Perovskites E-Book

Richard J. D. Tilley

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 551

Veröffentlichungsjahr: 2016

Beschreibung

Uniquely describes both the crystallography and properties of perovskite related materials.

• Practical applications in solar cells, microelectronics and telecommunications
• Interdisciplinary topic drawing on materials science, chemistry, physics, and geology
• Contains problems and answers to enhance knowledge retention 

Leseprobe

Perovskites

Structure–Property Relationships

RICHARD J. D. TILLEY

Professor Emeritus, Cardiff University, UK

This edition first published 2016© 2016 John Wiley & Sons, Ltd

First Edition published in 2016

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Preface

Perovskites are a class of compounds with structures related to that of the mineral Perovskite, CaTiO3, and can be considered to be derived from a parent phase of general formula ABX3. They have been intensively studied since the middle of the twentieth century because of their innate properties: initially dielectric, piezoelectric and ferroelectric. This range of behaviour has been expanded into areas which include magnetic ordering, multiferroic properties, electronic conductivity, superconductivity and thermal and optical properties. Apart from these purely physical aspects, the phases show a wide range of chemical attributes. They are used as electrode materials for solid oxide fuel cells where materials with high oxide ion conductivity, electronic conductivity and mixed ionic/electronic conductivity are required. Many perovskite phases show useful catalytic and redox behaviour, often dependent upon the presence of chemical defects in the phase.

This complexity is a result of two prime factors. Firstly the crystal structures encompassed by the term ‘perovskite’ embrace a huge range, from the simple cubic ‘aristotype’ SrTiO3 to cation- and anion-deficient phases, modular phases including the cuprate superconductors and hexagonal perovskites related to BaNiO3. In addition, both the chemical and physical properties of any member of these structural forms can be tuned over wide ranges by relatively simple substitution into all or some of the A-, B- and X-sites. This wide-ranging flexibility includes the formation of perovskites in which the A cation is replaced by an organic molecule, typified by the perovskite methylammonium lead iodide, now intensely studied as the core of ‘perovskite’ photovoltaic cells. Additionally, the properties of thin films, superlattices and nanoparticles show new and totally unexpected responses, when compared to the behaviour of the separate bulk phases.

The aim of this book is to provide a compact overview of this large body of knowledge. An outline of the structures of these phases is of primary importance as a prerequisite to an understanding of many physical properties. This material is contained in the earlier chapters. For the purposes of providing an overall vista, crystal structures are mostly represented as idealised forms. This has the advantage of bringing out the structural relationships between the phases described, but the obverse side is that it does obscure small details that may be of significance. However, the detailed crystallographic parameters of all the phases mentioned in this book are available via the CrystalWorks database (http://www.cds.rsc.org). This source will also provide literature references to the originally published crystallographic data of all the phases listed. Crystal structures are followed by a survey of physical and chemical properties. The properties emphasised are, where possible, those unique to perovskites or at least manifested in a singular way by these compounds. They are grouped into broad categories – chemical, dielectric, magnetic, electronic, thermal and optical properties – although these classifications cannot truly be cleanly separated from each other.

In a book of modest size, it is necessary to be somewhat selective in material content. For this reason, two areas have been omitted. The first of these concerns preparation techniques. In the main these are the normal techniques of solid-state chemistry, physics and ceramic science and are not unique to perovskites. Secondly catalysis has also been omitted. Again, the bulk of the catalytic reactions studied are not unique to perovskites and are better described and discussed within the broader perspective of catalysis rather than via the narrower standpoint of perovskites.

Although large numbers of papers are published each month detailing some aspect or other of perovskite physics and chemistry, it is felt that an extensive literature reference section would overwhelm any reader seeking a broad overview of the field. Because of this, the Further Reading sections contain mainly reviews or selected recent references that expand material in the text. These are sufficient to provide an entry point to the literature base for those needing additional information. In addition a few sources that explain the basic concepts of crystallography and structure–property relationships are added, and two appendices are included that explain two rather more specialist aspects of nomenclature.

As ever, I am grateful to my wife Anne for support and tolerance during the assembly of this work, without which the project could not have been undertaken. In addition I thank my family for continual encouragement. Finally I am indebted to the staff of the Trevithick Library, University of Cardiff, who helped with literature resources and related matters.

Richard TilleyJanuary 2016

1The ABX3 Perovskite Structure

1.1 Perovskites

Perovskite is a mineral of formula CaTiO3. It was discovered in 1839 by the Prussian mineralogist Gustav Rose in mineral deposits in the Ural Mountains and named after the Russian mineralogist Count Lev Aleksevich von Petrovski. Natural crystals have a hardness of 5.5–6 and a density of 4000–4300 kg m−3. They are usually dark brown to black, due to impurities, but when pure are clear with a refractive index of approximately 2.38. The crystal structure of this compound, initially thought to be cubic, was later shown to be orthorhombic (Table 1.1).

Table 1.1Representative ABX3 perovskite phasesa

Phase

Space group

b

Unit cell

a

(nm)

b

(nm)

c

(nm)

1, 2

AgMgF

3

C,

Pm

m

(221)

0.41162

CsPbI

3

C,

Pm

m

(221)

0.62894

KCuF

3

T,

I

4/

mcm

(140)

0.56086

0.76281

KMgF

3

C,

Pm

m

(221)

0.39897

KZnF

3

C,

Pm

m

0.40560

NaMgF

3

O,

Pbnm

(62)

0.48904

0.52022

0.71403

NaFeF

3

O,

Pnma

(62)

0.56612

0.78801

0.54836

NH

4

ZnF

3

C,

Pm

m

(221)

0.41162

1, 5

KTaO

3

C,

Pm

m

(221)

0.40316

KNbO

3

O,

Amm

2 (38)

0.3971

0.5697

0.5723

2, 4

SrTiO

3

C,

Pm

m

(221)

0.3905

BaTiO

3

T,

P

4

mm

(99)

0.39906

0.40278

CaTiO

3

O,

Pbmn

(62)

0.54035

0.54878

0.76626

BaSnO

3

C,

Pm

m

(221)

0.4117

CdSnO

3

O,

Pnma

(62)

0.52856

0.74501

0.51927

CaIrO

3

O,

Pbnm

(62)

0.52505

0.55929

0.76769

PbTiO

3

T,

P

4

mm

(99)

0.3902

0.4143

PbZrO

3

O,

Pbam

(55)

0.58822

1.17813

0.82293

SrCoO

3

C,

Pm

m

(221)

0.3855

SrMoO

3

C,

Pm

m

(221)

0.39761

SrRuO

3

O,

Pnma

(62)

0.55328

0.78471

0.55693

(Fe,Mg)SiO

3

O,

Pnma

(62)

0.5020

0.6900

0.4810

3, 3

BiFeO

3

Tr,

R

3

c

(161)

0.55798

1.3867

BiInO

3

O,

Pnma

(62)

0.59546

0.83864

0.50619

ErCoO

3

O,

Pbnm

(62)

0.51212

0.54191

0.73519

GdFeO

3

O,

Pbnm

(62)

0.53490

0.56089

0.76687

HoCrO

3

O,

Pnma

(62)

0.5518

0.7539

0.5245

LaAlO

3

Tr,

R

3

c

(161)

0.53644

1.31195

LaCoO

3

Tr,

R

c

(167)

0.54437

1.30957

LaMnO

3

O,

Pbnm

(62)

0.55367

0.57473

0.76929

LaTiO

3

O,

Pbnm

(62)

0.5576

0.5542

0.7587

NdAlO

3

Tr,

R

c

(167)

0.53796

1.31386

PrRuO

3

O,

Pnma

(62)

0.58344

0.77477

0.53794

YbMnO

3

O,

Pbnm

(62)

0.52208

0.58033

0.73053

4, 5

ThTaN

3

C,

Pm

m

0.4020

aMany of these phases are polymorphic, and lattice parameters vary with temperature and pressure.

bThe crystal system, here and throughout the other tables in this book, is abbreviated thus: C, cubic; H, hexagonal; M, monoclinic; O, orthorhombic; T, tetragonal; Tr, trigonal (often specified in terms of a hexagonal unit cell); Tri, triclinic.

As with many minerals, Perovskite has given its name to a family of compounds called perovskites, which have a general formula close to or derived from the composition ABX3. At present many hundreds of compounds are known that adopt the perovskite structure. In fact a perovskite structure mineral, Bridgmanite (Fe,Mg)SiO3, is the most abundant solid phase in the Earth’s interior, making up 38% of the total. The phase occurs between depths of approximately 660–2900 km but is only stable at high temperatures and pressures so that it is not found at the surface of the Earth.

To some extent the multiplicity of phases that belong to the perovskite family can be rationalised by assuming that perovskites are simple ionic compounds, where A is usually a large cation, B is usually a medium-sized cation and X is an anion. Naturally the overall ionic structure must be electrically neutral. If the charges on the ions are written as qA, qB and qX, then

Frequently encountered (but not exclusive) combinations are

The importance of perovskites became apparent with the discovery of the valuable dielectric and ferroelectric properties of barium titanate, BaTiO3, in the 1940s. This material was rapidly employed in electronics in the form of capacitors and transducers. In the decades that followed, attempts to improve the material properties of BaTiO3 lead to intensive research on the structure – property relations of a large number of nominally ionic ceramic perovskite-related phases with overall compositions ABO3, with a result that vast numbers of new phases were synthesised.

It was soon realised that, as a group, these materials possessed very useful physical and chemical properties far broader than those shown by BaTiO3, and research widened to include a range of structures and phases that could all be related structurally to the perovskite family, including nominally ionic nitrides and oxynitrides. In addition, a number of materials which are better described as alloys, of formula A3BX, where A and B are metals and X is an anion or semimetal, typically C, N, O and B are known. These are often said to adopt the so-called antiperovskite or inverse perovskite structure, because the metal A atoms occupy the positions corresponding to the anions in the ionic perovskites and the B and X atoms occupy sites corresponding to those occupied by the cations. The flexibility of the perovskite framework also allows it to include cations such as NH4+, which can often be considered to be spherical at normal temperatures. More complex phases, such as the inorganic–organic hybrid compounds (CH3NH3)PbX3, where X is typically Cl, Br, I or a combination of these anions, have also been synthesised.

As well as phases with an ABX3 composition, large numbers of modular structures have been prepared, all of which are built up, at least in part, from fragments, usually slabs, of perovskite-like structure. The formulae of these are not easily reconciled with a composition of ABX3 until the structural building principles have been found and the nature of the interfaces between the various slabs is clarified. For example, Bi2Ca2Sr2Cu3O10+δ, a superconducting oxide, is built from slabs of perovskite type separated by slabs of composition Bi2O2.

As would be expected, there is a close correlation between chemical and physical properties in these complex materials. It is this flexibility that makes the perovskites as a group, important, as the facile replacement of any of the atoms in this range of structures can be used to modify important physical properties in a controlled way. The flexibility comes at a structural cost. The ABX3 perovskite structure is beset by structural variations that depend upon exact composition as well as temperature and pressure, all of which have a profound significance for physical properties. Moreover, many multi-cation or anion materials show an intricate microdomain structure when examined by transmission electron microscopy. These microdomains are small volumes of differing structural complexity that exist within a coherent anion matrix. Often they show ordering of atoms over several unit cell volumes with the pattern of order changing from one microdomain to its neighbours. When these microdomains are arranged throughout the crystal in a more or less random fashion, dependent upon the symmetry of the phase, the microscopic ordering is hidden from normal X-ray and neutron diffraction structure solving methods and may not feature in the refined structure of the macroscopic crystal studied. This level of order is generally revealed by high-resolution transmission electron microscopy. Because of this divergence, exact structural details of many perovskite phases of complex composition are open to question, although the overall broad-brush structure is known.

Fortunately much of this diversity can be understood or rationalised in terms of an ideal cubic perovskite structure. In this chapter the ideal ABX3 perovskite structure is described together with some of the structural variations that occur which have significance for chemical and physical properties and which make precise structure determination a difficult task.

1.2 The Cubic Perovskite Structure: SrTiO3

The idealised or aristotype perovskite structure is cubic and is adopted by SrTiO3 at room temperature (but not at all temperatures). There are two general ways of listing the atoms in the cubic unit cell. The standard crystallographic description places the choice of origin at the Sr atom:

SrTiO3: cubic; a = 0.3905 nm, Z = 1; space group, Pmm (No. 221);

The Sr2+ ions lie at the corners of the unit cell. The Ti4+ ions lie at the cell centre and are surrounded by a regular octahedron of O2− ions (Figure 1.1a and b). For some purposes it is useful to translate the cell origin to the Ti4+ ions:

Figure 1.1The idealised perovskite structure of SrTiO3: (a) atom positions with Sr2+ at cell origin; (b) TiO6 octahedral coordination polyhedron; (c) atom positions with Ti4+ at cell origin; (d) TiO6 octahedral polyhedron framework with Sr2+ at the cell centre; (e) cuboctahedral cage site

The large Sr2+ ions are coordinated to 12 O2− ions and are now situated at the unit cell centre (Figure 1.1c). For a discussion of the chemical and physical properties of this (and other) perovskites, it is convenient to think of the structure as built-up from an array of corner sharing TiO6 octahedra (Figure 1.1d). The large Sr2+ ions are located at the unit cell centre and are surrounded by a cuboctahedral cage of O2− ions (Figure 1.1e). The TiO6 framework is regular and the octahedra are parallel to each other. All the Ti4+O2− bond lengths are equal and the six O2−Ti4+O2− bonds are linear.

The Sr2+ and O2− positions in the SrTiO3 structure are identical to that of the Au and Cu positions in the alloy Cu3Au, and if the difference between the Sr2+ and O2− ions (or Cu and Au atoms) is ignored, they form a cubic array identical to that of the Cu structure (Figure 1.2a and b). This latter is the simple A1 structure type, often described as the face-centred cubic (fcc) structure, which is made up of (111) planes that lie normal to the cell body diagonal [111], stacked in the normal face-centred sequence …ABCABC…. Thus the SrTiO3 structure can also be thought of as a built-up from close-packed layers of (111) planes containing ordered Sr2+ and O2− ions that lie normal to the cubic unit cell body diagonal [111]. The charge balance needed to maintain charge neutrality in this skeleton structure is provided by an ordered distribution of the Ti4+ ions in the available octahedral interstices that are bounded by O2− ions only (Figure 1.3).

Figure 1.2(a) The AuCu3 and (b) the Cu (A1, fcc) structure

Figure 1.3A single SrO3 (111) plane in SrTiO3. The Ti4+ ions, above and below the SrO3