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Intermetallic compounds , where stands for the rare earth or some ...

Intermetallic compounds , where stands for the rare earth or some ...

It is interesting to

It is interesting to note that hyperfine parameters (Mössbauer spectra) practically do notdepend on the structure type, since the mixed phase Ho based sample exhibits almost thesame kind of spectra as single phase ones.One of sites exhibit low magnetic hyperfine field of about 6 T ( 1. 5 T for Ce) regardless ofthe model used. The contribution c due to this site practically does not depend on the model,and it is constant across the series of compounds investigated despite the change of the spacegroup. Isomer shift and quadrupole splitting do not depend on the model, and they varyslightly with the rare earth except for the quadrupole splitting in the Ce based compound.They seem to be independent of the space group. Isomer shift for this site indicates rather lowelectron charge density. This is the site treated previously as non-magnetic and it originatesfrom iron located on the (4e) site [12] or unidentified site [13, 14]. Our results show that avery similar position exists in the R3 m group, as hyperfine parameters do not change whilegoing from one to another group. This position is called in this paper as (12e) . Once thesesites are treated as quadrupole doublets one obtains practically the same hyperfine parametersof the main iron sites, but the fits are somewhat poorer. The same (4e) site for the compoundsfree of silicon is characterized by the largest hyperfine magnetic field at least for R Fe 2 17(R = Dy [7], Ho[19], Er [3], Tm [20]) .It is assumed that the recoilless fraction is the same for all iron sites within a particularcompound. Under such an assumption one can use the contribution c due to (4e) or (12e)site to the total spectrum to estimate the non-stoichiometry parameter 2x for R2−xFe14+2xSi3compounds. We have used the average contribution following the models described above( 1+ 4 and 1+ 7 ) and obtained 2 x = 0. 5 ± 0.1 for all compounds investigated. It has beenfound earlier that the parameter x increases with addition of silicon to Lu Fe 2 17[21].Relative contributions of various Mössbauer sextets c due to regular iron sites ( 4 or 7 )could be used to estimate the silicon distribution over accessible sites. We have calculatediron occupancies of various sites accessible to silicon using above data. Average contributionsfrom both models were used. Results are summarized in Table 3. Regular iron dumbbellpositions (either (6c) or (4f) ) are fully occupied by iron except for the Ce, Nd and Er basedcompounds. Sites (9d) and (6g) are also silicon free in all compounds investigated except forthe Ce based compound. Remaining regular iron sites are more or less evenly occupied bysilicon. Such behavior is in good agreement with results obtained earlier [9, 22].10

Table 3Non-stoichiometry parameter and regular iron site occupancies derived from the Mössbauerspectra.Non-stoichiometryparameter 2x± 0.1Contribution (at. % Fe)± 4R3m6c 9d 18f or 18h 18h or 18fCe 2 Fe 14 Si 3 0.6 56 79 85 91Nd 2 Fe 14 Si 3 0.5 85 100 75 81Gd 2 Fe 14 Si 3 0.4 100 100 68 78P63/ mmcR3m4f6c6g9d12j or 12k18f or 18h12k or 12j18h or 18fHo 2 Fe 14 Si 3 0.5 100 100 69 776 / mmc4f 6g 12j or 12k 12k or 12jP 3Dy 2 Fe 14 Si 3 0.5 100 100 74 69Er 2 Fe 14 Si 3 0.5 93 100 76 74Lu 2 Fe 14 Si 3 0.4 100 100 68 77Y 2 Fe 14 Si 3 0.4 100 100 73 704. ConclusionsMain conclusions of the present contribution could be summarized as follows:1. Iron atoms partly replace rare earth atoms on the 6c − R sites forming dumbbells (12e ) inthe R3 m structure. Similar replacement takes place in the P6 3/ mmc structure, whererare earth atoms occupying 2b sites are replaced partly by iron atoms, the latter forming4e dumbbells. The hyperfine field on these iron atoms is very low. Such replacement isobserved for compounds with some iron atoms replaced by silicon atoms on the regulariron sites.2. Silicon atoms avoid 6c and 9d sites for the R3 m structure except for the Ce basedcompound. For the 6 / mmc structure silicon atoms try to avoid 4f and 6g sites.AcknowledgmentsP 3The work of A. V. Andreev is a part of the research project AVOZ10100520 of the Academyof Sciences, Czech Republic.References1. Bao-gen Shen, Bing Liang, Zhao-hua Cheng, Hua-yang Gong, Wen-shan Zhan, HongTang, F. R. de Boer, and K. H. J. Buschow, Solid State Commun. 103 (1997) 71.2. Bo-Ping Hu, Hong-Shuo Li, Hong Sun, and J. M. D. Coey, J. Phys.: Condens. Matter 3(1991) 3983.3. F. Grandjean, O. Isnard, Dimitri Hautot, and Gary J. Long, Phys. Rev. B 63 (2001)014406.4. J. P. Liu, F. R. de Boer, P. F. de Chatel, and K. H. J. Buschow, Phys. Rev. B 50 (1994)3005.5. P. C. M. Gubbens, A. M. van der Kraan, T. H. Jacobs, and K. H. J. Buschow, J. LessCommon Metals 159 (1990) 173.11

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