KLASSENGLEICHE PHASE TRANSITION IN CALCIUM AND STRONTIUM SULFAMATE TETRAHYDRATE, CA(NH2SO3)2 . 4H2O AND SR(NH2SO3)2 . 4H2O
G. Schetter, L. Bohatý and P. Held
Institute of Crystallography, University of
Cologne, Zülpicher Str. 49b, D-50674 Köln, Germany,
Email: bohaty@kri.uni-koeln.de
Keywords: calcium sulfamate
tetrahydrate, strontium sulfamate tetrahydrate, Ca(NH2SO3)2
. 4H2O, Sr(NH2SO3)2
. 4H2O, phase transition
We present crystal structures, thermoanalytical and crystal physical investigations of the new klassengleiche phase transition P21/c <-->C2/c in the isostructural sulfamates Ca(NH2SO3)2 . 4H2O and Sr(NH2SO3)2 . 4H2O. This reversible transition with transition temperatures of T = 218 K (Ca-salt) and T = 191 K (Sr-salt) was first discovered thermoanalytically (DSC). Furthermore, we detected anomalies in the temperature dependence of the dielectric tensor, birefringence, tensor of thermal expansion and in elastic properties. At room temperature both compounds crystallize in the monoclinic space group C2/c. The structure of the high temperature (HT) phase contains anionic sulfamate groups [NH2SO3]- and eightfold coordinated cations [CaO4(H2O)4] (or [SrO4(H2O)4]). A complex network of hydrogen bonds fixes the crystal water and reinforces the ionic part of the lattice energy [1]. The low temperature (LT) phase belongs to the monoclinic space group P21/c. The changes in the lattice parameters, atomic parameters and the average bond lengths are very small.
LT phase |
HT phase |
|
Ca(NH2SO3)2 . 4H2O | T = 173(1) K P21/c |
T= 293(2) K C2/c |
a = 11.5867(7) A b = 7.7396(5) A c = 11.5443(8) A b = 98.659(7)o |
a = 11.6260(6) A b = 7.7646(3) A c = 11.6207(4) A b = 98.927(6)o |
|
Sr(NH2SO3)2 . 4H2O | T = 161(1) K P21/c |
T = 293(2) K C2/c |
a = 11.8745(5) A b = 7.8581(4) A c = 11.7657(9) A b = 99.009(4)o |
a = 11.9274(4) A b = 7.8798(6) A c = 11.8505(8) A b = 99.497(6)o |
The HT and LT phases comprise the same structural units. The outstanding structural differences between both phases are:
DSC curves show typical characteristics of a l-transition. Within the experimental precision the transition is free of any hysteresis. Measurements of the thermal expansion aij(T) reveal a strong anisotropy. Significant correlation exists between the tensor of thermal expansion [aij] and the detected structural changes in the HT and LT phases. Detailed ultrasonic studies of the phase transition using the RPR-method [2] reveal only small anomalies in the temperature dependence of the eigenfrequencies.
For all measurements orientated samples were
cut from large single crystals (with dimensions up to 3 5 5 cm3)
which we have grown from aqueous solutions at 36o C by
evaporation of the solvent in a period of eight to ten weeks.