The crystal structure of Na3HP2O7 ⋅ H2O has been determined from X-ray powder diffraction data. The structure is monoclinic, space group P21/n (No. 14), with unit cell dimensions a = 10.4242(1) Å, b = 6.8707(1) Å, c = 10.0837(1) Å, β = 99.2258(7)°, V = 712.87(2) Å3, Z = 4, Dm = 2.43 g/cm3 and Dx = 2.41 g/cm3. The structure has been solved by direct methods using EXPO program and refined by FULLPROF package. The final RBragg value was 7.6%. Positions of hydrogen atoms were not defined. There are three sodium cations in the asymmetric unit. Two of them are five-coordinated and one cation has sixfold oxygen coordination. Each HP2O73– anion is hydrogen bonded to its two adjacent neighbours forming infinite chains parallel to the b axis. Water molecules are located in infinite channels extended along the b direction. The oxygen atom of the water molecule is connected with two sodium cations and hydrogen bonded with one oxygen atom of diphosphate group.
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