Structural chemistry and magnetic properties of Nd 18 Li 8 Fe 4 M′O 39 (M′ ¼ Al, Ga) and La 18 Li 8 Fe 4.5 In 0.5 O 39 Nirawat Thammajak, Peter D. Battle n , Catherine Brown, Katherine Higgon, Rhian Stansfield Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK article info Article history: Received 21 September 2013 Accepted 22 October 2013 Available online 29 October 2013 Keywords: Mixed-metal oxides Neutron diffraction Magnetism Spin glass abstract Polycrystalline samples of Nd 18 Li 8 Fe 4 M′O 39 (M′ ¼Al, Ga) and La 18 Li 8 Fe 4.5 In 0.5 O 39 have been prepared by the ceramic method and characterised by neutron diffraction and magnetometry. All three compounds adopt a cubic structure (space group Pm 3n, a 12 Å) based on intersecting 〈111〉 chains of cation sites with alternating octahedral and prismatic coordination geometry. These sites are occupied by Li, Fe and M′ or In; Nd or La cations occupy sites between the chains. The cation distribution over the octahedral and prismatic sites within the chains is disordered in all three compounds. The Nd-containing compositions show spin-glass behaviour below 4.5 K whereas small, weakly-ferrimagnetic domains form in La 18 Li 8 Fe 4.5 In 0.5 O 39 below 7.60 K. The dependence of the magnetic properties on the nature of the lanthanide cation is discussed. & 2013 Elsevier Inc. All rights reserved. 1. Introduction The synthesis of La 18 Li 8 Rh 5 O 39 [1] provided the first example of a compound containing an ordered arrangement of Rh 3 þ and Rh 4 þ cations. The cubic unit cell (space group Pm 3n) is shown in Fig. 1(a); polyhedral chains composed of alternating RhO 6 octahedra and LiO 6 trigonal prisms run parallel to the 〈111〉 axes of the cubic structure and intersect each other at intervals of (1/2, 1/2, 1/2). The inter- chain space is occupied by lanthanum cations and oxide anions which together form a network of rings through which the chains pass, see Fig. 1(b). The octahedral sites within the chains are not all equivalent. More specifically, those at the points where the chains intersect are smaller than those which lie between these points; the latter outnumber the former in the ratio 4:1. La 18 Li 8 Rh 5 O 39 was therefore described as a mixed-valence compound containing Rh 3 þ and Rh 4 þ in a 4:1 ratio with the Rh 4 þ cations occupying anion octahedra centred on the 2a sites at the points of intersection and the larger Rh 3 þ cations occupying octahedra centred on the 8e sites located halfway between nearest-neighbour 2a sites. The prisms containing the Li þ cations separate the two types of octahedra, sharing a common face with each. The cations occupy 16i sites on the 〈111〉 axes, not necessarily at the midpoint of the prism. Many isostructural compounds having the general formula Ln 18 Li 8 M 4 M′O 39 , where Ln is a lanthanide cation and M, M′ are d-block cations with different spin values, have been studied subse- quently [2–7] in an attempt to prepare new magnetic materials. It was hoped that cation ordering over the 2a and 8e sites would occur if the sizes and charges of M and M′ were sufficiently different, and that antiferromagnetic coupling in an ordered array of cations with different magnetic moments might result in ferrimagnetism, as it does in the case of Fe 3 O 4 . However, in the majority of the compounds studied to date cation disorder, involving not only mixed occupancy of the 2a and 8e sites by M and M′ but also disorder between Li and M over the 16i and 8e sites, has prevented the formation of a magnetically-ordered state and many Ln 18 Li 8 M 4 M′O 39 compositions have been found to show spin-glass behaviour at low temperatures. The two most obvious exceptions to this generalisation are the cation-deficient composition Nd 18 Li 8 Co 4 O 39 [5] and La 18 Li 8 Fe 5 O 39 [2]. The temperature dependence of the magnetic susceptibility of the former shows a maximum at 2.3 K. This was assumed to be a Néel point, below which the Nd sublattice is antiferromagnetically ordered. However, this has never been confirmed by neutron diffraction. Magnetometry and Mössbauer spectroscopy showed clearly that La 18 Li 8 Fe 5 O 39 undergoes a magnetic transition at 8 K, but the nature of the low-temperature phase could not be unam- biguously assigned. No magnetic Bragg scattering was detected in the neutron diffraction pattern, but the susceptibility did not show the behaviour normally associated with a spin glass. We report below an investigation of the consequences of introducing diamagnetic p-block cations, specifically Al 3 þ , Ga 3 þ and In 3 þ into the Ln 18 Li 8 M 4 M′O 39 structure. This study was undertaken in the knowledge that using p-block cations to dilute the concentration of magnetic species in a system can sometimes [8–10] lead to surprising changes in the magnetic behaviour. 2. Experimental Initially attempts were made to synthesize polycrystalline samples of Nd 18 Li 8 Fe 4 M′O 39 (M′ ¼ Al, Ga, In) by the standard Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.10.038 n Corresponding author. Tel.: þ44 1865 2726 12; fax: þ44 1865 272690. E-mail address: [email protected] (P.D. Battle). Journal of Solid State Chemistry 209 (2014) 120–126
Transcript
Structural chemistry and magnetic properties of Nd18Li8Fe4M′O39
(M′¼Al, Ga) and La18Li8Fe4.5In0.5O39
Nirawat Thammajak, Peter D. Battle n, Catherine Brown, Katherine Higgon, Rhian StansfieldInorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK
a r t i c l e i n f o
Article history:Received 21 September 2013Accepted 22 October 2013Available online 29 October 2013
Polycrystalline samples of Nd18Li8Fe4M′O39 (M′¼Al, Ga) and La18Li8Fe4.5In0.5O39 have been prepared bythe ceramic method and characterised by neutron diffraction and magnetometry. All three compoundsadopt a cubic structure (space group Pm3n, a �12 Å) based on intersecting ⟨1 1 1⟩ chains of cation siteswith alternating octahedral and prismatic coordination geometry. These sites are occupied by Li, Fe andM′ or In; Nd or La cations occupy sites between the chains. The cation distribution over the octahedraland prismatic sites within the chains is disordered in all three compounds. The Nd-containingcompositions show spin-glass behaviour below �4.5 K whereas small, weakly-ferrimagnetic domainsform in La18Li8Fe4.5In0.5O39 below 7.60 K. The dependence of the magnetic properties on the nature of thelanthanide cation is discussed.
& 2013 Elsevier Inc. All rights reserved.
1. Introduction
The synthesis of La18Li8Rh5O39 [1] provided the first example of acompound containing an ordered arrangement of Rh3þ and Rh4þ
cations. The cubic unit cell (space group Pm3n) is shown in Fig. 1(a);polyhedral chains composed of alternating RhO6 octahedra and LiO6
trigonal prisms run parallel to the ⟨1 1 1⟩ axes of the cubic structureand intersect each other at intervals of (1/2, 1/2, 1/2). The inter-chain space is occupied by lanthanum cations and oxide anionswhich together form a network of rings through which the chainspass, see Fig. 1(b). The octahedral sites within the chains are not allequivalent. More specifically, those at the points where the chainsintersect are smaller than those which lie between these points; thelatter outnumber the former in the ratio 4:1. La18Li8Rh5O39 wastherefore described as a mixed-valence compound containing Rh3þ
and Rh4þ in a 4:1 ratio with the Rh4þ cations occupying anionoctahedra centred on the 2a sites at the points of intersection andthe larger Rh3þ cations occupying octahedra centred on the 8e siteslocated halfway between nearest-neighbour 2a sites. The prismscontaining the Liþ cations separate the two types of octahedra,sharing a common face with each. The cations occupy 16i sites onthe ⟨1 1 1⟩ axes, not necessarily at the midpoint of the prism.
Many isostructural compounds having the general formulaLn18Li8M4M′O39, where Ln is a lanthanide cation and M, M′ ared-block cations with different spin values, have been studied subse-quently [2–7] in an attempt to prepare new magnetic materials. Itwas hoped that cation ordering over the 2a and 8e sites would occurif the sizes and charges of M and M′ were sufficiently different, and
that antiferromagnetic coupling in an ordered array of cations withdifferent magnetic moments might result in ferrimagnetism, as itdoes in the case of Fe3O4. However, in the majority of the compoundsstudied to date cation disorder, involving not only mixed occupancyof the 2a and 8e sites by M and M′ but also disorder between Li andM over the 16i and 8e sites, has prevented the formation of amagnetically-ordered state and many Ln18Li8M4M′O39 compositionshave been found to show spin-glass behaviour at low temperatures.The two most obvious exceptions to this generalisation are thecation-deficient composition Nd18Li8Co4O39 [5] and La18Li8Fe5O39
[2]. The temperature dependence of the magnetic susceptibility ofthe former shows a maximum at 2.3 K. This was assumed to be aNéel point, below which the Nd sublattice is antiferromagneticallyordered. However, this has never been confirmed by neutrondiffraction. Magnetometry and Mössbauer spectroscopy showedclearly that La18Li8Fe5O39 undergoes a magnetic transition at �8 K,but the nature of the low-temperature phase could not be unam-biguously assigned. No magnetic Bragg scattering was detected in theneutron diffraction pattern, but the susceptibility did not show thebehaviour normally associated with a spin glass.
We report below an investigation of the consequences ofintroducing diamagnetic p-block cations, specifically Al3þ , Ga3þ
and In3þ into the Ln18Li8M4M′O39 structure. This study wasundertaken in the knowledge that using p-block cations to dilutethe concentration of magnetic species in a system can sometimes[8–10] lead to surprising changes in the magnetic behaviour.
2. Experimental
Initially attempts were made to synthesize polycrystallinesamples of Nd18Li8Fe4M′O39 (M′¼Al, Ga, In) by the standard
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/jssc
Journal of Solid State Chemistry
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jssc.2013.10.038
“ceramic method”. Well-ground mixtures containing 50% excesslithium carbonate and stoichiometric quantities of pre-driedNd2O3, Fe2O3 and M′2O3 were pelletized and heated in aluminacrucibles, initially for 16 h at 800 1C. The reactants were thenreground with additional lithium carbonate, repelletized andheated again until the desired product formed. In the case of M′¼In, it was necessary to modify the target stoichiometry and toreplace Nd2O3 by pre-dried La2O3 in order to achieve a satisfactoryoutcome. Details are given below.
X-ray powder diffraction was used to monitor the progressof the reactions. Data were collected on a Philips X’pert diff-ractometer operating with Cu Kα1 radiation with a step size ofΔ2θ¼0.00841. High-intensity X-ray powder diffraction data werecollected over a small angular range (15r2θ⧸οr40) in order toincrease the likelihood of detecting low-level impurities. High-resolution X-ray powder diffraction data for use in quantitativeanalysis were collected over the angular range 5r2θ⧸οr125. TheX-ray scattering from these materials is dominated by the heavymetals and consequently it was not possible to perform a fullstructural analysis using these data. Limited Rietveld [11] refine-ments were carried out using the GSAS [12] suite of programs inorder to determine the unit-cell parameters and the distribution ofcations over the available sites. Backgrounds were fitted using a
Chebyshev polynomial of the first kind and the peak shape wasmodelled using a pseudo-Voigt function.
The diffractometer D2b at the Institut Laue Langevin, Grenoble,France was used to collect neutron powder diffraction data atroom temperature using a wavelength of �1.59 Å. The unit-cellparameters derived from X-ray diffraction data were used tocalibrate accurately the neutron wavelength. For one sample,further diffraction patterns were collected at 25 K and 2 K usinga wavelength of 2.4 Å. All data were collected over the angularrange 15r2θ⧸οr155 with a step size Δ2θ¼0.051. Samples(�0.5 g) were contained within vanadium cans (ϕ¼5 mm). Riet-veld refinements of the structures were carried out using the GSASprogram suite. The background level was refined using the soft-ware. Peak shapes were modelled using a pseudo-Voigt functiontogether with a correction for peak asymmetry at low angles.
Magnetic measurements were carried out using a QuantumDesign MPMS 5000 SQUID magnetometer. The magnetization (M)was measured as a function of temperature on warming from 2 to300 K after cooling both in zero field (ZFC) and in the measuringfield of 100 Oe (FC). AC susceptibility data were recorded at4 frequencies (1rω/Hzr1000) in a direct field of �2 Oe andan oscillating field of amplitude 3.5 Oe over an appropriatetemperature range with ΔT¼0.1 K.
3. Results
X-ray diffraction patterns containing only peaks attributableto Nd18Li8Fe4AlO39 or Nd18Li8Fe4GaO39 were obtained after therepelletized reaction mixture had been fired at 1000 1C (M′¼Al) or950 1C (M′¼Ga) for one hour. We were unable to find a set ofreaction conditions that resulted in the formation of monophasicNd18Li8Fe4InO39. The introduction of the larger La3þ cation pro-duced products that contained the target phase along with aLiInO2 impurity. The concentration of the impurity decreasedwhen the indium content of the target phase was reduced andwe were eventually able to prepare a sample of nominal composi-tion La18Li8Fe4.5In0.5O39 that was shown by X-ray diffraction tocontain only 0.14(8) wt % of LiInO2, i.e. �3 mol %. This sample wasprepared by firing the original reaction mixture at 975 1C for 2 h inpellet form, grinding with 25% excess Li2CO3, repelletizing andheating for a further 12 h, again at 975 1C. We note that we wereunable to prepare satisfactory samples with Ln¼La; M′¼Al or Ga.
Analysis of the X-ray diffraction data in the space group Pm3n,using the structure of Nd18Li8Fe5O39 as a starting model [5],resulted in unit-cell parameters of 11.95766(8), 11.96345(6) and12.22008(6) Å for Nd18Li8Fe4AlO39, Nd18Li8Fe4GaO39 and La18Li8-Fe4.5In0.5O39, respectively. Although complete structural refine-ments were not possible because of the presence of both heavyand light elements, our analyses suggested that whereas the Al3þ
cations occupy 8e sites, Ga3þ and In3þ occupy 16i sites within thetrigonal prisms.
The neutron diffraction data collected at room temperaturewere used to refine fully the crystal structures of these threecompositions. Bragg peaks attributable to unreacted lithium car-bonate were observed in all the diffraction patterns and wereincluded in the analysis. It was not possible to include a LiInO2
impurity in the analysis of La18Li8Fe4.5In0.5O39 because the Braggpeaks from this minority phase were lost in the background noise.However, weak peaks attributable to Li2O were included in theanalysis of this data set. The crystal structures of Li2CO3 and Li2Oare known [13] and their presence therefore does not impede thestructural analysis of the main phase. Furthermore, they arediamagnetic and so do not interfere with the interpretation ofthe magnetic data described below. We note that, to date, attemptsby ourselves and others [14] to prepare monophasic samples of
Fig. 1. (a) Polyhedral representation of the cubic (space group Pm3n) structure ofLa18Li8Rh5O39; LiO6 trigonal prisms are blue (16i site), RhO6 octahedra are green(2a) and red (8e), grey circles represent oxygen (O2 and O3), yellow circles La1 andorange circles La2. (b) The La–O2–O3 framework viewed along ⟨1 1 1⟩; thepolyhedral chains run through the channels.
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Ln18Li8M4M′O39 have been unsuccessful. The refined structuralparameters of Nd18Li8Fe4AlO39, Nd18Li8Fe4GaO39 and La18Li8-Fe4.5In0.5O39 are listed in Table 1; the corresponding bond lengthsare listed in Table 2. The 2a cation site is coordinated by O4, the 8esite by O1 and the 16i site by three O4 and three O1 anions; O2and O3, together with Ln1 and Ln2, form the network of ringsillustrated in Fig. 1(b). The distribution of the cations over the 2a,8e and 16i sites was allowed to vary in our preliminary refine-ments. On the basis of these refinements, and in accord with ourlimited analysis of the X-ray data, Al was subsequently constrainedto occupy the 8e site and Ga and In were assigned to the 16i site.Li/Fe disorder over the 16i/8e sites was found to be significant forM′¼Al and In, but not for M′¼Ga. Trial refinements provided noevidence of anion vacancies and it was assumed that the concen-tration of LiInO2 was low enough not to have a significant effect onthe composition of the main phase. The composition of eachcompound was therefore constrained to the ideal formulation. Inall three cases, in common with the majority of the Ln18Li8M4M′O39
phases studied previously, the O4 sublattice was found to showfour-fold disorder. O4 is thus located on a 48l (x, y, z) site witha fractional occupancy of 0.25 rather than on a 12f (x, 0, 0) site.The observed and calculated diffraction profiles are shown in Fig. 2.In the case of Nd18Li8Fe4AlO39 an additional weak maximum, toobroad to be a Bragg peak arising from a well-crystallized material,was also observed at 2θ�451. This peak was excluded from the dataanalysis but is included in Fig. 2.
The temperature dependence of the d.c. magnetic susceptibility,χ(T), and the field dependence of the magnetisation, M(H), ofNd18Li8Fe4AlO39 are shown in Fig. 3. The ZFC and FC susceptibilitiesare equal at high temperatures but hysteresis is apparent below themaximum in the ZFC susceptibility at Tc¼4.12(5) K; the gradient ofthe FC susceptibility decreases markedly below this temperature but
no maximum is observed. The parameters derived by fitting the datain the temperature range 150 KoTo300 K to the Curie–Weiss Law,χ¼C/(T�θ), are listed in Table 3. Hysteresis is also apparent at fieldsin the range �5oH/kOeo5 at 2 K, but not at 10 K and above.Nd18Li8Fe4GaO39 behaves in a similar manner, see Fig. 4 and Table 3,and shows a maximum in the ZFC susceptibility at 4.38(5) K.However, although La18Li8Fe4.5In0.5O39 apparently shows Curie–Weiss behaviour at high temperatures, with a negative Weissconstant, both the ZFC and FC susceptibilities increase markedlyon cooling below 8.4 K; the gradient of χ(T) is steepest at 7.60(5) K.
Table 1Structural parameters of Ln18Li8Fe5�xM′xO39 (Ln¼La, Nd; M′¼Al, Ga, In) at room temperature in space group Pm3n.
O1 48l x 0.8651(3) 0.8647(2) 0.8643(2)x y z y 0.8597(3) 0.8596(2) 0.8588(2)
z 0.6936(2) 0.6937(2) 0.6946(1)100�Uiso/Å2 0.82(4) 0.94(4) 0.85(3)
O2 6d 100�Uiso/Å2 0.52(1) 0.8(1) 1.0(1)¼ ½ 0O3 12g x 0.6313(5) 0.6307(4) 0.6323(3)x 0 ½ 100�Uiso/Å2 1.1(1) 0.78(9) 0.71(8)O4 48l x 0.1500(8) 0.1515(6) 0.1494(6)x y z y 0.018(2) 0.014(2) 0.022(1)
z 0.022(2) 0.021(1) 0.018(1)100�Uiso/Å2 4.2(4) 3.1(3) 3.9(3)occupancya 0.25 0.25 0.25
a The fractional occupancy.
Table 2Bond lengths (Å) and bond angles (1) in Ln18Li8Fe5�xM′xO39 (Ln¼La, Nd; M′¼Al, Ga,In) at room temperature.
a The average bond length to a disordered oxygen site.
N. Thammajak et al. / Journal of Solid State Chemistry 209 (2014) 120–126122
χ(T) shows hysteresis below this temperature as doesM(H), althoughthe effect is weak and the magnetisation does not approach satura-tion in the field range of the measurements, see Fig. 5.
The temperature and frequency dependence of the molar a.c.magnetic susceptibilities of the three compounds under considera-tion are shown in Figs. 6–8. The behaviours of Nd18Li8Fe4AlO39
and Nd18Li8Fe4GaO39 are very similar. They show a transition at afrequency-dependent temperature, and have a frequency-dependentimaginary component at temperatures below the transition. The ratioΔTc/(TcΔ(logω)), which parameterizes the frequency dependence ofthe transition temperature, takes values of 0.051 and 0.043 forNd18Li8Fe4AlO39 and Nd18Li8Fe4GaO39, respectively. The a.c. suscept-ibility of La18Li8Fe4.5In0.5O39 is markedly different. The susceptibility,but not the transition temperature, is weakly frequency dependent,and an imaginary component is observed below the transitiontemperature. The real part of the susceptibility reaches a maximumvalue at 7.40(5) K.
The neutron diffraction patterns collected from La18Li8Fe4.5In0.5O39
at 25 K and 2 K were compared, see Fig. S1, but no additionalscattering that could be attributed to the onset of long-range magneticordering was apparent in the data collected at the lower temperature.
4. Discussion
The successful synthesis of Nd18Li8Fe4AlO39 and Nd18Li8Fe4-GaO39 further demonstrates the flexibility of this crystal structureand, in particular, shows that it can accommodate p-block metals.Our failure to prepare Nd18Li8Fe4InO39 demonstrates the limits ofthis flexibility and is consistent with our previous suggestion [7]that size of the Ln cation must be matched to the M and M′ cationsin order to create Ln–O rings of the appropriate size to accom-modate the polyhedral chains. Our results suggest that in the caseof M′¼ In the use of the La3þ cation increases the compatibility,but still only limited substitution is possible. The data in Table 2show that the Ln–O bonds increase in length by �2% when Nd3þ
is replaced by La3þ .The bond lengths around the Ln sites are similar to those
determined previously in comparable compounds, for exampleNd18Li8Fe5O39 and La18Li8Fe5O39 [2,5], as are those around the 2a
Fig. 2. Observed and calculated neutron diffraction profiles for (a) Nd18Li8Fe4AlO39,(b) Nd18Li8Fe4GaO39 and (c) La18Li8Fe4.5In0.5O39; a difference curve is shown in eachcase. Vertical bars indicate reflection positions for the principal phase (lower),Li2CO3 (upper) and, in (c), Li2O (middle).
Fig. 3. (a) Temperature dependence of the d.c. molar magnetic susceptibility and(b) field dependence of the magnetisation of Nd18Li8Fe4AlO39.
Table 3Magnetic parameters of Ln18Li8Fe5�xM′xO39 (Ln¼La, Nd; M′¼Al, Ga, In).
N. Thammajak et al. / Journal of Solid State Chemistry 209 (2014) 120–126 123
and 8e sites. In all cases the Fe1–O4 bond length suggests thepresence of low-spin Fe4þ; only in Pr18Li8Fe4RuO39 [7] has this sitebeen large enough to accommodate high-spin Fe4þ . The bondlengths around the 8e and 16i sites are, as always, compatible withthe presence of high-spin Fe3þ . The mean M′–O bond lengths forsix-coordinated M′ in M′2O3 are 1.912, 1.999 and 2.183 Å for M′¼Al, Ga and In, respectively [15–17]. Thus Al3þ is too large to beaccommodated on the six-coordinate 2a site in Nd18Li8Fe4AlO39,where the mean bond length is 1.83 Å, and consequently it isfound on the larger 8e site, along with Fe3þ and Liþ . In3þ is toolarge to be accommodated on either of the octahedral sites inLa18Li8Fe4.5In0.5O39 and consequently the relatively low concentra-tion that the structure can tolerate is found on the prismatic 16isite. However, Ga3þ is essentially the same size as high-spin Fe3þ
and the location of the p-block metal on the prismatic site, ratherthan the 8e site, must be attributable to electronic factors ratherthan size effects. These factors would presumably also reinforcethe preference of In3þ for the 16i site. Interestingly, the refinedmean position of the cations within the prisms in Nd18Li8Fe4GaO39
lies very close to the centre of the polyhedron. In all the otherisostructural compounds studied, both in this work and pre-viously, the mean position of these cations has been displacedtowards the triangular face of the prism that is formed by O1
anions. The origin of this difference is not clear, although it mightstem from the relatively high concentration of small (compared toLiþ and In3þ), trivalent cations on the 16i site and the resultantchanges in the electrostatic repulsions between the three cationsites in the polyhedral chains. The presence of a relatively highconcentration of Ga3þ in the prisms appears to prevent the Fe/Lisite exchange that occurs in many isostructural compounds,although this exchange does occur in La18Li8Fe4.5In0.5O39 wherethe concentration of trivalent p-block cations on the 16i sitesis lower.
For all three compounds, the linear form of χ�1(T) above 150 K,see Figs. 3–5, invites interpretation using the Curie–Weiss Law. Ifthe Nd3þ cation is assumed always to be in the J¼9/2 ground state,then the contribution of this element to the molar Curie constantsof Nd18Li8Fe4AlO39 and Nd18Li8Fe4GaO39 will be 29.5 cm3 K mol�1.The calculated total Curie constant, assuming that the d-blockcations can be treated using the spin-only formula, is then45.6 cm3 K mol�1 if Fe4þ is present in the high-spin state and43.6 cm3 K mol�1 if it is in the low-spin state. As explained above,the short Fe–O bond lengths at the 2a site, together with Mössbauerdata collected on analogous compounds [2,5,6], suggest that the
Fig. 4. (a) Temperature dependence of the d.c. molar magnetic susceptibility and(b) field dependence of the magnetisation of Nd18Li8Fe4GaO39.
Fig. 5. (a) Temperature dependence of the d.c. molar magnetic susceptibility and(b) field dependence of the magnetisation of La18Li8Fe4.5In0.5O39.
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low-spin state is present and that the observed Curie constants aretherefore both significantly higher than the predicted values. Thereare many reasons why this situation could arise. For example, theassumption that low-spin Fe4þ can be described using the spin-only formula is unlikely to be valid. Furthermore, as a result of thehigh concentration of Nd3þ cations, a relatively small deviation ofthe effective magnetic moment of this cation from the calculatedvalue would lead to a significant change in the Curie constant.However, this explanation clearly cannot be extended to includeLa18Li8Fe4.5In0.5O39, where the observed Curie constant also exceedsany reasonable predicted value. A more likely explanation is that thecation–cation interactions within these compounds render aninterpretation in terms of the Curie–Weiss law physically mean-ingless, even if χ�1(T) is a linear function. It should be noted that,despite the decrease in the concentration of magnetic cations,χ(300) is larger for both Nd18Li8Fe4AlO39 and Nd18Li8Fe4GaO39
than for Nd18Li8Fe5O39. Similarly, χ(300) is larger for La18Li8-Fe4.5In0.5O39 than for La18Li8Fe5O39. The increase in room-tempe-rature magnetisation on doping with diamagnetic cations suggeststhat their introduction disrupts but does not eliminate spin pairingthat is present in the parent compound [5], and hence that thesecompounds cannot be treated as Curie–Weiss paramagnets withinthe measured temperature range. Unfortunately, the nature of theshort-range coupling cannot be deduced from the data available.
The temperature dependence of the d.c. magnetic susceptibilityand the onset of hysteresis in M(H) suggest that Nd18Li8Fe4AlO39
and Nd18Li8Fe4GaO39 behave as a spin glasses below 4.12 and4.38 K, respectively. The imaginary component and the frequencydependence of the transition temperature seen in the a.c. suscept-ibility support this conclusion. The rate of change of the transition
temperature with the measuring frequency is typical of aninsulating spin glass [18]. The susceptibility of La18Li8Fe4.5In0.5O39
behaves very differently. The rapid rise in the d.c. susceptibility at7.6 K, together with the onset of hysteresis, albeit weak, suggests
Fig. 6. Temperature dependence of the real (χ′) and imaginary (χ″) components ofthe a.c. molar magnetic susceptibility of Nd18Li8Fe4AlO39. Fig. 7. Temperature dependence of the real (χ′) and imaginary (χ″) components of
the a.c. molar magnetic susceptibility of Nd18Li8Fe4GaO39.
Fig. 8. Temperature dependence of the real (χ′) and imaginary (χ″) components ofthe a.c. molar magnetic susceptibility of La18Li8Fe4.5In0.5O39.
N. Thammajak et al. / Journal of Solid State Chemistry 209 (2014) 120–126 125
that the sample develops a spontaneous magnetisation at thistemperature. This is consistent with both the appearance of animaginary component and the absence of any frequency depen-dence of the transition temperature observed in the a.c. suscep-tibility. This compound is not behaving as a spin glass but theabsence of saturation in M(H) and the absence of magnetic Braggscattering in the neutron diffraction pattern recorded at 2 K indicatethat it is neither a simple ferromagnet nor a spin-canted antiferro-magnet. Coherent magnetic order can only be present in smalldomains that are too small to give rise to neutron Bragg scattering.The relatively low value of the net magnetisation leads us to suggestthat it is ferrimagnetic, rather than ferromagnetic, in origin.
The above discussion clearly does not provide a full explanationof the magnetic properties of these heavily-disordered materialsand more experimental work would be necessary in order to do so.There are, however, a number of significant observations thatshould be emphasised. Nd18Li8Fe4AlO39 and Nd18Li8Fe4GaO39
behave as spin glasses at low temperature, as did compositionscontaining Nd and other magnetic Ln cations studied previously inwhich the d-block sublattice was not diluted by p-block cations[5–7,19]; the concentration of diamagnetic Al3þ or Ga3þ cationson the 8e or 16i sites, respectively, is clearly too low to weakensignificantly the superexchange interactions present in thesecompounds. The absence of a strong paramagnetic component inthe susceptibility below the magnetic transition temperatureshows that spin freezing occurs at the Ln sites as well as at thosecontaining d-block cations. In contrast, La18Li8Fe4.5In0.5O39, with adiamagnetic Ln cation present, appears to support the formation ofsmall magnetic domains below Tc and is thus the first member ofthis structural family to show evidence of the sought-afterferrimagnetism. These results demonstrate that magnetic Lncations play an important part in determining the properties ofthese compounds; it cannot be assumed that the magneticbehaviour is dictated solely by the d-block cations. This isconsistent with our earlier [4,5] hypothesis that the Nd sublatticeshows long-range antiferromagnetic order in compositions whereM and M′ are diamagnetic or only weakly paramagnetic. Compar-ison of the magnetic behaviour of La18Li8Fe4.5In0.5O39 with that ofLa18Li8Fe5O39 [2] shows that the net magnetisation is sensitive tothe nature of the cations on the 16i site, and is enhanced by ahigher concentration of diamagnetic cations. It is possible that thisstems from the increased importance of ferrimagnetic couplingbetween 2a and 8e sites when there is less competition frommagnetic cations on the 16i sites. This coupling would also beexpected to be important in Nd18Li8Fe4GaO39, wherein the 16i sitesare occupied by Li and Ga, but it is apparently overwhelmed at lowtemperatures by interactions involving the Nd sublattice.
5. Conclusions
We have shown that the p-block cations Al3þ , Ga3þ and In3þ
can be introduced into the Ln18Li8M4M′O39 structure when M¼Fe
provided that Ln is compatible, in terms of size, with the set ofcations present. Thus for Ln¼Nd, Al or Ga but not In can fill therole of M′; the former occupies an octahedral site and displaces Feinto the trigonal prisms whereas the latter occupies sites in theprisms and displaces Li onto octahedral sites. When Ln¼La, In isable to occupy up to 6.25% of the prismatic sites. The Al- and Ga-containing compositions show spin-glass behaviour at low tem-peratures; the formation of this state involves coupling to the Ndsublattice. In contrast, La18Li8Fe4.5In0.5O39 composition showsevidence of the formation of small ferrimagnetic domains at lowtemperatures.
Acknowledgments
We are grateful to Dr. E. Suard for providing experimentalassistance at the ILL in Grenoble, France. N. T. was supported by aRoyal Thai Government Scholarship.
Appendix A. Supplementary material
Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2013.10.038.
References
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K. Oh-ishi, S. Nakanishi, Inorg. Chem. 49 (2010) 5912.[3] P.D. Battle, S.E. Dutton, P.A. vanDaesdonk, J. Solid State Chem. 183 (2010) 1620.[4] P.D. Battle, S.E. Dutton, F. Grandjean, G.J. Long, N. Thammajak,
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