jeanflop/post_ocr_correction2 huggingface dataset:
Nb2SiTe4. I 276.2 (1) I 277.1 (1) Nb-Te 295.3 (1) 295.8 (1) I 298.9 (1) V 299.8 (1) j 2x260.16 (6) Si-Te 2 x 260,34 (5) Nb-Nb 288.1 (2) Nb-Si 2x274.2 (1) Nb-Si 2x281.9 (1) I 360.11 (8) 360.09(8) Te-Te (II to the layer and <400) 387.54 (8) I 387.56 (8) 394.40 (7) Te-Te (1 to the layer and <400) 375.79 (8) (394.79 (8) Te-Te (through the van der Waals gap) 395.39 (8) 395.33 (8) The diffuse character of the superstructure streaks observed on Weissenberg photographs suggests stacking faults that destroy the out-of-plane long range order. Those defects probably correspond to sliding of the sheets. Considering, like in parent Nb3Ge0.9Te6 and many other silicon (or germanium-) containing chalcogenides, an oxidation state of II for Si and III for Nb in Nb2 groups, a straightforward charge balance is achieved with Te-III anions 2 1 e-. However, in view of short contact Te-Te distances ranging from approximately 360 pm to over 390 pm (the non-bonding contacts are normally expected around 400 pm), the question of interanionic bondings, true tellurium oxidation state and anion → cation charge transfer is to be raised. Only extended Hückel tight binding band structure calculations, although slightly biased because of the average nature of the known structure, can distinguish between short non-bonding and bonding Te-Te contacts, allowing also for the determination of the atoms' true oxidation states (vide infra). 5. ELECTRONIC STRUCTURE. Extended Hückel tight binding band structure calculations [5] were performed with the exponents and parameters summarized in Table V. Since only the average structure could be refined (vide supra), the calculations were carried out using the Nb2SiTe4 slab described on Figure 2 (b' = 2*b), that is, with the centres of the biprisms occupied by one Si or two Nb atoms alternating in an ordered way. TABLE V Exponents and parameters used in the Extended Hückel tight binding band structure calculations. Exposants et paramètres utilisés dans les calculs de structure de bandes par la méthode de Hückel étendue. Atom Orbital Hfi (eV) Çl C, C2 c Te 5s -10.10 1.90 Nb 5p -6.86 1.85 (4 d
because of the average nature of the known structure, can distinguish between short non-bonding and bonding Te-Te contacts, allowing also for the determination of the atoms' true oxidation states (vide infra). 5. ELECTRONIC STRUCTURE. Extended Hückel tight binding band structure calculations [5] were performed with the exponents and parameters summarized in Table V. Since only the average structure could be refined (vide supra), the calculations were carried out using the Nb2SiTe4 slab described on Figure 2 (b' = 2*b), that is, with the centres of the biprisms occupied by one Si or two Nb atoms alternating in an ordered way. TABLE V Exponents and parameters used in the Extended Hückel tight binding band structure calculations. Exposants et paramètres utilisés dans les calculs de structure de bandes par la méthode de Hückel étendue. Atom Orbital Hfi (eV) Çl C, C2 c Te 5s -10.10 1.90 Nb 5p -6.86 1.85 (4 d -12.10 4.08 0.6401 1.64 0.5516 Si 3s -17.3 1.383 3 P -9.2 1.383 5 s -20.78 2.51 Te 5p -13.20 2.16 Contraction coefficients used in the double- expansion. The calculated band structure of a Nb2SiTe4 slab is shown in Figure 5 a, where the dashed line indicates the highest occupied level. There is a semiconducting band gap of 0.6 eV. Analysis of the orbital character of the bands reveals that the levels of largely Si 3s-orbital character occur well below the band gap whereas those of largely Si 3p-orbital character are well above this band gap. Consequently, the oxidation state appropriate for Si is II. Four of the occupied bands of Figure 5 a are mainly built from Nb-4 d-orbitals. Since there are four Nb atoms per repeat unit of the slab, the oxidation state appropriate for Nb is III. The main orbital character of these bands is given by the CT+ and Ï orbitals of Figure 5 b. The σ+ orbital of the double prism is low in energy because of metal-metal bonding, while the Ï orbital is low in energy because the Te orbitals of the shared edges mix poorly with the Nb 4 d-orbitals due to the
level. There is a semiconducting band gap of 0.6 eV. Analysis of the orbital character of the bands reveals that the levels of largely Si 3s-orbital character occur well below the band gap whereas those of largely Si 3p-orbital character are well above this band gap. Consequently, the oxidation state appropriate for Si is II. Four of the occupied bands of Figure 5 a are mainly built from Nb-4 d-orbitals. Since there are four Nb atoms per repeat unit of the slab, the oxidation state appropriate for Nb is III. The main orbital character of these bands is given by the CT+ and Ï orbitals of Figure 5 b. The σ+ orbital of the double prism is low in energy because of metal-metal bonding, while the Ï orbital is low in energy because the Te orbitals of the shared edges mix poorly with the Nb 4 d-orbitals due to the nodal properties of the Ï orbital. The Ï orbital is further lowered in energy by the bonding interactions with the Si 3 p-orbitals which leads to sizeable Si-Te overlap populations (0.20). The overlap population associated with the short intralayer Te-Te contact is practically nil. All these conclusions are left unaltered when interlayer interactions are included in the calculations. Consequently, the oxidation formalism appropriate for Nb2SiTe4 is (Nb3+) (Si2+) (Te2−)4. It is interesting to compare these results with those for other layered transition metal tellurides ([3], [6]-[7]). It is quite clear now that intralayer Te−Te contacts of the order of 355 pm are not associated with positive overlap populations and that even shorter ones (i. e., 334 pm in Nb3Ge9Te6) ([21][3]) usually do not play any essential role in promoting the tellurium to metal electron transfers needed to adjust the electron counting. By contrast, interlayer Te−Te contacts of the order of 375 pm are short enough to be associated with important electron transfers ([3], [7]). The comparison between the structurally related Nb2SiTe4 and Nb3Ge9Te6 is quite revealing in that respect. In the latter, where Nb and Ge are in the 3+ and 2+ oxidation states, respectively, the Te oxidation state is
when interlayer interactions are included in the calculations. Consequently, the oxidation formalism appropriate for Nb2SiTe4 is (Nb3+) (Si2+) (Te2−)4. It is interesting to compare these results with those for other layered transition metal tellurides ([3], [6]-[7]). It is quite clear now that intralayer Te−Te contacts of the order of 355 pm are not associated with positive overlap populations and that even shorter ones (i. e., 334 pm in Nb3Ge9Te6) ([21][3]) usually do not play any essential role in promoting the tellurium to metal electron transfers needed to adjust the electron counting. By contrast, interlayer Te−Te contacts of the order of 375 pm are short enough to be associated with important electron transfers ([3], [7]). The comparison between the structurally related Nb2SiTe4 and Nb3Ge9Te6 is quite revealing in that respect. In the latter, where Nb and Ge are in the 3+ and 2+ oxidation states, respectively, the Te oxidation state is higher than 2 and consequently, the top of the tellurium band must be partially emptied. Extended Hückel calculations show that this partial emptying is realized through the creation of short interlayer Te−Te contacts (375-383 pm) [3]. By contrast, in Nb2SiTe4, the tellurium oxidation state is 2 and thus the tellurium band is full. Consequently, there is no need for short interlayer Te−Te bonding distances. In agreement with this reasoning our X-ray crystal structure for Nb2SiTe4 does not show any such interlayer Te−Te short contact. 6. CONCLUDING REMARK. — The structure of Nb2SiTe4 convincingly demonstrates that the occurrences of short Te−Te distances are not sufficient to lead to true anionic bonding and hence anion → cation charge transfer. Although considerably shorter than the sum of the van der Waals radii (360 pm instead of 400-410 pm), these short contacts do not raise the orbital levels high enough to lead to their depletion when located within the structure 2D slab and parallel to it. By comparison with other tellurides and parent phases like Nb3Ge9Te6, it appears that such bonding and such transfer are much more easily obtained for pz tellurium orbitals located on each site of the van der Waals gap and
band is full. Consequently, there is no need for short interlayer Te−Te bonding distances. In agreement with this reasoning our X-ray crystal structure for Nb2SiTe4 does not show any such interlayer Te−Te short contact. 6. CONCLUDING REMARK. — The structure of Nb2SiTe4 convincingly demonstrates that the occurrences of short Te−Te distances are not sufficient to lead to true anionic bonding and hence anion → cation charge transfer. Although considerably shorter than the sum of the van der Waals radii (360 pm instead of 400-410 pm), these short contacts do not raise the orbital levels high enough to lead to their depletion when located within the structure 2D slab and parallel to it. By comparison with other tellurides and parent phases like Nb3Ge9Te6, it appears that such bonding and such transfer are much more easily obtained for pz tellurium orbitals located on each site of the van der Waals gap and with pz perpendicular to the slab. Concerning the structure feature of the compound, one may wonder why Si (and Ge in Nb3Ge9Te6) is located in a square planar tellurium environment and not in a tetrahedral site between the prismatic slabs as the usual coordination of this element would lead one to expect. This is probably due, in part, to the high stability achieved through the setting up of Si−Nb bonds within the slabs. Note remise le 25 octobre 1992, acceptée le 5 novembre 1992. REFERENCES [1] S. JOBRIC, R. BREC and J. ROUXEL, J. Alloys Compounds, 178, 1992, p. 253. [2] L. MONCONDUIT, M. EVAIN, F. BOUCHER, R. BREC and J. ROUXEL, Z. anorg. allg. Chem., 616, 1992, p. 177. [3] E. CANADELL, L. MONCONDUIT, M. EVAIN, R. BREC, J. ROUXEL and M.-H. WHANGBO, Inorg. Chem. (in press). [4] C. KAY FAIR, MOLEN: Structure Determination Package, Enraf-Nonius, 1982. [5] M.-H. WHANGBO and R. HOFFMANN, J. Am. Chem. Soc., 100, 1978, p. 6093. A modified Wolfsberg-Helmholtz formula (J. AMMETER, H.-B. BURGI, J. THIBEAULT and R. HOFFMANN, J. Am. Chem. Soc., 100, 1978, p. 3686) was used to evaluate the nondiagonal Hij values. [6] E. CANADELL, S. JOBRIC, R. BREC and J.
to expect. This is probably due, in part, to the high stability achieved through the setting up of Si−Nb bonds within the slabs. Note remise le 25 octobre 1992, acceptée le 5 novembre 1992. REFERENCES [1] S. JOBRIC, R. BREC and J. ROUXEL, J. Alloys Compounds, 178, 1992, p. 253. [2] L. MONCONDUIT, M. EVAIN, F. BOUCHER, R. BREC and J. ROUXEL, Z. anorg. allg. Chem., 616, 1992, p. 177. [3] E. CANADELL, L. MONCONDUIT, M. EVAIN, R. BREC, J. ROUXEL and M.-H. WHANGBO, Inorg. Chem. (in press). [4] C. KAY FAIR, MOLEN: Structure Determination Package, Enraf-Nonius, 1982. [5] M.-H. WHANGBO and R. HOFFMANN, J. Am. Chem. Soc., 100, 1978, p. 6093. A modified Wolfsberg-Helmholtz formula (J. AMMETER, H.-B. BURGI, J. THIBEAULT and R. HOFFMANN, J. Am. Chem. Soc., 100, 1978, p. 3686) was used to evaluate the nondiagonal Hij values. [6] E. CANADELL, S. JOBRIC, R. BREC and J. ROUXEL, J. Solid State Chem., 98, 1992, p. 59. [7] E. CANADELL, S. JOBRIC, R. BREC, J. ROUXEL and M.-H. WHANGBO, J. Solid State Chem., 99, 1992, p. 189. L. M., M. E., R. B. and J. R. : Laboratoire de Chimie des Solides, I.M.N., 2, rue de la Houssinière, 44072 Nantes Cedex 02; E. C. : Laboratoire de Chimie théorique, Université de Paris-Sud, 91405 Orsay. The aqueous layer, when concentrated under vacuum, gives a white powder which is washed with acetone, water and finally dried at 120°C for 24 hrs. to yield 0.54 g (90%) of amorphous silica. In every case, the yields of silica are 90-95% and catechol is recovered quantitatively. Alternatively, the hydrolysis of a 0.25 M methanolic solution of 1 or 2 with a stoichiometric amount of water also leads to silica (reaction 4). The results for both methods are reported in Table. Silica obtained from both precursors exhibit similar characteristics: specific surface areas are of the same magnitude, but the porosities observed starting from 1 are larger. Results reported in Table show that it is possible to modify the porosity and the specific surface area of silica according to the experimental conditions. The nitrogen adsorption-desorption
44072 Nantes Cedex 02; E. C. : Laboratoire de Chimie théorique, Université de Paris-Sud, 91405 Orsay. The aqueous layer, when concentrated under vacuum, gives a white powder which is washed with acetone, water and finally dried at 120°C for 24 hrs. to yield 0.54 g (90%) of amorphous silica. In every case, the yields of silica are 90-95% and catechol is recovered quantitatively. Alternatively, the hydrolysis of a 0.25 M methanolic solution of 1 or 2 with a stoichiometric amount of water also leads to silica (reaction 4). The results for both methods are reported in Table. Silica obtained from both precursors exhibit similar characteristics: specific surface areas are of the same magnitude, but the porosities observed starting from 1 are larger. Results reported in Table show that it is possible to modify the porosity and the specific surface area of silica according to the experimental conditions. The nitrogen adsorption-desorption isotherms obtained for silica prepared in water, entries I and 2, are in all cases Langmuir isotherms without (or with a very weak) hysteresis (type I), and are characteristic of solids having very small pores: ultramicropores (<15 Å) and micropores (<30 Å). On the other hand, when the hydrolysis is performed in methanol, entries 3 and 4, only silica of higher porosity (mesopores) having an isotherm with hysteresis of type IV is observed from 1 whereas type I is formed from 2. Thus, hexacoordinated complexes 1 provide silica of various specific surface areas and porosity depending on the experimental conditions. With 2 as precursor only type I silica was observed. Results, entry 5, show that the hydrolysis with the stoichiometric amount of water yields silica having a type IV isotherm from both precursors. However, starting from 1 the specific surface area is always lower and the porosity higher. By analogy, we studied the formation of silsesquioxanes MeSiO3 from the hydrolysis of the pentacoordinated silicon complex 3 and methyltriethoxysilane 4. Under the same conditions described for 1 (reflux for 6 days), we obtained silsesquioxanes partially crystallized in the T8 structure. Sublimation of the reaction mixture provided crystalline T8 (60%) and a
performed in methanol, entries 3 and 4, only silica of higher porosity (mesopores) having an isotherm with hysteresis of type IV is observed from 1 whereas type I is formed from 2. Thus, hexacoordinated complexes 1 provide silica of various specific surface areas and porosity depending on the experimental conditions. With 2 as precursor only type I silica was observed. Results, entry 5, show that the hydrolysis with the stoichiometric amount of water yields silica having a type IV isotherm from both precursors. However, starting from 1 the specific surface area is always lower and the porosity higher. By analogy, we studied the formation of silsesquioxanes MeSiO3 from the hydrolysis of the pentacoordinated silicon complex 3 and methyltriethoxysilane 4. Under the same conditions described for 1 (reflux for 6 days), we obtained silsesquioxanes partially crystallized in the T8 structure. Sublimation of the reaction mixture provided crystalline T8 (60%) and a residue of amorphous material of composition MeSiO3. It is interesting to note that 3 is easily prepared from PMHS (polymethylhydrogenosiloxane), a byproduct of the organosilanes industry. In conclusion, we describe here an easy method for formation of silica powders. The reaction path is summarized in Scheme 2. This route represents an interesting alternative to prepare at low cost (temperature lower than 100°C) and from cheap starting materials (sand or Na2SiF6), silica with physical properties controlled by experimental conditions. Thus, crude sand can be converted into pure silica. Supplementary experiments are however, necessary in order to control the morphology of silica as effectively as by the classical methods, [5a], [8]. HC1/H2O/A 80°C/24 h type 1 (b) 847 <40 type I (a) 747 <15 2 HCl/H2O/0 amb/l h type 1 (b) 909 <15 3 HCl/MeOH/A 70°C/24 h type IV 441 80 4 HCl/MeOH/0 amb/24 h ou EtOH type IV 243 90 type 1 (a) 699 <40 5 H2O stoechio./ sol 0,25 M dans MeOH, O amb. type IV 307 135 type IV 602 75 (") Isotherme d'adsorption-désorption de type Langmuir. (b) Isotherme d'adsorption-désorption présentant une très légère hystérésis. ("") Nitrogen adsorption-desorption isotherm of type Langmuir. (b) Nitrogen adsorption-desorption isotherm with a weak
an interesting alternative to prepare at low cost (temperature lower than 100°C) and from cheap starting materials (sand or Na2SiF6), silica with physical properties controlled by experimental conditions. Thus, crude sand can be converted into pure silica. Supplementary experiments are however, necessary in order to control the morphology of silica as effectively as by the classical methods, [5a], [8]. HC1/H2O/A 80°C/24 h type 1 (b) 847 <40 type I (a) 747 <15 2 HCl/H2O/0 amb/l h type 1 (b) 909 <15 3 HCl/MeOH/A 70°C/24 h type IV 441 80 4 HCl/MeOH/0 amb/24 h ou EtOH type IV 243 90 type 1 (a) 699 <40 5 H2O stoechio./ sol 0,25 M dans MeOH, O amb. type IV 307 135 type IV 602 75 (") Isotherme d'adsorption-désorption de type Langmuir. (b) Isotherme d'adsorption-désorption présentant une très légère hystérésis. ("") Nitrogen adsorption-desorption isotherm of type Langmuir. (b) Nitrogen adsorption-desorption isotherm with a weak hysteresis in the desorption branch. conditions utilisées pour les entrées 1 et 2, les silices préparées présentent toutes des isothermes de Langmuir sans hystérésis ou avec une très légère hystérésis (type I). Ces caractéristiques sont celles de solides à pores très fins : ultramicropores ( < 15 Å) et micropores ( <30 Å). Par contre, la même réaction en solvant méthanol (entrées 3 et 4) conduit uniquement à partir de 1 à des silices de porosité plus élevée (mésopores) présentant un isotherme avec hystérésis de type IV. Les complexes hexacoordonnés 1 permettent ainsi l'obtention de silices de diverses surfaces et de porosité variable en fonction des conditions expérimentales alors qu'à partir de 2 dans tous les cas nous obtenons des silices de type I. Les résultats (entrée 5) montrent que l'hydrolyse par une quantité stoechiométrique d'eau donne des silices d'isotherme de type IV à partir des 2 précurseurs. Toutefois, à partir de 1, la surface est toujours plus faible et la porosité plus importante. Dans le but de généraliser ces réactions, nous avons comparé les silsesquioxanes MeSiO5 obtenus par hydrolyse du complexe pentacoordonné du silicium 3 et du méthyltriéthoxysilane 4. Dans les mêmes conditions que celles décrites pour 1 (reflux
( <30 Å). Par contre, la même réaction en solvant méthanol (entrées 3 et 4) conduit uniquement à partir de 1 à des silices de porosité plus élevée (mésopores) présentant un isotherme avec hystérésis de type IV. Les complexes hexacoordonnés 1 permettent ainsi l'obtention de silices de diverses surfaces et de porosité variable en fonction des conditions expérimentales alors qu'à partir de 2 dans tous les cas nous obtenons des silices de type I. Les résultats (entrée 5) montrent que l'hydrolyse par une quantité stoechiométrique d'eau donne des silices d'isotherme de type IV à partir des 2 précurseurs. Toutefois, à partir de 1, la surface est toujours plus faible et la porosité plus importante. Dans le but de généraliser ces réactions, nous avons comparé les silsesquioxanes MeSiO5 obtenus par hydrolyse du complexe pentacoordonné du silicium 3 et du méthyltriéthoxysilane 4. Dans les mêmes conditions que celles décrites pour 1 (reflux 6 jours), nous avons obtenu des silsesquioxanes partiellement cristallisés dans la structure T8. La sublimation du mélange conduit à 60 de silsesquioxane de structure T8, le résidu étant MeSiO5, amorphe. Il est intéressant de rappeler que 3 est facilement préparé à partir du PMHS (polyméthylhydrogénosiloxane) sous produit de l'industrie des organosilanes. En conclusion, nous décrivons ici une voie d'accès douce à des poudres de silice. Le chemin réactionnel est résumé sur le schéma 2. Schéma 2 Cette méthode représente une alternative intéressante qui permet d'obtenir, à faible coût énergétique (température maximum 100°C) et à partir de matières premières peu onéreuses (sable ou Na2SiF6) des silices dont les caractéristiques physiques semblent pouvoir être contrôlées par les conditions expérimentales. -Des expériences supplémentaires sont cependant nécessaires pour arriver à une maîtrise de la morphologie aussi complète que celle obtenue par les voies classiques [5 a], [S]. Nous remercions le Dr Laurent Seigneurin et M. Claude Giovannoni de la Société Rhône-Poulenc de Salindres qui ont effectué les analyses RX et les mesures de surface spécifique et de porosité. Note remise le 17 juillet 1992, acceptée après révision le 17 novembre 1992. RÉFÉRENCES BIBLIOGRAPHIQUES [1] Silices, silicates et dérivés, Informations chimie 243, 1983, p. 287-298.
organosilanes. En conclusion, nous décrivons ici une voie d'accès douce à des poudres de silice. Le chemin réactionnel est résumé sur le schéma 2. Schéma 2 Cette méthode représente une alternative intéressante qui permet d'obtenir, à faible coût énergétique (température maximum 100°C) et à partir de matières premières peu onéreuses (sable ou Na2SiF6) des silices dont les caractéristiques physiques semblent pouvoir être contrôlées par les conditions expérimentales. -Des expériences supplémentaires sont cependant nécessaires pour arriver à une maîtrise de la morphologie aussi complète que celle obtenue par les voies classiques [5 a], [S]. Nous remercions le Dr Laurent Seigneurin et M. Claude Giovannoni de la Société Rhône-Poulenc de Salindres qui ont effectué les analyses RX et les mesures de surface spécifique et de porosité. Note remise le 17 juillet 1992, acceptée après révision le 17 novembre 1992. RÉFÉRENCES BIBLIOGRAPHIQUES [1] Silices, silicates et dérivés, Informations chimie 243, 1983, p. 287-298. [2] A. BOUDIN, G. CERVEAU, C. CHUIT, R. J. P. CORRIU et C. REYÉ, Angew. Chem. Int. Ed. Engl., 25, 1986, p. 474-476; Organometallics, 7, 1988, p. 1165-1171 et références citées. [3] A. ROSENHEIM, B. RAIBMANN et G. SCHENDEL, Z. Anorg. Allg. Chem., 196, 1931, p. 160-176. [4] G. CERVEAU, C. CHUIT, R. J. P. CORRIU, L. GERBIER et C. REYÉ, C. R. Acad. Sci. Paris, 312, série II, 1991, p. 1311-1315.[5] (a) R. K. ILLER, The Chemistry of silica, Wiley, New York, 1979; (b) C. J. BRINKER et G. W. SCHERER, Sol-Gel Science the physics and chemistry of sol-gel processing, Academic press, San Diego, 1990, chap. 9, p. 515-532. [6] A. J. BARRY, W. H. DAUDT, J. J. DONNELAN et J. W. GILKEY, 77, 1955, p. 4248-4252. [7] A. BOUDIN, G. CERVEAU, C. CHUIT, R. J. P. CORRIU et C. REYÉ, Bull. Chem. Soc. Jpn, 61, 1988, p. 101-106. [8] W. STOBER, A. FINK et E. BOHN, J. of Colloid and Interface Science, 26, 1968, p. 62-69. Département de Chimie fine, U.R.A.-C.N.R.S. n° 1097, Université de Montpellier-II, Sciences et Techniques du Languedoc, place Eugène-Bataillon, 34095 Montpellier Cedex 5. Cristallochimie Crystal Chemistry Synthèse et structure cristalline du dihydrogène diphosphate de