Hydrothermal synthesis and formation of a synthetic analogue of reichenbachite, Cu5(PO4)2(OH)4

Reichenbachite (Cu5(PO4)2(OH)4) is a rare copper-hydroxy-phosphate, the synthetic accessibility and phase evolution of which are still poorly understood. Here we report the first known reproducible hydrothermal synthesis to yield high purity reichenbachite, facilitated by alkaline conditions at 180 °C. Systematic time-controlled experiments clearly reveal a kinetic-thermodynamic bias: pseudomalachite forms rapidly as the metastable kinetic phase and reichenbachite emerges as the thermodynamic product after extended reaction times. Powder X-ray diffraction experiments combined with Rietveld refinements confirm phase selectivity as an outcome of reaction conditions. Scanning electron microscopy shows characteristic morphological features of each polymorph. Through a combination of order–disorder theory, classical nucleation theory, and Ostwald’s rule of stages we propose a mechanism of formation through polymorphic interconversion driven by synthesis duration and alkalinity. This study not only establishes a new reproducible synthesis for a previously elusive mineral but also provides insights into fundamental metastable-equilibrium phase transitions in copper hydroxy-phosphate systems, with implications for crystal growth theory and design of functional phosphate-materials.