Source of material The new phase emerged as a by-product from a synthesis as described in [1] . 3 mmol (0.285 g) of 3-hydroxypyridine (pyOH = C 5H5NO) were dissolved in 20 ml distilled water and 1 mmol (0.354 g) of K 2Cu(ox)2 • 2H2O (ox = oxalate) was dissolved in 60 ml water, both under continuous stirring at slightly elevated temperature. 3-hydroxypyridine was a commercial product, but K 2Cu(ox)2 • 2H2O was prepared as a well crystallized precipitate from aqueous solutions of K 2(ox) • H2O

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... CuCl2 • 2H2O in the molar ratio 2:1. The pyOH solution was then added dropwise to the stirred potassium copper oxalate solution. The light blue solution was allowed to stand at room temperature. After three weeks two different crystal phases began to grow by evaporation of the solvent: (i) first, light-green elongated platelets formed with composition (pyOH)Cu(ox) • H 2 O. They proved to be a new phase, which is presented here. (ii) Afterwards blue crystals formed with isometric shape. They could be identified by means of X-ray diffraction as orthorhombic (pyOH) 2Cu(ox) [1] .