True composition and structure of hexagonal "YAlO3", actually Y3Al3O8CO3.

The discovery of a brilliant-blue color upon the introduction of Mn(3+) to the trigonal-bipyramidal (TBP) sites in YInO(3) has led to a search for other hosts for Mn(3+) in TBP coordination. An obvious choice would be YAlO(3). This compound, which has only been prepared through a citrate precursor route, has long been considered isostructural with YInO(3). However, Mn(3+) substitutions into YAlO(3) have failed to produce a product with the anticipated color. We find that the hexagonal structure for YAlO(3) 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 YAlO(3), with the perovskite structure liberating CO(2). The compound long assumed to be a hexagonal form of YAlO(3) is actually an oxycarbonate with the ideal composition Y(3)Al(3)O(8)CO(3). 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 (27)Al NMR, Fourier transform infrared, and transmission electron microscopy. Refinement of neutron diffraction data indicates a composition of Y(3)Al(3)O(8)CO(3). We find that the hexagonal structures of YGaO(3) and YFeO(3) from the citrate route are also stabilized by small amounts of carbonate. Surprisingly, Y(3)Al(3)O(8)CO(3) forms a complete solid solution with YBO(3) having tetrahedral borate groups. Other unlikely solid solutions were prepared in the YAlO(3)-YMnO(3), YAlO(3)-YFeO(3), YAlO(3)-YBO(3), YBO(3)-YMnO(3), YBO(3)-YFeO(3), and YBO(3)-YGaO(3) systems.