NiAs-Type and MnP-Type Structures (from a book )

NiAs-Type Structure

As the MnP-type structure is regarded as distorted NiAs-type structure with small shifts of atomic positions, these two structures are similar to each other. We explain below these crystal structures connecting them with each other.

Figure 1.2 shows the hexagonal NiAs-type structure. Pnicogen atoms form a lattice like the hexagonal close-packed structure, and 3d metal atoms a simple hexagonal lattice. Table 1.2 shows lattice constants of the MnX compounds at room temperature.

As seen in Table 1.2, with change of pnicogen from Bi to P, the lattice constants decrease. In particular, decrease of the a-axis is remarkable. It is noted that a-axis of MnP is 73% of that of MnBi, corresponding to the fact that radius of P atom is 75% of that of Bi. Hence we see that the a-axis is almost proportional to radius of the pnicogen atom. Such tendency is also found in the compounds CrP, CrAs, and CrSb.

MnP-Type Structure

The MnP-type structure is obtained by introducing an orthorhombic distortion as well as slight shifts of atom positions for the hexagonal NiAs-type structure. As distortion and shifts are small, these two structures are basically similar. However, even the slight difference affects magnetic properties through change of electronic structure.

We explain the difference between the MnP-type and the NiAs-type structure. Figure 1.3 shows atomic positions projected onto the c-plane for the NiAs-type and MnP-type structures, where displacements of atoms in the c-plane are also illustrated.

In Fig. 1.3, crystallographic parameters u and w indicate atomic displacement along the b-axis. The parameter w is one order smaller than u. Distortion from the NiAs-type to MnP-type structure is expressed with a parameter δ, which denotes deviation of the b-axis from √3a-axis :

Atoms also shift along the c-axis as shown in Fig. 1.4.

In summary, the MnP-type structure is characterized by the five crystallographic parameters: the displacement parameters of metal (u and x) and anion (v and w) and the distortion parameter (δ). The NiAs-type structure is thus expressed by setting u = v = x = w = δ = 0. The crystallographic parameters are listed in Table 1.3 for MnP and CrAs, which take the MnP-type structure.

Other MnP-type compounds have similar values of the parameters. Since w and x are generally smaller than u and v, only u, v, and δ can be determined in reliable accuracy by X-ray diffraction for powdered samples. Table 1.4 shows position of atoms (see Figs. 1.3 and 1.4). We note that the coordinates are based on the A-, B-, and C-axes (c-, a-, and b-axes) shown in Fig. 1.4.

Reference: Electronic Structure and Magnetism of 3d-Transition Metal Pnictides