T. Breczewski

Hidden Degrees of Freedom in Aperiodic Materials

Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other—an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal natures capacity to explore the increased number of phases allowed in aperiodic crystals.

X-ray diffraction study of the phase transitions of (CH3)4NCdCl3 between 293 and 80 K: a quantitative analysis of the ferroelastic domains distribution below 118 K

X-ray diffraction patterns of [N(CH3)4][CdCl3], tetramethylammonium trichlorocadmate(II), have been investigated in the temperature range 80-293 K, which includes two phase transitions at 118 and 104 K, respectively. The main interest in this compound is to establish the mechanism of the structural phase transitions common to other members of the isostructural family [(CH3)4N][MX3]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origin of the order-disorder mechanism would be the orientationally disordered distribution of the tetramethylammonium tetrahedra at room temperature. Maximum Entropy Methods suggest that the most probable distribution of the organic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with the crystallographic threefold axis of the structure. Below 118 K the reflections are split. However, the splitting cannot be fully explained by the ferroelastic domains expected to appear after the phase transitions. Recent NMR results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465-2476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.

X-ray Diffraction Study of the Ferroelectric Phase Transition of (CH3)4NCdBr3 (TMCB)

The crystal structure of tetramethylammonium tribromocadmate (II), [N(CH 3 ) 4 ][CdBr 3 ] (TMCB), has been studied at 295 and 85 K, in order to analyze the structural changes connected with the improper para-ferroelectric phase transition occurring at ∼160 K. The space group of the paraelectric (room - temperature) phase is P6 3 /m and that of the ferro - electric (low-temperature) phase is P6 1 -P6 5 . Final agreement factors are R (wR)=0.062 (0.075) and R (wR)=0.029 (0.038) for the data collected at 295 and 85 K respectively

Temperature-pressure phase diagram of an aperiodic host guest compound

This letter reports on the structural instabilities of an aperiodic composite crystal under pressure. The (P, T) phase diagram up to 0.55 GPa of nonadecane-urea is reported showing various symmetry breakings in crystallographic superspaces, towards three different orthorhombic phases. These structural phase transitions are characterized by a change in the intermodulation and are described by increasing the rank of the crystallographic superspaces.

The phase transition sequence in tetramethylammonium cadmium chloride (TMCC)

The structural phase transitions in were investigated by means of adiabatic calorimetry and thermal expansion measurements. Two phase transitions have been studied on cooling and on heating at about 104 K and 118 K. The shapes of the observed specific heat anomalies, as well as the existence of thermal hysteresis, confirm the first-order character previously assigned to these phase transitions. From the experimental data and the harmonic specific heat obtained from the known frequencies of the vibrational modes, a calculation of the Gruneisen parameter as a function of the temperature is also given. Finally, the suitability of different Landau potentials for the two phase transitions is briefly discussed.

Inelastic neutron scattering investigation of external modes in incommensurate and commensurate A2BX4 materials

Inelastic neutron scattering experiments in K2SeO4, Cs2SeO4 and K2CrO4 are reported. The lowest Sigma 3- Sigma 2 phonon branches along the alpha *-axis in their Pnam phase have been characterised for the three compounds. The results are compared with recent lattice dynamical simulations using rigid-ion models. Potassium chromate, that simulations suggested to be incommensurate at low temperatures, evidences no structural phase transition down to 12 K, but its lowest Sigma 2 phonon branch exhibits a significative softening and decreases more than 20% as the temperature is lowered from 300 K to 36 K. The softening mechanism in this compound and in potassium selenate is related to an increase of the effective interaction between an acoustic and an optical branch.

Characterization of the room-temperature phase of Tl2MoO4: crystal structure, symmetry mode analysis and second-harmonic generation ­measurements

The crystal structure of the glaserite-related compound dithallium(I)-molybdate(VI), which at 293 K crystallizes monoclinic, space group C121 with lattice parameters a = 10.565 (3), b = 6.418 (1), c = 8.039 (2) A, beta = 91.05 (4) degrees, has been determined. The structure was refined as an inversion twin to a final R(F(all)) value of 0.0611 for 1006 unique reflections [R(F(obs)) = 0.0285 for 644 observed reflections]. Second-harmonic generation measurements led to a value of d(eff) = 5.5 +/- 0.5 pm V(-1) as an estimation of the second-harmonic conversion efficiency at phase matching. Symmetry mode analysis shows that, in general, primary modes have the highest amplitudes, yet surprisingly some of the secondary modes assume amplitudes of comparable magnitude. A comparison of the phase at 293 K with that at 350 K (space group P3m1) shows that the main change can be described as a rotation of the molybdate tetrahedra around the trigonal a(b) axis. The molybdate tetrahedra as well as the octahedra around one of the symmetry-independent Tl atoms are more strongly distorted in the monoclinic phase. The coordination number for the other two Tl atoms is decreased from 12 and 10 in the high-symmetry phase to 10 and 9 in the monoclinic phase. Furthermore, the number of common edges between the Tl and Mo coordination polyhedra is reduced and the common face which is observed between them in the high-temperature phase is changed to a common edge in the low-temperature phase. The contribution of the primary symmetry modes leads exactly to this change in the coordination spheres of the atoms.

Crystal growth and twinned crystal structure of Sr2CaWO6

Single crystals of Sr(2)CaWO(6) have been prepared by sintering at high temperature. Powder samples were compressed into rods and heated up to 1953 K. This seems a promising new route for further studies of the structure and physical properties of double perovskites. The structural model of Sr(2)CaWO(6) includes a quantitative description of the twinning shown by the diffraction pattern that should be present in almost any single-crystal specimen for this type of compound.

Frozen Polarization of Piezoelectric Polyimides

We report a comparative frozen polarization study in three piezoelectric polymers: one polyimide without –CN dipolar groups (poly0CN), and two more that differ in the position of the –CN dipolar group present in the repetition unit (poly2-4 and poly2-6). The values of the dielectric permittivity and frozen polarization show that the participation of N-phenylphthalyimide group depends strongly on the curing process. For totally cured samples this group has little influence on the value of remnant polarization. The study was performed by dielectric measurements and by Thermally Stimulated Depolarization Current (TSDC) technique.

Confined linear molecules inside an aperiodic supramolecular crystal: The sequence of superspace phases in n-hexadecane/urea

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).