The perovskite-like structure of compounds is gaining attention for their potential applications in solar cells. The efficiency of these materials has grown rapidly, and their market potential seems to be near since the photochemical properties of CH3NH3PbI3 perovskite were first observed in 2006. This new “perovskite boom” has surpassed the superconductivity fever of the late 1980s, and the excitement about perovskite-related materials has reached a broader audience.
The study of perovskite-related materials dates back to 1839 when the mineral perovskite was first described. The recent perovskite solar cell “boom” is just one of many that have occurred in the field of perovskite-related materials research. The aim of this chapter is to show the historical evolution of perovskite research from the nineteenth-century mineralogical and crystallographic problem to the present date plethora of potential and real applications of these materials.
Establishing Perovskite Mineral’s Crystal System
(1839–1925)
The discovery of the mineral perovskite is closely related to the famous scientific journey through the Asian part of Russia carried out by Alexander von Humboldt, funded by Tsar Nicholas I. The journey covered around 15000 km, from May to December 1829, with the aims of increasing geological, geographical, and biological knowledge of the vast Russian region. Humboldt invited two of the best known German scientists of the time, Christian Gottfried Ehrenberg for the biological part and Gustav Rose as the expert in geology and mineralogy.
Gustav Rose’s responsibilities during the journey included chemical analyses of potentially new minerals, analysis of Caspian Sea waters, and microscopic examinations of sands and rocks in mines and mineral deposits of the Urals to detect the presence of diamonds, one of the greatest commodities of the time. He was also in charge of writing the expedition’s diary, which was published in two parts, one in 1837 and the other one in 1849. The scientific papers related to his discoveries would appear during the 20 years after the trip (1829–1849).
Among the private collections that Rose visited, the chief of the Mining Pharmacy of Saint Petersburg, August Kämmerer, was of special interest. In 1839, Kämmerer sent a piece of rock collected in the Ural Mountains to the leatter’s laboratory at the University of Berlin (now Humboldt University). After the examination, Gustav Rose confirmed that the crystals belonged to a new mineral with cubic symmetry, composed of calcium and titanium oxides. As suggested by Kämmerer, Rose accepted that the new mineral be named to honor Count Lev Alekseevich von Perovskiy, a well-known aristocrat and mineral collector that would later be an active political member of the Russian government.
Despite being the first to make public the description of the new mineral, Gustav Rose did not determine its exact chemical composition. His brother, Heinrich, in 1844, fully analyzed and deduced the actual chemical composition (CaTiO3), which was then given as a mixture of oxides, CaOTiO2. A more detailed description of the crystal morphology and crystal system was given the next year, in 1845, by Alfred Des Cloizeaux, a well-known French mineralogist of the time who concluded that Rose’s initial description of the crystals as being cubic was correct.
The matter was not a trivial one because in the nineteenth century, two characteristics of a mineral were needed in order to be properly classified and accepted as valid: one of such properties was the chemical composition and the other was its crystal system. The conundrum was only resolved at the beginning of the new century, thanks to the works of Bowman and Böggild. Bowman’s work is remarkable because he had to improve the goniometers of the time to tackle the problems presented with perovskite crystals. Böggild’s work resides on his critical review of all previous data and his own work, which concluded that CaTiO3 mineral had to be orthorhombic.
The Era of X-Ray Exploration and the Conceptual Framework for the Mineral Perovskite. (1925–1950s)
In 1912, the discovery of X-rays and diffraction by thematter revolutionized crystallography, providing chemists, physicists, mineralogists, and material scientists with a tool for investigating the internal structure of crystals. However, when this revolutionary technique was first applied to perovskite crystals, the controversy with its crystal system resurfaced, leading to a new debate among researchers. The first X-ray studies of perovskite crystals were independently made in 1925 by two different groups: one based in Italy, led by Giorgo Renato Levi and his student, Giulio Natta, and the other from Norway, led by Victor Moritz Goldschmidt.
Both groups worked without knowing each other’s existence but deduced the same basic arrangement of atoms in perovskite’s crystals: the proposed structure was represented by a cube with calcium lying at the cube’s center, titanium at the origin of the cube (cell corners), and oxygen atoms at the center of each face. However, they felt disappointed when the X-ray data indicated a cubic structure, as previous research had concluded that perovskite crystals were orthorhombic. Doubts initially focused on the technique, which was considered less sensitive to the true symmetry of the crystals than optical or etching experiments.
In 1936, German Otto Zedlitz, from the University of Tübingen, picked up the thread of these inconclusive works and decided to finish the literature published during the last 100 years. He realized that solving the structure and crystal system of the mineral perovskite was trickier than expected and devoted the next three years to study natural specimens and synthetic perovskite samples. Zedlitz started describing previous works as coming from just beginners and decided to start the crystallographic study from the very beginning: measuring crystal faces and angles to deduce the crystal system.
However, Zedlitz found himself in the same situation as his predecessors: the task of proposing a crystal system of perovskite only from goniometric measurements was extremely difficult because the crystals looked like cubic but some stripes and crystal sections did not obey cubic laws. After performing several optical measurements and various etching experiments with different solvents, he concluded that none of them were reliable enough to solve the symmetry problem of perovskite.
Finally, Zedlitz concentrated on the X-ray diffraction, but this task was not easy either. Perovskite crystals were not X-ray-friendly due to their numerous imperfections and stripes. From all his measurements, he was only able to say that perovskite’s crystal structure wasn’t cubic and could not find any systematics in the twining laws. He finally proposed a monoclinic unit cell, which he did not reconcile with Böggild axis ratios and was essentially the same unit cell that Levi and Natta had proposed.
The structure of perovskite was initially thought to be a distorted version of the ideal one proposed by Barth in 1925. However, the cause of the distortion remained unsolved until 1943 when Hungarian crystallographer István Náray-Szabó proposed a plausible explanation from his study of single crystals from the National Museum of Hungary. He proposed that perovskite’s structure should be considered as composed of a three-dimensional arrangement of corner-sharing TiO6 octahedra with Ca occupying the resulting dodecahedral voids. This idea, missed for years, is still correct and is still correct.
The majority of single-crystal X-ray determinations and neutron diffraction measurements were carried out mostly on synthetic CaTiO3 crystals. Detailed and definitive single-crystal studies on natural samples had to wait until the end of the 1990s. Most of the twinning laws were also completely determined by that decade. Modern techniques demonstrated that those laws had been correctly addressed by some of the earliest workers, such as Bowman in 1908 or even Kay and Bailey in 1957, although others were only determined after the use of modern techniques such as transmission electron microscopy (TEM).
The identification of most defects affecting the structure that so much had exasperated early researchers was also concluded in this period. Their complexity was finally demonstrated by a multicultural group of geologists from the University of California (USA). This led to the discovery of perovskites having the habit of doing the unexpected, which was later demonstrated by a multicultural group of geologists from the University of California (USA).
A subsequent work by the outstanding English crystallographer Helen Dick Megaw proved that Náray-Szabó had slightly underestimated Perovskite’s symmetry. Megaw’s work came at a time when other oxides with a crystal structure similar to the mineral perovskite were being investigated by hundreds of researchers around the world for their recently discovered ferroelectric properties. Megaw’s participation was crucial and her work would later become a reference for anyone working on perovskite-type compounds.
In 1957, more than 100 years after perovskite discovery, Kay and Bailey published their thesis results on synthetic and natural perovskite specimens. With some minor corrections, their structure proposal is still assumed as correct. They recognized that the task had not been easy for the preceding researchers due to the omnipresent twining in most of the specimens. By the time Kay and Bailey unambiguously determined perovskite’s crystal structure, the number of synthetic compounds found to crystallize in the same type of structure was so high that the word “perovskite” no longer remained associated to the mineral itself but to its crystal structure type.
The Perovskite Structure Emerges in Materials Chemistry (1930–1950s)
World War I marked the first major conflict involving chemical weapons. Post-war, countries recognized the importance of the chemical industry in their economies and their chances of victory in major conflicts. Governments sought to control raw materials for chemical industries and implemented taxes on chemicals and raw materials to favor their industries. This led to a policy of exploitation and control of the chemical industry.
During WWI, Norway was a major exporter of calcium carbide, used to produce acetylene for welding metals, and the United Kingdom was an important buyer. After the war, the United Kingdom imposed a new tax on the import of organic chemicals, including calcium carbide, which put at risk the Norwegian economy. This led to a lawsuit between both governments, as they did not agree that the carbide should be considered an organic chemical but rather an inorganic one.
Victor Moritz Goldschmidt, a well-known petrologist, was chosen to support Norway’s interests in the lawsuit. Goldschmidt proved to the British authorities that calcium carbide was a salt on the basis of its crystal structure, leading to the removal of the tax. In recognition, the Norwegian government equipped Goldschmidt’s laboratory with an X-ray diffractometer, which he started a revolution in the field of crystal chemistry.
With this new instrument, he and his working group (which included later well-known crystallographers and crystallochemists such as Thomas F. W. Barth, Gulbrand O. J. Lunde, Ivar W. Oftedal, and F. William Hurder Zachariasen) started studying the geochemical distribution and technical properties of economically important raw materials. By 1926, they had elucidated and rationalized the structure of at least 200 compounds of most known elements. Several AMO3 oxides were among those studied.
Goldschmidt’s group adopted and greatly expanded the idea of the radius ratios between ions that had been proposed a few years earlier by Gustav F. Hüttig for coordination compounds. They were able to classify into different structure types most of the AaMmOx oxides known. This groundbreaking idea is still in use today. The name “perovskite” would be used both for the mineral itself and its structure type.
The results of their investigation were published from 1922 to 1926 in a series of nine monographs under the common title “Geochemische Verteilungsgesetz der Elements.” William Zachariasen expanded the data of all known AMO3 oxides and classified them into four groups based on their B-site coordination and on the rB/rO and rA/rO ratios.
A similar but less thorough work was carried out by the group of the future 1963 Chemistry Nobel Prize Giulio Natta. Their work demonstrated that the structure of the mineral perovskite could also be observed in ABX3 compounds regardless of the nature of the X atom.
The increase of compounds classified as having the perovskite structure came parallel to the improvements of X-ray techniques, which posted a new problem. The structural model associated with the mineral perovskite, which was taken as a reference or ideal model for similar compounds, was cubic but the mineral was orthorhombic. Crystallographers and chemists working on ABX3 compounds increasingly realized that the X-ray diffraction patterns of most of them were not cubic. Consequently, the appropriateness of the name “perovskite” for such compounds was put in doubt: if the mineral perovskite was not cubic, its name should not be used for cubic structures.
Perovskite, a structural group with a cubic structure, was proposed as a name for cubic “perovskite” compounds by well-known crystallographers like Giulio Natta in 1928. However, this proposal was not accepted due to the fact that the first cubic perovskite described was that of KMgF3, discovered and published in 1925. Ralph Wyckoff, another important crystallographer, disagreed with the classification of perovskite structural models within the E21 group of cubic structures in Strukturbericht, the reference work on crystal structures of the time.
In 1934, Waldbauer and McCan proposed three options for using the term “perovskite structure” for mineral perovskite, discontinuing its use, or using the term “perovskite pattern” for pseudo-cubic powder diffraction patterns. This discussion crossed the field of crystal chemistry and became a material’s chemistry problem. The first commercially available pigment was Lead titanate (PbTiO3), commercialized as Titanox L in the USA. The first patents for an ABO3 compound for its use as a pigment were for CaTiO3, the artificial version of the mineral.
Barium titanate (BaTiO3) and lead titanate (PbTiO3) patents followed, with the lead compound initially developed by the Titanium Pigment Company in the United States. It gained extensive use in Europe in the 1940s and 1950s but was later banned for its toxicity associated with its lead content.
The term “perovskite structure” was used to describe cubic and non-cubic ABX3 compounds without distinction. This led to debates about the name for distorted phases, cubic compounds, and the group that would retain the name of perovskite. Some crystallographers did not propose alternatives but noted the incongruency, such as Otto Zedlitz in 1936 and in all of his later works. Others went further, such as Cole and Espenschied, who found that PbTiO3 had a tolerance factor within the range of the perovskite structure but its powder diffraction pattern was neither cubic nor similar to other distorted phases.
Itsván Náray-Szabó proposed a reasonable solution to the conflicting views in 1943. He recognized that the name perovskite was being used for many ABX3 compounds that did not show the ideal structure. Instead of disproving the perovskite name for all those compounds, he decided to classify them into different subgroups and proposed the concept of “sister structures” (Schwesterstrukturen in the original German). Within these sister structures, he included all known perovskite-like compounds within this supergroup, including compounds with BX3 formula, such as ReO3 and ScF3, with different degrees of distortion.
Náray-Szabó’s concept of Sister Structures was readily accepted, although some concerns existed around the suitability of the term “sister.” Harold Pierce Rooksby favored the term “pseudo-isomorphous,” but the debate did not last long and was ended by Helen Megaw. She favored the term “homotypic” over “pseudo-isomorphous” and the term ideal perovskite type (she would later use “aristotype”) for the undistorted phase. She also considered that the cubic structure could be referred to as the “ideal perovskite type,” as it is customary nowadays.
The appropriateness of the naming was resurrected again in the 1970s by Muller and Roy, who preferred to use the term “[perovskite] structure” when referring to the structure and just “perovskite” when the actual mineral is mentioned. The rationalization of the distortions observed in “simple perovskites” came later (at the beginning of the 1970s) as a result of the intense work carried out by Megaw and his postdoc student, Mike Glazer. These studies resulted in two of the most cited papers in the perovskite world and were based on the detailed analysis of the idea planted by Naray-Szabó: many of the perovskite distortions are a result of oxygen displacements considering that they form part of a rigid BX6 octahedron which adapts, by cooperative tilting, to the space left by the A-cation. Glazer produced a notation to describe these rotations, which has been refined over time and is still used by those studying the ABX3 compounds of the perovskite family.
Perovskites Rise to Prominence (1940–1950s)
The identification of remarkable dielectric characteristics in perovskite oxides, especially the ferroelectricity observed in barium titanate, is somewhat obscured, with multiple research groups globally recognised as having made concurrent discoveries. The uncertainty appears to stem from the urgent circumstances facing the globe: the onset of World War II loomed ahead, and any inquiry that could provide a strategic edge or potentially benefit the adversary was placed under strict publication restrictions. During the late 1930s and early 1940s, the scrutiny surrounding dielectric research made it challenging to determine the exact onset of studies that would eventually be published.
The remarkable properties of titanates were uncovered in the United States. Insulating materials known as dielectrics can be temporarily polarised when an external electric field is applied. In the era surrounding World War II, capacitors played a vital role in electric devices like radio transmitters, which were essential during the war. Prior to and throughout World War II, the capacitors used were made from sheets of mineral mica, while the most advanced versions utilised titanium dioxide (TiO2, titania). Nonetheless, the dependability of the initial models was lacking, and the capacitance along with the dielectric constant of the subsequent ones was rather low. As a result, the advancement of innovative dielectric materials became essential for any warring nation.
Amidst the turmoil of the world, Eugene Wainer and his team at the Titanium Alloy Manufacturing Company of New York made a significant discovery regarding the properties of barium titanate. In 1940, Wainer held the position of head at the chemical and physical research laboratory. A few years ago, Wainer focused on enhancing titanium dioxide dielectrics and discovered that incorporating alkaline earth titanates into titanium dioxide significantly boosted its dielectric properties. On April 27, 1939, Wainer and Norman R. Thielke submitted a patent (granted in 1942) asserting that the dielectric properties of titania-based dielectrics are significantly enhanced by incorporating a small amount of barium, strontium, or calcium titanate, or any combination of these, into a titanium dioxide foundation. The top choices among these are barium titanate and strontium titanate.
As certain patents emerged, numerous papers and reports from various regions, including the United Kingdom, Russia, and Japan, underscored the significance of Barium Titanate as an innovative dielectric material for capacitors. The revelation of these properties transformed the electronics industry, as the potential uses of barium titanate in the 1950s appeared limitless, leading to a surge in researchers embarking on new studies of its characteristics.
From the Idea of “Sister Structures” to the True Definition of “Perovskite-Type Structure” (1940s—Present)
The ferroelectric qualities of BaTiO3 and similar compounds were found. This caused a boom in the field of perovskite-type compounds, which are now a popular topic in solid-state and materials chemistry. To fully explain the ferroelectric and piezoelectric properties of these oxides, scientists had to fully understand the perovskite structure, including how the atoms in the unit cell were arranged, how the structure changed when different elements were present, and how the atoms changed positions as the temperature changed.
Because of these careful studies, the idea of the “perovskite structure family” came about. It changed from including all ABX3 phases to including a huge group of compounds that all had the same basic building block, which Naray-Szabó named a corner-shared BX6 octahedron. In the beginning, Náray-Szabó put perovskites and their sister structures into groups based on how they were distorted compared to the ideal cubic structures. He also changed Goldschmidt’s idea of the tolerance factor, which was used to set the limits for how perovskites could be shaped. Using new ionic radius values, he raised the limits from 0.71 to 1.08.
At the start of the 1950s, the ABX3 perovskite sections were better organised. Elizabeth A. Wood’s work was one of the most important in this area. She began studying variation in perovskites because of the different ways that the perovskite structure of two important ferroelectric crystals (KNbO3 and NaNbO3) could be interpreted. She finished by giving a full explanation of all known perovskites. Wood made field maps that showed the areas where a certain type of structure was most common. These maps were more accurate than the tolerance factor at predicting the presence of a perovskite structure for a certain ABX3 formula and even its displacement.
The perovskite structure’s ability to hold different elements and keep the balance between A and B ions was the first thing that showed this could happen. Early studies on the dielectric properties of barium titanate showed that there was a solid solution between BaTiO3 and SrTiO3. Over time, strontium slowly replaced barium in the A position, which changed the dielectric properties. In later research, it was shown that this could also change the features of B-site cations and the compounds that were made if the oxidation states of these cations could be changed because of the A-site replacement.
Some research by G.H. Jonker and J.H. Van Santen at the Phillips Research Laboratories in the Netherlands on perovskite manganites with the general formula Ln1−xAxMnO3 showed that there was a solid solution for all x values. They were able to change their magnetic properties, electrical conductivity, and structural distortion by changing x. For some x values, these oxides showed that there was a change from a ferromagnetic phase that conducted electricity to a paramagnetic phase that did not.
Great scientists like Zener, Goodenough, Anderson, and Kanamori came up with new physical ideas to explain things like superexchange and double exchange. Pierre Gilles de Gennes, who would later win the Nobel Prize in Physics, helped to build these ideas.
In the 1950s, synthetic studies on perovskites kept going, and new field maps were made. Roy and Keith’s research was especially impressive because they made a lot of compounds with more complicated chemical makes-ups. These compounds showed that the perovskite structure could handle almost any mix of A and B-cations in the A- and B-sites. This let material scientists look into all the qualities that could come from the endless combinations of ions on the A, B, or even X places. This led to new and possible uses for perovskite materials.
The idea of a perovskite-type structure has changed over time as a result of many studies. Between 1951 and 1953, Steward and Rooksby made a big step in the right direction. They realised that most of the compounds they had made with the formulas A3BX6 and A2BB’X6 were so similar that Naray-Szabó’s idea of calling them “sister-structures” seemed like a great way to describe them. Francis Galasso’s reading of the passage above made it possible to include most of the known A3BX6 and A2BB’X6 molecules in the perovskite family. Ralph Wyckoff, on the other hand, only put the Cs2BB’Cl6 halides (A = Cs, B and B’ = Au or Ag) in the perovskite group and not the ammonia-based ones. The ammonia-based ones were put in a separate group called “Ammonium ferric hexafluoride, (NH3)3FeF6, and closely related structures,” which included structures related to the mineral elpasolite, K2NaAlF6.
This separate and strange grouping was likely affected by people who found later combinations but never said anything about it. The first group was led by Linus Pauling, who would later win the Nobel Prize in Chemistry. In his earlier work at the California Institute of Technology, they had already noticed that the structure of the A2BB’X6 halides is similar to that of the mineral perovskite. However, they did not suggest that the two were similar to that of ammonium halides because they had already explained those differences in 1924. In the late 1960s and early 1970s, well-known books that talked about perovskite problems finally put them in the perovskite family.
The main thing that makes an A2BB’X6 molecule different from a regular ABX3 perovskite is that the cations in the B-site are arranged in a rock-salt-like way.
The most clear “sister structure” that wasn’t there was that of the perovskites that didn’t have enough B-sites, which can be written as A2B□O6 where □ stands for an empty crystalographic point. Linus Pauling came up with the idea first in his 1924 work on ammonium salts and again in 1928 when he looked into the structure of K2PtCl6. The B-site vacancy structures were probably not added to the perovskite family until Lewis Katz and Roland Ward did so in the mid-1960s. This was likely because these papers came out early.
More progress was made in the perovskite family thanks to Katz and Ward’s work. They looked at how A.F. Wells described many artificial crystal structures using the different types of packing spheres. They thought that the perovskite structure could be explained by both a corner-shared BX6 octahedra network and AX3 layers that were close together. This new group of structures is now called the hexagonal branch of the family. It is made up of a huge number of chemicals that stack in different ways.
In 1953, Balz and Plietz found that K2NiF4 was an intermediate phase in the KF-NiF2 system. It had a structure type related to the perovskite that had not been known before. This structure might be made up of a perovskite layer made up of BX6 octahedra that share edges and a close-packed KF layer. Oxides also used this structure, but it was more similar to the new oxide Sr3Ti2O7, which had a more difficult formula than the old one.
It was discovered by the British Ceramic Research Association that both shapes could be made from a general formula: An+1BnX3n+1, where n is the number of perovskite layers and AXE layers that are close together. As of now, compounds with the general formula An+1BnX3n+1 belong to the Ruddlesden-Popper branch of the perovskite family. This formula can also be written as (AnBnX3n)AX to emphasise the close connection with the perovskite formula.
In the 1960s, ferroelectric qualities were found on some members of this family. This sped up study into the family and led to the creation of several compounds of this type. Georg Bednorz and Alex Müller won the Nobel Prize in Physics in 1987 for finding high-temperature superconductivity in the twisted La1−xBaxCuO4 oxide. This discovery made these compounds popular all over the world.
A few years ago, Bengt Aurivillius, an expert in bismuth-related oxides from the University of Stockholm in Sweden, found a closely similar layered structure. It was found by Aurivillius that the oxides CaNb2Bi2O9 and Bi4Ti3O12 were made up of layers of perovskite that were divided by layers of bismuth oxide. It’s possible to write these new phases as (Bi2O2)(Bin−xBnO3n+1), where n is the number of perovskite layers that are in between the “bismuth oxide” layers.
The Aurivillius stages are still mostly interested in dielectric materials because lead-based dielectrics need to be replaced for health and environmental reasons.
After Bednorz and Müller’s finding in 1986, there was a second “boom” in perovskite study. This time, many oxides that were superconductors above the temperature of liquid nitrogen were found. There is another type of perovskite that is linked to the oxygen-deficient YBa2Cu3O7−d oxide. This can be thought of as a perovskite phase with ordered oxygen holes. There was a new branch added to the family a long time ago. Roland Ward’s group at the Polytechnic Institute of Brooklyn, New York, noticed that the perovskite structure could also hold anion gaps. They made a big finding in their research: they found that the metals’ ability to withstand different amounts of oxygen holes led to a changeable mixed valence in the B-site, which made them useful as catalysts.
The perovskite family grew a lot because Mössbauer spectroscopy made it possible to study iron compounds. In this decade, research on negatively charged SrFeO3−x oxides (0 < x < 0.5) oxides led to the discovery that oxides with the brownmillerite structure (A2BB’O5 formula) were actually a subgroup of the perovskite family, with the oxygen gaps arranged in a certain way. Later, a plan for how those openings should be filled would be made.
When the 1980s came around, the perovskite family had grown so big that it was hard to tell many of its members apart from the structure that Thomas Barrett had suggested for perovskite more than 50 years before. We really needed a new classification. There had been some attempts to group the perovskite structure into different categories before, but this reference book wasn’t well known. Donald Smyth from Pennsylvania’s Lehigh University tried to make sense of all the possible outcomes, such as the different options that come up when there are cation and anion gaps.
The best and most well-known way to classify and explain the perovskite structural family for manmade oxides was by Chintamani N.R. Rao and Bernard Raveau, who worked on it in 1995 and then updated it in 1998. But Kirill S. Aleksandrov and his colleagues at the Kirenskii Institute of Physics in Russia probably have the best explanation of the perovskite family, including both current and future members. From 1997 to 2001, he wrote a number of papers explaining all the different members of the family and how their structures changed. These papers used two different approaches: they looked at all the possible structure derivations from a polyhedral view of the perovskite (the three-dimensional network of corner-sharing BX6 octahedra) and they thought about how the perovskite structure came from the close packing (cubic or hexagonal) of AX3 units, which Katz and Ward had already done.
It was “probably impossible to say, at this point,” how many substances belong to the perovskite family, according to Aleksandrov’s work from 2001. This is because all the parts of the “perovskite family tree,” as he called it, could have mixtures of cations and anion in each position.
“Hierarchies of perovskite crystals,” a work he wrote with B.V. Beznosikov in 1997, used a system of windows to organise all known and newly suggested perovskite family members. The paper was an outline of their book, which didn’t get much attention around the world. Their description is the most complete perovskite “family tree” to date, even though it is hard to see. The classification is like the one in Roger Mitchell’s more well-known book, though it is less well-illustrated and has fewer sources. Mitchell’s book on the perovskite family is still the most important one that has been written so far. It is also a Roger Mitchell study that is the best collection of native perovskites.
From the 1970s to Now
In the late 1960s, there were so many publications and study groups working with perovskite-type substances that the data had to be put into large-scale studies in order to be understood. In his 1969 work called “Structure, Properties, and Preparation of Perovskite-Type Compounds,” Francis Galasso wrote about many different aspects of perovskites, such as their crystal structures, electrical conductivity, dielectric properties, optical properties, catalytic properties, thermal conductivity, and magnetic properties. The book also had steps for making more than 550 different chemicals, ranging from oxides to nitrides.
At that time, the insulating properties of perovskites were the most talked-about subject. However, Galasso’s book wasn’t an encyclopaedia and didn’t cover all the work that had been done in this area. At the same time, volume 3 (group III) of the Landolt-Börnstein series, a big reference book on ferroelectric materials, came out. It had information on over 400 compounds and thousands of references, with a lot of them being about oxides from the perovskite family.
The area of ferroelectrics grew very quickly, and perovskites are now the subject of about 40% of all papers released each year. Because of this unchecked increase, the Landolt-Börnstein book on ferroelectrics had to be updated three times, in 1974, 1981, and 1990.
A type of semiconductor material called perovskites has been studied a lot for their insulating and other qualities. Physicists, chemists, and material scientists were also very interested in their magnetic properties because they could be used in magnetic recording devices and showed complex magnetic phenomena. There is a full guide to the magnetic properties of artificial compounds in the perovskite family that was put together by John B. Goodenough and John M. Longo. It shows the crystal structure of the perovskite and all the magnetic interactions that can happen in it.
So quickly did things change in this field that by 1975, there were almost 2,000 papers on the subject. This work was updated and rewritten in 1978, the 1990s, and at the start of the 2000s. Newer changes were made after it was found in the early 1990s that perovskite manganites with the general formula Ln1−xAx′MnO3 have huge magnetoresistance. Magnetoresistance, which is the ability to change electric resistance by applying a magnetic field, or vice versa, was the basis of magnetic recording media. As a result, perovskite materials could be used for many things, from magnetic recording heads to sensors and actuators.
Once the excitement died down after the high-temperature superconductivity find in the late 1980s, a new “research boom” began. From 1994 onwards, works on this topic published at a rate of nearly 200 papers per year around the world. Even though a lot of work went into it, magnetoresistive perovskites did not find the uses that were hoped for. However, these compounds are still being studied to find new interesting features and possible uses.
People think that the 1970s were the best time to study perovskites in terms of their qualities and possible uses. This decade was when members of the perovskite family showed that the “structure–property” relationship that was thought to be the key to understanding materials science was not at all true. A lot of new and important qualities were found that will be used in future study. These include the first superconducting ABX3 perovskites, their ability to host lasing elements, their new electro-optic properties, and how well they stick to nuclear waste.
Scientists who study rocks and minerals are also having a great time in this decade. Since the 1940s, perovskite crystals have been almost forgotten in the huge amount of research on perovskite-structured chemicals. But a lot of the progress made in perovskite study over the next few years was based on qualities that were found in the 1970s or that were already known. The photochemical qualities of perovskite crystals are what make them stand out. This means that they could be used in the next generation of solar cells.
| Compound | Reference property | Application example |
| BaTiO3 | Ferroelectricity | Multilayer capacitors in almost every electronic device |
| Pb(Zr, Ti)O3 | Piezoelectricity | Piezoelectric transducers |
| Ba1−xSrxTiO3 | Positive temperature dependence of resistivity | P.T.C. Thermistor (used in temperature sensors, fuses, etc.) |
| (Pb1−xLax)(Zr1−xTix)O | Transparent under visible light and piezoelectricity | Electro-optical modulator (used in military pilot goggles) |
| LiNbO3 | Electro-optic effect (change in optical properties under an applied field) | Optical switches (used in lasers and optical fibers) |
| BaZrO3 | High dielectric constant and low dielectric loss in the microwave region | Dielectric resonator in communication devices and antennas |
| Nd3+ doped YAlO3 | Efficient laser Host | Lasers (common in dentistry) |
| KNbO3 | Nonlinear optical properties | Second harmonic generator (the ability to combine the energy of two photons to double the frequency) in lasers |
| Pb(Mg1−xNbx)O3 | Relaxor ferroelectric | Electrostrictive actuator (used in devices where high precision movement is required) |
| YBa2Cu3O7−x | Superconductivity | Thin superconducting wires |
| BaRuO3 | Electrical resistivity | Thin-film resistor (used in old printers) |
| La1−xSrxMnO3 | Mixed ionic electronic conductivity | Solid oxide fuel cell cathode in pre-commercial devices |
| LaCrO3 | Mixed ionic electronic conductivity and stability under oxidizing and reducing conditions | Solid oxide fuel cell interconnecting material in pre-commercial devices |
| PbTiO3 | High refractive index | Paints (discontinued for health reasons) |
| CH3NH3PbI3 | Photochemical activity | Photoactive die for semiconductors used in solar cells (prospective) |
Conclusion
There are a lot of different compounds that make up perovskites. Some have easy formulas, like the material itself, CaTiO3, while others are more complicated, like the ferroelectric Aurivillius compound Ba2Bi4Ti5O18. It was interesting to see how such a wide range of chemicals joined the same family while scientists learnt more about their useful qualities and properties. Each new finding sparked more study into these chemicals, and soon there were more members of the same family. It’s like a circle that never ends. Because the perovskite structure can change to different combinations of elements at each atomic point, they are great for studying a lot of different physical qualities and could also help us find new ones. Crystallography was first used to study a rare material. Since then, it has grown into a huge and interesting area of ongoing research.
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