chemical principles revisited
edited by
MURIELBOYD BISHOP Clernson Univemity Clemson. SC 29631
The Importance of Understanding Structure Frank ~ a l a s s o ' United Technologies Research Center, East Hartford, CT 061 08 Chemistry books today are filled with facts and knowledge that will make the student majoring in chemistry better able to work in the various fields of chemistry and will help even the students who take only the basic chemistry courses to appreciate and understand better the new technical developments they may read or hear about. However, there is one field of chemistry that still is being neglected in a student's education and that is solid-state chemistry and its link with atomic structure. This is unfortunate, because many of the most interesting scientific discoveries and developments that have grown into mature industries where students may be employed, and that affect everyday living, involve solid-state materials. Transistors, lasers, su~erconductors.and high-strenah mamete are iust a few of ihesc solid-state disco\.eries. How much better students could aooreciate these developments if they knew the structure itthese materials and how the properties of these materials are dependent on their atomic structures. The problem is that the students are not exposed to structures early enough in their education and are, therefore, not comfortable with visualizing even simple atomic arrangements. Advanced chemistry students have the same problem when they are exposed to complex structures in advanced courses or in a n industrial environment without having had the necessary background. One solution is to start introducing students in high school to simple structures and to proceed with more complex structures throughout their education in chemistry. With this knowledge of structures, together with information on the electronic configuration of the atoms, which affects the bonding and the form of materials, the students can obtain a better understanding of the properties of solids. Method of Visualizing Structures and Structure-Properly Relationships This paper suggests an easy method of visualizing structures that should enable beginning students to picture the simple structures and the advanced students to visualize complex structures. In this approach, the structures are correlated with properties to make them more meaningful, useful, and interesting to the student. Some of the essential features are given in this paper. This technique has been presentedinmore detail ( I ) .(Kato has translated this book for use by students in Japan.) In this approach, simple structures are described and then built up to produce a series of related structures. Each structure is a little more complicated than the former. To do this, the concept of a unit cell is used. An example of a unit cell in two dimensions is the smallest parallelogram on a wallpaper pattern that, when repeated over the whole wall, produces the observed overall pattern. In 'Presently at Depallment of Chemistry, University of Connecticut, Storrs, CT 06269.
Figure 1. Bravais lattices. three dimensions, a unit cell is determined by selecting a box (a parallelepiped) that, when repeated throughout a crystal, reproduces the observed overall structure. This box can have different dimensions x, y, and z and angles. The simplest box is a cube where all sides and angles are equal. Various shaped unit cells that are possible are shown in Figure 1. There is more than one unit cell type (Bravais lattice) for some shapes. In these cells, the surroundings of each dot shown are the same. The lattice concept is discussed elsewhere in greater detail (1). Cubic Unit Cell
The simplest structure is a cubic unit cell with an atom a t each corner (Fig. 2). The cell contains one atom because each of the eight comer atoms is,shared with eight other Volume 70 Number 4 April 1993
287
cubes around it. Unfortunately, there is only one element with the simple cubic structure so it is not a very important structure, but it is the first building block. Body-Centered Cubic Cell By adding an atom a t the center of the cube (Fig. 3a), a body-centered cubic cell is Figure 2. Simple cubic struc- formed. Elements such as iron and more exotic elements ture. such as tungsten and niobium have this structure. This is the first important structure of metals. If the unit cells around this one are considered [Fig. 3b), it can be seen that there are eizht iron atoms closest around each iron atom and six atoms farther away. These atoms are not u s closely oackcd as thcv muld bc: thcrcforc, the materials are not as heme as possible. The &oms also are not in smooth layers, so when the layers slide over one another, they do not slide as easily as they could (less ductile). However, iron still can be bent and drawn into wires. Face-Centered Cubic Cell Another important metal structure is obtained when one starts with the basic cubic cell with atoms a t the m r e r s , and an atom is added in the center of each face of the cube. This arrangement is a face-centered cubic structure (Fig. 4). By looking at any of the body diagonals (corner to opposite corner) of the cube, smooth planes of atoms perpendicular to the diagonals can be seen; these can slide over one another (Fig. 4a). Elements with this structure should be ductile and, indeed, the elements copper, silver, and gold, which have this structure, are ductile and can be drawn easily into wires. They also are soft. That is why jewelry is rarely made from pure gold, but instead, is alloyed with other elements to make the gold wear better. In addition, these layers of atoms are densely packed. Each atom is
Figure 3. Body-centered cubic structure
288
Journal of Chemical Education
Figure 4. Face-centered cubic structure. surrounded closely with 12 other atoms (Fig4b). . Ifwe look a t a series of layers perpendicular to the body diagonals, the atoms in the fourth layer lie directly below those in the first layer, and we describe this layer sequence a s ABCABC (Fig. 4b). The Hexagonal Close-PackedStructure By using combinations of the face-centered cubic structure and the body-centered cubic structure described above, a whole series of more complex structures can be built up. However, this procedure will be set aside for the moment while we consider t h e third important structure of metals, the hexagonal close-oacked structure. As can be seen i n Figure 5a, t h e angle between thex andy axis ofthe unit cell is 120' Y rather than 9W as it \ is i n a cubic structure. By looking a t several unit cells together perpendicular to the z direction, we can see t h a t t h e atoms i n t h e third layer lie below those in the first layer, the layer sequence being ABAB (Fig. 5b). Thus, there are not a s many layers of atoms that can slide b over each other easily, as is the case in Figure 5. Hexagonal close packed struc. : t h e face-centered ture.
portant structure, the p tungsten structure (Fig. 8).Before the dismvery of the supermnducting oxides discussed below, some materials with this structure, such as Nb3Sn had the hiehest known su~erconductinetransition tem~erature.T, (inthe 20 K ran&), The tranztion temperatu;e is defined as the tem~eraturewhere a substance loses all of its resistance to electricity. Powerful magnets now are made from wires of superconductor materials. When many unit cells are placed together, as is the case in an actual sample of material, pairs of atoms in each face line up with those in neighboring cells forming strings of atoms. I n NbaSn t h e strings are of niobium atoms. It is this feature that some scientists feel may be important i n making, 6-tuugstentype materials good superconductors. Figure 6. Diamond cubic structure.
Decoding More Complex Structures
cubic structure. Elements with this structure, like cobalt, tend to be more brittle. Pictures of these basic structures oRen are shown, even in articles that are not highly technical, but they are seldom explained clearly. It is only by looking a t the structure step-by-step that the student actually can visualize and remember these arrangements of atoms so common for metals.
By proceeding in the above manner, progressively more complex structures can be built. These should be intro- Figure 7. Zinc blende structure
The Diamond Cubic and Zinc Blende Structures
Now we will return to the building up of other structures, starting with the face-centered cubic structure. Ifwe connect an atom at a corner with one in each contiguous face, a tetrahedron is formed. This is repeated for all corners and then atoms are added (as shown in Figure 6 ) in four of the eight tetrahedrons to produce a new tetrahedral structure within the unit cell. This produces the diamond cubic structure, one of the structures of carbon. The important semiconducting elements silicon and germanium have this diamond cubic structure. Transistors and many other semiconductor devices are made from these types of materials by adding elements that produce electrons or holes. If the atoms at the corners of the original face-centered cube and the newly added atoms in the diamond cubic structure are different, the structure is the zinc blende structure (Fig. I ) , the structure of another important group of semiconductor materials such as gallium arsenide. These compounds can be formed from erouu I11 and V elements or group I1 and VI elements and are referred to simply as three-five or two-six compounds i n the I technical literature. ! The P Tungsten Structure and Superconductors -
Figure 8. A 1Sktungsten structure.
A
Starting with t h e body-centered cubic structure. which is w described by a cubic' unit cell with an atom a t each comer and a t the center of the cell, and adding pairs of different atoms in Figure 9. Carbowraphite structure. the faces produces another im-
b
Volume 70 Number 4 April 1993
289
Figure 10.Boron nitride structure duced in steps at different levels of a student's education. For the advanced student. this tvoe of training eventuallv will enable him or her tb take";ery complex structures aoart for easv visualization of their basic structures and fiker details (1). Most recentlv. high T, su~erconductorshave been formed that contain various numbers of conducting Cu-O lavers seoarated bv other oxide lavers (3). These struct&es aredifficult to visualize at first glance, but they can be broken down into substructures that are common in many of them. Because of the interest in these materials, a meat deal has been written on how these structures can be-visualized using structural building blocks -
"
&
Other Influences on Properties I t was stated that the structure, the electronic configuration of the atom, and the form of a solid also are important in determining properties. Up to now, structure-property relationshius have been emohasized. The structures of graphite add boron nitride dow will be examined to see how electronic configurations of the atom also must be considered to understand properties. The unit cell of the graphite structure is shown in Figure 9a. When several unit cells are placed together (Fig. 9b), it can be seen that this structure contains layers of carbon atoms separated by a relatively large spacing. These layers can slip over one another, which makes graphite a good lubricant and allows layers to slip off on paper to produce writing. Recently, compounds such a s antimony fluoride have been introduced into the large spacings between the layers, giving or
290
Journal of Chemical Education
taking electrons from the layers to form materials with conductivities as high as copper or silver. Because there are free electrons in graphite, the material is more conducting in the layers than perpendicular to the layers of atoms where the spacing is large. There is a large field known as intercalation chemistry growing from this work. Boron nitride h a s a similar structure (Fig. 101, but it does not have t h e free electrons. Therefore, it does not conduct electricity well, but as can be seen from t h e structure, i t should also be, and is, a good lubricant because the layers can slide over one another. Both graphite and boron nitride can be heated and pressed to form materials with a diamond structure. With the diamond structure, they are now very hard and are used in cutting tools. The form of a material also is important in determining properties. When all of the unit cells are lined up the same direction in a sample of a material, we call the material a single crvstal. A single crvstal of a material will show the prGerti& expected-from-the structure. Ceramics andlor cast metals often consist of a large number of erains rather than a single crystal. Each grain can be consGered to be a single crystal, the unit cells within each grain oriented differently in respect to those in other grains; thus a n averaging of certain properties can be expected. Another concept relating to form that also should be introduced is the idea of the absence of any regular arrangements of atoms. Materials with this form are glasses and are said to be amorphous. Recently, even metals have been prepared as glasses by cooling the moltenmaterial rapidly. Even in glasses we find some short range structural units that can give us a clue as to what properties we should expect. Literature Cited
