Jean-Christian Trombe

New concepts in the composition, crystallization and growth of the mineral component of calcified tissues

Abstract Several difficulties arise when studying the mineral component of calcified tissues: this material is complex, due to the large number of atomic components; it is poorly crystallized, heterogeneous, and varies with different factors (animal species, kind of bone, age, sampling zone, etc.); it is strongly linked to the organic component (collagen, etc.), and today no available technique allows a complete separation of these two components without alteration of one of the other. Research on synthetic materials allows the elaboration of some models to account, at least partially, for the nature and properties of the calcified-tissue mineral component. So, glycine fixation by apatite constitutes the first model of the collagen-apatite bond. The introduction of carbonate ions into the apatitic lattice can take place in two kinds of site, and under different forms. The replacement of PO3-4 ions by HPO2-4 can also be observed. The properties of phosphates depend on the presence of these various substituents, and therefore such substitutions can play an important role in phosphate behaviour in biological media. The study of the hydrolysis and crystallization of amorphous phosphate into apatite leads to new conceptions relative to the possible existence of an amorphous “phase” in calcified tissues. The conversion of amorphous phosphates to crystalline apatite is dependent on numerous ions (Mg2+, P2O4-7, CO32-, etc.). Studies on synthetic materials can be regarded as a basis for the further study of calcified tissues, partic ularly to determine their constitution and properties. Besides, such studies enable the synthesis of materials, for implants, very similar to calcified tissues.

ESR of CO 2 − in X-irradiated tooth enamel and A-type carbonated apatite

SummaryUsing both low microwave power and weak magnetic field modulation, we have shown that the asymmetric signal arising in X-irradiated tooth enamel as well as in A-type carbonated apatite exposed to X-rays or to excited oxygen has an orthorhombic character and must be attributed to CO2−. Effectively, the mean values found for the three g-tensor components are comparable to those quoted for this defect in single-crystal specimens of calcite and sodium formate.

f Element croconates. 1 : Lanthanide croconates : synthesis, crystal structure and thermal behaviour

Abstract Lanthanide croconates free of alkaline elements have been prepared by using croconic acid or triethanolammonium croconate. They are divided into two families: 1-Ln: [Ln(H2O)5]2(C5O5)3·4H2O with Ln(III)Ce, Pr, Nd, Sm, Eu, Gd; 2-Ln: [Ln(H2O)6]2(C5O5)3·3H2O with Ln(III)Tb, Dy, Ho, Er, Yb. In the general conditions used to synthesize such complexes, lanthanum, lutetium and yttrium belong neither to the family 1-Ln nor to the family 2-Ln. In the presence of oxygen and exposed to daylight the latter elements induce, after a few days, a degradation of the croconate ligand leading to oxalate complexes and to another phase still unknown. A possible hypothesis is presented. A structural study has been performed on a single-crystal representative on each family: Pr for 1-Ln, Er for 2-Ln. The first family crystallizes in the orthorhombic system, space group Pccn, while the second one crystallizes in the triclinic system, space group P1. The salient feature of both structures consists of discrete, neutral, dinuclear entities: [Ln(H2O)x]2(C5O5)3, with x=5 for 1-Ln and x=6 for 2-Ln. However, these entities differ markedly from one family to the other by the coordination mode of the croconate ligand. For 1-Ln the two independent croconates are either chelating or bis-chelating while for 2-Ln the three independent croconates are monodentate and transmonodentate. Between the two structures, a modification of the lanthanide coordination number takes place: 9 for Pr as a distorted trigonal tricapped prism, 8 for Er (Er1 and Er2) as a deformed antisquare prism. The crystal structure is assured, in both cases, by hydrogen bonding and van der Waals interactions between stacked croconate planes. Free water molecules are localized in tunnels. Dehydration of lanthanide croconates of each family takes place in a single step around 90 to 150 °C; anhydrous compounds are stable. The decomposition of the croconate ligand proceeds via oxycarbonate according to the considered lanthanide. It starts around 310 °C, and constant weight is achieved at a variable temperature up to 700 °C, leading to the corresponding oxides. An endotherm is found for the dehydration, a strong exotherm for the croconate decomposition.

Effect of some dopant elements on the low temperature formation of γ-Ce2S3

Abstract For the first time, it has been clearly shown that carbon is an excellent element to stabilize the γ-phase of cerium sesquisulfide at a low temperature (around 800°C) by using CS 2 . The most appropriate route is to use cerium molecular-complex precursors which contain some carbon (like acetate, succinate…), but this element can also be added to carbon-free precursors, such as nitrate. However, the γ-phases so-obtained are dark or brown due to the presence of graphitic carbon. Silicon presents the same properties as carbon. Likewise, the use of phosphate precursors leads to the γ-phase at low temperature (750–800°C). However, during the thermal treatment under CS 2 , some phosphorus is lost from the solid-phase resulting in a progressive transformation of the γ-phase into the α-phase. Contrary to the α-Ce 2 S 3 phase, the residual monazite (CePO 4 ) does not greatly affect the red color of γ-Ce 2 S 3 . The same results are observed by using borate and, to a lesser extent, vanadate and arseniate precursors. One can assume that the stabilization of the γ-phase is probably due to the insertion of a small amount of these dopants into the empty tetrahedral cavities of the γ-Ce 2 S 3 structure.

Low-temperature process of the cubic lanthanidesesquisulfides:remarkable stabilization of theγ-Ce2S3 phasei

Upon treating the corresponding oxalates with carbon disulfide (p CS2 =130 Torr) at a heating rate of 5 °C min -1 , it is shown that the cubic γ-phase of pure rare-earth sesquisulfides (γ-Ln 2 S 3 ) can be obtained at 800 °C from samarium to holmium (also yttrium) and at 1000 °C from neodymium to dysprosium. By working on ternary sulfides Ce 2-x Ln x S 3 , it has been shown that the stabilization of the γ-phase occurs when the average ionic radius ranges between 1.015 and 1.104 A at 800 °C. However, the nature of the observed sulfide phase also depends greatly on the experimental conditions, i.e. the nature of the precursor and the heating rate. At 800 °C and 5 °C min -1 , cerium oxalate leads to the β-phase while cerium nitrate leads to the α-phase. On the other hand, with cerium oxalate, the lower the heating rate the higher the amount of cubic γ-phase obtained.

The stabilization of γ-Ce2S3 at low temperature by heavy rare earths

Abstract The use of some heavy lanthanide(III) elements allows the stabilization of the γ-phase of Ce 2 S 3 synthesized in H 2 S at low temperatures (600°C–800°C). Among the lanthanides used, the best element is dysprosium followed by holmium, erbium and terbium. The stabilization is more pronounced when using mixed-precursors, either the crystallized oxalate or other less crystallized complexes. However this stabilization is only transitory: heating above 800°C yields the β form. It is the first time that the γ phase is observed at a lower temperature than the β form. Due to the low temperatures and the short treatment time no change of morphology is observed between the starting precursor and the final sulphide.

f element croconates 2. Thorium(IV) and dioxo-uranium(VI) croconates — synthesis, crystal structure and thermal behaviour

Abstract Two complexes of actinide croconates, thorium(IV) and dioxo-uranium(VI) were prepared: Th(H2O)7(C5O5)2 and UO2(H2O)K2(C5O5)2. The crystal structure of these complexes was determined by X-ray single crystal technique. The thorium croconate hydrated crystallizes in the orthorhombic system, space group Pnma. It is made of discrete, neutral entities. In this complex the croconate ligand is only monodentate. The uranyl hydrated potassium croconate crystallizes in the monoclinic system, space group C2/c. This complex has a 3D structure presenting tunnels in which are localized the water molecules bound to the uranium atom. The outstanding feature of this structure is the high number (seven) of metals bound to each croconate ligand. This latter is bis-monodentate towards the uranium atom, bis-chelating towards the potassium atom and the two oxygen atoms are bound to one supplementary potassium atom. The thermal behaviour of these complexes was studied. For the thorium croconate hydrated dehydration occurs first, followed by the decomposition of the croconate ligand which yields thorium oxide, ThO2. A more complex and interesting behaviour is noticed for the complex UO2(H2O)K2(C5O5)2. The products formed at the outset of the decomposition seem to influence the further stages.

Two novel families of lanthanide mixed-ligand complexes: synthesis, structure and characterization of [Ln(H2O)x]2(C2O4)[O(CH2CO2)2]2 with x=3, Ln=Ce–Gd and x=1, Ln=Ho–Lu, including yttrium

Abstract New lanthanide complexes associating two ligands, oxalate and oxydiacetate, have been prepared under an autogenous pressure at 200°C for 7 to 21 days by reacting an aqueous suspension of the corresponding lanthanide oxalate and oxydiacetic acid. These complexes are gathered into two families, [Ln(H2O)x]2(C2O4)[O(CH2CO2)2]2 with x=3, Ln=Ce–Gd I and x=1, Ln=Ho–Lu including Y II, according to the number of water molecules. The structures have been solved for neodymium I and for yttrium II, respectively, from X-ray diffraction data on single crystal. Both families crystallize in the monoclinic symmetry but with different space groups, P21/c and C2/m for I and II, respectively. I has a double-chained structure in the form of ladder while II has a double-layered structure. The oxydiacetate ligands and the metal atoms build a chain or a layer. The oxalate ligands bridge two chains or two layers, respectively. There is no connection between these entities, double-chain or double-layer, but hydrogen bonds. While I has a compact structure, II presents a relatively open framework; about 12% of the unit cell is empty. In both families, the oxalate ligand is bischelating while the coordination mode of the oxydiacetate ligand changes from tridentate and monodentate in I to tridentate and bismonodentate in II. The thermal behavior of some compounds of these two families is presented.

Study of some ternary and quaternary systems based on γ-Ce2S3 using oxalate complexes: stabilization and coloration

Abstract In the ternary system, Ce–Na–S, starting with oxalate precursors and using H2S as sulfurizing agent, the sodium doped γ-Ce2S3 becomes the main phase for Na/Ce=0.1 and is pure for Na/Ce=0.2 at 700°C. The addition of dysprosium (oxalate) to this system allows the stabilization of the γ-phase as long as the atomic ratios take the values Dy/Ce≤2 and Na/[Ce+Dy]≥0.1, and the temperature of treatment is equal or higher than 900°C. The secondary phases are the oxysulfide Ce2−xDyxO2S and the mixed sulfide NaDyS2. By substituting erbium for dysprosium the amount of secondary phases increases. To improve the coloration of the sulfide, the oxalate precursor must undergo a thermal treatment in air, before its sulfurization, in order to decrease the carbon amount of the sulfides. The addition of lanthanide(III) to the ternary system modifies slightly the color of the resulting sulfide.

Stabilization of the γ-Ce2S3 phase

Abstract The γ-Ce 2 S 3 phase is used as a safe red pigment for plastics. Therefore, its stabilization, at low temperatures (around 800°C), has been particularly studied during the last decade. This can be realized in different ways: (1) the presence of alkali metals leading to A 0.5 Ce 2.5 S 4 (A=Li, Na and K). (2) The presence of an important carbon or silicon concentration enhances the γ-phase formation. However the γ-phases so obtained are dark or brown due to the presence of traces of these two elements. (3) The use of phosphate precursors (or phosphate dopant) also leads to the γ-phase. However during the thermal treatment under CS 2 , some phosphorus is lost from the solid phase resulting in a progressive transformation of the γ-phase into the α-one. The residual monazite (CePO 4 ) does not much affect the red color of γ-Ce 2 S 3 . The γ-phase is also obtained by using borate precursors and to a lesser extent by vanadate and arseniate precursors or by treating some ceria associated with SiO 2 . (4) γ-Ce 2 S 3 , as a major phase, is also obtained by treating some specific ceria under CS 2 and its color is red.