Structures of room-temperature and ferroelectric (35 K) phases of deuterated betaine calcium chloride dihydrate

Abstract

The structures of the room-temperature and the lowesttemperature ferroelectric phases of fully deuterated betaine (trimethylammonioacetate) calcium chloride dihydrate have been determined by means of elastic neutron diffraction. In previous structural studies the structural distortions of the incommensurate phase at 130K and the fourfold phase at 90K were analyzed in terms of symmetry modes. The same approach has been applied in the present case and, as a result, a quantitative comparison of the distortions at different temperatures can be performed. This shows that the A 3 (for q ~ 0) and B2u (for q -0) modes, corresponding to the order parameter, are predominant for the three phases. In addition, it has been found that the structure of this primary mode remains essentially unaltered in the three phases. There is only an increase in its global amplitude as the temperature decreases. Accordingly, it can be expected that the structures of the remaining intermediate modulated phases in the transition sequence are similar to that of the first incommensurate phase, except for the change of wavevector values and greater amplitude of the distortion. Crystal data: (CD3)3NCD2COOCaC12.2D20, M r = 279.0, 2 = 0.8308 A, Z = 4, F(000) = 552. At room temperature: Pnma, a--10.95(1), b = 10.15(1), c= 10.82(1)A, V=1203(2)A 3, D x=l.54gcm -3, /z=0.10cm -1, final R = 0.048 for 748 unique observed reflections. At 35K: Pn21a, a = 10.905(9), b = 10.00(1), c =10.80 (1) A, V=1178(2),~ 3, D x=l.57gcm -3, /x--0.11cm -I, final R =0.042 for 1726 unique observed reflections.

Influence of bromine on the incomplete devil’s staircase behavior of betaine calcium chloride dihydrate

“…Betaine calcium chloride dihydrate (CH 3 ) 3 NCH 2 COO•CaCl 2 •2H 2 O ͑BCCD͒ is a remarkable example of a crystal that presents a rich sequence of one-dimensional modulated ͓qϭ␦(T)c*͔ commensurate ͑C͒ and INC phases ͑''incomplete devil's staircase''͒, occurring between a normal ͑N͒ paraelectric and a nonmodulated ferroelectric ͑FE͒ phase. [1][2][3][4][5][6][7][8][9][10] The temperature ranges of stability of the principal phases at normal pressure are displayed in Table I. The intrinsic properties of BCCD are well known as crystals of high quality can be obtained.…”

Structure of the X-phase of 38% brominated betaine calcium chloride dihydrate

“…The dielectric compound betaine calcium chloride dihydrate [BCCD, (CH 3 ) 3 NCH 2 COOCaCl 2 (H 2 O) 2 ], synthesized in 1984 (Rother et al, 1984), is a noteworthy experimental example of the Devil's staircase concept (Aubry, 1983). Indeed, its thermal phase diagram, between a paraelectric parent phase (space group Pnma; Ezpeleta et al, 1996; above T i 164 K) and a non-modulated ferroelectric phase (space group Pn2 1 a; Dernbach, 1993;Ezpeleta et al, 1996; below T 0 46 K), is constituted by a wealth of one-dimensional displacively modulated phases [k Tc à ], either incommensurate or commensurate (mainly 1 4 , 1 5 and 1 6 on cooling; Brill & Ehses, 1985;Chaves & Almeida, 1991;Ezpeleta et al, 1996;Schaack & le Maire, 1998).…”

“…At room temperature, the orthorhombic unit cell of the parent structure [a 10X95 1, b 10X15 1, c 10X82 1 A Ê ; Ezpeleta et al, 1996] contains four formula units. Each unit constitutes two entities: the polar betaine molecule [(CH 3 ) 3 N CH 2 COO À ] and a distorted OOCl 2 (H 2 O) 2 octahedron surrounding the Ca atom, which are linked via the COO carboxyl terminal group of the betaine molecule (see Fig.…”

“…Each unit constitutes two entities: the polar betaine molecule [(CH 3 ) 3 N CH 2 COO À ] and a distorted OOCl 2 (H 2 O) 2 octahedron surrounding the Ca atom, which are linked via the COO carboxyl terminal group of the betaine molecule (see Fig. 1a of Ezpeleta et al, 1996). This structure can be described by a stacking along the b axis of parallel layers at yab 1a4 and 3a4 (which thus coincide with the m mirror plane to which the H2, C1, N, C3, C4, O1 and O2 atoms of the betaine molecule and the Ca atom belong), interconnected along [011] through rather long and anti-symmetric hydrogen bonds between Cl atoms and the water molecules of neighbouring Ca octahedra.…”

“…Within these layers, the Ca octahedron and the carboxyl group form quasi-one-dimensional chains directed along the a axis, to which the betaine molecules are branched with their axis mainly oriented along the c axis. The Ca octahedra belonging to the above-mentioned one-dimensional chains are connected along [100] by ClÁ Á ÁHÐO hydrogen bonds, which are nevertheless slightly longer than those between layers (2.288 versus 2.248 A Ê ; Ezpeleta et al, 1996).…”