SOLID STATE SEARCH BY #JAMAL_KAZMI Occasionally one of the ions - TopicsExpress



          

SOLID STATE SEARCH BY #JAMAL_KAZMI Occasionally one of the ions in an ionic lattice is simply in the wrong position. An example of this phenomenon, called a Frenkel defect, is a cation that occupies a tetrahedral hole rather than an octahedral hole in the anion lattice The Two Most Common Defects in Ionic Solids ). Frenkel defects are most common in salts that have a large anion and a relatively small cation. To preserve electrical neutrality, one of the normal cation sites, usually octahedral, must be vacant. Frenkel defects are particularly common in the silver halides AgCl, AgBr, and AgI, which combine a rather small cation (Ag +, ionic radius = 115 pm) with large, polarizable anions. Certain more complex salts with a second cation in addition to Ag + and Br− or I − have so many Ag + ions in tetrahedral holes that they are good electrical conductors in the solid state; hence they are called solid electrolytes. Reactions in Aqueous Solution , most ionic compounds do not conduct electricity in the solid state, although they do conduct electricity when molten or dissolved in a solvent that separates the ions, allowing them to migrate in response to an applied electric field.) In response to an applied voltage, the cations in solid electrolytes can diffuse rapidly through the lattice via octahedral holes, creating Frenkel defects as the cations migrate. Sodium–sulfur batteries use a solid Al2 O 3 electrolyte with small amounts of solid Na 2 O. Because the electrolyte cannot leak, it cannot cause corrosion, which gives a battery that uses a solid electrolyte a significant advantage over one with a liquid electrolyte. EXAMPLE In a sample of NaCl, one of every 10,000 sites normally occupied by Na + is occupied instead by Ca2+ . Assuming that all of the Cl − sites are fully occupied, what is the stoichiometry of the sample? Given: ionic solid and number and type of defect Asked for: stoichiometry Strategy: A Identify the unit cell of the host compound. Compute the stoichiometry if 0.01% of the Na + sites are occupied by Ca2+ . If the overall charge is greater than 0, then the stoichiometry must be incorrect. B If incorrect, adjust the stoichiometry of the Na + ion to compensate for the additional charge. Solution: A Pure NaCl has a 1:1 ratio of Na + and Cl − ions arranged in an fcc lattice (the sodium chloride structure). If all the anion sites are occupied by Cl −, the negative charge is −1.00 per formula unit. If 0.01% of the Na + sites are occupied by Ca 2+ ions, the cation stoichiometry is Na 0.99 Ca 0.01 . This results in a positive charge of (0.99)(+1) + (0.01) (+2) = +1.01 per formula unit, for a net charge in the crystal of +1.01 + (−1.00) = +0.01 per formula unit. Because the overall charge is greater than 0, this stoichiometry must be incorrect. B The most plausible way for the solid to adjust its composition to become electrically neutral is for some of the Na + sites to be vacant. If one Na + site is vacant for each site that has a Ca 2+ cation, then the cation stoichiometry is Na 0.98 Ca 0.01 . This results in a positive charge of (0.98)(+1) + (0.01) (+2) = +1.00 per formula unit, which exactly neutralizes the negative charge. The stoichiometry of the solid is thus Na 0.98 Ca0.01 Cl 1.00 . Exercise In a sample of MgO that has the sodium chloride structure, 0.02% of the Mg 2+ ions are replaced by Na + ions. Assuming that all of the cation sites are fully occupied, what is the stoichiometry of the sample? Answer: If the formula of the compound is Mg0.98 Na 0.02 O 1−x , then x must equal 0.01 to preserve electrical neutrality. The formula is thus Mg0.98 Na 0.02 O 0.99 .
Posted on: Wed, 01 Oct 2014 18:49:52 +0000

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