Optical and elastic properties of linear metal-chain compounds: The 1,2-benzoquinonedioximates (bqd) of nickel, palladium, and platinum, M(bqd) ₂

Polarized single crystal reflectance spectra of the linear metal-chain complexes Pt(bqd)₂, α-Pd(bqd)₂, β-Pd(bqd) ₂, Pd(bqd)₂I_0.55, Ni(bqd)₂, and Ni(bqd)₂I_0.5 (bqd^-=ion of 1,2- benzoquinonedioxime, bqdH) have been recorded at room temperature. Well-defined sharp absorption bands polarized parallel to the metal atom chains are observed in the frequency region from 0.5 to 2.5 eV, and are most reasonably identified as the interband electronic transitions between the nd_z² valence and the (n+1)p_z conduction bands in a direct optical excitation process (n=3, 4, 5). The elastic behavior (Young's modulus and internal friction) examined using a vibrating reed technique in the temperature range 17-300 K reveals, in the case of α-Pd(bqd)₂, a peculiar phase transformation occurring along the metal atom chains below 100 K. From powder x-ray diffraction patterns recorded at ambient and at liquid nitrogen temperatures, this phase transition is interpreted as a lattice distortion resulting in the formation of stable chains of dimerized neutral metal complexes.