Growth of crystals, composite crystal structures and electrical resistance of high-pressure phases of Mg2B1+x (B=Sn,Ge)

Single crystals of high-pressure phases of Mg2B1+x (B=Sn, Ge; x similar to 0.1) were grown at 5.8 GPa within the temperature range 600-1500 K. The composite crystal structures were investigated by the X-ray technique. Both structures are found to be closely related and each is composed of two incommensurate atomic subsystems. Two identically oriented hexagonal subcells are characterized by a common period a and an irrational value of the ratio gamma=c(I)/c(II) between incommensurate periods. The values of the cell parameters an, for Mg2Sn1+x a=13.245(3) Angstrom, c(I)=4.450(2) Angstrom, c(II)=3.273(3) Angstrom, gamma similar to 1.36, and for Mg2Ge1+x, a=12.53(1) Angstrom, c(I)=4.175(3) Angstrom, c(II)=3.037(3) Angstrom, gamma similar to 1.375. All Mg atoms and most of the Sn(Ge) atoms form subsystem I close to the alpha-PbCl2 structure type (cotunnite). Both the structure type and the stoichiometry of the compounds are disturbed by Sn(Ge) atoms which form incommensurate subsystem II of the composite. Resistance measurements at low temperature confirm conduction of metallic character for both compounds. (C) 1998 Elsevier Science S.A. All rights reserved.