Pressure-tuning Raman microspectroscopic study of cobalt(II), manganese(II), zinc(II) and magnesium(II) pyrophosphate dihydrates

The effect of high external pressures on the Raman spectra of four metal pyrophosphate dihydrates, KMHP2O7·2H2O [M=Co(II), Mn(II), Zn(II) and Mg(II)], has been investigated by diamond-anvil cell, Raman microspectroscopy for pressures up to ∼42 kbar. At ∼29 kbar, the νasym POP band of the Co(II) compound disappears completely and irreversibly, suggesting the occurrence of a pressure-driven structural transformation or possibly even the first stages of amorphization. No such dramatic spectral changes were observed for the other three metal pyrophosphates. The previously published crystallographic data for the unit cells of the Co(II) and Mn(II) salts indicate that the former is more densely packed and, therefore, more likely to be affected by the application of high pressures than is the latter, as is observed. The vibrational modes of all four compounds shift gradually to higher wavenumbers with increasing pressure and the measured pressure dependences range from 0.08 to 1.10 cm−1 kbar−1.