Chem242. - Inorg. Chem. Lab Spring, 2006 UMass-Amherst 1/5 Experiment 9 Thermochromism in the Ionic Conductor, Cu2HgI4 This lab taken from “Teaching General Chemistry; A Materials Science Companion,” Eds. Ellis, A.B.; Geselbracht, M.J.; Johnson, B.J.; Lisensky, G.C.; Robinson, W.R.R.; American Chemical Society, 1993 Introduction A great variety of structure types are derived from filling cations into the voids of a face-centered cubic (FCC) anion lattice. One simple example is the zinc blende structure of ZnS where one half of the tetrahedral FCC voids are filled (Fig. 1, far left). The ordered tetragonal low-temperature structure of Cu2HgI4 (Fig. 1, far right) involves a more complex arrangement of cations in the tetrahedral voids, and closely resembles a doubled zinc blende unit cell. Cu+ and Hg2+ ions are arranged in separate layers, both sandwiched between close-packed layers of I- ions. The stability of this complex order is tenuous – when heated above room temperature, the cations begin to move freely and a degree of randomness will be incorporated into the manner in which the Cu+ and Hg2+ ions fill tetrahedral voids. As a result, Cu2HgI4 will undergo a structural phase transition to a disordered cubic structure of one-half the volume of the tetragonal phase. At high temperatures, the Cu+ and Hg2+ tetrahedral sites are indistinguishable to x-rays (because X-ray diffraction measures the cell contents averaged over all unit cells within a crystal, and on average, each of the four available sites has a 50% chance of being occupied by a Cu+ ion, a 25% chance of being occupied by a Hg+ ion, and a 25% chance of being vacant), giving the high temperature phase the same high symmetry cubic unit cell as ZnS. The phase change is accompanied by a color change (from red to brown) and a marked decrease in resistance. The thermochromic color change of Cu2HgI4 is due to a small decrease in the semiconducting band gap (2.1 to 1.9 eV) with the temperature-induced change in structure. Figure 1. Left: A single and doubled unit cell of zinc blende structure of ZnS. Right: The closely related disordered cubic high-temperature (HT) and ordered tetragonal low-temperature (LT) structures of Cu2HgI4. Despite the fact that four different tetrahedral voids in the cubic HT cell can host Cu+ and Hg2+ ions, only three of these sites actually contain an atom in a typical cell. Zn+2S-2HgCuIZnS Cu2HgI4Chem242. - Inorg. Chem. Lab Spring, 2006 UMass-Amherst 2/5 Ionic conduction Unlike most electrical conductors where electrons move in response to an applied voltage, ionic conductors such as Cu2HgI4 (and the related solid, Ag2HgI4) can transport a current due to the ability of their constituent ions to move in response to an applied voltage. Ionic conductors are the solid analogues of electrolyte solutions, which also conduct electricity via the motion of ions. Ionic conductors are generally broken into two classes – those which conduct cations, and those which conduct anions. The latter class is of crucial importance to fuel cell applications, which demand a material capable of conducting oxygen in the form of O2- ions through a solid barrier. Above its transition temperature, Cu2HgI4 exhibits ionic conductivity (with some electronic conductivity also). Five-eighths of the tetrahedral holes and all of the octahedral holes formed by the iodide ions are vacant, and these open sites provide a possible mechanism for the small copper cations to move through the crystal, carrying charge. It is easiest for a copper cation to jump between tetrahedral holes by moving to an octahedral hole and then to the new tetrahedral hole, rather than jumping directly between tetrahedral holes. Substantial ionic conductivity is rare in ionic solids. Chemical reactivity Despite having a solid state (and not molecular) structure, Cu2HgI4 is readily prepared through a solution synthesis, as are most nanoparticles. Copper(I) tetraiodomercurate(II), Cu2HgI4, is prepared by combining copper(I) iodide with mercury(II) iodide. Copper(I) iodide is formed by reacting copper(II) sulfate with potassium iodide, in which the iodide ion reduces Cu(II) to Cu(I), thereby forming solid CuI (see Appendix). 2Cu2+(aq) + 4I-(aq) ' 2CuI(s) + I2(aq) (1) In the presence of excess iodide ion, the iodine undergoes further reaction, forming triiodide ions I2(aq) + I- ' I3-(aq) (2) The net ionic equation for the formation of copper(I) iodide is shown below (eq. 3). 2Cu2+(aq) + 5I-(aq) ' 2CuI(s) + I3-(aq) (3) Triiodide can act as an oxidizing agent, by the reversal of equations 2 and 1, so it must be removed from solution. Sodium sulfite is used to reduce the triiodide ion back to iodide, eq. 4. I3-(aq) + SO32-(aq) + 3H2O ' 3I-(aq) + SO42-(aq) + 2H3O+(aq) (4) The solid copper(I) iodide can be separated from the reaction mixture by carefully pouring off the excess solution of supernatant liquid. A mercury(II) iodide precipitate is synthesized by an anion metathesis reaction, in which mercuric nitrate is combined with potassium iodide. The insolubility of HgI2 helps to pull the equilibrium toward the right of eq. 5. Hg2+(aq) + 2I-(aq) ' HgI2(s) (5) Finally, Cu2HgI4 is prepared by adding the solid copper(I) iodide to the mixture containing the mercury(II) iodide precipitate. 2 CuI(s) + HgI2(s) → Cu2HgI4(s) (6)Chem242. - Inorg. Chem. Lab Spring, 2006 UMass-Amherst 3/5 Procedure: CAUTION: The mercury-containing compounds, are toxic. Avoid creating or breathing dust. Use gloves when working with the mercury compounds, and avoid eye contact. Make extra efforts to ensure that all mercury waste ends up in the proper container. Keep your work surfaces clean at all times so that mercury waste is not inadvertently transferred. Synthesis of Cu2HgI4 CuI: Add 2.5ml of 0.5 M CuSO4 solution, 3.0ml of 1M KI solution and several drops of 6M acetic acid to 25ml of deionized water in a 100ml beaker. A precipitate of CuI will form. Add to this precipitate, with continuous stirring, a solution of 0.10g Na2SO3 dissolved in 5ml of water. Allow the precipitate of CuI to stand for 5 – 10 mins and then pour off as much as possible of the supernatant solution without losing much of the precipitate. HgI2 (in situ): Combine 12.5 ml of 0.05 M Hg(NO3)2, 1.5ml of 1M KI and 50ml of deionized water in a 200ml beaker. Note your observations. “in situ” is Latin for “in its original place.” In