structure and diffraction

Structure and Diffraction

Materials science is often described as being comprised of structure-property relation- 

ships. In this context structure refers not only to the arrangement of the basic building 

blocks, or long-range ordering but also to the chemical structure or short-range order- 

ing. This more complete notion of ordering is discussed early in Chapter 2 of this text 

with the appropriate nomenclature, and this theme is revisited many times throughout 

the book. Different structures can represent both different chemical bonding and differ- 

ent arrangements of atoms and/or molecules,and possibly even different states of aggre- 

gation (roughness, large grained, etc.). All these structural aspects can lead to different 

properties,including electronic and optical properties.It is important to use a consistent 

nomenclature to identify the unique structural features so that materials scientists 

communicate in a standard language. These topics are discussed in Chapter 2 on the 

structure of solids. 

In Chapter 3 on diffraction we study the determination of crystal structure.The basic 

idea that underlies this important family oftechniques,diffraction techniques,is the prin- 

ciple of superposition.It will be seen in the text that much of the fundamentals of mate- 

rials science can be understood by referring to a few the basic tenets of chemistry and 

physics.Among the tenets that are continually revisited is the superposition principle that 

is used for diffraction, mechanical properties, and electronic structure (with the first 

review of this tenet in Chapter 3 and again more thoroughly in Chapter 9).For example, 

the nature of a wave function that is used to describe an electron can be understood by 

considering the wave function to be made up of many waves in a complex blend,namely 

the notion of modulation. 

Later in Chapter 3 the concept of reciprocal space is introduced. The idea follows 

from the notion that it is important in science to operate in the coordinate space most 

appropriate to the system. It is found that for crystal structure obtained by diffraction, 

reciprocal distances correlate the structure with diffraction experiments. 

From a study of structure and diffraction one may glean the erroneous idea that only, 

or at least mostly, crystalline materials are important in materials science and electronic 

materials science.This is far from the truth,but it is a natural tendency that follows from

paying close and early attention to only perfect crystals.In fact a large fraction of useful 

materials in all fields are not crystalline at all (e.g.,the dielectrics used in microelectronic 

ICs),and another large fraction is partially crystalline (alloys used for contacts in micro- 

electronics) or at least defective in their crystalline nature.However,the nonperfectly crys- 

talline materials are more difficult to describe universally and simply.That is to say,each 

material must be described using a number of structural aspects where crystallinity may 

be one of the important aspects. However, as is usual in science, the ideal state is the 

easiest to describe thoroughly, and this is the reason why virtually all studies of materi- 

als science commence with a discussion of ideal or perfect crystals. 

Also electronic structure that is discussed in Chapter 9 on electronic structure is 

important for determining many properties particularly electrical properties. It will be 

seen in Chapter 9 that the structure of the material will greatly influence the electronic 

structure and in turn the electronic and optical properties. 

Ramon A. Carmona C.

C.I 17646653

CRF

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