Nonferroic Phase Research Articles

Phase transitions and ferroelectricity in NaSb3F10

Atomic coordinate analysis allows materials with appropriate but previously unrecognized dielectric properties to be predicted as new ferroelectrics if their crystal structure is known. An earlier such prediction that NaSb3F10is ferroelectric is confirmed herein without ambiguity. Its spontaneous polarizationPsis found to exhibit reproducible dielectric hysteresis at room temperature, withPs≃ 60 µC m−2, under the application of a field of 0.3 MV m−1or greater. The pyroelectric coefficient 〈p〉 = 17 (5) µC m−2 K−1at 298 K. NaSb3F10undergoes a phase transition atTC≃ 461 K, on correction for thermal hysteresis, with entropy change ΔS= 5.7 (3) J mol−1 K−1. The colorless crystals melt atTm ≃ 515 K and decompose above ∼600 K. The thermal hysteresis of ∼35 K inTC, on heating and cooling at 5–25 K min−1, is typical of first-order phase transitions. The space group in ferroelectric phase III isP63, and that in phase II is predicted to beP6322, a nonpolar supergroup ofP63; the supergroup expected in the prototypic nonferroic phase I isP63/mmc. The space group of phase III isnota direct subgroup of phase I. The dielectric permittivity ∊′ at 1 kHz increases over an order of magnitude between 300 K and a major inflection atTC, continuing to increase steadily thereafter toTm.

13C and 39K High-Resolution Solid-State NMR Study of the Nonferroic Phase Transition of Potassium Hydrogen Carbonate. Complementarity between NMR and Incoherent Neutron Scattering

Potassium hydrogen carbonate KHCO 3 is formed of centrosymmetric dimers (HCO 3 ) 2 linked by hydrogen bonds, the protons being disordered in an asymmetric double-well potential at ambient temperature. This compound is a model for 0-dimensional hydrogen bonds. It undergoes a nonferroic nonferroelectric phase transition at T c = 318 K, and the double-well potential becomes symmetric in the high-temperature phase. This phase transition is studied for the first time by variable-temperature 1 3 C and 3 9 K high-resolution solid-state NMR experiments. We present an original method to correlate NMR data to other local probes such as incoherent neutron scattering experiments to extract structural information from powder NMR experiments. Using this method, the eigenvalues and jump angle of the chemical shift tensor of the carbon 1 3 C and the quadrupolar interaction of the potassium 3 9 K could be extracted in the completely ordered phase, and their evolution with temperature is correlated to the proton-dynamic disorder within the hydrogen bond. This study also illustrates the complementarity between NMR and incoherent neutron-scattering experiments in the study of hydrogen bonding.

Thermal Conductivity of LiKSO4 between 10 and 250 K

en

The role of ferroelasticity in toughening of brittle materials

Abstract Ferroelastic materials, which are characterized by the existence of a hysteresis between strain and stress, can exhibit enhanced toughness relative to the paraelastic, prototypic phase. Domain switching in the near crack tip stress field provides the requisite energy absorption mechanism and thus a toughening mechanism. Experimental evidence (X-ray diffraction) is presented on three different types of ferroelastics; namely, tetragonal (t') zirconia, gadolinium molybdate (GMO) and its terbium (TMO) and dysprosium (DMO) analogs, and lead zirconate titanate (PZT). It is shown that ferroelastic domain switching occurs in these materials in the vicinity of a crack tip. The fracture toughness of the ferroic phase is shown to be two to three times that of the nonferroic phase in all three materials. The results of the present study demonstrate that ferroelastic domain switching is viable toughening mechanism.

Elastic behaviour of RbAlF4 in the vicinity of the non-ferroic and improper ferroelastic phase transitions

Abstract The elastic constants of RbA1F4 are studied in the vicinity of the non-ferroic P4/mmm - P4/mbm phase transition (at 553 K) and of the improper ferroelastic P4/mbm - Pmmn phase transition (at 282 K) by ultrasonics and by Brillouin scattering. The non-ferroic phase transition is characterized by a large softening of C11 which is interpreted below Tc1 in the framework of a Landau model involving two coupled order parameters. The elastic properties are connected to the soft mode behaviour. The improper ferroelastic phase transition is characterized by the softening of C33, C11 and (C11 - C12). The C33 softening is described using a Landau model involving a two component order parameter.

Nonferroic phase transitions

Nonferroic phase transitions are defined as the structural transitions occurring with a breaking of translational symmetry within the same crystal class. They involve no new macroscopic tensor components below the transition point, and they are generally identified experimentally through the onset of superlattice reflections denoting the multiplication of the crystal's unit cell. A theoretical analysis of these transitions is presented, based on Landau's symmetry criteria for continuous transitons, of the order-parameter symmetries, space-group changes, and free-energy expansions. We establish that the order parameters of such transitions are necessarily related to one-dimensional (real or complex) small representations of the group of the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ vector. Their symmetry characteristics are, in general, simpler than those of other types of structural transitions. Most of them possess a one-component order parameter inducing a doubling of the unit cell. The remaining ones are associated with order-parameter dimensions as high as six, and unit-cell multiplications up to thirty-two. The coupling of the order parameter to macroscopic quantities, illustrated by the example of dielectric ones, is shown to belong to two possible schemes. The relation between nonferroics and antiferroelectrics is discussed. The theoretical results are compared to the available experimental data which pertain mainly to organic compounds and metallic alloys.

Structural phase transitions in ferroelastic RbAlF4. I. DSC, X-ray powder diffraction investigations and neutron powder profile refinement of the structures

The layer compound RbAlF4 is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction. The structures of the three phases are determined by neutron powder profile refinement. The high-temperature phase has the ideal TlAlF4 structure P4/mmm which corresponds to the tilt system a0a0c0. The intermediate room-temperature phase is confirmed as P4/mbm, the tilt system is a0a0c+. The low-temperature phase is consistent with the space group Pmmn which corresponds to both ap +b0c+ (near the transition temperature) and ap +bp +c+ (at very low temperature). The 553K phase transition is a non-ferroic phase transition while the 282K transition is ferroelastic. According to Aizu the low-temperature phase is a 4/mmmFmmm species. The temperature behaviour of the structural parameters is discussed, particularly in the framework of ferroelasticity.

Non-ferroic phase transitions

Abstract A systematic investigation is presented of non-ferroic phase transitions, i.e.of transitions which break the translational symmetry but preserve the crystal class. Possible space group changes, unit cell multiplications, order parameter dimensions and associated free energies are given in table form for the transitions predicted in the framework of the Landau theory by the use of group theoretical methods. Illustrative examples of non-ferroic compounds are indicated.