Mixed crystals optical properties

Optical detection offers the most conventional technique to time-resolve the coherent phonons. It includes four-wave mixing [8], transient reflectivity [9,10] and transmission [7] measurements, as well as second harmonic generation (SHG) [15,32]. Coherent nuclear displacement Q induces a change in the optical properties (e.g., reflectivity R) of the crystal through the refractive index n and the susceptibility y,... 

Crystals of stoichiometric 1 1 mixtures of compounds that can complex with each other have been shown to form preferentially to pure crystals of the individual components. In some cases these crystals may have potential non-linear optical properties. An interesting example is the 1 1 mixture of p-aminobenzoic acid and 3,5-dinitrobenzoic acid. (15) A view of the crystal structure is shown in figure 3. Examination of this figure leads one to the hypothesis that the preference for the mixed crystal may be due to a) a more stable H-bonding interaction between the different benzoic acids in the hetero-dimer than in the homo-dimer b) the ability of the mixed crystal (hetero- dimers) to H-bond between their amino and nitro groups. It is likely that both of these factors play a role in the stability of the crystal structure. Calculational modelling can aid in determining the importance of these factors. 

The purpose of this paper is to examine by calculations of simple models the spectroscopic behaviour expected of mixed crystals under varying conditions of guest concentration and of guest optical properties. We shall begin by reviewing some points of the theory of pure crystals, then go on to introduce the onedimensional model, apply it to a series of representative examples, and finally consider its relevance to real mixed crystals. 

The polymer most commonly blended with SBC is crystal polystyrene. Crystal polystyrene and SBC do have a significant difference in refractive index, but the polystyrene is miscible in the polystyrene domains of the SBC. Hence the blended part, if well mixed, will have good optical properties. Crystal polystyrene is desirable as a blend resin for SBC because it is of lower cost and also offers advantages in temperature resistance, stiffness and surface hardness. The major disadvantage in blends of SBC with crystal polystyrene is a significant decrease in impact strength as the polystyrene content is increased. 

Nature furnishes us with a certain number of hydrated silicates which mineralogists call zeolites the dehydration of certain hydrates offers curious peculiarities analcime, for instance, may be completely dehydrated without any sudden variation in form or optical properties of the crystals being observed Georges Friedel has shown that analcime had not, at a given temperature, an invariable dissociation tension let us suppose the temperature constant in a first equilibrium state the tension of the water vapor which exists in equilibrium above the crystals has the value P remove a portion of this water vapor the analcime will undergo a certain dehydration and the tension of the water vapor will increase, but only to a value P, less than P and BO on analcime is therefore not a definite hydrate, but only a solid solution in which water is mixed with an anhydrous silicate. 

Optical antipodes. Inactive substances to which they may give rise.— The idea of mixed crystals assumes great importance in the discussions relative to the properties of substances... 

In this approximation (the additive refraction or mean polarizability approximation) eqn (5.42) with the addition of eqn (5.37) fully determines the dependence of the dielectric constant tensor on the impurity concentration c. The optical properties of mixed crystals with large impurity concentrations are discussed in Section 5.6. As we did above for crystals with one molecule per unit cell, we shall discuss here the case of small values of c when we can ignore terms of the order of c2, c3, etc. in the expansion of the tensor (5.42) in powers of c. Then we obtain ... 

Agranovich, V. M. and Dubovsky, O. A. (1988). Phonon multimode spectra biphonons and triphonons in crystals with defects. In R. J. Elliott and I. P. Ipatova (Eds.), Optical Properties of Mixed Crystals with defects. North-Holland, Amsterdam, pp. 297-398. 

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