Types of Crystals

Crystals are usually categorized by the type of interactions operating among the atoms, molecules, or ions of the substance. These interactions include ionic, metallic, and covalent bonds as well as intermolecular forces such as hydrogen bonds, dipole-dipole forces, and London dispersion forces. 

As you know from general chemistry, when two elements, one a metal with a low ionization energy and the other a nonmetal with a highly exothermic electron affinity, are combined, electrons are transferred to produce cations and anions. These 

The formation of sodium chloride. In a hypothetical view the constituent elements ate combined, electrons are transferred, and ionic bonds among the sodium and chloride ions are formed. 

Copyright 1976 Brools/CoL a part of Cengage Learning, Inc. Reproduced by permission, www.cengage.com/ permissions.) 

Electrical conductivity in a metallic lattice. A lattice of cations is held together by a sea of electrons. An electron enters from the left and bumps other electrons down through the lattice until an electron emerges from the right side. (From Ebbing and Gammon, 

The structures and properties of crystalline solids, such as melting point, density, and hardness, are determined by the kinds of forces that hold the particles together. We can classify any crystal as one of four types ionic, covalent, molecular, or metallic. 

It is important to realize that the lattice points used to define a unit ceii must aii be identical. We can define the unit ceii of NaCl based on the positions of the Na ions or the positions of the Cr ions. 

It is a common mistake to identify the CsCI structure as body-centered cubic. Remember that the lattice points used to define a unit cell must all be identical. In this case, they are all Cr ions. CsCI has a simple cubic unit cell. 

Sample Problems 12.5 and 12.6 show how to determine the number of ions in a unit cell and the density of an ionic crystal, respectively. 

How many of each ion are contained within a unit cell of ZnS  

CHAPTER 12 Intermolecular Forces and the Physical Properties of Liquids and Solids 

Think About It Make sure that the ratio of cations to anions that you determine for a unit cell matches the ratio expressed in the compound s empirical formula. 

Examples 11.5 and 11.6 show how to calculate the number of ions in and the density of a unit ceU. 

How many Na and Cl ions are in each NaCl unit cell  

Check This result agrees with sodium chloride s empirical formula. 

Practice Exercise How many atoms are in a body-centered cube, assuming that all 

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In many crystals there is sufficient overlap of atomic orbitals of adjacent atoms so that each group of a given quantum state can be treated as a crystal orbital or band. Such crystals will be electrically conducting if they have a partly filled band but if the bands are all either full or empty, the conductivity will be small. Metal oxides constitute an example of this type of crystal if exactly stoichiometric, all bands are either full or empty, and there is little electrical conductivity. If, however, some excess metal is present in an oxide, it will furnish electrons to an empty band formed of the 3s or 3p orbitals of the oxygen ions, thus giving electrical conductivity. An example is ZnO, which ordinarily has excess zinc in it. 

There are some alternatives to this HCI generator type of crystallization. There are, of course, canisters of HCI gas that can be purchased. Also, one can crystallize with very concentrated (fuming) HCI by pouring the stuff directly into the ether/freebase [26]. Regular 35% HCI can do this too, but the water content may dissolve the MDA.HCl or make the crystals sticky which means that the chemist will have to dry the solution by removing the water. 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

Crystal Structure. The structure type of crystals may be added in parentheses and in italics after the name the latter should be in accordance with the structure. When the typename is also the mineral name of the substance itself, italics are not used. 

The glass-ceramic phase assemblage, ie, the types of crystals and the proportion of crystals to glass, is responsible for many of the physical and chemical properties, such as thermal and electrical characteristics, chemical durabiUty, elastic modulus, and hardness. In many cases these properties are additive for example, a phase assemblage comprising high and low expansion crystals has a bulk thermal expansion proportional to the amounts of each of these crystals. 

Other factors also impact the type of crystals formed upon cooling of hot soap. Water activity or moisture content contribute to the final crystal state as a result of the different phases containing different levels of hydration. Any additive that changes the water activity changes the crystallization pathway. For example, the addition of salt reduces the water activity of the mixture and pushes the equiUbrium state toward the lower moisture crystal stmcture. Additionally, the replacement of sodium with other counter cations influences the crystallization. For example, the replacement of sodium with potassium drives toward the formation of 5-phase. 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

Another type of crystallizer is the Oslo-type unit shown in Figure 24. In units of this type, the object is to form a supersaturated solution in the upper chamber and then reHeve the supersaturation through growth in the lower chamber. The use of the downflow pipe in the crystallizer provides good mixing in the growth chamber. 

For large-scale continuous crystal production, several alternate types of crystallizer vessel are available (Figure 3.3). 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Preferential corrosion or attack at many other types of crystal defect may also be best illustrated during the etching of metallographically polished... 

Although Fig. 10 was drawn for a uni-univalent or di-divalent crystal, there is no reason why three or more ions should not be removed from the crystal along divergent lines, so that the ions are gradually separated from each other against their mutual attraction. In this way Fig. 11 can be constructed for uni-divalent or other types of crystal. In this case, when Fig. 13 is drawn the vertical arrow will represent the sum of the solvation energies of three or more ions. 

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

The type of crystal structure depends on the ratio X Me, where X is the total number of anions (oxygen and fluorine) and Me is the total number of all cations that can fit into/occupy octahedral voids (tantalum, niobium, lithium and other metals with similar ionic radii). 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

Figure 11 shows that the molecular weight distribution in the melt (presence of short chains) can account for the coexistence of two types of crystals in the absence of molecular orientation or at a slight stretching of the melt. However, there is a purely thermodynamic reason for the appearance of this main structural feature of samples crystallized under conditions of molecular orientation, even at high degrees of orientation, when virtually the whole distribution function is displaced into the region of /S > /3cr. 

Hence, it can be concluded that the formed intermediate oriented phase is an indispensable stage preceding extended-chain crystallization so that this type of crystallization occurs in two stages the first stage involves formation of the oriented phase during melt deformation and the second formation of ECC from this phase on cooling. 

The ionic solids NaCl and KC1 form the same type of crystal structure. In which solid are the ions bound together more strongly by coulombic interactions ... 

Madelung constant (A) A number that appears in the expression for the lattice energy and depends on the type of crystal lattice. Example A = 1.748 for the rock-salt structure. 

Fig. 3 shows that NaYl contains two types of crystals Y zeolite in cubic form and Pj zeolite in spherical form with diameter of about 5 pm and 3 pm, respectively. On the other hand, NaYs contains only crystals of Y zeolite in spherical form with diameter of about 0.5 pm. It is clearly demonstrated that the diameter of zeolite synthesized fi om kaolin is much larger than that of zeolite synthesized from pine chemiceils. 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

In the real world of defect chemistry, we find that in addition to the simple defects, other types of defects can also appear, depending upon the type of crystal we are dealing with. These may be summarized as shown in the following, given as 3.7.1. on the next page. 

Thermal stability is important in this method because the high temperatures reached may be sufficient to cause decomposition of the material. The Vemeuil Method of crystal growth is not generally applicable to all types of crystals. There are serious deficiencies in the method. For example, there is a large temperature drop of hundreds of degrees over a few millimeters within the crystal. This causes a large difference in thermal expansion within a limited space, and consequent... 

In these cases, one is limited to the growth of single crystals of the transition metals since it is those metals which form volatile carbonyl compounds. The alkali metals, alkaline earth metals and certain of those elements which are allotropic in nature are not at aH suited for this type of crystal growth. 

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