Crystal structure, phase transitions, anomalous thermal and elastic properties of bis (butylammonium) zirconium bis(nitrilotriacetate) dihydrate and bis(butylammonium) hafnium bis(nitrilotriacetate) dihydrate, [NH3(CH2)(3)CH3](2)X[N(CH2COO)(3)](2)center dot 2 H2O, (X=Zr, Hf)

The isotypic compounds [NH3(CH2)(3)CH3](2) . X[N(CH2COO)(3)](2) . 2 H2O, C2 0H40N4O14X (X: Zr; Hf) crystallize in space group P2(1)2(1)2(1) With lattic e constants a(1) = 10.314(4) Angstrom, a(2) = 13.018(6) Angstrom, a(3) = 21 .357(8) Angstrom, (Zr-salt) and a(1) = 10.308(4) Angstrom, a(2) = 12.996(6) Angstrom, a(3) = 21.377(8) Angstrom (Hf-salt), Z = 4. The structure is bui lt up from layers of X[N(CH2COO)(3)](2) anions with intralayer and interlay er Coulomb interactions to the NH3 groups of the butylammonium cations. In addition, hydrogen bonds of NH3 groups and water molecules with oxygen atom s of the nitrilotriacetate groups are present. Strong anomalies occur in thermal expansion and elastic properties between 340 K and 160 K in both species, e.g. the thermoelastic constants T-11, T-3 3, T-44, T-12, T-13 and T-23 attain increasing positive Values upon lowerin g the temperature. The result is a negative temperature derivative of the b ulk compressibility over a wide temperature range, a quite unique phenomeno n. The piezoelastic constants P-11, P-44 and P-13 show anomalies too. Ultra sonic waves are affected by an increasing attenuation below 250 K which cul minates at about 188 K (Zr-salt) and 193 K (Hf-salt), indicating a weakly f irst-order transition, which is also recognized in differential scanning ca lorimetry and thermal expansion. The observed anomalies and the transition passing from room temperature to lower temperatures appear to be related ma inly to distinct temperature-induced changes in bonding and position of one of the two nonequivalent butylammonium groups and the two water molecules.