Structure, phase transition and vibrational spectra of the NaH5(PO4)2 and NaD5(PO4)2 crystals

Abstract

The crystals of sodium pentahydrogenbis(phosphate), NaH5(PO4)2 [NaH2PO4·H3PO4] and its deuterated analogue, NaD5(PO4)2 [NaD2PO4·D3PO4], were obtained for the first time. They exhibit the second-order phase transition at ca. 200K slightly sensitive on deuteration (ca. 6–8K) as determined by the DSC experiments. Their structures were solved by the X-ray method at 150K (for NaH5(PO4)2) and at 240K (for deuterated) crystals. Both these crystals belong to the P21/c space group of the monoclinic system (a=8.464(4), b=7.817(5), c=10.337(5)Å, β=90.58(4)° for NaH5(PO4)2 and a=8.495(3), b=7.854(3), c=10.352(3)Å, β=90.67(3)° for deuterated one) and their structures are similar, although determined in different phases. The crystals are built of the layers of orthophosphoric acid molecules and of the layers of dihydrogenphosphate anions. The layers are built of the centrosymmetric dimers of the orthophosphoric acid molecules and of the dihydrogenphosphate anions, respectively. Two corresponding moieties are joined into dimers by two equivalent hydrogen bonds. The structures of title crystals differ from that of KH5 (PO4)2. The powder IR and Raman spectra were measured at room temperature. The IR spectra of the NaH5 (PO4)2 crystal were also measured at low temperatures. It is suggested that the phase transition is related to the changes in the motions of the protons in weak hydrogen bonds.