CONTROL OF SELECTED PHYSICAL-PROPERTIES OF MX SOLIDS - AN EXPERIMENTAL AND THEORETICAL INVESTIGATION

A series of eight materials of stoichiometry [Pt(L-L)(2)X(2)][Pt(L-L)(2)]Y-4 (X is Cl, Br; L-L is 1,2-diaminoethane (en) or 1,2-diaminocyclohexane (chxn); Y is ClO4-, X(-)) were synthesized. Crystal structures were determined for the compounds [Pt(chxn)(2)Cl-2][Pt(chxn)(2)](ClO4)(4) 1, [Pt(chxn)(2)Br-2][Pt(chxn)(2)](ClO4)(4) 2, and [Pt(chxn)(2)Br-2][Pt(chxn)(2)]Br-4 4. All three of these compounds crystallize in the orthorhombic space group I222. Compound 1 has a = 5.711(1) Angstrom, b = 7.804(1) Angstrom, c = 24.101(7) Angstrom, Z = 1, d(x) = 2.033 g cm(-3). Compound 2 has a = 5.781(1) Angstrom, b = 7.720(1) Angstrom, c = 24.036(5) Angstrom, Z = 1, d(x) = 2.174 g cm(-3). Compound 4 has a = 5.379(1) Angstrom, b = 7.028(1) Angstrom, c = 23.884(4) Angstrom, Z = 1, d(x), = 2.440 g cm(-3). These solids contain pseudo one-dimensional chains with a charge-density-wave (CDW) ground state structure: X-Pt(IV)-X ... Pt(II)... X. Single crystal resonance Raman experiments were performed on all compounds to measure the symmetric X-Pt-X stretching frequency nu(1) and the band edge. It is shown that the optical and electronic properties and, therefore, the CDW strength of these one-dimensional materials may be systematically varied over a wide range by employing different combinations of L-L and Y; templates composed of hydrogen bonded networks of L-L and Y were found to control the metal-metal separation, thereby controlling the X-Pt(IV)-X ... Pt(II)... X chain geometry. Relationships between the CDW strength, measured as the ratio of the short M(IV)-X distance to the long M(II)-X distance, the band gap energy nu(1) and the Pt-Pt separation are developed. The reaction coordinate is found to be dominated by changes in the M-M and Pt(II)-X separations over most of the range studied, with contributions from changes in the Pt-IV-X bonds becoming important only at the smallest M-M separations. Direct evidence demonstrating that MX systems are true Peierls distorted systems is also presented. These results are consistent with modeling based on Peierls-Hubbard hamiltonians. This work explains the unusual pressure and temperature dependences that have been observed for the structures and optical properties of this class of materials and also provides a wealth of information to benchmark many-body theoretical calculations modeling electron-electron and electron-phonon interactions in one-dimensional materials.