Single-Crystal Polarized Optical Absorption Spectroscopy of the One-Dimensional Ferrimagnet Mn<sup>II</sup>Cu<sup>II</sup>(pba)(H<sub>2</sub>O)<sub>3</sub>·2H<sub>2</sub>O (pba = 1,3-Propylenebis(oxamato))

Powder and single-crystal optical absorption of the ferrimagnet Mn<sup>II</sup>Cu<sup>II</sup>(pba)(H<sub>2</sub>O)<sub>3</sub>·2H<sub>2</sub>O (denoted MnCu) and the Mn-doped compound Mn<sub>0.1</sub>Mg<sub>0.9</sub>Cu(pba)(H<sub>2</sub>O)<sub>3</sub>·2H<sub>2</sub>O (denoted Mn<sub>0.1</sub>Mg<sub>0.9</sub>Cu) with pba standing for 1,3-propylenebis(oxamato) was investigated in the 10−300 K range. The crystal structure of MnCu was previously reported, and consists of bimetallic chains with octahedral Mn<sup>II</sup> and square pyramidal Cu<sup>II</sup> ions linked by oxamato bridges, MnCu and Mn<sub>0.1</sub>Mg<sub>0.9</sub>Cu being isostructural. The spectra of both MnCu and Mn<sub>0.1</sub>Mg<sub>0.9</sub>Cu show an important dichroism along the chain direction, due to the strong polarization of the Cu<sup>II</sup> band at around 16 000 cm<sup>-1</sup> in this direction. They exhibit narrow and intense spin-forbidden Mn<sup>II</sup> transitions in the 24000−25000 cm<sup>-1</sup> range, which are activated by an exchange mechanism. The polarization and thermal dependence of the <sup>6</sup>A<sub>1g</sub> → <sup>4</sup>A<sub>1g</sub>, <sup>4</sup>E<sub>g</sub>(G) Mn<sup>II</sup> transitions were recorded. The polarization along the chain axis was interpreted in the framework of the pair mechanism first introduced by Tanabe and co-workers. A theoretical expression for the thermal dependence of the intensity was derived by considering the Cu<sup>II</sup> spin as a quantum spin and the Mn<sup>II</sup> spin as a classical spin, and compared with the experimental data. The interaction parameter between the local ground states has been found to be <i>J</i> = −25 cm<sup>-1</sup> using the spin Hamiltonian <b>H</b> = −<i>J</i>∑<i><sub>i</sub></i>(<b>S</b><sub>Mn,</sub><i><sub>i</sub></i><b>s</b><sub>Cu,</sub><i><sub>i</sub></i> + <b>S</b><sub>Mn,</sub><i><sub>i</sub></i><sub>+1</sub><b>s</b><sub>Cu,</sub><i><sub>i</sub></i>). The spectra of Mn<sub>0.1</sub>Mg<sub>0.9</sub>Cu showed cold and hot bands, whose energy difference is directly related to <i>J</i> and the interaction parameter <i>J</i>* between the Cu<sup>II</sup> ion in its ground state and the Mn<sup>II</sup> ion in its spin-flip excited state. <i>J</i>* has been estimated to be +40 cm<sup>-1</sup>. These results have been compared to those obtained with other Mn<sup>II</sup>Cu<sup>II</sup> compounds. The complementarity between optical and magnetic properties has been discussed.