S. C. Abrahams

Nomenclature of polytype structures. Report of the International Union of Crystallography Ad hoc Committee on the Nomenclature of Disordered, Modulated and Polytype Structures

An earlier report [Acta Cryst. (1977), A33, 681-684] by a joint IUCr-IMA committee on the nomenclature of polytypism has been revised and extended. Two kinds of symbolism are recommended for use with either simple or complicated polytypic structures. The first consists of indicative symbols in a modified Gard notation, the second of descriptive symbols based on earlier proposals by Dornberger-Schiff, I)urovi~ and Zvyagin. The polytypism of ZnS and SrGeO3 provides examples for the use of descriptive symbolism.

Ferroelectricity and structure in the YMnO3 family

The 1963 discovery of ferroelectricity in YMnO(3) was accompanied by an experimental Curie temperature (T(c)) reported as 913 K; this value was revised to 1270 K in the following decade. Subsequently, YInO(3) was shown to be isostructural with YMnO(3) and later demonstrated to satisfy the structural criteria for ferroelectricity; recent unpublished measurements give T(c) (YInO(3)) = 835 (15) K. The experimental T(c) value of 913 K for YMnO(3) is in satisfactory agreement with the calculated 1220 (100) K value as derived from a very recent structural refinement, the experimental T(c) of 835 (15) K for YInO(3) with the calculated 760 (120) K. The full YMnO(3) family includes the AMnO(3) subfamily with A = Y, Ho, Er, Tm, Yb, Lu, Sc, In; the AInO(3) subfamily with A = Y, Gd, Dy, Ho, Tb; and the AGaO(3) subfamily with A = Y, Ho, Er. The T(c) values of six family members with known structure, in addition to YMnO(3) and YInO(3), have been structurally derived as 1310 (110) K for ErMnO(3), 1290 (165) K for LuMnO(3), 1270 (110) K for YbMnO(3), 1220 (105) K for ScMnO(3), 540 (375) K for InMnO(3) and 1020 (100) K for YGaO(3). The agreement between predicted and experimental T(c) values for ErMnO(3), LuMnO(3) and YbMnO(3), in addition to that for YMnO(3) and YInO(3), leads to the confident prediction that ScMnO(3), InMnO(3) and YGaO(3) are new ferroelectrics. The remaining six members of the full YMnO(3) family are also expected to be new ferroelectrics.

Statistical descriptors in crystallography: Report of the IUCr Subcommittee on Statistical Descriptors

The Subcommittee has attempted to elucidate the nature of problems encountered in the definition and use of statistical descriptors as applied to crystallography and to propose procedural improvements. The report contains (a) a dictionary of statistical terms established for use by experimentalists; (b) a description of the statistical basis for refinement procedures; (c) sections dealing with defects in the physical model used for refinement, and with the choice and significance of weighting schemes; and (d) recommendations, some of which may be readily implemented, whilst others may require a long-term effort to bring them into general use.

Fresnoite: A new ferroelectric mineral

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2u200aMVu200am−1 ac is ∼0.2u200aCu200am−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3)u200aJu200amol−1u200aK−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2u200aJu200amol−1u200aK−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60u200aK wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.

Ba6CoNb9O30 and Ba6FeNb9O30 : Two new tungsten-bronze-type ferroelectrics. Centrosymmetry of Ba5.2K0.8U2.4Nb7.6O30 at 300 K

Ba6CoNb9O30 and Ba6FeNb9O30 in space group P4bm are shown to satisfy the structural criteria for ferroelectricity. Ba6CoNb9O30 undergoes a diffuse phase transition at 660u2005(11)u2005K, as observed calorimetrically, in addition to a dielectric permittivity anomaly with an onset temperature of 685u2005(10)u2005K. The demonstration of dielectric hysteresis at room temperature under the application of a varied DC field reaching a maximum of ± 300 kV m−1, corresponding to a spontaneous polarization of 1.2 (5) × 10−2Cu2005m−2, provides unambiguous verification that it is a new ferroelectric. Ba6FeNb9O30 also undergoes a diffuse phase transition at 605u2005(16)u2005K, with a dielectric anomaly at 583 (5) K, and exhibits dielectric hysteresis at room temperature under a varied DC field ranging to ± 310u2005kV m−1 corresponding to a spontaneous polarization of 2.2(5) × 10−2Cm−2; it too is demonstrably a new ferroelectric. Although Ba5.2K0.8U2.4Nb7.6O30 has also been reported in space group P4bm, all atomic displacements from the corresponding centrosymmetric positions are less than their refined root-mean-square thermal or static amplitudes. Such an arrangement is likely to be thermodynamically unstable. Either its space group has been incorrectly assigned, and reinvestigation will show the space group is P4/mbm, or the structural refinement is incomplete.

Report of a Subcommittee on the Nomenclature of n-Dimensional Crystallography. I. Symbols for point-group transformations, families, systems and geometric crystal classes

The notation of crystallography in arbitrary dimensions is considered. Recommended symbols for point-group transformations, geometric crystal classes, families and systems are presented.

Structural Phase Transition Nomenclature. Report of an IUCr Working Group on Phase Transition Nomenclature

A compact and intuitive nomenclature is recommended for naming each phase formed by a given material in a sequence of phase transitions as a function of temperature and/or pressure. The most commonly used label for each phase in a sequence, such as [alpha], [beta], ..., I, II, ... etc., is included in the new nomenclature.

Atomic displacements at and order of all phase transitions in multiferroic YMnO3 and BaTiO3.

Coordinate analysis of the multiple phase transitions in hexagonal YMnO(3) leads to the prediction of a previously unknown aristotype phase, with the resulting phase-transition sequence: P6(3)cm(e.g.) <--> P6(3)cm <--> P6(3)/mcm <--> P6(3)/mmc <--> P6/mmm. Below the Néel temperature T(N) approximately = 75 K, the structure is antiferromagnetic with the magnetic symmetry not yet determined. Above T(N) the P6(3)cm phase is ferroelectric with Curie temperature T(C) approximately = 1105 K. The nonpolar paramagnetic phase stable between T(C) and approximately 1360 K transforms to a second nonpolar paramagnetic phase stable to approximately 1600 K, with unit-cell volume one-third that below 1360 K. The predicted aristotype phase at the highest temperature is nonpolar and paramagnetic, with unit-cell volume reduced by a further factor of 2. Coordinate analysis of the three well known phase transitions undergone by tetragonal BaTiO(3), with space-group sequence R3m <--> Amm2 <--> P4mm <--> Pm3m, provides a basis for deriving the aristotype phase in YMnO(3). Landau theory allows the I <--> II, III <--> IV and IV <--> V phase transitions in YMnO(3), and also the I <--> II phase transition in BaTiO(3), to be continuous; all four, however, unambiguously exhibit first-order characteristics. The origin of phase transitions, permitted by theory to be second order, that are first order instead have not yet been thoroughly investigated; several possibilities are briefly considered.

Report of a Subcommittee on the Nomenclature of n-Dimensional Crystallography. II. Symbols for arithmetic crystal classes, Bravais classes and space groups

The Second Report of the Subcommittee on the Nomenclature of n-Dimensional Crystallography recommends specific symbols for R-irreducible groups in 4 and higher dimensions (nD), for centrings, for Bravais classes, for arithmetic crystal classes and for space groups (space-group types). The relation with higher-dimensional crystallographic groups used for the description of aperiodic crystals is briefly discussed. The Introduction discusses the general definitions used in the Report.

Aminoguanidinium hexafluorozirconate: a new ferroelectric

Analysis of the atomic arrangement in anhydrous aminoguanidinium hexafluorozirconate, CN4H8ZrF6, reported by Bukvetskii, Gerasimenko & Davidxadovich [Koord. Khim. (1990), 16, 1479–1484], led to the prediction that it is a new ferroelectric [Abrahams, Mirsky & Nielson (1996). Acta Cryst. B52, 806–809]. Initial attempts to verify the prediction were inconclusive because of the variety of closely related materials produced under the original preparation conditions. Clarification of these conditions led to the formation of pure CN4H8ZrF6 and the growth of single crystals with dimensions as large as 7 × 7 × 2u2005mm. Highly reproducible calorimetric and dielectric permittivity anomalies reveal the Curie temperature Tc = 383u2005(1)u2005K. At this temperature, the heat capacity Cp exhibits an entropy change of 0.7u2005(1)u2005Ju2005mol−1u2005K−1, while the relative permittivity ∊r exhibits an inflection and the dielectric loss a distinct peak; the dielectric anomaly at Tc is observed only at the lowest (0.1–1u2005kHz) frequencies used. Dielectric hysteresis is demonstrable at 295u2005K under the application of ∼1u2005MVu2005m−1 alternating fields and remains observable at all T < Tc but not at T ≥ Tc; the prediction of ferroelectricity is hence confirmed. The value of the spontaneous polarization Ps is 0.45u2005(9) × 10−2u2005Cu2005m−2 at 298u2005K, with piezoelectric coefficient d33 = 1.9u2005(5)u2005pCu2005N−1 and pyroelectric coefficient p3 = 4u2005(1)u2005µCu2005m−2u2005K−1. Tilts of less than ∼11° by the two symmetry-independent CN4H{}_{8}^{2+} ions, combined with rotations of ∼20° or less by the N—NH3 and C—(NH2)2 groups about the central C—N bond in each cation, as all H atoms rotate into or become symmetrically distributed about the planes at z = 0 or ½, allow them to conform to mirror symmetry via polar atomic displacements of ∼0.4u2005A or less by N or C, and of 0.7u2005A or less by H. Corresponding displacements of less than 0.08u2005A within the two symmetry-independent ZrF{}_{6}^{2-} anions also result in mirror symmetry, satisfying the structural criteria required for the development of ferroelectricity.