Functional Oxides

Duncan W. Bruce, Dermot O'Hare, Richard I. Walton, Duncan W. Bruce, Dermot O'Hare, Richard I. Walton

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470997505
• eBook ISBN: 9781119972945
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Functional oxides have a wide variety of applications in the electronic industry. The discovery of new metal oxides with interesting and useful properties continues to drive much research in chemistry, physics, and materials science.

In Functional Oxides five topical areas have been selected to illustrate the importance of metal oxides in modern materials chemistry:

• Noncentrosymmetric Inorganic Oxide Materials
• Geometrically Frustrated Magnetic Materials
• Lithium Ion Conduction in Oxides
• Thermoelectric Oxides
• Transition Metal Oxides - Magnetoresistance and Half-Metallicity

The contents highlight structural chemistry, magnetic and electronic properties, ionic conduction and other emerging areas of importance, such as thermoelectricity and spintronics.

Functional Oxides covers these complex concepts in a clear and accessible manner providing an excellent introduction to this broad subject area.

Chapter 1

Noncentrosymmetric Inorganic Oxide Materials: Synthetic Strategies and Characterisation Techniques

P. Shiv Halasyamani

Department of Chemistry, University of Houston, Houston, Texas, USA

1.1 INTRODUCTION

Materials that are crystallographically noncentrosymmetric (NCS), or acentric, are of current interest attributable to their functional properties, including piezoelectricity, ferroelectricity, and second-harmonic generation. Numerous relationships occur between these properties and crystal classes.[1] These relationships are shown in Figure 1.1, along with several well-known materials. It is instructive if we examine this figure more closely. If we examine the left-side of Figure 1.1, the symmetry dependent property we encounter is enantiomorphism, and the chiral crystal classes. All chiral materials must crystallise in one of eleven crystal classes, 1 (C1), 2 (C2), 3 (C3), 4 (C4), 6 (C6), 222 (D2), 32 (D3), 422 (D4), 622 (D6), 23 (T), or 432 (O). Materials found in any of these crystal classes have a ‘handedness’, and a nonsuperimposable mirror image. The well-known chiral material α-SiO2[2, 3] crystallises in crystal class 32 (D3). If we examine the right-side of Figure 1.1, we encounter the ten polar crystal classes, 1 (C1), 2 (C2), 3 (C3), 4 (C4), 6 (C6), m (Cs), mm2 (C2v), 3m (C3v), 4mm (C4v), and 6mm (C6v). Materials found in these crystal classes have a permanent dipole moment. In fact LiIO3,[4, 5] which crystallises in crystal class 6 (C6) is both chiral and polar. The other materials shown: KTiOPO4 (KTP)[6] and Ba2NaNb5O15 (mm2 for both),[7] LiNbO3 [8, 9] and β-BaB2O4 (3m for both),[10, 11] and BaTiO3 (4mm) are all ‘purely’ polar. They all have a dipole moment, but are not chiral. Examples are also given of materials, CO(NH2)2 (urea)[12] and (NH4)H2PO4 (ammonium dihydrogen phosphate, ADP)[13] that crystallise in crystal class

, that are neither chiral nor polar, but are still noncentrosymmetric. Other symmetry-dependent properties that are of importance are second-harmonic generation and piezoelectricity. Except for materials that are found in crystal class 432 (O), all NCS materials exhibit the correct symmetry for second-harmonic generation and piezoelectric behaviour.

Figure 1.1 The relationships with respect to symmetry-dependent property between the noncentrosymmetric crystal classes are given along with representative compounds. Note that only five crystal classes, 1 (C1), 2 (C2), 3 (C3), 4 (C4), and 6 (C6) have the proper symmetry for all of the symmetry dependent properties. Adapted from Halasyamani and Poeppelmeier, 1998 [71].

Copyright 1998 American Chemical Society

Determining if a crystalline material is centrosymmetric or noncentrosymmetric is usually straightforward. From Friedel’s law it is known that, during the diffraction process, if the incident wavelength is small compared with the absorption edge of any atom in the crystal, a centre of symmetry is introduced between oppositely related reflections. In other words I(hkl) = I(−h−k−l). Friedel’s law fails when the incident wavelength is similar to an atom’s absorption edge. This anomalous scattering, when the imaginary part of the scattering factor becomes large, has been exploited to address a host of crystallographic problems.[14] Also, with the diffraction data the intensity distribution between a centric and acentric crystal differs. Statistical indicators of centricity have been developed by Wilson and Howell,[15, 16] but have been shown to be incorrect if the structure contains heavy atoms on special positions. Marsh has emphasised the importance of weak reflections if the centricity is in question.[17, 18] If weak reflections are removed, the statistical distribution tests can be strongly biased toward an acentric indication. Marsh also argues that when the diffraction data do not provide a clear choice between centrosymmetric and noncentrosymmetric space groups the centrosymmetric space group is preferred, even if disorder occurs.[17] The Platon suite of programs, specifically Addsym, can be used on refined structures to check for missing symmetry, e.g. inversion centres, as well as mistakes in crystal system or Laue class.[19]

1.2 STRATEGIES TOWARD SYNTHESISING NONCENTROSYMMETRIC INORGANIC MATERIALS

In the past decade or so a number of strategies have been described whose aim was to increase the incidence of acentricity in any new material. In one manner or another, each of these strategies involves crystal engineering.[20] One question that needs to be addressed is why there are so few (relatively) NCS materials? It is estimated that only ~15% of all inorganic materials are NCS. This would indicate that in the vast majority of inorganic materials, the ‘building blocks’ of the structure are centrosymmetric, i.e. made up of regular polyhedra. These regular polyhedra are usually related by inversion symmetry. Thus, in order to design inorganic NCS materials, two challenges must be overcome. First, the building blocks of the structure must necessarily be intrinsically acentric. In other words, there must be a distortion that requires or forces the metal cation not to be on an inversion centre. If local centricity occurs, macroscopic centricity is observed. Secondly, these building blocks must be connected or related in the structure by noninversion-type symmetry. In other words, it is not sufficient to have only acentric polyhedra; these polyhedra must be related by acentric symmetry elements. Numerous researchers have developed strategies to address both issues.

The purpose of this chapter is to discuss noncentrosymmetric materials, their synthetic strategies as well as their symmetry dependent properties. We will begin by discussing the various strategies employed in synthesising new NCS materials, and then move on to physical property characterisation. Although we will be unable to discuss in detail all of the proposed strategies for synthesising NCS materials, we will describe the major ideas in the field. Finally, we will discuss the outlook for this field with multifunctional materials in mind.

1.3 ELECTRONIC DISTORTIONS

One manner in which the incidence of acentricity may be increased in any oxide material is to use cations susceptible to second-order Jah...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Inorganic Materials Series Preface
• Preface
• List of Contributors
• Chapter 1: Noncentrosymmetric Inorganic Oxide Materials: Synthetic Strategies and Characterisation Techniques
• Chapter 2: Geometrically Frustrated Magnetic Materials
• Chapter 3: Lithium Ion Conduction in Oxides
• Chapter 4: Thermoelectric Oxides
• Chapter 5: Transition Metal Oxides: Magnetoresistance and Half-Metallicity
• Index