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Preparation, Crystal Structure, and Properties of the Lanthanoid Carbides Ln4C7 with Ln = Ho, Er, Tm, and LuRalf Czekalla, Wolfgang Jeitschko*, Rolf-Dieter Hoffmann, Helmut Rabeneck
Anorganisch-Chemisches Institut der Universität Münster,Wilhelm-Klemm-Straße 8, D-48149 Münster, Germany
Z. Naturforsch. 51 b, 646-654 (1996); received August 28, 1995Binary Lanthanoid Carbides, Magnetic Properties, Hydrolyses of Carbides, Crystal Structure
The isotypic carbides Ln4C7 (Ln = Ho, Er, Tm, Lu) were prepared by arc-melting of the elemental components, followed by annealing at 1300 °C. The positions of the metal and of some carbon atoms of the monoclinic crystal structure of LU4C7 were determined from X-ray powder data, and the last carbon positions were found and refined from neutron powder diffraction data: P2,/c, a = 360.4(1), b = 1351.4(3), c = 629.0(2) pm, ß = 104.97(2)°, Z = 2, R = 0.026 for 429 structure factors and 15 positional parameters. The structure contains isolated carbon atoms with octahedral lutetium coordination and linear Cvunits, with C-C bond lengths of 132( 1) and 135( 1) pm. This carbide may therefore be considered as derived from methane and propadiene. The hydrolysis of LU4C7 with distilled water yields mainly methane and propine, while the hydrolyses of the corresponding holmium and erbium carbides resulted in relatively large amounts of saturated and unsaturated C2-hydrocarbons in addition to the expected products methane and propine. The structure comprises two-dimensionally infinite NaCl-type building elements, which are separated by the C3-units. It may be described as a stacking variant of a previously reported structure of H04C7, now designated as the a-modification. The Lu4C7-type /^-modification was obtained at higher temperatures. Its structure was refined by the Rietveld method from X-ray powder data to a residual R = 0.037 for 320 F values and 15 positional parameters. Lu4C7 is Pauli paramagnetic; /3-H04C7 and Er4C7 show Curie-Weiss behavior with magnetic ordering temperatures of less than 20 K.
Introduction
The phase diagrams of the binary systems of the heavy rare earth elements with carbon have not been determined, but several phases o f those systems have been known for some time [1]. At the metal-rich sides of these system s defect NaCl- type carbides are known with com positions varying around Ln^C and Lm C. They are stable at high tem peratures and can be retained as metastable phases at ambient temperature [1, 2 ] , At lower tem peratures a hexagonal modification of H 0 2 C with (anti-) C dC ^-type structure has been reported [1, 3, 4], With higher carbon contents several compounds have been published with the approxim ate com positions Ln : C = 15 : 19. Their Sc3C 4-type structure has been established more recently [5]. H 0 2 C 3 , E nC ^, T 1TI2C 3 , and LU2C 3 are reported to be isotypic with PU2C 3 [ 1 ,2 , 6 ,7]. The latter three carbides have been prepared at high pressure and temperature [8 , 9].
* Reprint requests to W. Jeitschko.
At the carbon-rich sides of these systems various modifications of the compositions LnC 2 have been characterized [ 1 , 2 , 10-13].
Very recently the crystal structures of Y4C 7 and H0 4C 7 have been established [14]. The latter com pound was prepared at lower temperature (1450 K) than the modification of H0 4C7 reported here. We therefore designate the earlier characterized m odification as Q-H0 4C 7 , while the presently reported com pound is designated as /3 -H0 4C 7 . At least one additional phase exists in that binary system with the approximate composition H0 5C6 or H0 4C 5 [7, 15].
Sample Preparation and Lattice Constants
Starting materials for the preparation of the b inary carbides were ingots of the rare-earth elements (Kelpin, 99.9%) and graphite flakes (Ven- tron, 99.5 %). The samples were prepared by arc- melting cold-pressed pellets of the elemental com ponents with the composition of Ln:C = 2:5. The pellets were melted several times from both sides to ensure good homogeneity. Subsequently the
samples were placed in closed carbon crucibles and annealed in an argon atmosphere at 1300 °C for four days. Samples of LU4C7 were also wrapped in tantalum foils and annealed in evacuated silica tubes at 900 °C and 1000 °C for four weeks.
The Guinier powder diagrams of the quenched arc-melted samples — recorded under dried (Na) paraffin oil — showed diffuse diffraction lines, which seemed to be different from the sharp diffraction patterns of the annealed samples. The patterns of LU4C7 annealed at 900, 1000, and 1300 °C were practically identical. Monoclinic indices for these patterns could be assigned with the aid of a computer program [16]. Subsequently the proper assignment of these indices was checked by intensity calculations [17] using the positional parameters of the refined structures. The lattice constants (Table I) were obtained by least-squares fits o f the Guinier powder data using Cu Kct\ radiation with a -quartz (a = 491.30, c = 540.46 pm) as an internal standard. The cell volumes show the expected lanthanoid contraction.
Structure Determination and Refinements
The structure analyses were based on X-ray and neutron powder diffraction data o f the lutetium com pound. Since the samples are very sensitive to the humidity of the air, the annealed ingots were ground to a fine powder in a glove box under dry nitrogen. The samples used for the X-ray investigations were mixed with a-quartz to minimize absorption and filled with the aid o f ultrasonic sound into silica tubes with an outer diameter of 0.1 mm. The sample of LU4C7 investigated by neutron diffraction was prepared from 13 ingots with a total weight of 8 g. The powder was sealed in an aluminum tube.
X-ray intensity data were recorded from a rotating sample on a focussing powder diffractometer (STOE Stadi P) using monochromated C uK «i radiation and a linear position-sensitive detector in Debye-Scherrer geometry. The neutron powder diffraction data of LU4C7 were measured on the E2/1 diffractometer at the Hahn-Meitner-Institut in Berlin. A collimator with an opening angle of 30’ was placed in front of the monochromator and a flat cone multidetector was used. The sample absorption was determined by a separate experiment. Further data of these experiments and the structure refinements are summarized in Table II.
The positions of the lutetium, the C l, and the C4 atoms could be determined from 60 structure factors extracted from the X-ray data using Patterson and difference Fourier syntheses [18-20]. The C2 and C3 atoms were located by difference
X-rayLu4C7
neutron/3-H04.C7X-ray
Radiation Cu Kqi A = 121.66 pm Cu Kq ,Monochromator Ge (111) Ge (311) Ge (111)Formula weight 783.9 743.8Formula units per cell Z = 2 Z = 2Calculated density (g/cirr) 8.80 7.68Container (0) silica (0.1 mm) A l(8 mm) silica (0.1 mm)Range in 26 12° - 100° 9°-89° 12°-98°Stepwidth in 26 0 .02° 0 . 1° 0 .02°Total measuring time 69 h 23 h 96 hTotal number of steps 4401 793 4301Intensity range 1214-52084 1734-20943 1650-35894Number of reflections 307 429 320Number of structural parameters 15 15 15Goodness of fit ( \ 2) 4.99 10.1 3.19Bragg residual /?Bragg = 0.039 / ? B r a g g = 0.047 B̂rasg = 0.054Conventional residual Rf = 0.034 Rf = 0.026 Rf =0.037
Table II. Crystallographic data of LU4C7 and /3-Ho4C7.
648 R. Czekalla et al. • Lanthanoid Carbides
Table III. Atomic parameters for /3-H04C7 and L114C7. The positional parameters were standardized by the program STRUCTURE TIDY [30]. The last column lists the isotropic displacement parameters B (x 10-4, in nm2 units) of the metal atoms. The corresponding parameters of the carbon atoms were held at the indicated values in all refinements.
Atom P2|/c .V V z B
/3-Ho4C7Hoi 4e 0.0426(5) 0.1645(1) 0.1603(2) 0.46(4)Ho2 4e 0.3312(5) 0.4377(1) 0.2146(3) 0.69(4)Cl 4e 0.289(6) 0.608(2) 0.089(4) 0.6C2 4e 0.405(6) 0.696(2) 0.062(3) 0.6C3 4e 0.464(6) 0.285(2) 0.421(3) 0.6C4 2e 0 0 0 0.6LU4C7Lul 4e 0.0582(4) 0.16026(9) 0.1620(2) 0.53(3)Lu2 4e 0.3625(4) 0.4449(4) 0.2377(2) 0.73(4)Cl 4e 0.262(2) 0.6047(5) 0.046(1) 0.6C2 4e 0.376(2) 0.6974(5) 0.0418(9) 0.6C3 4e 0.454(2) 0.2869(4) 0.4192(9) 0.6C4 2e 0 0 0 0.6
Fourier maps using the neutron powder diffraction data. Eventually the structure o f LU4C 7 was refined
Fig. 1. Rietveld refinement plots for /j-Ho-jC? (CuKoi) and LU4C7 (neutron diffraction data). In the uppermost parts of the plots the measured values are indicated by dots and the calculated fits by lines. Below the peak positions of the carbides, the diluent a-quartz, and the impurity phases graphite and aluminum are shown. The difference profiles between the calculated and the observed plots are also given.
Table IV. Interatomic distances in the structures of /3-Ho4 C7 and L114C7 . All distances shorter than 460 pm (Ln-Ln), 325 (Ln-C), and 250 pm (C-C) are listed.
with the program FULLPROF [21] using both the X-ray and the neutron diffraction data. In iterating cycles the positions of the carbon atoms were not allowed to vary when the X-ray data were refined, and the metal positions were fixed in refining the positions of the carbon atoms from the neutron diffraction data (Fig. 1).
The atomic form factors for the X-ray data [22] were corrected for anomalous dispersion [23]. and the nuclear scattering lengths of 6(Lu) = 7.210 fm and 6(C) = 6.646 fm [24] were used for the refinement of the neutron powder profiles. In addition to the positional parameters of all atoms, the thermal parameters of the metal atoms, the zero point, and the lattice constants, four parameters optimizing the pseudo-Voigt profile function with symmetrical
R. Czekalla et al. ■ Lanthanoid Carbides 649
Fig. 2. The L114C7 structure viewed approximately along [100],
peak shape were refined for both the X-ray and the neutron diffraction data. Furthermore we fitted the scale factor (15.9 %) of the diluent a-quartz [25] and the scale factors, lattice constants, positional and profile parameters of the impurity phases graphite ( 1.1 %) [26] and the cubic modification of LU2O 3
(7.3 %) [27, 28] in the case of the X-ray data. During the refinement of the neutron diffraction data the scale factors and the profile fitting parameters of graphite (16.6% ) and aluminum (8.1 %) [29] were optimized.
The structure of /i-H o4C 7 was refined only from X-ray data (Fig. 1), obtained in the same way as described for LU4C7. Interestingly, it was possible to refine also the positions of the carbon atoms to meaningful values, albeit with relatively large standard deviations. This sample contained the diluents Q-quartz (12.8 %) and graphite (4.0 %). The results are summarized in the Tabels II—IV. A stereoplot of the structure is shown in Fig. 2.
Magnetic Properties
The magnetic susceptibilities o f LU4C7, ß- H 0 4 C 7 , and Er4C 7 have been investigated with a SQUID magnetometer (superconducting quantum m terference device; Quantum Design, Inc.) as described earlier [31, 32], LU4C7 shows a low magnetic susceptibility, which is almost independent of the temperature. This behavior can be rationalized
as Pauli param agnetism . The upturn in the susceptibility curve of LU4C7 below 100 K (Fig. 3) might be rationalized as due to a minor amount of a paramagnetic impurity. /J-H0 4 C 7 shows Curie-Weiss behavior and orders antiferrom agnetically with a Neel tem perature Tn = 7±1 K and a Weiss constant 0 = -A ±2 K. Er4C 7 also behaves like a Curie- Weiss param agnet down to 20 K with a Weiss constant o f ( 9 = - 7 ± 2 K . The effective magnetic moments //exp for both com pounds were calculated from the linear portions o f the l / \ vs. T plots above 100K (Fig. 3). As could be expected, they agree well with the theoretical moments /ietf calculated for the free Ln3+ ions [33]: //exp = 10.59 ± 0.05 p b /H o atom, //eft(Ho3+) = 10.60 / / ß ; / i exP = 9.53 ± 0.05 //.#/Er atom, //,etf(Er3+) = 9.58 p B.
Hydrolysis Results
Com pact as well as powdered samples of the Lu4C 7-type com pounds are dark gray and very sensitive to the hum idity of the air. Powdered samples of /3-H 0 4C 7 , Er4C 7 , and LU4C 7 were hydrolyzed with distilled water at room temperature. The gaseous reaction products were separated in fractions by a gas chrom atograph (Dani, 8521-a) and im m ediately thereafter analyzed by mass spectrometry (Finnigan MAT, ITD800). The results (Table V) were com pletly reproducible. However they are affected by errors, especially for methane. This
650 R. Czekalla et al. ■ Lanthanoid Carbides
T [K]
T[K]-----►
T[K]
Fig. 3. Magnetic properties of /3-H04C7, Er4C7, and LU4C 7 . The reciprocal susceptibility l / \ for the Curie- Weiss magnets /3-H04C7 and Er4C7 are plotted as a function of temperature. The insets show the behavior at low temperatures. The temperature dependence of the magnetic susceptibility \ for the Pauli-paramagnetic carbide LU4C 7 is also shown.
was taken into account, nevertheless the results shown for methane in Table V may have relative errors of up to 2 0 %, e. g. for /3 -Ho4C 7 the methane content is 15.9±3.2 wt-%.
Discussion
The structure determined here for the isotypic compounds Lu4C 7 und (?-Ho4C 7 is closely related
Table V. Hydrolysis results of the carbides /3-Ho4C7, Er4C7 and LU4C7 as analyzed by gas chromatography and mass spectrometry. The gaseous reaction products are listed in weight-%.
Reaction product /3-Ho4C7 Er4C7 Lu4C7
c h 4 16.0 35.8 20.0CHvCH^ 14.6 4.7 —
CHvCH-> 4.7 2.6 —
CH CH 6.9 5.5 1.5C H vO TC H , 7.4 3.8 —
CHvCH CHi 26.1 16.1 1.7CHt C CH2 3.3 6.8 5.8CHvCCH 20.3 22.7 71.0CHvCH2 CH c h 2 0.6 2.1 —
to that reported earlier for a -H o 4C 7 [14]. We obtained the more accurate carbon positions from the neutron diffraction data of the lutetium compound, which we consider to be the prototype. Nevertheless, in discussing the differences between the structures of q -H o 4C 7 and Lu4C7 it is more instructive to directly compare the low temperature (a ) m odification of Ho4C 7 with the high temperature (ß ) modification of that compound (Fig. 4). Both structures crystallize in the space group P 2 j/c , and the structures are closely related.*
As is usually the case, the high temperature (ß) modification needs the smaller number of variable param eters for its description. In our case it has only half the cell content (Z = 2) of the low tem perature (a ) modification (Z = 4) and hence only half as many positional parameters. Correspondingly, the cell volume of the /^-modification has to be doubled (2 - 2 x 0.3213 nm 3 = 0.6426 nm3) to be com parable with that of the a-m odification (V Q =0.6245 nm 3). As is usually observed, the cell volume per form ula unit is larger for the high- tem perature form, allowing more space for the thermal m otion in that modification.
Both structures contain puckered, two-dimen- sionally infinite sheets o f rare earth and carbon atoms, which have an atomic arrangement well known from the NaCl-type structure o f the early transition metals (e. g. TiC, ZrC, HfC) and ThC. These NaCl-type sheets are indicated by shading
*In the earlier work on the a-modification the equivalent nonstandard setting P 2 \ / n o f that space group was used. The corresponding lattice constants o f q-H o4C7 in the standard setting P 2 \ / c are: a = 368.06(3), 6= 1251 .8 (1 ), c = 1371.7(2)pm, ß = 98 .85(1)°.
R. Czekalla et al. ■ Lanthanoid Carbides 651
Fig. 4. The earlier determined structure of a-Ho4C7 [14] as compared to the presently reported Ln4C7 structure for /3-H04C7 and L U 4C 7 . The shaded parts of the structures have atomic arrangements similar to that of NaCl. The two-dimensionally infinite slabs A and B are the same in both structures.
in Fig. 4. They are separated by the C 3 -groups derived from propadiene. In that sense the two structures of H 0 4 C 7 resem ble the structures of SC3C4 [5] and Sc5Re2C 7 [34], which also contain building elements o f the N aCl-type structure and C 3 groups with two C -C double bonds.
It can also be seen from Fig. 4 that the lattice constant c (651 pm) o f ß-Wo+Ci is approximately half as large as the lattice constant b (1252 pm) of the ^-m odification. The two structures may be considered as stacking variants. The two-dimensionally infinite slabs extending perpendicular to the ~ and y direction of the ß- and a-m odification, respectively, are practically the same in both structures. In the /^-modification we find the stacking sequence AB, AB, and in the a-m odification the stacking sequence AABB, AABB. M ore com plicated stacking
sequences seem to be possible, e. g. AAB, AAB or AAABBB, AAABBB.
In Fig. 5 we show the near-neighbor coordinations of the L u ;C 7 -type structure. The lutetium atoms are coordinated by nine (L u l) and seven (Lu2) carbon atoms, respectively, at distances covering the range between 238 and 292 pm. In addition, the lutetium atoms have 10 (L u l) and 11 (Lu2) lutetium neighbors with L u-L u distances between 341 and 399 pm. At least the shorter ones of these should be considered as bonding, as is discussed further below. These coordination polyhedra of the lanthanoid atoms in LU4C 7 and /3 -H 0 4 C 7 correspond exactly to those o f the Ho3 and Ho4 atoms of Q-H0 4C 7 . The other two coordination polyhedra of the holmium atoms in Q-H0 4C 7 are com posed of two half-shells, which need to be interchanged to correspond to the polyhedra of the holm ium atoms in the /^-modification.
The isolated carbon atoms (C4 in ß-Wo^Ci and C l in CÜ-H0 4 C 7 ) are situated in octahedral voids formed by the lanthanoid atoms. The other carbon atoms form Q vunits in both modifications of H04C7 . The bond lengths of the C -C bonds in the two modifications of H 0 4C 7 are sim ilar with average values of 132.0(10) and 130.5(15) pm for the Q- and /3-form. The corresponding value for LU4C7 is 133.5(6) pm. These distances are all slightly smaller than the typical C -C double bond distance of 134 pm in olefins. In Fig. 6 we show the nearneighbor coordination of the Q vunits in all structurally characterized com pounds containing these units. In SC3C4 and Sc5Re2C7 [34] the Q -u n its are coordinated by ten metal atoms, in Ca3C l2C 3 [35] and the other carbides by eight. In M g2Q? [36] and Sc5Re2C 7 the C -C -C angle is required by sym m etry to be exactly 180°. This is not the case in the other compounds, and consequently these angles are found to deviate somewhat from the ideal value.
Chemical bonding in LU4C7 may be rationalized very roughly by oxidations numbers, where all bonding electrons of the lutetium -carbon interactions are counted at the carbon atoms. In assum ing that the carbon atoms obey the octet rule and in assigning double bonds to the C -C interactions one may write the form ula as [8 Lu3+]24+ [4(C 1— C 2-C 3)4“ ] 16- [2C44 - ]8 - . If this form ulation were entirely correct no electrons would be left for lutetium -lutetium bonding. In view o f the shortest L u-Lu distances of 341.5 pm we believe that
652 R. Czekalla et al. ■ Lanthanoid Carbides
this bonding should not be neglected. We remind the reader of the isotypic series CaO, ScN and TiC with NaCl-type structure. The form ulas Ca2+0 2~, Sc3+N 3 - , and Ti4+C4~ correspond to the formula given above for LU4C 7 . Nevertheless, it is well known, that the T i-T i bonding is very important for the stability of the high-m elting carbide TiC [37, 38]. In the isotypic carbide HfC (with a melting point o f almost 4000 °C [39]) each hafnium atom has (besides the six carbon neighbors) twelve hafnium neighbors at a distance of 326 pm [40]. For the nonexisting isotypic carbide LuC a corresponding L u-Lu bond distance of 341 pm can be calculated (The metallic radii o f the lutetium and hafnium atoms for the coordination number 12 are173.4 and 158.0 pm, respectively 141]). As can be seen from Table IV there are many L u-L u distances, which are only slightly greater than 341 pm, and we believe that the corresponding interactions should all be considered as bonding.
The hydrolysis results also deserve some com ments. It is well known that AI4C 3 and CaC 2 are hydrolyzed by water to give methane and acetylene. respectively, the reaction products expected
Fig. 5. Near-neighbor environments in L114C7.
from the crystal structures o f these carbides. To our knowledge these are the only carbides, where the hydrocarbons resulting from hydrolysis correspond to the bonding of the carbon atoms in the solid. The hydrolysis o f M g2C 3 yields essentially propine and only up to 20 % propadiene [42], the product expected from the crystal structure [36]. The hydrolyses of the earlier characterized binary carbides of the lanthanoids resulted in various hydrocarbons, depending on the reaction conditions [43-45]. A great variety of hydrocarbons is obtained in the hydrolysis of ternary lanthanoid and actinoid transition metal carbides [46, 47], even though some of these solids contain only carbon atoms, which are isolated from each other [48, 49].
Similarly, the hydrocarbons resulting from the hydrolysis o f J -H 0 4 C 7 , Er4C 7 , and LU4C 7 with distilled water (Table V) do not correspond to the arrangements of the carbon atoms established by the structure determination. The structure contains 6
carbon atoms per form ula unit form ing two C vunits corresponding to propadiene, and one carbon atom, which is isolated. Therefore, one could expect the hydrolyses to yield propadiene and methane in the
R. Czekalla et al. • Lanthanoid Carbides 653
Fig. 6. Environments of the Ci-units in SciC4 [5], Ca,CbC’, [35], Ms^O [36] ScsRe,C7 [34], o-Ho4C7 [14], and Lu4C7.
ratio 83: 17. This is approximately in agreement with the results obtained for LU4C-7, if we do not differentiate between the various C3-species. The predominance o f propine (instead of propadiene) in the reaction products of all three binary lanthanoid carbides Ln4C7 correlates with the results of the above mentioned hydrolysis o f MgoCv
In contrast to the results obtained for LU4C-7, the hydrolyses of J-H o4C7 and E14C7 resulted in relatively large amounts of C2-hydrocarbons (Table V). These differences are difficult to rationalize. Obviously the hydrocarbons are formed by heterogeneous reaction on the surface of the Ln4C7 crystals. Possibly, minor amounts of unknown impurities at the grain boundaries or within the crystals become important. It seems also possible, that the different hydrolysis behavior of the erbium and the holmium carbides on the one hand, and that of the lutetium carbide on the other hand, is due to the open and filled f shells, respectively, o f the corresponding
elements. This difference is not reflected in the structural chemistry of these elements, where all differences between these elements can be rationalized by the differences in the atomic volumes of these elements. However, the filled or unfilled f shells may become important in the transition states o f the reaction kinetics.
Acknowledgements
We thank Dipl.-Phys. H.-M. Meyer and Dr. D. Hohlwein (Hahn Meitner Institut. Berlin) for the collection of the neutron diffraction data. The magnetic susceptibilities were competently measured by Dipl.- Phys. K. Hartjes. We are also indebted to Mr. K. Wagner, who characterized our samples by scanning electron microscopy and Dr. R. Pöttgen for preliminary preparative work. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
654 R. Czekalla et al. • Lanthanoid Carbides
[1] K. A. Gschneidner (Jr.), F. W. Calderwood. in "Binary Alloy Phase Diagrams" (T. B. Massaiski, ed.) Amer. Soc. for Metals, Metals Park, Ohio (1986).
[2] F. H. Spedding, K. A. Gschneidner (Jr.), A. H. Daane, J. Am. Chem. Soc. 80, 4499 (1958).
[3] G. L. Bacchella, P. Meriel, M. Pinot, R. Lallement, Bull. Soc. Fr. Mineral. Cristallogr. 89. 226 (1966).
[4] M. Atoji, J. Chem. Phys. 74, 1893 (1981).[5] R. Pöttgen, W. Jeitschko, Inorg. Chem. 30, 427
(1991).[6] M. Atoji, Y. Tsunoda, J. Chem. Phys. 54, 3510
(1971).[7] J. Bauer, P. Vennegues, J. L. Vergneau, J. Less-
Common Met. 110, 295 (1985).[8] M. C. Krupka, N. H. Krikorian, Proc. 8th Rare
Earth Research Conference, Vol. 2, T. A. Henrie, R. E. Lindstrom, Eds., National Technical Information Service, Springfield, VA, p. 382 (1970).
[9] V. I. Novokshonov, Zh. Neorg. Khim. 25,684 (1980).[10] M. Atoji, J. Chem. Phys. 35, 1950 (1961).[11] M. Atoji, J. Chem. Phys. 46. 1891 (1967).[12] M. Atoji, R. H. Flowers, J. Chem. Phys. 52, 6430
(1970).[13] N. H. Krikorian, T. C. Wallace, M. G. Bowman.
Colloques Internationaux du Centre National de la Recherche Scientifique, Paris. 157, 489 (1967).
[14] H. Mattausch, T. Gulden, R. K. Kremer, J. Horakh, A. Simon, Z. Naturforsch. 49b, 1439 (1994).
[15] R. Czekalla, R.-D. Hoffmann, T. Hüfken, R. Pöttgen, W. Jeitschko, Z. Kristallogr. Suppl. 10, 96 (1995).
[16] J. W. Visser, J. Appl. Crystallogr. 2, 89 (1969).[17] K. Yvon, W. Jeitschko, E. Parthe, J. Appl. Crystal
logr. 10. 73 (1977).[18] G. M. Sheldrick, SHELXS-86, Univ. of Cambridge,
England (1986).[19] G. M. Sheldrick, SHELXL-93, A program for crystal
structure determination, Göttingen (1993).[20] H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1965).[21] J. Rodriguez-Carvajal, FULLPROF version 2.6.1.
Oct. 94, ILL (unpublished) based on the original code provided by D. B. Wiles and A. Sakthivel, J. Appl. Crystallogr. 14, 149 (1981).
[22] D. T. Cromer. J. B. Mann, Acta Crystallogr. A 24. 321 (1968).
[23] D. T. Cromer, D. Liberman, J. Chem. Phys. 53. 1891(1970).
[24] V. F. Sears, Atomic Energy of Canada Limited, Report AECL-8490 (1984).
[25] Y. Le Page. L. D. Calvert, E. J. Gabe, J. Phys. Chem. Solids 41, 721 (1980).
[28] H. R. Hoekstra, K. A. Gingerich. Science 146. 1163 (1964).
[29] P. P. Ewald, C. Hermann. Strukturbericht 1. 43 (1931).
[30] L. M. Gelato, E. Parthe. J. Appl. Crystallogr. 20, 139 (1987).
[31] M. Reehuis, T. Vomhof, W. Jeitschko, J. Phys. Chem. Solids. 55, 625 (1994).
[32] M. E. Danebrock, W. Jeitschko, A. M. Witte, R. Pöttgen, J. Phys. Chem. Solids. 56, 807 (1995).
[33] S. Legvold, in “Ferromagnetic Materials” (E. P. Wohlfarth, ed.) Vol. 1, pp. 183-295. North-Holland, Amsterdam (1980).
[34] R. Pöttgen, W. Jeitschko, Z. Naturforsch. 47b, 358(1992).
[35] H.-J. Meyer, Z. Anorg. Chem. 593, 185 (1991).[36] H. Fjellväg. P. Karen. Inorg. Chem. 31, 3260 (1992).[37] S. D. Wijeyesekera. R. Hoffmann, Organometallics
3, 949(1984).[38] P. Blaha, K. Schwarz, F. Kübel, K. Yorn, J. Solid
State Chem. 70. 199 (1987).[39] R. C. Weast (Ed.), Handbook of Chemistry and
[40] P. Villars, L.-D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, 2nd edn., American Society for Materials International, Materials Park, Ohio (1991).
[41] E. T. Teatum, K. A. Gschneidner (Jr.), J. T. Waber, LA-2345, U.S. Department of Commerce, Washington, D.C. (1960). See: W. B. Pearson: The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New York (1972).
[42] J. F. Cordes, K. Wintersberger, Z. Naturforsch. 12b, 136(1957).
[43] J. S. Anderson, N. J. Clark. I. J. McColm, J. Inorg. Nucl.Chem. 30. 105 (1968).
[44] F. H. Pollard, G. Nickiess, S. Evered, J. Chromatog. 15,211 (1964).
[45] H. J. Svec, J. Capellen, F. E. Saalfeld, J. Inorg. Nucl. Chem. 26, 721 (1964).
[46] W. Jeitschko, M. H. Gerss, R.-D. Hoffmann, S. Lee, J. Less-Common Met. 156. 397 (1989).
[47] N. J. Clark, R. Mountford, I. J. McColm, J. Inorg. Nucl. Chem. 34. 2729(1972).
[48] D. T. Cromer. A. C. Larson. R. B. Roof (Jr.), Acta Crystallogr. 17, 272 (1964).
[49] M. H. Gerss, W. Jeitschko, Z. Naturforsch. 41b, 946(1986).
