Dielectric dispersion and vibrational studies of a new ferroelectric, glycinium phosphite crystal

Abstract

The dielectric dispersion in glycine phosphite crystal over the frequency range 30-1000 MHz is presented. Ferroelectric dispersion of the Debye type along the b-axis occurs in the microwave-frequency region and is caused by a single relaxational soft mode. The powder IR (at low temperatures between 300 and 14 K) and FT Raman spectra in the region between 4000 and 80 cm −1 are measured. DSC measurements for the GPI and deuterated crystals are performed. The results confirm the order-disorder nature of a proper ferroelectric phase transition of second order.

Dielectric Investigation of Glycine Phosphite Crystal

“…The sponta neous p ol ari zati on P s i s para l l el to two f old b-axi s and p erp endi cul ar to the i nÙnite hydro gen b onded chai ns of the pho sphi te ani ons di rected al ong the crysta l l ogra phi c c -axi s. The v alue of the sponta neous p ola ri zati on obta i ned fro m the hysteresi s l oop m easurem ent was equa l to ab out 5 È 1 0 À 2 C m À 2 at 170 K. The sym m etry of ferro electri c pha se is expected to be P 2 1 [ 2 ] . The very l arg e i soto pi c e˜ect ( ¤ 9 8 K) found in G PI [4] and a signi Ùcant e˜ect o f externa l pressure on tem p erature of the pha se tra nsi ti on T c give (8 97) evi dences tha t the orderi ng of pro to ns pl ay a very i mp orta nt rol e i n the mol ecular m echani sm of pha se tra nsiti on [6,7]. The ul tra soni c studi es of the pha se tra nsiti on i n G PI crysta l s i ndi cate a pseudopro p er chara cter of the ferro electri c pha se tra nsiti on [8].…”

“…G l yci ne pho sphite crysta l s underg o a ferro electri c, conti nuous, second-order pha se tra nsi ti on o f the order{ di sorder typ e at T c = 2 24:7 K whi ch wa s observed i n di el ectri c [2,3], di˜erenti al scanni ng cal ori metri c (D SC) [4], and l i near bi refri ng ence (LB) [5] m easurem ents. The sponta neous p ol ari zati on P s i s para l l el to two f old b-axi s and p erp endi cul ar to the i nÙnite hydro gen b onded chai ns of the pho sphi te ani ons di rected al ong the crysta l l ogra phi c c -axi s. The v alue of the sponta neous p ola ri zati on obta i ned fro m the hysteresi s l oop m easurem ent was equa l to ab out 5 È 1 0 À 2 C m À 2 at 170 K. The sym m etry of ferro electri c pha se is expected to be P 2 1 [ 2 ] .…”

“…The i nfra red and R am an spectra [4] i ndi cate m ore com pl ex na ture of the pha se tra nsiti on. There i s a suggestio n tha t o rderi ng of the pro to ns i s coupl ed to the moti on of the glyci ne cati ons.…”

Proton Ordering Model of Phase Transitions in Hydrogen Bonded Ferrielectric Type Systems: The Gpi Crystal

“…However, the observed dynamics of the soft relaxational mode connected with the proton flipping in the double potential well is more complicated in GPI than in the other similar H-bonded ferroelectrics (for example, BPI). The estimates of the corresponding activation energy and relaxation time imply that para-ferroelectric phase transition in GPI does not seem to be strictly connected only with the movement of protons [9,10]. A reorientation and deformation of the ionic group (phosphite anions) could also play an important role (the corresponding structure changes can be extracted from the neutron scattering data [3]).…”

“…Investigations of the IR and Raman spectra [10], the thermal expansion coefficients, spontaneous electrostriction and refraction indices measurements were also performed [16]. Relatively large values of the thermal expansion coefficients, especially in the c-axis direction, give some more evidence of the existence of the noticeable lattice anharmonicity.…”