A NEW CRITERION FOR EXPLAINING THE POLAR AND RETICULAR MICROHARDNESS ANISOTROPY OF TETRABORIDES AND HEXABORIDES OF LANTHANIDES AND ACTINIDES

Knoop microhardness measurements were made on single crystals of hexa- and tetraborides of lanthanides and actinides prepared by the solutio n-melt method, Polar and reticular anisotropy of the microhardness was established (P=20 g), The highest polar anisotropy was observed for N dB6 (410 kg/mm(2)) and for SmB4 (760 kg/mm(2)), The highest reticular anisotropy was observed for SmB6 (620 kg/mm(2)) and for HoB4 (1120 kg/ mm(2)), To explain the microhardness anisotropy, a new criterion of in verse density of atoms in a layer within a unit cell (unit volume of i dentity in the crystal structure or in a block of layers in the crysta l structure) was proposed, It was found that the inverse density of me tal or boron atoms in a layer correlates with the reticular and polar microhardness anisotropy of hexa- and tetraborides of lanthanides and actinides. (C) 1997 Academic Press.