Infrared and Raman studies of manganese dihydrogen phosphate dihydrate, Mn(H2PO4)2·2H2O. I: Region of the vibrations of the phosphate ions and external modes of the water molecules

Infrared and Raman spectra of Mn(H2PO4)2·2H2O and of series of deuterated analogues recorded at room temperature (RT) and the boiling temperature of liquid nitrogen (LNT) have been presented and analyzed in detail in respect to the internal vibrations of the H2PO4- ions and the external modes of the water molecules. Some vibrational couplings of the stretching and bending PO4 modes have been discussed. The stretching PO4 modes appear to be coupled with the in-plane δ(OH) and out-of-plane γ(OH) bending POH vibrations, while the bending PO4 modes are coupled with the water librations. The mutual exclusion rule is obeyed for all vibrations under consideration. The large frequency separation between the γ(OH) modes of the two POH groups evidences for the considerable difference in the strength of the hydrogen bonds which they form. The observed A–B and g–u splittings for the γ  (OH) vibrations show that both intra-chain and inter-chain interactions of the H2PO4- ions are significant. Three bands of water librations are found in the IR and Raman spectra of Mn(H2PO4)2·2H2O and the observed g–u correlation splittings are smaller than 10 cm−1. Strong interactions of ν4 and ν2 modes of PO4 with the librations of H2O and D2O molecules have been found.