Thermal Polymorphism and Decomposition of Y(BH₄)₃

Abstract

The structure and thermal decomposition of Y(BH 4 ) 3 is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), 11 B MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH 4 and YCl 3 in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH 4 ) 3 , denoted β-Y(BH 4 ) 3 , is discovered besides the Y(BH 4 ) 3 polymorph previously reported, denoted R-Y(BH 4 ) 3 . β-Y(BH 4 ) 3 has a cubic crystal structure and crystallizes with the space group symmetry Pm3m and a bisected a-axis, a = 5.4547(8) Å , as compared to R-Y(BH 4 ) 3 , a = 10.7445(4) Å (Pa3). β-Y(BH 4 ) 3 crystallizes with a regular ReO 3 -type structure, hence the Y 3þ cations form cubes with BH 4 -anions located on the edges. This arrangement is a regular variant of the distorted Y 3þ cube observed in R-Y(BH 4 ) 3 , which is similar to the high pressure phase of ReO 3 . The new phase, β-Y(BH 4 ) 3 is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from R-to β-Y(BH 4 ) 3 , at ∼180°C. Upon further heating, β-Y(BH 4 ) 3 decomposes at ∼190°C to YH 3 , which transforms to YH 2 at 270°C. An unidentified compound is observed in the temperature range 215-280°C, which may be a new Y-B-H containing decomposition product. The final decomposition product is YB 4 . These results show that boron remains in the solid phase when Y(BH 4 ) 3 decomposes in a hydrogen atmosphere and that Y(BH 4 ) 3 may store hydrogen reversibly.

Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH

“…However, it requires very tough conditions for rehydrogenation [3] and suffers from capacity loss on cycling due to the formation of higher boranes [4]. Another class of materials to be considered is rare earth (RE) borohydrides, with hydrogen capacities varying between 9.0 wt % for Y(BH 4 ) 3 and 5.5 wt % for Yb(BH 4 ) 3 [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Li-containing RE borohydrides are also considered as solid state electrolytes for new battery applications, due to their high Li-ion conductivities [20][21][22][23].…”

Structure and Characterization of KSc(BH4)4

“…13 However, no experimental evidence supporting the existence of Sc(BH 4 ) 3 is reported up to now. In contrast, Y(BH 4 ) 3 is found to crystallize in two different cubic polymorphs, one at room temperature 20 and one above 453 K. 22,23 On the other hand, scandium readily forms bimetallic homoleptic borohydrides based on the [Sc(BH 4 …”

Powder diffraction methods for studies of borohydride-based energy storage materials

“…The two phases have closely relatedcubic structures,the major difference is in the ordering schemes for the BH 4 anions and the deformation of the octahedral environment for the Yatom: in the low-temperature Pa " 3 3 phase it is distorted (Sato et al,2008), while in the high-temperature phase it is undistorted.F urthermore, the high-temperature phase is 4.6% less dense than the lowtemperatureo ne and contains relatively large unoccupied voids of 39 # A 3 ,accounting for almost aquarter of the volume. The metal atoms ubstructure in the high-temperature phase,d etectedb ys ynchrotron X-rayp owder diffraction, has the regularR eO 3 type (Ravnsbaek et al, 2010c). An independent studyb yn eutron powder diffraction (Frommen et al,2 010) revealed a Fm " 3 3 c superstructure with an ordered arrangement of the BH 4 groups,w here half of these are flipped to minimize H--H repulsion on the cost of al ess densely packeds tructure (Fig.…”