Cent. Eur. J. Phys. • 11(11) • 2013 • 1559-1566DOI: 10.2478/s11534-013-0316-z

Central European Journal of Physics

Synthesis and spectroscopic properties of LiIn(WO4)2nanopowders and single crystals doped withchromium(III) ions

Research Article

Tomasz Skibiński1∗, Hubert Fuks1, Sławomir M. Kaczmarek1† , Tomasz Bodziony1,Krzysztof Hermanowicz2, Mirosław Maczka2, Maciej Ptak2, Jerzy Hanuza2,3

1 Institute of Physics, Faculty of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology,Al. Piastow 48, 70-311 Szczecin, Poland

2 Institute of Low Temperature and Structure Research, Polish Academy of Sciences,P.O. Box 1410, 50-950 Wroclaw 2, Poland

3 Department of Bioorganic Chemistry, Institute of Chemistry and Food Technology, University of Economics,Komandorska 118/120 Str., Wroclaw, Poland

Received 19 August 2013; accepted 14 October 2013

Abstract: LiIn(WO4)2 single crystals and nanopowders doped with chromium(III) ions were synthesized and investi-gated, including their EPR spectra and magnetic properties. The EPR spectra have shown low and highfield lines attributed to isolated chromium ions with an electron spin S=3/2 and complex chromium centerswith higher spin value, respectively. Magnetic susceptibility measurements revealed an almost linear de-pendence on the magnetic field. The electron resonance and magnetic properties have been related to thestructural and spectroscopic data of the studied material.

PACS (2008): 75., 75.75.+a

Keywords: EPR spectra • chromium (III) • nanomaterials • double tungstate • single crystal© Versita sp. z o.o.

1. Introduction

LiIn(WO4)2 is a member of a large class of inorganic com-pounds with general formula MIMIII (MV IO4)2 (MI = alkalimetal, MIII = Al, In, Sc, Fe, Bi, lanthanide, MV I = Mo,W), which has been applied in the field of quantum elec-∗E-mail: tskibinski@zut.edu.pl (Corresponding author)†E-mail: skaczmarek@zut.edu.pl

tronics [1–3]. Recently, indium containing tungstates havealso been shown to be very promising room temperaturescintillators [4, 5]. According to data in the literature,LiIn(WO4)2 crystallizes in the wolframite type structure,space group C2/c, which is built from WO6 octahedra con-nected to form infinite chains [6, 7]. This kind of structuremay imply specific magnetic properties of the compoundsrevealing both ferromagnetic and antiferromagnetic kindsof magnetic interactions depending on the physical state(single crystal, nanopowder), kinds of dopants, and theirconcentration. As it was shown previously, the optical1559

Synthesis and spectroscopic properties of LiIn(WO4)2 nanopowders and single crystals doped with chromium(III) ions

and magnetic properties of the compounds change in asignificant manner when post growth thermal processesare applied, especially in the case of nanopowders [8]. In-creasing the annealing temperature leads to a change inthe nanoparticle size affecting the structural and physicalproperties of the compounds. Thus, we decided to synthe-size single crystals and nanocrystalline LiIn(WO4)2:Cr3+in order to investigate the influence of the concentrationof doped chromium ions as well as the value of annealingtemperature (in the case of nanopowders) on the magneticproperties of the compounds.

Figure 1. a) Orientation of experimental axes for the investigated sin-gle crystal: c axis - along larger size of the crystal, a axis -perpendicular to the c axis. b) Unit cell of LiIn(WO4)2 ori-ented according to crystalographic axes a, b, c (generatedusing Diamond 3 program).

2. Experimental setupLiIn(WO4)2:Cr3+ nanoparticles and single crystals inthe form of thin plates were prepared by Pechini andflux methods, respectively. Detailed descriptions ofthe growth processes were presented elsewhere [8, 9].LiIn(WO4)2:Cr3+ 0.5 mol.% single crystal and three typesof nanopowders: LiIn(WO4)2:Cr3+ 2 mol.% annealed at 520(6), 550 (2), and 700℃ (4) were analyzed for their mag-netic properties. The numbers in brackets numerate indi-vidual samples from 1 to 9, excluding numbers 1, 3, 5, 7,8 and 9 assigned to NaIn(WO4)2: Cr 2 mol. % samples,quoted in the paper for purpose of comparison. The firstderivatives of the absorption spectra of the nanopowdersand single crystals were recorded as a function of the ap-plied magnetic induction on a conventional X-band BrukerELEXSYS E 500 CW-spectrometer operating at 9.5 GHzwith 100 kHz magnetic induction modulation. The tem-perature dependencies of the EPR spectra of the sampleswere recorded using an Oxford Instruments ESP helium-

flow cryostat. Optimization of the spin-Hamiltonian pa-rameters and EPR data simulation was achieved by usingthe software package EPR-NMR [10]. Owing to the pe-culiarities of single crystals, angular dependencies wereregistered only for two perpendicular crystal directions:along and perpendicular to the symmetry axis (Fig. 1a).Static (DC) magnetic susceptibility measurements wereperformed using a Quantum Design MPMS XL-7 withEverCool Magnetic Property Measurement System. Themeasurements were performed at temperature up to 305 Kand magnetic fields in the range 100 – 5000 Oe. DCsusceptibilities were measured in the zero-field-cooling(ZFC) and field-cooling (FC) modes.3. Results and discussion

3.1. Spectroscopic studiesLiIn(WO4)2 crystallizes in the monoclinic C2/c = C 62hspace group with Z = 4, where Z is the number ofmolecules in a unit cell. The unit cell dimensions area = 9.57 Å, b = 11.59 Å, c = 4.95 Å and β = 91.10°[11, 12]. The experimental axes from Fig. 1a are not thesame as crystallographical axes representing the cell di-mension (see Fig. 1b). The crystal is built up of WO2−4ions connected to each other by means of intermolecu-lar interactions of the WO

OW type, forming chains alongthe c-direction. The two different W· · ·O distances are2.36 and 2.07 Å. The WOOW bridges are related to eachother by a c glide plane and the symmetry of the doubleoxygen bridge is C1. All oxygen bridges in the crystalare crystallographically equivalent and the resulting co-ordination of the tungstate ion is octahedral. Li and Inatoms occupy the C2 symmetry site of the unit cell. Theoptical absorption and emission spectra of the electronic

Cr3+ transitions of the studied crystal were reported in[8]. Two characteristic broad bands are observed in theranges 430 – 640 nm and 590 – 840 nm (i.e. 15600 –23300 and 12000 – 17000 cm−1). They correspond to thespin-allowed 4A2 →4T1 and 4A2 →4T2 transitions of theCr3+ ion in the octahedral crystal field. Due to the verylarge bandwidth of the 4A2 →4T2 bands and partial over-lapping of the 4A2 →4T1 band by the band at about 300nm, the spectral parameters of Cr3+ ions were obtainedfrom a Gaussian fitting procedure. The broadband 4T2 -4A2 luminescence from the low-crystal field of Cr3+ ionswas recorded [12]. The following spectral parameters wereobtained for LiIn(WO4)2:Cr3+ 0.5 mol. % crystal at roomtemperature:• Energy levels: 4T2 = 13680 cm−1, 2E =13890 cm−1, 2T1 = 14370 cm−1, 4T1(F) =

1560

T. Skibiński et al.

19150 cm−1, 2T2 = 19050 cm−1, 4T1(P) =28940 cm−1; ∆E = −2330 cm−1;• Octahedral crystal field strength Dq = 1368 cm−1,Racah parameters: B = 534 cm−1, C = 3294cm−1, the nephelauxetic ratio (delocalization of d-electrons) β = 0.582, Dq/B = 2.56, C/B = 6.17,Stokes shift SS = 4240 cm−1;• The Huang Rhys parameter S = 9.33; Sh̄ω =2270 cm−1;• Emission 4A2 →4T2 9440 cm−1.

The nanopowders of the studied material were preparedby the Pechini method [9]. Some samples were ob-tained at annealing temperatures in the range 500℃ –800℃. Two clear and jumping changes of crystallite sizeversus annealing temperature were observed at about530℃ and 675℃; these have been explained by the exis-tence of critical sizes of about 30 and 100 nm, respec-tively [9]. Some irregularities were found in the XRDpowder diagrams vs. crystallite sizes. On this basisthe existence of polymorphs which were not reported ear-lier of the studied compound was postulated. Two size-induced phase transitions change the structure from that ofLiFe(WO4)2-type, through that of LiYb(WO4)2-type struc-ture to LiGa(WO4)2-type structure [9]. These polymorphsdiffer in the distribution of the sites occupied by Li+ andIn3+ ions.The spectral parameters determined from the spectra mea-sured at 9 K for the sample annealed at 500℃ (crystallitesize 18 – 25 nm) are as follows [9]: 4T2 = 13890 cm−1,4T1(F) = 19504 cm−1, Dq = 1389 cm−1, B = 563 cm−1,Dq/B = 2.47, β = 0.62, C = 3235 cm−1, C/B = 5.74,SS = 4210 cm−1, S = 6.7 and Sh̄ω = 1675 cm−1, ∆E =−2120 cm−1. The parameters of this sample obtained fromthe emission studies are the following: E00 = 11380 cm−1,hν = 250 cm−1, and S = 6.7. Comparing these data withthose listed above for LiIn(WO4)2:Cr3+ single crystals itshould be noted that they do not differ substantially fromthose of the bulk material. The strength of the crystalfield for the nanopowders is slightly larger in comparisonwith that of the bulk material. The chromium environmentof the Cr3+ ions in nanoparticles has less covalent charac-ter. It results from comparison of the values for B - inter-electronic repulsion, and β - delocalization of d-electrons,for both kinds of matter. The electron-phonon coupling isclearly reduced. This feature is explained as a result ofa confinement effect observed in nanocrystalline materi-als. The most effective phonons with energy ca. 250 cm−1are involved in this coupling and they correspond to thebending modes of tungstate ions. The following conclu-sions follow from the spectroscopic studies:

1. The optical study allowed determination of theground state of Cr3+ as 4A2 and suggested incor-poration on the (distorted) octahedral In3+ sites.2. The optical data at low temperature suggestedthe presence of two incorporation sites for Cr3+ inLiIn(WO4)23. Cr3+ ions mainly occupy the sites of the C2 sym-metry and only the spectra of such sites are seenin the electron absorption and emission studies;4. Two size-induced phase transitions can be observedfor LiIn(WO4)2:Cr3+ nanopowders.

3.2. EPR spectraFig. 2a shows examples of EPR spectra of LiIn(WO4)2: 0.5mol. % Cr3+ single crystals at two crystal orientations,i.e. when the magnetic induction was operating in the bcplane (axis of rotation parallel to the whisker axis a) and inthe ac plane (axis of rotation perpendicular to whisker axisa). The overall observed signal can be divided into twogroups of lines: very narrow lines of different intensity atmagnetic inductions below 400 mT (low field signal) andwider lines of equal intensity at magnetic inductions above700 mT (high field signal). The double field axis (differentfor top and bottom spectra) may appear somewhat con-fusing, but in our opinion it enables showing interestingdetails of both EPR spectra. In the case of a uniformrange of magnetic induction of 100 – 950 mT the upperspectrum could be unclear. A low field signal consistingof at least three lines couldn’t be ascribed to one Cr3+ ionwith S=3/2 [13–17]. It can be concluded from the differentintensities of the lines and from the fact that spectroscopicgeff value calculated in this case should be much higherthan expected for chromium trivalent ions. Calculationsperformed assuming a combination of allowed and forbid-den transitions also don’t fit in this case. So, detailedanalysis of low field EPR spectra allowed us to concludethat indeed, we observed three different magnetic centersascribed to nonequivalent locations of Cr3+ paramagneticions in the crystal. The centers denoted as I,II, and III giveonly specific transitions being observed, including a for-bidden transition for center II. Figs 2b,c show the angulardependence of the resonance line position as a functionof rotation angle. The presented roadmap confirmed C2local symmetry of all three chromium centers. Simula-tions of three roadmaps were done using an EPR-NMRprogram [10], where for Cr3+ ions with S= 3/2, the spinHamiltonian was applied in the following form:

Hs = µBBgS ′ + SDS, (1)1561

Synthesis and spectroscopic properties of LiIn(WO4)2 nanopowders and single crystals doped with chromium(III) ions

where the first term is a Zeeman term and the second oneis a Zero Field Splitting second order term. B denotesthe external magnetic induction and µB is the Bohr mag-neton. g and D are matrices. Hyperfine terms of the 53Crisotope (I=3/2, 9.5 % natural abundance) are not includedin Eq. (1), as being too weak in the observed spectra. Re-sults of the simulation presented in Figs. 2b,c as solidlines are very satisfying, especially for the centers I andII. EPR-NMR simulation yielded the principal values ofthe g and D matrices and their spatial orientation for allthree of the centers, gathered in Tab. 1.Mutual configuration of experimental abc and calculatedprincipal directions xyz of the g matrix are presented inFig. 3.The roadmap presented in Fig. 2b is significantly sim-plified, but indeed the visible signals are more asymmet-ric. In particular, in the bc plane the signal denoted asI revealed a splitting into at least five lines (Fig. 4a),where the splitting is best resolved at ca. 72°. This split-ting could be related to super hyperfine interactions ofchromium ions with ligand nuclei, probably oxygen, havingnuclear spin I= 5/2, giving six split lines. A real roadmapincluding satellite lines is presented in Fig. 4b, wheresome weak signal around the main lines could arise fromhyperfine interactions of the 53Cr isotope. Another expla-nation for the complicated roadmap presented in Fig. 4bcould be in terms of site splitting (maybe in combinationwith imperfection of the crystal). As the crystals exhibitmonoclinic symmetry, in arbitrary magnetic field orienta-tions one should observe various magnetically inequiva-lent sites of the same center (site splitting).Integral intensities of the low-field lines fulfill theCurie-Weiss law with a slightly positive Curie-Weiss param-eter θ = 0.66 K (Fig. 5a). Such behavior indicates aweak ferromagnetic kind of magnetic interaction betweenchromium ions. In this figure there are presented exper-imental results registered for temperatures below 100 K,but EPR signal from chromium ions was observed up toroom temperature. Due to specific experimental condi-tions, like some troubles with gas flow control, we presentEPR results only below 100 K. The reported large de-viations from g = 1.97 (g is slightly smaller than gefffor the 4A2 ground state of Cr3+) for all three centers inTable 1 are not expected. Very probably, for all cen-ters (maybe there are less than three dominant types) thezero-field splitting is much larger than the microwave fre-quency, making it difficult to estimate the D tensor fromspectra recorded in one microwave frequency band. It maybe that recording EPR spectra at higher microwave fre-quencies (Q, W-band) could help in more precise interpre-tation. Figs 5b (perpendicular orientation) and 5c (paral-lel orientation) show angular dependencies registered for

high-field EPR lines. We assumed that these lines cor-respond to more complex chromium centers, proposing S= 2 as the more common case for chromium ions. Resultsof the simulation with supposed model S = 2 seems tobe adequate for the experimental points in Fig. 5b. Theeffective spin S = 2 for chromium complex centers is wellknown in the literature, for example in [18]. We didn’tpresent any spin Hamiltonian parameters resulting fromthe fitting as the results for both planes were not com-patible. Solid lines in Fig. 5b present the fitting obtainedusing the EPR-NMR program. As it can be seen the fit-ting is quite reasonable. However, we could not find agood fitting for both rotation planes simultaneously. Oneof the explanations suggests that the mentioned magneticcenters could indeed be assigned to interstitial chromiumions.The registered EPR spectra of nanopowders are simplerthan those of single crystal. However, interpretation ofthese spectra is more difficult. Figs 6a and 6b show thespectra in the low (<20 K) and higher (<80 K) tempera-ture range for LiIn(WO4)2 : Cr3+ 2 mol. % nanopowders.For lower temperatures clear recognition is possible of thechromium origin of the spectra, especially at low magneticinduction. They represent all investigated samples, whichwere annealed at different temperature. Low temperaturespectrum was simulated using an EPR-NMR powder al-gorithm (Fig. 6c), employing spin Hamiltonian parametersgathered in Table 1. As can be seen from Fig. 6c, bothsignals reveal good enough agreement. Some discrep-ancies observed above 600 mT result from existence ofmore complex chromium centers and presence of oxygen.Two ranges were chosen due to a specific temperaturedependence of integral intensity of the registered EPRspectra. This intensity obeys the Curie-Weiss law below20 K and reveals a maximum in the 20 – 80 K range (seeFig. 7a). From Figs 6a and 6b one can see that the pres-ence of chromium dopants in the nanopowders implies thepresence of both EPR low-field and high field signals, aswas found to be the case in single crystals. The signalsare partially overlapped by some strong external signals(chamber 850 mT and oxygen 1200 mT), so separationof the low-field and high-field signals seems to be verydifficult. To overcome this difficulty, we first calculatedthe overall integral intensity of the EPR spectra. Fig. 7ashows that the integral intensity changes distinctly, show-ing a maximum starting at about 20 K and then suddenlyvanishing at about 80 K. Such a behavior suggests a com-plex nature of the EPR signal including the influence oflattice vibrations due to the thermal field.The above EPR results allow us to conclude that in thecase of nanopowders we deal with chromium complexesrather than isolated ions. We tried to confirm this con-1562

T. Skibiński et al.

Figure 2. Low-field and high-field EPR spectra of LiIn(WO4)2:Cr3+ 0.5 mol. % single crystal in both directions (a) and angular dependencies of lowfield lines and fittings (solid lines) derived from the EPR-NMR program, T=6.5 K (b, c). Angles of rotation mean that magnetic inductionwas operating in the: cb and ca planes in Fig. 2b and 2c, respectively.

Table 1. Elements of the matrices g and D calculated using EPR-NMR program for LiIn(WO4)2:Cr3+ 0.5 mol. % single crystal. θ and φ are thepolar and azimuthal angles of principal axes with respect to experimental abc system.

Center I (D values in mT)gxx = 2.523 gyy = 2.193 gzz = 1.510 Dxx = 169.3 Dyy = 60.9 Dzz = 230.2θ = 159 θ = 75 θ = 104 θ = 97 θ = 171 θ = 96φ = 95 φ = 50 φ = 324 φ = 4 φ = 224 φ = 95

Center II (D values in mT)gxx = 2.207 gyy = 2.001 gzz = 1.836 Dxx = 27.3 Dyy = -2.4 Dzz = -24.9θ = 86 θ = 103 θ = 166 θ = 87 θ = 177 θ = 89φ = 359 φ = 270 φ = 72 φ = 58 φ = 69 φ = 148

Center III (D values in mT)gxx = 3.919 gyy = 2.126 gzz = 1.438 Dxx = 132.0 Dyy = 1.3 Dzz = -133.3θ = 81 θ = 50 θ = 139 θ = 96 θ = 22 θ = 111φ = 304 φ = 206 φ = 224 φ = 335 φ = 261 φ = 243

clusion in another way by performing deconvolution of theEPR lines. We were able to select low-field and high-field lines. We then performed calculation of integral in-tensities. Low-field integral intensities obeyed the Curie-Weiss law giving positive Curie-Weiss temperature as inthe case of single crystals, while the high-field inensitiesdid not obey the Curie-Weiss law, revealing a maximumof the intensity in the 20 – 80 K range. For the low-field

1563

Synthesis and spectroscopic properties of LiIn(WO4)2 nanopowders and single crystals doped with chromium(III) ions

Figure 3. Orientation of experimental abc and calculated directionsof the principal axes of the g tensor for the three Cr3+ mag-netic centers in LiIn(WO4)2 single crystals.

Figure 4. a) Evolution of the main signal in bc plane at differentangles of rotation. (Magnetic induction values are properonly for spectrum at θ= 4Ť ). b) Real roadmap in bc planeincluding splitting of lines.

lines we calculated the Curie-Weiss temperature for all ofthe investigated nanopowders. The results of the calcula-tions are presented in Fig. 7b vs. annealing temperature.As can be seen, with increase of annealing temperature theCurie-Weiss parameter θ increases up to 550℃ and thendecreases. We observed a similar phenomenon for parentNaIn(WO4)2 materials doped with Cr3+ ions (squares inFig. 7b).The parameter θ is a measure of the strength of magneticinteractions between chromium ions. Increase of θ indi-cates an increase in the amount of isolated centers at theexpense of more complex centers. We believe that iso-lated centers give contributions to low-field EPR spectraand complex centers give contribution to high-field EPRspectra. Among complex centers there may be interstitialions, pairs of ions or ions placed on the surface of grains.Above 550℃ the number of isolated centers decreases atthe expense of more complex ones.

3.3. Magnetization measurementsIn Figs 8a and 8b we present the magnetic susceptibil-ity vs. temperature measured in FC and ZFC modes forLiIn(WO4)2:Cr3+ 2 mol. % (sample No 6) nanopowder. Fit-ting according to the C-W law was performed. All calcu-lated Curie-Weiss constants are negative. Their relationvs. magnetic field is listed in Figs 8a, b. The same con-clusion with negative θ values is derived from the overallintegral intensities of EPR spectra (Fig. 7a) taking intoaccount only the temperature range below 20 K. Both theEPR and magnetic susceptibility results confirm that inthe case of nanopowders we deal mainly with chromiumcomplexes.We calculated the effective magnetic moment of the com-pounds according to the following relation:

µeff = √ 3 · kBµ0 · µ2

B ·NA·√χm · T , (2)

where kB - Boltzmann constant, NA - Avogadro constant,µB - Bohr magneton, χm - magnetic susceptibility, and T- temperature. From the temperature dependence of theeffective magnetic moment of the sample the result is thatthe maximal value of the moment does not exceed 0.7 µBin the temperature range from liquid helium up to 300 K.It seems that only part of the chromium ions contributesto this value.4. SummaryEPR spectra of both single crystals and nanopow-ders reveal low-field and high-field signals attributed tochromium ions at low, C2 symmetry. For single crystalswe have calculated elements of the g and D matrices aswell as the azimuthal angles of the principal axes in re-spect to the experimental ones. Values of g close to 2 aretypical for Cr3+ ions. We believe that the low-field EPRsignal, both in single crystal and nanopowders spectra,originates from isolated chromium centers. Magnetic in-teractions between these ions are of a ferromagnetic type,because the ions locate and interact with each other alonga chain. The high-field EPR lines reveal antifferomag-netic interactions between chromium ions both for singlecrystals and nanopowders. The lines registered for crys-tals originate rather from isolated, interstitial ions thanchromium complexes. In contrast, in the case of nanopow-ders, the same type of lines seems to originate mainlyfrom chromium complexes. Among them chromium com-plexes forming on the particle surface may also be found.The sudden vanishing of all EPR lines at about 80 K isan effect of vanishing of paramagnetic contribution due

1564

T. Skibiński et al.

Figure 5. (a) Temperature dependence of EPR integral intensity for low-field lines, where solid lines describe the Curie-Weiss law, and theinset shows the same relation as a function of reciprocal temperature, (b, c) Angular dependencies of EPR resonance lines forLiIn(WO4)2:Cr3+ 0.5 mol. % at high-fields, T = 7 K.

Figure 6. Temperature dependence of EPR lines in selected ranges for LiIn(WO4)2:Cr3+ 2 mol. % nanopowders at temperature ∼10 K (a), and∼40 K (b). Comparison of an experimental EPR signal with that simulated for powders applying spin Hamiltonian parameters calculatedfor single crystals (c).

to the thermal field. Decomposition of the EPR lines ofnanopowders allowed selection of low-field and high-fieldEPR lines, giving for the former a clear Curie-Weiss de-pendence of integral intensity. Basing on the dependencewe drew a conclusion regarding increase of the content of

isolated ions with increasing annealing temperature at theexpense of chromium complexes up to the critical tempera-ture, found to be 550℃. Above this annealing temperaturethe number of isolated ions decreases at the expense ofchromium complexes.1565

Synthesis and spectroscopic properties of LiIn(WO4)2 nanopowders and single crystals doped with chromium(III) ions

Figure 7. (a). Overall integral intensity of EPR spectrum forLiIn(WO4)2:Cr3+ 2 mol. % nanopowders numbered as 2, 4and 6, (b). Curie-Weiss temperature calculated for severalsamples annealed at different temperatures of LiIn(WO4)2and NaIn(WO4)2 nanopowders.

Figure 8. Magnetic susceptibility of LiIn(WO4)2:Cr3+ 2 mol. %nanopowder (sample No 6) in FC (a) and ZFC (b) modes.

Acknowledgements

This work was supported by Ministry of Science andHigher Education in the framework of grant numberN N202 206637.

References

[1] C. Cascales, A.M. Blas, M. Rico, V. Volkov, C. Zaldo,Opt. Mater. 27, 1672 (2005)[2] M. Rico et al., Appl. Phys. B: Lasers Opt. 81, 621(2005)[3] I. Nikolov et al., Opt. Mater. 25, 53 (2005)[4] A.E. Borisevich, M.V. Korzhik, G.Yu. Drobychev, J.-F. Cavaignac, R. Chipaux, Nucl. Instrum. MethodsPhys. Res. A 537, 228 (2005)[5] A.E. Borisevich, M.V. Korzhik, G.Yu. Drobychev, J.-F. Cavaignac, R. Chipaux, J. High Energy Phys. 7,28 (2006)[6] P.V. Klevtsov, R.F. Klevtsova, J. Solid State Chem. 2,278 (1970) (crystal structure data in ICSD 16263)[7] V.B. Kravchenko, Zh. Struct. Khim. 12, 1108 (1971)(crystal structure data in ICSD 28099)[8] K. Hermanowicz, J. Phys. Condens. Matter. 18, 10601(2006)[9] K. Hermanowicz, M. Maczka, P.E. Tomaszewski,L. Krajczyk, J. Hanuza, J. Baran, J. Nanosci. Nan-otechno. 8, 1 (2008)[10] K.N.R. Taylor, M.I. Darbi, Physics of Rare EarthSolids (Chapman and Hall Ltd., London, 1972)[11] P.V. Klevtsov, A.V. Demenew, R.F. Klevtsova, Kristal-lografiya 16, 520 (1971)[12] I.G. Avaeva, V.B. Kravchenko, T.N. Kobzyeva, Neorg.Mater. 8, 586 (1972)[13] R. Kripal, I. Mishra, Mat. Chem. Phys. 119, 230(2010)[14] B.D.R. Raju, J.L. Rao, K.V. Narasimhulu, N.O. Gopal,C.S. Sunandana, Spectrochimica A 61, 2195 (2005)[15] R. Kripal, H. Govind, Physica B 403, 3345 (2008)[16] B.V. Padlyak, M. Grinberg, T. Lukasiewicz,J. Kisielewski, M. Swirkowicz, J. Alloys Comp.361, 6 (2003)[17] R. Kripal, S. Pandey, Spectrochimica A76, 62 (2010)[18] J.C.M. Henning, J.H. Den Boef, G.G.P Van Gorkom,Phys. Rev.B 7, 1825 (1973)

1566