Low-temperature structural phase transition in synthetic libethenite Cu2PO4OH

Low-temperature structural properties of the synthetic mineral libethenite Cu2PO4OH were investigated by single-crystal X-ray diffraction, synchrotron X-ray powder diffraction, specific heat measurements, and Raman spectroscopy. A second-order structural phase transition from the Pnnm symmetry (a=8.0553(8) Å, b=8.3750(9) Å, c=5.8818(6) Å at 180 K) to the P21/n symmetry (a=8.0545(8) Å, b=8.3622(9) Å, c=5.8755(6) Å, β=90.0012(15) at 120 K) was found at 160 K during cooling. At 120 K, the monoclinic angle is 90.0012(15) from single crystal X-ray data vs 90.083(1) from powder X-ray diffraction data. The P21/n–to–Pnnm transition may be a general feature of the adamite-type compounds, M2XO4OH.

Graphical AbstractFragments of experimental synchrotron X-ray powder diffraction patterns of Cu2PO4OH between 100 and 280 K. Arrows show additional reflections that appear below 160 K in the monoclinic P21/n phase.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A low-temperature phase transition was found in the mineral libethenite Cu2PO4OH. ► No magnetic anomalies and weak specific heat anomalies are detected. ► Phase transition is of the second order. ► Libethenite may exemplify a general feature of the adamite-type compounds.