Class Note for CHEM 242 at UMass(7)
Class Note for CHEM 242 at UMass(7)
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Date Created: 02/06/15
Lecture Experiment 7 Thermochromism and Ionic Conductivity in Solid State CqugI4 This experiment studies the properties that can be obtained in what is called inorganic solid state chemistry Solid state compounds are in nite non molecular solids There are no separate molecules of CqugI4 present in the conductive thermochromic solid we will study in this experiment but instead in nite arrays of atoms packed together in different arrangements The subscripts in the formula CqugI4 simply give the ratios of the atoms in the solid You are used to molecular chemistry either in solution or in molecular species that just happen to be solids at room temperature The difference between such compounds and non molecular ones is that the packing structure or the ph of the infinite solid is what determines its properties The most common example of this is diamond and graphite Both are infinite solids with exactly the same formula Cn Diamond s phase is a cubic one in which each carbon forms four bonds to every other Graphite is hexagonal an in nite twodimensional array of stacked fused rings in which the carbons are doublebonded The properties of these two phases of carbon could not be more different because of their differences in structure Diamond is the hardest material known and the best electrical and thermal insulator Graphite is one of the softest of materials and is a good electrical and thermal conductor While in molecular solids the identity and placement of the functional groups the structure of the molecule determines its properties in solid state chemistry it is the phase of the nonmolecular solid that does so The objectives of this experiment slide 1 is to synthesize one phase of CqugI4 at room temperature then convert it to a different phase using high temperature The difference between the two phases colors and electrical conductivities caused by the phase change will then be observed Structural Descriptions of Solids The simplest structural description of an in nite solid is that it consists of close packed layers slide 2 In this description all the closepacked atoms are the same the closepacked atoms in CqugI4 are the iodide ions Every atom is coordinated by six other atoms in the same layer and three more atoms are placed in the layer above in the depressions made by the first layer There is room for only half the depressions to be lled The third layer can now be placed in two ways First all the closepacked atoms can be placed directly above the atoms in the first layers slide 2 This leads to hexagonal symmetry and therefore to hexagonal closepacking Second the third layer of atoms can be placed above the unfilled depressions of the first layer This leads to cubic symmetry and therefore to cubic close packing In each kind of close packing the space filled by the closepacked atoms is 74 This is the most efficient way to pack any spheres of the same size in space In both packings the 26 of unfilled space is organized into what are called sites voids vacancies or holes slide three Octahedral Oh sites are formed when six closepacked atoms in two different layers come into contact slides 3 and 4 These sites can hold another type of atom of a radius up to 41 of the radius of the closepacked atoms There is one Oh site for every close packed atom Tetrahedral Td sites are formed when four closepacked atoms in two different layers come into contact slides 3 and 4 These sites can hold another type of atom of a radius up to 23 of the radius of the closepacked atoms There are two Td sites for every close packed atom The organization of the vacancies in solids allows the formation of organized solid state phases such as the roomtemperature phase of CqugI4 that we will study As an example some crystals may form a cubic array with the anions closepacked because they are larger and the cations because they are smaller sitting in the Oh or Td sites Slide 5 shows sodium chloride in which the cubic closepacked array of Cl39 ions has a Na ion occupying each Oh site Its unit cell shown on slide 5 is defined as the smallest repeating volume element in the structure The translation of the unit cell in three dimensions therefore generates the entire structure For NaCl the unit cell is facecentered because one Cl39 ion lies on every face of the cube that makes up the cell slide 5 There are many shapes of unit cells some can be rectangular or hexagonal or shaped like other polygons Similarly the zinc sul de ZnS unit cell is found to have a facecentered cubic array of 8392 ions with Zn2 sitting in half the available Td sites slide 6 The doubled ZnS unit cell is analogous to the ordered roomtemperature cell of CqugI4 slide 6 The unit cell of this phase of CqugI4 consists of a closepacked array of 139 ions with Cu and Hg 2 cations sitting in the various sites in a tetragonal unit cell The unit cell is shaped like a rectangle with square ends slide 6 CqugI4 Structures We can see from slide 6 that two Cu1 and one Hg2 ions are distributed in three of the eight available Td sites in the roomtemperature structure of CqugI4 Each cation is coordinated by four 139 ions This structure is ordered because the di erent cations occur in layers first a layer of I39 then one of Cu then I39 then one of Hg2 etc The structure occurs at room temperature and lower It is an organized structure because these particular cations occur at these particular organized sites But note that this structure is defective one Hg2 ion is missing from each of its layers This is because the compound s stoichiometry demands only one Hg2 per every four I39 But leaving this Td site empty is like pulling a brick out of a wall it generates a large structural defect This phase is more unstable and therefore more reactive because of the defect Therefore at higher temperatures the compound can change its phase The 139 ions collapse to a facecentered cubic unit cell slide 6 The Cu1 and Hg2 ions are still in Td sites but their placements are not organized but randomly scattered A stronger structure is formed Three out of four Td sites are filled with either a Cu1 or a Hg2 or are empty but each unit cell can have these ions placed in di erent sites Each unit cell can be different This is the hightemperature disordered phase Now that we have had a structure change we may expect quite different properties of the two phases as we see with diamond and graphite Differences Between the Ordered and Disordered Phases A color change arises when the lowtemperature ordered phase is converted to the high temperature disordered phase This change in color with temperature is called thermochromism It arises from a change in energy between lowlying lled electron sites and highlying empty electron sites in the two phases These are called semiconducting bands and are analogous to lowlying lled and highlying empty molecular orbitals in molecular compounds The energy needed to promote an electron from the lled band to the empty one changes from 21 eV for the room temperature phase to 19 eV for the high temperature phase Thus the color change that is seen between the two phases This color change will allow you to determine the phasechange temperature of CqugI4 using a melting point apparatus The electrical conductivity of the phases of CqugI4 also changes as the high temperature phase is capable of ionic conductance slide 7 This is electrical conduction generated not by the movement of electrons through the solid but by the movement of ions which are also charged particles Ions in some phases can hop from one vacant site to another thus moving across the solid and carrying electrical charge through it When a voltage is applied the charged ions move through the in nite structure and transport a current CqugI4 conducts cations in the disordered phase Many of the Td and all of the Oh holes are empty and the small cations can hop from one end of the solid to the other Ions are much bigger and harder to move than are electrons so there are not very many good ionic conductors known In the hightemperature phase it is easier for the cations to move because the iodide closepacked lattice is more stable yet many vacant sites still remain Resistance Resistivity and Conductivity What you will measure experimentally is the resistance of the two phases of CqugI4 you will then be able to calculate their resistivity and conductivity The resistance is measured slide 8 by making a cylindrical sample of CqugI4 and applying voltage to it The resistance for both phases in ohms is then read off of an ohmmeter However resistance is not independent of sample geometry It must be converted to the dimensionless resistivity p slide 9 a measure of resistance that is not dependent on the sample s geometry The conductivity of both phases slide 10 can then be calculated In conclusion an infinite nonmolecular solid CqugI4 will be synthesized in two different phases and the difference in physical properties that different phase structure has on the same nonmolecular compound will be determined
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