Synthesis and single crystal structure are reported for a new gadolinium acid diphosphate tetrahydrate HGdP(2)O(7)center dot 4H(2)O. This salt crystallizes in the monoclinic system, space group P2(1)/n, with the following unit-cell parameters: a=6.6137(2) angstrom, b = 11.4954(4) angstrom, c = 11.377(4) angstrom, beta = 87.53(2) and Z = 4. Its crystal structure was refined to R = 0.0333 using 1783 reflections. The corresponding atomic arrangement can be described as an alternation of corrugated layers of monohydrogendiphosphate groups and GdO8 polyhedra parallel to the ((1) over bar 0 1) plane. The cohesion between the different diphosphoric groups is provided by strong hydrogen bonding involving the W4 water molecule. IR and Raman spectra of HGdP(2)O(7)center dot 4H(2)O confirm the existence of the characteristic bands of diphosphate group in 980-700 cm(-1) area. The IR spectrum reveals also the characteristic bands of water molecules vibration (36003230 cm(-1)) and acidic hydrogen ones (2340 cm(-1)). TG and DTA investigations show that the dehydration of this salt occurs between 79 and 900 degrees C. It decomposes after dehydration into an amorphous phase, Gadolinium diphosphate Gd-4(P2O7)(3) was obtained by heating HGdP(2)O(7)center dot 4H(2)O in a static air furnace at 850 degrees C for 48 h, (C) 2005 Elsevier Ltd. All rights reserved.