Synthesis, Structure, and Properties of BaCO3-(hR5)-Type PbCO3

A new recoverable form of PbCO 3 has been synthesized at a pressure of 10.8 GPa and at temperatures higher than 735 K with a trigonal structure ( a = 5.20727(14) Å and c = 8.4326(3) Å in a hexagonal setting at 1 bar and room temperature). This new structure of PbCO 3, which has been derived from in situ diffraction data and then confirmed and refined using DFT calculations, is of the BaCO 3 -( hR 5)-type, also known for BaCO 3 at 1 bar and high temperatures. Data collected during its coexistence with aragonite-type PbCO 3 indicate an increase of density of 4.8% at the transition. Our observations of the transformation are compatible with the transformation in the 6–10 GPa range to the field labeled PbCO 3 -III in previous studies of the phase diagram. DFT calculations have been employed to understand the Raman signature of this phase and predict its properties, such as its nonlinear optical effect. The latter was then experimentally studied, and here, we show that PbCO 3 -( hR 5) is a chemically simple carbonate with very strong SHG effect.