Single crystal synthesis of δ-(Al,Fe)OOH

Single crystals of delta-AlOOH, delta-(Al-0.953,Fe-0.047)OOH, and delta-(Al-0.878,Fe-0.122)OOH with dimensions up to similar to 0.4-0.6 mm were synthesized by the high-pressure hydrothermal method. Synthesis experiments were performed at 21 GPa and 1470 K for 4 h using a Kawai-type multi-anvil apparatus. The crystals of delta-AlOOH, delta-(Al-0.953,Fe-0.047)OOH, and delta-(Al-0.878, Fe-0.122)OOH were colorless, yellowish green, and brown, respectively. Mossbauer spectra showed 95-100% Fe3+/Sigma Fe at the octahedral site in delta-(Al,Fe)OOH. Chemical compositions of delta-(Al-0.953,Fe-0.047)OOH and delta-(Al-0.878,Fe-0.122)OOH are homogeneous with Fe/(Al+Fe) of 0.0469(8) and 0.122(3), respectively. Unit-cell parameters of delta-AlOOH are consistent with those of previous studies, and they increase with Fe/(Al+Fe). These results confirm that delta-AlOOH can form a solid solution with epsilon-FeOOH. The crystals contained a small number of fluid inclusions. The syntheses of large single crystals of delta-(Al,Fe)OOH will facilitate investigation of their phase stability, physical properties including elasticity and elastic anisotropy, behavior of hydrogen bonding, and spin state of Fe, which will improve models of the water and oxygen cycles in the deep Earth.