Synthesis, crystal structure and spectroscopic properties of the NH,NiPO,vzH,O (n = 1,6) compounds; magnetic behaviour of the monohydrated phase

Aintzane Goiii," JosC Luis Pizarro,b Luis M. Lezama,' Gaston E. Barberis,' Maria Isabel Arriortuab and Teofilo Rojo*" "Dpto. Quimica Inorgcinica (ISEM), Universidad del Pais Vasco, Bilbao 48080, Spain bDpto. Mineralogia y Petrologia (ISEM), Universidad del Pais Vasco, Bilbao 48080, Spain

NH,NiPO4-6H,O and NH4NiP04.H,0 have been obtained by adding different concentrations of H$O, to dilute solutions of NiCl2-6H20 with special attention to the control of the pH in the solvent medium, which was regulated by addition of NH40H. The NH,NiP04.6H,0 compound crystallizes in the orthorhombic Pmn2, space group with cell parameters a = 6.9032( 8), b=6.0907(5) and c= 11.1402(8) A, V=468.39(7) A3, 2 = 2 , R=2.3 and R,=2.3%. The structure is three-dimensional and consists of Ni[O(W)]6 octahedra [O(w) = oxygen from a water molecule] linked to PO, and NH, tetrahedra by hydrogen bonds. All polyhedra are quite regular in this compound. The NH4NiPO4*H20 phase crystallizes in the P m r ~ 2 ~ space group with cell parameters a = 5.5698(2), b = 8.7668( 2) and c = 4.7460( 2) A. The structure of this compound has been refined with the Rietveld method using the coordinates of the KMnPO,.H,O phase as a starting model. The final residual factors were R , = 7.37, RB = 2.68%. The structure is formed from sheets of distorted NiO, corner-sharing octahedra bridged through the oxygen atoms of the phosphate tetrahedra. These layers are pillared along the b direction and are interconnected by hydrogen bonds with the NH4+ cations, which are inserted between the sheets. The spectroscopic properties of both compounds are in good agreement with the symmetry observed in each phase. The values of the nephelauxetic ratio, p, are 0.89 and 0.94 for the hexahydrated and monohydrated compounds respectively. Magnetic susceptibility and specific heat results obtained for NH,NiPO4-H20 show an essentially two-dimensional antiferromagnetic exchange coupling, which becomes of a more three-dimensional behaviour with decreasing temperature.

Nickel(I1) phosphates offer a considerable number of different structures which can give rise to practical applications such as ion exchange, ionic conductivity, etc.,' and interesting magnetic properties. The choice of synthetic method is important, as it can lead to the production of several phases with predetermined structure types. Water solution chemistry procedures can gener- ate nickel phosphates with a variable number of H 2 0 molecules coordinated to the metal. This number depends on the reaction conditions, such as pressure and temperature. The affinity of Ni" and other divalent transition metals to coordinate water molecules sometimes prevents other ligands from forming intermetallic bridges. When there is a large number of water molecules in the formula of a compound, most of the coordi- nation positions of the metal are occupied by water molecules, leading to three-dimensional (3D) structures built via hydrogen

However, a small number of water molecules in the coordination sphere of the metal allows the bonding of other groups as PO,, which can form strong intermetallic bridges. This can lead to the formation of compounds with interesting magnetic behaviour. 5,6

The well known series of compounds M'M"PO,.H,O (MI = NH,, K; M"=Mn, Fe, Co, Ni),7 has been of interest because of the strongly defined layered crystal structures of these phases. The divalent metal ions are bridged by the oxygen atoms of the phosphate groups, leading to the formation of magnetic planes separated by NH,' ions. This arrangement should afford interesting two-dimensional (2D) magnetic inter- actions. However, the study of the magnetic behaviour of the manganese compound' showed a crossover in the power law dependence of the magnetization with temperature. This result was attributed to a crossover in the lattice dimensionality from 2D to 3D at low temperatures. On the other hand, magnetic and Mossbauer studies of the NH4FeP0,.H,0 c o m p o ~ n d ~ - ' ~ showed, at low temperatures, two different regimes. One of them involves a short-range ordered region at 70 > T/K > 26 and the second is a long-range ordered region below 26 K, with an uniaxial antiferromagnetic state with the axis nearly parallel to the layer stacking direction. These results may be

explained by the existence of superexchange pathways between the magnetic layers through the intercalated NH,' ions.

In this article, we report the phase diagrams in water solution obtained for the [Ni2+ /H,PO,/NH,OH] system, in which the presence of three different nickel@) phosphates has been observed: Ni3( P04)2*8H20, NH4NiPO4.H2O and NH4NiPO,.6H,O. The Ni3( P04)2.8H20 compound has the vivianite structure and has been studied extensively." In this work, the other two ammonium nickel phosphates are ana- lysed, and their magnetic properties and the specific heat of the monohydrated phase are discussed.

Experimental Synthesis

A systematic investigation of the reactions in water solution for the [Ni2+/H3P04/NH40H] system was carried out. The phase diagrams obtained are shown in Fig. 1. The precipitates were characterised by elemental analysis. The nickel and phosphorus content were determined by atomic absorption spectroscopy (Perkin-Elmer 3030B) and thermogravimetric techniques, respectively. Characterisation by X-ray powder diffraction was also performed. As can be seen in Fig. l(a), mixtures of phases exist in different pH ranges and Ni :P ratios, which may due to the overlap of areas of thermodynamic equilibrium or the low solubility of the nickel phosphates.

The phase diagram undergoes important variations when the precipitate is maintained in contact with the solvent medium at room temperature and pressure for a week [see Fig. l(b)]. In this case, mixtures of phases and the NH,NiPO,-H,O phase are not observed. This result indicates that the solid $ solution equilibrium favours the evolution of this species to form the NH4NiP04*6H20 compound which is thermodynamically more stable.

The title compounds were synthesized by adding NiCl2-6H,O ( 4 x mol dm-3) to different solutions of H,PO, and NH,OH at 90°C. Green and yellow polycrystal-

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I :40 1:40 - 5 130 1:30 -

1:20 - z

1:lO . . . . . . . . 1:lO - . . . . . . 1:20 . . . . . . . . , . . . . . .

1:1 - . . . . . , . . . . . . . . . . 1 1 I

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 7 A S 6 7 8 9 1 0 1 1

NH4NiP0,.6€120 0 Ni3(P0,).8H,0 0 No precipitate

Fig. 1 Phase diagrams in water solution obtained for the [NiZf/H,PO,/NH4OH] system: (a) the precipitate was filtered off immediately; (b) one week after synthesis (see text)

line samples were obtained for NH,NiP0,.6H20 and NH,NiPO,-H,O respectively, but only recrystallization of NH,NiP0,.6H20 from water solution gave crystals of X-ray diffraction quality.

Structure refinement of NH4NiP04*6H20j-

A prismatic single crystal of NH,NiP0,.6H20 (0.12 x 0.15 x 0.22 mm3) was selected for structure determination. Preliminary cell dimensions were calculated by oscillation and Weissenberg photographs. Diffraction experiments were per- formed on an Enraf-Nonius CAD-4 automatic diffractometer using graphite-monochromated Mo-Ka radiation. The orien- tation matrix and the final lattice constants were determined from 25 high-angle reflections ( 14 < 28 < 27"). Crystal and data collection parameters are summarized in Table 1. Two standard reflections were recorded every 2 h. Their intensities showed no statistically significant change over the duration of the data collection. Lorentz and polarization corrections, as well as an empirical absorption correction (DIFABS program12) were applied to the data.

The crystal structure was refined using as the starting structural model the fractional coordinates and the space group of the struvite mineral, NH,MgP04.6H20,13-15 by the full-matrix least-squares method ( SHELX76I6). The origin was fixed by keeping the z coordinate of the P atom constant. Further anisotropic refinements followed by a difference Fourier synthesis allowed the location of all H atoms. Atomic scattering factors were taken from the International Tables for X-ray Crystal10graphy.l~ All non-hydrogen atoms were refined with anisotropic thermal parameters and the hydrogen atoms with isotropic ones. The final difference Fourier map showed no peaks higher or deeper than k0.38 e A-3.

The final fractional atomic coordinates and displacement parameters are given in Table 2. Selected interatomic distances are shown in Table 3. The geometric calculations were per- formed with the programs PARST" and BONDLAI9 and molecular illustrations were drawn with the ATOMS2' program.

Structure refinement of NH4NiP04*H,0j-

The X-ray powder diffraction pattern of NH,NiPO,.H,O was measured at room temperature on a Stoe (Darmstadt) diffractometer using Ge-monochromated Cu-Ka, radiation in reflection mode. The data were collected in the 28 range 5-1 10" in steps of 0.02" (Table 1). The Rietveld refinement of the crystal structure was undertaken by starting from the structural

t Single-crystal data are available from Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany.

model of KMnP04.H20.21 The structure was refined using the program FULLPROF2' with a pseudo-Voigt function used to model the peak shape. A March model for preferred orientation correction was applied because of the plate habit of the microcrystals, parallel to the (010) plane, as well as an asymmetry correction to the low-angle reflection. Scale and background variables were refined initially followed, in sub- sequent iterations, by the zero point of 28, the cell constants, the peak-shape parameters and the atomic and isotropic thermal parameters. The origin was fixed by keeping the z coordinate of the Ni atom constant. The residual factors dropped by successive refinement cycles to R,, = 7.37% and RB = 2.68%.

Fig. 2 shows the observed, calculated and difference powder X-ray diffraction (XRD) patterns of NH,NiPO,.H,O. The final fractional atomic coordinates, equivalent displacement parameters and selected interatomic distances are given in Tables 2 and 3.

Physical measurements

IR spectra were obtained using KBr pellets (0.5%) on a Nicolet FTIR 740 spectrophotometer in the 4000-400 cm-I region. Reflectance spectra were measured at room temperature on a Cary 2415 spectrometer in the range 5000-40 000 cm-'. Thermogravimetry (TG) measurements were carried out with a Perkin-Elmer 7 system. Crucibles containing 20 mg of sample were heated at 5°C min-' under a nitrogen atmosphere. Magnetic measurements were performed on a polycrystalline sample using a Quantum SQUID magnetometer, in the 300-2K temperature range. The magnetic field used for the magnetic measurements was 1000 G. Specific heat measure- ments were carried out using a quasi-adiabatic method, in the temperature range 1.5-30 K. The calorimeter was made of an adiabatic chamber with a germanium resistance thermometer and an evaporation deposit Cr/Ti heater. The sample holder was suspended with a Nylon thread.

Results and Discussion Crystal structure of NH4NiP04.6H20

The crystal structure of NH,NiP0,.6H20 consists of isolated Ni[O(w)16 octahedra bonded to the PO, and NH, tetrahedra by hydrogen bonds (Fig. 3). The Ni[O(w)16 octahedra are fairly regular, with a range of Ni - 0 (w) distances from 2.092( 5) to 2.040(3) A, and O(w)-Ni-O(w) angles ranging from 95.9(1) to 87.8(1)". The PO, tetrahedra show a high local symmetry caused by the special position of the P atom, located in a mirror plane. The P-0 distances with a mean value of 1.535A do not show important variations (< la) , and the

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Table 1 Crystallographic data for the NH,NiPO,.nH,O (n = 1,6) compounds

NH4NiP0,.6H,O NH,NiPO,.H,O

Mr crystal system sp!ce group (no.) a/+ b/+ CIh VIA3 z F (000) Pobsli? cm - Pcalclg cm - TIK p/cm - diffractomete; radiation (A/A) scan type h,k,l range 28 rangeldegrees reflections collected unique reflections reflections with 13 2.541) no. of parameters R R w

279.8 orthorhombic Pmn2, (31) 6.9032( 8) 6.0907( 5 ) 11.1402( 8) 468.39( 7) 2 292 1.96(3) 1.98 295 22.81 Enraf-Nonius CAD4 Mo-Ka (0.71069) wf28 9, f8 , k15

3035 77 1 675 105 0.023 0.023

4-60

189.74 orthorhombic Pmn2, (31) 5.5698(2) 8.7668(2) 4.7460( 2) 231.75(1) 2

2.68( 5 ) 2.72 295

Stoe (Darmstadt) Cu-Ka, (1.5406) w/28

5-110

R P 0.052 R W P 0.074 R B 0.027

Table 2 Fractional atomic coordinates and equivalent displacement parameters (A2) for the NH,NiPO,.nH,O (n = 1,6) compounds

NH,NiPO, .6H,O NH,NiPO,.H,O

atom X Y Z Beqa/A2 atom X Y z B,,"/A2

0 0 0 0 0 0.1826( 4) 0 0 0.2194(4) 0.2055( 4)

0.37285( 9)

0.3696( 10) -0.00548( 18)

-0.0222(5) - -0.2379(6)

0.11 57( 4) 0.6779( 7) 0.0661(7) 0.2675( 5 ) 0.4785( 5 )

0.37222( 13) 0.00190 0.7347( 6)

0.0557( 3) 0.0443 (3) 0.2907( 5 ) 0.4566( 5 ) 0.2639( 3) 0.491 3 (3)

- 0.1355 (4)

1.62(1) Ni 0 - 0.0189( 2) 0.00427 1.26( 2) 1.47(2) P 0 0.1902(2) -0.4278(8) 1.26(2) 2.93(11) N 0 0.5290(6) 0.1255( 15) 1.26(2) 2.14(7) 0(1) 0 0.1624(5) -0.7481( 13) 1.26(2) 2.03(7) O(2) 0 0.3659(4) -0.3628( 12) 1.26(2) 2.07(5) O(3) 0.2239(9) 0.1135(3) -0.2779( 10) 1.26(2) 3.24(12) O(w) 0 -0.2010(5) -0.3015( 12) 1.26(2) 2.91 (10) 2.20(6) 2.72( 7)

Table 3 Selected bond lengths (A) and angles (degrees) for the NH,NiPO,.nH,O (n = 1,6) compounds

NH,NiP0,.6H20 NH,NiPO,.H,O

Ni coordination octahedra Ni- 0 (w 1 ) 2.068(5) Ni-O( l)b 1.977( 5 ) Ni - 0 (w2) 2.092(5) Ni-O(w) 2.158( 5 ) Ni-O(w3)/O(w3)" 2.040( 3) Ni- O( 3)/0( 3 r 2.172(4) Ni- 0 (w4)fO (w4)" 2.046( 3) Ni-O( 3)d/0( 3)" 2.023( 5 ) 0-Ni-0 (ca. 90") 90(2) 0-Ni-0 (ca. 900) 90(7) 0-Ni-0 (ca. 180") 177(2) 0-Ni-0 (ca. 180") 169(5)

phosphate tetrahedra P-O(1) 1.534(5) P-0(1) 1.540( 7) P-O(2) 1.538(4) P-0(2) 1.57 1 (4) P-O(3)/O(w3)" 1.535(2) P-0(3)/0(3)' 1.593( 5 ) O( 1)-P-0(2) 109.2(1) 0(1)-P-0(2) 110.4( 3) O( 1)-P-0(3)/0(~3)' 109.8(2) O(l)-P-0(3)/0(3~ 112.0(2) O( 2) - P- O( 3)/0(w3 )" 108.8 (2) O( 2) - P- O( 3)/0( 3 r 109.1 (2) 0(3)-P-0(3)" 110.4(1) 0(3)-P-0(3)' 104.2(3)

ammonium tetrahedra and hydrogen bonds N-H 0.88(2) (hydrogen atoms not located) H-N-H 109( 3) N-acceptor 2.8(2) N.- .0(2) 2.85( 1) H- donor - H 107(5) 0(2)*. .N..-0(2) 109( 18)

Symmetry codes: a -x, y , z; b ~ , y , l + z ; ' -x, y , z; d ~ - + 7 - Y7 s+z; =+-x, --y, + + z .

0-P-0 angles are in the range from 108.8( 1) to 110.4( 1)O. So, the PO4 groups can be described as low-distorted tetra- hedra. The NH4+ tetrahedFa are rather regular too, with mean N-H distances of 0.88A and mean H-N-H angles of 109( 3)".

The 3D structure of the title compound is formed by a complicated scheme of hydrogen bonds established by the water molecules and ammonia groups. The hydrogen- bonding arrangement of the four independent water molecules in the structure contains eight different bonds. Seven of them are among the strongest hydrogen bridges given by the H 2 0 molecule in crystalline hydrates, with values ranging from 2.607(4) to 2.691(4) A. The eighth bond, O ( w l ) . - . O ( ~ 2 ) ~ (f= x, 1 + y , z ) , however, is near to !he upper limit for this type of bond,23 with a value of 3.00(7) A. The hydrogen bridges of the ammonium group are rather different, ranging from a shorte!t one N.. .O(l)g (g=x, y, ++z) with a distance of 2.790(7)A, to two bifurcated weak bonds, N-..O(w4)/0(w4)', and N...O(w3)j/O(w3)" ( h = + - x , 1-y, ++z; i = -x, y , z ; j=x-3 , 1-y, z++), with distances of 3.131(7) and 2.957(6) A, respect- ively. No bond was detected between N-..O(w2), in contrast to the model proposed for NH4MgP04.6H20r1' because the observed distance, 3.607( 8) A, is too long.

Crystal structure of NH4NiP04*H20

NH4NiP04.H20 presents a layered structure, with nickel phosphate sheets separated by NH4+ cations, as shown in

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Fig. 2 Observed, calculated and difference powder XRD pattern of NH,NiPO,*H,O

Fig. 4 View of the crystal structure of NH,NiPO,.H,O

The PO4 groups can be described as distorted tetrahedra, even though the special position of the P atom is located in a mirror plane as in NH4NiP04.60H,0. The P-0 distances with a mean value of 1.57(2)A range from 1.540(7) to 1.593(5) A, and the 0 - P - 0 angles are in the range 104.2(3)-112.0(2)". The PO4 groups are connected to the nickel octahedra by three oxygen vertices, 0(1), 0(3), 0(3)', with one common edge, 0(3) . . .0 (3 )". The fourth oxygen corner, 0 (2 ) , is directed towards the interlayer space.

Fig. 3 View of the crystal structure of NH,NiP0,-6H20

Fig. 4. The inorganic layers are formed by the NiO, corner- sharing octahedra, crosslinked by the phosphate tetrahedra.

The Ni(O), octahedra are highly distorte!, with a range of Ni-0 distances from 1.977(5) to 2.172(4) A, and 0-Ni-0 angles ranging from 98.1(2) to 86.4(2)". The four equatorial

- - The NH4+ ions inserted between the inorganic layers estab-

lish hydrogen bonds with the symmetrical O(2) atoms of the four adjacent phosphate groups of two facing layer!. The N-0(2) distances range from 2.733(8) to 2.933(2)A, and the 0(2)-P-0(2) angles are in the range 98.5(2)-143.4(2)".

bonds Ni-0(3)/o(3)~/o(3)d/0(3)e are 2.172(4), 2*172(4), Thermogravimetric study 2.023(5) and 2.023(5) A respectively (Table 3) are responsible for the. 'crosslinking in the sheets. The axial 'oxygen Oj l )b is provided by the PO4 tetrahedron [Ni-O( l)b 1.977(5) A] and the remaining axial vFrtex is the O(w) of the coordinated water [Ni-O(w) 2.158(5) A].

The thermogravimetry and differential thermogravimetry (TG and DTG) curves of the title compounds obtained under a nitrogen atmosphere from room temperature to 600 "C are represented in Fig. 5. The thermal decomposition study of

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I I I I I I 1 100 200 3 00 LOO 500

T I T

Fig. 5 TG (-) and DTG (---) curves of (a) NH4NiP04-6H,0 and (b) NH4NiP04.H20

NH4NiP04-6H20 shows only one endothermic step in the temperature range 80-300 "C, which corresponds to the mass loss of one ammonia and six water molecules (exptl. 44.0%, calc. 44.72%).

The TG study of NH4NiP04.H20 shows two non-solved steps, in the temperature range 170-300°C, with a total mass loss of 19%. This loss can be attributed to the water and ammonium molecules present in the structure (calc. 18.5%). The two overlapped steps can be solved by using atmospheres of ammonia or humid

Finally, a gradual mass loss is observed in the TG curves of both compounds to give nickel(I1) pyrophosphate, Ni2P20,, at temperatures higher than 400 "C.

Spectroscopic properties

Selected bands obtained from the IR spectra of NH4NiP04.6H20 and NH4NiP04.H20 are given in Table 4. Both phases present a set of bands in the range 3500-3000cm-', which can be assigned to the stretching vibration modes of the OH and NH groups, in good agreement with the presence of the NH4+ group and the water molecules coordinated to the metal in the structures. In the case of NH4NiP04.6H20, these bands are strong because of the presence of six water molecules in the compound.

The IR spectra of both compounds show the bending mode of the 0 - H group around 1615cm-'. The bands which appear at 1470 and 1440cm-' in the spectrum of NH4NiP04.H20 and at 1455 cm-' in the NH4NiP04-6H20 phase can be ascribed to the bending mode of the NH4+ group. In the case of the hexahydrated compound this band is not split because of the NH4+ tetrahedra present high symmetry in the structure. However, for the monohydrated

compound a splitting is observed which is indicative of the existence of distortions in the NH, + polyhedra.

The vSt( PO4) bands appear in the range 1100-950 cm- ' for NH4NiP04.H20. These bands are split owing to the distortion of the PO4 tetrahedra observed in the structure. However, in the hexahydrated compound only one band assigned to this vibration mode is present at 1015 cm-', which indicates that the phosphate tetrahedra are quite regular, in good agreement with the structural features. These results are confirmed by the presence of the PO4 bending modes which are located at 625-560 cm-', and 575 cm-' for the monohydrated and hexa- hydrated compounds, respectively.

The reflectance spectra of both compounds exhibit three strong absorptions corresponding to the three spin-allowed transitions expected for a d8 ion in octahedral symmetry C3A2, j3T2,, 3A2, j3T1,, 3A2, +3Tl,(P)], and two more sig- nals which can be attributed to the spin-forbidden tran- sitions 3A2g -*'Eg and 3A2, +'T2,, respectively. For the NH4NiP04.6H20 compound strong absorption bands were observed at 8400, 13800 and 25000 cm-', whereas in the spectrum of the monohydrated compound these signals are displaced to lower frequencies, 7200, 12900 and 23900 cm-', as a consequence of the distortion in the NiO, octahedra. The values determined for the lODq (8400 cm-') and Racah param- eters, B (918 cm-') and C (4229 cm-'), corresponding to the NH4NiP04.6H20 compound are typical for an Ni2+ ion in an octahedral environment of six water with a value C/B=4.61 whcih close to the value for the ideal octahedral geometry (C/B = 4.46). However, for the NH4NiP04.H20 compound these values are lODq = 7200cm-', B=967cm-' and C=3720cm-', which give a value of C/B = 3.85, characteristic of the Ni2+ ion in a distorted

Table 4 Selected bands (v/cm-') obtained from the IR spectra for the NH4NiP04~nH,0 (n = 1,6) compounds

NH,NiP04 .H,O 3390m 3210w 3060w 2930w

NH4NiP04 .6H,O 3450m 32 15s 3100s 2935s

1610w

1620w

1470m 1 lOOm 1440m 1085m

1050s 950s

1455m 1015s

625m 560m

575s

w =weak, m =medium, s = strong.

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octahedral environment These results are in good agreement with the structural data of both phases

6 -

5 - L z 4 - 3 3 0 2 -

Magnetic properties of the NH,NiPO,*H,O compound

The thermal variations of both the magnetic susceptibility and x,T for the NH4NiP04 H 2 0 compound are shown in Fig 6 The thermal evolution of xrn satisfies the Cune-Weiss law at high temperatures with C = 1 2 emu K mol-I and 8= - 27 44 K, and exhibits a xmax centred at 14 0 K This result, together with the continuous decrease in the x,T values, is indicative of antiferromagnetic exchange couplings in the compound

Owing to structural findings of this phase, two different antiferromagnetic models could be considered for the analysis of the magnetic behaviour the 2D square-planar Heisenberg system for S = 1, considering only magnetic interactions between the nickel(1r) ions arranged inside the layers, or the 3D Heisenberg system for S = 1, which also considers inter- actions between layers through the NH4+ groups In the first case, the validity of the 2D Heisenberg model was tested by using the expressions described by De Jongh and M ~ e d e m a ~ ~ for the calculation of the exchange coupling value, J /k

( a )

-

Xmax I J I Ns2P2 =00521

':.L . , . . I . . . . I . , . . I ' . . . I . . . , *

Two different J /k values, 3 18 K and 3 68 K, were obtained using expressions (1) and (2), respectively These results indi- cate no agreement between the experimental data and the used model In the same way, it was not possible to fit the xm experimental values (Fig 6) using the analytic expression reported by Lines28 for a 2D square-planar Heisenberg system from a high temperature expansion series studied by Rushbrooke and Wood 29

x= ~ 2Ng2P2 [ 1 + A X + Bx2 + Cx3 + Dx4 + Ex5 + Fx6] -' (3) 3kT

where x = J/kT, A = 5 333333, B = 9 777778, C = 9 481482, D = 19 06173, E = 45 08971 and F = 25 46392

On the other hand, the magnetic behaviour of this phase was also studied with a 3D Heisenberg model using the expression of Rushbrooke and given in eqn (3), with the values A = 8, B = 14 66667, C = 14 2222, D = 61 185, E = 162 449 and F = 1127 96 As can be seen in Fig 6 these results show no good agreement between the experimental data and the 3D model

In order to clarify the magnetic behaviour of NH4NiP04 H20, a study of specific heat for this compound

0 025

- 002 L z 0015

z N

0 01

0 0 0 5

0 t 0 50 100 150 200 250 300"

TIK

0

- a $

E Y

6 3 E ," t-

4 N

2

Fig. 6 Thermal variation of xrn and x,T for NH4NiP04 H 2 0 the circles are the expenmental values and the full lines represent the theoretical values for 2D and 3D Heisenberg systems

has been also carned out The thermal variation of the magnetic contribution (C,) to the specific heat, calculated by substrac- tion of the lattice contribution to the expenmental values, showed a broad maximum centred at 8 8 K [Fig 7(u)] The magnetic entropy calculated for the reached maximum tem- perature gave a value of AS= 8 46 J rno1-l K-l , which is close to the theoretical value for A S = R ln(2S+ 1) =9 13 J mol-' K-', with S = l [Fig 7(b)] 59% of the total entropy is acquired above the maximum (at 8 8 K) indicating a high degree of short-range interactions

Taking into account that the 3D ordering causes in the magnetic specific heat a A-type second-order transition, the rounded maximum observed in Fig 7(a) for NH,NiPO, H 2 0 cannot be interpreted as being due to the existence of an order of 3D character So, the existence of a 3D order at temperatures higher than 1 8 K, which is the lower temperature limit reached in the measurements, should be discarded However, this fact does not exclude the existence of weak antiferromagnetic 3D interactions In this way, it was not possible to fit the exper- imental values of the thermal variation of C, by using the 2D model for magnetic specific heat of Rushbrooke and as was also observed in the case of the thermal variation of magnetic suscepti bill ty

Considering the structural data, the exchange pathways of the 3D interactions must involve the NH4 groups This path- way implies that the hydrogen bonds between the oxygen atoms of the P O roups are directed toward the interlayer space and the NH4 ion inserted between the layers, leading to weak antiferromagnetic interactions With respect to the intralayer magnetic exchange, two different pathways could be deduced One of them involves the d,z-yz orbitals from the N106 octahedra linked through the O(3) atoms (see Fig 4), leading to antiferromagnetic couplings The second pathway involves the PO roup as was observed for other transition metal phosphate:! giving ferromagnetic interactions

One can conclude that the 2D simple model does not explain the magnetic behaviour of the layered NH4NiP04 H 2 0 com- pound and that an increase of the 3D interlayer ordering occurs when the temperature decreases, which agrees with the results obtained for other related compounds 25 These results lead us to deduce that a 2D-3D intermediate model for the

4 g +

0 5 10 15 20 25 30 TIK

Fig.7 Thermal variation of (a) the magnetic specific heat, C,, and (b) A S for the NH4NiP04 H 2 0 compound

426 J Mater Chem, 1996,6(3), 421-427

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fitting of the magnetic behaviour of this compound might be necessary.

5 6

7 Conclusions

8 Two phase diagrams in water solution were obtained for the [Ni2+ /H3P04/NH40H] system, in which the presence of two ammonium nickel(I1) phosphates is observed: NH4NiP04-H20 and NH4NiP04.6H20. The crystal structure of the hexahy- drated compound consists of isolated Ni [0(w)l6 octahedra bonded to the PO4 and NH, tetrahedra by hydrogen bridges. It adopts a three-dimensional structure in which all polyhedra are linked together by a complicated scheme of hydrogen bonds. The NH4NiP04.H20 compound presents a layered structure, with nickel phosphate sheets separated by NH4+ cations. The inorganic layers are formed by the NiO, corner- sharing octahedra, crosslinked by the phosphate tetrahedra. The spectroscopic data for the hexahydrated compound con- firm the high symmetry of the NiO,, PO, and NH, groups, whereas the data for monohydrated compound show the existence of important distortions in these polyhedra. The antiferromagnetic behaviour observed in the layered NH4NiP04 .H20 compound cannot be explained by either the 2D or the 3D Heisenberg models because of the existence of weak exchange interactions between layers.

This work was carried out with the financial support of the Basque Government (PI-9439) which we gratefully acknowl- edge. (G. E. B., on sabbatical from UNICAMP (Brazil) acknowledges the Ministerio de Educacibn (Spain) and CNPq (Brazil) for financial help. We thank R. Kuentzler and Y. Dossmann (Strasbourg) for the specific heat measurements.

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Paper 5/04974A; Received 26th July, 1995

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