Crystal forms, existence

Only one carbide, CaCj, with four crystal forms exists in this system . The particular form depends on the temperature from which the sample is quenched, the mechanical treatment and the presence of [CNj] or xs Ca Cubic CaCj-IV is only stable above 500 C and cannot be quenched to RT CaCj-III is metastable and is observed in the absence of CaCj-Il is stable below RT but can be stabilized to higher T by the presence of dissolved The tetragonal CaC2-I is observed in commercial... 

Thus, on the basis of quantity and size required to play a role, once a crystal form exists in a certain locale the presence of seeds is almost always possible, indeed often unavoidable. One can think of local seeding, where the unintentional source may come from e.g. the experimentalists clothing, a portion of the room, an entire room, a building, or even, with increasing degrees of improbability, increasingly larger environments. ... 

There is a transition point at the 11- 4 isotherm from a monolayer with crystalline chain in a tilted arrangement to a monolayer with vertical (and crystalline) chains. The monolayer phase with vertical chains is unstable which is not surprising as no stable crystal form exists with vertical chains. At variations in compression rate (Table 8.14) this transition pressure is constant whereas the collapse pressure is reduced with decreasing compression rate. The equilibrium spreading pressure of these fatty acids is equal to the transition pressure in Table 8.14. As discussed in Section 8.4 the reduction in surface tension, when crystalline fatty acids are present, is equal to the equilibrium spreading pressure. 

Poly(vinylidine fluoride) has attracted much attention because of its piezoelectric properties. It is also complicated in that a number of crystal forms exist and have been carefully studied by X-ray diifrnction. The various crystal forms arise because the chain can exist in three distinct conformations and the chain alignment can be up or down along the axis and parallel or antiparalld about the axis. The three conformations are all trans, TG" TG and T3G+T3G-. 

HNOC-CeH4-CONH-] . Similar to the case of quenched PET (see Fig. 2.43), the glass transition is followed by an exothermic peak. This peak corresponds to an amorphous melt crystal transition. However, several crystal forms exist for this polymer, and crystal crystal transitions take place as the temperature increases. These crystal forms have not been identified, but were clearly seen by wide-angle X-ray diffraction (Menczel et al. 1996). So if the three existing crystal forms are designated with A, B, and C, then the lowest-temperature exotherm (the cold crystallization exotherm) corresponds to the melt crystal form A transition, followed by melting of crystal form A. After the melting of crystal form A is completed, the sample crystallizes into crystal form B. Then crystal form B melts, and the melt crystallizes into crystal form C, and finally crystal form C will melt. Some of these transitions can be avoided... 

Cobalt is a bluish silvery metal, exhibits ferromagnetism, and can exist in more than one crystal form it is used in alloys for special purposes. Chemically it is somewhat similar to iron when heated in air it gives the oxides C03O4 and CoO, but it is less readily attacked by dilute acids. With halogens, the cobalt(II) halides are formed, except that with fluorine the (III) fluoride, C0F3, is obtained. 

Manganese Oxides. Manganese(IV) dioxide rarely corresponds to the expected stoichiometric composition of Mn02, but is more reahsticaHy represented by the formula MnO y 2 q, because invariably contains varying percentages of lower valent manganese. It also exists in a number of different crystal forms, in various states of hydration, and with a variety of contents of foreign ions. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Anhydrous Oxalic Acid. The anhydrous form of oxaUc acid is odorless and colorless. It exists in two crystal forms, ie, the rhombic or a-form and the monoclinic or P-form (3). The rhombic crystal is thermodynamically stable at room temperature, but the monoclinic form is metastable or slightly stable. The main difference between the rhombic and monoclinic forms exists in the melting points which are 189.5 and 182°C, respectively (Table 1)-... 

Physical Properties of PBT. Unlike PET, the polymer PBT exists in two polymorphs called the a- and p-forms, which have distinctly different crystal stmctures. The two forms are interconvertible under mechanical stress (158,159). Both crystal forms are triclinic and the crystal parameters are shown in Table 7. 

Anhydrous aluminum bromide, AIBr., forms colorless trigonal crystals and exists in dimeric form, in the crystal and Hquid phases (1). Dissociation... 

In the presence of excess fatty acid, different soap crystalline phase compounds can form, commonly referred to as acid—soaps. Acid—soap crystals are composed of stoichiometric amounts of soap and fatty acid and associate in similar bilayer stmctures as pure soap crystals. There are a number of different documented acid—soap crystals. The existence of crystals of the composition 2 acid—1 soap, 1 acid—1 soap, and 1 acid—2 soap has been reported (13). The presence of the acid—soaps can also have a dramatic impact on the physical and performance properties of the finished soap. The presence of acid—soaps increases the plasticity of the soap during processing and decreases product firmness, potentially to the point of stickiness during processing. Furthermore, the presence of the acid—soap changes the character of the lather, decreasing the bubble size and subsequently increasing lather stabiUty and... 

Cadmium Iodide. Two crystal morphologies exist for Cdl2 [7790-80-9] the white a-form (see Table 1) and the brown p-form. The latter... 

The small precious metal crystals can exist as metal crystallites or as metal oxides, both of which are catalytic (31). Rodium oxide has a tendency to react with alumina to form a soHd solution (35). To minimize this reaction, zirconia is used with the alumina (36). PubHcations regarding the TWC function of precious metals abound (37—42). 

As mentioned above, the unit operation of crystallization rarely exists in isolation but is normally part of a wider particulate processing system as illustrated schematically, in a very simplified form, in Figure 9.2. A particular feature of such processes is the variety of unit operations and the range of equipment types that may be employed at any stage. 

IR spectra in conjunction with theoretical calculations (B3LYP/6-31G ) indicate that the n /2ydro-(2-hydroxy-4-oxo-4//-pyrido[l, 2-n]pyrimidinium)-hydroxide form exists in solution and in the crystal, but the 2-hydroxy-4//-pyrido[l,2-n]pyrimidin-4-one tautomer dominates in the gas phase (00JCS(P2)2096). 

An alloy is said to be of Type II if neither the AC nor the BC component has the structure a as its stable crystal form at the temperature range T]. Instead, another phase (P) is stable at T, whereas the a-phase does exist in the phase diagram of the constituents at some different temperature range. It then appears that the alloy environment stabilizes the high-temperature phase of the constituent binary systems. Type II alloys exhibit a a P phase transition at some critical composition Xc, which generally depends on the preparation conditions and temperature. Correspondingly, the alloy properties (e.g., lattice constant, band gaps) often show a derivative discontinuity at Xc. 

Fig. 2b. The appearance of two crystal forms shows that the protein in the membrane exists in equilibrium between the protomeric aj8 unit and oligomeric (aj8>2 forms. The high rate of crystal formation of the protein in vanadate solution shows that transition to the E2 form reduces the difference in free energy required for self association of the protein. This vanadate-method for crystallization has been very reproducible [34-36] and it also leads to crystalline arrays of Ca-ATPase in sarcoplasmic reticulum [37] and H,K-ATPase from stomach mucosa [38]. 

The Zag meteorite fell in the western Sahara of Morocco in August 1998. This meteorite was unusual in that it contained small crystals of halite (table salt), which experts believe formed by the evaporation of brine (salt water). It is one of the few indications that liquid water, which is essential for the development of life, may have existed in the early solar system. The halite crystals in the meteorite had a remarkably high abundance of 128Xe, a decay product of a short-lived iodine isotope that has long been absent from the solar system. Scientists believe that the iodine existed when the halite crystals formed. The xenon formed when this iodine decayed. For this reason, the Zag meteorite is believed to be one of the oldest artifacts in the solar system. In this lab, you will use potassium-argon radiochemical dating to estimate the age of the Zag meteorite and the solar system. 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

A lattice is a three-dimensional array, and there are eight systems known. Inorganic substances are usually defined by one crystal system by the so-called radius-ratio rule [22], but organic compounds often have the capability of existing in more than one crystal form, a phenomenon referred to as polymorphism. 

The literature lists numerous examples of polymorphism i.e., the existence of several crystal forms of a given chemical that exhibit different physical properties [7]. The conversion of one polymorph to another may cause a significant change in the physical properties of the drug and in critical quality attributes of drug products. 

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