Divalent impurities

Asay and Gupta [25] measure elastic precursor amplitudes as functions of propagation distance for two different divalent impurity concentrations in <100)-loaded LiF. It is shown that not only does the presence of divalent ions affect the precursor amplitude, but also that the state of the dispersion plays an important part. It is concluded that, for a given concentration of defects, the rate of precursor attenuation is reduced if the defects are clustered. 

The compound MgGa204, when activated by divalent impurities such as Mn +, is used in ultraviolet-activated powders as a brilliant green phosphor. Another very important application is to improve the sensitivity of various bands used in the spectroscopic analysis of uranium. Minor uses are as high-temperature liquid seals, manometric fluids and heat-transfer media, and for low-temperature solders. 

As mentioned above alkali halide crystals are strongly hardened by small additions of divalent impurities. Data are available for 12 cases, the host crystals NaCl, NaBr, KC1, and KBr with additions of Ca2+, Sr2+, and Ba2+ (Chin, et al., 1973). It was found that the hardness increases depend only on the concentrations of the additions and not on the divalent specie (Ca, Sr, or Ba). However, a dependence on the valence (1, 2, or 3) is observed. It was also found that hardness increment is proportional to the square root of the concentration, (C1/2). 

Figure 9.7 Schematic diagram of the vicinity of a divalent impurity in the NaCl crystal structure.

The effect of atomic motion in the solid state on nuclear resonance line width is illustrated by the behavior of Na resonance from NaCl as a function of temperature 97). In Fig. 9 is shown the variation of the Na line width with temperature for pure NaCl and NaCl doped with an atomic fraction concentration of 6 X 10 of CdCU. As discussed in Section II,A,2 the low-temperature, rigid-lattice line width will narrow when the frequency of motion of the nuclei under observation equals the line width expressed in sec.-. The number of vacancies present should be equal to the concentration of divalent impurities and the jump frequency of Na+ is the product of the atomic vacancy concentration and the vacancy jump frequency... 

Of particular interest is the possibility of decontaminating actinides from impurities by washing the loaded column with dilute acid or monovalent salts to elute the divalent impurities before stripping the column with 7M HNO3. This method is presently being evaluated for lead removal from americium using Bio-Rad AG MP-50. 

The pH sensitive reversible nature of the phosphate gel provides a continuously renewable surface for the entrapment of divalent impurities moving toward the membrane from the brine during operation and it eliminates the need for expensive ion exchange equipment for purifying the brine. 

A more recent approach by Simanek and Sroubek [21] has been to interpret the pressure dependence of the chemical isomer shift for a single compound. The first attempt involved the calculation of the overlap distortion effect on I y s(0) 1 due to a decreasing Fe-O bond distance for an Fe divalent impurity atom in CoO. Incorporation of 4j-bonding into the orbitals used... 

Figure 6.27 Effect of various divalent impurities on the critical resolved shear stress in various alkali halides. (After Cook and Pharr, 1994.)...

Several authors of papers published in the 1960s and 1970s came to the conclusion that divalent impurities, includ g R ions, form in alkali-halide hosts impurity-vacancy (I-V) dipoles. Kao and Perlman (1979) have also accepted the 1-V dipole model for KCl Eu. They studied the relaxation and aggregation processes in these dipoles. It has been observed that X-irradiation destroys the dipoles and reduces the Eu " to Eu. References to earlier papers dealing with the I-V dipole model are cited in the above paper. 

At this point let us briefly consider the formation of associates. The formation of associates between cation vacancies and divalent impurities in alkali halides has already been given as an example. Such reactions are homogeneous solid state reactions, and so the relaxation time for the formation of associates can be calculated in a completely analogous manner to the calculation of the relaxation time for the equilibration of Frenkel defects. The result of such calculations is precisely the same as the result given in eq. (6-5). It is only necessary, in the case of association, to replace the concentration c (eq) = in the denominator by the nearly constant concentration of the corresponding majority defect. In general, in the case of the formation of defect associates, we can conclude that the equilibrium concentration is attained rapidly compared to the time required by defect reactions which occur at sites of repeatable growth. 

The serial sectioning technique was used to study the diffusion of 45ca in single crystals which contained about 18ppm of divalent impurities. At 260 to 440C, the data could be described by ... 

The serial sectioning technique was used to investigate the diffusion of Sr and the simultaneous diffusion of Sr and Co into single crystals with a divalent impurity concentration of about 5 x 10"2/cm. The data for the diffusion of alone, at... 

The effect of the divalent impurity ions (eg. Cd" " in AgCl) may be represented in a formal way within our energy level scheme by the introduction of a new level at energy B above that associated with a normal lattice site as shown in figure (16). B is then the energy required for dissociation of a divalent ion-vacancy complex it may alternatively be viewed as the energy of a silver ion next to the impurity. In the bulk of the crystal where charge neutrality exists we have... 

The minus sign originates from the particular way A ,i , was defined above. Fii P (III) is the solution energy of the appropriate divalent impurity in the trivalent host. Again, obviously, the impurity term is not the thermodynamical one, but rather a restricted quantity where the atomic positions remain fixed in thfe electron capture process. Naturally, also here a thermodynamical impurity energy will give a lower limit to the vertical excitation energy in eq. (3). 

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