PB-2[B5O9](OH)H2O - A NEW CENTROSYMMETRIC MODIFICATION OF NATURAL HILGARDITE - THE HILGARDITE-GROUP STRUCTURES AS MEMBERS OF THE OD FAMILY

Two modifications of new lead berate Pb-2[B5O9](OH) H2O are prepared b y the hydrothermal synthesis. One of these modifications is isostructu ral with the mineral hilgardite Ca-2[B5O9]Cl . H2O: it is monoclinic, polar, and exhibits nonlinear-optical properties. The other modificati on is new; it also belongs to the monoclinic system, but has a centros ymmetric space group. The crystal structure of the latter modification is determined. The structures of the natural and synthetic compounds in the hilgardite family are discussed within the Dornberger-Schiff OD theory. The variety of structures of this family, including hypotheti cal structures, is described based on the OD groupoid.