Thermal Polymorphism and Decomposition of Y(BH4)(3)

The structure and thermal decomposition of Y(BH4)(3) is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), B-11 MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH4 and YCl3 in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH4)(3), denoted beta-Y(BH4)(3), is discovered besides the Y(BH4)(3) polymorph previously reported, denoted alpha-Y(BH4)(3). beta-Y(BH4)(3) has a cubic crystal structure and crystallizes with the space group symmetry Pm (3) over barm and a bisected a-axis, a = 5.4547(8) angstrom, as compared to alpha-Y(BH4)(3), a = 10.7445(4) angstrom (Pa (3) over bar). beta-Y(BH4)(3) crystallizes with a regular ReO3-type structure, hence the Y3+ cations form cubes with BH4 anions located on the edges. This arrangement is a regular variant of (he distorted Y3+ cube observed in alpha-Y(BH4)(3), which is similar to the high pressure phase of ReO3. The new phase, beta-Y(BH4)(3) is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from alpha- to beta-Y(BH4)(3), at similar to 180 degrees C. Upon further heating, beta-Y(BH4)(3) decomposes at similar to 190 degrees C to YH3, which transforms to YH2 at 270 degrees C. An unidentified compound is observed in the temperature range 215-280 degrees C, which may be a new Y B H containing decomposition product. The final decomposition product is YB4. These results show that boron remains in the solid phase when Y(BH4)(3) decomposes in a hydrogen atmosphere and that Y(BH4)(3) may store hydrogen reversibly.