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Magnetic clusters and magnetic interactions in single-crystal studies of K(RE)Nb6Cl18 and (RE)Nb6Cl18 (RE = Lu and Tm)
Solid State Communications, Vol. 74, No. 4, pp. 285-290, 1990. Printed in Great Britain.
0038-1098/90 $3.00 + .00 Pergamon Press plc
M A G N E T I C CLUSTERS A N D M A G N E T I C I N T E R A C T I O N S IN S I N G L E - C R Y S T A L STUDIES O F K(RE)Nb6Clls A N D (RE)Nb6CII8 (RE = Lu A N D Tm) Octavio Pefia,* Saadia Ihma'ine, Christiane Perrin and Marcel Sergent Laboratoire de Chimie Min6rale B. U.R.A.C.N.R.S. no 254, Universit6 de Rennes I, 35042 Rennes C6dex, France
(Received 17 September 1989 by P. Burlet) Recently isolated ternary and quaternary niobium chlorides containing rare earth (RE = Lu and Tm) atoms were studied. Single-crystal magnetic data were analyzed as a superposition of effects due to Nb 6 clusters and RE ions. One unpaired electron localized in an INb6CI~813- unit is observed in ternary RENb6CI~8 compounds. In addition, for RE = Tm, a paramagnetic behaviour due to the Tm 3÷ moment is found at high temperatures, while a VanVleck susceptibility due to a singlet non-magnetic ground state is obtained at low temperatures. Magnetic interactions were observed in LuNb6Clt8 single crystals.
1. I N T R O D U C T I O N
magnetic ground state. Then, the magnetic effects should be easily separated out at low temperatures.
IN T H E C H E M I S T R Y of low oxidation states of niobium, several well-known compounds exhibit non2. E X P E R I M E N T A L magnetic or paramagnetic INb6Xl8 I" units (X = halogen), depending on the number of valency 2.1. Sample preparation electrons per Nb 6 cluster [1]. When the cluster contains RENb6CI~8 (RE = Tm, Lu) are prepared from 16 or 14 electrons (n = 4 or 2), the unit is non mag- RECI3 (synthesized from RE203 treated under HC1 as netic. On the other hand, one unpaired electron per described in [4], NbCI 5 (VENTRON, purity 99.998%) cluster is found when n = 3 (15e-/Nb6 units); in this and Nb (VENTRON, purity 99.8%) in stoichiometric latter case, the clusters are magnetic with an effective proportions. The mixture, handled under inert atmosmoment/~efr = grS(S + 1)1~/2 = 1.73/a B, well localized phere, is ground, pressed into pellets and heated at in the unit. 750°C during 24 h in an evacuated silica tube. Some Magnetic entities such as rare-earth atoms intro- pieces of niobium foil (VENTRON, 0.025 mm thick, duced as counter cations among these INb6Xisl"- purity 99.8%) are put together with the starting anions are of great interest as they lead to four poss- pellets to make the synthesis under reducing conible magnetic behaviours, depending on the n-value ditions (the niobium foil is removed afterwards to and on the nature of the rare earth. These different avoid a superconducting contribution at low temperapossibilities are found in the recently isolated tures). KRENb6CI~8 are prepared at 800°C from KC1 chlorides RENb6CII8 and MRENb6CII8 (RE = rare (PROLABO, purity 99.5%), REC13, NbC15 and Nb, e a r t h , a n d M = monovalent cation), which exhibit following a similar procedure as described above respectively 15 and 16 valency electrons per Nb6 clus- for the ternary compounds. For all compounds, the ter [2, 3]. In addition, magnetic interactions at low resulting powder is black and stable in open air. Powder temperature may eventually occur between clusters samples were characterized by X-rays diffraction. and/or magnetic rare earths. Single crystals of LuNb6CIIs, TmNb6Cll8 and In this work, we present magnetic studies on KTmNb6CI~8 (about l mm 3 in size) were obtained LuNb6CI~8 and KLuNb6CII8, in which the rare earth i~ directly by heating the starting materials for several non magnetic, and on TmNb6Clt8 and KTmNb6CIIs, days at the synthesis temperature [5]. In the case of in which the rare-earth atom is magnetic. The case KLuNb6CiI8 however, crystals were slightly smaller of thullium is particularly interesting because of its and magnetic measurements had to be performed on non-Kramers nature, which usually provides a non- sintered pellets, as discussed below. All single crystals studied in this work were selected after Scanning* Author to whom correspondence should be addressed. Electron-Microscopy observations (JEOL JSM 35 CF) 285
STUDIES OF K(RE)Nb6CII8 A N D (RE)Nb6Clj8 (RE = Lu A N D Tm)
286
and microprobe analysis by using Energy-DispersiveSpectrometry EDS ( T R A C O R - N O R T H E R N T N 2000). 2.2. Crystallographic characterization Ternary and quaternary chlorides are is•typic, and crystallize in the space group R3. Lattice parameters, interatomic distances and a full description of the structure are given elsewhere [2, 3, 5, 6] (as an example, a = 9.948(1)A, ~ = 55.201(5) °, for LuNb6CI~8 ). All these compounds exhibit INb6Cllg[" units with Nb 6 clusters (average N b - N b intracluster distances are 2.92 A for KRENb6CII8 and 2.96 A for RENb6Clj8 ). The units are located at the centre of the rhombohedral unit-cell, with the rare-earth atoms lying at the origin of the lattice. In the quaternary compounds, the potassium atom is situated on the ternary axis with a statistical site occupancy of ½. This latter site is empty in the ternary chlorides. For all these compounds, the Nb 6 clusters are about 9.22A apart (centre to centre), and the shortest R E - N b distances are about 4.85 A [7].
Vol. 74, No. 4
where [6]. An ensemble of single crystals (total weight between 3.5 and 4mgs) were measured in the case of TmNb6CI~8 and KTmNb6CII8 compounds. For the lutetium-based compounds, susceptibility measurements were carried out under magnetic fields of 5 and 10kOe, after verification of a linear fielddependence of the magnetization at low temperatures. In the case of lutetium compounds, experimental data were fitted only below 50 K, since a large signal due to the sample holder precludes any reasonable analysis of results at higher temperatures. For thullium-based ternary and quaternary niobium chlorides, a l - k • • field was large enough to get good sensitivity, even at room temperature. 3. RESULTS 3.1. A non-magnetic rare-earth: R E = Lu
3.1.1. Low temperature susceptibility. Results obtained for KLuNb6CI~8 and LuNb6C1]8 were presented in a separate report [6], but are here recalled for further comparison with the thullium compounds. 2.3. Magnetic measurements The most important results obtained are summarized Magnetic susceptibility measurements were per- in Fig. 1: firstly, a nearly temperature-independent formed in a SHE-906 SQUID magnetometer, between behaviour for the quaternary compound KLuNb6CI~s, room temperature and 2 K. Single crystals were used with a suceptibility value of + 7.8 × 10 4 (emu/mol), in most of the experiments, except in the case of which corresponds to the VanVleck contribution KLuNb6CI~8, where a sintered-powder pellet of Zvv estimated by Converse and McCarley [1] for 100mgs was used because of the small paramagnetic the ]M6XI21 x+ cluster compounds. Secondly, a parasignal. In the case of LuNb6CI~8, measurements were magnetic susceptibility for the ternary compound performed on several single crystals separately (the LuNb6CI~8, which may be fitted by a Curie-Weiss largest one weighing about 900 micrograms), in order behaviour (insert, Fig. 1), with an effective moment of to confirm the intrinsic character and reproducibility 1.50#B, and a paramagnetic Curie temperature of of the magnetic results exposed below. Other experi- about - 1 K. Extrinsic contributions to the paramagnetic mental details concerning sampling were given elseCurie-Weiss susceptibility of LuNb6Cit8 were ruled out [6] on the basis of materials elaboration: paramagnetic impurities are less likely to occur in single crystals of LuNb6CI~8 than in sintered powder of 120 KLuNb6CIjg, both prepared from the same starting • ,° components. It was concluded then, that such a mag4 ; 80 .." netic contribution observed in the case of LuNb6CI~8 x Io i oe • comes, in fact, from the presence of the magnetic • T(K: 1Nb6Cl1813 units in this compound. • ••o 'i0 20 30 t.O o°
r• 0
KLoNb-Ct-'''' u°O00°O°~o
0
10
•
.
_
I
0 0 0
20
30
40
50
T(K) Fig. 1. Magnetic susceptibilities of single crystals of LuNb6CI~8 and powdered sample of KLuNb6CI~8. Insert shows the reciprocal susceptibility of LuNb6CI~8.
3.1.2. Magnetic interactions at low temperatures. From Fig. 1, it is seen that a maximum occurs at about 2 K for LuNb6CI~8 single crystals. Such an effect was confirmed in several single crystals coming from different preparations. Figure 2 shows a detail of the susceptibility peak after two kinds of subtraction of paramagnetic contributions: (a) the ZDIA and XTJPcomponents [6] were subtracted from the experimental values ( - 7 1 7 × 10 6 and 780 x 10 -6 emu/
287
STUDIES OF K(RE)Nb6CI,8 AND (RE)Nb6C1,8 (RE = Lu AND Tm)
Vol. 74, No. 4
7..(em u/mole) x 10-3 60
~gl (em u/g) -I
*+'4-++
04[-
T=2K
o
4
x10
6 °
5 0.20~
++
.
r, Ooo + o i; (b)
40
~°
Tm N b 6 C / 1 8
4
o +
3
o
~ -
(a) 2
o +
30
° ++ o o
Lu Nb 6 Cl18
lUe~: 7.87~u B
+ o
O : -1.5 K
1 +
j'"
T(K)
o T(K) 2%
I
I
I
I
I
2
4
6
8
10
:~
Fig. 2. Maximum of the susceptibility of LuNb6Clt8 corrected by: (a) temperature independent ZmA and Zvv, or (b) using the KLuNb6CI~s values. Insert shows the linear dependence of the magnetization versus magnetic field for LuNb6CIIs single crystals. mol, respectively); (b) the magnetic susceptibility of KLuNb6CI~s (Fig. l) was subtracted from Ztot,~, in order to correct any extrinsic contribution coming from the non-magnetic compound• In both cases, a clear maximum is observed, situated at the N6el temperature of about 2.3 and 2.5 K, depending on the type of correction made. Magnetization measurements were performed at 2K in LuNb6CI~8 to determine the nature of such interactions (insert, Fig. 2). A strict linearity of M = f(H) was observed upto 50kOe, with no hysteresis or remanent magnetization with decreasing field. These magnetization and susceptibility results suggest an antiferromagnetic character for such interactions. 3.2. A magnetic rare-earth: R E = Tm 3.2.1. High temperature susceptibility. Figures 3 and 4 show the reciprocal susceptibility below room
5
2 o
o
o
X
(era u / g )
0
300
-1
xlO 3
K Tm _ _ ~ [
m )- o ° o o • " •
° o° 0mmoc~OO od~°°
4
.
•
.'~-z
•
Tm Nb6CIm
o.~-'"
T(K)
T(K) i
I
200
o
1 i"" 0
I
3.2.2. Low temperature susceptibility. Figure 5 shows the reciprocal susceptibility below 50K for KTmNb6CI~8 and TmNb6CI~8 single crystals. Mag-
4
0:-3.3K
I
temperature for KTmNb6CI,8 and TmNbgCI,s single crystals. No subtraction of temperature-independent components (Zing or ZTm)was made because of their negligible values compared to the experimental magnetic susceptibility. In both cases, data were fitted by Curie-Weiss laws above 50 K, with effective moments of 7.54 and 7.87 #a for KTmNb6Clls and TmNb6CI~8, respectively. The paramagnetic Curie temperatures are quite small ( - 3.3 and - 1.5 K, respectively). Only in the first case, the effective moment agrees perfectly well with the free-ion value of Tm 3+ (#th = 7.56#B). In the case of the ternary compound, the enhancement of the effective moment comes from the superposed contributions of both the trivalent rareearth ion and the magnetic cluster, as discussed below.
6
o °TLLeff=7.54}4 B
I
Fig. 4. Reciprocal magnetic susceptibility of single crystals of TmNb6Clls.
8
KTmNb6C/18
I
100
10
X g (e m u/g )-I 6,\x104
I
0
i
100 ' 260 ' 3~0>
Fig. 3. Reciprocal magnetic susceptibility of single crystals of KTmNb6Clu.
0
0
t
~
i
~
i
10
20
30
40
50
:>
Fig. 5. Reciprocal magnetic susceptibilities below 50 K of single crystals of TmNb6Cl, 8 and KTmNb6CIts.
STUDIES O F K(RE)Nb6CII8 A N D (RE)Nb6Clt8 (RE = Lu A N D Tm)
288 AX(emu/~ ~1C)-~
,, x-l(e,.,,u/g) -~ ~,,{ 10 4
<
25 }
20 ,~
/
0=0K
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3.2.4. Magnetization. Figure 7 shows the magnetization of TmNb6Cl~8 single crystals at 2.6K. Neither hysteresis nor a remanent magnetization were observed, meaning that no magnetic order exists at low temperatures. No apparent anisotropy was observed, but further work on oriented single crystals should allow us to confirm this assertion [8].
15
4. DISCUSSION l0
As discussed above, appropriate combinations of rare-earth cations and cluster units may yield quite Oo %°°°°°° different magnetic behaviours. The simplest situations .... r(K) L I t i > 0 J correspond to non-magnetic rare earths (e.g. Lu) or 40 60 80 100 0 20 those having non-magnetic ground states (e.g. Tm). In Fig. 6. Magnetic susceptibility of the cluster AZ those cases, the magnetic state of the cluster may be {z(TmNb6CI~8)-z(KTmNb6CI~8)} below 100 K. Insert obtained by straightforward subtraction of the non shows the reciprocal of the cluster susceptibility below magnetic "matrix" (quaternary compound). It must 50K. b e emphasized that this procedure is strictly rigorous from a magnetic point of view, due to the isotypisme netic susceptibility of KTmNb6CI~8 does not vary of the crystallographic structures and the similarities much below 10K, and reaches a value of 2.3 × 10 -4 of the interatomic distances between the ternary and (emu g-~) at 2 K, indicating a non-magnetic ground quaternary series. state. Magnetic behaviour of TmNb6Clj8 is quite different: a positive curvature at about 20 K suggests the 4.1. Cluster contribution beginning of a plateau similar to the one observed in From Figs. 1 and 6 (inserts), it becomes clear that KTmNb6CI~8, but at lower temperature the reciprocal an additional magnetic contribution occurs in ternary susceptibility decreases much faster and an inflection compounds, which is not observed in the quaternaries. point is clearly observed at about 10 K. When subtracting the magnetic susceptibilities of the 3.2.3. Magnetic cluster contribution. Figure 6 "matrix", an effective moment of 1.50/~B or 2.4/~B is shows the difference A • c l u s t e r between the magnetic obtained for RE = Lu and Tm respectively, which susceptibilities of TmNb6CI~8 and KTmNb6Clts. The corresponds approximately to the effective moment inverse susceptibility follows a linear behaviour (1.73/~) of one unpaired electron per cluster. below 40K, with an effective moment of 2.4/~B. It is noteworthy the fact that such magnetic this value agrees with the effective moment (2.2/tB) moments are obtained in single crystals of ternary obtained after subtraction of the Curie constants in compounds which are, a priori, less exposed to extrinthe range 50 K < T < 300 K, and indicates the same sic impurities. Hence, such magnetic contributions are origin for the additional contribution (magnetic clus- due to an intrinsic behaviour connected to the oxiters) in TmNb6CI~8 at both high and low temperatures. dation state of the INb6CI~81" unit. It is possible however that slight deviations to stoichiometry may M(emu/g) /% influence the exact value of the observed moment. For instance, slight potassium deficiencies may occur in 12 / "°f / ~vv : 23, ~0-4 the quaternaries, related to the statistical occupancy of the potassium site in the structure (6), then creatj o/ ing paramagnetic units which will "contaminate'" the non-magnetic matrix: this may be the case of Tm Nb6CI18 single crystals KLuNb6CI~8 powdered sample, where a small moment 4 d °~ % of 0.4/~B may be due to a maximum of 5% vacancies T:2.6 K o Z_~ / o in the structure (6). ¢ I I I l I 1 I I I I Other authors have also reported a reduction 3O 40 50 0 10 20 of magnetic moments in compounds containing H (kOe) Fig. 7. Magnetization at 2.6 K for TmNb6Cl~8 single IMe6X~2["+ (Me = Ta or Nb; x = 4, 3, 2) units crystals. For estimations of Zvv and saturation mag- (1, 9): moments ranged from 1.45 to 1.67/~B, and the reduction attributed to small residual orbital contrinetization, see text. 5
Vol. 74, No. 4
STUDIES OF K(RE)Nb6Clj8 AND (RE)Nb6Cll8 (RE = Lu A N D Trn)
butions resulting from spin-orbit coupling (1). In our case, such an orbital contribution to the resultant magnetic moment is evidenced by Land6 g factors smaller than 2, as found in EPR experiments performed in these chlorides (g between 1.943 and 1.993) (10). On the contrary, it is more difficult to explain an enhanced moment of 2.4ttB, as observed in the thullium-based system. No magnetic interactions are seen at low temperatures which could account for such an increase of the effective moment, and so far we have no explanations to this phenomenon. A similar result, even more pronounced, was also observed in the holmium-based compounds of these series [5], and suggests enhancement mechanisms due to the presence of two magnetic entities. 4.2. Low temperature magnetic behaviour Figures 3 and 5 suggest a non-magnetic singlet ground state for the thullium ions in these niobium chlorides. Such an hypothesis agrees quite reasonably with the non-Kramers nature of Tm 3+, as also observed in other ternary systems containing cluster units (e.g. TmMo6S 8 [11]). The almost constant value observed below 10K for KTmNb6CI~8 characterizes a temperature-independent term :~vv of the Tm 3+ ion. Thus, the magnetic susceptibility behaviour of TmNb6Cl~s may be described as: )~ = C / T +
Zvv
where the paramagnetic Curie term C/T due to the magnetic cluster is superposed to the VanVleck susceptibility due to Tm 3+. The quantity gvv can also be estimated from the experimental values of TmNb6CI~8 in a gT-versus-T plot, and yields 2.1 < Zvv x 104 emug ~ < 2.6, depending on the range of temperatures used for the fitting. We have preferred to take the experimental value of 2.3 x 10 4 (emu/g) (0.32emumol ~) obtained for KTmNb6Clls, and we have calculated the paramagnetic constant C ( ~ 0.75 e m u m o l - ~) and the effective moment #c~ (~2.45ttB). These values confirm our previous estimations for the cluster contribution. By the same way, we may write the low temperature magnetization as: M
=
M cluster
+ M 3+ = M d"stcr + zvvH
(where M 3+ is the contribution of Tm 3+ in its crystalfield ground state), provided no admixing occurs between excited states of Tm 3+ due to the applied field. In Fig. 7 we have drawn (dotted line) a tangent line corresponding to the M 3+ (ZvvH) term. Extrapolation to zero field should give the cluster contribution M c~u~terto the magnetization, which is of the
289
order of 0.5 #B. This value is half the one expected for one unpaired electron (gS = I#B; g = 2), and probably comes from the fact that the magnetization is not saturated at these fields and temperatures. 4.3. Magnetic interactions Low temperature magnetic interactions apparently exist in LuNb6CI18 , as suggested from: the maximum in the magnetic susceptibility, the negative value of the Curie temperature ( - 1 K), and the linear behaviour of the magnetization (Fig. 2). Interesting enough is the fact that no such interactions were observed in other members of the series [5, 12], independent of the nature of the rare-earth atoms. This result may suggest that an important parameter for the existence of such interactions must be the intercluster distances, which are smaller in the case of lutetium. Attempts to reduce this distance by appropriate choice of other cations or anions are currently under way [13]. CONCLUSION The new families of chlorides MRENb6CI18 and RENb6CI~s recently isolated [2, 3, 5, 7] allowed the study of the magnetic behaviours of either the Nb6 cluster or the rare-earth ion separately, as well these two magnetic entities together. In this report we have presented two examples in which the cluster contribution is clearly distinguished: in the first case (RE = Lu), magnetic properties depend only on the oxidation state of the cluster; in the second (RE = Tm), the magnetic state of trivalent thullium is added to the cluster. At low temperatures, a VanVleck susceptibility due to the Tm 3+ singlet ground state is observed in KTmNb6Cl18, and is confirmed in the corresponding ternary compound TmNb6Cl~s. By subtracting this term, we obtain again the contribution of the magnetic cluster. Other interesting combinations are possible, and constitute separate works published elsewhere [8, 12]. REFERENCES 1. 2. 3. 4. 5. 6. 7.
J.G. Converse & R.E. McCarley, Inorg. Chem. 9, 1361 (1970). S. Ihmaine, C. Perrin & M. Sergent, C.R. Acad. Sc. Paris 303, Sbrie II, 1293 (1986). C. Perrin, S. Ihmaine & M. Sergent, New J. Chem. 12, 321 (1988). G. Meyer & P. Ax, Mat. Res. Bull. 17, 1447 (1982). S. Ihmaine, University Thesis, Universitb de Rennes I (France), 1988. S. Ihmaine, C. Perrin, O. Pefia & M. Sergent, J. Less Common Met. 137, 323 (1988). S. Ihmaine, C. Perrin & M. Sergent, Acta Cryst. C43, 813 (1987).
290 8. 9. 10.
STUDIES OF K(RE)Nb6Cll8 AND (RE)Nb6CIt8 (RE = Lu AND Tm) O. Pefia, C. Perrin & M. Sergent, to be published. N. Brni6evi6, Z. Ru~i6-Toro~ & B. Koji6-Prodi6, J. Chem. Soc. Dalton Trans. 3, 455 (1985). M. Rabii, G. Alqui6 & C. Perrin, Proceed. Colloque O.H.D., Rennes (France) September 1989.
11. 12.
13.
Vol. 74, No. 4
D.R. Noakes, G.K. Shenoy & D.G. Hinks, Phys. Rev. B31, 5712 (1985). S. Ihmaine, C. Perrin, O. Pefia & M. Sergent, Proceed. Int. Conf. Physics Highly Correlated Electron Systems. Santa Fe (New Mexico, USA). Sept. 1989. Physica B, in press. C. Perrin et al., work in progress.
0038-1098/90 $3.00 + .00 Pergamon Press plc
M A G N E T I C CLUSTERS A N D M A G N E T I C I N T E R A C T I O N S IN S I N G L E - C R Y S T A L STUDIES O F K(RE)Nb6Clls A N D (RE)Nb6CII8 (RE = Lu A N D Tm) Octavio Pefia,* Saadia Ihma'ine, Christiane Perrin and Marcel Sergent Laboratoire de Chimie Min6rale B. U.R.A.C.N.R.S. no 254, Universit6 de Rennes I, 35042 Rennes C6dex, France
(Received 17 September 1989 by P. Burlet) Recently isolated ternary and quaternary niobium chlorides containing rare earth (RE = Lu and Tm) atoms were studied. Single-crystal magnetic data were analyzed as a superposition of effects due to Nb 6 clusters and RE ions. One unpaired electron localized in an INb6CI~813- unit is observed in ternary RENb6CI~8 compounds. In addition, for RE = Tm, a paramagnetic behaviour due to the Tm 3÷ moment is found at high temperatures, while a VanVleck susceptibility due to a singlet non-magnetic ground state is obtained at low temperatures. Magnetic interactions were observed in LuNb6Clt8 single crystals.
1. I N T R O D U C T I O N
magnetic ground state. Then, the magnetic effects should be easily separated out at low temperatures.
IN T H E C H E M I S T R Y of low oxidation states of niobium, several well-known compounds exhibit non2. E X P E R I M E N T A L magnetic or paramagnetic INb6Xl8 I" units (X = halogen), depending on the number of valency 2.1. Sample preparation electrons per Nb 6 cluster [1]. When the cluster contains RENb6CI~8 (RE = Tm, Lu) are prepared from 16 or 14 electrons (n = 4 or 2), the unit is non mag- RECI3 (synthesized from RE203 treated under HC1 as netic. On the other hand, one unpaired electron per described in [4], NbCI 5 (VENTRON, purity 99.998%) cluster is found when n = 3 (15e-/Nb6 units); in this and Nb (VENTRON, purity 99.8%) in stoichiometric latter case, the clusters are magnetic with an effective proportions. The mixture, handled under inert atmosmoment/~efr = grS(S + 1)1~/2 = 1.73/a B, well localized phere, is ground, pressed into pellets and heated at in the unit. 750°C during 24 h in an evacuated silica tube. Some Magnetic entities such as rare-earth atoms intro- pieces of niobium foil (VENTRON, 0.025 mm thick, duced as counter cations among these INb6Xisl"- purity 99.8%) are put together with the starting anions are of great interest as they lead to four poss- pellets to make the synthesis under reducing conible magnetic behaviours, depending on the n-value ditions (the niobium foil is removed afterwards to and on the nature of the rare earth. These different avoid a superconducting contribution at low temperapossibilities are found in the recently isolated tures). KRENb6CI~8 are prepared at 800°C from KC1 chlorides RENb6CII8 and MRENb6CII8 (RE = rare (PROLABO, purity 99.5%), REC13, NbC15 and Nb, e a r t h , a n d M = monovalent cation), which exhibit following a similar procedure as described above respectively 15 and 16 valency electrons per Nb6 clus- for the ternary compounds. For all compounds, the ter [2, 3]. In addition, magnetic interactions at low resulting powder is black and stable in open air. Powder temperature may eventually occur between clusters samples were characterized by X-rays diffraction. and/or magnetic rare earths. Single crystals of LuNb6CIIs, TmNb6Cll8 and In this work, we present magnetic studies on KTmNb6CI~8 (about l mm 3 in size) were obtained LuNb6CI~8 and KLuNb6CII8, in which the rare earth i~ directly by heating the starting materials for several non magnetic, and on TmNb6Clt8 and KTmNb6CIIs, days at the synthesis temperature [5]. In the case of in which the rare-earth atom is magnetic. The case KLuNb6CiI8 however, crystals were slightly smaller of thullium is particularly interesting because of its and magnetic measurements had to be performed on non-Kramers nature, which usually provides a non- sintered pellets, as discussed below. All single crystals studied in this work were selected after Scanning* Author to whom correspondence should be addressed. Electron-Microscopy observations (JEOL JSM 35 CF) 285
STUDIES OF K(RE)Nb6CII8 A N D (RE)Nb6Clj8 (RE = Lu A N D Tm)
286
and microprobe analysis by using Energy-DispersiveSpectrometry EDS ( T R A C O R - N O R T H E R N T N 2000). 2.2. Crystallographic characterization Ternary and quaternary chlorides are is•typic, and crystallize in the space group R3. Lattice parameters, interatomic distances and a full description of the structure are given elsewhere [2, 3, 5, 6] (as an example, a = 9.948(1)A, ~ = 55.201(5) °, for LuNb6CI~8 ). All these compounds exhibit INb6Cllg[" units with Nb 6 clusters (average N b - N b intracluster distances are 2.92 A for KRENb6CII8 and 2.96 A for RENb6Clj8 ). The units are located at the centre of the rhombohedral unit-cell, with the rare-earth atoms lying at the origin of the lattice. In the quaternary compounds, the potassium atom is situated on the ternary axis with a statistical site occupancy of ½. This latter site is empty in the ternary chlorides. For all these compounds, the Nb 6 clusters are about 9.22A apart (centre to centre), and the shortest R E - N b distances are about 4.85 A [7].
Vol. 74, No. 4
where [6]. An ensemble of single crystals (total weight between 3.5 and 4mgs) were measured in the case of TmNb6CI~8 and KTmNb6CII8 compounds. For the lutetium-based compounds, susceptibility measurements were carried out under magnetic fields of 5 and 10kOe, after verification of a linear fielddependence of the magnetization at low temperatures. In the case of lutetium compounds, experimental data were fitted only below 50 K, since a large signal due to the sample holder precludes any reasonable analysis of results at higher temperatures. For thullium-based ternary and quaternary niobium chlorides, a l - k • • field was large enough to get good sensitivity, even at room temperature. 3. RESULTS 3.1. A non-magnetic rare-earth: R E = Lu
3.1.1. Low temperature susceptibility. Results obtained for KLuNb6CI~8 and LuNb6C1]8 were presented in a separate report [6], but are here recalled for further comparison with the thullium compounds. 2.3. Magnetic measurements The most important results obtained are summarized Magnetic susceptibility measurements were per- in Fig. 1: firstly, a nearly temperature-independent formed in a SHE-906 SQUID magnetometer, between behaviour for the quaternary compound KLuNb6CI~s, room temperature and 2 K. Single crystals were used with a suceptibility value of + 7.8 × 10 4 (emu/mol), in most of the experiments, except in the case of which corresponds to the VanVleck contribution KLuNb6CI~8, where a sintered-powder pellet of Zvv estimated by Converse and McCarley [1] for 100mgs was used because of the small paramagnetic the ]M6XI21 x+ cluster compounds. Secondly, a parasignal. In the case of LuNb6CI~8, measurements were magnetic susceptibility for the ternary compound performed on several single crystals separately (the LuNb6CI~8, which may be fitted by a Curie-Weiss largest one weighing about 900 micrograms), in order behaviour (insert, Fig. 1), with an effective moment of to confirm the intrinsic character and reproducibility 1.50#B, and a paramagnetic Curie temperature of of the magnetic results exposed below. Other experi- about - 1 K. Extrinsic contributions to the paramagnetic mental details concerning sampling were given elseCurie-Weiss susceptibility of LuNb6Cit8 were ruled out [6] on the basis of materials elaboration: paramagnetic impurities are less likely to occur in single crystals of LuNb6CI~8 than in sintered powder of 120 KLuNb6CIjg, both prepared from the same starting • ,° components. It was concluded then, that such a mag4 ; 80 .." netic contribution observed in the case of LuNb6CI~8 x Io i oe • comes, in fact, from the presence of the magnetic • T(K: 1Nb6Cl1813 units in this compound. • ••o 'i0 20 30 t.O o°
r• 0
KLoNb-Ct-'''' u°O00°O°~o
0
10
•
.
_
I
0 0 0
20
30
40
50
T(K) Fig. 1. Magnetic susceptibilities of single crystals of LuNb6CI~8 and powdered sample of KLuNb6CI~8. Insert shows the reciprocal susceptibility of LuNb6CI~8.
3.1.2. Magnetic interactions at low temperatures. From Fig. 1, it is seen that a maximum occurs at about 2 K for LuNb6CI~8 single crystals. Such an effect was confirmed in several single crystals coming from different preparations. Figure 2 shows a detail of the susceptibility peak after two kinds of subtraction of paramagnetic contributions: (a) the ZDIA and XTJPcomponents [6] were subtracted from the experimental values ( - 7 1 7 × 10 6 and 780 x 10 -6 emu/
287
STUDIES OF K(RE)Nb6CI,8 AND (RE)Nb6C1,8 (RE = Lu AND Tm)
Vol. 74, No. 4
7..(em u/mole) x 10-3 60
~gl (em u/g) -I
*+'4-++
04[-
T=2K
o
4
x10
6 °
5 0.20~
++
.
r, Ooo + o i; (b)
40
~°
Tm N b 6 C / 1 8
4
o +
3
o
~ -
(a) 2
o +
30
° ++ o o
Lu Nb 6 Cl18
lUe~: 7.87~u B
+ o
O : -1.5 K
1 +
j'"
T(K)
o T(K) 2%
I
I
I
I
I
2
4
6
8
10
:~
Fig. 2. Maximum of the susceptibility of LuNb6Clt8 corrected by: (a) temperature independent ZmA and Zvv, or (b) using the KLuNb6CI~s values. Insert shows the linear dependence of the magnetization versus magnetic field for LuNb6CIIs single crystals. mol, respectively); (b) the magnetic susceptibility of KLuNb6CI~s (Fig. l) was subtracted from Ztot,~, in order to correct any extrinsic contribution coming from the non-magnetic compound• In both cases, a clear maximum is observed, situated at the N6el temperature of about 2.3 and 2.5 K, depending on the type of correction made. Magnetization measurements were performed at 2K in LuNb6CI~8 to determine the nature of such interactions (insert, Fig. 2). A strict linearity of M = f(H) was observed upto 50kOe, with no hysteresis or remanent magnetization with decreasing field. These magnetization and susceptibility results suggest an antiferromagnetic character for such interactions. 3.2. A magnetic rare-earth: R E = Tm 3.2.1. High temperature susceptibility. Figures 3 and 4 show the reciprocal susceptibility below room
5
2 o
o
o
X
(era u / g )
0
300
-1
xlO 3
K Tm _ _ ~ [
m )- o ° o o • " •
° o° 0mmoc~OO od~°°
4
.
•
.'~-z
•
Tm Nb6CIm
o.~-'"
T(K)
T(K) i
I
200
o
1 i"" 0
I
3.2.2. Low temperature susceptibility. Figure 5 shows the reciprocal susceptibility below 50K for KTmNb6CI~8 and TmNb6CI~8 single crystals. Mag-
4
0:-3.3K
I
temperature for KTmNb6CI,8 and TmNbgCI,s single crystals. No subtraction of temperature-independent components (Zing or ZTm)was made because of their negligible values compared to the experimental magnetic susceptibility. In both cases, data were fitted by Curie-Weiss laws above 50 K, with effective moments of 7.54 and 7.87 #a for KTmNb6Clls and TmNb6CI~8, respectively. The paramagnetic Curie temperatures are quite small ( - 3.3 and - 1.5 K, respectively). Only in the first case, the effective moment agrees perfectly well with the free-ion value of Tm 3+ (#th = 7.56#B). In the case of the ternary compound, the enhancement of the effective moment comes from the superposed contributions of both the trivalent rareearth ion and the magnetic cluster, as discussed below.
6
o °TLLeff=7.54}4 B
I
Fig. 4. Reciprocal magnetic susceptibility of single crystals of TmNb6Clls.
8
KTmNb6C/18
I
100
10
X g (e m u/g )-I 6,\x104
I
0
i
100 ' 260 ' 3~0>
Fig. 3. Reciprocal magnetic susceptibility of single crystals of KTmNb6Clu.
0
0
t
~
i
~
i
10
20
30
40
50
:>
Fig. 5. Reciprocal magnetic susceptibilities below 50 K of single crystals of TmNb6Cl, 8 and KTmNb6CIts.
STUDIES O F K(RE)Nb6CII8 A N D (RE)Nb6Clt8 (RE = Lu A N D Tm)
288 AX(emu/~ ~1C)-~
,, x-l(e,.,,u/g) -~ ~,,{ 10 4
<
25 }
20 ,~
/
0=0K
Vol. 74, No. 4
3.2.4. Magnetization. Figure 7 shows the magnetization of TmNb6Cl~8 single crystals at 2.6K. Neither hysteresis nor a remanent magnetization were observed, meaning that no magnetic order exists at low temperatures. No apparent anisotropy was observed, but further work on oriented single crystals should allow us to confirm this assertion [8].
15
4. DISCUSSION l0
As discussed above, appropriate combinations of rare-earth cations and cluster units may yield quite Oo %°°°°°° different magnetic behaviours. The simplest situations .... r(K) L I t i > 0 J correspond to non-magnetic rare earths (e.g. Lu) or 40 60 80 100 0 20 those having non-magnetic ground states (e.g. Tm). In Fig. 6. Magnetic susceptibility of the cluster AZ those cases, the magnetic state of the cluster may be {z(TmNb6CI~8)-z(KTmNb6CI~8)} below 100 K. Insert obtained by straightforward subtraction of the non shows the reciprocal of the cluster susceptibility below magnetic "matrix" (quaternary compound). It must 50K. b e emphasized that this procedure is strictly rigorous from a magnetic point of view, due to the isotypisme netic susceptibility of KTmNb6CI~8 does not vary of the crystallographic structures and the similarities much below 10K, and reaches a value of 2.3 × 10 -4 of the interatomic distances between the ternary and (emu g-~) at 2 K, indicating a non-magnetic ground quaternary series. state. Magnetic behaviour of TmNb6Clj8 is quite different: a positive curvature at about 20 K suggests the 4.1. Cluster contribution beginning of a plateau similar to the one observed in From Figs. 1 and 6 (inserts), it becomes clear that KTmNb6CI~8, but at lower temperature the reciprocal an additional magnetic contribution occurs in ternary susceptibility decreases much faster and an inflection compounds, which is not observed in the quaternaries. point is clearly observed at about 10 K. When subtracting the magnetic susceptibilities of the 3.2.3. Magnetic cluster contribution. Figure 6 "matrix", an effective moment of 1.50/~B or 2.4/~B is shows the difference A • c l u s t e r between the magnetic obtained for RE = Lu and Tm respectively, which susceptibilities of TmNb6CI~8 and KTmNb6Clts. The corresponds approximately to the effective moment inverse susceptibility follows a linear behaviour (1.73/~) of one unpaired electron per cluster. below 40K, with an effective moment of 2.4/~B. It is noteworthy the fact that such magnetic this value agrees with the effective moment (2.2/tB) moments are obtained in single crystals of ternary obtained after subtraction of the Curie constants in compounds which are, a priori, less exposed to extrinthe range 50 K < T < 300 K, and indicates the same sic impurities. Hence, such magnetic contributions are origin for the additional contribution (magnetic clus- due to an intrinsic behaviour connected to the oxiters) in TmNb6CI~8 at both high and low temperatures. dation state of the INb6CI~81" unit. It is possible however that slight deviations to stoichiometry may M(emu/g) /% influence the exact value of the observed moment. For instance, slight potassium deficiencies may occur in 12 / "°f / ~vv : 23, ~0-4 the quaternaries, related to the statistical occupancy of the potassium site in the structure (6), then creatj o/ ing paramagnetic units which will "contaminate'" the non-magnetic matrix: this may be the case of Tm Nb6CI18 single crystals KLuNb6CI~8 powdered sample, where a small moment 4 d °~ % of 0.4/~B may be due to a maximum of 5% vacancies T:2.6 K o Z_~ / o in the structure (6). ¢ I I I l I 1 I I I I Other authors have also reported a reduction 3O 40 50 0 10 20 of magnetic moments in compounds containing H (kOe) Fig. 7. Magnetization at 2.6 K for TmNb6Cl~8 single IMe6X~2["+ (Me = Ta or Nb; x = 4, 3, 2) units crystals. For estimations of Zvv and saturation mag- (1, 9): moments ranged from 1.45 to 1.67/~B, and the reduction attributed to small residual orbital contrinetization, see text. 5
Vol. 74, No. 4
STUDIES OF K(RE)Nb6Clj8 AND (RE)Nb6Cll8 (RE = Lu A N D Trn)
butions resulting from spin-orbit coupling (1). In our case, such an orbital contribution to the resultant magnetic moment is evidenced by Land6 g factors smaller than 2, as found in EPR experiments performed in these chlorides (g between 1.943 and 1.993) (10). On the contrary, it is more difficult to explain an enhanced moment of 2.4ttB, as observed in the thullium-based system. No magnetic interactions are seen at low temperatures which could account for such an increase of the effective moment, and so far we have no explanations to this phenomenon. A similar result, even more pronounced, was also observed in the holmium-based compounds of these series [5], and suggests enhancement mechanisms due to the presence of two magnetic entities. 4.2. Low temperature magnetic behaviour Figures 3 and 5 suggest a non-magnetic singlet ground state for the thullium ions in these niobium chlorides. Such an hypothesis agrees quite reasonably with the non-Kramers nature of Tm 3+, as also observed in other ternary systems containing cluster units (e.g. TmMo6S 8 [11]). The almost constant value observed below 10K for KTmNb6CI~8 characterizes a temperature-independent term :~vv of the Tm 3+ ion. Thus, the magnetic susceptibility behaviour of TmNb6Cl~s may be described as: )~ = C / T +
Zvv
where the paramagnetic Curie term C/T due to the magnetic cluster is superposed to the VanVleck susceptibility due to Tm 3+. The quantity gvv can also be estimated from the experimental values of TmNb6CI~8 in a gT-versus-T plot, and yields 2.1 < Zvv x 104 emug ~ < 2.6, depending on the range of temperatures used for the fitting. We have preferred to take the experimental value of 2.3 x 10 4 (emu/g) (0.32emumol ~) obtained for KTmNb6Clls, and we have calculated the paramagnetic constant C ( ~ 0.75 e m u m o l - ~) and the effective moment #c~ (~2.45ttB). These values confirm our previous estimations for the cluster contribution. By the same way, we may write the low temperature magnetization as: M
=
M cluster
+ M 3+ = M d"stcr + zvvH
(where M 3+ is the contribution of Tm 3+ in its crystalfield ground state), provided no admixing occurs between excited states of Tm 3+ due to the applied field. In Fig. 7 we have drawn (dotted line) a tangent line corresponding to the M 3+ (ZvvH) term. Extrapolation to zero field should give the cluster contribution M c~u~terto the magnetization, which is of the
289
order of 0.5 #B. This value is half the one expected for one unpaired electron (gS = I#B; g = 2), and probably comes from the fact that the magnetization is not saturated at these fields and temperatures. 4.3. Magnetic interactions Low temperature magnetic interactions apparently exist in LuNb6CI18 , as suggested from: the maximum in the magnetic susceptibility, the negative value of the Curie temperature ( - 1 K), and the linear behaviour of the magnetization (Fig. 2). Interesting enough is the fact that no such interactions were observed in other members of the series [5, 12], independent of the nature of the rare-earth atoms. This result may suggest that an important parameter for the existence of such interactions must be the intercluster distances, which are smaller in the case of lutetium. Attempts to reduce this distance by appropriate choice of other cations or anions are currently under way [13]. CONCLUSION The new families of chlorides MRENb6CI18 and RENb6CI~s recently isolated [2, 3, 5, 7] allowed the study of the magnetic behaviours of either the Nb6 cluster or the rare-earth ion separately, as well these two magnetic entities together. In this report we have presented two examples in which the cluster contribution is clearly distinguished: in the first case (RE = Lu), magnetic properties depend only on the oxidation state of the cluster; in the second (RE = Tm), the magnetic state of trivalent thullium is added to the cluster. At low temperatures, a VanVleck susceptibility due to the Tm 3+ singlet ground state is observed in KTmNb6Cl18, and is confirmed in the corresponding ternary compound TmNb6Cl~s. By subtracting this term, we obtain again the contribution of the magnetic cluster. Other interesting combinations are possible, and constitute separate works published elsewhere [8, 12]. REFERENCES 1. 2. 3. 4. 5. 6. 7.
J.G. Converse & R.E. McCarley, Inorg. Chem. 9, 1361 (1970). S. Ihmaine, C. Perrin & M. Sergent, C.R. Acad. Sc. Paris 303, Sbrie II, 1293 (1986). C. Perrin, S. Ihmaine & M. Sergent, New J. Chem. 12, 321 (1988). G. Meyer & P. Ax, Mat. Res. Bull. 17, 1447 (1982). S. Ihmaine, University Thesis, Universitb de Rennes I (France), 1988. S. Ihmaine, C. Perrin, O. Pefia & M. Sergent, J. Less Common Met. 137, 323 (1988). S. Ihmaine, C. Perrin & M. Sergent, Acta Cryst. C43, 813 (1987).
290 8. 9. 10.
STUDIES OF K(RE)Nb6Cll8 AND (RE)Nb6CIt8 (RE = Lu AND Tm) O. Pefia, C. Perrin & M. Sergent, to be published. N. Brni6evi6, Z. Ru~i6-Toro~ & B. Koji6-Prodi6, J. Chem. Soc. Dalton Trans. 3, 455 (1985). M. Rabii, G. Alqui6 & C. Perrin, Proceed. Colloque O.H.D., Rennes (France) September 1989.
11. 12.
13.
Vol. 74, No. 4
D.R. Noakes, G.K. Shenoy & D.G. Hinks, Phys. Rev. B31, 5712 (1985). S. Ihmaine, C. Perrin, O. Pefia & M. Sergent, Proceed. Int. Conf. Physics Highly Correlated Electron Systems. Santa Fe (New Mexico, USA). Sept. 1989. Physica B, in press. C. Perrin et al., work in progress.