Two Bravais lattices

The hexagonal and trigonal systems yield special complications owing to the relationship between crystal axes and angles. One finds that face-centering or body-centering the primitive lattice types with simultaneous preservation of the threefold or sixfold axes is not possible. It suffices to say that only the two Bravais lattices of Fig. 1 are... 

Monoclinic System It has two Bravais lattices, i.e., primitive (P) and base-centered C, and three point groups 2, m, and 2/m. In detailed study of symmetry, the array of atoms that constitutes the structure of the crystal, a macroscopic mirror plane m, might be a glide plane c, while twofold rotation axis might be a screw axis as 2i. Considering these aspects of possible symmetry, the complete set is given as follows ... 

Figure Bl.21.4. Direct lattices (at left) and reciprocal lattices (middle) for the five two-dimensional Bravais lattices. The reciprocal lattice corresponds directly to the diffraction pattern observed on a standard LEED display. Note that other choices of unit cells are possible e.g., for hexagonal lattices, one often chooses vectors a and b that are subtended by an angle y of 120° rather than 60°. Then the reciprocal unit cell vectors also change in the hexagonal case, the angle between a and b becomes 60° rather than 120°.

Figure BT2T4 illustrates the direct-space and reciprocal-space lattices for the five two-dimensional Bravais lattices allowed at surfaces. It is usefiil to realize that the vector a is always perpendicular to the vector b and that is always perpendicular to a. It is also usefiil to notice that the length of a is inversely proportional to the length of a, and likewise for b and b. Thus, a large unit cell in direct space gives a small unit cell in reciprocal space, and a wide rectangular unit cell in direct space produces a tall rectangular unit cell in reciprocal space. Also, the hexagonal direct-space lattice gives rise to another hexagonal lattice in reciprocal space, but rotated by 90° with respect to the direct-space lattice.

Fig. 2.47. Direct (left) and reciprocal (right) lattices for the five two-dimensional Bravais lattices (2.243).

The common periodic structures displayed by surfaces are described by a two-dimensional lattice. Any point in this lattice is reached by a suitable combination of two basis vectors. Two unit vectors describe the smallest cell in which an identical arrangement of the atoms is found. The lattice is then constructed by moving this unit cell over any linear combination of the unit vectors. These vectors form the Bravais lattices, which is the set of vectors by which all points in the lattice can be reached. 

In two dimensions, five different lattices exist, see Fig. 5.6. One recognizes the hexagonal Bravais lattice as the unit cell of the cubic (111) and hep (001) surfaces, the centered rectangular cell as the unit cell of the bcc and fee (110) surfaces, and... 

Greek indices a, p = x,y,z of the Cartesian coordinate axes is meant). Minimization of expression (2.1.1) for an arbitrary two-dimensional Bravais lattice becomes possible since the Fourier representation in q implies the reduction of the double sum over j and/ to the single sum over q. Then the ground state energy is given by... 

Two-dimensional Bravais lattices with no higher than second-order axes of symmetry are characterized by a non-degenerate dipole ground state. On a rectangular lattice, the dipoles are oriented along the chains with the least intersite distances ax and antiferroelectric ordering in neighboring chains. As an example, for... 

In the general case of arbitrary two-dimensional Bravais lattices (not rectangular and rhombic), the ground state, depending on the lattice parameters (x0 and y0 in Fig. 2.13), is characterized by ferroelectric (0.25 < x0 <0.5) or stratified bisublattice antiferroelectric ordering (0 < x0 < 0.25). 

Fig. 2.13. Two-dimensional Bravais lattice with the basis vectors a)s a2, and the reciprocal lattice vectors bi, b2. The solid and dashed arrows at angles A and 0A give the ferroelectric (k = 0) and antiferroelectric (k = bi/2) configurations of dipoles in the ground state.

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

Consider an arbitrary two-dimensional Bravais lattice, with its sites R occupied by adsorbed molecules and molecular vibrations representing two modes, of a high and low frequency. Frequencies (ohh reduced masses mh/y vibrational coordinates w/,/(R), and momenta pA /(R) are accordingly labeled by subscripts h and / referring to the high-frequency and the low-frequency vibration. The most general form of the Hamiltonian appears as140... 

The dye molecules are positioned at sites along the linear channels. The length of a site is equal to a number ns times the length of c, so that one dye molecule fits into one site. Thus ns is the number of unit cells that form a site we name the ns-site. The parameter ns depends on the size of the dye molecules and on the length of the primitive unit cell. As an example, a dye with a length of 1.5 nm in zeolite L requires two primitive unit cells, therefore ns = 2 and the sites are called 2-site. The sites form a new (pseudo) Bravais lattice with the primitive vectors a, b, and ns c in favorable cases. 

Figure 4.8 The 14 Bravais lattices. Black circles represent atoms or molecules. P cells contain only one lattice point, while C- and /-centred cells contain two and / -centred cells contain four.

An example is the (110) plane of III-V semiconductors, such as GaAs(llO). The only nontrivial symmetry operation is a mirror reflection through a line connecting two Ga (or As) nuclei in the COOl] direction, which we labeled as the X axis. The Bravais lattice is orthorhombic primitive (op). In terms of real Fourier components, the possible corrugation functions are... 

When these four types of lattice are combined with the 7 possible unit cell shapes, 14 permissible Bravais lattices (Table 1.3) are produced. (It is not possible to combine some of the shapes and lattice types and retain the symmetry requirements listed in Table 1.2. For instance, it is not possible to have an A-centred, cubic, unit cell if only two of the six faces are centred, the unit cell necessarily loses its cubic symmetry.)... 

Table 1.1 gives the structures of the elements at zero temperature and pressure. Each structure type is characterized by its common name (when assigned), its Pearson symbol (relating to the Bravais lattice and number of atoms in the cell), and its Jensen symbol (specifying the local coordination polyhedron about each non-equiyalent site). We will discuss the Pearson and Jensen symbols later in the following two sections. We should note,... 

In Section 11.4 the fourteen 3D lattices (Bravais lattices) were derived and it was shown that they could be grouped into the six crystal systems. For each crystal system the point symmetry of the lattice was determined (there being one point symmetry for each, except the hexagonal system that can have either one of two). These seven point symmetries are the highest possible symmetries for crystals of each lattice type they are not the only ones. 

We can now complete our answer to the question, What information is conveyed when we read that the crystal structure of a substance is monodime P2JC7" The structure belongs to the monoclinic crystal system and has a primitive Bravais lattice. It also possesses a two-fold screw axis and a glide plane perpendicular to it. The existence of these two elements of symmetry requires that there also be a center of inversion. The latter is not specifically included in the space group notation as it would be redundant. 

FIGURE 5.33 The 14 Bravais lattices. P denotes primitive I, body-centered F, face-centered C, with a lattice point on two opposite faces and R, rhombohedral (a rhomb is an oblique equilateral parallelogram). 

Crystalline surfaces can be classified using the five two-dimensional Bravais lattices and a basis. Depending on the surfaces structure, the basis may include more than just the first surface layer. The substrate structure of a surface is given by the bulk structure of the material and the cutting plane. The surface structure may differ from the substrate structure due to surface relaxation or surface reconstruction. Adsorbates often form superlattices on top of the surface lattice. 

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