From crystalline to amorphous calcium pyrophosphates: A solid state Nuclear Magnetic Resonance perspective.

Hydrated calcium pyrophosphates (CPP, Ca2P2O7·nH2O) are a fundamental family of materials among osteoarticular pathologic calcifications. In this contribution, a comprehensive multinuclear NMR (Nuclear Magnetic Resonance) study of four crystalline and two amorphous phases of this family is presented. (1)H, (31)P and (43)Ca MAS (Magic Angle Spinning) NMR spectra were recorded, leading to informative fingerprints characterizing each compound. In particular, different (1)H and (43)Ca solid state NMR signatures were observed for the amorphous phases, depending on the synthetic procedure used. The NMR parameters of the crystalline phases were determined using the GIPAW (Gauge Including Projected Augmented Wave) DFT approach, based on first-principles calculations. In some cases, relaxed structures were found to improve the agreement between experimental and calculated values, demonstrating the importance of proton positions and pyrophosphate local geometry in this particular NMR crystallography approach. Such calculations serve as a basis for the future ab initio modeling of the amorphous CPP phases. STATEMENT OF SIGNIFICANCE: The general concept of NMR crystallography is applied to the detailed study of calcium pyrophosphates (CPP), whether hydrated or not, and whether crystalline or amorphous. CPP are a fundamental family of materials among osteoarticular pathologic calcifications. Their prevalence increases with age, impacting on 17.5% of the population after the age of 80. They are frequently involved or associated with acute articular arthritis such as pseudogout. Current treatments are mainly directed at relieving the symptoms of joint inflammation but not at inhibiting CPP formation nor at dissolving these crystals. The combination of advanced NMR techniques, modeling and DFT based calculation of NMR parameters allows new original insights in the detailed structural description of this important class of biomaterials.

Introducing a large polar tetragonal distortion into Ba-doped BiFeO3 by low-temperature fluorination.

This article reports on the synthesis and crystallographic and magnetic structure of barium-doped BiFeO3 compounds with approximate composition Bi(1-x)Ba(x)FeO(3-x/2), as well as those of the fluorinated compounds Bi(1-x)Ba(x)FeO(3-x)F(x) (both with x = 0.2, 0.3), prepared by low-temperature fluorination of the oxide precursors using polyvinylidenedifluoride. Whereas the oxide compounds were obtained as cubic (x = 0.2) and slightly tetragonal (x = 0.3, c/a ≈ 1.003) distorted perovskite compounds, a large tetragonal polar distortion was observed for the oxyfluoride compounds (c/a ≈ 1.08 for x = 0.2 and ∼1.05 for x = 0.3), being isostructural to tetragonal PbTiO3. Although described differently in previous reports on Ba-doped BiFeO3, the observed remanent magnetization is found to agree well with the amount of BaFe12O19 only detectable by neutron diffraction and the well-known magnetic properties of BaFe12O19. The oxyfluoride compounds show G-type antiferromagnetic ordering with magnetic moments lying in the a/b plane.

Crystallographic and magnetic structure of the perovskite-type compound BaFeO2.5: unrivaled complexity in oxygen vacancy ordering.

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group P2(1)/c [a = 6.9753(1) Å, b = 11.7281(2) Å, c = 23.4507(4) Å, ß = 98.813(1)°, and Z = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe([6])1Fe([5])2Fe([4])4O17.5 and is in agreement with the (57)Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of "G-type" antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements.

The effect of amorphous pyrophosphate on calcium phosphate cement resorption and bone generation.

Pyrophosphate ions are both inhibitors of HA formation and substrates for phosphatase enzymes. Unlike polyphosphates their hydrolysis results simultaneously in the complete loss of mineral formation inhibition and a localised elevation in orthophosphate ion concentration. Despite recent advances in our knowledge of the role of the pyrophosphate ion, very little is known about the effects of pyrophosphate on bone formation and even less is known about its local delivery. In this work we first developed a self setting pyrophosphate based calcium cement system with appropriate handling properties and then compared its in vivo degradation properties with those of a non-pyrophosphate containing control. Contrary to expectation, the presence of the pyrophosphate phase in the cement matrix did not inhibit mineralisation of the healing bone around the implant, but actually appeared to stimulate it. In vitro evidence suggested that enzymatic action accelerated dissolution of the inorganic pyrophosphate ions, causing a simultaneous loss of their mineralisation inhibition and a localised rise in supersaturation with respect to HA. This is thought to be a rare example of a biologically responsive inorganic material and these materials seem to be worthy of further investigation. Bioceramics to date have mainly been limited to orthophosphate, silicate and carbonate salts of calcium, here we report the successful application of a pyrophosphate material as a degradable osteoconductive bone repair cement.

On the soft magnetic properties of the compounds of the series Na(x)Mn(4.5-x/2)(VO4)3 and the magnetic structure of h.t.-Mn3(VO4)2 (x = 1).

The high temperature phase of manganese vanadate h.t.-Mn3(VO4)2 and the solid solution with NaMn4(VO4)3 (Na(x)Mn(4.5-x/2)(VO4)3) were shown to order ferrimagnetically below 55 K for x < 1, whereas NaMn4(VO4)3 is an antiferromagnet. The materials show very soft magnetic properties with low coercitive fields required for demagnetisation (Hc < 0.001 T). The magnetic structure of h.t.-Mn3(VO4)2 was determined by Rietveld analysis of low temperature powder neutron diffraction data, and shows antiferromagnetic alignment of the magnetic moments of the Mn(2+) ions on the 8c and 8d sites. The ferrimagnetic moments were shown to result from the magnetic moments of the Mn(2+) cations located on the 4b site in unusual dodecahedral coordination (Hoard dodecahedron). This coordination can be understood as two penetrating oxygen coordination tetrahedra, one showing shorter and one showing longer Mn-O distances. The magnetic moments of the Mn(2+) ions on the 4b site are aligned parallel to the ones on 8d and antiparallel to the ones on 8c, being in good agreement with the GKA rules. The local exchange interactions between the Mn(2+) ions on the 4b to those on the 8c/8d sites are likely to be similar in strength and competitive and therefore probably contribute to the soft magnetic properties.

Brushite cement additives inhibit attachment to cell culture beads.

Brushite-forming calcium phosphate cements are of great interest as bone replacement materials because they are resorbable in physiological conditions. Cell-attached culture beads formed from this material could be of great use for cell therapy. Despite a significant amount of work on optimizing the physicochemical properties of these materials, there are very few studies that have evaluated the capacity of the materials to facilitate cell adhesion. In this study, we have formed resorbable calcium phosphate (brushite) culture beads and for the first time we showed that cell attachment to the surface of the brushite cement (BC) could be inhibited by the presence of an intermediate dicalcium phosphate-citrate complex, formed in the cement as a result of using citric acid, a retardant and viscosity modifier used in many cement formulations. The BC beads formed from the mixture of ß-TCP/orthophosphoric acid using citric acid did not allow cell attachment without further treatment. Ageing of BC beads in serum-free Dulbecco's Modified Eagle's Medium (DMEM) solution at 37°C for 1 week greatly enhanced the cell adhesion capacity of the material. Scanning electron microscopy, X-ray diffraction (XRD), and confocal Raman microspectrometry indicated the increased capacity for cell adhesion was due to the changes in phase composition of BC. XRD patterns collected before and after ageing in aqueous solution and a high initial mass loss, suggest the formation of a dicalcium phosphate-citrate complex within the matrix. Since compacts formed from brushite powder supported cell attachment, it was hypothesized that the dicalcium phosphate-citrate complex prevented attachment to the cement surface.

Jahn-Teller distorted frameworks and magnetic order in the Rb-Mn-P-O system.

Two previously uncharacterized members of the Rb-Mn-P-O system, RbMnP(2)O(7) and beta-RbMnHP(3)O(10), have been synthesized using a phosphoric acid flux synthetic route and their crystal and magnetic structures determined using neutron powder diffraction. The crystal structure of RbMnP(2)O(7) (space group P2(1)/c, a = 7.3673(2) A, b = 9.6783(2) A, c = 8.6467(2) A, and beta = 105.487(1) degrees) was found to be isostructural with RbFeP(2)O(7). The polymorph beta-RbMnHP(3)O(10) was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) A, b = 8.5243(3) A, c = 8.8530(4) A, beta = 107.233(2) degrees). Both structures consist of frameworks of corner-sharing MnO(6) octahedra linked together by condensed phosphate anions, with Rb(+) cations located in the intersecting channels. In both cases the Mn(3+) octahedra exhibit unusual Jahn-Teller distortions indicative of a plasticity effect driven by the steric requirements of the condensed phosphate anions, and this causes a strong violet coloration similar to that observed in the manganese violet pigment; the structure of this has yet to be determined. Magnetic susceptibility measurements show that both RbMnP(2)O(7) (T(N) = 20 K) and beta-RbMnHP(3)O(10) (T(N) = 10 K) undergo a phase transition at low temperatures to an antiferromagnetically ordered state. Low-temperature neutron powder diffraction studies show that the magnetic ground states of each of these materials involve both ferromagnetic and antiferromagnetic super-superexchange interactions between orbitally ordered Mn(3+), which are mediated by PO(4) tetrahedra. These interactions are compared and discussed.

Fluorination of perovskite-related phases of composition SrFe(1-x)Sn(x)O(3-δ).

Perovskite-related compounds of composition SrFe(1-x)Sn(x)O(3-δ) (x = 0.31, 0.54) have been prepared. X-ray powder diffraction shows that the materials adopt orthorhombic unit cells. The lattice parameters increase with the incorporation of increasing amounts of tin, which is shown by x-ray absorption near edge structure investigation to be present as Sn(4+). (57)Fe Mössbauer spectroscopy indicates that iron in these phases is present as Fe(5+) and Fe(3+) and that the materials adopt the compositions SrFe(0.69)Sn(0.31)O(2.94) and SrFe(0.46)Sn(0.54)O(2.88). We propose that the disproportionation of Fe(4+) in SrFeO(3-δ) to Fe(5+) and Fe(3+) in SrFe(1-x)Sn(x)O(3-δ) is driven by the reduction of local lattice strain. The materials have been fluorinated by reaction with poly(vinylidene fluoride) to give products of composition SrFe(0.69)Sn(0.31)O(2.31)F(0.69) and SrFe(0.46)Sn(0.54)O(2.54)F(0.46). The increased iron to oxygen or fluorine distances as revealed by the extended x-ray absorption fine structure are associated with the reduction of Fe(5+) to Fe(3+) as evidenced by (57)Fe Mössbauer spectroscopy. The (57)Fe Mössbauer spectra recorded from the fluorinated materials at low temperature show the coexistence of magnetic sextet and non-magnetic doublet components corresponding to networks of Fe(3+) coupled through oxide ions. The Sn(4+) ions disrupt the coupling and the size of the networks. The magnetic susceptibility measurements and Mössbauer spectra recorded between 4.2 and 300 K are used to model the magnetic properties of these materials, with the larger networks appearing to possess random spin orientations consistent with spin glass-type materials.

Synthesis and crystal structure of AlH2P3O10.2H2O; a new structure-type for layered acid phosphates.

The crystal structure of the layered acid phosphate, AlH2P3O10.2H2O, has been determined and provides a new structure-type for a series of metal phosphates with interlamellar regions likely to be highly suited to intercalation behaviour.

The influence of simulated masticatory loading regimes on the bi-axial flexure strength and reliability of a Y-TZP dental ceramic.

OBJECTIVES: The purpose of the current study was to examine the influence of simulated masticatory loading regimes, to which all-ceramic crown or bridge restorations will routinely be subjected during their service-life, on the performance of a yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) dental ceramic. METHODS: Ten sets of 30 Y-TZP ceramic discs (13 mm diameter, 1.48-1.54 mm thickness) supplied by the manufacturer were randomly selected. Six groups were loaded for 2000 cycles at 500 N (383-420 MPa), 700N (536-588 MPa) and 800 N (613-672 MPa) with three groups maintained dry and the remaining three groups loaded while immersed in water at 37+/-1 degrees C. A further two groups underwent extended simulated masticatory loading regimes at 80 N (61-67 MPa) for 10(4) and 10(5)cycles under dry conditions. The mean bi-axial flexure strengths, standard deviations and associated Weibull moduli (m) were determined. The surface hardness was also determined using the Vickers hardness indentation technique. RESULTS: No significant difference (P>0.05) was identified in the bi-axial flexure strength of the simulated masticatory loading regimes and the control specimens loaded dry or wet. A significant increase in m was identified for the Y-TZP specimens following loading while immersed in water (8.6+/-1.6, 8.5+/-1.6 and 10.3+/-1.9) compared with the control (7.1+/-1.3). However, the extended loading regime to 10(5)cycles resulted in a significant reduction in the m of the Y-TZP specimens (5.3+/-1.0) compared with the control. Localised areas of increased surface hardness were identified to occur directly beneath the spherical indenter. CONCLUSIONS: The occurrence of localised areas of increased surface hardness could be the result of either a transformation toughening mechanism or crushing and densification of the material beneath the indentor manifested as the formation of a surface layer of compressive stresses that counteracted the tensile field generated at the tip of a propagating crack which increased the Weibull modulus of the Y-TZP specimens. The reduced reliability of the Y-TZP specimens loaded to 80 N for 10(5)cycles was associated with the accumulation of subcritical damage as a result of the extended nature of loading.

The influence of surface modification techniques on the performance of a Y-TZP dental ceramic.

OBJECTIVES: The purpose of the current study was to examine the influence of the pre-cementation surface modification techniques, namely alumina abrasion and surface grinding, routinely employed by dental practitioners prior to cementation and placement of crown and bridge restorations on the performance of a Y-TZP dental ceramic. METHODS: Twelve sets of 30 Lava ceramic discs (13 mm diameter, 1.5mm thickness) supplied by the manufacturer were randomly selected. Six groups were abraded utilising 25, 50 and 110 microm alumina and stored dry or in a water bath at 37+/-1 degrees C for 24 h. Four groups were ground utilising a fine or a coarse grit diamond bur, specimens were ground dry or while using water as a coolant. The mean bi-axial flexure strengths, standard deviations and associated Weibull moduli (m) were determined. The surface roughness, hardness and phase composition were assessed utilising profilometry, the Vicker's hardness indentation and X-ray diffraction, respectively. RESULTS: No significant difference (P > 0.05) was identified in the bi-axial flexure strength of the 25, 50 and 110 microm alumina-abraded and the control specimens stored dry and wet for 24h. However, a significant increase in m was identified for the alumina-abraded specimens stored dry (10.7+/-1.9, 10.6+/-1.9 and 10.6+/-1.9) compared with the control (7.5+/-1.3) and the specimens stored in a water bath. In addition, the alumina abrasion regimes reduced the surface roughness compared with the controls. The coarse grinding regime significantly reduced both the bi-axial flexure strength and the associated m compared with the control whilst no significant difference was identified for the fine grinding regimes. The surface modification techniques initiated a phase transformation mechanism and resulted in the formation of a layer of compressive stresses on the surface of the disc-shaped specimens. CONCLUSIONS: The combination of the reduced surface roughness and the formation of a surface layer of compressive stress as a result of the alumina abrasion regimes investigated increased the reliability of the bi-axial flexure strength. The presence of water in the current study did not detrimentally influence the performance of the Y-TZP ceramic under investigation. Coarse grinding significantly reduced the bi-axial flexure strength and m due to the increased surface roughness. The Y-TZP specimens in the current investigation underwent a toughening mechanism as a result of a phase transformation mechanism which generated a transformation compressive stress that opposes the externally applied, crack-propagating tensile stress.

Biologically mediated resorption of brushite cement in vitro.

A new calcium phosphate cement is reported, which sets to form a matrix consisting of brushite, dicalcium pyrophosphate dihydrate and an amorphous phase following the mixture of beta-tricalcium phosphate with an aqueous pyrophosphoric acid solution. This reactant combination set within a clinically relevant time-frame (approximately 10 min) and exhibited a higher compressive strength (25 MPa) than previously reported brushite cements. The in vitro degradation of the beta-tricalcium phosphate-pyrophosphoric acid cement was tested in both phosphate buffered saline and bovine serum. The pyrophosphate ion containing cement reported here was found not to be hydrolysed to form hydroxyapatite in vitro like beta-tricalcium phosphate-orthophosphoric acid solution cements. This finding is significant since the formation of hydroxyapatite by hydrolysis is thought to retard in vivo degradation of brushite cements. When aged in bovine serum, the cement lost considerably more mass than when aged in phosphate buffered saline, indicating that proteins, most likely phosphatase enzymes played an important role in the degradation. As pyrophosphate ions are thought to be the source of orthophosphate ions during bone mineralisation, this new class of bone cement offers a route to new degradable synthetic bone grafting materials.