Introductory Chapter: Structure-Processing-Properties Relationships in Stoichiometric and Nonstoichiometric Oxides

1. Introduction

The research in the area of stoichiometric and nonstoichiometric compounds has grown considerably in the past decades forming an emerging scientific issue with a great impact in the materials science. According to the law of definite proportions, stoichiometric compounds (also referred to as daltonides) are chemical compounds in which atoms are combined in exact whole-number ratios. By contrast, nonstoichiometric compounds (also known as berthollides) are chemical compounds deviating from stoichiometry, and therefore their elemental composition cannot be represented by a ratio of well-defined natural numbers [1, 2, 3, 4]. The nonstoichiometry occurs most often in solids due to defects in the lattice of their crystalline structures, and it is most common in the transition metal oxides, but also the group of nonstoichiometric compounds includes nitrides, fluorides, hydrides, carbides, metal sulfides, tellurides, and so on [2, 4, 5, 6, 7, 8].

The focus of the present book is on metal oxides, which present a large diversity of electrical, magnetic, optical, optoelectronic, thermal, electrochemical, and catalytic properties, making them suitable for a wide range of applications including sensors, solid-state electronic devices, thermoelectric power generation, and energy harvesting. This richness of properties is owed to the oxides’ structure flexibility (especially of transition metal oxides) that makes them easily distort/adapt to the relative sizes of the ions forming the compound [9]. This implies a large chemical diversity providing for a complex interplay of intrinsic materials properties (related to the constituent elements) and extrinsic (defect-driven) properties (related to the presence of impurities and/or dopants).

Recent developments in the solid-state chemistry motivated by the prospect of new applications with topics such as colossal magnetoresistance, multiferroics, high-entropy stabilization, and superconductivity have uncovered rich complexities [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21] that had not previously been recognized. These studies bring up some important issues that should be taken into account when the development of new multifunctional materials was intended.

The core focus that is implicit in most of today’s studies in this field is firmly on the fundamental understanding of the materials chemistry and microstructures and how they are related to the thermodynamic, electronic, and transport properties of stoichiometric and nonstoichiometric compounds.

Several basic textbooks address this topic in relation to the defect chemistry and nonstoichiometry, many of them emphasizing on oxides [22, 23, 24, 25, 26]. Nonstoichiometric crystals were first interpreted, as regards their structure and their thermodynamics, in terms of the statistical thermodynamics of point defects [1, 2]. At small deviations from the stoichiometric composition, an approach assuming noninteracting and randomly distributed point defects was used [24]. In order to describe the higher deviations from stoichiometry, the interactions between the defect complexes, clustering, and long-range ordering into superstructures had to be taken into account [3], and extended defects structure models were developed. Particularly, in variable valence transition metal oxides, there is a strong coupling between defect structures, charge ordering, and orbital degrees of freedom that impacts property evolution. In fact, the transition from the point defect concept in highly dilute defect systems to extended defects models applicable in highly defective systems reflect the need to better define important phenomena in the real world and to determine to a large extend, the fundamental properties of a large range of advanced materials used in heterogeneous catalysis, fuel cells, sensors, oxygen and hydrogen separation membranes, battery materials, electrochromics and so on.

Searching for novel approaches and major breakthroughs in the materials properties, important factors that influence materials stoichiometry, e.g., the variation of the defect structure under controlled conditions (various kinds of doping, temperature, oxygen pressure), were excellently addressed in a series of books, papers, and reviews, and several key physicochemical descriptors (measured as well as calculated ones) showing a good correlation between stoichiometry, structure, and properties have been described [27, 28, 29, 30, 31, 32, 33, 34, 35].

Providing insights into new possibilities to control and optimize the properties based on the correlation between the thermochemical stability, the preparation routes, and characterization of different oxide-based compositions is also a topic that underpins the development of emergent devices and technology.

Besides the classical synthesis approaches based on high-temperature synthesis (solid-state reaction, thermal decomposition, high-temperature/high-pressure preparation) or on electrochemical methods (anodic electrocrystallization, direct current electrolysis), new materials synthesis techniques have evolved, such as mechanosynthesis, microwave hydrothermal synthesis, and atomic layer deposition [4]. In addition, advances in materials synthesis techniques, such as molecular beam epitaxy, reflection high-energy electron diffraction (RHEED)-assisted growth, ion implantation, or nanopatterning of defects by focused ion beams, allow the production of materials with controlled concentrations of point or planar defects and create interstitial doping and local strain fields that can enable patterning of circuits and magnetic domains [13, 36, 37, 38, 39, 40, 41, 42, 43].

One of the challenging problems related to the understanding and practical exploitation of the enhanced properties of nanocrystalline materials is the thermal stabilization of a nanoscale grain size. The thermal stability of these microstructures is essential for adopting nanocrystalline materials in commercial processes and applications. Because the refinement in grain size is accompanied by a significant increase in volume fraction of grain boundaries, the thermal stability involves not only the stability of the grain structure, i.e., microstructure, but also the stability of the structure of the grain boundaries in nanocrystalline materials [44]. The lowering of interfacial energy with grain refinement and lattice strain in nanometer-sized crystallites plays an important role in controlling grain size stability during the grain growth in nanocrystalline phases [45, 46].

In parallel with the development of the synthesis methods, the characterization of different oxides based compositions is a crucial issue. It was argued that, due to the particularities of nonstoichiometric compounds, only after a thorough analysis of the composition, structure, and properties one can conclude that the compound is a nonstoichiometric compound rather than a stoichiometric compound [4]. There is a complex task for which a combination of different methods is required. The results of the classical methods for the composition analysis, e.g., iodometry, cerimetry or electroanalysis methods, should be correlated with the crystallographic structure and microstructure information coming from application of X-ray diffraction techniques, neutron diffraction, high-resolution electron microscopy (HRTEM), laser Raman spectroscopy (LRS) and electron paramagnetic resonance (EPR), together other advanced technique imaging at the atomic level and allowing a detailed study of local defect structures and chemistry, e.g., scanning transmission electron microscopy (STEM) and in situ electron energy loss spectroscopy (EELS) [47, 48, 49, 50, 51, 52]. In addition, many advances have come by measuring the physical properties such as electrical conductivity, magnetic, optical, and optoelectronic properties that strongly depend on stoichiometry and on types and concentration of the defects.

The oxide microstructure modification by using different synthetic methods and the modification of various compositional variables such as the nature and concentration of donor- or acceptor-type dopants are essential for obtaining optimum electrical and transport characteristics. Heat treatment is also an important step not only to ensure stability but also to control structural defects and grain size, also contributing to sensitivity and selectivity of the new materials. Previous reports on the substituted perovskites indicate that the mismatch at the A and B sites in the ABO₃ structure creates strain on grain boundaries which affect not only the electrical but also the thermodynamic properties [9], phase stability, and oxygen stoichiometry [53, 54]. It was also pointed out that the remarkable behavior of the multiferroic and magnetoresistive materials, as well as of the mixed ionic-electronic conducting ceramic membranes obtained by substitution of A and B sites, could be explained not only qualitatively by the structural changes upon doping but also by the fact that the thermodynamic properties are extremely sensitive to the chemical defects in oxygen sites [55, 56, 57]. An interesting relationship between the energetics of growth film conditions and the subsequent materials properties was observed when the pulsed laser deposition (PLD) was used in the synthesis of complex oxide films. Variations in the energetics of growth process can enable fine-tuning and control stoichiometry, dielectric response, thermal and electrical conductivity of films and heterointerfaces [58, 59].

The role of the energetic parameters in understanding the physical- and chemical-modified properties associated with the rise of the surface/volume ratio at a nanometer scale is also a topic of paramount importance [60]. Shifts in thermodynamics at the nanoscale and the strong interplay between the thermodynamic properties and electrical and structural characteristics in the hydrothermally prepared perovskite materials have been revealed [61]. In addition, at the nanometric scale, a large variety of morphologies and related surface properties can exist for the same metal oxide. This means that a great deal of attention must be turned to the energetic parameters which play an important role in the overall properties and behavior of materials. Exploring the relationships between morphology and thermodynamic properties of nanocrystalline BaTiO₃, it was shown that the enhancement of the dielectric properties for the BaTiO₃ hydrothermal-prepared powders with 1D morphology, comparatively with nanocubes or hollow-type morphologies, is strongly correlated with the increase in the binding energy of oxygen in the perovskite structure [62, 63].

Computational approaches, such as DFT-based calculations, phase-field modeling, molecular dynamics (MD) and Monte Carlo simulations) integrated at different stages of materials development have demonstrated to be important tools to address the complexity of based oxides stoichiometric and nonstoichiometric compounds and provide information on phase competition and stability, defect dynamics and kinetics and so on. Thermodynamic Databases, such as CALPHAD (CALculation of PHAse Diagrams) or Databases including high-throughput DFT calculations, read-across and QSAR approaches, together with machine learning platforms have been developed contributing to the prioritization and screening of materials properties for applications as electronics, fuel cells, multiferroics, piezoelectrics, magnetocalorics, thermoelectrics [51, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73].

Critical advances in discovery and design of next-generation materials are expected by applying the concept of the Materials Genome Initiative (MGI) [74, 75] that tightly integrates high-throughput experiment (including both synthesis and characterization), theory, and computation. The development of advanced materials by using emerging synthetic and processing approaches should be based on the understanding of all factors affecting the reliability of these materials for specific applications (including a thorough thermodynamic analysis). The development of robust experimental methods to quantify microstructure and interfaces and to identify descriptors that strongly correlate with rearrangement dynamics on multiple lengthscales is also necessary. Multiscale and lifetime modeling should be accompanied by designing new tools and data analytics enabling high-content analysis and automated data evaluation and thus increasing the ability to understand and tailor the physical properties of materials. The strong interplay between these components offers great opportunities to establish support at the different tiered workflows directed toward emergent applications.

Driven by this concept, we have chosen in this book to concentrate on a limited number of chemical systems that exemplify the complex bridging between materials structure, synthesis, and properties. A special focus is on the role of thermodynamic parameters on the stabilization of the phase and physical properties in oxides. Various methods of synthesis are employed, each of these methods leaving their own mark on the properties of the resulting materials. The strong structure-processing-property relationship is emphasized in each of the chapters of this book, as can be seen from the brief overview of the main topics developed in these chapters: (i) The synthesis and complex characterization of a transitional metal oxide extensively used in industry, e.g., nickel oxide, are discussed. The understanding of the conditions of synthesis effect on the degree of nonstoichiometry provides clues for controlling the properties evolution. (ii) The stoichiometry and nonstoichiometry from crystal structure point of view are introduced along with some examples relevant for the importance of nonstoichiometry in the application - oriented research. Several advanced techniques available to ascertain stoichiometry are presented with a special emphasis on neutron diffraction techniques. Finally, important results obtained using neutron diffraction and scattering in identifying the structural modification which leads to superconductivity in the compounds are described. (iii) Particular aspects of the thermodynamic concepts related to associated phase equilibria in oxides exhibiting variable stoichiometry are emphasized. Insights into the equilibrium studies and construction of thermodynamic models of nonstoichiometric phases with application in high temperature superconducting materials are providing. (iv) The scientific and technological importance of the stoichiometry variation in the lead-free perovskite-structure materials, such as SrTiO₃ (ST) and KTaO₃ (KT), pure or modified, are defined. The strong relationship between the grain growth, the Sr/Ti or K/Ta ratio, the phase structure, morphology and dielectric response of ST and KT ceramics is overviewed. (v) The strong correlation between structure, nonstoichiometry and thermodynamic properties of some mixed conducting perovskite-type oxides Ba_xSr_1-xCo_1-yFeyO_3-δ (BSCF) studied as potential high-performance solid oxide fuel cells cathode materials is discussed and the effect of A- and B-site dopants concentration and of the oxygen stoichiometry change on the thermodynamic stability and morphology of the BSCF samples was evidenced.

We hope the approach adopted on this book would give an account about the significance of the structure-processing-property relationship in stoichiometric and nonstoichiometric compounds as an important issue for both scientific and applicative reasons.