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

Area of Science:

• Solid-state chemistry and materials science.
• Exploration of rare-earth aluminates and their structural properties.

Background:

• The trigonal-bipyramidal (TBP) coordination of Mn(3+) in YInO(3) yields a blue color, prompting searches for similar hosts.
• YAlO(3) was considered a potential host due to its presumed structural similarity to YInO(3).

Purpose of the Study:

• To investigate the structural and compositional properties of hexagonal YAlO(3) prepared via a citrate route.
• To determine the suitability of YAlO(3) as a host for Mn(3+) in TBP coordination.

Main Methods:

• Powder neutron and X-ray diffraction as a function of temperature.
• Magic-angle-spinning (27)Al Nuclear Magnetic Resonance (NMR).
• Fourier transform infrared spectroscopy and transmission electron microscopy.

Main Results:

• The hexagonal phase, previously assumed to be YAlO(3), was identified as an oxycarbonate, Y(3)Al(3)O(8)CO(3).
• Analysis indicated that aluminum (Al) predominantly exhibits a coordination number of 6, not TBP.
• The oxycarbonate transforms to perovskite YAlO(3) upon heating, releasing CO(2).
• Hexagonal YGaO(3) and YFeO(3) from citrate routes are also stabilized by carbonate.
• Y(3)Al(3)O(8)CO(3) forms a complete solid solution with YBO(3).

Conclusions:

• The long-held structural model for hexagonal YAlO(3) was incorrect; it is an oxycarbonate.
• Mn(3+) doping in this phase does not yield the expected blue color due to the absence of TBP sites.
• The study reveals new insights into the formation and stability of rare-earth oxycarbonates and their solid solutions.