G.S. Kanner

Control of selected physical properties of MX solids: an experimental and theoretical investigation

Received 17 January 1995; accepted in final form 2 May 1995 Abstract A series of eight materials of stoichiometry [Pt(L-L)2X2][Pt(L-L)2]Y 4 (X is Cl, Br; L-L is 1,2-diaminoethane (en) or 1,2- diaminocyclohexane (chxn); Y is CIO4, X-) were synthesized. Crystal structures were determined for the compounds [Pt(chxn)zC12][Pt(chxn)2](C104) 4 1, [Pt(chxn)2Brz][Pt(chxn)2](CIO4)4 2, and [Pt(chxn)zBrz][Pt(chxn)z]Br 4 4. All three of these compounds crystallize in the orthorhombic space group I222. Compound 1 has a = 5.711(1) ,~, b = 7.804(1) A, c = 24.101(7) A,, Z = 1, dx = 2.033 g cm 3. Compound 2 has a = 5.781(1) A, b = 7.720(1) ,~, c = 24.036(5) ,~, Z = 1, d× = 2.174 g cm -3. Compound 4 has a = 5.379(1) ,~, b = 7.028(1) A, c = 23.884(4) ,~, Z = 1, dx = 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 sym- metric X Pt X stretching frequency u~ 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. Rela- tionships 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 ul 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 ptlV-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. 1. Introduction There has been recent interest in halogen bridged * Corresponding authors.

Pressure tuning of the charge-density wave in the halogen-bridged transition-metal solid Pt2Br6(NH3)4.

We report the pressure dependence up to 95 kbar of Raman-active stretching modes in the quasi-one-dimensional MX chain solid

Time-resolved photoinduced absorption and raman scattering in the MX chain solid PtCl

{\mathrm{Pt}}_{2}

Excitation dependence of the subpicosecond dynamics of excitons and polarons in the MX chain solid PtCl

{\mathrm{Br}}_{6}

Surface structure of MX-chains studied by atomic force microscopy

(

Pressure and Temperature Induced Frequency Shifts of Raman-Active Phonons in the MX Solid Pt2Br6(NH3)4

{\mathrm{NH}}_{3}

Ultrafast Relaxation Upon One- and Two-Photon Absorption in the >Halogen-Bridged Transition Metal (MX) Compound PtCl

{)}_{4}

Time-resolved and pressure dependent studies of MX chain solids

. The data indicate that a predicted pressure-induced insulator-to-metal transition does not occur, but are consistent with the solid undergoing either a three-dimensional structural distortion or a transition from a charge-density wave to another broken-symmetry ground state. We show that such a transition can be well modeled within a Peierls-Hubbard Hamiltonian.