Properties of H₂O at elevated pressure and temperature are of fundamental importance in both condensed matter physics and planetary sciences. We studied behavior of H₂O in externally heated diamond anvil cells (DACs) at pressures up to 50 GPa and temperatures to 1150 K by combining visual observations, Raman spectroscopy, and X-ray powder diffraction. The melting curve of H₂O was found to be well described by the Simon equation P(GPa) = 2.2 + 1.31{[T(K)/364]^3.3 – 1}. Above 30 GPa and 950 K, using visual observations and Raman spectroscopy, we found an...