Dielectric relaxation in ferroelectric (CH₃)₂NH₂AL(so₄)₂· 6H₂O crystal

27Al and71Ga NMR Measurements on Single Crystals of DMAAS and DMAGaS

“…The first-order character of phase transition at T c1 was confirmed in ultrasonic studies [13]. The mechanism of the transitions to the ferroelectric phase is regarded to be of an order±disorder type according to the results from dielectric dispersion measurements [3,4] and Raman spectroscopic [14] studies. Structural investigations showed that the dimethylammonium cations are disordered in the room temperature phase.…”

“…Crystals of a volume of about 2.5 cm 3 and with a good optical quality were obtained in the period of one month. 27 Al and 71 Ga NMR measurements were run at resonance frequencies of 78.205 and 91.531 MHz, respectively, using a BRUKER MSL 300 NMR spectrometer.…”

“…DMAAS undergoes a ferroelectric phase transition at T c 153 K [1,3,5] whereas DMAGaS shows two successive phase transitions, i.e. at T c1 136 K to the ferroelectric phase and at T c2 118 K to the low temperature nonferroelectric phase [4,6].…”

“…In the ferroelectric phases of both crystals the dimethylammonium cations are found to be ordered. The spontaneous polarization is along the crystallographic direction [101] [1,3,4,5].…”

Dielectric properties of a DMAGaS/DMAAS mixed crystal

“…Dimethylammonium gallium sulphate hexahydrate (DMAGaS) (CH 3 ) 2 NH 2 Ga (SO 4 ) 2 Á 6H 2 O belongs to a crystal family with the further members dimethylammonium aluminum sulphate hexahydrate (DMAAS) (CH 3 ) 2 NH 2 Al(SO 4 ) 2 Á 6H 2 O and dimethylammonium aluminum sulphate selenate hexahydrate (DMAASSe) (CH 3 ) 2 NH 2 Al(S 1Àx Se x O 4 ) 2 Á 6H 2 O [1][2][3]. The crystal structure is build up of Ga or Al atoms co-ordinating six water molecules, regular SO 4 or SeO 4 tetrahedra and [(CH 3 ) 2 NH 2 ] þ (DMA) cations, where all these units are interconnected by a threedimensional framework of hydrogen bonds.…”