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Introduction of the Metastable‐Phase Materials

Qi Shao1 and Mingwang Shao2

1Soochow University, College of Chemistry, Chemical Engineering and Materials Science, Suzhou, 215123, Jiangsu, China

2Soochow University, Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon‐Based Functional Materials & Devices, Suzhou, 215123, Jiangsu, China

1.1 Introduction

From the point of classical thermodynamics, no metastable‐phase can exist [1]. Yet, when we look around our world, the number of metastable‐phase materials is so huge that it is predominantly larger than that of stable‐phase ones. For simple chemical compounds, the ratio of metastable‐phases to stablephases is not large. However, the number of metastable compounds becomes dramatically large for complex chemicals, such as organic ones and polymers [2]. For biological materials, nearly 100% phases are metastable ones [3]. Therefore, we can say with certainty: The more complex the materials, the larger the number of metastable‐phases.

Such a huge number of metastable‐phase materials will certainly bring out interesting and important properties, which may find wide applications in the energy, material, industry, agriculture, biology, environmental, and catalysis‐related fields [4]. It should be pointed out that in this book the authors have tried to emphasize on classification, synthetic methodology, characterization, and catalytic performance of different metastable‐phase materials.

1.2 What Are Metastable‐Phase Materials?

A metastable‐phase material is the matter located in a state that corresponds to a local minimum in energy separated by a barrier from the state corresponding to the global minimum. Metastable‐phases are in a nonequilibrium state and thus show thermodynamic instability [5]. The energy barrier ensures the metastability and keeps the metastable‐phase materials from transformation to stable states, as shown in Figure 1.1.

Figure 1.1 The schematic of metastable‐phase and stable‐phase. The red ball and the blue ball point to metastable‐phase and stable‐phase, respectively.

Some metastable‐phase materials may exist for a long time, such as diamond, which may exist virtually infinitely at room temperature. However, some only exist for a very short time, such as β‐Sn, which may exist only for a few days at a temperature of −20 °C and then transform to α‐Sn [6].

1.3 The Categories of Metastable‐Phase Materials

Considering the large number of metastable‐phase materials, several classifications for metastable‐phase materials are highly recommended.

From the point of crystallinity, metastable‐phases may be classified as crystalline metastable‐phases, microcrystalline metastable‐phases, quasicrystalline metastable‐phases, and amorphous metastable‐phases.

Microcrystalline metastable‐phase materials, or nanocrystalline metastable‐phase materials, have large surface energy, which is favorable to the stability of metastable‐phases [7]. These materials have numerous grain boundaries and defects, which bring out larger values of hardness, strength, heat capacity, electrical resistivity, and magnetism [8].

Quasicrystalline metastable‐phase materials show lack of transition symmetry [9]. These materials usually have low surface energy, low friction ecoefficiency, but high values of wear resistance, hardness, high‐temperature plasticity, thermal resistance, corrosion resistance [10].

Amorphous metastable‐phase materials have short‐range order and long‐range disorder [11]. They have excellent soft magnetic performance and high values of strength and corrosion resistance [12, 13].

This book will focus on the crystalline metastable‐phase materials only.

As metastable‐phases, together with their corresponding stable‐phases, form polymorphs, we classified polymorphs according to the chemical bonds and the coordination environment.

Figure 1.2 The crystal structures of (a) α‐Ni and (b) β‐Ni.

1.3.1 Different Packing Orders

These polymorphs have the same layers with identical composition, coordination environment, connection mode of coordination. Yet, there are differences in the packing order of these layers.

The typical examples are α‐Ni and β‐Ni (Figure 1.2), where α‐Ni is the stable‐phase, while β‐Ni is a metastable‐phase one. Both of them are coordinated with the number of 12. Each layer of these two materials is composed of the closest packing Ni atoms, with one Ni atom surrounded by six Ni in one layer. Their difference is in the packing order, α‐Ni is packed with the order of ABC and β‐Ni is AB [14]. The ABC packing endows α‐Ni with strong ferromagnetism, while the magnetism of β‐Ni is so weak that it can be ignored [15, 16].

The number of this kind of polymorphs is extraordinarily large because only a small amount of energy is needed to alter the packing order. Both SiC and ZnS have hundreds of polymorphs with different packing orders [17, 18].

1.3.2 Different Connecting Modes

These polymorphs have the same coordination polyhedron but different connection modes of these polyhedra. The typical example is TiO2. Rutile‐, anatase‐, and brookite‐TiO2 have the same TiO6 coordination octahedron (Figure 1.3) [19]. Although their connection mode shares edge and corner together, rutile‐, anatase‐, and brookite‐TiO2 share 2, 4, and 3 edges, respectively, among which rutile‐TiO2 is the most stable material [20].

As the energy to alter connection mode is larger than that for packing order, the number of this kind of polymorphs is less than that of previous ones.

1.3.3 Different Coordination Number

Different coordination number indicates the formation or breaking of chemical bonds, which often involves a large amount of energy [21]. Therefore the number of this kind of polymorphs is small.

Figure 1.3 The crystal structures of (a) rutile‐, (b) anatase‐, and (c) brookite‐TiO2.

Figure 1.4 The crystal structures of (a) α‐ and (b) γ‐Fe.

The typical example of this kind of polymorphs is iron (Fe). The α‐ and γ‐Fe have body‐centered cubic and face‐centered cubic phases (Figure 1.4), with the coordination numbers of 8 and 12, respectively [22].

1.3.4 Different Kinds of Chemical Bonds

There are only a few examples for this kind of polymorphs. For example, α‐ and β‐Sn are formed with covalent bonds and metallic bonds, respectively (Figure 1.5) [23]. Another example is carbon, where graphite has a mixture of covalent bonds and van der Waals bonds, while diamond is composed of pure covalent bonds (Figure 1.6) [24, 25].

Figure 1.5 The crystal structures of (a) α‐ and (b) β‐Sn.

Figure 1.6 The crystal structures of (a) graphite and (b) diamond.

1.3.5 Order and Disorder Polymorphs

Order and disorder polymorphs generally exist in alloys and metallic compounds. At low temperatures, different kinds of atoms were arranged orderly to form crystals with low symmetry; and at high temperature, these atoms occupy the positions disorderly to obtain crystals with high symmetry [26].

For example, FeAl alloy is a simple cube at low temperatures and becomes a body‐centered cube at high temperatures (Figure 1.7) [27, 28].

1.3.6 Molecular Thermal‐Motion‐Related Polymorphs

Temperature also has a significant effect on crystals with ionic groups [29]. As the temperature rises, the thermal vibration of ionic groups becomes so obvious that these groups may rotate freely showing spherical symmetry, leading to the high symmetry of crystal [30]. For example, NaCN and KCN have low symmetry at low temperatures and form rocksalt structure at high temperatures [31].

Figure 1.7 The crystal structures of FeAl alloy at (a) low and (b) high temperatures.

Organic compound C29H60, whose carbon atoms are connected by the chain‐like mode, belongs to the orthorhombic system at low temperatures [32]. As the temperature increases, the molecule rotates around the long axis and has cylindrical symmetry, leading to hexagonal crystal system.

1.3.7 Spin‐Related Polymorphs

The polymorphs have the same in X‐ray diffraction patterns. Yet, neutron diffraction can discover their differences due to the spin variation.

For example, α‐Fe becomes β‐Fe when temperature is over 770 °C [33]. Although both of them...