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S C Abrahams

Publications and source records attributed to S C Abrahams.

9 recordsLinked to original sources
PubMed2002

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.

Journal Article↗
PubMed2002

Aminoguanidinium(1+) pentafluorozirconate: multiple redetermination and comparisons.

The structure of CN(4)H(7)ZrF(5) reported by Bukvetskii et al. [Koord. Khim. (1992). 18, 576-579] has been independently redetermined on the basis of measurements on three different crystals. Assuming all four resulting structures are drawn from a normal distribution, normal probability analysis of the atomic coordinates taken in pairs reveals joint standard uncertainties that are underestimated by factors as large as 16.5 for the x(Zr) coordinate. Unit-cell parameters in the four crystals similarly have joint uncertainties, under the same assumption, that are underestimated by factors as large as 83.0 for the b axis. The variations in axial lengths from crystal to crystal and the declines in standard reflection intensities by 13-15% in at least two of the crystals measured are consistent with the inference that the distribution is not normal. Rather, the differences observed may be assumed to be caused by small but highly significant radiation-induced structural changes. The large underestimations hence reflect physical differences among the four irradiated crystals. The determinations show that the CN(4)H(7)(+1) cation is exactly planar except for the two H atoms bonded to the terminal N atom; the plane of this NH(2) group is normal to that of the cation. The average length of the three independent C-N bonds is 1.318 (11) A; the N-N bond length is 1.397 (3) A. Distorted ZrF(7) pentagonal bipyramids share edges, forming chains linked by N-H...F bonds to the CN(4)H(7)(+1) ions.

Guanidines↗
PubMed2001

Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions. II. Report of an IUCr Working Group on Phase Transition Nomenclature.

A general nomenclature applicable to the phases that form in any sequence of transitions in the solid state has been recommended by an IUCr Working Group [Acta Cryst. (1998). A54, 1028-1033]. The six-field notation of the first Report, hereafter I, was applied to the case of structural phase transitions, i.e. to transformations resulting from temperature and/or pressure changes between two crystalline (strictly periodic) phases involving modifications to the atomic arrangement. Extensive examples that illustrate the recommendations were provided. This second Report considers, within the framework of a similar six-field notation, the more complex nomenclature of transitions involving magnetic phases, incommensurate phases and transitions that occur as a function of composition change. Extension of the nomenclature to the case of phases with less clearly established relevance to standard schemes of transition in equilibrium systems, namely polytype phases, radiation-induced and other transient phases, quasicrystalline phases and their transitions is recommended more tentatively. A uniform notation for the translational periodicity, propagation vector or wavevector for magnetic and/or incommensurate substances is specified. The notation adopted for incommensurate phases, relying partly on the existence of an average structure, is also consistent with that for commensurate phases in a sequence. The sixth field of the nomenclature is used to emphasize the special features of polytypes and transient phases. As in I, illustrative examples are provided for each category of phase sequence.

Journal Article↗
PubMed2001

Ferroelectricity and structure in the YMnO(3) 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.

Journal Article↗
PubMed2001

Structurally ferroelectric SrMgF4.

The crystal structure of 0.06% Ce-doped SrMgF4, strontium magnesium tetrafluoride, reported by Ishizawa et al. [(2001), Acta Cryst. C57, 784-786] is shown to satisfy the structural criteria for ferroelectricity and to have a predicted Curie temperature T(c) approximately l450 K. The estimated spontaneous polarization P(s) approximately 11 x 10(-2) C x m(-2) is consistent with classification as a two-dimensional ferroelectric in which minor Delta(x) and major Delta(y), Delta(z) atomic coordinate component displacements are required for ferroelectric switching.

Journal Article↗
PubMed2000

Systematic prediction of new ferroelectrics in space group P3.

The current release of the Inorganic Crystal Structure Database contains a total of 57 entries under space group P3 that correspond to 50 different materials. There are 21 structures reported with this space group that satisfy the criteria for ferroelectricity, at a confidence level that depends on the reliability of the underlying structural determination. One ferroelectric discovered earlier is also listed. In addition, the database contains 19 entries that probably should be assigned to a centrosymmetric space group, seven that are polar but probably not ferroelectric and two that are without atomic coordinates. Seven entries are either duplicates or present additional structural studies of the same material. Structures in space group P3 identified as potentially new ferroelectrics include LiAsCu(0.93), Na(2)UF(6), BiTeI, BaGe(4)O(9), alpha-UMo(2)O(8), Cu(2)SiS(3), Co(IO(3))(2), Sr(7)Al(12)O(25), KSn(2)F(5), YbIn(2)S(4), Na(5)CrF(2)(PO(4))(2), Sn(ClO(2))(2)(ClO(4))(6), Eu(3)BWO(9), Li(H(2)O)(4)B(OH)(4).2H(2)O, Mn(3)V(1/2)(SiO(4))O(OH)(2), Ca(6)(Si(2)O(7))(OH)(6), Na(6. 9(2))[Al(5.6(1))Si(6.4(1))O(24)](S(2)O(3))(1.0(1)).2H(2)O, BaCa(2)In(6)O(12), Ni(H(2)O)(6)[Sb(OH)(6)](2), Sr(4)Cr(3)O(9) and Cu(5)O(2)(VO(4))(2).CuCl(2).

Journal Article↗
PubMed2000

Symmetry elements in space groups and point groups. Addenda to two IUCr reports on the nomenclature of symmetry.

The definition of 'symmetry element' given in the Report of the IUCr Ad-Hoc Committee on the Nomenclature of Symmetry by de Wolff et al. [Acta Cryst. (1989). A45, 494-499] is shown to contain an ambiguity in the case of space groups P6/m, P6/mmm, P6/mcc and point groups 6/m and 6/mmm. The ambiguity is removed by redefining the 'geometric element' as a labelled geometric item in which the label is related to the rotation angle of the rotation or rotoinversion symmetry operation. The complete set of different types of glide plane is shown to contain three more than the 15 that are illustrated in the 1992 Report by de Wolff et al. [Acta Cryst. (1992). A48, 727-732].

Journal Article↗
PubMed1999

Anhydrous ammonioguanidinium(2+) and dihydrated bis[aminoguanidinium(1+)] hexafluorosilicates: new co-products of preparing ferroelectric ammonioguanidinium(2+) hexafluorozirconate.

Ammonioguanidinium hexafluorosilicate, CH8N4(2+).SiF6(2-), and bis(aminoguanidinium) hexafluorosilicate dihydrate, 2CH7N4+.SiF6(2-).2H2O, are new materials formed as by-products in course of preparing ferroelectric CH8N4ZrF6 in the presence of glassware. Their structures were determined for comparison with the corresponding hexafluorozirconates. All atoms including the eight H atoms in the CH8N4(2+) cation and the seven H atoms in the CH7N4+ cation have been located and refined with wR(F2) = 0.0653, R = 0.0255, S = 1.146 and wR(F2) = 0.0745, R = 0.0301, S = 1.065, respectively. The N2C-N-N backbone of the 2+ cation is close to planarity, while that of the 1+ cation does not differ significantly from planarity. The SiF6(2-) octahedron is nearly ideally regular in both materials, with < Si-F > = 1.684 (unbiassed estimator of standard uncertainty = 0.016) A in the anhydrous hexafluorosilicate and 1.6801 (unbiassed estimator of standard uncertainty = 0.0006) A in the dihydrate. The combination of coulombic and NH...F interactions in CH8N4SiF6 results in a relatively dense variant of the NaCl structure. In addition to similar forces, the dihydrate is also characterized by the role of the water molecule with its strong NH...O interactions; its packing efficiency is, however, appreciably less than that of the anhydrous hexafluorosilicate with an approximately 8% increase in void space. Cleaved crystals of the dihydrate are frequently twinned across the (001) composition plane, with a twofold rotation about the b axis as the twin operation.

Crystallization↗