The crystal chemistry of the STACK family of orthoborates having the n
ominal formula A6MM'(BO3)6 where A = Sr, Ba, Pb, or Ln (Ln = lanthanid
e), and M, M' = +2, +3, or +4 metal cation is described. Over 150 indi
vidual members of the family have been synthesized; they crystallize i
n the trigonal space group R3BAR with unit-cell volumes ranging from 1
143.8(1) to 1365.6(3) angstrom3. In this report, the metal site prefer
ences, disorder, and solid solubility of these phases are discussed. I
nterrelationships between this structure and the layered structure typ
e of Ba3Sc(BO3)3 are also detailed.