J. M. Igartua

PSEUDO: a program for a pseudo- symmetry search

The program PSEUDO provides tools for the systematic search of pseudosymmetry, based on group±subgroup relations between space groups (Igartua et al., 1996). For a crystal structure L, speci®ed by its space group U, the cell parameters and the coordinates of the atoms in the asymmetric unit, the program searches for pseudosymmetry among all minimal supergroups Gk > U of the group U. The interpretation of a structural pseudosymmetry as a small distortion of a higher symmetric (prototype) structure H allows the following. (i) The prediction of phase transitions at higher temperature. If the distortion is small enough, it can be expected that the crystal acquires the more symmetric con®guration at a higher temperature after a phase transition (Igartua et al., 1996, 1999). (ii) The search for new ferroelastic and ferroelectric materials. Polar structures having atomic displacements smaller than 1 AE with respect to a hypothetical non-polar con®guration are considered as possible ferroelectrics (Abrahams & Keve, 1971; Kroumova et al., 2000). (iii) The detection of false symmetry assignments (overlooked symmetry) in crystal structure determination.

Systematic search of displacive ferroelectrics

Abstract Polar structures with pseudosymmetry related to a hypothetical non-polar configuration are considered as good candidates for displacive ferroelectrics. Recently we have developed a procedure for the systematic search of pseudosymmetric structures in the available databases. The aim of the present contribution is the generalization of the pseudosymmetry search procedure to include possible displacive ferroelectric structures which implies an additional optimization of the continuous parameter(s) describing the location of the non-polar symmetry elements. Here we present the results of the search for possible displacive ferroelectrics among the compounds with space group Pba 2, listed in the Inorganic Crystal Structure Database.

The specific heat of the ferroelectric phase transition in N(CH3)4CdBr3

The specific heat of N(CH3)4CdBr3 from 50 to 300 K has been measured by adiabatic calorimetry, using both static and dynamic methods. The obtained results have permitted a careful study of the ferro-paraelectric phase transition the crystal shows at 160 K. The available spectroscopic data have been used to generate a reliable baseline which accounts for the normal lattice contribution to the specific heat. These results allow for an accurate estimation of the phase transition thermodynamic functions: ΔH=2620 J·mol−1 and ΔS=18.04 J·(mol°C)−1. These high values are in agreement with the predictions of the 6 well potential Frenkel model.ZusammenfassungUnter Einsatz sowohl dynamischer als auch statischer Methoden wurde mittels adiabatischer Kalorimetrie die spezifische Wärme von N(CH3)4CdBr3 zwischen 50 und 300 K gemessen. Die erhaltenen Resultate erlaubten eine behutsame Untersuchung der ferroparaelektrischen Phasenumwandlung, die das Kristall bei 160 K aufweist. Die zur Verfügung stehenden spektroskopischen Angaben wurden benutzt, um eine zuverlässige Basislinie zu erhalten, welche den normalen Gitterbeitrag zur spezifischen Wärme berücksichtigt. Diese Resultate erlauben eine genaue Schätzung der thermodynamischen Funktionen der Phasenumwandlung: ΔH=2620 J·mol−1 und ΔS=18.04 J·(mol°C)−1. Diese hohen Werte stimmen mit den Vorhersagen der 6 besten Frenkelschen Modelle überein.

Growth and characterization of Pb3Ni1.5Mn5.5O15 single crystal

Single crystals of the mixed valence compound Pb₃Ni₁.₅Mn₅.₅O₁₅ were successfully grown. Polarized Raman spectroscopy revealed that they belong to the point group D(3d), in accordance with single-crystal x-ray diffraction data which were refined within the trigonal space group P3c1 (No. 165), with lattice parameters a = 9.941(3) Å and c = 13.543(3) Å. Strongly anisotropic long range magnetic order is established below 65 K.

A specific heat study on the phase transitions in Cs2ZnI4 below room temperature

Calorimetric measurements of Cs2ZnI4 showed four distinct phase transitions at 117, 108, 104 and 94 K. The shape of the observed specific heat peaks indicates that those at 108 and 94 K are first order in nature. This sequence is in agreement with previous results using different experimental techniques, and supports the hypothesis that the phase transition sequence of Cs2ZnI4 is the same as that of Cs2HgBr4 and can be written as Pnma (Z=4) 2nd order to T1=117 K incommensurate 1st order to T2=108 K ferroelastic P21/n (Z=8) 2nd order to T3=104 K ferroelastic P1 1st order to T4=94 K ferroelastic P1.

Specific heat and thermodynamic functions for Cs2CdBr4 phase transitions

Adiabatic calorimetry measurements have been performed on Cs2CdBr4 from 90 to 270 K. Phase transitions at 252.03, 237.02, and 154.88 K were found. The thermal expansion coefficients and the elastic constant values were used to separate the harmonic and anharmonic contributions to the specific heat, which permitted us to establish a baseline for the precise determination of the thermodynamic functions associated with the phase transitions. Anharmonic quantities, such as the Gruneisen parameter and the isothermal compressibility, were also calculated. Finally, no experimental evidence of the suggested transitions at 208 and 130 K was found.

Supergroups of space groups and their application to the prediction of structural phase transitions

Abstract A systematic procedure for the determination of supergroups of space groups is developed and used for the search of pseudosymmetric structures within crystal structure databases. The method is applied to P212121 structures listed in the Inorganic Crystal Structure Database. A significant number of compounds with known phase transitions at high temperatures are clearly detected as pseudosymmetric with differences between the transformed and original structure lower than 1·0 A. Taking this value as a threshold about 20 additional compounds are detected having a high probability of exhibiting a phase transition at higher temperatures.

Ferroelectric phase transition in (CH3)4NCdBr3 (TMCB)

Abstract The structural phase transition occurring below room temperature in the chain compound TMCB is studied by means of X-ray diffraction, Raman scattering and adiabatic calorimetry. A phenomenological model based on Landau theory is proposed.