First-principles phase diagram calculations for the carbonate quasibinary systems CaCO(sub3)-ZnCO(sub3), CdCO(sub3)-ZnCO(Sub3), CaCO(sub3)-CdCO(sub3) and MgCO(Sub3)-ZnCO(sub3)
Z.T.Y. Liu, Benjamin P. Burton, S. V. Khare, P. Sarin
Quasibinary solid solutions of calcite-structure carbonate minerals play an important role in rock formation. We have investigated solid solutions with cations such as Ca, Cd, Mg and Zn by performing first-principles phase diagram calculations for Ca1-xZnxCO3, Cd1-xZnxCO3, Ca1-xCdxCO3 and Mg1-xZnxCO3 (0 < x < 1) with density functional theory (DFT), cluster expansion and Monte Carlo simulations. The end members and the dolomite structure in each system were individually studied to analyze their structural parameters and bonding characteristics. The consolute temperature/ continuous transition temperature is 1450 K for Ca1-xZnxCO3 and 1000 K for Cd1-xZnxCO3, but below 100 K for Ca1-xCdxCO3 and Mg1-xZnxCO3. In agreement with existing literature, the consolute temperature increases with increasing difference in cation sizes. Including the vibrational free energy contribution (vib) lowers the consolute temperature of Ca1-xZnxCO3 to 1200 K, but left that of Cd1-xZnxCO3 unchanged. The formation energy of the dolomite structure in each of these two systems is close to 0 within DFT uncertainty. If the dolomite structure is assumed to be stable, the phase diagram calculation predicts that it persists to 1150 K (without vib) and 1000 K (with vib) for Ca1-xZnxCO3, 900 K (without vib) and 450 K (with vib) for Cd1-xZnxCO3. In addition, the formation energy of the dolomite structure was decomposed by a two-step process, first a volume change and then a chemical-exchange-relaxation, and compared with a random solution. This division of energy demonstrates the stability of the dolomite structure relative to the random solution.
carbonates, calcite, dolomite, first principles phase diagram calculation
, Burton, B.
, Khare, S.
and Sarin, P.
First-principles phase diagram calculations for the carbonate quasibinary systems CaCO(sub3)-ZnCO(sub3), CdCO(sub3)-ZnCO(Sub3), CaCO(sub3)-CdCO(sub3) and MgCO(Sub3)-ZnCO(sub3), Chemical Geology
(Accessed December 8, 2023)