Crystallography and Crystal Defects. Anthony Kelly

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      (1.26)

      using the Einstein summation convention (Section A1.4).

      An equivalent, more elegant, way is to use quaternion algebra. The rotation matrix described by Eq. (1.23) is homomorphic (i.e. exactly equivalent) to the unit quaternion:

(1.27)

      satisfying:

      (1.28)

      In quaternion algebra, the equation equivalent to Eq. (1.24) is one in which two quaternions q_B and q_A are multiplied together. The multiplication law for two quaternions p and q is (Section A1.4):

(1.29)

The quaternion p · q is also a unit quaternion, from which the angle and the direction cosines of the axis of rotation of this unit quaternion can be extracted using Eq. (1.27). Therefore, if:

(1.30)

      the angle γ′ satisfies the equation:

      (1.31)

      The term (n_1A n_1B + n_2A n_2B + n_3A n_3B) is simply the cosine of the angle between n_A and n_B, and therefore n_A · n_B, since n_A and n_B are of unit length. Defining γ = 2π − γ′, so that γ and γ′ are the same angle measured in opposite directions, and rearranging the equation, it is evident that:

(1.32)

      Permitted values of γ are 60°, 90°, 120° and 180°, as shown in Table 1.1, and so possible values of cos γ/2 are:

(1.33)

      To apply these results to crystals, let us assume that the rotation about n_A is a tetrad, so that α = 90° and α/2 = 45°. Suppose the rotation about n_B is a diad, so that β = 180° and β/2 = 90°. Then, in Eq. (1.32):

(1.34)

      Since n_A · n_B has to be less than one for non‐trivial solutions of Eq. (1.34), the possible solutions of Eq. (1.34) are when n_A and n_B make an angle of (i) 90° or (ii) 45° with one another, corresponding to values of γ of 180° and 120°, respectively. From Eq. (1.29) the direction cosines n_1C, n_2C and n_3C of the axis of rotation, n_C, are given by the expressions:

(1.35)

      because sin γ = sin γ′. If we choose n_A to be the unit vector [001] in the reference orthonormal coordinate system then for case (i) we can choose n_B to be a vector such as [100]. Hence in Eq. (1.30):

      (1.36)

      and so:

      (1.37)

That is, γ = 180°, γ′ = 180°, and n_C is a unit vector parallel to [10]. This possibility is illustrated in Figure 1.17a, with axes A, B and C lettered according to these assumptions. The other diad axes marked in Figure 1.17a must automatically be present, since A is a tetrad.

Figure 1.17 Examples of possible allowed combinations of rotational symmetries in crystals. In (a) a tetrad, A, is perpendicular to two diads, B and C, at 45° to one another, while in (b) the tetrad, A, is 45° away from a diad, B, and 54.74° from a triad, C, with B and C 35.26° apart. In (a) other diad axes which must be present are also indicated, and in (b) other triad axes (but not other tetrad and diad axes) which must be present are also indicated

For case (ii), we can again choose n_A to be [001]. n_B can be chosen to be a vector such as:

      (1.38)

      Therefore, in Eq. (1.30):

      (1.39)

      and so:

      (1.40)

      i.e. γ = 120°, γ′ = 240°, and n_C is a unit vector parallel to [111], making an angle of 54.74° with the fourfold axis and 35.26° with the twofold axis. This arrangement is shown in Figure 1.17b,

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