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)3CH3]2X[N(CH2COO)3]2 · 2 H2O, (X=Zr, Hf)

Abstract

Abstract The isotypic compounds [NH3(CH2)3CH3]2 · X[N(CH2COO)3]2 · 2 H2O, C20H40N4O14X (X: Zr; Hf) crystallize in space group P212121 with lattice constants a 1 = 10.314(4) Å, a 2 = 13.018(6)Å, a 3 = 21.357(8) Å, (Zr-salt) and a 1 = 10.308(4) Å, a 2 = 12.996(6) Å, a 3 = 21.377(8) Å (Hf-salt), Z = 4. The structure is built up from layers of X[N(CH2COO)3]2 anions with intralayer and interlayer Coulomb interactions to the NH3 groups of the butylammonium cations. In addition, hydrogen bonds of NH3 groups and water molecules with oxygen atoms 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 33, T 44, T 12, T 13 and T 23 attain increasing positive values upon lowering the temperature. The result is a negative temperature derivative of the bulk compressibility over a wide temperature range, a quite unique phenomenon. The piezoelastic constants P 11, P 44 and P 13 show anomalies too. Ultrasonic waves are affected by an increasing attenuation below 250 K which culminates at about 188 K (Zr-salt) and 193 K (Hf-salt), indicating a weakly first-order transition, which is also recognized in differential scanning calorimetry and thermal expansion. The observed anomalies and the transition passing from room temperature to lower temperatures appear to be related mainly to distinct temperature-induced changes in bonding and position of one of the two non-equivalent butylammonium groups and the two water molecules.