Liquid-Liquid Critical Point in Heavy Water

According to the liquid-liquid critical-point hypothesis about water, two liquid waters exist at low temperatures and are supposed to be merged at a critical point. The low-temperature metastable melting curves of ${\mathrm{D}}_2\mathrm{O}$ ices have been measured. It is found that the melting curve of ${\mathrm{D}}_2\mathrm{O}$ ice III is smoothly curved around 25 MPa and 238 K, whereas the melting curve of ${\mathrm{D}}_2\mathrm{O}$ ice IV undergoes an abrupt change of slope at 100 MPa and 220 K. This is consistent with the existence of a liquid-liquid critical point in the region between the melting curve of ${\mathrm{D}}_2\mathrm{O}$ ice III and the melting curve of ${\mathrm{D}}_2\mathrm{O}$ ice IV.