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Synthesis and crystal structure of the α polytype of HfNBr
Solid State Sciences 4 (2002) 475–480 www.elsevier.com/locate/ssscie
Synthesis and crystal structure of the α polytype of HfNBr Judith Oró-Solé a , Mikhail Vlassov a,1 , Daniel Beltrán-Porter b , Maria Teresa Caldés c , Vicent Primo b , Amparo Fuertes a,∗ a Institut de Ciència de Materials de Barcelona (C.S.I.C.), Campus U.A.B., 08193 Bellaterra, Spain b Institut de Ciència de Materials de la Universitat de València, P.O. BOX 2085, Polígono “La Coma” s/n, 46980 Paterna, Spain c Institut des Matériaux Jean Rouxel UMR 6502, 2, rue de la Houssinière, 44072 Nantes cedex 03, France
Accepted 19 December 2001
Abstract α-HfNBr has been prepared at 760 ◦ C in a sealed evacuated fused silica tube by reaction between NH4 Br and Hf followed by purification through chemical vapour transport under a temperature gradient. The crystal structure of this compound at room temperature has been determined for the first time by Rietveld refinement of X-ray powder diffraction data, electron diffraction and high resolution electron microscopy. It crystallises in the orthorhombic space group Pmmn with the unit cell parameters a = 4.1165(2), b = 3.5609(2), c = 8.6440(3) Å. α-HfNBr is isotypic to FeOCl and is built from layers of composition Br–Hf–N–N–Hf–Br stacked along c that are separated by a Van der Waals gap. The hafnium atoms are six-coordinated by four nitrogen atoms and two bromine atoms. The resulting coordination polyhedron may be described as a highly distorted octahedron. Electron diffraction and high resolution electron microscopy studies reveal that the material is almost free of defects in the inner part of the crystals and confirm the structural model refined by the Rietveld method. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
1. Introduction The synthesis of zirconium nitride halides was first reported by Juza et al. who prepared the α or β polytypes of ZrNCl, ZrNBr and ZrNI by different methods including the ammonolysis of mixtures of the metal halides and the reaction of the metal nitrides with halogens under nitrogen atmosphere [1,2]. Their structure consist of sheets X– M–N–N–M–X separated by a Van der Waals gap, stacked with different packing for the α and β types. Intercalation of alkaline ions or cobaltocene in the β-MNX polytypes induces superconductivity with a maximum critical temperature of 25 K for Lix HfNCl [3–5]. The crystal structure of the β-MNX compounds is SmSI-type [6] and consists of a cubic close-packing of double layers of metal and halide atoms, yielding the sequence X–M–M–X with relative orientations AbcA [5,7,8]. The nitrogen atoms in β-MNX occupy tetrahedral sites between the metal layers as sulphur in SmSI. The compound ZrCl shows the same X–M–M–X ¯ as β-MNX, layer packing and the same space group (R3m) * Correspondence and reprints. 1 Permanent address: Earthcrust Research Institute, St. Petersburg Uni-
versity, Russia.
and thus the β polytypes may be considered as intercalation derivatives of parent ZrCl, as a result of the occupation of N atoms of the tetrahedral interstices [9]. The crystal structures of α-ZrNCl, α-ZrNBr and α-TiNX (X = Cl, Br, I) were first reported by Juza and Heners from powder X-ray diffraction data [2]. These compounds show the orthorhombic structure of FeOCl and decompose in ambient air because of hydrolysis. The alpha polytypes of HfNBr and HfNI have been recently prepared for the first time by Yamanaka et al. [10] by reaction of the metals with ammonium bromide or iodide in flowing ammonia, followed by transport purification under a temperature gradient. The same authors showed that these polytypes are transformed into the corresponding β phases at high pressure. In previous articles we have reported the synthesis of β-MNX phases by a new method that avoids the use of flowing NH3 using evacuated sealed fused silica tubes [11,12]. We have also reported the structural characterization of the host compounds and their chemical and electrochemical intercalation behaviour in relation with the superconducting properties of the doped phases [12–14]. In this paper we report the preparation of the compound α-HfNBr by the same synthetic method we applied to the β polytypes, as well as the first determination of its crystal structure from
1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 2 ) 0 1 2 7 2 - 4
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X-ray powder diffraction and the electron diffraction and high resolution electron microscopy studies.
2. Experimental 2.1. Synthesis of α-HfNBr α-HfNBr was prepared by the reaction of Hf (Aldrich 99.5%) with NH4 Br (Aldrich, 99.999%) as described in Ref. [11]. Typically 300 mg of Hf were mixed with 165 mg of NH4 Br and placed in one end of a fused silica tube of 25 cm of length and 8 mm of inner diameter that was subsequently sealed under vacuum. The tube was heated at 120 ◦ C min−1 to 760 ◦ C, held at this temperature for 12 hours, and then the end of the tube free of sample was heated to 860 ◦ C for the recrystallization process via vapour transport under a temperature gradient (transport direction: 760 ◦ C → 860 ◦ C). The recrystallization time was 72 hours. After cooling, pure α-HfNBr was found at the hot zone of the tube whereas in the cold zone—where some attack on the tube walls was observed—a mixture of α-HfNBr and ammonium bromide was identified by powder X-ray diffraction. Handling of the reactants and sample preparation for the different subsequent studies were carried out in an Ar filled glove box. 2.2. X-ray diffraction X-ray diffraction patterns were generally taken in a Siemens D-5000 diffractometer using Cu Kα radiation. Powder X-ray diffraction data collection for the crystal structure determination was performed on a powder diffractometer equipped with an INEL curved position sensitive CPS120 in a horizontal Debye–Scherrer geometry using a rotating capillary of diameter 0.1 mm as sample holder. The angular range was 120 degrees and the radiation was Cu Kα1 (λ = 1.540598 Å), obtained with a Ge (1 1 1) monochromator. The dimensions of the incident beam as controlled by a cross slit collimator were 0.1 × 4.0 mm2 . The sample was sieved to 65 µm and mixed with glass powder before filling the capillary that was sealed under argon. The structure was determined by the Rietveld method with the help of the F ULLPROF [15] program, using the coordinates reported by Juza et al. for α-ZrNBr [2] and the cell parameters obtained by electron diffraction as starting parameters. The profile fitting of the data was performed with a pseudoVoigt function, including asymmetry and preferred orientation corrections; the background was refined to a 5th degree polynomial. Preferred orientation and asymmetry were corrected, respectively, by the March–Dollase and the Berar– Bardinozzi expressions. (See Ref. [15].)
2.3. Electron diffraction and high resolution electron microscopy Samples for transmission electron microscopy were prepared by dispersing the powder in n-hexane and depositing the solution on a holey carbon aluminium grid. Electron diffraction patterns and XEDS analyses were obtained in a JEOL 1210 transmission electron microscope operating at 120 kV and equipped with a side-entry 60◦ /30◦ double tilt GATAN 646 analytical specimen holder and a Link QX2000 XEDS element analysis system. High resolution electron microscopy images were obtained in a HITACHI H9000NAR electron microscope operating at 300 kV. The Scherzer resolution was 1.8 Å for the HITACHI instrument. Calculated images for different defocus and different thickness were obtained using the M ACTEMPAS program (V.1.70, Roar Kilaas, Berkeley).
3. Results and discussion After the chemical transport step of the synthesis the phase α-HfNBr is found to crystallize at the high temperature zone of the quartz tube forming transparent light yellow hexagonal platelets. The X-ray diffraction pattern of the ground platelets (Fig. 1) is similar to that reported by Yamanaka [10] for the same phase prepared by treating the mixture of reactants in flowing ammonia previously to the chemical transport, but important differences in the intensity of some diffraction peaks are clearly observed. These differences are probably caused by preferential orientation perpendicularly to [0 0 1] that is frequently observed for zirconium and hafnium nitride halides and is due to the platelet morphology of the crystallites [5]. In our X-ray diffraction experiments the preferential orientation was minimised because of the special conditions taken for the sample preparation (see experimental section). Exposition of the sample to humid air promotes its hydrolysis and the decomposition of
Fig. 1. Observed and calculated X-ray diffraction patterns for α-HfNBr.
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Table 1 Crystallographic parameters for α-HfNBr Crystal data Pmmn (No. 59), Z = 2 4.1165(2) 3.5609(2) 8.6440(3) 126.71(1) 7.14 298 2.5
Space group a (Å) b (Å) c (Å) Cell volume (Å3 ) Calculated density (g cm−3 ) Temperature (K) µ × rc
Atomic coordinates and isotropic displacement parameters (in Å2 ) Atom
Wyckof site
x
y
z
B
2b 2a 2a
1/4 1/4 1/4
3/4 1/4 1/4
0.1000(2) 0.3393(5) 0.959(3)
2.73(6) 3.25(12) 2.4(6)
Hf Br N
Selected bond distances (Å) and angles (deg)
Fig. 2. Representative electron diffraction patterns along the [0 0 1], [0 1 0] and [1 0 0] zone axes for α-HfNBr. Multiple diffraction spots are marked with arrows.
α-HfNBr, yielding NH4 Br and an unknown phase showing the most intense peak at 10.8 Å. The decomposition process induces a change of colour of the sample from light yellow to white. Hydrogen contents for air exposed samples during three days were about 2%. As the peaks of α-HfNBr remain in the X-ray diffraction pattern taken after several weeks to air exposition, we suppose that either the hydrolysis reaction could lead to a phase structurally-related to α-HfNBr (for instance, obtained by exchange of Br− and OH− ) and/or the decomposition might have simply slow kinetics. Fig. 2 shows representative electron diffraction patterns along [0 0 1], [0 1 0] and [1 0 0] zone axes for crushed crystals of α-HfNBr. Indexation of the reflections in these planes,
d(Hf–N) (×2) d(Hf–N) (×2) d(Hf–Br) (×2)
2.152(18) 2.123(9) 2.728(3)
Br–Hf–Br Br–Hf–N (×2) N–Hf–N N–Hf–N
81.44(12) 165(6) 111.6(12) 151.6(12)
Br–Hf–N (×2) Br–Hf–N (×4) N–Hf–N (×4)
83.5(12) 100.7(9) 82.1(6)
N p , N irefl d P p, P i, P ge R Bragg , R F , χ 2 R p , R wp , R exp a f Pb
5000, 128 17, 8, 0 5.5, 4.0, 2.2 6.7, 8.6, 5.8 3.15
a Conventional Rietveld R-factors (R , R p exp ) are calculated by using background corrected counts. b Standard deviations in the table are multiplied by the Pawley parameter f P (to get realistic values) (see Ref. [15]). c Thermal vibrations where restricted to be isotropic and the product µ × r for the absorption correction was fixed in the last steps of the refinement. d Np , N irefl refer to the number of experimental points and independent reflections. e Pp , Pi , Pg , refer to the number of profile, intensity-dependent and global refined parameters, respectively. The profile fitting of the data was performed with a split pseudoVoigt function, including asymmetry and preferred orientation corrections. Preferred orientation and asymmetry were corrected, respectively, by the March–Dollase and the Berar–Bardinozzi expressions (see Ref. [15]).
and in others obtained by tilting around the three crystallographic axes, lead to an orthorhombic cell with dimensions a∼ = 4.11, b ∼ = 3.62 and c ∼ = 8.80 Å. The observed reflection conditions (h 0 0, h = 2n; 0 k 0, k = 2n; h k 0, h + k = 2n) are consistent with the space groups P21 mn (No. 31) and Pmmn (No. 59) [16]. The last group was subsequently confirmed after the successful structure refinement by the Rietveld method. Additional spots originated by multiple diffraction were observed in the patterns along the zone axes [0 1 0] and [1 0 0] (see Fig. 2). The results of the best fitting from Rietveld refinement are shown in Table 1. The observed and calculated patterns corresponding to this fitting are depicted in Fig. 1. The structural model for α-HfNBr consists
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Fig. 4. Experimental HREM image of α-HfNBr along [0 1 0].
(a)
(b) Fig. 3. (a) Perspective view of the structure of α-HfNBr. Hf atoms are placed at the centers of the octahedra. (b) Perspective view of the distorted coordination octahedron for Hf.
of layers of composition Br–Hf–N–N–Hf–Br stacked along c that are separated by a Van der Waals gap (Fig. 3a). The hafnium atoms are hexacoordinated by four nitrogen atoms and two bromine atoms. The resulting polyhedron shows the typical geometry of the FeOCl structure type and may be described as a highly distorted octahedron. (See Table 1 and Fig. 3b.) There are two pairs of slightly different M– N bond distances (around 2.1 Å), very close to those observed by Juza et al. in α-ZrNBr [2]. These distances agree with the sum of ionic radii for hexacoordinated Zr+4 or Hf+4 and tetracoordinated N−3 (r(Zr+4 ) = 0.78, r(Hf+4 ) = 0.71, r(N−3 ) = 1.46 Å [17,18]). The observed Hf–Br bond distance (2.728(1) Å) is consistent with the sum of ionic radii for hexacoordinated Hf4+ and Br− (r(Br− ) = 1.96 Å [18]), being close to the Zr–Br bond distance in ZrBr (2.74 Å) [19]
Fig. 5. Enlarged HREM image of α-HfNBr along [0 1 0], and (inset) computer overlay of the projected crystal structure and the computer-simulated image (thickness = 20 nm and defocus = −80 nm). The bright dots are identifying the Br configuration.
and significantly lower than that reported by Juza et al. for α-ZrNBr (2.85 Å) [2]. The Hf–Br distance is lower than the Zr–Br bond length observed for β-ZrNBr in our recent determination of the crystal structure (2.87 Å) [5], according with the different coordination number shown by the two metals in both structures (CN = VII for β-ZrNBr and VI for α-MNBr). A typical experimental HREM image along [0 1 0] zone axis of α-HfNBr is shown in Fig. 4. The inner part of this image is in relative good agreement with the calculated image for a thickness of 20 nm and a defocus of 800 Å
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Fig. 6. Computer-simulated HREM images of α-HfNBr as function of defocus (nm) (top) and thickness (nm) (bottom).
(Fig. 5). The calculated images in function of defocus and thickness based on the structural model of Table 1 are shown in Fig. 6. The intense bright dots in Fig. 5 correspond to the Br configuration, and the diffuse maxima are assigned to the Hf double layer configuration. The material is rather stable under the electron beam and shows a regular stacking of the Br–Hf–N–N–Hf–Br double layers, being almost free of defects at the inner part of the crystals. In the thin parts, however, we observed an image contrast similar to that found in crystals of β-HfNCl contaminated with HfO2 [13,14]. In that case the presence of HfO2 was understood as a product of the sample decomposition under the electron beam that took place in some samples containing small amounts of oxygen. The origin of these impurities was interpreted as caused by the oxygen scavenging by the Hf used in the synthesis. The intercalation of organic molecules and lithium in layered transition metal oxyhalides with FeOCl structure has been extensively established [20]. In the case of zirconium nitride halides, Fogg et al. reported superconductivity for lithium intercalated α-ZrNCl and α-ZrNI showing critical temperatures of 11 K [21]. More recently, Yamanaka
has reported superconductivity below 4–6 K in pyridineintercalated α-TiNCl [22]. In order to get further insight into the mechanisms of superconductivity in this new class of materials, and considering the high Tc shown by intercalated β-HfNCl, it would be relevant to study the physical properties of intercalated samples of the α polytype for the hafnium derivatives and, if superconductivity is induced, to compare the critical temperatures for both, α and β intercalated compounds. Thus, research in this direction by using different reducing agents (chemical and electrochemical) certainly deserves further attention.
Acknowledgements
This study was supported by the MEC (grant PB98-1424C02), the Picasso program from the Ministère de l’Education Nationale de la Recherche et de la Technologie (grant 99102) and the Comissionat per Universitats i Recerca de la Generalitat de Catalunya (grant 2000SGR00113).
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References [1] R. Juza, H. Friedrichsen, Z. Anorg. Allg. Chem. 332 (1964) 173. [2] R. Juza, J. Heners, Z. Anorg. Allg. Chem. 332 (1964) 159. [3] S. Yamanaka, H. Kawaji, K. Hotehama, M. Ohashi, Adv. Mater. 8 (1996) 771; H. Kawaji, K. Hotehama, S. Yamanaka, Chem. Mater. 9 (1997) 2127. [4] S. Yamanaka, K. Hohetama, H. Kawaji, Nature 392 (1998) 580. [5] A. Fuertes, M. Vlassov, D. Beltrán-Porter, P. Alemany, E. Canadell, N. Casañ-Pastor, M.R. Palacín, Chem. Mater. 11 (1999) 203. [6] F. Hulliger, in: F. Lévy (Ed.), Structural Chemistry of Layer-Type Phases, Vol. 5, D. Reidel Publishing Company, Dordrecht, 1976. [7] A.M. Fogg, S.O. Evans, D. O’Hare, Chem. Commun. (1998) 2269. [8] S. Shamoto, T. Kato, Y. Ono, Y. Miyazaki, K. Ohoyama, M. Ohashi, Y. Yamaguchi, T. Kajitani, Physica C 306 (1998) 7. [9] D.G. Adolphson, J.D. Corbett, Inorg. Chem. 15 (1976) 1820. [10] S. Yamanaka, K. Itoh, H. Fukuoka, M. Yasukawa, Inorg. Chem. 39 (2000) 806.
[11] A. Fuertes, M. Vlassov, D. Beltrán-Porter, M.R. Palacín Peiró, Patent application number 9901978, Oficina Española de Patentes y Marcas, Spain, 1999. [12] M. Vlassov, M.R. Palacín, D. Beltrán-Porter, J. Oró-Solé, E. Canadell, P. Alemany, A. Fuertes, Inorg. Chem. 38 (1999) 4530. [13] J. Oró-Solé, M.T. Caldés, M.R. Palacín, M. Vlassov, D. Beltrán-Porter, A. Fuertes, Chem. Mater. 11 (1999) 3425. [14] J. Oró-Solé, M.T. Caldés, M.R. Palacín, M. Vlassov, D. Beltrán-Porter, B. Martínez, A. Fuertes, Solid State Sci. 2 (2000) 77. [15] J. Rodríguez-Carvajal, Program FULLPROF, Version 2.5, April 1994, ILL, unpublished. [16] T. Hahn (Ed.), International Tables for X-Ray Crystallography, Volume A, 3rd edn., Kluwer Academic Publishers, London, 1992. [17] W.H. Baur, Crystallogr. Rev. 1 (1987) 59. [18] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [19] R.L. Daake, J.D. Corbett, Inorg. Chem. 16 (1977) 2029. [20] J. Rouxel, P. Palvadeau, Rev. Chim. Miner. (1982) 317. [21] A.M. Fogg, V.M. Green, D. O’Hare, J. Mater. Chem. 9 (1999) 1547. [22] S. Yamanaka, Annu. Rev. Mater. Sci. 30 (2000) 53.
Synthesis and crystal structure of the α polytype of HfNBr Judith Oró-Solé a , Mikhail Vlassov a,1 , Daniel Beltrán-Porter b , Maria Teresa Caldés c , Vicent Primo b , Amparo Fuertes a,∗ a Institut de Ciència de Materials de Barcelona (C.S.I.C.), Campus U.A.B., 08193 Bellaterra, Spain b Institut de Ciència de Materials de la Universitat de València, P.O. BOX 2085, Polígono “La Coma” s/n, 46980 Paterna, Spain c Institut des Matériaux Jean Rouxel UMR 6502, 2, rue de la Houssinière, 44072 Nantes cedex 03, France
Accepted 19 December 2001
Abstract α-HfNBr has been prepared at 760 ◦ C in a sealed evacuated fused silica tube by reaction between NH4 Br and Hf followed by purification through chemical vapour transport under a temperature gradient. The crystal structure of this compound at room temperature has been determined for the first time by Rietveld refinement of X-ray powder diffraction data, electron diffraction and high resolution electron microscopy. It crystallises in the orthorhombic space group Pmmn with the unit cell parameters a = 4.1165(2), b = 3.5609(2), c = 8.6440(3) Å. α-HfNBr is isotypic to FeOCl and is built from layers of composition Br–Hf–N–N–Hf–Br stacked along c that are separated by a Van der Waals gap. The hafnium atoms are six-coordinated by four nitrogen atoms and two bromine atoms. The resulting coordination polyhedron may be described as a highly distorted octahedron. Electron diffraction and high resolution electron microscopy studies reveal that the material is almost free of defects in the inner part of the crystals and confirm the structural model refined by the Rietveld method. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
1. Introduction The synthesis of zirconium nitride halides was first reported by Juza et al. who prepared the α or β polytypes of ZrNCl, ZrNBr and ZrNI by different methods including the ammonolysis of mixtures of the metal halides and the reaction of the metal nitrides with halogens under nitrogen atmosphere [1,2]. Their structure consist of sheets X– M–N–N–M–X separated by a Van der Waals gap, stacked with different packing for the α and β types. Intercalation of alkaline ions or cobaltocene in the β-MNX polytypes induces superconductivity with a maximum critical temperature of 25 K for Lix HfNCl [3–5]. The crystal structure of the β-MNX compounds is SmSI-type [6] and consists of a cubic close-packing of double layers of metal and halide atoms, yielding the sequence X–M–M–X with relative orientations AbcA [5,7,8]. The nitrogen atoms in β-MNX occupy tetrahedral sites between the metal layers as sulphur in SmSI. The compound ZrCl shows the same X–M–M–X ¯ as β-MNX, layer packing and the same space group (R3m) * Correspondence and reprints. 1 Permanent address: Earthcrust Research Institute, St. Petersburg Uni-
versity, Russia.
and thus the β polytypes may be considered as intercalation derivatives of parent ZrCl, as a result of the occupation of N atoms of the tetrahedral interstices [9]. The crystal structures of α-ZrNCl, α-ZrNBr and α-TiNX (X = Cl, Br, I) were first reported by Juza and Heners from powder X-ray diffraction data [2]. These compounds show the orthorhombic structure of FeOCl and decompose in ambient air because of hydrolysis. The alpha polytypes of HfNBr and HfNI have been recently prepared for the first time by Yamanaka et al. [10] by reaction of the metals with ammonium bromide or iodide in flowing ammonia, followed by transport purification under a temperature gradient. The same authors showed that these polytypes are transformed into the corresponding β phases at high pressure. In previous articles we have reported the synthesis of β-MNX phases by a new method that avoids the use of flowing NH3 using evacuated sealed fused silica tubes [11,12]. We have also reported the structural characterization of the host compounds and their chemical and electrochemical intercalation behaviour in relation with the superconducting properties of the doped phases [12–14]. In this paper we report the preparation of the compound α-HfNBr by the same synthetic method we applied to the β polytypes, as well as the first determination of its crystal structure from
1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 2 ) 0 1 2 7 2 - 4
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X-ray powder diffraction and the electron diffraction and high resolution electron microscopy studies.
2. Experimental 2.1. Synthesis of α-HfNBr α-HfNBr was prepared by the reaction of Hf (Aldrich 99.5%) with NH4 Br (Aldrich, 99.999%) as described in Ref. [11]. Typically 300 mg of Hf were mixed with 165 mg of NH4 Br and placed in one end of a fused silica tube of 25 cm of length and 8 mm of inner diameter that was subsequently sealed under vacuum. The tube was heated at 120 ◦ C min−1 to 760 ◦ C, held at this temperature for 12 hours, and then the end of the tube free of sample was heated to 860 ◦ C for the recrystallization process via vapour transport under a temperature gradient (transport direction: 760 ◦ C → 860 ◦ C). The recrystallization time was 72 hours. After cooling, pure α-HfNBr was found at the hot zone of the tube whereas in the cold zone—where some attack on the tube walls was observed—a mixture of α-HfNBr and ammonium bromide was identified by powder X-ray diffraction. Handling of the reactants and sample preparation for the different subsequent studies were carried out in an Ar filled glove box. 2.2. X-ray diffraction X-ray diffraction patterns were generally taken in a Siemens D-5000 diffractometer using Cu Kα radiation. Powder X-ray diffraction data collection for the crystal structure determination was performed on a powder diffractometer equipped with an INEL curved position sensitive CPS120 in a horizontal Debye–Scherrer geometry using a rotating capillary of diameter 0.1 mm as sample holder. The angular range was 120 degrees and the radiation was Cu Kα1 (λ = 1.540598 Å), obtained with a Ge (1 1 1) monochromator. The dimensions of the incident beam as controlled by a cross slit collimator were 0.1 × 4.0 mm2 . The sample was sieved to 65 µm and mixed with glass powder before filling the capillary that was sealed under argon. The structure was determined by the Rietveld method with the help of the F ULLPROF [15] program, using the coordinates reported by Juza et al. for α-ZrNBr [2] and the cell parameters obtained by electron diffraction as starting parameters. The profile fitting of the data was performed with a pseudoVoigt function, including asymmetry and preferred orientation corrections; the background was refined to a 5th degree polynomial. Preferred orientation and asymmetry were corrected, respectively, by the March–Dollase and the Berar– Bardinozzi expressions. (See Ref. [15].)
2.3. Electron diffraction and high resolution electron microscopy Samples for transmission electron microscopy were prepared by dispersing the powder in n-hexane and depositing the solution on a holey carbon aluminium grid. Electron diffraction patterns and XEDS analyses were obtained in a JEOL 1210 transmission electron microscope operating at 120 kV and equipped with a side-entry 60◦ /30◦ double tilt GATAN 646 analytical specimen holder and a Link QX2000 XEDS element analysis system. High resolution electron microscopy images were obtained in a HITACHI H9000NAR electron microscope operating at 300 kV. The Scherzer resolution was 1.8 Å for the HITACHI instrument. Calculated images for different defocus and different thickness were obtained using the M ACTEMPAS program (V.1.70, Roar Kilaas, Berkeley).
3. Results and discussion After the chemical transport step of the synthesis the phase α-HfNBr is found to crystallize at the high temperature zone of the quartz tube forming transparent light yellow hexagonal platelets. The X-ray diffraction pattern of the ground platelets (Fig. 1) is similar to that reported by Yamanaka [10] for the same phase prepared by treating the mixture of reactants in flowing ammonia previously to the chemical transport, but important differences in the intensity of some diffraction peaks are clearly observed. These differences are probably caused by preferential orientation perpendicularly to [0 0 1] that is frequently observed for zirconium and hafnium nitride halides and is due to the platelet morphology of the crystallites [5]. In our X-ray diffraction experiments the preferential orientation was minimised because of the special conditions taken for the sample preparation (see experimental section). Exposition of the sample to humid air promotes its hydrolysis and the decomposition of
Fig. 1. Observed and calculated X-ray diffraction patterns for α-HfNBr.
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Table 1 Crystallographic parameters for α-HfNBr Crystal data Pmmn (No. 59), Z = 2 4.1165(2) 3.5609(2) 8.6440(3) 126.71(1) 7.14 298 2.5
Space group a (Å) b (Å) c (Å) Cell volume (Å3 ) Calculated density (g cm−3 ) Temperature (K) µ × rc
Atomic coordinates and isotropic displacement parameters (in Å2 ) Atom
Wyckof site
x
y
z
B
2b 2a 2a
1/4 1/4 1/4
3/4 1/4 1/4
0.1000(2) 0.3393(5) 0.959(3)
2.73(6) 3.25(12) 2.4(6)
Hf Br N
Selected bond distances (Å) and angles (deg)
Fig. 2. Representative electron diffraction patterns along the [0 0 1], [0 1 0] and [1 0 0] zone axes for α-HfNBr. Multiple diffraction spots are marked with arrows.
α-HfNBr, yielding NH4 Br and an unknown phase showing the most intense peak at 10.8 Å. The decomposition process induces a change of colour of the sample from light yellow to white. Hydrogen contents for air exposed samples during three days were about 2%. As the peaks of α-HfNBr remain in the X-ray diffraction pattern taken after several weeks to air exposition, we suppose that either the hydrolysis reaction could lead to a phase structurally-related to α-HfNBr (for instance, obtained by exchange of Br− and OH− ) and/or the decomposition might have simply slow kinetics. Fig. 2 shows representative electron diffraction patterns along [0 0 1], [0 1 0] and [1 0 0] zone axes for crushed crystals of α-HfNBr. Indexation of the reflections in these planes,
d(Hf–N) (×2) d(Hf–N) (×2) d(Hf–Br) (×2)
2.152(18) 2.123(9) 2.728(3)
Br–Hf–Br Br–Hf–N (×2) N–Hf–N N–Hf–N
81.44(12) 165(6) 111.6(12) 151.6(12)
Br–Hf–N (×2) Br–Hf–N (×4) N–Hf–N (×4)
83.5(12) 100.7(9) 82.1(6)
N p , N irefl d P p, P i, P ge R Bragg , R F , χ 2 R p , R wp , R exp a f Pb
5000, 128 17, 8, 0 5.5, 4.0, 2.2 6.7, 8.6, 5.8 3.15
a Conventional Rietveld R-factors (R , R p exp ) are calculated by using background corrected counts. b Standard deviations in the table are multiplied by the Pawley parameter f P (to get realistic values) (see Ref. [15]). c Thermal vibrations where restricted to be isotropic and the product µ × r for the absorption correction was fixed in the last steps of the refinement. d Np , N irefl refer to the number of experimental points and independent reflections. e Pp , Pi , Pg , refer to the number of profile, intensity-dependent and global refined parameters, respectively. The profile fitting of the data was performed with a split pseudoVoigt function, including asymmetry and preferred orientation corrections. Preferred orientation and asymmetry were corrected, respectively, by the March–Dollase and the Berar–Bardinozzi expressions (see Ref. [15]).
and in others obtained by tilting around the three crystallographic axes, lead to an orthorhombic cell with dimensions a∼ = 4.11, b ∼ = 3.62 and c ∼ = 8.80 Å. The observed reflection conditions (h 0 0, h = 2n; 0 k 0, k = 2n; h k 0, h + k = 2n) are consistent with the space groups P21 mn (No. 31) and Pmmn (No. 59) [16]. The last group was subsequently confirmed after the successful structure refinement by the Rietveld method. Additional spots originated by multiple diffraction were observed in the patterns along the zone axes [0 1 0] and [1 0 0] (see Fig. 2). The results of the best fitting from Rietveld refinement are shown in Table 1. The observed and calculated patterns corresponding to this fitting are depicted in Fig. 1. The structural model for α-HfNBr consists
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Fig. 4. Experimental HREM image of α-HfNBr along [0 1 0].
(a)
(b) Fig. 3. (a) Perspective view of the structure of α-HfNBr. Hf atoms are placed at the centers of the octahedra. (b) Perspective view of the distorted coordination octahedron for Hf.
of layers of composition Br–Hf–N–N–Hf–Br stacked along c that are separated by a Van der Waals gap (Fig. 3a). The hafnium atoms are hexacoordinated by four nitrogen atoms and two bromine atoms. The resulting polyhedron shows the typical geometry of the FeOCl structure type and may be described as a highly distorted octahedron. (See Table 1 and Fig. 3b.) There are two pairs of slightly different M– N bond distances (around 2.1 Å), very close to those observed by Juza et al. in α-ZrNBr [2]. These distances agree with the sum of ionic radii for hexacoordinated Zr+4 or Hf+4 and tetracoordinated N−3 (r(Zr+4 ) = 0.78, r(Hf+4 ) = 0.71, r(N−3 ) = 1.46 Å [17,18]). The observed Hf–Br bond distance (2.728(1) Å) is consistent with the sum of ionic radii for hexacoordinated Hf4+ and Br− (r(Br− ) = 1.96 Å [18]), being close to the Zr–Br bond distance in ZrBr (2.74 Å) [19]
Fig. 5. Enlarged HREM image of α-HfNBr along [0 1 0], and (inset) computer overlay of the projected crystal structure and the computer-simulated image (thickness = 20 nm and defocus = −80 nm). The bright dots are identifying the Br configuration.
and significantly lower than that reported by Juza et al. for α-ZrNBr (2.85 Å) [2]. The Hf–Br distance is lower than the Zr–Br bond length observed for β-ZrNBr in our recent determination of the crystal structure (2.87 Å) [5], according with the different coordination number shown by the two metals in both structures (CN = VII for β-ZrNBr and VI for α-MNBr). A typical experimental HREM image along [0 1 0] zone axis of α-HfNBr is shown in Fig. 4. The inner part of this image is in relative good agreement with the calculated image for a thickness of 20 nm and a defocus of 800 Å
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Fig. 6. Computer-simulated HREM images of α-HfNBr as function of defocus (nm) (top) and thickness (nm) (bottom).
(Fig. 5). The calculated images in function of defocus and thickness based on the structural model of Table 1 are shown in Fig. 6. The intense bright dots in Fig. 5 correspond to the Br configuration, and the diffuse maxima are assigned to the Hf double layer configuration. The material is rather stable under the electron beam and shows a regular stacking of the Br–Hf–N–N–Hf–Br double layers, being almost free of defects at the inner part of the crystals. In the thin parts, however, we observed an image contrast similar to that found in crystals of β-HfNCl contaminated with HfO2 [13,14]. In that case the presence of HfO2 was understood as a product of the sample decomposition under the electron beam that took place in some samples containing small amounts of oxygen. The origin of these impurities was interpreted as caused by the oxygen scavenging by the Hf used in the synthesis. The intercalation of organic molecules and lithium in layered transition metal oxyhalides with FeOCl structure has been extensively established [20]. In the case of zirconium nitride halides, Fogg et al. reported superconductivity for lithium intercalated α-ZrNCl and α-ZrNI showing critical temperatures of 11 K [21]. More recently, Yamanaka
has reported superconductivity below 4–6 K in pyridineintercalated α-TiNCl [22]. In order to get further insight into the mechanisms of superconductivity in this new class of materials, and considering the high Tc shown by intercalated β-HfNCl, it would be relevant to study the physical properties of intercalated samples of the α polytype for the hafnium derivatives and, if superconductivity is induced, to compare the critical temperatures for both, α and β intercalated compounds. Thus, research in this direction by using different reducing agents (chemical and electrochemical) certainly deserves further attention.
Acknowledgements
This study was supported by the MEC (grant PB98-1424C02), the Picasso program from the Ministère de l’Education Nationale de la Recherche et de la Technologie (grant 99102) and the Comissionat per Universitats i Recerca de la Generalitat de Catalunya (grant 2000SGR00113).
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