Crystal structures and comparisons of huntite aluminum borates REAl(3)(BO(3))(4) (RE = Tb, Dy and Ho)

Three huntite-type aluminoborates of stoichiometry REAl(3)(BO(3))(4) (RE = Tb, Dy and Ho), namely, terbium/dysprosium/holmium trialuminium tetrakis(borate), were synthesized by slow cooling within a K(2)Mo(3)O(10) flux with spontaneous crystallization. The crystal structures were determined using single-crystal X-ray diffraction (SC-XRD) data. The synthesized borates are isostructural to the huntite [CaMg(3)(CO(3))(4)] structure and crystallized within the trigonal R32 space group. The structural parameters were compared to literature data of other huntite REAl(3)(BO(3))(4) crystals within the R32 space group. All three borates fit well into the trends calculated from the literature data. The unit-cell parameters and volumes increase linearly with larger RE cations whereas the densities decrease. All of the crystals studied were refined as inversion twins.