Pressure-induced phase transition of ammonium fluosilicate (NH4)2SiF6 by in situ X-ray diffraction and Raman spectroscopy

The ammonium fluorosilicate has been investigated by Synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy experiments with diamond anvil cells (DAC) up to 20 GPa at room temperature. The experimental results show that a phase transition from the cubic phase (Fm (3) over barm) to the trigonal phase (P (3) over bar m1) occurs at 0.3 GPa with a volume collapse of similar to 6.3 %. The phase transition mechanism is related to the change in the ordering of hydrogen atoms of ammonium fluosilicate. The pressure-volume data of ammonium fluosilicate are fitted to the second-order Birch-Murnaghan equation of state, yielding V-0 = 592.7(15) A(3) and K-0 = 12.2(9) GPa for the cubic phase and V-0 = 134.5(2) A(3), K-0 = 53.8(11) GPa for the trigonal phase. The axial compression ratio of trigonal ammonium fluosilicate, alpha(c)/alpha(a), is approximately 1.36, indicating anisotropy. The high-pressure phase of ammonium fluorosilicate exhibits improved compression properties and catalytic activity, suggesting its potential for industrial applications.