International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shumeli

International Tables for Crystallography (2006). Vol. B, ch. 3.1, pp. 349-350   | 1 | 2 |

## Section 3.1.9. Planes

D. E. Sandsa*

aDepartment of Chemistry, University of Kentucky, Chemistry–Physics Building, Lexington, Kentucky 40506-0055, USA
Correspondence e-mail: sands@pop.uky.edu

### 3.1.9. Planes

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Among several ways of characterizing a plane in a general rectilinear coordinate system is a description in terms of the coordinates of three non-collinear points that lie in the plane. If the points are U, V and W, lying at the ends of vectors u, v and w, the vectors , and are in the plane. The vector is normal to the plane. Expansion of (3.1.9.1) yields Making use of (3.1.7.1) , If now x is any vector from the origin to the plane, is in the plane, and From (3.1.9.2) , Rearrangement of (3.1.9.4) with on the left and on the right, and using (3.1.9.3) for z on the left leads to If, in particular, the points are on the coordinate axes, their designations are , and , and (3.1.9.6) becomes which may be written or in which the vector h has coordinates That is, the covariant components of h are given by the reciprocals of the intercepts of the plane on the axes. The vector h is normal to the plane it describes (Sands, 1982a ) and the length of h is the reciprocal of the distance d of the plane from the origin; i.e., If the indices are relatively prime integers, the theory of numbers tells us that the Diophantine equation (3.1.9.8) has solutions that are integers. Points whose contravariant components are integers are lattice points, and such a plane passes through an infinite number of lattice points and is called a lattice plane. Thus, the for lattice planes are the familiar Miller indices of crystallography.

Calculations involving planes become quite manageable when the normal vector h is introduced. Thus, the distance l from a point P with coordinates to a plane characterized by h is where a negative sign indicates that the point is on the opposite side of the plane from the origin.

The dihedral angle between planes with normals h and is A variation of (3.1.9.13) expresses in terms of vector u in the first plane, vector w in the second plane, and vector v, the intersection of the planes, as (Shmueli, 1974 ) A similar calculation gives angles of torsion. Let and be, respectively, the projections of vectors t and u onto the plane with normal h. The angle between and represents a torsion about h (Sands, 1982b ). Another approach to the torsion angle, which gives equivalent results (Shmueli, 1974 ), is to compute the angle between and using (3.1.8.3) .

### References

Sands, D. E. (1982a). Vectors and tensors in crystallography. Reading: Addison Wesley. Reprinted (1995) Dover Publications.
Sands, D. E. (1982b). Molecular geometry. In Computational crystallography, edited by D. Sayre, pp. 421–429. Oxford: Clarendon Press.
Shmueli, U. (1974). On the standard deviation of a dihedral angle. Acta Cryst. A30, 848–849.