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Introduction: A growing number of dialysis patients is treated with home haemodialysis. Our current pre-analytical protocols require patients to centrifuge the blood sample and transfer the plasma into a new tube at home. This procedure is prone to errors and precludes accurate bicarbonate measurement, required for determining dialysate bicarbonate concentration and maintaining acid-base status. We therefore evaluated whether cooled overnight storage of gel separated plasma is an acceptable alternative. Materials and methods: Venous blood of 34 haemodialysis patients was collected in 2 lithium heparin blood collection tubes with gel separator (LH PSTTM II, REF 367374; Becton Dickinson, New Jersey, USA). One tube was analysed directly for measurement of bicarbonate, potassium, calcium, phosphate, glucose, urea, lactate, aspartate aminotransferase (AST), and lactate dehydrogenase (LD); whereas the other was centrifuged and stored unopened at 4 °C and analysed 24 h later. To measure analyte stability after 24 h of storage, the mean difference was calculated and compared to the total allowable error (TEa) which was used as acceptance limit. Results: Potassium (Z = - 4.28, P < 0.001), phosphate (Z = - 3.26, P = 0.001), lactate (Z = - 5.11, P < 0.001) and AST (Z = - 2.71, P = 0.007) concentrations were higher, whereas glucose (Z = 4.00, P < 0.001) and LD (Z = 3.13, P = 0.002) showed a reduction. All mean differences were smaller than the TEa and thus not clinically relevant. Bicarbonate (Z = 0.69, P = 0.491), calcium (Z = - 0.23, P = 0.815) and urea (Z = 0.81, P =0.415) concentrations were stable. Conclusions: Our less complex, user-friendly pre-analytical procedure resulted in at least 24 h stability of analytes relevant for monitoring haemodialysis, including bicarbonate. This allows shipment and analysis the next day.

Coordinate analysis of the multiple phase transitions in hexagonal YMnO3 leads to the prediction of a previously unknown aristotype phase, with the resulting phase-transition sequence: P63′cm′(e.g.) ↔ P63 cm ↔ P63/mcm ↔ P63/mmc ↔ P6/mmm. Below the Néel temperature T N ≃ 75 K, the structure is antiferromagnetic with the magnetic symmetry not yet determined. Above T N the P63 cm phase is ferroelectric with Curie temperature T C ≃ 1105 K. The nonpolar paramagnetic phase stable between T C and ∼ 1360 K transforms to a second nonpolar paramagnetic phase stable to ∼ 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 BaTiO3, with space-group sequence R3m ↔ Amm2 ↔ P4mm ↔ Pm\overline 3m, provides a basis for deriving the aristotype phase in YMnO3. Landau theory allows the I ↔ II, III ↔ IV and IV ↔ V phase transitions in YMnO3, and also the I ↔ II phase transition in BaTiO3, 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.

ICSD Release 2007/1 contains 47 families of inorganic crystal structures, some single-member only, within the 311 entries listed under the polar space group P3m1. Coordinate analysis reveals 12 such families to be candidate ferroelectrics, over a range of confidence levels. Selection is based on the detection of an approach to nonpolar supergroup symmetry, within specified limits, by the atomic arrangement as reported in a confirmed polar space group. The primary source of uncertainty in such predictions is the reliability of the underlying structural determination. The candidates include In2ZnS4, TlSn2F5, Cu7Te4, NaMnSe2, Na2In2(Mo3O8)(MoO4)2, Nb3Br7S, Nb3TeI7, fencooperite, Bi(HCOO)3, Li(NpO2)(CO3)(H2O)2, LiPtD0.66 and Ag3(MoO3F3)(Ag3(MoO4)Cl). A total of 20 structures examined are likely to be nonpolar, a further 20 have reduced predictive properties and three others are more likely to retain P3m1 symmetry over a wide thermal range. Substantial uncertainties associated with many of the listed 163 CdI2, 69 ZnS and 10 SiC polytype structures, together with their low potential for use as possible ferroelectrics, led to their exclusion from fuller analysis.

Release 2006/1 of the Inorganic Crystal Structure Database contains 155 entries under space group R3. Atomic coordinate analysis of the first 81 structures, with 52 different structure types, in Part I [Abrahams (2006). Acta Cryst. B62, 26–41] identified a total of 18 new types that satisfy the structural criteria for ferroelectricity, five that are more likely to have or undergo a transition to 3m symmetry, 19 more likely to be or undergo a transition to nonpolar symmetry and ten with a lower property predictability. Coordinate analysis of the remaining 71 entries with 54 different structure types in Part II leads to 11 materials including Al4B6O15, PbTa3(PO4)(P2O7)3.5, the KCd4Ga5S12 family, the LiZnPO4 family, Ca3Nb1.95O8V0.05 and Mn4Ta2O9 as new candidates which satisfy the structural criteria, together with the three known ferroelectrics Na3MoO3F3, Pb2ScTaO6, and RbTi2(PO4)3 at 6.2 GPa. Two additional ferroelectric predictions are at a lower level of confidence. The analysis also reveals nine materials, two of which are isostructural, that more likely belong or are capable of undergoing a transition to crystal class 3m. There are 14 additional structure types which are more likely to be nonpolar or undergo a transition to nonpolarity, ten have reduced predictive properties, with a further nine for which the space group is expected to remain R3 over the full thermal stability range. The increasing use of methods for identifying overlooked inversion centers in structural determinations may be extended by using coordinate analysis for detecting additional commonly overlooked symmetry elements.

The proposed crystalline phase identifier consists of a number of components (layers) describing enough properties of the phase to allow a unique identification. These layers consist of the chemical formula, a flag indicating the state of matter, the space-group number and the Wyckoff sequence. They are defined in such a way that they can be incorporated into the IUPAC International Chemical Identifier (InChI) proposed by the International Union of Pure and Applied Chemistry (IUPAC).

Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K(2)[HCr(2)AsO(10)], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch-Pouchot, DurifGuitel [Acta Cryst. (1978), B34, 3725-3727], led to an independent redetermination of the structure using two separate crystals. The resulting improved accuracy allows the inference that the H atom is located in the hydrogen bonds of length 2.555 (5) angstroms which form between the terminal O atoms of shared AsO(3)OH tetrahedra in adjacent HCr(2)AsO(10)(2-) ions. The largest atomic displacement of 0.586 angstroms between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius approximately 0.60 A about the 3(1) or 3(2) axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates.

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.

Analysis of the atomic arrangement in anhydrous aminoguanidinium hexafluorozirconate, CN4H8ZrF6, reported by Bukvetskii, Gerasimenko & Davidovich [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 CN4H8ZrF6and the growth of single crystals with dimensions as large as 7 × 7 × 2 mm. Highly reproducible calorimetric and dielectric permittivity anomalies reveal the Curie temperatureTc= 383 (1) K. At this temperature, the heat capacityCpexhibits an entropy change of 0.7 (1) J mol−1 K−1, while the relative permittivity ∊rexhibits an inflection and the dielectric loss a distinct peak; the dielectric anomaly atTcis observed only at the lowest (0.1–1 kHz) frequencies used. Dielectric hysteresis is demonstrable at 295 K under the application of ∼1 MV m−1alternating fields and remains observable at allT

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 ≃ l450 K. The estimated spontaneous polarization P s ≃ 11 × 10−2 C m−2 is consistent with classification as a two-dimensional ferroelectric in which minor Δx and major Δy, Δz atomic coordinate component displacements are required for ferroelectric switching.

The atomic coordinates of K2(NbO)2Si4O12reported in space groupP4bmsatisfy the structural criteria for ferroelectricity. The estimated phase-transition temperature,Tc= 2230 (250) K, is substantially in excess ofTmelting= 1476 (5) K; the hypothetical paraelectric phase in space groupP4/mbmis hence experimentally inaccessible. A strong indication that the phase transition does not occur atT

The possibility that the structure of the novel semiconducting perovskite-related material strontium niobium oxide, Sr5Nb5O17, refined by Schmalle et al. [Acta Cryst. (1995), C51, 1243–1246] in space group Pnn2, might instead belong to space group Pnnm has been investigated following an analysis of the atomic coordinates that indicated the latter space group to be more likely. All I obs were carefully remeasured, first those within a hemisphere containing c *, then all that lay within the full sphere of reflection. Refinement was undertaken, with each of two different sets of weights, in each space group. Each data set was used under three limiting intensity conditions: I obs > 4σ(I obs), I obs > 2σ(I obs) and finally with all reflections, but setting magnitudes with I obs ≤ 0 equal to 0. Fourteen separate tests based only upon the X-ray diffraction data may be used to distinguish between Pnn2 and Pnnm. Nine tests favored the latter choice, four were indeterminate and one was not used. Seven further tests may be made on the basis of physical measurement; of these, three strongly indicated Pnnm, one was indeterminate and three could not be used. The evidence clearly suggests the space group is Pnnm. The use of all reflections, including those with negative magnitude set equal to zero, is essential to avoid ambiguity in the X-ray diffraction tests and achieve the highest reliability. Refinement with weights based on variances of Type A and Type B [Schwarzenbach et al. (1995). Acta Cryst. A51, 565–569] resulted in improved reliability compared with that obtained from a popular empirical weighting scheme. The revised structure differs in several respects from that published previously.

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 R 1 = 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.

The ac (alternating current) conductivity of Pb5Al3F19 between 293 and 743 K has been analyzed within a combined complex impedance, modulus, and permittivity formalism. The antiferroelectric phase IV to ferroelastic phase III, and the phase III to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by higher F− ion mobility above T=368±5 K. A new dielectric anomaly observed isochronously in e′(f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb5Al3F19. A 19F NMR investigation between 125 and 430 K of F− ion diffusion properties in Pb5Al3F19 shows that half the anions have become mobile at the ferroelectric phase V to phase IV transition temperature and that almost all the anions are mobile at the phase IV to phase III transition temperature. These motions are shown to be short range, and no correlation is found between phase formation and F− ion diffusion properties in Pb5Al3F19...

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.

Resume Pb 5 Al 3 F 19 est un membre de la famille isostructurale Pb 5 M 3 F 19 (M = Al, Ti, V, Cr, Fe, Ga); il differe des autres compositions par l'exisrence de trois phases supplementaires entre la variete ferroelectrique V et la variete paraelectrique I. La phase IV est antiferroelectrique, la phase III ferroelastique et la phase II paraelastique. Chaque phase a ete caracterisee. Ces nombreuses varietes distordues proviennent de l'association du doublet 6(sp) 2 de l'ion Pb 2+ et de la faible taille de l'ion Al 3+ .

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.

Despite several methods to measure H 2 S, physiological H 2 S serum concentrations remain highly variable and method-dependent. While modifying the classic methylene blue (MB) method to improve detection of serum H 2 S levels, we also probed potential sulfur-binding characteristics of serum albumin. To avoid loss of serum H 2 S and to trap sulfide ions effectively in the MB method serum was strongly alkalized to completely ionize gaseous H 2 S, followed by incubation with zinc acetate at 37°C, to trap sulfide ions effectively. After incubation, N,N-Diethyl-p-phenylenediamine and FeCl 3 were added to form MB product. Subsequently, trichloroacetic acid was added to remove matrix effects. Absorbance of MB product was at 670nm and compared to a dilution range of NaHS. Spectra (400-900nm) were taken to check quality of MB product and exclude matrix effects. Adaptations in experimental setup are mentioned below. Applying the modified MB method produced the unexpected finding that yield of serum H 2 S increases with higher pH and longer incubation time. H 2 S synthesizing enzymes cystathionine-β-synthase and cystathionine -lyase were both ruled out as sources of serum H 2 S by separately treating serum with their respective inhibitors hydroxylamine and DL-propargylglycine. Human serum protein standard containing only albumin and -globulin, or pure bovine serum albumin (BSA), also yielded H 2 S. Albumin was confirmed as source of H 2 S by absence of increasing H 2 S yield from analbuminemic rat serum, as compared to normal rat serum. That yield of H 2 S increased at least twofold after treating both BSA and human serum with dithiolthreitol (DTT), a disulphide reducer, and close correlation between albumin level and yield of H 2 S during DTT incubation (r 2 =0.99), both suggest H 2 S storage in albumin as “labile sulphurs”. The modified MB method allows measurement of release of H 2 S from albumin. Treating albumin with a disulphide reducer revealed H 2 S storage in albumin as sulfanes. Our work is consistent with suggestions that circulating 'pools' of biologically relevant sulfur/sulfide species exist. Thus, albumin may be a source of H 2 S, as has been observed for NO. How this is affected by disease states (e.g. chronic renal failure) remains to be seen.

The Inorganic Crystal Structure Database containing a total of 209 entries for 128 different materials reported in point group 4 mm, of which eight are for previously known ferroelectrics. Analysis of the remaining entries, assuming the structure determinations reported are correct, leads to the prediction of an additional 43 new ferroelectric materials. 15 were determined in space group P4mm, 11 in P4bm, one in P42 nm, one in P4cc, two in P42 mc, one in P42 bc, seven in I4mm, two in I4cm, one in I41 md and two in I41 cd. Numerous other structures are shown to have been assigned, most likely, to an incorrect space group. All but one of the materials reported in space group P4bm are predicted to be ferroelectric. New ferroelectrics predicted in point group 4 mm include K2[Pt(CN)4]Br0.3.3H2O, Nd0.33TaO3, YBaCuFeO5+δ, Ba2(TiO)(Si2O7), K2(NbO)2(SiO3)4, Ba6CoNb9O30, Sr2SbMnO6, Hg2AlF5.(H2O)2, Rb5Nb3OF18 and SrNi2(VO4)2.

Pb5Al3F19 at 160 K is ferroelectric, crystallizing in the tetragonal system. With Mr = 1477.9, the space group is I4cm and a = 14.07 (2), c = 7.30 (1) Å, V = 1445 (3) Å3, Z = 4, Dx = 6.793 Mg m−3. For λ(Mo Kα) = 0.71073 Å, μ = 56.36 mm−1. F(000) = 2480 Pb5Al3F19 undergoes a first-order phase transition on cooling at 120 K and on heating at 270 K. The structure was determined from 656 independent F(hkl) with I> 3σ(I) and refined by least squares to R = 0.0527, wR = 0.0480, S = 1.065. No atom is further than 0.83 Å, from the hypothetical I4/mcm atomic positions, hence the crystal is structurally ferroelectric. The mean atomic polar displacement of the two independent Al atoms from the zero spontaneous-polarization location, allowing for the F atoms, is 0.207 (28) Å, which leads to a predicted Tc = 860 (230) K. Several additional phase transitions are known to form before the hypothetical phase is attained. Each independent Al atom occupies a fluorine octahedron with average Al—F distance either 1.818 (5) or 1.824 (1) Å. One Pb atom is nine coordinated, occupying a distorted tricapped trigonal prism with average Pb—F = 2.66 (22) Å. The other is ten coordinated, forming a distorted bicapped cuboid, with average Pb—F = 2.61 (28) Å. Pyroelectric pulses are readily detectable in crystals cooled below 120 K, or subsequently on heating to 270 K. Similarly, numerous piezoelectric resonances are observed in the same thermal ranges; the compression mode at ~1197 kHz has been identified by its characteristic admittance circle. The mechanical quality factor Qm for ferroelectric Pb5Al3F19, determined from the admittance-circle-derived equivalent circuit, is ~423.

The crystal structure of Cu(OH)2, recently redetermined at room temperature by Oswald, Reller, Schmalle & Dubler [Acta Cryst. (1990), C46, 2279–2284], is shown to satisfy the structural criteria for ferroelectricity. The Cu2+ ion at 295 K is displaced 0.131 Å from the most likely paraelectric atomic arrangement, allowing the prediction to be made that the ferroelectric Curie temperature is Tc = 343 K. This prediction is in excellent agreement with an earlier report of a reversible structural transition at 325–335 K in Cu(OH)2. The presence of a λ-shaped anomaly in the heat capacity at about 335 K has been confirmed, and a dielectric anomaly is also observed at similar temperatures over a wide frequency range; the frequency dependence is dispersive, owing at least partly to ionic conductivity. The prediction is hence verified, and Cu(OH)2 is demonstrated experimentally to be a new ferroelectric.

The alignment in Tables 1–4 of the paper by Abrahams (2003) was incorrect. The corrected tables are reproduced herein.

Crystallography provides a major approach toward reaching an understanding of the relationship between crystal structure and material physical properties. The specific relationships of five highly structure dependent physical properties, namely piezoelectricity, pyroelectricity, ferroelectricity, ferroelasticity and optical activity, to atomic arrangement are developed in this Lead Article. The major attributes of each property, together with the structural measurements required to establish the property–structure relationship, are introduced and illustrative results for each are given. Predictive criteria that emphasize the insight leading to new property-structure relationships are presented as is an expanded experimental access to the crystallographic study of materials.

Pb5Al3F19 undergoes three first-order phase transitions; on cooling from phase I (4/m) to phase II (2/m) at 365 K, from phase II to phase III (4/m) at 305 K, and from phase III to phase IV (4mm) at 120 K. The lengths of the a- and c-axes undergo continuous change at the highest temperature phase transition as the β-angle increases from 90[unk] below 365 K; no difference is detectable between the lengths of the a- and b-axes in monoclinic phase II, within an accuracy of 0.007 Å. The β-angle temperature dependence follows a power law with exponent that does not differ significantly from 1/2. At the 305 K first-order phase transition, the β-angle abruptly changes from its maximum value of 90.41° in phase II to 90[unk] in tetragonal phase III; the a- and c-axes, however, maintain their continuous change in length through this transition. The 120 K first-order phase transition is characterized by an abrupt 0.77% decrease in the length of the normalized a-axis, and an equally abrupt 1.89% increase in the c-axis length, on cooling from phase III to phase IV. The a- and c-axes in all four phases are characterized by linear thermal coefficients, with magnitudes presented below. The normalized a-axis lengths in phases I, II and III extrapolated to 0 K differ by less than 0.021 Å from 14.174 Å, the corresponding c-axis lengths at 0 K differ by less than 0.004 Å from 7.194 Å. The normalized unit cell volume thermal dependence of phases I, II and III is linear and continuous between 400 and 120 K but undergoes a discontinuity of 0.40% at the transition from phase III to phase IV.

The Pb5(Cr1 − xAlx)3F19 system forms four phases. Ferroelectric phase IV, with point group 4mm, undergoes a first-order transition on heating to paraelectric phase I, point group 4/m, in the composition range 0 ≤ x ≲ 0.80. The Curie temperature varies linearly with x over this range, from 555 K at x = 0 to 365 K at x = 0.80. Phase IV transforms to monoclinic phase II in the range 0.80 ≲ x ≲ 0.88 in a first-order transition before undergoing a second-order transition at higher temperatures to phase I. In the range 0.88 ≲ x ≲ 1, phase IV initially transforms on heating to phase III, point group 4/m, in a first-order transition; at higher temperatures, this phase transforms to phase II in another first-order transition; at still higher temperatures, it transforms to phase I in a second-order transition. Multiple phase formation at x ≲ 0.80 is a result of structural instabilities that are related to the small size of the Al3+ cation and to the decreased interaction strength between the 6(sp)2 electron lone pair of Pb2+ and the residual 3d electron spin moment at the Cr-atom site as it is increasingly substituted by aluminum.

Antiferroelectric phase III of lead aluminium fluoride, Pb 5 Al 3 F 19 , M r =1477.9, tetragonal space group P4/n. At T=295 K, a=20.1738 (4) and c=7.2205 (1) A V=2939 (1) A 3 , Z=8, D m =6.66 (5), D x =6.681 Mg m -3 . For λ(Mo Kα)=0.71069 A, μ=58.0 mm -1 , F(000)=4960. The structure was determined from 18502 (1276 independent) F m 2 ≥3σ(F m 2 ) with (sin θ)/λ≤0.703 A -1 . Least-squares refinement on wF m 2 resulted in R(F m )=0.0579 with R int (F m )=0.048. Pb 5 Al 3 F 19 undergoes a first-order phase transition from antiferroelectric to ferroelectric at about 110 K, with a wide thermal hysteresis

Ferroelectricity has been reported in six families of complex fluorides that contain fluorine octahedra and in four other families that contain both oxygen and fluorine atoms in the octahedra. The experimental Curie temperatures found are in good agreement with those predicted on the basis of structure in the case of the BaMnF4, SrAlF5, Sr3Fe2F12, Pb5Cr3F19 and K3Fe5F15 fluorine-octahedra containing families and are also in agreement in the case of the Pb5W3O9F10 and Na5W3O9F5 oxygen and fluorineoctahedra containing families. The structures of Na2MgAlF7, Bi2TiO4F2, and K3MoO3F3 in their ferroelectric phases are not known with sufficient accuracy to allow prediction of Tc but each family exhibits the experimental attributes of ferroelectricity. Dielectric, calorimetric, and structural properties are presented for all ten ferroelectric families. The range of Curie temperatures and dielectric permittivities in fluorine-octahedra containing ferroelectrics is comparable to that in oxygen-octahedra fer...

Phase transitions have been found in Pb5Al3F19 at Tc = 285 K, Pb5Ti3F19 at 695 K, Pb5V3F19 at 630 K, Pb5Cr3F19 at 545 K (reported previously), Pb5Fe3F19 at 740 K, and Pb5Ga3F19 at 945 K. In striking contrast to the variation of Tc with M atom in Sr5M3F19 and Ba5M3F19 (reported previously), for which materials the phase transition temperature increases with increasing atomic number from M=Ti to M=Cr but with a lower Tc for M=Fe before increasing again, Tc in Pb5M3F19 decreases from M=Ti to M=Cr with a sharp increase for M=Fe before it decreases again. The transition temperature for Pb5Al3F19 is below room temperature, that for isotypic Sr5Al3F19 is not detectable before reaching a dissociation temperature around 1000 K. All phase transitions in the Pb5M3F19 family are first order. Each new material has been fully characterized. Differences between materials with A=Pb, M=Al or Cr, and those with A=Sr, M=Al or Cr have been investigated through the formation of solid solutions. Six separate systems have been ...

Na5W3O9F5 is a coupled ferroelectric-ferroelastic with a structure at 295 K that consists of three symmetry-independent corner-sharing W-(O, F)6 octahedra separated by three independent Na cations in a monoclinic unit cell with a = 7.3597(7), b = 10.6342(15), c = 7.3618(10) A and β = 90.77° (based on previous single crystal measurements). Phase transitions have been reported at about 200, 530 and 800 K. The powder diffraction pattern of Na5W3O9F5 has been measured with X-rays and neutrons over a wide thermal range and the structure at 20, 90, 160, 230, 300, 455, 563, 700 and 741 K determined by means of profile analysis. The lattice constants vary monotonically through the lowest temperature transition but exhibit inflections at about 500 K. The tilt angle between two independent W-(O, F)6 octahedra, related to the order-parameter, varies from 35° at 5 K to 7° at the highest transition (the Curie temperature). The rotation of the remaining octahedron about the a axis varies from 15° at 5 K to zer...

Synthesis of the title compounds by present literature methods produces compositional mixtures that are difficult to separate. Controlled temperatures, times, and reactant concentrations produce gram quantities of pure, colorless, transparent crystals. Thermogravimetry and differential scanning calorimetry show that heating to 400 °C in a nitrogen atmosphere causes stepwise decomposition involving first the loss of water (for the hydrated compounds), followed by liberation of HF, then decomposition to ZrF4.

K3Fe5F15 has been predicted to be both ferroelectric and ferroelastic, with a phase transition at 535 K, on the basis of the atomic coordinates. Subsequently, the dielectric permittivity has been found to reach a maximum at 495 (10) K as the dielectric loss undergoes a change in slope, characteristic of ferroelectric behavior. Furthermore, the heat capacity exhibits a Λ-type anomaly at 490 (10) K, with a corresponding entropy change of Δ S = 5.5 (2) J mol−1 K−1. Ferroelastic domains present at room temperature disappear sharply on heating above 490 (10) K, as K3 Fe5 F15 transforms from orthorhombic to tetragonal symmetry, and reappear on cooling below 480 (10) K. The phase transition was also predicted to be accompanied by a change from order to disorder among the Fe2+, Fe3+ ions on heating above Tc, and the prediction is confirmed by the thermal dependence of the Mossbauer effect in which lines due to Fe2+ broaden as an anomaly appears in the hyperfine structure at Tc.

A single crystal of stoichiometric lithium niobate has been studied, using high-resolution synchrotron X- rays, under an electric field applied along the polar crystallographic c direction. A crystalline plate, 0.2 mm thick along the c direction, was polarized by a ±900 V square wave of frequency 50 and 230 Hz. Electronic gating ensured that the scattered intensities were recorded only during the central 50% of the square-wave duration. The results show that the stoichiometric crystals have surface layers under the Al electrodes that differ in the c cell dimension from the bulk by about Δc/c = 6 × 10-4, The resulting c lattice vector is close in length to that of congruent Li0.941Nbl.012O3, although the composition of the surface layer may only be inferred; modification by Al in-diffusion may also be possible. The surface-layer thickness is estimated to be of the order of 0.01 mm.

The prediction made in J. Appl. Phys. 65, 3987 (1989) that the ferroelectric–ferroelastic phase transition at Tc =490 K in K3Fe5F15 will be accompanied by a change from order to disorder among the Fe2+/Fe3+ ions has been confirmed by Mossbauer spectroscopy. The thermal dependence of both the Fe3+ quadrupolar splitting and the Fe3+ isomer shift exhibits a smooth decrease with increasing temperature to Tc, each with a sharp and reproducible change in slope above Tc. The average Fe2+ quadrupolar splitting (QS) similarly undergoes a major decrease with increasing temperature, with a clear change in slope above Tc. Furthermore, the thermal dependence of the Fe2+ QS distribution shows that the Fe2+/Fe3+ order required crystallographically below Tc is not conserved at higher temperatures. The proportion of Fe2+ present below Tc is close to the stoichiometric 60% value; the proportion decreases above Tc rapidly with temperature and becomes less than 50% by 543 K, consistent with increasing delocalization of an Fe...

Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K2[HCr2AsO10], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch-Pouchot, Durif & Guitel [Acta Cryst. (1978), B34, 3725–3727], led to an independent redetermination of the structure using two separate crystals. The resulting improved accuracy allows the inference that the H atom is located in the hydrogen bonds of length 2.555 (5) A which form between the terminal O atoms of shared AsO3OH tetrahedra in adjacent HCr2AsO102− ions. The largest atomic displacement of 0.586 A between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius ∼0.60 A about the 31 or 32 axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates.

Crystals of phase II K2Cr2O7, potassium dichromate, space group P\overline 1, grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, T PT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at T PT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher T PT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629–630]. The first-order transition between phases I and II arises from the ca 60° relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.

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 amino-guanidinium polyfluorozirconates may result from minor displacements of H atoms in weak N-H...F bonds to new potential minima and subsequent anionic realignment.

Release 2002/2 of the Inorganic Crystal Structure Database (FIZ Karlsruhe, Germany, and Institute Laue–Langevin, Grenoble, France) contains 62 entries in space group P31 and ten entries in space group P32 for 49 different materials including eight families with two or more isostructural members. The structural criteria for ferroelectricity are satisfied for 16 new structure types at a confidence level that depends on the reliability of each determination. LaBGeO5, a mineral with stillwellite structure, was previously reported as ferroelectric and forms an additional family with seven other members or related structures that satisfy the criteria. Ten structures reported in space group P31 or P32 are dubious or incorrect, with atomic coordinates that satisfy supergroup symmetry. One material is probably pyroelectric but is unlikely to become ferroelectric, and three others are either incompletely solved or incompletely refined. Among the predicted new ferroelectrics are Cu2BaGeS4, Fe3(Fe,Si)O4(OH)5, Se4S5, K2HCr2AsO10, IV-RbNO3, Rb2Sc(NO3)5, Na3ReO5, Nd14(GeO4)2(BO3)6O8, CsHgCl3, Ba2Cu2AlF11, KYF4, SrS2O6·4H2O, Cu3PbTeO6(OH)2, ReH(CO)4, Ni2(NH3)9Mo(CN)8·2H2O and the 6T polytype of Ca1.89Ta1.80Sm0.16Ti0.10O7, in addition to β-LaBSiO5, PbBAsO5 and BaBAsO5 in the stillwellite family.

Large colorless crystal of the Pb/sub 5/Al/sub 3/F/sub 19/ type with dimensions approaching 1 cm/sup 3/, have been grown in sealed platinum tubes by the Bridgman technique. The phase transitions in Pb/sub 5/Al/sub 3/F/sub 19/ have been studied by optical microscopy on crystals oriented along different crystallographic directions. Ferroelastic domains have been observed; a reversible and reproducible ferroelastic-prototype transition has been shown. The thermal dependence of the birefringence has also been studied. The ferroic properties of Pb/sub 5/Al/sub 3/F/sub 19/ are discussed in relation to the crystal symmetry. >

The calorimetric and dielectric properties of Pb5Al3F19 in the five phases stable under ambient pressure are correlated with structure for fuller characterization of each phase. The first-order transition between ferroelectric phase V and antiferroelectric phase IV at T V,IV = 260 (5) K exhibits a thermal hysteresis of 135 (5) K on heating, with a maximum atomic displacement Δ(xyz)max = 1.21 (6) Å; the transition from phase IV to ferroelastic phase III at 315 (5) K is also first order but with a thermal hysteresis of 10 (5) K and Δ(xyz)max = 0.92 (7) Å; that from phase III to paraelastic phase II at 360 (5) K is second order without hysteresis and has Δ(xyz)max = 0.69 (4) Å; and the transition from phase II to paraelectric phase I at 670 (5) K is second or higher order, with Δ(xyz)max = 0.7 (4) Å. The measured entropy change ΔS at T V,IV agrees well with ΔS as derived from the increased configurational energy by Stirling's approximation. For all other phase transitions, 0.5 ≥ ΔS > 0 J mol−1 K−1 is consistent with an entropy change caused primarily by the changes in the vibrational energy. The structure of phase III is determined both by group theoretical/normal mode analysis and by consideration of the structures of phases II, IV and V reported previously; refinement is by simultaneous Rietveld analysis of the X-ray and neutron diffraction powder profiles. The structure of prototypic phase I is predicted on the basis of the atomic arrangement in phases II, III, IV and V. The introduction of 3d electrons into the Pb5Al3F19 lattice disturbs the structural equilibrium, the addition of 0.04% Cr3+ causing significant changes in atomic positions and increasing T IV,III by ∼15 K. Substitution of Al3+ by 20% or more Cr3+ eliminates the potential minima that otherwise stabilize phases IV, III and II.

IntroductionEncapsulating peritoneal sclerosis (EPS) is a devastating complication of peritoneal dialysis (PD). The pathogenesis is not exactly known and no preventive strategy or targeted medical therapy is available. CCN2 has both pro-fibrotic and pro-angiogenic actions and appears an attractive target. Therefore, we studied peritoneal expression of CCN2, as well as TGFβ1 and VEGF, in different stages of peritoneal fibrosis.Materials and methodsSixteen PD patients were investigated and compared to 12 hemodialysis patients and four pre-emptively transplanted patients. Furthermore, expression was investigated in 12 EPS patients in comparison with 13 PD and 12 non-PD patients without EPS. Peritoneal tissue was taken during kidney transplantation procedure or during EPS surgery. In a subset of patients, CCN2 protein levels in peritoneal effluent and plasma were determined. Samples were examined by qPCR, histology, immunohistochemistry, and ELISA.ResultsPeritoneal CCN2 expression was 5-fold higher in PD patients compared to pre-emptively transplanted patients (P < 0.05), but did not differ from hemodialysis patients. Peritoneal expression of TGFβ1 and VEGF were not different between the three groups; neither was peritoneal thickness. Peritoneum of EPS patients exhibited increased expression of CCN2 (35-fold, P < 0.001), TGFβ1 (24-fold, P < 0.05), and VEGF (77-fold, P < 0.001) compared to PD patients without EPS. In EPS patients, CCN2 protein was mainly localized in peritoneal endothelial cells and fibroblasts. CCN2 protein levels were significantly higher in peritoneal effluent of EPS patients compared to levels in dialysate of PD patients (12.0 ± 4.5 vs. 0.91 ± 0.92 ng/ml, P < 0.01), while plasma CCN2 levels were not increased.ConclusionsPeritoneal expression of CCN2, TGFβ1, and VEGF are significantly increased in EPS patients. In early stages of peritoneal fibrosis, only CCN2 expression is slightly increased. Peritoneal CCN2 overexpression in EPS patients is a locally driven response. The potential of CCN2 as biomarker and target for CCN2-inhibiting agents to prevent or treat EPS warrants further study.

The 1963 discovery of ferroelectricity in YMnO3 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, YInO3 was shown to be isostructural with YMnO3 and later demonstrated to satisfy the structural criteria for ferroelectricity; recent unpublished measurements give T c (YInO3) = 835 (15) K. The experimental T c value of 913 K for YMnO3 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 YInO3 with the calculated 760 (120) K. The full YMnO3 family includes the AMnO3 subfamily with A = Y, Ho, Er, Tm, Yb, Lu, Sc, In; the AInO3 subfamily with A = Y, Gd, Dy, Ho, Tb; and the AGaO3 subfamily with A = Y, Ho, Er. The T c values of six family members with known structure, in addition to YMnO3 and YInO3, have been structurally derived as 1310 (110) K for ErMnO3, 1290 (165) K for LuMnO3, 1270 (110) K for YbMnO3, 1220 (105) K for ScMnO3, 540 (375) K for InMnO3 and 1020 (100) K for YGaO3. The agreement between predicted and experimental T c values for ErMnO3, LuMnO3 and YbMnO3, in addition to that for YMnO3 and YInO3, leads to the confident prediction that ScMnO3, InMnO3 and YGaO3 are new ferroelectrics. The remaining six members of the full YMnO3 family are also expected to be new ferroelectrics.

The ferroelectric-ferroelastic phase transition predicted in K3Fe5F15 from the previously determined atomic positions has been demonstrated experimentally. K3Fe5F15 is the first ferroelectric fluoride with a structure derived from that of tetragonal tungsten bronze. The transition is accompanied by reversible order-disorder among the Fe2+ and Fe3+ cations located near the centers of their fluorine octahedra. The predicted Curie temperature (Tc ) is 535 K whereas that determined by x-ray diffraction, optical, calorimetric and dielectric measurements is 490 K. The Curie temperature, spontaneous strain and entropy change in solid solutions of K3-x Fe5F15 (0 ≤ x ≤ 0.2) are found to decrease with increasing values of x as the transition becomes increasingly diffuse. The broadened transition may be related to a statistical distribution in the ionic ratio population.

Ferroelectric Pb5Cr3F19 forms a complete series of solid solutions both with Pb5Al3F19 and Pb5Fe3F19. The end and intermediate compositions are isostructural with the ferroelectric A5M3F19 family previously reported [J. Appl. Phys. 62, 4299 (1987)]. The a and c axial lengths in Pb5(Cr0.2Al0.8)3F19 undergo discontinuities of +0.5% and −0.8%, respectively on heating through the Curie temperature (Tc =325 K), and exhibit a hysteresis of about 40 K between the phase transition temperature on heating and on cooling, characteristic of a first‐order phase change. The value of Tc may be varied continuously between 555 and 280 K for the solid solution Pb5(Cr1−xAlx) 3F19, and between 555 and 725 K for Pb5(Cr1−xFex) 3F19, by increasing x from 0 to 1. The entropy change at the phase transition in Pb5Fe3F19, Pb5Cr3F19, and Pb5(Cr0.8Al0.2)3 F19 is high, at about Rln8. The maximum dielectric permittivity in Pb5Cr3F19 is about 2000.

Five compositions of K6−2x[FeII2 (FeII4−2xFeIII4+2x)]F30 with 0≤x≤0.20 have been investigated. The Curie temperature Tc decreases from 490 K for x=0 to 230 K for x=0.20. The spontaneous strain es=45×10−4 for x=0 at 300 K, and decreases with increasing x to es=0 for x≥0.075. The entropy change at the transition from the ferroelectric–ferroelastic to the paraelectric–paraelastic phase is 5.5 J mol−1 K−1 for x=0, decreasing to 1.5 J mol−1 K−1 for x=0.20. Calorimetric and dielectric measurements show that the phase transition broadens as x increases. The composition dependence of the phase transition is a function of the Fe2+/Fe3+ distribution. The presence of one Fe3+ ion on the Fe2+ sites of ten unit cells reduces Tc by 150 K. The entropy change at Tc for x≂0.2 corresponds to significant remaining displacements, from the tetragonal unit‐cell positions, being associated only with two of the three independent Fe atoms and with not more than two of the eight independent F atoms in each unit cell. UFAIPXR

The latest release of the Inorganic Crystal Structure Database contains a total of 87 entries corresponding to 70 different materials in point group 4. The structures reported for 11 materials in space group P4 satisfy the criteria for ferroelectricity, as do four in P41, one each in P42 and P43, 12 in I4, including seven that form three families, and another three in I41. Three previously known ferroelectrics were also listed in I4 and one in I41. In addition, the listing for point group 4 contains 22 entries for nonferroelectric materials and three with misassigned space groups. Among the newly predicted ferroelectrics in point group 4, assuming the validity of the underlying structural reports, are Ce5B2C6, modulated NbTe4, Na3Nb12O31F, Ca2FeO3Cl, K4CuV5O15Cl, TlBO2, CrOF3, PbTeO3, VO(HPO3)(H2O).3H2O, MgB2O(OH)6, β-tetragonal boron, CuBi2O4, WOBr4, Na8PtO6, SbF2Cl3, Ba1.2Ti8O16, Ni[SC(NH2)2]4Cl2, Ca2SiO3Cl2, the mineral caratiite, NbAs, β-NbO2 and Ag3BiO3.