High-pressure studies of NaCo(H2PO3)3·H2O phosphite by Raman spectroscopy

High-pressure investigations of NaCo(H2PO3)3·H2O dihydrogenphosphite, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 69Ag + 69B1g + 69B2g + 69B3g Raman active modes for NaCo(H2PO3)3·H2O, of which only 36 bands are observed at ambient conditions. The pressure-induced phase transition sequence of NaCo(H2PO3)3·H2O dihydrogenphosphite to pressures up to 20 GPa was explored. Raman spectra showed that the investigated compound, NaCo(H2PO3)3·H2O, compressed smoothly up to 4.2 GPa and starts amorphizing at 6.8 GPa. The amorphous phase persists down to 1 atm upon release of pressure. The amorphization was due to a chemical decomposition and also might be due to pressure-induced distortions of the lattice upon compression associated with the Na+ and Co2+ cation displacements and strongly coupled with rigid rotations of the (PO3) units. No P–H vibrations were seen after pressure release which means that P–H bonds were broken. The frequencies seen in the Raman spectra for the amorphous phase are characteristic of (PO4)3− or P2O74− tetrahedral short-range structure.