High-pressure studies of SrNi3(P2O7)2 pyrophosphate by Raman spectroscopy and X-ray diffraction

High-pressure investigations of SrNi3(P2O7)2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy and X-ray synchrotron radiation source. 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 60 Raman active modes 30Ag+30Bg for SrNi3(P2O7)2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi3(P2O7)2 pyrophosphate to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, SrNi3(P2O7)2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The new high-pressure phase persists metastably down to 1 atm. upon release of pressure. We also measured the pressure dependence of the lattice parameters of the polycrystalline SrNi3(P2O7)2 sample up to a pressure of 48 GPa. The compressibility of SrNi3(P2O7)2 along the b-axis is larger than that along the a-axis. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to a tetragonal structure started. The bulk modulus of SrNi3(P2O7)2 (P21/c) is 124±2 GPa with a pressure derivative of 4.37±0.12.