True Composition and Structure of Hexagonal “YAlO3”, Actually Y3Al3O8CO3

The discovery of a brilliant-blue color upon the introduction of Mn3+ to the trigonal-bipyramidal (TBP) sites in YInO3 has led to a search for other hosts for Mn3+ in TBP coordination. An obvious choice would be YAlO3. This compound, which has only been prepared through a citrate precursor route, has long been considered isostructural with YInO3. However, Mn3+ substitutions into YAlO3 have failed to produce a product with the anticipated color. We find that the hexagonal structure for YAlO3 with Al in TBP coordination proposed in 1963 cannot be correct based on its unit cell dimensions and bond-valence sums. Our studies indicate instead that all, or nearly all, of the Al in this compound has a coordination number (CN) of 6. Upon heating in air, this compound transforms to YAlO3, with the perovskite structure liberating CO2. The compound long assumed to be a hexagonal form of YAlO3 is actually an oxycarbonate with the ideal composition Y3Al3O8CO3. The structure of this compound has been characterized by powder neutron and X-ray diffraction data obtained as a function of temperature, magic-angle-spinning 27Al NMR, Fourier transform infrared, and transmission electron microscopy. Refinement of neutron diffraction data indicates a composition of Y3Al3O8CO3. We find that the hexagonal structures of YGaO3 and YFeO3 from the citrate route are also stabilized by small amounts of carbonate. Surprisingly, Y3Al3O8CO3 forms a complete solid solution with YBO3 having tetrahedral borate groups. Other unlikely solid solutions were prepared in the YAlO3–YMnO3, YAlO3–YFeO3, YAlO3–YBO3, YBO3–YMnO3, YBO3–YFeO3, and YBO3–YGaO3 systems.