J. MATER. CHEM., 1991, 1(6),915-918 (C6H,CH2NH3)2CrBr3.310.7:A New Insulating Ferromagnet with a Curie Temperature of 51 K Gigliola Staulo and Carlo Bellitto" lstituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del C.N.R., Area della Ricerca di Roma, Via Salaria Km.29.5, 1-00016 Monterotondo Staz., Roma, Italy A mixed-halide bis(benzylammonium)tetrahalogenochromate(II), (C,H,CH,NH3),CrBr3~310~7,has been synthesized and characterized. The compound crystallizes in the layered K,NiF, perovskite structure. It orders ferromag- netically at Tc=51 K, one of the highest critical (Curie) temperatures shown by magnetic insulators. A hysteresis loop typical of a soft ferromagnet has been obtained. At T=6 K, the remnant magnetization is 4.2 x lo3cm3 mol-' G and the coercive field is 130 G.Keywords: ferrornagnet; Insulator; Hysteresis The search for new insulators that order ferromagnetically with as high a critical temperature, T,, as possible represents an interesting target in solid-state chemistry. To date, few well characterized bulk ferromagnets are known and they are characterized by having low T,.' Very few exceptions occur; one example is represented by the series bis(alky1ammonium)- tetrachlorochromate(u), which have been found to order ferro- magnetically in the temperature range 37-42 K.' These materials are typical examples of two-dimensional Heisenberg ferromagnets. They crystallize in space groups closely related to the perovskite K2NiF4 str~cture.~ The deviation from this structure is mainly brought about by cooperative Jahn-Teller distortion of the corner-sharing C1- octahedra surrounding the high-spin (3d)4 Cr2 + ion.An antiferrodistortive displace- ment of the C1- ions in the basal plane is observed, i.e. the [CrCl,] octahedra are tetragonally elongated, with the princi- pal axes alternately along [loo] and [OlO] directions of the parent K2NiF4 unit cell.4 Layers of [CrCl,] corner-sharing octahedra are separated by double layers of alkylammonium cations, CnHzn+ 1NH3f. van der Waals forces operate between the ends of the organic substituents of two adjacent alkyl- ammonium ions. The corresponding bromide derivatives have also been isolated, and the Curie temperatures of these materials were found to be slightly higher than 50 K.5 So far no iodo-derivatives have been reported.In the literature only completely ionic A,CrI, are known,6 where A=T1+, In+, but they have a different structure, i.e. the crystal structure consists of isolated [CrI6I4- octahedra with the T1+ ions being eight co-ordinate. In an attempt to isolate and stabilize the corre- sponding alkylammonium iodo-compounds in the perovskite structure we thought it was worthwhile to synthesize solid- state solutions (RNH3)2CrXxY4-x, where X =Br and Y =I, 0 <x <4. Another reason for doing this is to clarify the role of the halide in the superexchange mechanism in these ferro- magnetic materials and therefore to determine T,.Here we report on the synthesis and magnetic properties of the first of the series: (C6H5CH2NH3)2CrBr3 .JO .,. Experimental Elemental analyses were performed by Malissa and Reuter Mikroanalytische Laboratorium, Elbach, Germany. Benzyl- ammonium iodide was prepared by a direct reaction between benzylamine in ethanol and HI in water (56%) in the ratio 1:1. The resulting solution was concentrated to a quarter of the total volume. White crystals were isolated and collected, washed with ether and dried under vacuum. The purity of the compound was checked by elemental analysis. Since the Cr" compounds are very sensitive to oxygen and moisture, all manipulations were carried out using Schlenck tube techniques7 under oxygen-free nitrogen.Synthesis of (C6HSCH2NH3)2CrBr3.310.~ The synthesis was carried out by using a method described previously.' To a hot solution of Cr" in glacial acetic acid (90 cm'), prepared by passing HBr gas through a suspension of finely divided electrolytic Cr metal (1 g, 1.94 x lo-' mol), was slowly added a stoichiometric quantity of benzylam-monium iodide (4 g, 3.0 x lo-' mol) in the same solvent. As the sample cooled, a yellow-brown polycrystalline powder of the title compound was isolated. Found: C, 26.49; H, 3.49; N, 4.23; Cr, 10.45; Br, 41.53; I, 12.94%. CI4HZON2CrBr3 requires: C, 27.09; H, 3.22; N, 4.51; Cr, 8.38; Br, 42.48; I, 14.31%. X-Ray Powder Diffraction The polycrystalline sample was sealed under nitrogen in a Lindemann tube, i.d.0.3 mm, and diffraction patterns were collected with a Philips Debye-Scherrer camera (Ni-filtered Cu-Ka radiation). Magnetic Susceptibility Measurements Magnetic susceptibility was measured by means of a commer- cial SQUID magnetometer, Quantum Design model MPMS down to 4.2 K. The polycrystalline sample was placed inside a Perspex cylinder, sealed under nitrogen and placed in a long polyethylene tube. This tube was then inserted in the sample holder. An external magnetic field of up to 5 T was supplied by a superconducting solenoid. Temperature measurements were based on a calibrated carbon glass resistor up to 40 K, and on a Pt sensor above 40 K. The susceptibility measurements were corrected for diamag- netism as calculated from the known diamagnetic suscepti- bility of the constituents of the title compound, i.e.-2.72 x lo-, emu mol- '. The temperature-independent susceptibility, zTrpwas also introduced and it was found to be 1 x~O-~emu mol-' Electronic Spectra Near-infrared and visible spectra were recorded on a Beckman DK 2A recording spectrophotometer. Powdered samples were placed between two quartz glasses and sealed in a dry box with glue. MgO was used as reference. The absence of a broad intense band centred at 450nm, typical of Cr"' impurities, was taken as a further purity check. Results The mixed halide (C6H5CH2NH3)2CrBr3 .310.7 was isolated and characterized by elemental analysis and X-ray powder diffraction. The golden brown compound is very sensitive to air and moisture, and samples must be handled in an inert atmosphere.The X-ray diffraction patterns were indexed in the space group Pbca, (Dl:),with the following unit-cell parameters:? a=7.9, A;b =32.3, A;c =7.8,, A.These data are similar to those found in the pure bromide derivative, the crystal structure of which has been solved.' The compound shows a crystal structure similar to that of (RNH3)2MX4, where R is an alkyl group, M a divalent ion, and X a halide." As mentioned in the introduction, they consist of layers of MX6 octahedra linked in a square array by sharing equatorial vertices. Two adjacent layers of [MX4];"- are separated by the diamagnetic alkylammonium cations, RNH;. In the pre- sent case the magnetic unit is represented by the chromophore [CrBr,I].The longest unit-cell parameter is related to the distance between the magnetic layers by a factor of 112. Electronic Spectra The diffuse electronic reflectance spectrum of the title com- pound, reported in Fig. 1, is characterized by an edge in the visible UV region, on which are superimposed two sharp spin-forbidden bands at 18.7 and 16.2 kK; there is a broad band in the near-infrared at 11.2 kK. This optical spectrum is typical of a six-co-ordinated Cr2+ ion, thus confirming the t The space group assigned for (CnHZn+,NH,),CrCl,, n= 1,2,3 is Cmca. The unit-cell c parameter for this space group corresponds to the b parameter of the Pbac space group. J. MATER. CHEM., 1991, VOL. 1 presence of a [CrX,] chromophore.The main feature of the optical spectrum is represented by the two spin-forbidden bands, which appear at the same frequencies as those observed previously in the tetrachloro- and tetrabromo-chromate(I1) salts. These are pure spin-flip transitions and have been assigned as electronic transitions in D,, symmetry: on the basis of the exchange intensity mechanism." The other feature is the presence of an absorption edge in the visible region, which in the corresponding chloro- and bromo-deriva- tives is shifted to the near UV. This feature can be ascribed to an LMCT charge-transfer band.'* Magnetic Properties High-temperature Region The thermal variation of the inverse of the magnetic suscepti- bility, recorded at 100 G, in the temperature range 50-300 K is shown in Fig.2. Above 100 K the magnetic susceptibility follows the Curie-Weiss law: =C/(T-0) with Curie and Weiss constants of 2.98 emu K mol-' and 74 K, respectively. The effective magnetic moment is nearly equal to the spin-only value, i.e. 4.90pB, and it is typical of Cr" in a high-spin d4 configuration. The positive value of 6 indicates that ferromagnetic interactions (Jij>0) dominate. The magnetic data above 60 K were analysed in terms of Rushbrook and Wood series expansion, appropriate for an S =2, two-dimensional quadratic layer ferromagnet, as modi- fied by Lines13 with the exchange Hamiltonian H = -JS1.S2: Ng2pi/x= 1/2k,T+ J(-4+9x-9.O72x2 +55.728~~ +160.704~~+1 16.64~~) (1) where x= J/k,T, J is the nearest-neighbour exchange constant and g is the Lande factor.The best fit for the curve (solid line in Fig. 2), obtained by using a non-linear least-squares Marquardt algorithm, gave the parameters J = 13 K, g= 2.00. The values of J for the series are reported in Table 1, together with the other magnetic and structural parameters. 0 100 200 I I I IIII TiK 400 600 800 1000 2000 A/nm Fig. 2 Thermal variation of the inverse magnetic susceptibility in the temperature range 50-300 K for a polycrystalline sample of Fig. 1 Room-temperature diffuse electronic spectrum of (C6H,CH2NH,),CrBr, ,,Io., [the solid line is a least-squares fit to (C,H,CH,NH,),CrBr, .,I0 .,; peak values shown in kK eqn. (1)l J. MATER.CHEM., 1991, VOL. 1 Table 1 Magnetic parameters and interlayer spacings of (RHN3),CrX,, R =alkyl and X =C1, Br, I (CH ,NH 3)2CrCl, (C2H5NH3)2CrC14 42 41 13.0 10.1 59 58 9.44 10.71 (C6H,CH ,NH &CrCl, (C6H5CH,NH3)2CrBr3 .3c10.7 37 49 10.6 12.5 58 62 15.71 16.10 (C,H5CH2NH3),CrBr, 52 13.1 77 16.03 (C6H5CH2NHd,CrBr3 .3IO.7 51 13.0 74 16.15 Low-temperat ure Region A sharp increase in magnetic susceptibility is observed as the sample is cooled below 70 K. To ascertain the presence of a ferromagnetic phase transition, the variation of the molar magnetization, M, us. temperature in a powdered sample has been measured. The zero-field cooled (ZFCM) and field- cooled (FCM) molar magnetization curves are shown in Fig. 3.The first is obtained by cooling to 6 K in a zero field, applying an external field of 5 G and then heating. The ZFCM curve shows a peak at T=51 K. The FCM curve, obtained by cooling the sample in a field of 100 G, shows a feature typical of a ferromagnetic transition. This behaviour has been observed previously on polycrystalline ferromagnets. l4 The variation of magnetization us. external field was then measured for the same sample below and above T,; the plots for two temperatures well below T,, i.e. T=6 and 20 K are shown in Fig. 4. An isothermal magnetization at T>>T,, i.e. at T= 100 K is also reported. Above T, the plot is linear, confirming that the system is in the paramagnetic state. The experimental l 000000000000 oooo +-J 7 -0 00 (3 0 12000 Y 0 0 20 40 60 T/K Fig.3 Magnetization us. temperature plots at applied magnetic field of 5 G (U)and of 100G (0) 0 0 000 values fit the magnetization M: for S=2, g= 1.95 and 8=71 K. Below T,, at T =6 K, the magnetization increases with increasing field and reaches 80% of the saturation value at 5 T. The saturation value is expected to be 23 300 emu G mol-for an S =2 system, as calculated from the relation Ms =NAgpB where NA is Avogadro's number. At T= 20 K the magnetization increases with increasing field and reaches half of the saturation value at 2 T. At 6 K the increase of M us. H is faster and the saturation is reached more quickly. To verify that the title compound is a bulk ferromagnet, a hysteresis loop has been performed at both temperatures.The plot recorded at T=6 K, in the external field range +0.2 to -0.2 T, is shown in Fig. 5. At this temperature the remnant magnetization, 2.e. the value at zero applied field, is 4.22 x lo3 cm3 mol-' G, i.e. ca. 20% of the saturation value. The coercive field, H,, corresponding to the value of that where M =0, is 130 G. At T=20 K the remnant magnetization is 1.75 x lo3 cm3 mol-'G and the coercive field is 50 G. Discussion (C6H,CH2NH3)2CrBr3.310.7 is a ferromagnet with T,=51 K. This value is high compared to that shown by other known ferromagnetic insulators.' From X-ray powder diffraction data a similar crystal structure to the pure bromide derivative is suggested.In this structure the Cr" ion is tetragonally elongated and six-co-ordinate. The long axes of the octahedra alternate at right angles in the basal plane. The compound features ferromagnetic nearest-neighbour exchange resulting from the cooperative Jahn-Teller effect, which constrains the unpaired eg electron of Cr" to the dZ2 orbital. The super- exchange pathway is via bridging halide anions between the half-filled d: orbital on one Cr" ion and the empty d,2-,,2 orbital on its neighb~urs.~ The halide ion in the plane plays a key role in the superexchange mechanism between the neighbouring Cr" ions, and the nearest-neighbour exchange constant J increases on passing from the pure chloride to the bromide derivative^.'^ This is mainly due to a larger extension of the bromide valence electron shell.Here the remarkable result is that the substitution of the bromide by iodide does not change the value of J and therefore the value of T,. A -1 5' I I 1 2 3 4 5 -0.2 -0.1 0 0.1 0.2 HIT HIT Fig. 4 Isothermal magnetization us. magnetic field at T =6 K (O), Fig. 5 Hysteresis loop M=f (H) for a polycrystalline sample of the 20 K (n)and 100 K (0) title compound at T=6 K more marked increase in T, would have been expected, as observed previously on passing from the chloride to the bromide derivatives. This suggests that the substitution of the iodide in the chromophore [CrBrJ] occurs in the axial position and not in the basal plane (where the nearest-neighbour exchange J is effective).A similar effect has been observed previo~sly.'~*'~ The peak observed at the magnetic phase transition deserves comment. In general, for a simple ferromagnet, the magnetization below T, should be constant and equal to the reciprocal demagnetizing factor multiplied by the applied magnetic field. This is caused by the spon- taneous formation of domains exactly compensating for the increase in spontaneous magnetization below T,. However, if the sample displays significant anisotropy and/or hysteresis, below T, the formation of domain walls will be prevented, leading to the magnetization curve shown in Fig. 3 (ZFCM curve).14 In the present case the low value of the remnant magnetization observed in the hysteresis loop excludes the latter hypothesis.Previously, a.c. magnetic susceptibility measurements as a function of temperature on polycrystalline samples of pure tetrabromo-and tetrachlorochromate(I1) derivatives showed similar peak^.^^,^ Furthermore, a.c. mag- netic susceptibility measurements on a single crystal of (C6H5CH2NH3)2CrBr3&lo ., showed a strong anisotropy between xII and xI to the c axis of the Cmca unit cell, with constant and ca. two orders of magnitude smaller than that in the c plane below Tc.I6The compound behaves then as a 2D easy-plane ferromagnet.? In conclusion we have obtained a new ferromagnetic insulator with a reasonably high value of T,. This has been achieved by using para- magnetic ions with a high S value, i.e. Cr", and by diluting t One of the referees suggested the possibility that the compound is a metamagnet.This is not the case because of the absence of a typical 'S' curve in the magnetization us. applied magnetic field. A nice example is reported in the literature," where metamagnetic +[Fe"'(C,Me,),]. [TCNQ]. -and ferromagnetic [Fe"'(C,Me,),]. + [TCNE].- compounds are reported. Fig. 3 and 4 of this paper provide evidence of the difference in the magnetic behaviour in these compounds. J. MATER. CHEM., 1991, VOL. 1 heavier halogens in extended two-dimensional perovskite structure. Attempts to isolate pure (RNH3)2Cr14, R =alkyl, are in progress. We thank C. Veroli for X-ray data collection and Dr. A. Testa for the SQUID measurements. References 1 See e.g.R. L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1985, p. 142. 2 (a) C. Bellitto and P. Day, J. Chem. SOC., Dalton Trans., 1978 1207; (b) C. Bellitto, P. Day and T. E. Wood, J. Chem. SOC., Dalton Trans., 1986, 847. 3 A. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 4th edn., 1975, p. 172. 4 P. Day, M. T. Hutchings, E. Janke and P. J. Walker, J. Chem. SOC., Chem. Commun., 1979, 71 1. 5 C. Bellitto, P. Filaci and S. Patrizio, Inorg. Chem., 1987, 26, 191. 6 (a)H. W. Zandbergen, Acta Crystallogr., Sect. B, 1979,35, 2852; (b)J. Solid State Chem., 1981, 38, 239. 7 D. F. Shriver, The Manipulation of Air-sensitive Compounds, McGraw-Hill, New York, 1969. 8 C. Bellitto, Inorg. Synth., 1986, 24, 188. 9 D. M. Halepoto, L. F. Larkworthy, D. C. Povey and V. Ramdas, Inorg. Chim. Acta, 1989, 162, 71. 10 See e.g. G. Chapuis, R. Kind and M. Arend, Phys. Status Solidi A, 1976, 36, 285. 11 C. Bellitto, H. Brunner and H. V. Gudel, Inorg. Chem., 1987, 24, 2750. 12 A. B. P. Lever, in Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1984, p. 203. 13 (a) G. S. Rushbrooke and P. J. Wood, Mol. Phys., 1958, 1, 257; (b)M. E. Lines, J. Phys. Chem. Solids, 1970, 31, 101. 14 (a) D. W. Carnegie Jr., C. J. Tranchita and H. Claus, J. Appl. Phys., 1979,50,7318; (b)R. L. Carlin, L. J. Krause, A. Lambrecht and H. Claus, J. Appl. Phys., 1982, 53, 2634; (c) M. Hitzfeld, P. Ziemann, W. Buckel and H. Claus, Phys. Rev. B, 1984, 29, 5023. (d) 0. Kahn, Y. Pei, M. Verdaguer, J. P. Renard and J. Sletten, J. Am. Chem. SOC., 1988, 110, 782. 15 P. J. Fyne and P. Day, Muter. Res. Bull., 1985, 20, 197. 16 C. Bellitto, Mol. Cryst. Liq. Cryst., 1989, 176, 465. 17 J. S. Miller, A. J. Epstein and W. M. Reiff, Science, 1988, 240, 40. Paper 1100332A; Received 23rd January, 1991