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

The crystals of sodium pentahydrogenbis(phosphate), NaH5(PO4)(2) [NaH2PO4. H3PO4] and its deuterated analogue, NaD5(PO4)(2) [NaD2PO4. D3PO4], were obt ained for the first time. They exhibit the second-order phase transition at ca. 200 K slightly sensitive on deuteration (ca. 6-8 K) as determined by t he DSC experiments. Their structures were solved by the X-ray method at 150 K (for NaH5(PO4)(2)) and at 240 K (for deuterated) crystals. Both these cr ystals belong to the P2(1)/c space group of the monoclinic system (a = 8.46 4(4), b = 7.817(5), c = 10.337(5) Angstrom, beta = 90.58(4)degrees for NaH5 (PO4)(2) and a = 8.495(3), b = 7.854(3), c = 10.352(3) Angstrom, beta = 90. 67(3)degrees for deuterated one) and their structures are similar, although determined in different phases. The crystals are built of the layers of or thophosphoric acid molecules and of the layers of dihydrogenphosphate anion s. The layers are built of the centrosymmetric dimers of the orthophosphori c acid molecules and of the dihydrogenphosphate anions, respectively. Two c orresponding moieties are joined into dimers by two equivalent hydrogen bon ds. 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 spect ra 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 mot ions of the protons in weak hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved.