CRYSTAL-STRUCTURE, ELASTIC PROPERTIES AND PHASE-TRANSITION OF TRICLINIC AMMONIUM HYDROGEN SUCCINATE, NH4HC4H4O4

Large optical quality, single crystals of triclinic ammonium hydrogen succinate have been grown from aqueous solutions. An X-ray structure analysis yields space group P1BAR, a = 7.484(2), b = 8.876(2), c = 4.726(1) angstrom, alpha = 91.25(2), beta = 93.26(2), gamma = 100.26(3)-degrees, Z = 2, rho(calc) = 1.456(1), rho(obs) = 1.454(2) g cm-3. The structure contains strong hydrogen bonds between adjacent succinate ions which form a periodic bond chain along [110]. Another strong bond chain results from hydrogen bonds and ionic interactions between NH4 ions and the carboxylic groups along [001]. The succinate ion exhibits no intrinsic symmetry as found in most other acid salts of succinic acid. In the final refinement one hydrogen atom had to be split into two positions related by a centre of symmetry. Complete elastic and thermoelastic tensors have been determined from ultrasonic resonance frequencies of thick plates and their temperature dependence, respectively. Further, the tensor of thermal expansion has been studied in a wide temperature range. The existence of a second-order phase transition has been observed by thermal expansion and DSC measurements at about 169 K.