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:

  1. A different spatial orientation in a part of the amido-groups of the sulfamate anions [NH2SO3]-. In the LT phase this orientation leads to an antiparallel ordering within planes perpendicular to the [010] direction.
  2. The transition of the Ca (Sr) cations from a special position in the HT phase to a general position in the LT phase.

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.

  1. Bohatý, L., Held, P. and Tillmanns, E.: to be published.
  2. Haussühl, S., Lüghausen, K. and Siegert, H.: Z. Kristallogr. Suppl. Issue 3 (1991), 102