An extended high pressure-temperature phase diagram of NaBH4.

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).