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Polymorphism and Thermodynamics of Y(BH(4))(3) from First Principles

Author:
Lee, Young-Su  Shim, Jae-Hyeok  Cho, Young Whan  


Journal:
JOURNAL OF PHYSICAL CHEMISTRY C


Issue Date:
2010


Abstract(summary):

Structure and thermodynamics of the recently discovered two polymorphs of Y(BH(4))(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 [BH(4)](-) groups is considered and compared with NaBH(4), which has a disordered [BH(4)](-) arrangement at room temperature. In the case of NaBH(4), the structure stays in a local energy minimum irrespective of the [BH(4)](-) orientation, but in Y(BH(4))(3), [BH(4)](-) reorientation is suppressed by a strong repulsive force created by close H-H contacts and the structure becomes unstable, thus favoring an ordered [BH(4)](-) arrangement. The calculated high energy barrier for the [BH(4)](-) reorientation partly accounts for the slow phase transition observed in Y(BH(4))(3), again making a good contrast with the facile [BH(4)](-) flipping in NaBH(4). The thermodynamics of Y(BH(4))(3) appears quite attractive, exhibiting a lower dissociation temperature than Mg(BH(4))(2) under 1 bar of H(2).


Page:
12833---12837


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