Solid–solid phase transitions in K2Cr2O7 at pressures below 0.4 GPa from differential thermal analysis under hydrostatic pressure

Differential thermal analyses of reagent-grade K2Cr2O7, under Ar pressure up to 0.4 GPa, yield accurate temperatures for two reversible solid–solid transitions and some data for melting. Onset temperatures for the transition of the low-temperature polymorph (low) to the high-temperature polymorph (high) increase linearly with pressure from 255 °C at 0.1 MPa, with slope 0.289µK Pa–1; for the high → low transition onset temperatures also increase linearly with pressure, but from 237 °C at 0.1 MPa and with slope 0.303µK Pa–1. A new phase, γ, was discovered, with the low–high–γ triple point located near 0.135 GPa and 279 °C. The γ–high transition was followed to melting, near 0.41 GPa and 457 °C, and also metastably below the triple point with low; hystereses are ≲ 4 K for the γ–high transition. Relative transition entropies were deduced from areas under the peaks of the curves of differential temperature against time; near the low–high–γ triple point the γ–high transition entropy appears slightly greater than that for low–high. The low–γ transition was not detected by these techniques and a quench from the γ field to ambient conditions did not yield an X-ray powder diffraction pattern distinctly different from low. Variation of volume with temperature at 0.1 MPa is deduced from the critically evaluated 0.1 MPa data plus the constraints imposed by the high-pressure data.