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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).

The structural stability of Mg(BH4)2, a promising hydrogen storage material, under pressure has been investigated in a diamond anvil cell up to 22 GPa with combined synchrotron X-ray diffraction and Raman spectroscopy. The analyses show a structural phase transition around 2.5 GPa and again around 14.4 GPa. An ambient-pressure phase of Mg(BH4)2 has a hexagonal structure (space group P61, a = 10.047(3) Å, c = 36.34(1) Å, and V = 3176(1) Å3 at 0.2 GPa), which agrees well with early reports. The structure of high-pressure phase is found to be different from reported theoretical predictions; it also does not match the high-temperature phase. The high-pressure polymorph of Mg(BH4)2 is found to be stable on decompression, similar to the case of the high-temperature phase. Raman spectroscopic study shows a similarity in high-pressure behavior of as-prepared Mg(BH4)2 and its high-temperature phase.

We have studied the structural stability of NaBH(4) under pressures up to 17 GPa and temperatures up to 673 K in a diamond anvil cell and formed an extended high P-T phase diagram using combined synchrotron x-ray diffraction and Raman spectroscopy. Even though few reports on phase diagram of NaBH(4) are found in current literature, up to our knowledge this is the first experimental work using diamond anvil cell in a wide pressure/temperature range. Bulk modulus, its temperature dependence, and thermal expansion coefficient for the ambient cubic phase of NaBH(4) are found to be 18.76(1) GPa, -0.0131 GPa K(-1), and 12.5x10(-5)+23.2x10(-8) T/K, respectively. We have also carried out Raman spectroscopic studies at room temperature up to 30 GPa to reinvestigate the phase transitions observed for NaBH(4). A comparative symmetry analysis also has been carried out for different phases of NaBH(4).

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