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Control of selected physical properties of MX solids: an experimental and theoretical investigation |
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| Authors: | Brian Scott Steven P Love Gary S Kanner Sabina R Johnson Marianne P Wilkerson M Berkey Basil I Swanson A Saxena X Z Huang A R Bishop |
| Institution: | a Spectroscopy and Biochemistry (CST-14), Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Condensed Matter Theory (T-11), Los Alamos National Laboratory, Los Alamos, NM 87545, USA |
| Abstract: | A series of eight materials of stoichiometry Pt(L-L)2X2]Pt(L-L)2]Y4 (X is Cl, Br; L-L is 1,2-diaminoethane (en) or 1,2-diaminocyclohexane (chxn); Y is ClO?4, X?) were synthesized. Crystal structures were determined for the compounds Pt(chxn)2Cl2]Pt(chxn)2](ClO4)4 1, Pt(chxn)2Br2]Pt(chxn)2](ClO4)42, and Pt(chxn)2Br2]Pt(chxn)2]Br4 4. All three of these compounds crystallize in the orthorhombic space group I222. Compound 1 has a = 5.711(1) Å, b = 7.804(1) Å, c = 24.101(7) Å, Z = 1, dx = 2.033 g cm?3. Compound 2 has a = 5.781(1) Å, b = 7.720(1) Å, c = 24.036(5) Å, Z = 1, dx = 2.174 g cm?3. Compound 4 has a = 5.379(1) Å, b = 7.028(1) Å, c = 23.884(4) Å, 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 symmetric X---Pt---X stretching frequency v1 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 v1 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 PtIV---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. |
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