An extended high pressure-temperature phase diagram of NaBH4.

We have studied the structural stability of NaBH₄ 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 NaBH₄ 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 NaBH₄ are found to be 18.76(1) GPa, -0.0131 GPa K^-1, and 12.5×10^-5+23.2×10^-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 NaBH₄. A comparative symmetry analysis also has been carried out for different phases of NaBH₄. [ABSTRACT FROM AUTHOR]