Structure of Complex Materials

Abstract

Investigating the structure of a material is the first essential step in understanding its macroscopic properties. Getting insights of the interatomic and inter/intra-molecular interactions that influence the stability and the chemical behavior of a given compound allows its application in the best possible conditions and opens the way to design new materials with tailored properties. Neutron diffraction is a fundamental tool in structural characterization. Few examples concerning chemical crystallography, will be given to illustrate the properties that make neutrons the ideal probe for locating light atoms in the presence of heavy, electron-rich ones, and accurately determine atomic position and displacement parameters.

Introduction

Up to 1912, the world of atoms was out of reach to direct observation, but their role was already deployed in scientific theories like the kinetic theory of gases or the lattice theory of crystals. The experiment by Friedrich, Knipping, and Laue, we are celebrating the centennial of, showed the phenomenon of diffraction of X-rays by crystals [1], and it started a revolution in solid-state physics: atoms became real physical objects, experimentally “visible”. A very rapid development followed this first observation, both in the direction of explaining the diffraction process and in trying to understand the physical nature of X-rays. The intuition by William Lawrence Bragg that the diffraction pattern obtained by Laue and colleagues was due to the reflection of short wavelength electromagnetic waves from planes of atoms in the crystal [2], set the first milestone in the process of understanding materials from their structure, and offered a preliminary experimental confirmation of the physical quantum theories that were being developed in those years. The basis of modern chemistry and solid-state physics were framed.

With the discovery of the neutron in 1932 by Chadwick [3], neutron diffraction became feasible and was demonstrated by Halban and Preiswerk using a radium–beryllium radioactive-decaying source, in an experiment that aimed to prove the particle-wave duality of the neutron [4]. Although the radioisotope-driven sources of neutrons allowed the first neutron measurements of Bragg intensities from inorganic crystals, see, for example, MgO [5], it was only after the remarkable work of Enrico Fermi on the characterization of neutrons [6] and with the construction of the first graphite-moderated reactor in Chicago as part of the Manhattan Project, that neutron diffraction started to taking off (for a nice historical summary, see Ref. [7]).

By 1920 most of the theory of diffraction was available, and many structures had been solved by photographic methods. However, it was only after the solution of the phase problem by Hauptman and Karle in 1956 [8], and with the increasing availability of automatic diffractometers and electronic computers, that X-rays and neutron crystallography started to be a fundamental tool in chemical, physical and material science research.

The last 30 years have seen a tremendous impact of crystallographic work on the study of materials, as can be seen, for example, in the number of organic and metallorganic structures deposited with the Cambridge Structure Database [9] (Figure 4.1). During this period, the number of X-rays structures has grown from 52,363 to 595,276, while the number of neutron structures has increased much less: only from 567 to 1534, an astonishing discrepancy. Many reasons have contributed to such a colossal difference. We have to admit it: photons have always run much faster than neutrons, but in recent years, the technical development of sources, optical components and detectors for X-rays has been remarkable and has made X-rays techniques much more accessible than neutron ones. The impressive development and availability of laboratory equipment with integrated microfocus X-ray tubes and CCD-type detectors, has transformed X-ray crystallography into a standard investigation tool in many university departments, prompting some companies to produce table-top “push-button” instruments for routine structure determination [10] for which the expertise of a crystallographer is considered almost unnecessary. The increasing interest in structural analysis of protein and nucleic acids has brought the field a lot of funding that was quickly spent in designing more and more automated crystallographic screening and data collection procedures, especially targeted at macromolecular beamlines at synchrotron sources [11], [12]. The determination in developing the field is quite visible in the new generation of liquid metal jet anode tubes [13], together with the new fast acquisition, large surface area detectors [14].

Neutron crystallography did not benefit of comparable technical progress as did X-rays. The production of neutrons is only the first in a series of important and technically challenging steps, like the need of focussing a largely divergent neutron beam onto a relatively small target, detecting a neutral particle, and growing samples with a volume that ensures a reasonable duration of the experiments. The combination of these factors have discriminated against the development of neutron techniques as compared to X-rays ones.

The “ability” of X-rays to see the electron distribution in materials has opened the way to many studies of the chemical bond, the “stabilizing interaction” that holds atoms together and accounts for the formation of molecules and solids (see the pioneering work of Debye in 1915 [15]). In this context, charge density analysis and other methods for extracting physical properties from structural data have emphasized the different types of information obtainable with X-rays and neutrons in chemical analysis and relegated neutrons to the secondary role of providing accurate nuclear positions and anisotropic displacement parameters for H atoms [16], [17].

Solid-state physicists, for their part, were aiming at understanding lattice vibrations and magnetic properties in solids; they found in neutrons “the most valuable probe of a solid we posses”, as Ashcroft and Mermin put it in their milestone book [18]. Inelastic scattering of neutrons then became the field of solid-state physicist while neutron crystallography was mainly viewed as the tool to help understanding magnetic structures.

The rest of this chapter presents a brief discussion of the use of neutron diffraction in structural science from the point of view of a chemical crystallographer. The aim is not to be exhaustive about the fields in which neutron diffraction gives valuable contributions but to reinvigorate discussion about what used to be called “neutron chemical crystallography”, but is no longer recognizable as a (sub-)discipline in itself. After recalling some properties of neutrons (section 2), a few selected examples will show where neutron diffraction can play a fundamental role in understanding properties and applications of various materials (section 3). The chapter concludes with an outlook (section 4). Readers interested in magnetic neutron scattering are referred to Chapter 6 of this book, completely dedicated to the subject. Here the emphasis is to chemical problems for which neutron diffraction is not the “unique” technique that can be applied but a tool that can often “make the difference”.