CRYSTAL-STRUCTURE OF (CH3)(2)NH2GA(SO4)(2)CENTER-DOT-6H(2)O IN THE FERROELECTRIC PHASE AT 125-K AND LOW-TEMPERATURE PHASE AT 100-K

(CH3)(2)NH2Ga(SO4)(2) . 6H(2)O (DMAGaS) in the ferroelectric phase bel ow T-e1=122K is monoclinic (space group Pn) and isomorphic with (CH3)( 2)NH2Al(SO4)(2) . 6H(2)O. At T =125 K, a=6.413(1) Angstrom, b=10.657(2 ) Angstrom, c=11.398(2) Angstrom, beta=100.29(3)degrees. At Tc-2 = 114 K (on cooling) DMAGaS undergoes a first order phase transition accomp anied by a discontinuous decrease of volume to the low temperature pha se, monoclinic (space group P2(1)). At T=100 K, a=6.234(1) Angstrom, b =10.891(2) Angstrom c=11.113(2) Angstrom, beta=103.87(3)degrees In the para- and ferroelectric phases dimethylammonium ions are located in a mple voids which allows an almost free rotation in the paraelectric ph ase and flipping in the ferroelectric phase. In the low temperature ph ase the volume of the voids decreases preventing dimethylammonium ions from any changes of their positions. These results have been confirme d by lattice parameters measurements in vicinity of Tc-2 indicating an abrupt decrease of the lattice cell volume at Tc-2.