to the number of (3-Sn-like pseudo-cells stacked in the c direction of the super-cell. The arrangement of the atoms in these phases can be compared to that found in the structure of TiSi2. 

The following is a list of some chimney-ladder phases (more complex phases containing as many as 600 atoms in the unit cell have been described). 

In V17Ge31, for instance, there are 17 3 -Sn like pseudo-cells of V atoms and 31 Ge pseudo-cells stacked along the c-axis. 

Two empirical rules, moreover, have been enunciated. The first rule concerns the number of valence electrons for the Nowotny, T Xm, phases formed by the late transition metals, it is that the total number of valence electrons per T atom is 14. With reference to previously cited phases we have, for instance  

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The fact that the electron concentration appears to play some role in the control of this family of structures was noted by Nowotny (Schwomma et al. 1964a, b, Jeitschko and Parthe 1967, Flieher et al. 1968a, b, Parthe 1969) and reported by Pearson (1972). The second rule, discernible in the electron diffraction of the T Xm Nowotny chimney-ladder phases, is the appearance, along the c-axis of a... 

Figure 3.44. Views, according to Fredrickson et al. (2004a, b), of the Ru2Sn3 structure type, an example of the chimney-ladder Nowotny phases. On the left, a lateral view of the Ru helix (black balls) and of the Sn helix (white balls). The periods of the two helices (cn and cm) are given together with the lattice parameter c.

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