Dry autoclaving of the metal acetates under similar conditions resulted in various phases and morphologies of metal carbonates, oxycarbonates, and oxides. The acetates of calcium (Group IIA) gave ∼100 nm thick CaCO 3 nanosheets, and 3d transition metals (Mn and Fe) gave MnCO 3 microcubes and Fe 3 O 4 tetragonal bipyramids. The lanthanides (La, Ce, and Pr) presented hierarchical flower-shaped structures with self-assembled 85−250 nm thick nanosheets comprising of La 2 O 2 CO 3 and LaOHCO 3 mixed phases, CeO 2 and Pr 2 O 2 CO 3 , respectively. The Gibb's free energy of formation (ΔG f o ) of the oxide or decomposition of the carbonates at elevated temperatures >700 °C controls the final phase. Elemental line scans showed carbon coating on the nanosheets, whereas carbon existed as separate microspheres whenever the micron-sized morphologies were obtained. The solidification kinetics of the supercritical metal intermediates and carbon were comparable when the freezing point (FP) of the metals is <1000 °C such that once the nanosheets formed from stacking of the solidified nuclei, carbon could wrap and stabilize the nanosheets. The autoclaved products were air heated to obtain the metal oxide phases. Defect-related emissions were observed from the rare-earth oxides.