The monoclinic/orthorhombic phase transition in Ba2Cu(HCOO)6: An optical and x-ray powder diffraction study

α-Ba₂Cu(HCOO)₆ grown at room temperature crystallizes in space group P2₁. On heating a reversible, hysteretic, equitranslational, first-order phase transition at about 60-90°C takes place. Here we discuss the influence of thermal treatments on the occurrence, coexistence and stability ranges of the two observed phases, as studied by several techniques (optical microscopy, differential scanning calorimetry and X-ray diffraction). On cooling a single set of domain walls parallel to (001) have been observed. Taking into account experimental results and the crystal pseudosymmetry of the α phase we propose that the high-temperature β phase crystallizes in space group Pbnm.