Polymorphism and Thermodynamics of Y(BH4)(3) from First Principles

Abstract

Structure and thermodynamics of the recently discovered two polymorphs of Y(BH4)(3) are investigated by first-principles calculation. Simulated X-ray and neutron diffraction patterns combined with structure analysis from first principles enable us to assign a space group Fm (3) over barc to the high-temperature polymorph among the several proposed space groups. An orientational disorder of [BH4](-) groups is considered and compared with NaBH4, which has a disordered [BH4](-) arrangement at room temperature. In the case of NaBH4, the structure stays in a local energy minimum irrespective of the [BH4](-) orientation, but in Y(BH4)(3), [BH4](-) reorientation is suppressed by a strong repulsive force created by close H-H contacts and the structure becomes unstable, thus favoring an ordered [BH4](-) arrangement. The calculated high energy barrier for the [BH4](-) reorientation partly accounts for the slow phase transition observed in Y(BH4)(3), again making a good contrast with the facile [BH4](-) flipping in NaBH4. The thermodynamics of Y(BH4)(3) appears quite attractive, exhibiting a lower dissociation temperature than Mg(BH4)(2) under 1 bar of H-2.