Mixed Crystal Systems

Mixed Crystal Systems. - 2.6.1 Co-crystals. l-Methyluracil 9-ethyladenine (Radicals 31-37)... 

Radical formation in a mixed crystal system of cytosine monohydrate doped with small amounts of thiocytosine (ca. 0.5%) was investigated on order to gain insight into hole transfer in a well-defined crystalline system.31 Also of interest was whether the protonation state of the thiocytosine radical(s) was the same as that of the cystosine radical(s). Crystals were X-irradiated (ca. 30 kGy) and ESR and ENDOR spectra recorded at ca. 15 K. After irradiation, many types of free radicals were formed. Among these, the low field resonance from a sulfur centered radical (42), with g-tensor (2.132, 2.004, 2.002), was clearly visible. Radical 42 constituted approximately 10% of the total cohort of radicals formed in the crystal and is apparently the only sulfur-centred radical observed in this experiment. Six weakly coupled protons were observed, two of which are shown... 

Without stepping into any nomenclature controversies, for the purposes of this review, cocrystal systems will be regarded as those mixed crystal systems where the individual components exist as solids under ambient conditions [11]. Aakeroy has summarized guidelines for cocrystal formation from supramolecular synthons as being constructed from discrete neutral molecular species that are solids at ambient temperatures, and where the cocrystal is a structurally homogeneous crystalline material that contains the building blocks in definite stoichiometric amounts [12],... 

There are still challenges BrUUant, inorganic, nontoxic, stable and cheap green or blue pigments are amongst these. Probably, the mixed crystal systems are promising fields for new discoveries. 

The mixed-crystal system formed by indomethacin and saccharin (l,2-benzisothiazol-3(2H)-one-l,1-dioxide) has been used to evaluate the feasibility of using supercritical fluids as media for the design and preparation of new cocrystals [44]. In this work, the relative merits of supercritical fluid processes (i.e., cocrystallization with a supercritical solvent, supercritical fluid as anti-solvent, and the atomization and anti-solvent technique) were evaluated, as well as the influence of processing parameters on product formation and particle properties of the yields. It was reported that while the anti-solvent and atomization procedures yielded pure cocrystal products, only partial to no cocrystal formation took place when using the crystallization process. 

Similarly, the symmetry of the mixed crystal system composed of (E)-cin-namamide (host) and (E)-2-thienylacrylamide (guest, 8% occluded) is lower than that of the host crystal (P2j/c). The selective occlusion of the guest arises from repulsive sulfur- n interactions. The -l-b end and the -b end of the mixed crystal are enantiomorphic (space group PI) and underwent topochemical [2-1-2] photodimerization in 40-69% enantiomeric excess (Scheme 8) [29]. 

In a mixed-crystal system, ket differs from donor to donor according to each donor s specific acceptor environment. The total quantum efficiency for energy transfer from the ensemble of donors is given by... 

Other Azides. Preliminary absorption spectra of CUN3 [86], CdNe, and HgNg [121] have been reported by Deb. Thin films were prepared by the solid-solid reaction technique described for PbNg. Mixed crystal systems of metal azide halides were probably formed in the reactions, since the results differed for each combination of alkali azide-metal halide. An earlier CUN3 spectrum was interpreted by Evans and Yoffe [82] to indicate an w = 1 exciton, but here again that appears theoretically unsound (Section C.3.b). 

Mixed crystal systems themselves are interesting to speculate upon since one can imagine numerous combinations, for example KjcCui jfN3, which should be unstable in the limit of small concentrations of potassium but would perhaps exhibit decreased sensitivity compared to pure CUN3 [151]. 

Sukhushin and Zakharov [16] investigated the shock sensitivity and detonation rates for lead azide-silver azide, lead azide-copper azide, and silver azide-copper azide mixed-crystal systems. The most striking effect occurred when the minor constituent was less than 10% of the composition. Such systems are perhaps better viewed as being heavily doped than as mixed crystals. 

Koshima and his co-workers have studied the reactivity of co-crystals of acridine and 1-phenyIpropionic acid. This work is analogous to that described last year for the reactivity of acridine with diphenylacetic acid. The present study is summarized in Scheme 2. Again irradiation brings about decarboxylation of the propionic acid and the resulting radical bonds to the acridine. The yields of the compounds and their optical activities are shown below the appropriate structures. When acridine and the acid are irradiated in acetonitrile solution racemic (71) is formed along with the dimer (72). A review of the photochemistry in two-component and mixed crystals systems has been published. ... 

Fig. 6.14 The anthracene-pyromellitic acid dianhydride (A-PMDA) mixed-crystal system as an example of a weak donor-acceptor crystal with mixed stacks. The ground state is neutral, nearly without charge transfer. The lowest CT state Is formed by a transition of an electron...

The first experiments with ESR on electronically excited molecular triplet states were published by Hutchison et al. [1]. A few years later, the first optical detection of ESR in a magnetic field followed [2], as well as the first optical detection of the zero-field resonance [3]. Eor these experiments, molecular crystals (host crystals doped with guest molecules) were investigated. The host crystal in these mixed-crystal systems serves mainly as an orienting matrix and the result of these ESR measurements are values for the fine structure and the dynamic properties of the Ti state of the guest molecules. 

Von Hippel, Arthur Robert (1898-2003) reported the ferroelectric properties of BaTiOj in 1946. Since then over a hundred pure materials and many more mixed crystal systems that are ferroelectric have been found. He was on the faculty of MIT from 1936 until he retued in 1964. His starting salary was only 3,500 per year and he is reported to have sold textbooks to pay for medical bills for his children. The Materials Research Society has named one of its major awards in recognition of von Hippel s contribution to dielectrics, semiconductors, ferromagnetics, and ferroelectrics. ... 

An important contribntion to solid hot atom chemistry came from H. Muller s laboratory in Freiburg with rhenium (osmium) compounds (Muller 1979). Muller performed his studies on mixed crystal systems involving rhenium, osmium, etc., for many years. An advantage of rhenium (n,y) recoil reaction is its relatively small recoil energy ( max 102 eV, average energy may be much lower for Re), which results in a reaction zone with a radius of ca. 0.5 nm (one or two unit cells of the crystal lattice). 

When Wagner had recognised the mechanism of conduction in the Nernst glower, he pointed out in 1943 For fuel cells with solid electrolytes anion conductors are to be considered exclusively. From this point of view a systematic investigation of the mixed crystal systems of the type of the Nernst mass with roentgenographic and electrical methods seems to be desirable [22]. This was the start of concentrated work on solid oxide fuel cells (SOFCs). 

In this section, we consider how the optical spectra of an impurity ion mixed-crystal system can be understood in terms of a lattice-dynamical model [6]. In particular, we consider the d manganese (IV) MnF ion doped in a CsjSiFg lattice. The sharp lines and detailed vibronic structure of the absorption and emission spectra of the A2g - Eg transition makes the Mn ion in cubic symmetry an ideal system for study [7]. 

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