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Journal of Alloys and Compounds 253 ­ 254 ( 1997 ) 339 ­ 342

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Ё Sn Mossbauer study of Mn 3 SnH

a, a b G. Grosse *, F.E. Wagner , V.E. Antonov , T.E. Antonova
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a Physics Department E 15, Technical U niversity of Munich, 85747 Garching Germany Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Distr., Russia

Abstract Hydrides of the intermetallic compound Mn 3 Sn have been prepared at 3508C and pressures of molecular hydrogen between 2.8 and 4.0 119 Ё GPa. The hydrides with an approximate composition Mn 3 SnH were found to have a cubic perovskite structure. A Sn Mossbauer study revealed that they are magnetically ordered between 4.2 and 300 K and undergo a magnetic transition at T M &200 K.
Ё Keywords: Mossbauer spectroscopy; Hydride; Magnetic transition; Perovskite structure

1. Introduction A cubic phase with the approximate composition Mn 3 SnH was found in a recent study of hydrides of Mn 1 2 y Sn y alloys [1]. It formed together with hcp eMn( Sn) ­ H solid solutions upon high pressure hydrogenation of alloys with Sn contents y *0.05, and its metal lattice was found to be of the Cu 3 Au type. The crystal structure of the Mn 3 SnH x hydrides was recently studied by neutron diffraction [2]. Hydrogen atoms were found to occupy the position at the cube center of the Cu 3 Au unit cell, which leads to a structure of the perovskite type, as detailed in Table 1. Mn 3 SnH can be considered as the hydride of the intermetallic compound Mn 3 Sn. Like the isostructural compounds Mn 3 Ge and Mn 3 Ga, Mn 3 Sn has the hexagonal DO 19 structure. It exhibits a complex magnetic behaviour and has been subject to a number of studies (see e.g. [3 ­ 9] ґ and references therein). Below its Neel temperature, T N 5 420 K, Mn 3 Sn was shown to have a triangular anti-

ferromagnetic spin structure [4 ­ 6]. A weak ferromagnetic remanence below T N , first observed by Yasukochi et al. [3], disappears below a temperature T 1 , which has been reported to lie in the range 150 ­ 270 K, depending on stoichiometry and details of thermal treatment [6]. Below T 1 an incommensurate modulation of the spin structure is observed [6,8,10], which becomes commensurate below about 120 K [10]. Despite their different symmetry, the structures of Mn 3 Sn and its hydride are closely related. This relationship, which is analogous to that of the fcc and the hcp structure, is illustrated in Fig. 1, which shows the two

Table 1 Proposed structure of the hydrides Mn 3 1 y Sn 1- y H x . The occupied sites of space group Pm 3m with Wykoff notation, site symmetry, coordinates x, y, z, and occupancy N. For the studied samples: x 50.92( 3 ) and 0&y #0.2. Atom Sn Mn1 Mn2 H Site 1a 3c 1a 1b m 3m 4 / mmm m 3m m 3m x 0 0 0 1/2 y 0 1/2 0 1/2 z 0 1/2 0 1/2 N 1-y 1 y x Fig. 1. [001] projection of the hexagonal DO 19 structure of Mn 3 Sn. Filled 1 circles represent Mn atoms, the smaller ones at z 5 ] ( layer A), the larger 4 3 ones at z 5 ] ( layer B ). Of the Sn atoms shown as empty circles, the 4 smaller ones are again in layer A and the larger ones in layer B. This projection corresponds to a [111] projection of the cubic perovskite structure of Mn 3 SnH x . The cross indicates the octahedral interstitial site between layers A and B which is occupied by hydrogen in the cubic structure, and which is surrounded by six Mn atoms. The position of the Sn atoms in the C layer (not shown) is above this hydrogen site.

*Corresponding author. 0925-8388 / 97 / $17.00 © 1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02980-5


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hexagonal layers (A and B ) of a [001] projection of the hexagonal DO 19 structure. This projection corresponds to a [111] projection of the cubic perovskite structure. The cubic symmetry requires that the Mn atoms occupy the positions indicated by dashed circles in Fig. 1, and that a third layer, C, is added. The DO 19 structure of Mn 3 Sn is generally stable only in the presence of excess Mn, which replaces Sn atoms on 2c sites of the space group P 6 3 / mmc [5]. Likewise, the excess Mn atoms in the hydride randomly occupy 1a sites of the space group Pm 3m of Mn 3 SnH x [2], which are regularly occupied by Sn ( Table 1 ). By its cubic perovskite structure, Mn 3 SnH is also related to a large group of nitrides and carbides of Mn with the general formula Mn 3 MX, where M is a metal and X5N or C. An extensive review on these compounds, many of which have interesting magnetic properties, can be found in Ref. [11].

investigated Mn 3 1 y Sn 1 2 y H x samples had a hydrogen content of x 50.92( 3 ). Ё The Mossbauer spectra were obtained with a source of 119 m Sn in CaSnO 3 . The temperature of the Mn 3 SnH x absorbers was varied between 4.2 and 300 K. Measurements at higher temperatures have not been performed since hydrogen losses during the measurement time of about one day per spectrum could not be ruled out. The spectra were analysed by fitting transmission integral curves comprising Gaussian distributions of magnetic sextet patterns to the measured data. The effective absorber thicknesses were determined by simultaneous measurement of a b-Sn reference absorber of known thickness.

3. Results Over the whole investigated temperature range from 4.2 Ё to 300 K, the 119 Sn Mossbauer spectra of Mn 3.0 Sn 1.0 H x , Mn 3.08 Sn 0.92 H x and Mn 3.2 Sn 0 .8 H x ( Fig. 2 ) show a distribution of magnetic hyperfine splittings. At ambient temperature the corresponding hyperfine field distributions ( Fig. 3 ) are essentially single-peaked with hyperfine fields around 5.9 T depending only slightly on the Sn content. As the temperature is lowered, the distributions broaden and new peaks with smaller hyperfine fields emerge. In the case of Mn 3.2 Sn 0 .8 H x the distribution also exhibits higher fields. At 4.2 K the spectra of Mn 3.0 Sn 1.0 H x and Mn 3 .08 Sn 0 .92 H x show a main peak at 1.3 ­ 1.7 T and two smaller peaks at 3.2 ­ 3.5 T and 5.4 ­ 6.0 T, the larger hyperfine fields being observed in the sample with the lower Sn content. The 4.2 K spectrum of Mn 3.2 Sn 0 .8 H x is considerably more complex and shows significant peaks at hyperfine fields as high as 20 T and 24 T, a main peak at 7 T, and smaller peak at 1.7 T. All components have the same isomer shift of 11.67 mm / s relative to CaSnO 3 with both source and absorber at 4.2 K. This shift is slightly smaller than that of the initial Mn 3 Sn compounds ( 1.69 ­ 1.70 mm / s). The spectra are symmetric around their center of gravity. This implies that electric field gradients caused by local deviations from the cubic m 3m symmetry of the Sn sites are small and randomly oriented and therefore do not lead to an asymmetry of the unresolved sextet patterns. Ё The complex variation of the Mossbauer spectra with temperature is reflected in the temperature dependence of the average hyperfine fields ( Fig. 4 ). For Mn 3.0 Sn 1 .0 H x and Mn 3.08 Sn 0.92 H x , after a slight increase which is typical of magnetically ordered substances, the average hyperfine field drops with decreasing temperature between 200 and 100 K from about 6 T to about 2 T. In the case of Mn 3.2 Sn 0 .8 , a somewhat less pronounced increase of the average hyperfine field below T Ї110 K is connected with the appearance of the high-field peaks in the hyperfine field distribution.

2. Experimental details Compounds of nominal composition Mn 3.0 Sn 1 .0 , Mn 3.08 Sn 0.92 and Mn 3.2 Sn 0 .8 were prepared by arc-melting from the elements in an argon atmosphere and subsequent annealing in an evacuated quartz tube at 8508C for 10 h in the case of Mn 3.0 Sn 1.0 and at 9008C for 140 h in the case of Mn 3.08 Sn 0.92 and Mn 3.2 Sn 0.8 . The starting materials, Mn flakes and Sn pellets, were of 99.98% and 99.96% purity, respectively. Prior to the melting, a MnO 2 layer covering the Mn flakes was removed by reduction to MnO in a H 2 flow at 8008C and subsequent etching in 1 N HCl. The samples were ground to a powder and characterized by X-ray diffraction. Mn 3.08 Sn 0.92 and Mn 3.2 Sn 0.8 were single phase samples with the DO 19 structure, whereas the sample of nominal composition Mn 3.0 Sn 1 .0 contained a few percent of Mn 2 Sn with a B8 structure. 119 Sn Ё Mossbauer spectra of the initial samples confirmed these findings. Hydrides of these compounds were prepared by exposing the samples to molecular hydrogen at 3508C and a pressure of 2.8 GPa in the case of Mn 3.08 Sn 0.92 and of 4 GPa in the case of Mn 3.0 Sn 1 .0 and Mn 3.08 Sn 0.92 in a high pressure cell described previously [12]. Before releasing the pressure, the samples were cooled to below room temperature. To avoid hydrogen losses, the samples were stored in liquid nitrogen. It turned out, however, that hydrides stored at ambient temperature did not lose hydrogen within several months. X-ray powder patterns of the hydrides showed that these had a cubic structure with a lattice parameter a 53.985( 5 ) ° A at 100 K, in agreement with the structure proposed in Table 1 and Fig. 1. The diffraction lines of the hydrides were slightly broadened compared to those of the initial samples. The hydrogen content of the samples was determined with an accuracy of 2 ­ 3% by hot extraction into a calibrated volume at temperatures around 5008C. All


G. Grosse et al. / Journal of Alloys and Compounds 253 ­ 254 ( 1997 ) 339 ­ 342

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Ё Fig. 2. Typical 119 Sn Mossbauer spectra of Mn 3 SnH x hydrides at different temperatures. The shown temperatures were choosen to illustrate the limiting cases of the high and the low temperature magnetic state as well as the spectra observed at intermediate temperatures.

4. Discussion The temperature dependence of the average 119 Sn hyperfine fields clearly indicates that a transition between different magnetic states occurs in the Mn 3 SnH x hydrides. The temperature dependence of the hyperfine fields suggests a somewhat smeared out second order phase transition with a critical temperature of T C Ї200, 160 and 110 K for the hydrides Mn 3.0 Sn 1 .0 H x , Mn 3.08 Sn 0.92 H x and Mn 3.2 Sn 0 .8 H x , respectively. Several reasons may account for the complexity of the hyperfine field distributions at low temperatures. One possibility is, of course, a complex magnetic state giving rise to several magnetically inequivalent Sn sites. There are, however, two factors that are more likely to lead to a complex distribution of sites with different hyperfine fields: the partial occupation of 1b sites by hydrogen and the excess Mn atoms replacing Sn on 1a sites. Since the

hydrogen content of the studied samples is largely the same, we believe that additional magnetic moments carried by excess Mn atoms on 1a sites are the essential cause, as this can account also for the different distributions observed in the samples with different amounts of excess Mn. ( Note that the Mn 3.0 Sn 1 .0 H x sample is only nominally stoichiometric due to the presence of the Mn 2 Sn impurity.) Direct influences of the 1a Mn atoms on the isomer shift or the quadrupole splitting of the Sn probes are expected to be small because of the strong shielding by the twelve nearest Mn neighbours of the Sn atoms. The same holds for the hydrogen atoms on 1b sites. The additional magnetic moments may, however, influence the local magnetic order and thus the hyperfine field of the nearby Sn probes. A quantitative estimation can be endeavoured for the samples Mn 3.08 Sn 0.92 H x and Mn 3.2 Sn 0.8 H x , for which the concentration of excess Mn is approximately known:

Ё Fig. 3. Hyperfine field distributions corresponding to the Mossbauer spectra shown in Fig. 2. The distribution curves are normalized to equal maximum intensity.


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Fig. 4. Average

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Sn hyperfine field kBHF l of the Mn 3 SnH x hydrides as a function of temperature.

assuming a binomial distribution for the number of Mn atoms on the 6 1a sites at a distance of one lattice constant from a Sn probe, one finds that only 60% and 26% of the Sn atoms in Mn 3.08 Sn 0.92 H x and Mn 3.2 Sn 0 .8 H x , respectively, have an undisturbed local surrounding up to this distance. These fractions largely agree with the relative intensity of the 1.3 ­ 1.7 T peak in the hyperfine field distributions of these samples at 4.2 K, which is the main peak in the Bhf distribution of Mn 3.0 Sn 1.0 H x and Mn 3.08 Sn 0.92 H x , and might thus be associated with an undisturbed local environment of the Sn probes. It must be noted, however, that at ambient temperature the relative intensity of the main peak is much higher. Furthermore, excess Mn is also present in the parent Ё compound Mn 3 Sn. The Mossbauer spectra of Mn 3 Sn, however, show essentially one hyperfine field, Bhf Ї4 T, at room temperature and a relatively narrow field distribution at 4.2 K, which is only somewhat broader for samples with a higher concentration of excess Mn. The reason for this different behaviour is unknown and therefore further studies of this system by complementary methods such as NMR or neutron diffraction are highly desirable.

References
[1] G. Grosse, M. Baier, G.F. Schneider, F.E. Wagner, V.E. Antonov and T.E. Antonova, in I. Ortalli (ed.), Conf. Proc. Vol. 50: Int. Conf. on Ё the Applications of the Mossbauer Effect, Rimini, Italy, September 10 ­ 16, 1995, Italian Physical Society, Bologna, 1996, p. 215. [2] V.K. Fedotov, private communication. [3] K. Yasukochi, K. Kanematsu and T. Ohoyama, J. Phys. Soc. Jpn., 16 ( 1961 ) 1123. [4] J.S. Kouvel and J.S. Kaspar, Proc. Conf. on Magnetism, Nottingham, 1964, Institute of Physics, London, 1965, p. 169. [5] S. Tomiyoshi, J. Phys. Soc. Jpn., 51 ( 1982 ) 803. [6] P.J. Brown, V. Nunez, T. Tasset, J.B. Forsyth and P. Radhakrishna, J. Phys.: Condens. Matter, 2 ( 1990 ) 9409. [7] P. Rhadakrishna and S. Tomiyoshi, J. Phys.: Condens. Matter, 3 ( 1991 ) 2523. [8] P. Rhadakrishna and J.W. Cable, J. Magn. Magn. Mater., 104 ­ 107 ( 1992 ) 1065. [9] J.W. Cable, N. Wakabayashi and P. Radhakrishna, Solid State Commun., 88 ( 1993 ) 161. [10] J.W. Cable, N. Wakabayashi and P. Radhakrishna, J. Appl. Phys., 75 ( 1994 ) 6601. [11] J.B. Goodenough and J.M. Longo, in K.-H. Hellwege (ed.), Numerical Data and Functional Relationships in Science and Technology, Ё Landolt-Bornstein, New Series, Group III, Vol. 4, Part a, SpringerVerlag, Berlin, 1970, p. 263. [12] E.G. Ponyatovsky, V.E. Antonov and I.T. Belash, in A.M. Prokhorov and A.S. Prokhorov (eds.), Problems in Solid State Physics, Mir, Moscow, 1984, p. 109.

Acknowledgments This work was funded in part by Grant No.30-02-17522 of the Russian Foundation for Basic Research.