High-Pressure Investigation on Calcium Borohydride

Abstract

Structural properties of calcium borohydride, Ca(BH 4 ) 2 , containing R and β polymorphs are investigated under pressure by in situ synchrotron X-ray diffraction up to 13 GPa and by Raman spectroscopy up to 25 GPa. Results show that the β phase transforms to a highly disordered structure above 10.2 GPa, whereas the R phase is stable in the pressure range studied. Rietveld analysis of the X-ray diffraction pattern collected at ambient conditions shows that the R phase is an orthorhombic F2dd structure with lattice parameters a ) 8.82(7) Å, b ) 13.05 (10) Å, c ) 7.404(5) Å, and V ) 851.8(10) Å 3 (64.13 cm 3 /mol). The β phase was checked for all the suggested tetragonal structures, and the best match is obtained with space group P4 2 /m; the lattice parameters are a ) 6.88(4) Å, c ) 4.32(5) Å, and V ) 204.82(18) Å 3 (61.68 cm 3 /mol). Pressure volume data obtained for R-Ca(BH 4 ) 2 are fitted with the third-order Birch-Murnaghan equation of state that yields a bulk modulus value of K 0 ) 22.95(4) GPa with K 0 ′ ) 2.63(0.5).

High-pressure crystal structure prediction of calcium borohydride using density functional theory

“…Two experimental studies of Ca(BH 4 ) 2 under high pressure have so far been published, 14,15 which investigated the structural behavior of the α and β phases under pressures up to 25 GPa and room temperature by x-ray powder diffraction and Raman spectroscopy; no transitions into different crystalline polymorphs were observed for either phase, but the β phase was observed to transform into an unknown disordered or amorphous phase. …”

High-Pressure Structures and Transformations of Calcium Borohydride Probed by Combined Raman and Infrared Spectroscopies

“…Because all the fundamental vibrational frequencies of BH 4 -lie coincidentally in the proximity of the very intense (both the first-and the secondorder) Raman modes of diamond, structural characterization using Raman spectroscopy only is difficult and subject to ambiguity. The current study represents a significantly improved experimental work in which high-quality Raman spectroscopic data in both the BH 4 -deformation region and the lattice region that were missing from the previous studies 15 were carefully obtained and analyzed. More importantly, all of the fundamental internal vibrational frequencies were clearly resolved in IR spectroscopy without interference.…”

Structural Characterization of Metal Hydrides for Energy Applications

“…This is higher than the calculated bulk modulus for the polymorphs of Ca(BH 4 ) 2 by first-principles method which was ~15 GPa [163]. The compressibility of Ca(BH 4 ) 2 is of the order of many metal borohydrides [96,97]. Any α to β phase transition was not observed in this experiment, around 5.3 GPa, as reported by…”

“…Ca(BH 4 ) 2 has an orthorhombic-F2dd and Mg(BH 4 ) 2 has a complex hexagonal structure with space group P6 1 22 and both of them exhibits polymorphism [94,97,160]. Therefore it is interesting to study structural behavior of metal borohydrides under various conditions of pressures and temperatures.…”