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Chem. Mater. 1996, 8, 1052-1060
Synthesis, Structure, and Magnetic Properties of the New Layered Compound HNiPO4‚H2O. Study of Alkylamine Intercalated Compounds Aintzane Gon˜i,† Jordi Rius,‡ Maite Insausti,† Luis M. Lezama,† Jose´ Luis Pizarro,§ Marı´a Isabel Arriortua,§ and Teo´filo Rojo*,† Departamento Quı´mica Inorga´ nica, Universidad del Paı´s Vasco, Bilbao 48080, Spain; Institut de Cie` ncia de Materials de Barcelona, CSIC, 08193-Cerdanyola, Catalunya, Spain; and Departamento Mineralogı´a y Petrologı´a, Universidad del Paı´s Vasco, Bilbao 48080, Spain Received November 13, 1995. Revised Manuscript Received March 5, 1996X
The new layered HNiPO4‚H2O compound has been prepared and characterized, and its crystal structure resolved. It crystallizes in the monoclinic P21 space group with cell parameters a ) 8.069(3), b ) 4.726(1), c ) 5.597(1) Å, β ) 109.62(2)°, V ) 201.04 Å3, Z ) 2. The structure consists of (100) sheets of NiO6 corner-sharing octahedra cross-linked by (HPO4) groups. These sheets are linked by a zig-zag system of hydrogen bonds along the [010] direction, involving the (OH) group of the (HPO4) tetrahedra arranged toward the interlayer space. Five different alkyl amines, CnH2n+1NH2 (n ) 3-7), have been intercalated in this compound to form the (CnH2n+1NH2)xHNiPO4‚H2O phases. The intercalated derivatives present the inserted alkylamines arranged in bilayers and tilted at an angle of approximately 53.2° with respect to the inorganic sheets. The extent of intercalation is approximately constant (0.75) with the only exception being butylamine. No changes in particle size and morphology have been observed during the intercalation reactions. Thermal studies suggest that the alkyl amine desorption in these compounds takes place in only one step. Magnetic measurements of (C7H15NH2)0.76HNiPO4‚H2O show the existence of essential 2D antiferromagnetic intralayer interactions. In the case of HNiPO4‚H2O, an increase 3D interlayer ordering appears when temperature decreases due to the presence of hydrogen bonds between layers in the structure of this compound.
Introduction From the known phosphate compounds, only a limited number forms two-dimensional layered structures. The low dimensionality and the intercalation properties of some of these layered phosphates make them attractive and exciting for materials scientists. Since the layered host lattice is deformable along the third dimension, a variety of guest molecules and ions of different shapes and sizes can be accommodated in the interlayer space. They are of considerable interest because of potential applications as ion-exchangers, heterogeneous catalyst, catalyst supports, ionic conductors, etc. An example of this interest is the extensive study of the intercalation behaviors of the layered vanadyl phosphates or R-zirconium phosphates.1,2 To induce the formation of new types of composite materials, the intercalation of alkylamines into layered phases has now become a common technique in the first step of attainment of pillaring compounds.3 This reaction increases the spacing between the inorganic sheets by formation of an amine bilayer or monolayer that could then be replaced by * To whom all correspondence should be addressed. † Dpto. Quı´mica Inorga ´ nica. ‡ Institut de Cie ` ncia de Materials de Barcelona. § Dpto. Mineralogı´a y Petrologı´a. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Whittingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press: New York, 1982. (2) Kanazawa, T. Inorganic Phosphate Materials; Elsevier: Amsterdam, 1989. (3) Sylvester, P.; Cahill, R.; Clearfield, A. Chem. Mater. 1994, 6, 1890.
S0897-4756(95)00539-4 CCC: $12.00
other interesting species. The inorganic layers appear to act as a template, directing the growth of the inserted guest lamellas. Following this procedure, Shpeizer et al.4 reported a ferromagnetic composite containing sheets of NiO between the R-zirconium phosphate layers. On the other hand, the formation of new twodimensional layered phosphates of magnetic ions, such as Ni2+, Co2+, etc., and related intercalation compounds, might be very interesting in order to study the magnetic properties together with the effects of the intercalation on their magnetic behaviors, assuming that the magnetic layers are left chemically unchanged during the intercalation process. In this way, real magnetic layered compounds can be considered as 2D model magnetic systems in which the ratio of the inter- to intralayer exchange coupling, J′/J, is less than ≈10-2. Sometimes, however, one may observe a crossover between different lattice dimensionalities near the transition temperature caused by the interlayer coupling J′. In this way, the KNiAsO4 arsenate5,6 exhibits a 2D antiferromagnetic behavior, and the intercalation of alkylamines with a great number of carbons as in the case of C10H21NH3NiAsO4, does not modify its bidimensional magnetic behavior.7 However, in the layered (4) Shpeizer, B.; Poojary, D. M.; Ahn, K.; Runyan, C. F.; Clearfield, A. Science 1994, 266, 1357. (5) Beneke, K.; Lagaly, G. Clay Minerals 1982, 17, 175. (6) Buckley, A. M.; Bramwell, S. T.; Visser, D.; Day, P. J. Solid State Chem. 1987, 69, 240. (7) Bramwell, S. T.; Buckley, A. M.; Visser, D.; Day, P. Phys. Chem. Minerals 1988, 15, 465.
© 1996 American Chemical Society
A New Layered Compound, HNiPO4‚H2O
Chem. Mater., Vol. 8, No. 5, 1996 1053
Table 1. Analytical Data for the (RNH2)xHNiPO4‚H2O Derivatives
a
RNH2
%C
%H
%N
xa
C3H7 C4H9 C5H11 C6H13 C7H15
12.32 11.85 18.55 21.74 24.34
4.71 4.48 5.57 5.90 6.29
4.79 3.51 4.41 4.23 4.10
0.74 0.53 0.74 0.75 0.76
x is the extent of intercalation of alkylamine in every case.
HMnAsO4‚H2O8,9 compound, which exhibits antiferromagnetic interactions, when the interlayer spacing increases by inserting methylamine, CH3NH3MnAsO4‚ H2O, the Neel’s temperature (TN) decreases, suggesting that the interlayer coupling is important and the magnetic order is strongly affected by J′.7 As far as we are aware, a similar study in phosphate compounds has not been carried out. In this paper, we report the structure and properties of a new layered phosphate, HNiPO4‚H2O, together with the results of a systematic study of the intercalation reactions with aliphatic amines. Likewise, a study of the magnetic behavior of this compound and its intercalation derivative (C7H15NH2)0.76HNiPO4‚H2O is carried out, and the results discussed in the context of lowdimensional systems. Experimental Section Preparation of HNiPO4‚H2O. HNiPO4‚H2O was obtained by heating at 170 °C the NH4NiPO4‚H2O phase, in atmospheric conditions (humidity of 80% in the syntheses room). The precursor compound was previously prepared by slowly addition of NiCl2‚6H2O to a concentrated aqueous solution of (NH4)2HPO4 at 90 °C.10 Both compounds exhibit yellow color. The elemental analysis shows the no presence of nitrogen in the HNiPO4‚H2O. Nickel content (exp ) 33.7%, calc ) 33.9%) was confirmed by atomic absorption (Perkin-Elmer 3030B) and phosphorous content (exp ) 17.6%, calc ) 17.9%) by gravimetric techniques.11 Alkylamine Intercalation Procedure. Intercalation compounds with N-propyl-, butyl-, pentyl-, hexyl-, and heptylamines were obtained by placing the HNiPO4‚H2O phase in an atmosphere saturated with N-alkylamine vapor at room temperature and pressure. The contact times of solid with the CnH2n+1NH2 alkylamines vapors were 3, 6, 8, 9, and 10 days for the n ) 3, 4, 5, 6, and 7, respectively. Intercalation experiments in polar solvents, such as acetone, methanol, etc., gave samples with a low degree of crystallinity. In the case of acid or basic aqueous solutions the HNiPO4‚H2O phase forms Ni3(PO4)2‚8H2O which is very stable. The intercalated samples were dried to the air at 50 °C. The amount of alkylamine intercalated was determined by elemental analysis (Perkin-Elmer CHN 2400) and the results are given in Table 1. Experiments with longer time contacts to increase the degree of intercalation (x) were carried out and the results obtained were similar to those given before. Characterization and Physical Measurements. X-ray diffraction patterns of the intercalated compounds were recorded on a Philips 1470 diffractometer using Cu KR radiation. Infrared spectra were performed with a Nicolet FT-IR 740 spectrometer using the KBr disk technique. Thermogravimetric analyses of HNiPO4‚H2O and intercalated phases were performed with a Perkin-Elmer System-7 DSC-TGA. Crucibles containing 20 mg were heated at 2 °C min-1 under (8) Beneke, K.; Lagaly, G. Am. Mineral. 1981, 66, 432. (9) Buckley, A. M.; Bramwell, S. T.; Day, P. Am. Mineral. 1990, 75, 1140. (10) Fraissard, J.; Etienne, J. J. Bull. Soc. Fr. Mine´ral. Crystallogr. 1967, 90, 162. (11) Kolthoff, I. M.; Sandell, E. B. Quantitative chemical analysis, 4th ed.; 1969, p 382.
nitrogen atmosphere. Scanning electron microscopic (SEM) analyses were carried out on a JEOL JSM-6400 furniture, employing 20 kV and 6 × 10-11-6 × 10-10 A of intensity. Magnetic susceptibility measurements were performed at 0.1 T with a Quantum SQUID Magnetometer, in the 1.8-350 K temperature range. Crystal Structure Resolution of HNiPO4‚H2O. The X-ray powder diffraction data were collected on a D500 Siemens powder diffractometer using a Bragg-Brentano geometry with Cu KR1 radiation (in steps of 0.02°(2θ) over the 5-110°(2θ) angular range and a fixed-time counting of 20 s) at two different temperatures, 25 and 170 °C. The powder diffraction pattern measured at 25 °C was indexed with the TREOR12 program. The monoclinic cell found, with a ) 8.166 Å, b ) 4.746(1) Å, c ) 5.625(1) Å, and β ) 110.56°, is characterized by the figures of merit M(20) ) 12 and F(20) ) 10(0.024,87). The lattice parameters of HNiPO4‚H2O are close to those of the NH4NiPO4‚H2O precursor (a ) 5.574(1) Å, b ) 8.765(1) Å, c ) 4.755(1) Å, space group Pmn21).13 This similarity suggests the following unit cell relationship between both compounds (precursor ) prec): aprec f c, bprec f a, cprec f b, with a reduction of the bprec parameter and a change of the system from orthorhombic to monoclinic. If the topology of the layers is preserved (Figure 1), the most probable space group for HNiPO4‚H2O will be P21, with the layers parallel to (100). This solution was used for to review the quality of the powder diffraction data by FULLPROF14 program (pattern matching analysis), and the refined cell dimensions were found to be a ) 8.069(3) Å, b ) 4.726(1) Å, c ) 5.597(1) Å, β ) 109.62(2)°, V ) 201.04 Å3. The same procedure was employed to evaluate the 170 °C pattern, obtaining the lattice parameters a ) 8.136(1) Å, b ) 4.740(1) Å, c ) 5.612(1), β ) 110.83(1)°, V ) 202.28 Å3. The observed anisotropic lattice thermal expansion, with a variation of the a parameter 4.7 times higher than b or c, is consistent with the supposed orientation of the layers, perpendicular to the a crystallographic axis, which give rise to the largest increment of the interlayer distance d100, the weak bond direction. The integrated intensities were extracted with the wholepowder-pattern decomposition program AJUST.15 Only the Fobs values of the reflections with 2θ e 50° were used as input data for the direct-methods program XLENS,16 which is based on the modified tangent formula described in ref 17. Inspection of the E-map of the solution with the best combined figures of merit clearly showed three maxima, which were assigned to the Ni and P atoms and to the water molecule, respectively. The structure was refined with the Rietveld refinement program RIBOLS18 using the data in the interval 5° e 2θ e 75°. The diffraction peaks were described with Pearson VII profile functions with shape parameters m ) 1. The peak widths (fwhm) range from 0.30° at 2θ ) 25° to 0.53° at 2θ ) 65°. This produces severe peak overlap and, consequently, loss of information at higher 2θ angles. Since this region of the powder pattern is very important for the accurate determination of the atomic parameters, loss of information in this region will result in large esd’s for the refined variables. Crystallographic data and further details of the Rietveld refinement are given in Table 2. Figure 2 shows the best agreement obtained between calculated and observed profiles. Final atomic position parameters are given in Table 3, and selected bond distances and angles are shown in Table 4. (12) Werner, P. E. TREOR-4 Program; Arrhenius Laboratory, University of Stockholm, Sweden, 1984. (13) Durif, A.; Averbuch-Pouchot, M. T. Bull. Soc. Fr. Mineral. Crystallogr. 1968, 91, 495. (14) Rodriguez Carvajal, J. FULLPROF Program. Rietveld Pattern Matching Analysis of Powder Patterns, 1994. (15) Rius, J. AJUST. A whole-powder-pattern decomposition program; Institut de Cie`ncia de Materials de Barcelona (CSIC), Catalunya, Spain, 1994. (16) Rius, J. XLENS. A direct-methods program; Institut de Cie`ncia de Materials de Barcelona (CSIC), Catalunya, Spain, 1994. (17) Rius, J. Acta Crystallogr. 1993, A49, 406. (18) Rius, J. A rigid body Rietveld refinement program; Institut de Cie`ncia de Materials de Barcelona (CSIC), Catalunya, Spain, 1989.
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Figure 1. Crystal structure of HNiPO4‚H2O and structural relationship with the NH4NiPO4‚H2O precursor. Table 2. Summary of Crystallographic Data and Least-Squares Refinement Details for HNiPO4‚H2O formula
HNiPO4‚H2O
Mr crystal system space group (No.) a(Å) b(Å) c(Å) β (deg) V (Å3) Z 2θ range (deg) temp (C) radiation (λ(Å)) step scan increment (°2θ) time counting (s) Bover (Å2) no. of reflections no. of structural parameters no. of profile parameters Wi ) 1/yiobs RF ) ∑|Iobs1/2 - Icalc1/2|/∑Iobs1/2 RB ) ∑|Iobs - Icalc|/∑Iobs Rp ) ∑|yiobs - (1/c)yicalc|/∑yiobs Rwp ) [∑wi[yiobs - (1/c)yicalc]2/ ∑wi[yiobs]2]1/2
172.69 monoclinic P21 (4) 8.069(3) 4.726(1) 5.597(1) 109.62(2) 201.04 2 5.0-75.0 25 Cu KR1 (1.5406) 0.02 20 2.9 127 22 1 0.097 0.107 0.054 0.071
Results and Discussion Structure of HNiPO4‚H2O. The structure of HNiPO4‚H2O consists of (100) sheets of NiO6 corner-sharing octahedra cross linked by (HPO4) groups. These sheets are linked by a zig-zag system of hydrogen bonds along the [010] direction, involving the (OH) group of the (HPO4) tetrahedra arranged toward the interlayer space. Figure 1 shows the structural transformation from precursor to HNiPO4‚H2O. The sheet topology is similar to that observed in the NH4NiPO4‚H2O phase19. The Ni(II) ions exhibit a distorted octahedral geometry with four Ni-O(2)i, O(3), O(3)ii, O(4)iii (i ) x, y, 1 + z; ii ) 1 - x, y - 1/2, 1 - z; (iii ) 1 - x, y + 1/2, 1 - z) distances near 2.0 Å and two others with lower NiOwii (1.88 Å) and higher Ni-O(2)ii (2.20 Å) distances. (19) Tranquic, D.; Durif, A.; Guitel, J. C.; Averbuch-Pouchot, M. T. Bull. Soc. Fr. Mineral. Crystallogr. 1968, 91, 10.
Figure 2. Experimental and calculated X-ray diffraction pattern of HNiPO4‚H2O. Table 3. Fractional Atomic Coordinates for the HNiPO4‚H2O Compound (298 K) atom
x
y
z
Ni P O(1)H O(2) O(3) O(4) Ow
0.538(2) 0.710(3) 0.918(9) 0.623(9) 0.613(9) 0.704(7) 0.302(7)
0.5 0.550(8) 0.592(14) 0.698(15) 0.693(15) 0.226(11) 0.697(9)
-0.228(4) 0.357(7) 0.447(22) 0.105(16) 0.504(16) 0.342(19) 0.149(16)
The deformation observed in the coordination polyhedra is determined by the phosphate groups which distort the O-Ni-O bond angles significantly from 90° (see Table 4). The four equatorial Ni-O(3), O(2)i, O(3)ii, O(2)ii bonds are responsibles for the cross-linking in the sheet. The axial oxygen O(4)iii is provided by a PO4 group, and the coordinated water Ow is the remaining axial vertex. The PO4 tetrahedra are connected with the nickel octahedra by the three oxygen vertex, O(2), O(3), O(4) and one common edge, O(2)‚‚‚O(3). The observed average P-O distance is 1.53 Å. The fourth oxygen corner, O(1)H, is directed toward the interlayer space. The P-O(1)H distance is 1.60 Å (see Table 4). Unlike NH4NiPO4‚H2O, where the layers are separated by the NH4+
A New Layered Compound, HNiPO4‚H2O
Chem. Mater., Vol. 8, No. 5, 1996 1055
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the HNiPO4‚H2O Compound
Ni-O(2)i Ni-O(2)ii
Ni Coordination Octahedron (σ(Ni-O) ≈ 0.05 Å; σ(O-Ni-O) ≈ 1.5°) 2.00 Ni-O(3) 2.01 Ni-O(4)iii 2.20 Ni-O(3)ii 2.17 Ni-Owii
2.13 1.88
O(2)i-Ni-O(2)ii 94 O(3)-Ni-O(3)ii 91 O(2)i-Ni-O(3) 113 O(2)i-Ni-O(3)ii 152 O(3)-Ni-O(2)ii 156 O(2)ii-Ni-O(3)iii 62 O(2)i-Ni-Owii 98 O(3)-Ni-Owii 100 Owii-Ni-O(4) 160 O(2)ii-Ni-Owii 82 O(3)ii-Ni-Owii 80 O(2)i-Ni-O(4)iii 92 O(3)-Ni-O(4)iii 91 O(2)ii-Ni-O(4)iii 81 O(3)ii-Ni-O(4)iii 83 symmetry codes i: x, y, 1 + z ii: 1 - x, y - 1/2, 1 - z iii: 1 - x, y + 1/2, 1 - z Phosphate Tetrahedron (σ(P-O) ≈ 0.06 Å; σ(O-P-O) ≈ 2.0°) P-O(1)H 1.60 P-O(3) P-O(2) 1.52 P-O(4)
1.48 1.53
O(1)H-P-O(2) 110 O(1)H-P-O(3) 118 O(1)H-P-O(4) 99 O(2)-P-O(4) 115 O(3)-P-O(4) 118 O(2)-P-O(3) 98 Interlayer Hydrogen Bond (δ(O‚‚‚O) ≈ 0.07 Å; δ(O‚‚‚O‚‚‚O) ≈ 3.0°) O(1)H‚‚‚O(1)Hiv 2.68 O(1)Hv‚‚‚O(1)H 2.68 O(1)Hv‚‚‚O(1)H‚‚‚O(1)Hiv symmetry codes iv: 2-x, y-1/2, 1-z v: 2-x, y +1/2, 1-z
124
cations, in HNiPO4‚H2O the sheets are connected by the zig-zag system of hydrogen bonds, involving the O(1)H group of the phosphate tetrahedra. The length of the hydrogen bond O(1)H‚‚‚O(1)iv is 2.68 Å and the angle O(1)Hv‚‚‚O(1)H‚‚‚O(1)Hiv is 124° (iv ) 2 - x, y - 1/2, 1 - z; v ) 2 - x, y + 1/2, 1 - z), see Table 4). Strategy of Synthesis of the Intercalated Compounds. Compounds of type NH4MIIPO4‚H2O (where MII ) Mg, Mn, Co, Cu, ...) may undergo, on heating, several types of transformation,20 including in the first step, thermal loss of the water molecule and formation of anhydrous salt (eq 1)
NH4MIIPO4‚H2O f NH4MIIPO4 + H2Ov
(1)
or deintercalation of the ammonium molecule inserted between layers to form the acid phosphate monohydrate (eq 2).
NH4MIIPO4‚H2O f HMIIPO4‚H2O + NH3v
(2)
Nonisothermal thermogravimetric studies performed in inert atmosphere of nitrogen do not show any difference between dehydration and deammoniation steps and exhibit one endothermic effect between 170 and 300 °C, with a mass loss corresponding to one H2O and one NH3 molecule. Nevertheless, structural considerations allow us to deduce that when NH4MIIPO4‚ H2O is heated in an atmosphere of humid air it is possible the supression of the reaction 1 and to enable the elimination of interlayer NH3, which is bonded to structure only by hydrogen bridges. Unfortunately, this is not valid for all isostructural NH4MIIPO4‚H2O compounds, and reactions 1 and 2 may replace each other depending on the nature of the utilized metal. For the Co2+ ion, there is no preference for hexa- or tetracoordination, and consequently, there is no problem in losing (20) Pysiak, J.; Prodan, G. A.; Samuskevich, V. V.; Dacewska, B.; Shkorik, N. A. Thermochim. Acta 1993, 222, 91.
Figure 3. SEM photographs of (a) NH4NiPO4‚H2O, (b) HNiPO4‚H2O and (c) [C7H15NH2]0.76HNiPO4‚H2O.
the coordinated water molecule to form anhydrous salt (this phase presents blue color, indicating tetracoordination for Co2+). The Ni2+ ion, however, shows a high preference for hexacoordination, and therefore it is easier to remove the noncoordinated NH3 molecule than the H2O molecule directly bonded to metal. In this manner, by heating NH4NiPO4‚H2O at 170 °C, which is the lowest temperature necessary to initiate the first mass loss, it was possible to obtain the HNiPO4‚H2O compound. As shown before, its layer structure is basically preserved, and only an ammonia deintercalation reaction occurs. The obtention of this compound promted us the preparation of five intercalated derivatives with formula (RNH2)xHNiPO4‚H2O (R ) C3H7, C4H9, C5H11, C6H13, C7H15), stables in air, by means of the absorption of the N-alkylamines vapors by the layered phosphate HNiPO4‚H2O. The mechanism may be considered as an acid-base topotactic solid-state reaction between a layered acid host and Bro¨nsted base guest due to the host matrix retains its integrity with respect to the structure and composition in the course of the intercalation (eq 3).
HNiPO4‚H2O + xRNH2 f (RNH2)xHNiPO4‚H2O (3) Samples of the intercalated derivatives of HNiPO4‚H2O were characterized using scanning electron microscopy
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Figure 4. X-ray powder diffraction spectra of five different alkylamine intercalated derivatives of HNiPO4‚H2O: (a) propylamine, (b) butylamine, (c) pentylamine, (d) hexylamine, and (e) heptylamine. Table 5. d100 Reflexion and Packing Parameters of Different Alkylamine Derivatives (RNH2)xHNiPO4‚H2O NC 3 4 5 6 7
d100
Vc
Vt
17.5012 19.0140 21.1612 23.6986 25.1938
3.9 × 3.7 × 10-8 6.5 × 10-8 8.0 × 10-8 9.3 × 10-8
6.4 × 7.9 × 10-8 10.0 × 10-8 12.6 × 10-8 14.0 × 10-8
10-8
Vp 10-8
0.61 0.47 0.65 0.63 0.66
(SEM), X-ray diffraction, IR spectroscopy, and thermogravimetric techniques. The extent of intercalation, x, was calculated from the percentage of C and N in the sample and is given in Table 1. The x values are similar for all the intercalated compounds (x ≈ 0.75) except for the butylamine intercalated derivative that surprisingly presents an x value of 0.5. Scanning Electron Microscopy. Figure 3 shows the scanning electron microscopic (SEM) photographs of the NH4NiPO4‚H2O precursor, the new nickel phosphate acid obtained by NH3 deintercalation (HNiPO4‚ H2O), and the [C7H15NH2]0.76HNiPO4‚H2O compound as representative of the intercalate derivatives. The rectangular microcrystals are about 5 µm wide and 10 µm long, but extremely thin. The crystal morphology of all these phases is clearly lamellar, in good agreement with the structural characteristics. No change in morphology and size is observed in these materials, although the samples have been submitted to processes of intercalation and deintercalation with the consequent modification of the interlayer distance. These results are very important in order to obtain pillared compounds with a high degree of crystallinity. Study of the N-Alkylamine Intercalated Compounds. The powder X-ray diffraction profiles of the different alkyl amines (CnH2n+1NH2; n ) 3-7) intercalated phases are shown in Figure 4. The intercalation of amines between the nickel phosphate layers causes an expansion in the d100 direction and, therefore, separation of the inorganic sheets. The interlayer distance is characterized by the intense basal (100) reflection, which is given for each intercalated compound in Table 5. The relationship between the basal spacing (d100) and the number of carbons in the amine molecules (NC) is shown in Figure 5 and could be described by the following expression:
Figure 5. d(100) interlayer distance vs carbon number of intercalated alkylamines.
d100 ) 11.135 + 2.0339NC
(4)
Thus, the mean increase of the basal spacings, ∆d100/ ∆NC, was found to be 2.03 Å, which is in good agreement with the value of 1.96 Å obtained by Buckley et al.9 for the alkylamine intercalates of the mineral Krautite (HMnAsO4‚H2O). In the case of straight (trans-trans) alkyl chain, the increase of alkyl chain length per carbon atom is estimated to be 1.27 Å.21 Then, the mean increase per carbon atom of 2.03 Å observed in our compounds supposes that the alkyl chains must be arranged in bilayers tilted at an angle R ) 53.2° to the basal plane (100). This angle agrees well with those given for other layered hydrogen phosphates,22 such as 58.7° in R-TiP, 54.2° in R-ZrP, 66° in R-SnP, and 59.6° in VOHPO4‚0.5H2O, and also with 53.8°, the theoretical tilt angle of alkylamines when the N-C bond is nearly perpendicular to the basal plane (see Figure 6a). This arrangement suggests that more than one H bond may be formed between the NH3 group and the layer-surface oxygen atoms, and not only one N-H vertical bond (see Figure 6b), as Beneke and Lagaly5 proposed for the HMnAsO4‚H2O compound. This result can be also corroborated for the distance observed, 4.73 Å, between two oxygen atoms on the surface of the inorganic sheet in the structure of HNiPO4‚H2O (see Figure 7a). The free area surrounding each phosphate acid in the layer of HNiPO4‚H2O is 26.87 Å2, and considering that the cross-sectional area of a trans-trans alkyl chain is evaluated at 18.6 Å,2,23 it can be deduced that the accommodation of one molecule of amine per phosphate is possible. However, there is not room for the interpenetration of alkyl-chain amines bonded to the phosphate groups of two facing layers. This effect gives rise to the formation of a bilayer of alkylamines in our intercalated compounds. On the other hand, taking into account the type of bonding between the phosphate ions and the amine groups in the title compounds (see Figure 6a), the different increase of the interlayer distance when the number of carbon passes from odd to even (Figure 7b) should give rise to an odd-even alternation in the (21) Lagaly, G. Solid State Ionics. 1986, 22, 43. (22) Gulianst, V. V.; Benzinger, J. B.; Sundaresan, S. Chem. Mater. 1994, 6, 353 and references herein. (23) Kitagorodsky, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973.
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Chem. Mater., Vol. 8, No. 5, 1996 1057
Vc/Vt, where Vc is the volume occupied by the alkyl chains and Vt the volume available in the interlayer space24 (Table 5). Vt was calculated for 1 cm2 of layer, by subtracting the space occupied by the nickel phosphate sheets and the terminal -NH3+ groups (d′) from the interlayer distance d100:
Vt ) d100 - d′
Figure 6. Possible orientations of alkylamines in the (RNH2)xHNiPO4‚H2O compounds (a) N-C1 vertical and (b) N-H vertical.
(5)
Assuming that the penetration of the -NH3+ groups of different amines within the layer surface is independent of carbon number in the alkyl chain, d′ value is equal to d100 for NC ) 0 in expression 4, d′ ) 11.14 × 10-8 cm. Vc, referring to the 1 cm3 of the layer, will be given by the expression:
Vc ) (alkyl-chain length) × in cm (the cross sectional area of amine) × in cm2 (number of amines/cm2) The number of alkylamine units/cm2 of layer (7.44 × 1014) was obtained from the data of the HNiPO4‚H2O unit cell, but considering that this value depends on the intercalation degree (x) for each alkylamine-derivated compound, the Vc value must be expressed Figure 7. (a) Hydrogen bonds between alkylammonium groups and two O atoms of the surface. (b) Odd-even alternation in the increase of interlayer distance.
Vc )
relationship between d100 and the NC of the amine chains, which is not observed in our compounds (see Figure 5). This result could be explained by the existence of disorder and random interstratifications in the structures originated for the alkylamine vacancies between layers, because of the extent of the intercalation in the title compounds is approximately 0.75. This fact causes a broadening in the X-ray diffraction peaks, which gives rise to the inaccurancy of the d values corresponding to the basal reflection (d100) for the different compounds and so, the predicted odd-even alternation cannot be observed in our compounds. The NH4NiPO4‚H2O compound can be considered as an amine intercalated species, where the amine intercalated would be the ammonia. This compound exhibits an orthorhombic structure, whereas the deammoniated phase, HNiPO4‚H2O, is monoclinic. One might suspect that the intercalation of alkylamines might convert the structure back to orthorhombic. Unfortunately, the poor quality of the X-ray diffraction patterns obtained for the intercalated derivatives difficults the peak indexations, and consequently, to verify the existence of a structural monoclinic-orthorhombic transition is complicated. Nevertheless, this transition involves only a displacement between layers. Since the intense first reflexion (d100) corresponds to the interlayer spacing in both crystallographic systems and taking into account that the topology of the inorganic layer remains unchanged in all cases, the hypothetical structural transition would not affect the spacing between layers. The packing density of the N-alkylamines inserted between the layers of HNiPO4‚H2O was determined by the packing parameter (Vp), using the expression Vp )
The packing parameters, Vp, obtained from the Vc and Vt values for the different intercalated phases are given in Table 5. It can be deduced that the maximum volume in the interlayer space is not occupied, as opposed to other alkylamine systems25 in which all space is full. It could be explained for the high stability of the two hydrogen bonds between the NH3+ groups and the inorganic layers (hydrogen bond distance ) 2.90 Å), which constrains the alkylamine to tilt an angle close to 53.8° given as result a N-C bond perpendicular to the layer, (see Figure 6). This high stability does not make possible other arrangements of alkylamine chains in spite of the occupied volume decreases due to the reduction of the intercalation degree. Infrared Spectroscopy. Selected bands obtained from the IR spectra of NH4NiPO4‚H2O, HNiPO4‚H2O, and the heptylamine intercalated compound, as representative of intercalation derivatives, are given in Table 6. All phases present a set of bands in the range 35003000 cm-1, which can be assigned to the stretching vibration modes of the OH groups in good agreement with the presence of water molecules coordinated to the metal in the structures. In the case of HNiPO4‚H2O, these bands are splitted due to the presence of two different types of OH groups corresponding to both the water molecule and the P-O-H group of the phosphate acid. A sharp band at 3590 cm-1 is observed in the spectra of all alkylamine intercalated derivatives. This band
NC(1.27 × 10-8)(18.6 × 10-16)(7.44 × 1014x) cm3 (6)
(24) Casciola, M.; Costantino, U.; Di Croce, L.; Marmottini, F. J. Inclus. Phenom. 1988, 6, 291. (25) Menendez, A.; Barcena, M.; Jaimez, E.; Garcia, J. R.; Rodriguez, J. Chem. Mater. 1993, 5, 1078.
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can be assigned to the stretching frequency corresponding to the N-H bond present in the amino group. The other two hydrogens of this group are bridged to the inorganic sheets. The band observed at 1570 cm-1 seems to confirm the existence of the free N-H bond in these alkyl amine intercalated compounds. The bands which appear at 2930 and 2770 cm-1 in the NH4NiPO4‚H2O compound can be ascribed to the 2δ(N-H) vibration of the NH4+ groups.26 In the case of (C7H15NH2)0.76HNiPO4‚H2O the bands at 2950, 2915, and 2850 cm-1 can be assigned to the overlap between the 2δ(N-H) vibrations of the RNH3+ groups and the vst(C-H) vibrations due to the presence of the alkylamine groups. These bands are not observed in the HNiPO4‚H2O compound confirming the complete deintercalation of the NH3 group in the NH4NiPO4‚H2O phase during the synthesis of the nickel phosphate acid monohydrate. The bending modes of the PO-H and N-H bridge groups in all compounds appear in the range 1470-1250 cm-1. The νst(PO4) bands appear in the range 1100-940 cm-1 for NH4NiPO4‚H2O. These bands are split in the HNiPO4‚H2O compound due to the distortion of the PO4 tetrahedra observed in the structure in which the HPO4 group appears. However, in the intercalated derivative compounds similar bands to those observed in NH4NiPO4‚H2O are present, which is indicative of similar structural dispositions of the phosphate groups in the inorganic sheets of these compounds. This result is confirmed for the PO4 bending modes which are located
at 625 and 560 cm-1 for the NH4NiPO4‚H2O and intercalated derivative compounds. In the case of HNiPO4‚H2O, these bands are split as a consequence of the decrease in the symmetry of the PO4 groups due to the H-O-P bonding. Thermogravimetric Study. The TGA curves of HNiPO4‚H2O and the intercalated (RNH2)xHNiPO4‚H2O compounds from room temperature to 600 °C are represented in Figure 8. The experimental curve of HNiPO4‚H2O shows a weight loss of 10.50% in the range 180-310 °C, which can be attributed to the water molecule (calc 10.43%) from the layers. This result is similar to that obtained for the precursor NH4NiPO4‚H2O phase,27 in which the weight loss of both ammonia and water molecules was also observed among 170 and 300 °C. However, a small weight loss was observed at low temperatures that was attributed to water absorbed from the atmosphere.27 In the case of alkylamine intercalated derivatives a progressive loss of mass with the existence of at least four overlapped processes of weight loss could be considered. In these compounds, a continuous weight loss is observed at low temperatures. This process could be attributed to the loss of the intercalated alkylamines together with a small amount of water absorbed from the atmosphere during the intercalation process. This result is in good agreement with the experimental weight loss, which is higher than the correspondent only to the alkylamine groups and the water from the layers. This process was not observed for HNiPO4‚H2O, probably due to the little separations among layers in this compound. However, is similar to that observed in the precursor NH4NiPO4‚H2O phase,27 considering the different size of the ammonium molecule respect to the alkylamine groups. In the range 150-450 °C, the slow alkylamine deintercalation process is overlapped with the loss of the water molecule from the layers. Finally, at 600 °C, the condensation of the hydrogen phosphate groups takes place to obtain the nickel(II) pyrophosphate. The long range of temperature necessary for the alkylamines deintercalation seems to indicate that the process is complex and could be explained due to the existence of several desorption steps at intermediate temperatures. However, the HNiPO4‚H2O compound can be obtained as a process of complete deintercalation of amines by isothermal heating of the (RNH2)xHNiPO4‚H2O intercalated compounds at 170 °C for 2 weeks. On the other hand, unstable phases with lower intercalation degree can also be obtained using shorter heating times. These results could be kinetically explained because in the alkylamine deintercalation processes not enough time has been employed. In this sense, the long way that the big alkylamines need to run over the host matrix in the deintercalation step could be an important factor in the slow deintercalation process. So, these results suggest that the alkyl amine desorption in these compounds could occur in only one step. Magnetic Properties. The thermal variations of both the magnetic susceptibility and χmT for the HNiPO4‚H2O and (C7H15NH2)0.76HNiPO4‚H2O compounds are shown in Figures 9 and 10, respectively. The thermal variation of χm for both compounds satisfies the
(26) Nakamoto, K. Infrared spectra of inorganic and coordination compounds; John Wiley and Sons: New York, 1986.
(27) Gon˜i, A.; Pizarro, J. L.; Lezama, L.; Barberis, G. E.; Arriortua, M. I.; Rojo, T. J. Mater. Chem. 1996, 6, 421.
Table 6. Selected Bands (cm-1) Obtained from the IR Spectra of NH4NiPO4‚H2O, HNiPO4‚H2O, and (C7H15NH2)0.76HNiPO4‚H2Oa assignment
NH4NiPO4‚ H2O
HNiPO4‚ H2O
νst (N-Hfree)
3590(m)
νst(O-H) and νst(N-Hbridge)
3390(s) 3210(m) 3060(w)
2δ(N-H)
2930(w)
3500(s) 3450(s) 3380(s) 3360(s) 3280(s) 3230(s)
νst(C-H) δ(N-Hfree)
νst(PO4)
1570(m) 1470(m) 1440(m) 1380(w)
1470(w) 1420(w) 1400(m) 1390(w) 1250(m)
1470(m) 1430(w) 1390(w)
1085(s)
1135(s) 1097(s) 1055(s) 1025(s) 1005(m) 943(m)
1094(s)
1049(s) 950(s) 625(m)
δ(PO4) 560(m) a
3340(s) 3250(m)
2950(m) 2915(m) 2850(m) 2δ(N-H) and νst(C-H)
2770(w)
δ(PO-H) and δ(N-Hbridge)
(C7H15NH2)0.76HNiPO4‚H2O
690(w) 596(m) 550(m) 540(m)
1049(s) 940(s) 625(m) 560(m)
w ) weak, m ) medium, s ) strong.
A New Layered Compound, HNiPO4‚H2O
Chem. Mater., Vol. 8, No. 5, 1996 1059
Figure 8. TGA curves of the HNiPO4‚H2O compound and different alkyl amine intercalated derivatives: (a) HNiPO4‚H2O, (b) propylamine, (c) butylamine, (d) pentylamine, (e) hexylamine, and (f) heptylamine.
decrease in the χmT values are indicative of antiferromagnetic exchange couplings in both compounds. Considering that the arrangement of the Ni atoms inside the layer of HNiPO4‚H2O exhibits not a very great deviation from the square-planar geometry, and taking into account that the layer structure is not modified during the intercalation process, an antiferromagetic square-planar Heisenberg system of S ) 1 could be considered as the model in both cases. The validity of the proposed 2D Heisenberg model was tested by using the expressions described by De Jongh and Miedema28 for the calculation of the exchange coupling value, |J|/ k: Figure 9. Thermal variation of χm and χmT for HNiPO4‚H2O: the circles are the experimental values, and the full lines represent the theoretical values for 2D and 3D Heisenberg systems.
kT(χmax)/|J|(S + 1)S ) 2.20
(7)
χmax|J|/Ng2β2 ) 0.0521
(8)
The |J|/k values calculated for (C7H15NH2)0.76HNiPO4‚ H2O by using the expressions 7 and 8 were 2.50 and 2.63 K, respectively. These values are very close and confirm the validity of the 2D magnetic model proposed for the intercalated compound. In this way, the analytical expression reported by Lines29 for a 2D square-planar Heisenberg system from a high-temperature expansion series studied by Rushbrook and Wood29 was used to study the magnetic behavior of this compound:
Figure 10. Thermal variation of χm and χmT for (C7H15NH2)0.76HNiPO4‚H2O: the circles are the experimental values and the full lines represent the theoretical values for a 2D Heisenberg system.
Curie-Weiss law at high temperatures, and exhibits a χmax centered at 12.0 and 11.0 K temperatures for HNiPO4‚H2O and (C7H15NH2)0.76HNiPO4‚H2O, respectively. These results together with the continuous
χ) 2Ng2β2 [1 + Ax + Bx2 + Cx3 + Dx4 + Ex5 + Fx6]-1 3kT (9) where x ) J/kT, A ) 5.333 333, B ) 9.777 778, C ) 9.481 482, D ) 19.061 73, E ) 45.089 71, and F ) 25.463 92. This equation is valid only up to a certain temperature, since it becomes not quantitative when the (28) De Jongh, L. J.; Miedema, A. R. Adv. Phys. 1974, 23, 1. (29) Lines, M. E. J. Phys. Chem. Solids 1970, 31, 101.
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ratio kT/J nears unity; due to that only a small number of terms are known (six) in the infinite serie. Excellent agreements between the observed and calculated values of χm and χmT vs T of (C7H15NH2)0.76HNiPO4‚H2O have been obtained, considering g ) 2.15 as usual value for octahedral Ni(II) ions (see Figure 10). The magnetic exchange value J/k is -2.63 K. In the case of the HNiPO4‚H2O compound two different |J|/k values, 2.72 and 3.37 K, were obtained by using the expressions 7 and 8, respectively. These results indicate a not good agreement between the experimental data and the used model. In the same way, it was not possible to fit the experimental values using expression 9 for a 2D magnetic model (see Figure 9). These results could be indicative of the presence of 3D magnetic interactions in this compound. To corroborate this hypothesis, the magnetic behavior of this phase was also studied with a 3D Heisenberg model, using Rushbrook and Wood’s30 expression 9, where A ) 8, B ) 14.666 67, C ) 14.2222, D ) 61.185, E ) 162.449, and F ) 1127.96. As can be seen in Figure 9, it was not possible to otain good agreements between the experimental data and the three-dimensional model. So, these results lead us to deduce that a 2D-3D intermediate model to explain the magnetic behavior for the HNiPO4‚H2O compound might be necessary. Considering the structural features in the HNiPO4‚H2O compound, at least two different intralayer exchange pathways could be deduced. One of them involves the dx2-y2 orbitals from the NiO6 octahedra linked through an oxygen atom belonging to the phosphate group, leading to an antiferromagnetic coupling. The second pathway implies the PO4 group as was observed for other transition-metal phosphates,31 giving ferromagnetic interactions. So, the intralayer magnetic system could be described as two interpenetrate ferromagnetic nets which are antiferromagnetically coupled. Magnetic interactions along the b axis between the dz2 orbitals of two nickel(II) ions of different layers could be also considered. This pathway involves the hydrogen bonds between the facing HPO4 groups and leads to antiferromagnetic interactions. In this way, the 2D simple model does not explain the magnetic behavior of the layered HNiPO4‚H2O compound and an increased 3D interlayer ordering appears when the temperature decreases due to the presence in the structure of hydrogen bonds between the layers, which is indicative of 2D-3D magnetic interactions in this compound. These results have been also observed in other layer related compounds such as NH4MPO4‚H2O (M ) Mn, (30) Rushbrook, G. S.; Wood, P. J. Mol. Phys. 1963, 6, 409. (31) Lezama, L.; Suh, K. S.; Villeneuve, G.; Rojo, T. Solid State Commun. 1990, 79, 449.
Gon˜ i et al.
Ni).32,33 In the case of the intercalated (C7H15NH2)0.76HNiPO4‚H2O compound, the structure of the inorganic sheet remains unchanged and the interlayer distance becomes nearly 25 Å, making negligible the interlayer interactions, which favors the 2D intralayer antiferromagnetic interactions as were observed in the magnetic study. No indication of long-range magnetic order is observed for this compound above 2 K, which is the lower temperature reached in the experimental measurements. Concluding Remarks The new layered nickel phosphate HNiPO4‚H2O has been obtained by the deintercalation of ammonium in NH4NiPO4‚H2O, without loss of the water molecule. It is possible to insert different alkylamines in the interlayer space of HNiPO4‚H2O by acid-base reaction between the amines and inorganic sheets. However, during the intercalation and deintercalation processes the layered crystal structure of the inorganic matrix as well as the morphology of the microcrystals is basically maintained. This fact leads us to assert that they are topotactic reversible reactions. The disposition of the alkyl amine chains in bilayers with a tilt angle of 53.2° inside the inorganic host is determined by the hydrogen bonds established between the amino group of alkyl amine and the surface oxygens in the nickel phosphate sheet. The strength of these bonds restrains the magnitude of the volume occupied by amines in available space between layers, according to the extent to the intercalation in each case. The comparative study of magnetic properties of the HNiPO4‚H2O and (C7H15NH2)0.76HNiPO4‚H2O phases confirmed the hypothesis of the presence of essential 2D antiferromagnetic intralayer interactions in the last compound and an increase of the 3D magnetic character when the temperature is decreasing in the case of HNiPO4‚H2O. Acknowledgment. This work has been carried out with the financial support of the Eusko JaurlaritzaGobierno Vasco (PGV-9239, PI-9439) which we gratefully acknowledge. We also wish to thank Dr. X. Solans and J. Bassas (Universidad de Barcelona) for the measurements of X-ray diffraction pattern of HNiPO4‚ H2O. J.R. thanks the “Comissionat per a Universitat i Recerca” de la Generalitat de Catalunya for financial support (GRQ93-8036). CM950539+ (32) Carling, S. G.; Day, P.; Visser, D. Solid State Commun. 1993, 88, 135. (33) Carling, S. G.; Day, P.; Visser, D. Inorg. Chem. 1995, 34, 3917.
