J. Phys. Chem. B 2004, 108, 20351-20354
20351
Magnetic Studies on a New Low-Dimensional Antiferromagnetic Iron Phosphate Sukhendu Mandal,†,| Mark A. Green,‡ Srinivasan Natarajan,†,| and Swapan K. Pati*,§ Framework Solids Laboratory, Chemistry and Physics of Materials Unit, and Theoretical Sciences Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Bangalore 560 064, India, and The DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K. ReceiVed: August 17, 2004; In Final Form: October 6, 2004
A quasi-one-dimensional three-legged iron phosphate ladder, containing iron atoms connected through F bridges, giving rise to a three-legged Fe core, which are linked through P atoms, has been synthesized. Magnetic susceptibility data show the characteristics of Curie behavior at low temperature, which is attributed to an impurity concentration of 0.46% made free by lattice imperfections. The magnetic susceptibility attains saturation far below room temperature, possibly due to very weak superexchange interactions along the chains. The observed room temperature magnetic moment corresponds to the spin-only moment of Fe3+ with S ) 5/2 and passes through a minimum of 4.16 µB per Fe at 40 K, before increasing again as T f 0. The magnetization as a function of the magnetic field shows complete polarization of the magnetic ions around 5 T at low temperature.
Introduction One-dimensional and quasi-one-dimensional spin systems have been investigated in great detail during the past two decades, due to the many interesting properties exhibited by them. Such systems undergo large quantum fluctuations leading to various interesting magnetic phenomena, such as a finite spin gap or its absence and short-range magnetic order in the ground state.1-4 The observation of a spin-Peierls transition in CuGeO3,5,6 a Haldane gap in Y2BaNiO57 and Ni(C2H8N2)2NO2(ClO4)[NENP],8 and gapped and gapless behavior in a homologous series of ladder systems, Srn-1Cun+1O2n (n ) 3,5,7, ...)4,9,10 has generated considerable interest in this area of research. Though the Haldane spin gap or its absence is purely of electronic origin, the spin-Peierls transition results from an interplay of the localized electrons and the lattice. The analysis using the O(3) nonlinear σ model and various numerical methods indicate that the antiferromagnetic spin ladders with an even number of legs have a finite spin gap, while the ladders with an odd number of legs, depending on their topological arrangements, are either gapped or gapless.11,12 It has been argued that the odd-legged ladders with open boundary conditions in the rung direction are gapless, while the cylindrical ladder (with the periodic boundary conditions) exhibits a finite gap. This is to say that the open boundary condition does not introduce any frustrations, and the system can be described effectively by an antiferromagnetic one-dimensional chain.13,14 In this paper, we report the synthesis and structure of a new three-legged one-dimensional ladder, which has open boundary conditions along the rung directions. The geometry is complex, and the antiferromagnetic pathways leading to the final lattice appear to have very weak superexchange interactions along the * Corresponding author. E-mail:
[email protected]. † Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research. ‡ The Royal Institution of Great Britain. § Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research. | Present address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India.
chains. Magnetic susceptibility measurements suggest that there is a finite concentration of impurity at very low temperatures. Iron phosphates of wide compositional and structural variety, prepared using hydrothermal methods, have been described and discussed during the past few years.15 The use of fluoride in the synthesis mixture gave rise to a variety of new materials.16,17 In many of these compounds, fluorine becomes part of the structure linking the metal centers. We have presently prepared a new iron phosphate, [H3N(CH2)3NH2(CH2)2NH2(CH2)3NH3][Fe3F6(HPO4)2(PO4)]‚3H2O, I. The structure consists of small segments of tancoite chains,18 connected by phosphate units, giving rise to a three-legged iron core. Experimental Section I was prepared by a hydrothermal reaction of iron acetylacetonate, [Fe(acac)3] (0.198 g), H3PO4 (85 wt %, 0.14 mL), HF (48 wt %, 0.08 mL), N,N′-bis(3-aminopropyl)ethylenediamine (0.21 mL), and water (2 mL), in a molar ratio of 1:4:4:2:200, carried out in a PTFE-lined acid digestion bomb (7 mL). The mixture, with an initial pH of ∼4, was heated at 150 °C for 96 h, resulting in large quantities of colorless needle-shaped crystals (yield 60% based on Fe). The final mixture did not show any appreciable change in the pH. A suitable single crystal was used to determine the structure by single-crystal X-ray diffraction. Crystal data for I: orthorhombic, space group I212121, a ) 9.9042(11) Å, b ) 12.8865(14) Å, c ) 19.783(2) Å, V ) 2524.9(5) Å3, Z ) 4, Mr ) 800.85, Dc ) 2.107 g mm-3, µ(Mo KR) ) 20.1 mm-3. A total of 7933 reflections were collected in the θ range of 1.89-28.28 and merged to give 3035 unique data (Rint ) 0.0445), 2743 of which with I > 2σ(I) were considered to be observed. Final R1 ) 0.0324, wR2 ) 0.0702, and S ) 1.044 were obtained for 192 parameters. The structure was solved and refined using the SHELXTL-PLUS program.19 I was characterized by powder XRD, TGA, and IR characterizations. Anal. Found: C, 11.89; N, 6.93; H, 4.32. Calcd: C, 11.99; N, 6.99; H, 4.25. Infrared (IR) spectra were recorded in the range 400-4000 cm-1 using the KBr pellet method. The IR spectra showed typical peaks. IR bands for I: ν(H2O) )
10.1021/jp0463013 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/02/2004
20352 J. Phys. Chem. B, Vol. 108, No. 52, 2004
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3515 cm-1, δ(H2O) ) 1600 cm-1, δa(NH) ) 1519 cm-1, δs(NH) ) 1240.7 cm-1, Fr(NH) ) 817.6 cm-1, δ(CH) ) 1478 cm-1, ν(P-OH) ) 917 cm-1, ν(Fe-Fterminal) ) 611.6 cm-1, ν(FeFbridgeing) ) 532.1 cm-1. A photoluminescence study using an excitation wavelength of 330 nm shows a strong emission at 402 nm. Similar behavior has been observed recently in a magnesium-substituted aluminum phosphate (UCSB-10Mg).20 Thermogravimetric analysis (TGA) in oxygen showed mass loss in two steps. In the first step around 100 °C, a mass loss of 4.1% was observed, which corresponded to the loss of two water molecules (calcd 4.5%). The second step was broad with a long tail starting at 250 °C. A mass loss of 37.6% corresponds to the loss of the amine and the condensation of the hydrogen phosphate along with the loss of the fluorine (calcd 40.87%). It is likely that the water molecules formed during the combustion of the organic molecules hydrolyze the fluoride in the framework, resulting in the formation of volatile hydrogen fluoride. The calcined sample is amorphous to powder XRD. Results and Discussion The asymmetric unit of I contains 21 non-hydrogen atoms and two crystallographically independent Fe and P atoms. Fe(1) and P(2) atoms occupy special positions with a site occupancy of 0.5. The Fe(1) is coordinated with four oxygen and two fluorine atoms, and Fe(2) has three oxygen and fluorine neighbors, with average Fe-O/F distances of 1.970 Å. The O/F-Fe-O/F bond angles are in the range 78.05(12)177.04(9)°. The average P-O distances and O-P-O bond angles are 1.534 Å and 109.4°, respectively. The terminal P(1)O(9) with a distance of 1.589(3) Å is a P-OH linkage, as seen from the difference Fourier maps. The structure of I is built up from Fe(1)O4F2, Fe(2)O3F3 octahedral, and P(1)O3(OH) and P(2)O4 tetrahedral units linked through their vertexes. Thus, Fe(1) is connected to two Fe(2) ions through the fluorine bridges and two P(1) and P(2) units through the oxygen atoms. Fe(2), on the other hand, is connected to Fe(1) by a fluorine bridge and to P atoms through the oxygen atoms [two P(1) atoms and one P(2) atom] and possess two terminal Fe-F linkages. This type of connectivity gives rise to two distinct tancoite-type units, each Fe atom being connected to two different P atoms and also linked through F- ions. The tancoite units are mutually connected together to form the structure of I. The arrangement of the three FeIII centers and their connectivity to the neighboring units through the phosphate tetrahedra resemble a three-legged ladder (Figure 1). The view down the ladder reveals that four chains surround the central amine molecule and also interact through the water molecules. This type of structure, to our knowledge, has not been made before. A large number of hydrogen bond interactions involving the hydrogen atoms of the amine and the oxygen and fluorine atoms of the chain have been observed in I. Most of the donor (D) and acceptor (A) distances are in the range 2.670(3)3.472(5) Å, and the D-H and A angles are >150°. The magnetic susceptibility measurements were performed using a SQUID magnetometer in a magnetic field of 100 G on cooling from 300 to 4 K. The magnetization (M) as a function of the magnetic field (B) for a range of temperatures has also been measured. The temperature dependence of the magnetic susceptibility, χ, normalized to 1 mol of Fe3, indicates typical low-dimensional antiferromagnetic behavior up to 40 K (Figure 2). The rapid rise in susceptibility below ∼40 K suggests a possible Curie-like inverse temperature behavior (1/T) suitable for ferromagnets due to impurity effects. The low-temperature data were fitted to a Curie-Weiss law of the form χ ) (3.77 ×
Figure 1. (a) Structure of I showing the three-legged Fe3 ladder. Note that the ladders are connected through PO4 tetrahedra. (b) Structure of I showing two ladders along with the organic amine molecule. The dashed lines represent possible hydrogen bond interactions.
Figure 2. Variation of the magnetic susceptibility per mole of Fe3 as a function of temperature. The solid line represents the best fit obtained to the experimental data by considering the form χ ) (1 - η)χ0 + ηχCW + χVan Vleck. The dotted line represents the susceptibility obtained by subtraction from the low-temperature Curie-like fit to the susceptibility (T < ∼35 K). The dashed line shows the Curie-Weiss fit to the high-temperature data (T ) 300-150 K). See the text for details. The inset shows the variation of the effective magnetic moment, µeff (µB), as a function of temperature.
10-4)/(T - 2.473) emu (mol of Fe3)-1. When this susceptibility was subtracted from the experimental raw data, we found that the value of the susceptibility was very close to zero (within the experimental resolution) throughout the temperature range. This is shown as a dotted line in Figure 2. The core diamagnetism was estimated using the available literature values (χdia
Magnetic Studies on a New Iron Phosphate
J. Phys. Chem. B, Vol. 108, No. 52, 2004 20353
Figure 3. Structure showing the three-legged ladder along with the interaction schemes used in our evaluation of the magnetic properties. The arrow-shaped bracket indicates the position of the ith rung. The iron atoms within a rung are numbered numerically (1, 2, 3). The exchanges are shown as J1, J2, and J3.
× 104 ) -6.8 × 10-6 emu (mol of Fe3)-1). The lowtemperature magnetic susceptibility assumes a small positive value, once the core diamagnetic contribution is subtracted from the experimental raw data. This leads to an inconclusive situation with regard to the magnetic gap. Since the open chain threelegged ladder is believed to be gapless, we have carried out a rather detailed analysis of the data, especially at low temperatures. The Bethe ansatz analysis of Eggert et al. for a general halfodd-integer spin chain suggests that the susceptibility for an infinite chain is of the form χ0 ) 1/2πV + 1/{4πV ln(T0/T)}, where V is related to the exchange coupling J as V ) Jπ/2, for a spin 1/2 chain.21 This form of susceptibility applies to some values of V and T0. It also ensures that, for a gapless system, χ has a logarithmic dependence. On the other hand, if there is a finite gap, the susceptibility vanishes exponentially to zero as T f 0. Apart from the above, the susceptibility also should have a Curie-Weiss component, as discussed above, of the form χCW ) C/(T - Θ), with C ) g2µB2S(S + 1)/3kB. In addition, a small contribution from temperature-independent Van Vleck paramagnetic susceptibility (χVan Vleck) also has to be considered. With these three components, along with an impurity concentration of η, the experimental raw data were fitted with the form χ ) (1 - η)χ0 + ηχCW + χVan Vleck. The resulting fit appears to be quite good and is presented as a solid line in Figure 2, for J = 1/8 of the value of T0. The impurity concentration was found to be close to 0.46%. The theoretical fit, correctly reproducing the data especially near the minimum, suggests that the system is, most likely, gapless.22,23 The effective magnetic moment (µeff) as a function of temperature is shown as an inset in Figure 2. The room temperature value of the moment is 18 µB, which decreases monotonically upon cooling to a value of 12.54 µB at T ) 40 K. The moment, however, increases exponentially upon further cooling and reaches a value of 19.84 µB at T ) 4 K. The room temperature magnetic moment compares fairly well with the spin-only moment derived from the three high-spin Fe3+ ions. A careful analysis of the structure suggests three types of possible superexchange pathways as shown in Figure 3. From the distances and the angles between the corresponding magnetic centers, it is clear that the exchanges within the rung are stronger than the exchanges between the rungs, and all the exchanges are antiferromagnetic in nature. From Figure 3, the total Hamiltonian for this system can be expressed as H ) ∑i{J1[S2,i(S1,i + S3,i) + J2[S1,i‚S1,i+1 + S3,i+1‚S3,i+2] + J3[S2,i+1(S1,i + S2,i + S3,i) + S2,i(S1,i+1 + S3,i+1)]}. Since all the
Figure 4. 4. Magnetization (M) as a function of magnetic field (B, T) at T ) 5 K (dashed line), 10 K (filled circles), 15 K (filled squares), 20 K (filled inverted triangles), 25 K (open left triangles), 30 K (filled upright triangles), 35 K (open right triangles), and 40 K (asterisks).
half-integer spin chains behave similarly, and J2 and J3 are believed to be insignificant compared to J1, we solved the Hamiltonian using a first-order perturbation theory for a spin 1/2 three-chain open ladder. Using two doublet states with S ) 1/2, |1〉 ) 1/(x6)[2|vvV〉 - |Vvv〉 - |vvV〉] and |2〉 ) 1/(x6)[2|VVv〉 - |vVV〉 - |VVv〉], as the unperturbed states for one rung (three spin 1/2 sites; v and V represent the Sz components of the 1/2 spin) and considering J3 ) CJ2 as perturbation (C being a positive constant of less than 1), we find that the resulting effective Hamiltonian is always isotropic with varying effective exchange values. In general, the effective low-energy Hamiltonian to first order, in interchain exchanges, can be written as H ) Jeff(1 + 2.5C)∑iSi‚Si+1. From this, it is clear that the system has extremely weak interchain exchanges, and essentially behaves as an almost unconnected three-legged rung ladder. Also note that any isotropic half-integer spin chain is gapless. To estimate the exchange interactions within the rungs, we fit the high-temperature part of the susceptibility in Figure 2 (dashed line) with a Curie-Weiss equation of the form χ ) (9.989 × 10-4)/(T - Θ). The Θ value, which gives the estimate of J1, is -148.25 K. The dependence of the magnetization (M) as a function of the magnetic field (B) for the temperature range T ) 4-40 K is shown in Figure 4. For all the T values, M(B) increases with an increase in B. It is clear from the figure that, at T ) 4 K, each Fe3+ ion is completely aligned by the field (full saturation) at B > 5 T. We have verified the nature of the M(B) curves by comparing them with all the possible Brillouin functions. At T ) 4 K, only the ground state is significantly populated, and as expected, the M ) f(B) curve closely follows the Brillouin function for the large unit cell spin of S ) 15/2, three combined 5/2 spins. On the other hand, above T ) 30 K, the excited states are significantly populated, and the magnetization curves are just between the Brillouin function for the three independent S ) 5/2 spins and a large spin with S ) 15/2. The gapless excitation is found to be the spin triplet state. The rise of magnetization with the magnetic field corresponds to the Zeeman coupling assisted increase in population of this triplet state. To conclude, the synthesis, structure, and magnetic properties of a new three-legged iron phosphate ladder are presented. The magnetic studies indicate the presence of a small percentage of impurities created by lattice imperfections. The low-temperature
20354 J. Phys. Chem. B, Vol. 108, No. 52, 2004 magnetic behavior was fitted theoretically, which indicates that the system is gapless. The low-temperature magnetization data, as a function of varying magnetic field, indicate complete polarization of the magnetic ions around 5 T, possibly due to very weak interchain superexchange interactions. The present study, in addition to establishing the richness of iron phosphate chemistry, also provides opportunities to evaluate quantitatively the competing magnetic interactions between Fe centers, leading to low-temperature phase transitions in magnetic solids of lower dimensionality. Our continuing investigations reveal that related compounds can be synthesized, and efforts are presently under way to understand and correlate structure, dimensionality, and various exotic magnetic phases. Acknowledgment. S.M. thanks the UGC, Government of India, for the award of a research fellowship. S.N. thanks the CSIR, Government of India, and S.K.P. the DST, Government of India, for research grants. References and Notes (1) Haldane, F. D. M. Phys. Lett. 1983, 93A, 464; Phys. ReV. Lett. 1983, 50, 1153. (2) Schulz, H. J. Phys. ReV. 1986, B34, 6372. (3) Barnes, T.; Riera, J. Phys. ReV. 1994, B50, 6817. (4) Dagotto, E.; Rice, T. M. Science 1996, 271, 618. (5) Hase, H.; Terasaki, I.; Uchinokura, K. Phys. ReV. Lett. 1993, 70, 3651. (6) Nishi, M.; Fujita, O.; Akimitsu, J. Phys. ReV. 1994, B50, 6508. (7) Darriet, J.; Regnault, L. P. Solid State Commun. 1993, 86, 409. (8) Buyers, W. J. L.; Morra, R. M.; Armstrong, R. L.; Hogaa, M. J.; Gerlach, P.; Hirakawa, K. Phys. ReV. Lett. 1986, 56, 371. Ma, S.; Troyer, M.; Broholm, C.; Reich, D. H.; Sternlieb, B. J.; Erwin, R. W. Phys. ReV. Lett. 1992, 69, 3571.
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