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 simila r 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 tec hnique. Both structures are found to be closely related and each is co mposed of two incommensurate atomic subsystems. Two identically orient ed hexagonal subcells are characterized by a common period a and an ir rational value of the ratio gamma=c(I)/c(II) between incommensurate pe riods. 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 simil ar to 1.36, and for Mg2Ge1+x, a=12.53(1) Angstrom, c(I)=4.175(3) Angst rom, 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 st ructure type (cotunnite). Both the structure type and the stoichiometr y of the compounds are disturbed by Sn(Ge) atoms which form incommensu rate subsystem II of the composite. Resistance measurements at low tem perature confirm conduction of metallic character for both compounds. (C) 1998 Elsevier Science S.A. All rights reserved.