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Hyperfinc Interactions 69 (1991) 467-470

467

STFe MI)SSBAUER STUDY OF HYDRIDES AND DEUTERIDES OF MANGANESE
G. SCHNEIDER, M. BALER, F.E. WAGNER

Physics Department. Technical University of Munich, D-8046 Garching. Germany
V.E. ANTONOV, T.E. ANTONOVA

htstitute of Solid State Physics, Academy of Sciences of the USSR, Chernogolovka District 142432, USSR
Yu KOPILOVSKII and E. MAKAROV

hlstitute of Chemical Physics, Acudenty of Sciences of the USSR, Moscow USSR

llydrides and deuterides of manganese with hydrogen-to-metal ratios z between 0.65 and 0.93 were studi(~d by M6ssbauer spectroscopy on dilute substitutional STFe probes. No traces of magnetic ordering,were found. Experiments on STFe in MnD0.7"t in external magnetic fields show that the iron has no measurable magnetic moment. The shapes of the MSssbauer patterns are compatible with superstructure ordering of the interstitials of the anti-Cdl2 type near 9 = 0.5, while the dependence of the isomer shifts on z shows that there are no substantial iron-hydrogen interactions.

1. Introduction Under high pressures of molecular hydrogen or deuterium, manganese forms hcp hydrides and deuterides with hydrogen-to-metal ratios in the range of 0.65 "%~: ,% 0.95 [1-3]. According to neutron diffraction experiments [4], these are antiferromagnetic. Their N~el temperatures presumably lie near 350 K, where a direct observation has been impossible because tile hydrides decompose rapidly above 300 K at ambient pressure. A weakly ferromagnetic behaviour observed in both MnH, and MaD, [5,6] has been attributed to either spin canting or structural defects causing parasitic ferromagnetism [4I. The present M6ssbauer experiments on dilute substitutional 57Fe probes in MnIl, and MnD, were prompted by the open questions concerning the magnetic properties of these systems. 'I'o avoid decomposition of the hydrides above room temperature, part of the experiments were performed in a high pressure cell permitting ineasurements at temperatures up to about 600 K. Additional experiments were performed at a,nbient pressure and 4.2 I( in external magnetic fields Ul) to 6 T. 2. Experimental details

The hydrides were prepared as described by Ponyatovsky et al. [3] at temperatures between 325 and 350 ~ and pressures between 2.5 and 3.8 GPa from arc-melted alloys of a-Mn with 0.2 or 0.5 at.% of enriched SrFe. The deuterides contained a few percent of hydrogen. In order to avoid hydrogen losses during storage, the samples were kept in liquid nitrogen except during

c~ J.C. Baltzcr A.G., Scientific I~ublishing Company


468

G. Schneider et al. / ~TFe M.S. of Io,drides and deuterides of manganese

the M6ssbauer measurements. The M6ssbauer experiments at high pressure were performed in a pressure cell using B4C anvils [7], in which the powdered samples were enclosed in teflon capsules of 7 mm diameter with methanol as the pressure transmitting medium and silica as a fdler. The hydrogen contents were determined by outgassing into a calibrated volume at elevated temperatures. 3. Results and Discussion

The M6ssbauer spectra of hydrides and deuterides of manganese taken at 4.2 I( (fig. 1) show no evidence of a magnetic hyperfine splitting. The isomer shifts increase with the hydrogen content, while the linewidths are largest near ~ = 0.75 (fig. 2). As for STFe in hydrides of Ni and Pd [8-10], the broadening of the patterns can be attributed to a superposition of absorption lines with different isomer shifts resulting from the different numbers of nearest hydrogen ncighbours around the 57Fe probes in the disordered hydride phases, with at best very small electric quadrupole splittings. The fits shown in fig. 1 were made with a maximum of 7 single lines corresponding to between zero and 6 hydrogen nelghbours with a shift of +0.12 mm/s per hydrogen nelghbour like in the case of the PdH, and NiH~ systems [8-10]. A magnetic contribution to the linewidths should be most clearly revealed in the vicinity of the N~el temperature, where any such broadening should disappear. Therefore the temperature dependence of the spectra was measured for MnD0.~ at a pressure of about 2 GPa, by which decomposition could be avoidec~ up to about 600 K, the highest temperature that could be reached in the pressure cell, although some loss of deuterium occurred above 450 I(. The latter can be seen from the temperature dependence of the isomer shift (fig. 2), which increases slightly with temperature up to about 450 K and then decreases steeply. This decrease is irreversible, as is shown by the shift obtained at 100 K after the sample had been kept at 584 K for about 4 days (fig. 2). The hydrogen content of the sample after this treatment could not be detcrnfined because of the contanfination with debris from the pressure cell, but is estimated to be near the lower limit of existence of the hcp hydride phase of z ~ 0.65, since the MGssbauer pattern taken at 4.2 1( (fig. 1) shows the presence of about 10 % of a-Mn. The temperature dependence of the linewidths (fig. 2) decreases smoothly with increasing temperature but shows no anomaly near the supposed [4-6] magnetic ordering temperature of about 350 K. If MnD~ is indeed antiferromagnetic, the absence of an observable magnetic hyperfine interaction of the iron nuclei is surprising since iron in antiferromagnetic hcp Mn-Fe alloys exhibits magnetic hyperfine fields between 1 and 2 T [11-14}, which cause at least an easily detectable line broadening. In a further search for magnetic hyperfine effects, M6ssbauer spectra of STFe in MnD0.77 were measured at 4.2 I( in external magnetic fields of 2, ,1 and 6 T. The resulting hyperfine patterns can be explained by a magnetic field at the iron nuclei that is equal to the external field within the limits of error of about 1%. The iron thus bears no measurable localized magnetic moment, in agreement with the rigid band model, which predicts non-zero magnetic moments at iron in hcp phases only at substantially higher electrtm numbers than those reached in manganese hydrides. 'the isomer shift in Mnlt, and MnD= increases with the hydrogen content (fig. 2). Since the shift of iron in hydrogen-free hcp Mn is not known because of the instability of this phase, we take the mean isomer shift of the complex MSssbauer pattern of STFe in ce-Mn [15] as representative for unloaded manganese. For this we found a shift of -0.26 mm/s with respect the source of SVCo in Rh. Relative to this value, the hydrogen-induced shift is +0.27 mm/s at


G. Schneider et aL / ~TFeM.S. of hydrides and deuterides of manganese
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469

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91,

87.
100

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-',

6

i

-1

6
Velocity (rnna/s)

i

Velocity (ram/s)

Fig. 1: M6ssbauer spectra of S'tFe in hydrides and deuterides of manganese measured at 4.2 K with a source of SVCo in Rh at the same temperature.

~.. -.~

0.7
O

0.6 0.5 0.4.

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O
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200

300

400

500

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Temperature

(K)

Fig. 2: Centre shifts (CS) and full linewidths at half maximum (LWIIM) of the STFe MSssbauer patterns of hydrides (fifll symbols) and deuterides (open symbols) of manganese as a function of the hydrogento-metal ratio z at 4.2 K (left) and as a function of temperature (right) for a sample with an initial hydrogen content of z = 0.77. The centre shifts are given with respect to the source of STCo in Rh at the same temperature as the absorber, which largely eliminates the second order Doppler shift. z ~ 0.65' and +0.43

mm/s

at z = 0.93. The nearly linear dependence of these shifts on tile

hydrogen content indicates that, other than for STFe ill Ni and Pd [7-9], there is no sizeable influence of a repulsive interaction between the iron solutes and the interstitials, which would lead to a reduced hydrogen site occupancy near tile iron probes and to a nonlinear dependence of the centre shift on z [7-9]. For a distribution of hydrogens near the iron that is unperturbed by iron-hydrogen interactions and superstructure ordering, the width of tile M6ssbauer pattern


470

G. Schneider et al. / 57Fe M.S. of Io,drides and deuterides of manganese

should be largest near x = 0.5 and decrease towards higher hydrogen contents, ltowever, fig. 2 shows that the width is small at 9 ~ 0.65, has a maximum near z = 0.75 and then decreases again when all interstitial sites become filled as x approaches unity. Tiffs behaviour suggests a superstructure ordering of the hydrogen near ~c = 0.5, confirming the neutron diffraction results [4] that have been interpreted in terms of an anti-CdI2 type superstructure, in wtdch alternate hexagonal layers of octahedral interstitial sites are filled and each metal atom has 3 nearest hydrogen neighbours at x = 0.5.

4. Conclusions
The magnetic magnetic studies to SVFe MSssbauer data for the hydrides and deuterides of manganese do not reveal ordering and show that iron solutes in these systems have no measurable localized moment. Considering these results, it will be of interest to extend the SVFe Mgssbauer the fee manganese hydrides described recently [16,17], which have also been found

to be antiferromagnetic [16]. Acknowledgement This work has bccn funded by the Deutsche Forsclmngsgcmcinschaft.

References
[1] M. Krukowski and B. Baranowski, Roczn. Chem. 49 (1975) 1183; J. Less-Common Met. 49 (1976) 385. [2] l':. G. Ponyatovsky and I. T. 13clash, Dokl. Akad. Nauk SSSIt 224 (1975) 607. [3] E. G. Ponyatovsky, V. E. Antonov and I. T. Bclash, in: Problems in Solid Slate Physics, eds. A. M. Prokhorov and A. S. Prokhorov (Mir, Moscow, 1984) p. 109. [4J A. V. lrodova, V. P. Glazkov, V. A. Somenkov, S. Six. Shil'shtein, V. E. Antonov and E. G. Ponyatovskii, Fiz. Tvcrd. Tcla 29 (1987) 2714; Sov. Phys. Solid State 29 (1987) 1562. [5] I. T. Belash, B. K. Ponomarev, V. G. Tissen, N. S. Afonikova, V. Sh. Shekhtman and E. G. Ponyatovskii, Fiz. Tverd. Tcla 20 (1978) 422; Soy. Phys. Solid State 20 (1978) 244. [6] B. Baranowski, Z. Phys. Chem. N. F. 11,t (1979) 71. [7] J. Willncr and J. Moscr, J. Phys. l'~: Sci. Instr. 12 (1979) 886. [81 F. Pr6bst and P. B. Wagner, J. Phys. F: Met. Phys. 17 (1987) 2459. [9] M. Amer, l". E. Wagner and It. J. Itaucr, Hyp. ]nt. ,11 (1988) 539. [10] M. Amer, M. Baler, }i. J. Bauer and F. E. Wagner, Z. Phys. Chem. N. F. 164 (1989) 773. Ill] C. Kimball, W. D. Gerber and A. Arrott, J. Appl. Phys. 34 (1963) 1046. [12] 11. Ohno and M. Mekata, J. Jal,. Phys. Soc. 31 (1971) 102. [13] G. G. Amigud, V. V. Ovchinnikov and M. A. Filipl,ov , Fiz. Metal. Mctalloved. 51 (1981) 878; Phys. Met. Metall. 51 (1981) 179. [14] G. G. Amigud, V. V. Ovchinnikov, V. S. Listinov and M. A. Filippov, Fiz. Metal. Metallovcd. 51 (1981) 955; Phys. Met. Mctall. 51 (1981) 45. [15] C. W. Kimball, W. C. Phillips, M. V. Ncvitt and It. S. l'rcston, Phys. Rev. 146 (1966) 375. [16] Y. Fukai, 1|. Ishikawa, J. L. Soubcyroux and l). Fruchart, Z. Phys. Chem. NF 163 (1989) ,179. [17] S. M. Filipek, H. Sugiura and A. 13. Sawaoka, in: Iligh Pressure I~cscarch, vol. ,1 (Gordon and Breach, London, 1990) p. 354.