R. M. Nielson

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 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K 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 660 (11) K, as observed calorimetrically, in addition to a dielectric permittivity anomaly with an onset temperature of 685 (10) K. 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) × 10−2C m−2, provides unambiguous verification that it is a new ferroelectric. Ba6FeNb9O30 also undergoes a diffuse phase transition at 605 (16) K, with a dielectric anomaly at 583 (5) K, and exhibits dielectric hysteresis at room temperature under a varied DC field ranging to ± 310 kV 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.

Sr2SbMnO6: A new semiconductor ferroelectric

Sr2SbMnO6 is predicted to be ferroelectric with Curie temperature Tc=450(175) K on the basis of a 1990 structure determination. Calorimetry reveals a stepwise heat capacity change at Tc=431(5) K, indicative of a phase transition close to first order. A further small calorimetric anomaly occurs reproducibly at 743(10) K, consistent with an earlier report of a higher-temperature phase transition to cubic symmetry. The dielectric permittivity e′ increases sharply at Tc=466(10)K in the range 0.1–100 kHz; e′ rises ∼3 orders of magnitude at 0.1 kHz, ∼1.0 order at 100 kHz, before a stable maximum is reached at T>Tc. The conductivity σ of Sr2SbMnO6 at 295 K is 0.23(5) S m−1, too high for dielectric hysteresis measurement. The linear dependence of ln σ on 1/T between 295 K and Tc in this semiconductor ferroelectric corresponds to a band-gap EG=0.50(2) eV; above Tc, EG increases abruptly to 0.70(2) eV.

Aminoguanidinium(2+) Hexafluorozirconate Monohydrate: A Co-Product of Preparing the Ferroelectric Anhydrous Salt

Preparation of anhydrous aminoguanidinium(2+) hexafluorozirconate, CN4H8ZrF6, shown previously to satisfy the structural criteria for ferroelectricity [Abrahams et al. (1996). Acta Cryst. B52, 806–809], generally results in the co-formation of a series of related fluorozirconates. The structure of the monohydrate salt, one of the co-products, has been redetermined to improve understanding of the preparation pathway, locate the H atoms and compare corresponding atom positions [Gerasimenko et al. (1989). Koord. Khim. 15, 130–135]. The positions of the H atoms were not established in the latter study. All 16 H atoms in the two symmetry-independent CN4H8(2+) ions are now located and refined, with R1 = 0.0299 and S = 1.119. Both independent water molecules are disordered. Normal probability analysis reveals uncompensated error and/or underestimated uncertainty associated with ten non-H-atom position coordinates. The relative concentrations of HF, CN4H7Cl and H2ZrF6 are among the major variables controlling the formation of the related fluorozirconates.

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.

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.

Bis[aminoguanidinium(1+)] hexafluorozirconate(IV): redeterminations and normal probability analysis

The crystal structure of bis[aminoguanidinium(1+)] hexafluorozirconate(IV), (CH(7)N(4))(2)[ZrF(6)], originally reported by Bukvetskii, Gerasimenko & Davidovich [Koord. Khim. (1990), 16, 1479-1484], has been redetermined independently using two different samples. Normal probability analysis confirms the reliability of all refined parameter standard uncertainties in the new determinations, whereas systematic error detectable in the earlier work leads to a maximum difference of 0.069 (6) A in atomic positions between the previously reported and present values of an F-atom y coordinate. Radiation-induced structural damage in aminoguanidinium polyfluorozirconates may result from minor displacements of H atoms in weak N-H...F bonds to new potential minima and subsequent anionic realignment.

K2(NbO)2Si4O12 : a new ferroelectric

The atomic coordinates of K2(NbO)2Si4O12 reported in space group P4bm satisfy the structural criteria for ferroelectricity. The estimated phase-transition temperature, Tc = 2230 (250) K, is substantially in excess of Tmelting = 1476 (5) K; the hypothetical paraelectric phase in space group P4/mbm is hence experimentally inaccessible. A strong indication that the phase transition does not occur at T < Tmelting is provided by the absence of both calorimetric and dielectric permittivity anomalies below Tmelting, the linear thermal dependence of unit-cell parameters with expansion coefficients α11 = –0.53 (12) × 10–6 and α33 = 24.66 (11) × 10–6 K–1 between 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and alternating current, with spontaneous polarization Ps ≃ 0.2 C m–2, provides unambiguous verification of the ferroelectric property.

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

A printers error in the paper by Foster, Brown, Nielson & Abrahams [J. Appl. Cryst. (1997). 30, 495–501] is corrected. In §4.2 on p. 497, the displacement Δz(Fe1/Nb1) was given incorrectly as 0.5353 − 0.5063 = 0.1 (7) A. The correct value for the displacement Δz(Fe1/Nb1) is 0.5353 − 0.5063 = 0.12 (7) A.