  We obtain monatomic glass formers in simulations by modifying the tetrahedral
character in a silicon potential to explore a triple point zone between
potentials favoring diamond (dc) and bcc crystals. dc crystallization is always
preceded by a polyamorphic transformation of the liquid, and is frustrated when
the Kauzmann temperature of the high temperature liquid intersects the
liquid-liquid coexistence line. The glass forming liquids are extraordinarily
fragile. Our results suggest that Si and Ge liquids may be vitrified at a
pressure close to the diamond-beta-tin-liquid triple point.
