Solubility in water of an atomic crystal lattice. Crystal cell
There are two types of solids in nature, which differ markedly in their properties. These are amorphous and crystalline bodies. And amorphous bodies do not have an exact melting point; during heating, they gradually soften and then pass into a fluid state. An example of such substances is resin or ordinary plasticine. But the situation is completely different with crystalline substances. They remain in a solid state until a certain temperature, and only after reaching it do these substances melt.
It's all about the structure of such substances. In crystalline solids, the particles of which they are composed are located at certain points. And if you connect them with straight lines, you get some kind of imaginary frame, which is called a crystal lattice. And the types of crystal lattices can be very different. And according to the type of particles from which they are “constructed,” lattices are divided into four types. These are ionic, atomic, molecular and
And at the nodes, accordingly, ions are located, and there is an ionic bond between them. can be either simple (Cl-, Na+) or complex (OH-, SO2-). And these types of crystal lattices may contain some metal hydroxides and oxides, salts and other similar substances. Take, for example, ordinary sodium chloride. It alternates negative chlorine ions and positive sodium ions, which form a cubic crystal lattice. Ionic bonds in such a lattice are very stable and substances “built” according to this principle have fairly high strength and hardness.
There are also types of crystal lattices called atomic lattices. Here, the nodes contain atoms between which there is a strong covalent bond. Not many substances have an atomic lattice. These include diamond, as well as crystalline germanium, silicon and boron. There are also some complex substances that contain and have, accordingly, an atomic crystal lattice. These are rock crystal and silica. And in most cases, such substances are very strong, hard and refractory. They are also practically insoluble.
And the molecular types of crystal lattices have a variety of substances. These include frozen water, that is, ordinary ice, “dry ice” - solidified carbon monoxide, as well as solid hydrogen sulfide and hydrogen chloride. Molecular lattices also contain many solid organic compounds. These include sugar, glucose, naphthalene and other similar substances. And the molecules located at the nodes of such a lattice are connected to each other by polar and non-polar chemical bonds. And despite the fact that inside the molecules there are strong covalent bonds between atoms, these molecules themselves are held in the lattice due to very weak intermolecular bonds. Therefore, such substances are quite volatile, melt easily and do not have great hardness.
Well, metals have a variety of types of crystal lattices. And their nodes can contain both atoms and ions. In this case, atoms can easily turn into ions, giving up their electrons for “common use.” In the same way, ions, having “captured” a free electron, can become atoms. And this lattice determines such properties of metals as plasticity, malleability, thermal and electrical conductivity.
Also, the types of crystal lattices of metals, and other substances, are divided into seven main systems according to the shape of the elementary cells of the lattice. The simplest is the cubic cell. There are also rhombic, tetragonal, hexagonal, rhombohedral, monoclinic and triclinic unit cells that determine the shape of the entire crystal lattice. But in most cases, crystal lattices are more complex than those listed above. This is due to the fact that elementary particles can be located not only in the lattice nodes themselves, but also in its center or on its edges. And among metals, the most common are the following three complex crystal lattices: face-centered cubic, body-centered cubic, and hexagonal close-packed. The physical characteristics of metals also depend not only on the shape of their crystal lattice, but also on the interatomic distance and other parameters.
Solids exist in crystalline and amorphous states and are predominantly crystalline in structure. It is distinguished by the correct location of particles at precisely defined points, characterized by periodic repetition in the volume. If you mentally connect these points with straight lines, we get a spatial framework, which is called a crystal lattice. The concept of “crystal lattice” refers to a geometric pattern that describes the three-dimensional periodicity in the arrangement of molecules (atoms, ions) in crystalline space.
The locations of particles are called lattice nodes. There are internodal connections inside the frame. The type of particles and the nature of the connection between them: molecules, atoms, ions determine a total of four types: ionic, atomic, molecular and metallic.
If ions (particles with a negative or positive charge) are located at lattice sites, then this is an ionic crystal lattice, characterized by bonds of the same name.
These connections are very strong and stable. Therefore, substances with this type of structure have a fairly high hardness and density, are non-volatile and refractory. At low temperatures they act as dielectrics. However, when such compounds melt, the geometrically correct ionic crystal lattice (the arrangement of ions) is disrupted and the strength bonds decrease.
At temperatures close to the melting point, crystals with ionic bonds are already capable of conducting electric current. Such compounds are easily soluble in water and other liquids that consist of polar molecules.
An ionic crystal lattice is characteristic of all substances with an ionic type of bond - salts, metal hydroxides, binary compounds of metals with non-metals. has no directionality in space, because each ion is associated with several counterions at once, the strength of interaction of which depends on the distance between them (Coulomb's law). Ionic-bonded compounds have a non-molecular structure; they are solid substances with ionic lattices, high polarity, high melting and boiling points, and are electrically conductive in aqueous solutions. Compounds with ionic bonds are practically never found in their pure form.
The ionic crystal lattice is inherent in some hydroxides and oxides of typical metals, salts, i.e. substances with ionic
In addition to ionic bonds, crystals contain metallic, molecular and covalent bonds.
Crystals that have a covalent bond are semiconductors or dielectrics. Typical examples of atomic crystals are diamond, silicon and germanium.
Diamond is a mineral, an allotropic cubic modification (form) of carbon. The diamond crystal lattice is atomic and very complex. At the nodes of such a lattice there are atoms connected to each other by extremely strong covalent bonds. Diamond consists of individual carbon atoms, arranged one at a time in the center of a tetrahedron, the vertices of which are the four nearest atoms. This lattice is characterized by a face-centered cubic structure, which determines the maximum hardness of diamond and a fairly high melting point. There are no molecules in the diamond lattice - and the crystal can be viewed as one impressive molecule.
In addition, it is characteristic of silicon, solid boron, germanium and compounds of individual elements with silicon and carbon (silica, quartz, mica, river sand, carborundum). In general, there are relatively few representatives with an atomic lattice.
Structure of matter.
It is not individual atoms or molecules that enter into chemical interactions, but substances.
Our task is to get acquainted with the structure of matter.
At low temperatures, substances are in a stable solid state.
☼ The hardest substance in nature is diamond. He is considered the king of all gems and precious stones. And its name itself means “indestructible” in Greek. Diamonds have long been looked upon as miraculous stones. It was believed that a person wearing diamonds does not know stomach diseases, is not affected by poison, retains his memory and a cheerful mood until old age, and enjoys royal favor.
☼ A diamond that has been subjected to jewelry processing - cutting, polishing - is called a diamond.
When melting as a result of thermal vibrations, the order of the particles is disrupted, they become mobile, while the nature of the chemical bond is not disrupted. Thus, there are no fundamental differences between solid and liquid states.
The liquid acquires fluidity (i.e., the ability to take the shape of a vessel).
Liquid crystals.
Liquid crystals were discovered at the end of the 19th century, but have been studied in the last 20-25 years. Many display devices of modern technology, for example, some electronic watches and mini-computers, operate on liquid crystals.
In general, the words “liquid crystals” sound no less unusual than “hot ice”. However, in reality, ice can also be hot, because... at a pressure of more than 10,000 atm. water ice melts at temperatures above 2000 C. The unusualness of the combination “liquid crystals” is that the liquid state indicates the mobility of the structure, and the crystal implies strict order.
If a substance consists of polyatomic molecules of an elongated or lamellar shape and having an asymmetrical structure, then when it melts, these molecules are oriented in a certain way relative to each other (their long axes are parallel). In this case, the molecules can move freely parallel to themselves, i.e. the system acquires the property of fluidity characteristic of a liquid. At the same time, the system retains an ordered structure, which determines the properties characteristic of crystals.
The high mobility of such a structure makes it possible to control it through very weak influences (thermal, electrical, etc.), i.e. purposefully change the properties of a substance, including optical ones, with very little energy expenditure, which is what is used in modern technology.
Types of crystal lattices.
Any chemical substance is formed by a large number of identical particles that are interconnected.
At low temperatures, when thermal movement is difficult, the particles are strictly oriented in space and form a crystal lattice.
Crystal cell is a structure with a geometrically correct arrangement of particles in space.
In the crystal lattice itself, nodes and internodal space are distinguished.
The same substance, depending on conditions (p, t,...), exists in different crystalline forms (i.e., they have different crystal lattices) - allotropic modifications that differ in properties.
For example, four modifications of carbon are known: graphite, diamond, carbyne and lonsdaleite.
☼ The fourth variety of crystalline carbon, “lonsdaleite,” is little known. It was discovered in meteorites and obtained artificially, and its structure is still being studied.
☼ Soot, coke, and charcoal were classified as amorphous polymers of carbon. However, it has now become known that these are also crystalline substances.
☼ By the way, shiny black particles were found in the soot, which were called “mirror carbon”. Mirror carbon is chemically inert, heat-resistant, impervious to gases and liquids, has a smooth surface and is absolutely compatible with living tissues.
☼ The name graphite comes from the Italian “graffito” - I write, I draw. Graphite is dark gray crystals with a weak metallic luster and has a layered lattice. Individual layers of atoms in a graphite crystal, connected to each other relatively weakly, are easily separated from each other.
TYPES OF CRYSTAL LATTICES
Properties of substances with different crystal lattices (table)
If the rate of crystal growth is low upon cooling, a glassy state (amorphous) is formed.
The relationship between the position of an element in the Periodic Table and the crystal lattice of its simple substance.
There is a close relationship between the position of an element in the periodic table and the crystal lattice of its corresponding elemental substance.
The simple substances of the remaining elements have a metallic crystal lattice.
FIXING
Study the lecture material and answer the following questions in writing in your notebook:
- What is a crystal lattice?
- What types of crystal lattices exist?
- Describe each type of crystal lattice according to the plan:
What is in the nodes of the crystal lattice, structural unit → Type of chemical bond between the particles of the node → Interaction forces between the particles of the crystal → Physical properties determined by the crystal lattice → Aggregate state of the substance under normal conditions → Examples
Complete tasks on this topic:
- What type of crystal lattice does the following substances widely used in everyday life have: water, acetic acid (CH3 COOH), sugar (C12 H22 O11), potassium fertilizer (KCl), river sand (SiO2) - melting point 1710 0C, ammonia (NH3) , salt? Make a general conclusion: by what properties of a substance can one determine the type of its crystal lattice?
Using the formulas of the given substances: SiC, CS2, NaBr, C2 H2 - determine the type of crystal lattice (ionic, molecular) of each compound and, based on this, describe the physical properties of each of the four substances.
Trainer No. 1. "Crystal lattices"
Trainer No. 2. "Test tasks"
Test (self-control):
1) Substances that have a molecular crystal lattice, as a rule:
a). refractory and highly soluble in water
b). fusible and volatile
V). Solid and electrically conductive
G). Thermally conductive and plastic
2) The concept of “molecule” is not applicable to the structural unit of a substance:
b). oxygen
V). diamond
3) The atomic crystal lattice is characteristic of:
a). aluminum and graphite
b). sulfur and iodine
V). silicon oxide and sodium chloride
G). diamond and boron
4) If a substance is highly soluble in water, has a high melting point, and is electrically conductive, then its crystal lattice is:
A). molecular
b). atomic
V). ionic
G). metal
Any substance in nature, as is known, consists of smaller particles. They, in turn, are connected and form a certain structure, which determines the properties of a particular substance.
Atomic is characteristic and occurs at low temperatures and high pressure. Actually, it is precisely thanks to this that metals and a number of other materials acquire their characteristic strength.
The structure of such substances at the molecular level looks like a crystal lattice, each atom in which is connected to its neighbor by the strongest connection existing in nature - a covalent bond. All the smallest elements that form the structures are arranged in an orderly manner and with a certain periodicity. Representing a grid in the corners of which atoms are located, always surrounded by the same number of satellites, the atomic crystal lattice practically does not change its structure. It is well known that the structure of a pure metal or alloy can be changed only by heating it. In this case, the higher the temperature, the stronger the bonds in the lattice.
In other words, the atomic crystal lattice is the key to the strength and hardness of materials. However, it is worth considering that the arrangement of atoms in different substances may also differ, which, in turn, affects the degree of strength. So, for example, diamond and graphite, which contain the same carbon atom, are extremely different from each other in terms of strength: diamond is on Earth, but graphite can exfoliate and break. The fact is that in the crystal lattice of graphite, atoms are arranged in layers. Each layer resembles a honeycomb, in which the carbon atoms are joined rather loosely. This structure causes layered crumbling of pencil leads: when broken, parts of the graphite simply peel off. Another thing is diamond, the crystal lattice of which consists of excited carbon atoms, that is, those that are capable of forming 4 strong bonds. It is simply impossible to destroy such a joint.
Crystal lattices of metals, in addition, have certain characteristics:
1. Lattice period- a quantity that determines the distance between the centers of two adjacent atoms, measured along the edge of the lattice. The generally accepted designation does not differ from that in mathematics: a, b, c are the length, width, height of the lattice, respectively. Obviously, the dimensions of the figure are so small that the distance is measured in the smallest units of measurement - a tenth of a nanometer or angstroms.
2. K - coordination number. An indicator that determines the packing density of atoms within a single lattice. Accordingly, its density is greater, the higher the number K. In fact, this figure represents the number of atoms that are as close as possible and at an equal distance from the atom under study.
3. Lattice basis. Also a quantity characterizing the density of the lattice. Represents the total number of atoms that belong to the particular cell being studied.
4. Compactness factor measured by calculating the total volume of the lattice divided by the volume occupied by all the atoms in it. Like the previous two, this value reflects the density of the lattice being studied.
We have considered only a few substances that have an atomic crystal lattice. Meanwhile, there are a great many of them. Despite its great diversity, the crystalline atomic lattice includes units that are always connected by means (polar or non-polar). In addition, such substances are practically insoluble in water and are characterized by low thermal conductivity.
In nature, there are three types of crystal lattices: body-centered cubic, face-centered cubic, and close-packed hexagonal.
The structure of matter is determined not only by the relative arrangement of atoms in chemical particles, but also by the location of these chemical particles in space. The most ordered arrangement of atoms, molecules and ions is in crystals(from Greek " crystallos" - ice), where chemical particles (atoms, molecules, ions) are arranged in a certain order, forming a crystal lattice in space. Under certain conditions of formation, they can have the natural shape of regular symmetrical polyhedra. The crystalline state is characterized by the presence of long-range order in the arrangement of particles and symmetry crystal lattice.
The amorphous state is characterized by the presence of only short-range order. The structures of amorphous substances resemble liquids, but have much less fluidity. The amorphous state is usually unstable. Under the influence of mechanical loads or temperature changes, amorphous bodies can crystallize. The reactivity of substances in the amorphous state is much higher than in the crystalline state.
Amorphous substances
Main sign amorphous(from Greek " amorphos" - formless) state of matter - the absence of an atomic or molecular lattice, that is, the three-dimensional periodicity of the structure characteristic of the crystalline state.
When a liquid substance is cooled, it does not always crystallize. under certain conditions, a nonequilibrium solid amorphous (glassy) state can form. The glassy state can contain simple substances (carbon, phosphorus, arsenic, sulfur, selenium), oxides (for example, boron, silicon, phosphorus), halides, chalcogenides, and many organic polymers.
In this state, the substance can be stable for a long period of time, for example, the age of some volcanic glasses is estimated at millions of years. The physical and chemical properties of a substance in a glassy amorphous state can differ significantly from the properties of a crystalline substance. For example, glassy germanium dioxide is more chemically active than crystalline one. Differences in the properties of the liquid and solid amorphous state are determined by the nature of the thermal movement of particles: in the amorphous state, particles are capable only of oscillatory and rotational movements, but cannot move through the thickness of the substance.
There are substances that can only exist in solid form in an amorphous state. This refers to polymers with an irregular sequence of units.
Amorphous bodies isotropic, that is, their mechanical, optical, electrical and other properties do not depend on direction. Amorphous bodies do not have a fixed melting point: melting occurs in a certain temperature range. The transition of an amorphous substance from a solid to a liquid state is not accompanied by an abrupt change in properties. A physical model of the amorphous state has not yet been created.
Crystalline substances
Solid crystals- three-dimensional formations characterized by strict repeatability of the same structural element ( unit cell) in all directions. The unit cell is the smallest volume of a crystal in the form of a parallelepiped, repeated in the crystal an infinite number of times.
The geometrically correct shape of crystals is determined, first of all, by their strictly regular internal structure. If, instead of atoms, ions or molecules in a crystal, we depict points as the centers of gravity of these particles, we get a three-dimensional regular distribution of such points, called a crystal lattice. The points themselves are called nodes crystal lattice.
Types of crystal lattices
Depending on what particles the crystal lattice is made of and what the nature of the chemical bond between them is, different types of crystals are distinguished.
Ionic crystals are formed by cations and anions (for example, salts and hydroxides of most metals). In them there is an ionic bond between the particles.
Ionic crystals may consist of monatomic ions. This is how crystals are built sodium chloride, potassium iodide, calcium fluoride.
Monatomic metal cations and polyatomic anions, for example, nitrate ion NO 3 −, sulfate ion SO 4 2−, carbonate ion CO 3 2−, participate in the formation of ionic crystals of many salts.
It is impossible to isolate single molecules in an ionic crystal. Each cation is attracted to each anion and repelled by other cations. The entire crystal can be considered a huge molecule. The size of such a molecule is not limited, since it can grow by adding new cations and anions.
Most ionic compounds crystallize in one of the structural types, which differ from each other in the value of the coordination number, that is, the number of neighbors around a given ion (4, 6 or 8). For ionic compounds with an equal number of cations and anions, four main types of crystal lattices are known: sodium chloride (the coordination number of both ions is 6), cesium chloride (the coordination number of both ions is 8), sphalerite and wurtzite (both structural types are characterized by the coordination number of the cation and anion equal to 4). If the number of cations is half the number of anions, then the coordination number of cations must be twice the coordination number of anions. In this case, the structural types of fluorite (coordination numbers 8 and 4), rutile (coordination numbers 6 and 3), and cristobalite (coordination numbers 4 and 2) are realized.
Typically ionic crystals are hard but brittle. Their fragility is due to the fact that even with slight deformation of the crystal, cations and anions are displaced in such a way that the repulsive forces between like ions begin to prevail over the attractive forces between cations and anions, and the crystal is destroyed.
Ionic crystals have high melting points. In the molten state, the substances that form ionic crystals are electrically conductive. When dissolved in water, these substances dissociate into cations and anions, and the resulting solutions conduct electric current.
High solubility in polar solvents, accompanied by electrolytic dissociation, is due to the fact that in a solvent environment with a high dielectric constant ε, the energy of attraction between ions decreases. The dielectric constant of water is 82 times higher than that of vacuum (conditionally existing in an ionic crystal), and the attraction between ions in an aqueous solution decreases by the same amount. The effect is enhanced by solvation of ions.
Atomic crystals consist of individual atoms held together by covalent bonds. Of the simple substances, only boron and group IVA elements have such crystal lattices. Often, compounds of non-metals with each other (for example, silicon dioxide) also form atomic crystals.
Just like ionic crystals, atomic crystals can be considered giant molecules. They are very durable and hard, and do not conduct heat and electricity well. Substances that have atomic crystal lattices melt at high temperatures. They are practically insoluble in any solvents. They are characterized by low reactivity.
Molecular crystals are built from individual molecules, within which the atoms are connected by covalent bonds. Weaker intermolecular forces act between molecules. They are easily destroyed, so molecular crystals have low melting points, low hardness, and high volatility. Substances that form molecular crystal lattices do not have electrical conductivity, and their solutions and melts also do not conduct electric current.
Intermolecular forces arise due to the electrostatic interaction of the negatively charged electrons of one molecule with the positively charged nuclei of neighboring molecules. The strength of intermolecular interactions is influenced by many factors. The most important among them is the presence of polar bonds, that is, a shift in electron density from one atom to another. In addition, intermolecular interactions are stronger between molecules with a larger number of electrons.
Most nonmetals in the form of simple substances (for example, iodine I 2 , argon Ar, sulfur S 8) and compounds with each other (for example, water, carbon dioxide, hydrogen chloride), as well as almost all solid organic substances form molecular crystals.
Metals are characterized by a metallic crystal lattice. It contains a metallic bond between atoms. In metal crystals, the nuclei of atoms are arranged in such a way that their packing is as dense as possible. The bonding in such crystals is delocalized and extends throughout the entire crystal. Metal crystals have high electrical and thermal conductivity, metallic luster and opacity, and easy deformability.
The classification of crystal lattices corresponds to limiting cases. Most crystals of inorganic substances belong to intermediate types - covalent-ionic, molecular-covalent, etc. For example, in a crystal graphite Within each layer, the bonds are covalent-metallic, and between the layers they are intermolecular.
Isomorphism and polymorphism
Many crystalline substances have the same structures. At the same time, the same substance can form different crystal structures. This is reflected in the phenomena isomorphism And polymorphism.
Isomorphism lies in the ability of atoms, ions or molecules to replace each other in crystal structures. This term (from the Greek " isos" - equal and " morphe" - form) was proposed by E. Mitscherlich in 1819. The law of isomorphism was formulated by E. Mitscherlich in 1821 in this way: “The same numbers of atoms, connected in the same way, give the same crystalline forms; Moreover, the crystalline form does not depend on the chemical nature of the atoms, but is determined only by their number and relative position."
Working in the chemical laboratory of the University of Berlin, Mitscherlich drew attention to the complete similarity of the crystals of lead, barium and strontium sulfates and the similarity of the crystalline forms of many other substances. His observations attracted the attention of the famous Swedish chemist J.-Ya. Berzelius, who suggested that Mitscherlich confirm the observed patterns using the example of compounds of phosphoric and arsenic acids. As a result of the study, it was concluded that “the two series of salts differ only in that one contains arsenic as an acid radical, and the other contains phosphorus.” Mitscherlich's discovery very soon attracted the attention of mineralogists, who began research on the problem of isomorphic substitution of elements in minerals.
During the joint crystallization of substances prone to isomorphism ( isomorphic substances), mixed crystals (isomorphic mixtures) are formed. This is only possible if the particles replacing each other differ little in size (no more than 15%). In addition, isomorphic substances must have a similar spatial arrangement of atoms or ions and, therefore, similar crystals in external shape. Such substances include, for example, alum. In potassium alum crystals KAl(SO 4) 2 . 12H 2 O potassium cations can be partially or completely replaced by rubidium or ammonium cations, and aluminum cations by chromium(III) or iron(III) cations.
Isomorphism is widespread in nature. Most minerals are isomorphic mixtures of complex, variable composition. For example, in the mineral sphalerite ZnS, up to 20% of zinc atoms can be replaced by iron atoms (while ZnS and FeS have different crystal structures). Isomorphism is associated with the geochemical behavior of rare and trace elements, their distribution in rocks and ores, where they are contained in the form of isomorphic impurities.
Isomorphic substitution determines many useful properties of artificial materials of modern technology - semiconductors, ferromagnets, laser materials.
Many substances can form crystalline forms that have different structures and properties, but the same composition ( polymorphic modifications). Polymorphism- the ability of solids and liquid crystals to exist in two or more forms with different crystal structures and properties with the same chemical composition. This word comes from the Greek " polymorphos"- diverse. The phenomenon of polymorphism was discovered by M. Klaproth, who in 1798 discovered that two different minerals - calcite and aragonite - have the same chemical composition CaCO 3.
Polymorphism of simple substances is usually called allotropy, while the concept of polymorphism does not apply to non-crystalline allotropic forms (for example, gaseous O 2 and O 3). A typical example of polymorphic forms is modifications of carbon (diamond, lonsdaleite, graphite, carbines and fullerenes), which differ sharply in properties. The most stable form of existence of carbon is graphite, however, its other modifications under normal conditions can persist indefinitely. At high temperatures they turn into graphite. In the case of diamond, this occurs when heated above 1000 o C in the absence of oxygen. The reverse transition is much more difficult to achieve. Not only high temperature is required (1200-1600 o C), but also enormous pressure - up to 100 thousand atmospheres. The transformation of graphite into diamond is easier in the presence of molten metals (iron, cobalt, chromium and others).
In the case of molecular crystals, polymorphism manifests itself in different packing of molecules in the crystal or in changes in the shape of molecules, and in ionic crystals - in different relative positions of cations and anions. Some simple and complex substances have more than two polymorphs. For example, silicon dioxide has ten modifications, calcium fluoride - six, ammonium nitrate - four. Polymorphic modifications are usually denoted by the Greek letters α, β, γ, δ, ε,... starting with modifications that are stable at low temperatures.
When crystallizing from steam, solution or melt a substance that has several polymorphic modifications, a modification that is less stable under given conditions is first formed, which then turns into a more stable one. For example, when phosphorus vapor condenses, white phosphorus is formed, which under normal conditions slowly, but when heated, quickly turns into red phosphorus. When lead hydroxide is dehydrated, at first (about 70 o C) yellow β-PbO, which is less stable at low temperatures, is formed, at about 100 o C it turns into red α-PbO, and at 540 o C it turns back into β-PbO.
The transition from one polymorph to another is called polymorphic transformation. These transitions occur when temperature or pressure changes and are accompanied by an abrupt change in properties.
The process of transition from one modification to another can be reversible or irreversible. Thus, when a white soft graphite-like substance of composition BN (boron nitride) is heated at 1500-1800 o C and a pressure of several tens of atmospheres, its high-temperature modification is formed - Borazon, close to diamond in hardness. When the temperature and pressure are lowered to values corresponding to normal conditions, borazone retains its structure. An example of a reversible transition is the mutual transformations of two modifications of sulfur (orthorhombic and monoclinic) at 95 o C.
Polymorphic transformations can occur without significant changes in structure. Sometimes there is no change in the crystal structure at all, for example, during the transition of α-Fe to β-Fe at 769 o C, the structure of iron does not change, but its ferromagnetic properties disappear.
