The monoclinic/orthorhombic phase transition in Ba2Cu(HCOO)(6): An opticaland X-ray powder diffraction study

alpha-Ba2Cu(HCOO)(6) grown at room temperature crystallizes in space group P2(1). On heating a reversible, hysteretic, equitranslational, first-order phase transition at about 60-90 degrees C takes place. Here we discuss the influence of thermal treatments on the occurrence, coexistence and stabilit y ranges of the two observed phases, as studied by several techniques (opti cal microscopy, differential scanning calorimetry and X-ray diffraction). O n cooling a single set of domain walls parallel to (001) have been observed . Taking into account experimental results acid the crystal pseudosymmetry of the alpha phase we propose that the high-temperature beta phase crystall izes in space group Pbnm.