Crystal structure and magnetic ordering in dimorphic EuZn2Sn2

Solid State Communications 149 (2009) 68–72

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Crystal structure and magnetic ordering in dimorphic EuZn2 Sn2 S.K. Dhar a , P. Paulose a , R. Kulkarni a , P. Manfrinetti b,c,∗ , M. Pani b , N. Parodi b a

Condensed Matter Physics & Materials Science Department, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India

b

Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy

c

LAMIA Laboratory-CNR-INFM, Corso Perrone 24, 16152 Genova, Italy

article

info

a b s t r a c t

Article history: Received 27 June 2008 Received in revised form 7 October 2008 Accepted 10 October 2008 by C. Lacroix Available online 21 October 2008

We report on the synthesis and magnetic properties of the new Eu-based intermetallic EuZn2 Sn2 . The compound shows dimorphism; its formation and dimorphic nature have been investigated by powder and single crystal X-ray diffractometry, as well as by differential thermal analyses. The high-temperature form (HT-EuZn2 Sn2 ), stable above 645 ◦ C, is tetragonal CaBe2 Ge2 type [P4/nmm, tP10, a = 4.528(1) Å,

PACS: 61.50.Ks 61.66.Dk 65.40.Ba 75.50.Ee

90.85(1)◦ , V = 237.0(1) Å ]. The two structures are closely related, the monoclinic structure being a deformation derivative of the tetragonal one. Detailed magnetization, heat capacity, electrical resistivity and 151 Eu-Mössbauer measurements have been carried out on the two polymorphs. In both forms, the Eu ions are in the divalent state, 4f7 − Eu2+ , and, as inferred from the peak in the susceptibility, order antiferromagnetically at 10.4 and 9.8 K in HT- and LT-EuZn2 Sn2 , respectively. The 151 Eu-Mössbauer spectra of the two stannides are practically identical, both in the paramagnetic and in the magnetically ordered state. © 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Rare-earth alloys and compounds, EuZn2 Sn2 A. Magnetically ordered materials C. Crystal structure and symmetry E. Magnetic measurements and Mössbauer spectroscopy

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c = 11.370(2) Å, V = 233.1(1) Å ]; the low-temperature form (LT-EuZn2 Sn2 ), stable below 645 ◦ C, is monoclinic LaPt2 Ge2 type [P21 , mP10, a = 4.596(1) Å, b = 4.552(1) Å, c = 11.331(2) Å, β =

1. Introduction Due to the high volatility of europium metal and its strongly oxidizable nature, leading to difficulties in the synthesis of europium based compounds by ordinary methods, relatively few europium compounds have been studied. In particular, in the ternary systems Eu–Zn–X with X = Si, Ge, Sn, Pb, the only known phases are the four equiatomic compounds EuZnSi, EuZnGe, EuZnSn, EuZnPb [1–3] and the three 1:2:2 compounds [4,5]. EuZn2 Si2 and EuZn2 Ge2 have been synthesized by Grytsiv et al., grown from Zn or Ga(In)/Zn flux, obtaining crystals with the ThCr2 Si2 and the CaBe2 Ge2 types, respectively for the silicon and the germanium compound [4]. In another work, Pöttgen et al. found the ThCr2 Si2 structure for EuZn2 Ge2 , obtained by direct synthesis [5]. The divalent behaviour of Eu has been revealed in most of these compounds by magnetic and Mössbauer measurements [2–5]. In particular, EuZn2 Si2 orders

∗ Corresponding author at: Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy. Tel.: +39 010 3536081; fax: +39 010 3628252. E-mail address: [email protected] (P. Manfrinetti). 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.10.014

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antiferromagnetically first at 16 K, followed by a second transition at 13 K with a canting of the moments. The Néel temperature of EuZn2 Ge2 is 10 K [5]. During the course of our investigations on novel ternary europium compounds, we have discovered the new compound EuZn2 Sn2 . It is dimorphic, with the high temperature form adopting the tetragonal CaBe2 Ge2 -type structure and the low temperature form crystallizing in the related monoclinic LaPt2 Ge2 type. 2. Experimental The metals used were commercial products with high purity (99.9 wt% for Eu; 99.999 wt% for Zn and Sn). To avoid any weight losses due to possible volatilisation during reaction and melting (of either Eu or/and Zn, because of the high vapour pressure of both these elements), the samples had to be prepared in sealed containers. The weighed amounts of the metals (in form of freshly prepared and surface cleaned small pieces of Eu and Sn and of turnings of Zn; total mass of about 2 g) were directly pressed together into out-gassed tantalum crucibles which were sealed by arc welding a lid under a pure-Ar flow. The alloys were then melted by slowly heating the crucibles in a high-frequency induction furnace up to about 1100 ◦ C (taking care not to cause

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Table 1 Single crystal data and selected parameters of data collection and structure refinement for the two polymorphic forms of EuZn2 Sn2 . Compound

HT-EuZn2 Sn2

LT-EuZn2 Sn2

Structure type Pearson code—Space group a (Å) b (Å) c (Å)

CaBe2 Ge2 tP10 – P4 /nmm 4.528(1) – 11.370(2) – 233.1(1), 2 7.44 Prism 0.05 × 0.10 × 0.10 33.88

1178 353 0.138 0.047 287 3.29 Ψ -scans 15 0.0077(10) 0.093 0.035 1.21 –

LaPt2 Ge2 mP10 – P21 4.596(1) 4.552(1) 11.331(2) 90.85(1) 237.0(1), 2 7.28 Platelet 0.03 × 0.07 × 0.08 33.16 ω– θ 2–35 0 to 7; 0 to 7; −18 to 18 plus Friedel opposites 2546 2103 0.036 0.034 1423 3.18 Gaussian 47 0.0123(9) 0.124 0.049 0.999 0.6(1)

+3.5, −4.0

+3.5, −3.2

β (◦ ) V (Å3 ), Z

Calculated density (Mg m−3 ) Crystal shape Max dimensions (mm) µ (Mo Kα) (mm−1 ) Scan mode θ range (◦ ) Range in h, k, l Measured reflections Independent reflections Rint (before absorption correction) Rint (after absorption correction) Reflections with Fo > 4σ (Fo ) Transmission ratio Tmax /Tmin Absorpion correction Refined parameters Extinction coefficient wR (F2o ) all data R (Fo > 4σ (Fo )) Goodness-of-fit TWIN/BASF −3

1ρmax , 1ρmin (e Å )

ω

2–35

−7 to 7; 0 to 7; 0 to 18

Lattice constants from Guinier powder patterns; the corresponding values from single crystal measurements are equivalent within standard deviations. Cell parameters of LT-EuZn2 Sn2 are obtained from the sample quenched and then annealed at 400 ◦ C—7 days.

possible bombing or explosion of the crucible), shaken to ensure homogenization and re-melted twice. Neither contamination of the sample by the container material, nor reactivity towards it, was noticed. The crucibles were then sealed under vacuum in quartz tubes and annealed in resistance furnaces; then, the alloys were either air-cooled or water-quenched (this latter treatment carried out by suddenly breaking the tube in a cold-water bath after having extracted it from the furnace). The alloys prepared as above were subjected to thermal analyses: about 0.8 g closed by arc welding into Mo crucibles, or 0.4–0.5 g sealed into Ta crucibles, were respectively transferred to a DTA (differential thermal analysis) or a DSC (differential scanning calorimetry) equipment and subjected to heating–cooling cycles at the rates of 5 or 10 ◦ C/min; the temperature was measured to an accuracy of ±5 and ± 1 ◦ C, respectively for DTA and DSC. Metallographic specimens were prepared by standard techniques and examined by both optical and electron microscopy (SEM); semi-quantitative analysis of the phases present was performed on selected samples by microprobe (EDX). X-ray analysis was carried out on powders and single crystals. Powder patterns were obtained by a Guinier–Stoe camera using the Cu Kα1 radiation and pure Si as an internal standard (a = 5.4308 Å); these were compared with the calculated ones (Lazy–Pulverix program [6]). Then, the lattice parameters were determined by the least squares methods. Single crystal intensities were collected at 297 K on a Bruker–Nonius MACH3 diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.7107Å). The structure refinements were accomplished by using the SHELXL-97 program [7]: see Table 1 for further details of data collections and refinements. Magnetisation, as a function of temperature and applied magnetic field, was measured on a superconducting quantum interference device, SQUID (Quantum Design) and vibrating sample VSM (Oxford Instruments) magnetometers. Low temperature heat capacity was measured using a semi-adiabatic, heat pulse method on a home-built setup. Electrical resistivity was measured using fourprobe D.C. method; a D.C. current of 30–50 mA was applied in the

resistivity measurements. 151 Eu Mössbauer spectra were recorded at various temperatures using a conventional constant acceleration spectrometer with a 500 mCi 151 SmF3 source. 3. Results and discussion 3.1. Phase formation and crystal structure DTA analyses showed that EuZn2 Sn2 forms by a peritectic reaction at 755 ◦ C; a first and much weaker exothermic peak, detected on cooling at 815 ◦ C, is supposed to be due to the congruent formation of the equiatomic EuZnSn compound. The alloy annealed at 740 ◦ C—8 days, and then quenched, was found to be homogeneous when examined micrographically. Its Guinier powder pattern was easily and fully indexed on the basis of the primitive tetragonal CaBe2 Ge2 -type cell. The same sample, after a further annealing at 400 ◦ C—7 days, showed a powder pattern which was broadly similar to the earlier one but with a shifting in the positions of some lines, along with splitting of some others; moreover, similar X-ray powder patterns were obtained from either slowly-cooled or annealed specimens (at 600, 580, 530, 400, 300 ◦ C). This was clearly attributed to a structural transition, and the low-temperature patterns could be interpreted assuming the monoclinic symmetry of the LaPt2 Ge2 cell; this latter structure type has been reported by Venturini et al. [8]. The doubled cell, obtained more recently for LT-LaPt2 Ge2 [9], was also checked but no superstructure lines were detected in the Guinier patterns. DTA and DSC analyses in the temperature range between 600 ◦ C (highest temperature observed up to this moment for the occurrence of the low temperature form) and 755 ◦ C (temperature of peritectic decomposition) revealed no relevant thermal effect. At this point, further annealing tests done between 620 and 650 ◦ C, followed suddenly by quenching, converged at a transition temperature of 645 ◦ C, thus confirming the dimorphic nature of EuZn2 Sn2 . With the aim of comparison,

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Table 2 Standardised atomic coordinates and equivalent displacement parameters for the high-temperature (HT) and the low-temperature (LT) forms of EuZn2 Sn2 . Atom

Wyckoff site

2

x

y

z

Ueq ( Å )

1/4 1/4 3/4 1/4 3/4

1/4 1/4 1/4 1/4 1/4

0.75271(8) 0.3628(2) 0 0.13526(11) 1/2

0.0138(2) 0.0264(6) 0.0183(4) 0.0152(3) 0.0218(3)

0.25852(11) 0.2608(3) 0.7213(3) 0.22061(13) 0.75955(15)

1/4 0.2581(18) 0.2493(15) 0.2500(9) 0.2512(12)

0.75363(5) 0.36104(13) 0.00089(12) 0.13268(6) 0.49974(7)

0.0140(1) 0.0198(4) 0.0163(3) 0.0126(2) 0.0153(2)

HT-EuZn2 Sn2 Tetragonal, tP10, P4/nmm Eu Zn1 Zn2 Sn1 Sn2

2c 2c 2a 2c 2b

LT − EuZn2 Sn2 Monoclinic, mP10, P21 Eu Zn1 Zn2 Sn1 Sn2

2a 2a 2a 2a 2a

In LT-EuZn2 Sn2 the y coordinate of Eu was fixed to 1/4. In both structures, the ratios between maximum and minimum Uij’s values are within 2. Table 3 Interatomic distances [Å] in HT and LT EuZn2 Sn2 . HT-EuZn2 Sn2

LT-EuZn2 Sn2

Eu

4 Sn1 4 Zn1 4 Zn2 4 Sn2 4 Eu

3.440(1) 3.455(1) 3.608(1) 3.656(1) 4.519(1)

Zn1

Sn1 4 Sn2 4 Eu

2.588(3) 2.746(1) 3.455(1)

Zn2

4 Sn1 4 Eu

2.733(1) 3.608(1)

Sn1

Zn1 4 Zn2 4 Eu

2.588(3) 2.733(1) 3.440(1)

Sn2

4 Zn1 4 Sn2 4 Eu

2.746(1) 3.195(1) 3.656(1)

Eu

Zn1

Zn1 2 Sn1 Zn1 Zn2 Zn1 2 Sn1 Zn1 Zn2 Zn2 Sn2 Sn2 Sn2 Sn2 Zn2 2 Eu 2 Eu

3.417(6) 3.433(3) 3.466(6) 3.492(2) 3.509(5) 3.534(3) 3.557(6) 3.594(4) 3.598(5) 3.653(1) 3.662(4) 3.669(3) 3.712(1) 3.762(2) 4.556(1) 4.596(1)

Sn1 Sn2 Sn2 Sn2 Sn2 Eu Eu Eu Eu

2.592(2) 2.747(9) 2.760(2) 2.799(9) 2.808(2) 3.417(6) 3.466(6) 3.509(5) 3.557(6)

in Fig. 1 the room-temperature X-ray powder patterns of the two polymorphs are shown. The high-temperature (HT) form [tetragonal CaBe2 Ge2 -type, P4/nmm, tP10; a = 4.528(1) Å, c = 11.370(2) Å] is stable above T = 645 ◦ C while the low-temperature (LT) form is stable below T = 645 ◦ C [monoclinic LaPt2 Ge2 -type [8], P21 , mP10; a = 4.596(1) Å, b = 4.552(1) Å, c = 11.331(2) Å, β = 90.85◦ ]; the transition proved to be reversible. The subsequent single crystal analyses confirmed the structural assignments in both cases. In particular, for the monoclinic phase, the initial cell automatically obtained by the diffractometer was that reported above, which was transformed following Ref. [9] by means of the matrix [001, 010, 200]. A data collection of the hkl reflections with l odd in the θ range 2◦ –15◦ showed null intensities (within experimental errors) for all of them, thus allowing us to exclude the occurrence of the superstructure in the present case. The refined positions and displacement parameters, after standardisation [10], are reported in Table 2. The origin was chosen in the centre of symmetry for HT-EuZn2 Sn2 , and fixing at 1/4 the y coordinate of Eu in LT-EuZn2 Sn2 , in such a way as to have a direct comparison between the two sets of coordinates. While the

Zn2

Sn1 Sn1 Sn1 Sn1 2 Zn2 Eu Eu Eu Eu

2.720(2) 2.747(7) 2.753(6) 2.762(2) 3.054(2) 3.492(2) 3.594(4) 3.598(5) 3.762(2)

Sn1

Zn1 Zn2 Zn2 Zn2 Zn2 2 Eu 2 Eu

2.592(2) 2.720(2) 2.747(7) 2.753(6) 2.762(2) 3.433(3) 3.534(3)

Sn2

Zn1 Zn1 Zn1 Zn1 2 Sn2 2 Sn2 Eu Eu Eu Eu

2.747(9) 2.760(2) 2.799(9) 2.808(2) 3.174(1) 3.299(1) 3.653(1) 3.662(4) 3.669(3) 3.712(1)

atom distribution over the different crystallographic sites remains unchanged on going from the HT- to the LT form, with Zn1 and Sn1 occupying the antiprismatic positions, and Zn2 and Sn2 in the tetrahedral ones, a slight variation in the atomic coordinates occurs, breaking the tetragonal symmetry with a corresponding broadening of the bond lengths values (Table 3). CaBe2 Ge2 and LaPt2 Ge2 are closely related structures, the monoclinic structure being a deformation derivative of the tetragonal one: they are two representatives of the numerous structural family derived from the aristotype BaAl4 , recently collected and discussed in a general article [11]. The three structures, together with the ThCr2 Si2 type (ternary ordered version of BaAl4 ), are drawn in the Fig. 2. Following the group-subgroup relationship in the Bärnigausen formalism, as reported in [11], the transformation from BaAl4 (ThCr2 Si2 ) to CaBe2 Ge2 to LaPt2 Ge2 occurs by transitions t2 (translationgleiche of index 2): I 4/m2/m2/m (BaAl4 /ThCr2 Si2 ) → P4/n21 /m2/m(CaBe2 Ge2 )

→ P21 /m21 /m21 /n → P121 /m1 → P121 1(LaPt2 Ge2 ). The deformation of the tetragonal cell causes a symmetry reduction with consequent formation of the monoclinic cell. The

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Fig. 1. Comparison of the X-ray powder patterns of HT- and LT-EuZn2 Sn2 , showing the reflections which split due to the structural change. Both patterns are collected at room temperature: the upper plot refers to the DTA sample annealed at 740 ◦ C—8 days and then water quenched; the lower plot is obtained from the same sample, after remelting and annealing at 520 ◦ C—4 days.

Fig. 2. Drawing of HT- and LT-EuZn2 Sn2 , showing the close similarity of the two structures. For comparison, the aristotype BaAl4 and its ternary ordered variant, ThCr2 Si2 , are also shown.

structural transition likely occurs by a displacive mechanism, i.e. it only requires the movement of some atoms without breaking and/or formation of new bonds, therefore without significant changes in the coordination and bond distances, thus accounting for the negligible thermal effects during the structural transition. 3.2. Magnetic properties Fig. 3 shows the magnetic susceptibility data taken on both phases. The susceptibility χ shows a relatively broad peak in both HT- and LT-EuZn2 Sn2 (upper inset of Fig. 3), characteristic

of an antiferromagnetic transition: at low temperatures both forms order antiferromagnetically, respectively at 10.3 and 9.8 K, as inferred from the peak temperature. The peak is slightly broader in HT-EuZn2 Sn2 , this could likely be due to the intrinsically metastable nature of this phase at room and low temperatures (obtained by quenching). The slight upturn in the susceptibility of LT-EuZn2 Sn2 below 7 K may either be intrinsic due to some complicated antiferromagnetic arrangement of the magnetic moments or it may arise due to the presence of trace magnetic impurities. The inverse susceptibility, χ −1 , varies linearly with temperature in a substantial range in the paramagnetic region below 300 K. A least squares fitting of the Curie–Weiss expression χ = C /(T − θp ) to the data between 70 and 300 K (shown by the solid line drawn through the data points in Fig. 3) gives µeff = 7.6 and 8.0 µB (derived from the fitted values of C), and θp = −0.3 and −2.4 K, for HT-EuZn2 Sn2 and LT-EuZn2 Sn2 , respectively. The value of µeff in LT-EuZn2 Sn2 is close to the Hund’s rule derived value of 7.94 µB for the free Eu2+ ion, whilst it is slightly lower than the free Eu2+ ion in HT-EuZn2 Sn2 . A negative θp is in conformity with the antiferromagnetic transition seen in the two forms. However, the antiferromagnetic transition occurs at substantially higher temperatures than suggested by the values of θp . For divalent Eu ions with total orbital angular momentum L = 0, crystal field effects vanish in the first order and hence do not have any influence on θp . The magnetic isotherms at 1.8 K up to 7 T are shown in the lower inset of Fig. 3; they are in conformity with the antiferromagnetic ordering observerd in these phases. The magnetic isotherm of LT-EuZn2 Sn2 increases relatively rapidly at low fields (<0.1 T) indicating the possible presence of trace magnetic impurity in the sample, which may also be responsible for the upturn in the susceptibility of this compound at low temperatures as mentioned above. It may be noted that the magnetic transition temperatures of EuZn2 Sn2 is nearly the same as that of EuZn2 Ge2 (10 K) [5] which crystallises (like HT-EuZn2 Sn2 ) in the CaBe2 Ge2 -type structure. The bulk nature of the magnetic ordering in the two polymorphs is confirmed by the observation of huge peaks in the heat capacity, C , at temperatures lying close to the peak temperatures of the corresponding magnetic susceptibility. The data are plotted in Fig. 4 as C versus the temperature T . The peak in the heat capacity of LT-EuZn2 Sn2 is sharper and occurs at a slightly lower temperature compared with that for HT-EuZn2 Sn2 ; qualitatively, this observation is in good agreement with the susceptibility data. Since the corresponding non-magnetic La analog is not available

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Fig. 3. Inverse susceptibility of LT- and HT-EuZn2 Sn2 vs. temperature. The solid lines are fits of the Curie–Weiss expression to the data. The upper inset shows the susceptibility below 15 K; the lower inset shows magnetic isotherms at 1.8 K measured up to 7 T.

Fig. 5. The Mössbauer spectra of EuZn2 Sn2 at selected temperatures between 300 and 4.2 K (the spectra of the two forms are practically indistinguishable).

Fig. 4. The variation of the heat capacity of both forms of EuZn2 Sn2 with temperature.

as reference compound for the lattice heat capacity, the entropy associated with the magnetic transitions could not be estimated. The electrical resistivity of the two polymorphs of EuZn2 Sn2 is very similar. This shows that the slight monoclinic distortion of the LT form does not affect the phonon spectrum in any significant way, such that the electron phonon scattering has a similar behaviour in both phases. The resistivity does not show any discernible anomaly at the magnetic transition unlike the magnetisation and the heat capacity data presented above. At lower temperatures there is a superconducting transition in both the samples which occurs close to the known superconducting transition temperature of Sn and suggests the presence of trace amounts of free Sn in the samples. 151 Eu Mössbauer spectroscopy is an excellent tool to probe microscopically Eu compounds as the isomer shifts of the Eu2+ and Eu3+ states are markedly different giving rise to well separated peaks in the Mössbauer spectra corresponding to these two integer valence states. The single peak spectrum of the paramagnetic

state splits in the magnetically ordered state due the presence of magnetic and quadrupolar hyperfine fields at the Eu nucleus. The spectra recorded on the two polymorphs are practically indistinguishable (Fig. 5). In the paramagnetic state the spectra consist of a single line with an isomer shift of about −11.9 mm/s, characteristic of Eu2+ . A strong hyperfine split spectrum is observed below 10 K; this is in agreement with the magnetisation data confirming the magnetic ordering in both these systems. The well resolved spectrum was fit taking into account a magnetic hyperfine field and an electric quadrupolar splitting. The hyperfine field obtained by fitting is about 29 T at 4.2 K, comparable with the field of 33 T found in a typical divalent compound EuO. However there is a strong asymmetry displayed by the spectrum which can be attributed to a significant electric field gradient. The quadrupolar splitting is found to be 2.5 mm/s. To conclude, we report the existence of a new compound EuZn2 Sn2 and its dimorphism. The various physical measurements are consistent with the divalent nature of the Eu ions, which order antiferromagnetically near 10 K in both forms. References [1] F. Merlo, M. Pani, M.L. Fornasini, J. Less-Common Met. 171 (1991) 329–336. [2] R. Pöttgen, Z. Kristallogr. 210 (1995) 924–928. [3] U. Ernet, R. Müllmann, B.D. Mosel, H. Eckert, R. Pöttgen, G. Kotzyba, J. Mater. Chem. 7 (1997) 255–257. [4] A. Grytsiv, D. Kaczorowski, A. Leithe-Jasper, P. Rogl, C. Godart, M. Potel, H. Noël, J. Solid. State Chem. 163 (2002) 37–43. [5] C. Kranenberg, D. Johrendt, A. Mewis, R. Pöttgen, G. Kotzyba, H. Trill, B.D. Mosel, J. Solid. State Chem. 167 (2002) 107–112. [6] K. Yvon, W. Jeitschko, E. Parthé, J. Appl. Crystallogr. 10 (1977) 73–74. [7] M. Sheldrick, SHELXL-97, Program for Refinement of Crystal Structures, University of Göttingen, Germany, 1997. [8] G. Venturini, B. Malaman, B. Roques, J. Less-Common Met. 146 (1989) 271–278. [9] A. Imre, A. Hellmann, A. Mewis, Z. Anorg. Allg. Chem. 632 (2006) 2217–2221. [10] L.M. Gelato, E. Parthé, J. Appl. Crystallogr. 20 (1987) 139–143. [11] D. Kußmann, R. Pöttgen, U.Ch. Rodewald, C. Rosenhahn, B.D. Mosel, G. Kotzyba, B. Künnen, Z. Naturforsh. 54b (1999) 1155–1164.