FIRST ORDER MAGNETIC TRANSITION, MAGNETIC STRUCTURE AND VACANCY DISTRIBUTION IN Fe2P

HAL Id: jpa-00215741

https://hal.archives-ouvertes.fr/jpa-00215741

Submitted on 1 Jan 1974

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

FIRST ORDER MAGNETIC TRANSITION, MAGNETIC STRUCTURE AND VACANCY

DISTRIBUTION IN Fe2P

R. Wäppling, L. Häggström, T. Ericsson, S. Devanarayanan, E. Karlsson, B.

Carlsson, S. Rundqvist

To cite this version:

R. Wäppling, L. Häggström, T. Ericsson, S. Devanarayanan, E. Karlsson, et al.. FIRST ORDER MAGNETIC TRANSITION, MAGNETIC STRUCTURE AND VACANCY DISTRIBUTION IN Fe2P. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-597-C6-601. �10.1051/jphyscol:19746128�.

�jpa-00215741�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Dkcembre 1974, page (26-597

FIRST ORDER MAGNETIC TRANSITION,

MAGNETIC STRUCTURE AND VACANCY DISTRIBUTION IN Fe2P

R. WAPPLING, L. HAGGSTROM, T. ERICSSON S. DEVANARAYANAN (*) and E. KARLSSON (**)

B. CARLSSON and S. RUNDQVIST (***)

R6sum6. - La transition paramagnktique-ferromagnetique dans Fe2P a CtB etudiee par effet Mossbauer. Les champs magnktiques hyperfins chutent brusquement & 214,5 K d'une valeur d'environ moitie du champ & saturation jusqu'a zkro, ce qui indique une transition de premier ordre.

Les dkplacements isomkriques varient de mani6re discontinue k la transition. Pour certains kchan- tillons la transition sYCtale sur un large domaine de temperature, probablement & cause d'impuretks et d'autres imperfections dans les Bchantillons. Du champ magnetique hyperfin a 15 K on peut dkduire que les moments magnktiques sont 1,14 ^,UB pour Fe(1) et 1,78 ^,UB pour Fe(2). L'attribution des composants des spectres Mossbauer aux deux sites de fer cristallographiquement inequivalents a Bte faite sur la base de l'ktude de la variation des spectres en fonction de la temperature.

L'arrangement des lacunes de metal a etk 6tudik par spectrometric Mossbauer d'un echantillon non stcechiornetrique de Fe2P et par diffraction de rayons X sur un cristal non stcechiometrique de Mn2P.

Abstract. - The para- to ferromagnetic transition in Fe2P has been studied using Mossbauer spectroscopy. The magnetic hyperfine fields drop abruptly from about half of their saturation values to zero at 214.5 K indicating a first order transition. The isomer shifts show a discontinuous change at the transition point. For some samples the transition takes place over a wide temperature range, probably due to impurities and other imperfections in the samples. From the magnetic hyperfine fields at 15 K the magnetic moments can be deduced to be 1.14 ,UB and 1.78 ,UB for Fe(1) and Fe(2) respectively. An assignment of the components in the Mossbauer spectra to the two crystallo- graphically non-equivalent iron positions has been made from the temperature variation of the spectra.

The ordering of metal vacancies has been investigated by a Mossbauer study of a non-stoickio- metric Fe2P sample and by an X-ray diffraction study of a non-stoichiometric Mn2P crystal.

1. Introduction. - 1 .1 PREVIOUS WORK. - The chemical and physical properties of Fe2P have been studied in several investigations. While the actual occurrence of a paramagnetic/ferromagnetic transition in FezP is well established, there is considerable dis- agreement between the various determinations of the Curie temperature and the saturation magnetic moment for the ferromagnetic phase [I-91. I t appears that these discrepancies might, at least t o some extent, depend on the lack of rigorous chemical and physical definition of the test samples. Small amounts of impu- rities and deviations from stoichiometry might affect the measurements seriously 181.

I n a recent study [lo] the problems in preparing high-purity F 2 P in stoichiometric or non-stoichiome-

(*) Present address : Physics Department, University of Kerala, Trivandrum, India.

(**) Institute of Physics, Uppsala University, Box 530, S-751 21 Uppsala 1, Sweden.

(***) Institute of Chemistry, Uppsala University, Box 531, S-751 21 Uppsala 1, Sweden.

tric form were critically examined. The homogeneity range of the Fe2P phase was carefully determined for temperatures up t o 1 100 OC. I n addition, the crystal structure of pure, stoichiometric FezP was accurately refined from room temperature X-ray diffraction data.

With these new results a t hand we decided t o re-examine Fe2P by Mossbauer spectroscopic methods.

A number of Mossbauer studies of Fe2P have been reported previously [5, 6, 9, 11-14], but only limited conclusions have been drawn from the experimental information.

1.2 THE CRYSTAL STRUCTURE OF Fe2P. -Accurate crystallographic data for the Fe2P structure are present- ed in [lo]. The hexagonal unit cell of Fe2P contains six iron atoms situated on two threefold positions : Fe(1) and Fe(2), and three phosphorus atoms situated on one twofold : P(l), and one singlefold : P(2), position. The Fe(1) atoms have four near phosphorus neighbours situated a t the corners of a distorted tetrahedron, and the Fe(2) atoms have five phosphorus neighbours a t the corners of a distorted square-based

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746128

AND COLL.

pyramid. In addition, the Fe(1) atoms have eight near iron neighbours and the Fe(2) atoms ten near iron neighbours.

2. Experimental details. - For the Mossbauer spec- troscopic measurements we used four different Fe2P samples. Samples A and B were selected from the material synthesized by Carlsson et al. Detailed information on the synthetic techniques and chemical analytical results is given in [lo]. Sample A, which is identical with alloy number 2 in [lo], represented the closest approach to pure stoichiometric Fe2P attained in the work by Carlsson et al., its composition being Fe,.,,,P. Sample B was a non-stoichiometric Fe2P specimen. Strongly exposed powder photographs for this material revealed no diffraction lines from phases other than Fe2P. The lattice parameters : a = 5.858 5(3) P\ ; c = 3.452 7(3)

A

Sample D was kindly provided by Dr. D. Bellavance.

It consisted of single crystals prepared by molten salt electrolysis. The material was later reported to contain appreciable amounts of impurities, mainly carbon contamination introduced from the graphite electrode and crucible during electrolysis [8]. We determined the Fe2P lattice parameters for the electrolytic crystals by the technique described in [lo]. The values deviated insignificantly from those reported in [lo] for stoi- chiometric Fe2P. A wetchemical analysis using the methods described in [lo] gave 77.53 weight % iron and 21.90 weight % phosphorus.

The samples were crushed t o a fine powder, tho- roughly mixed with boron nitride powder and pressed to circular discs with 5 or 10 mg/cm2 of natural iron.

The available amount of sample B was so small that the average surface density for this absorber was only 2 mg/cm2.

The discs were mounted in a variable temperature cryostat or a furnace, and transmission Mossbaur spectra were recorded at typically ten different tem- peratures in the range 15-900 K, using a room tempe- rature s7CoPd source. The temperatures were control- led to withinA- 0.3 degrees.

In the analysis of the Mossbauer spectra the experi- mental data were fitted to various sets of Lorentzian lines by the least squares method, using an IBM 3701155 computer.

3. Results. - 3.1 ASSIGNMENT OF THE COMPONENTS IN THE MOSSBAUER SPECTRUM OF STOICHIOMETRIC

Fe2P. - The room-temperature Mossbauer spectrum

of Fe2P can be resolved into four absorption lines, two of which have a lower intensity than the remaining pair. Since there are two crystallographically non- equivalent iron atoms, Fe(1) and Fe(2), in the Fe2P structure it seems reasonable to associate one pair of lines in the Mossbauer spectrum with each of the two types of iron atom. An accurate structure refinement of a strictly stoichiometric Fe,P crystal [lo] has shown that there is a significant difference between the thermal vibrations for Fe(1) and Fe(2). The isotropic thermal vibration parameters obtained were B = 0.42(2) A 2

for Fe(1) and B = 0.58(2) A2 for Fe(2). The corres- ponding room temperature recoil-free fractions are 0.75(1) and 0.68(1) respectively.

The analysis of the intensities in Mossbauer spectra, particularly in cases where the lines are overlapping, is complicated by the saturation effects occurring in absorbers of finite thickness. There are several ways of estimating the intensities corresponding to the ideal case of zero thickness of the absorber. One way is to measure the intensities for several absorber thicknesses and extrapolate to zero thickness.

It turns out that in order to make reasonably thin homogeneous absorbers the grain size should, due to the high recoilfree fractions in Fe2P, be less than 1 ym.

Such absorbers were prepared, but they showed very broad absorption lines probably due to strains and deformations introduced during the grinding, and they could not be used for reliable intensity measurements.

An alternative way is then to try to correct the spectra (recorded with slightly larger grains

-

using theoretical expressions for the transmission of the gamma radiation through the absorbers. We used the fast Fourier transform technique to reduce the room temperature spectra to the corresponding absorption probabilities in the absorber. Figure 1 shows one measured spectrum and the corresponding absorp- tion probability obtained using the non-resonant background estimated from the recoil-free fractions given above and an average grain size as found in a microscopic investigation of the powdered samples used. The relative intensities of the two pairs of lines in the Mossbauer spectrum change from 0.90(2) : 1 to 1.06(2) : 1.

A third way to attack the problem is to study the temperature variation of the intensities in the Moss- bauer spectra, as examplified in figure 2.

It is evident that for the smallest thicknesses good agreement is obtained only if Fe 1 in table I is assigned to the Fe(1) position in the crystal.

3.2 THE DEFECT STRUCTURE OF NON-STOICHIOMETRIC

Fe2P AND RELATED COMPOUNDS. - Studies on the lattice parameter variation in non-stoichiometric Fe2P [6, 101 show that the unit cell dimensions decrease with decreasing iron/phosphorus atomic ratio.

A similar contraction of the unit cells occurs in the isostructural phases Mn2P and Ni2P and also in the closely related compound Co2P. The contraction of the

FIRST ORDER MAGNETIC TRANSITION, MAGNETIC STRUCTURE AND VACANCY C6-599

Fe1.99aP SAMPLE A

FIG. 1. - Mossbauer spectrum of FezP (sample A) recorded at 295 K, and the deconvoluted absorption probability of the same spectrum (below) using the Fast Fourier Transformation (FFT).

Fe1,998

SAMPLE A

FIG. 2. - Mossbauer spectra of FezP (sample A) recorded at 490 K and 900 K.

unit cells might be caused either by metal/phosphorus substitution or by the creation of metal vacancies.

Replacement of iron atoms by phosphorus atoms in Fe2P would give P-P distances of 2.2-2.3 A, while P-P distances in metal-rich transition metal phosphides normally exceed 3 A. The defect structure of Fe2P is not easily susceptible to analysis by X-ray diffraction methods, since the preparation of non-stoichiometric crystals suitable for X-ray work is a difficult experi- mental problem. We therefore tried to use Mossbauer spectroscopy for the defect structure analysis.

The composition of sample B corresponded to the formula Fel ,9,,P. Assuming that the recoil-free fractions are unchanged on passing from sample A to B, the expected intensity ratio I(Fe(1)) : I(Fe(2)) was calculated in the thin absorber approximation in the paramagnetic region for a) all vacancies on the Fe(1) position, b) vacancies equally distributed over the two metal positions and c) all vacancies on the Fe(2) position. A comparison of the high temperature expe- rimental ratio with the corresponding values for the three different models shows that model c) gives the best agreement.

One would assume, in view of the thermal vibration parameters, that the Fe(l) atoms are more tightly bound in the structure, and the energy required for vacancy formation would accordingly be larger for Fe(1) than for Fe(2). It is interesting to observe that an analogous situation seems to prevail in Co2P.

In order to provide material for further discussion, we undertook a structure refinement of a non-stoj- chiometric Mn,P crystal with an estimated composi- tion of Mn,.,P. The scattering parameter value of 0.88(3) obtained for Mn(1) strongly indicates a vacancy concentration of about 10 % on this position, which is entirely compatible with the estimated composition of Mn, .,P for the crystal concerned.

It is furthermore evident from this investigation that the thermal vibration parameters change insignifi- cantly on passing from the stoichiometric to the non-stoichiometric crystal. This supports indirectly the assumption we made for Fe2P in interpreting the Mossbauer spectrum of sample B.

The difference in vacancy distribution between Mn2P on the one hand and Fe,P and Co2P on the other can be accounted for in terms of free energy differences.

For Fe2P and Co2P the vibration entropy contribu- tion to the free energy is not large enough to balance the internal energy term at those temperatures, where an appreciable number of vacancies begin to be formed.

However, for Mn2P which has the largest difference in thermal vibration parameters, the vibration entropy contribution dominates.

3.3 THE MAGNETIC STRUCTURE OF Fe2P. -As is evident from figure 3 there is a rather large difference in magnetic hyperfine field for the two iron atoms in the structure. If we assume that the same relation

8~ (Ti 2 4

FIG. 3. - Magnetic hyperfine fields as function of temperature.

between magnetic moment and hyperfine field holds for both Fe(1) and Fe(2) we can derive values for the individual moments. Using the magnetic hyperfine field values obtained at low temperatures in the present study, and the recent magnetic moment value of 1.46(4) ^pB per iron atom obtained in our laboratory we obtain 1.14(4) ^pB and 1.78(4) ^pB for Fe(1) and Fe(2) respectively. From single crystal magnetization measu- rements it is known that the moments are directed along the c-axis.

Goodenough [15] has recently discussed the magnetic properties of transition metal pnictides M2X in terms of the itinerant electron model assuming that only the 3d-bands have a finite density of states at the Fermi level

whereas the 4s band is empty. The magnetic properties therefore reflect only the number of 3d electrons per transition metal atom. In Fe,P it is suggested that this number is (6.5 - 6) and (6.5

+

+

Using AR/R = - 8.7 x [16] and the atomic Hartree-Fock electron densities calculated by Blom- quist et at. [17] for 3d6.1a 4s0 4p0 and 3d6.82 4s' 4p0 and a relativistic correction [18], a difference in isomer shift of 0.27(4) mm/s is predicted between Fe(1) and Fe(2), the latter having the largest isomer shift with respect to iron metal. This is in good agreement with the difference found experimentally (0.26(1) mm/s) and the relative order is also correctly described.

3 . 4 THE ^MAGNETIC TR~NSITION. - AS is evident from figure 4 the transition between the para- and ferromagnetic states does not display the usual Bril- louin type behaviour. The transition is well defined for sample A occurring at 214.5(1.0) K whereas for samples C and D there is a rather large temperature region in which both a para- and a ferromagnetic structure is seen in the Mossbauer spectra.

Since all hyperfine parameters are found to be closely the same for samples A, C and D, the origin of the transition region is due to microstrains, due for instance, to the presence of impurities or otherwise

Results of the Mossbauer measurements on sample A. BhP is the magnetic hyperJineJield in Tesla,

e2 qQ ² 112

6 the isomer shiji in mm/s relative to iron metal at 295 K. A = - ₂ (1

+ %)

\

A=--.--- ' I ⁺ " " ⁺ " for T < T,, where vi is the velocity at the i th absorption line.

r

2 2

in mm/s and I the relative intensity in percent.

Temperature in K

- 900 760 600 490 295 220 217 215

FIRST ORDER MAGNETIC TRANSITION, MAGNETIC STRUCTURE AND VACANCY C6-601

Rel intensity

051 _SarnpleC . - - - Sample D

-

Sample A ' - - -

I

FIG. 4. - Relative intensity of the ferromagnetic phase as a function of temperature.

introduced during the thermal history of the samples.

This view is further supported by the fact that for

non-stoichiometric Fe2P, sample B, the transition region starts at -- 160 K and proceeds to much lower temperatures.

In all the Fe,P samples there is a change in electron density at the iron nuclei at the transition giving a discontinuous increase in isomer shift for Fe(1) of

-

-

The magnetic transition in Fe2P, is of the first order, and since a transition region is found for all samples (except for sample A where insufficient temperature stability is the most likely cause of the occurrence of two components at 214 K) magneto-elastic effects [19]

are quite important in this compound. Preliminary X-ray diffraction studies performed in our laboratory indicate that there is a change in lattice parameters of the order of 0.1 % at the transition.

References

[I] CHIBA, S., J. Phys. SOC. Japan 15 (1960) 581.

121 MEYER, A. J. P. and CADEVILLE, M. C., J. P h y ~ . SOC. Japan Suppl. B-1 17 (1962) 223.

[3] CADEVILLE, M.-C. and MEYER, A. J. P., C . R. Hebd. Sian.

Acad. Sci. 252 (1961) 1124.

[4] CADEVILLE, M.-C.,,Thesis, University of Strasbourg (1965).

[5] FRUCHART, R., ROGER, A. and SENATEUR, J. P., J. Appl.

Phys. 40 (1969) 1250.

[6] ROGER, A., Thesis, University of Paris (1970).

[7] BELLAVANCE, D., MIKKELSEN, J. and WOLD, A., J. Solid State Chem. 2 (1970) 285.

[a] ^CATALANO, A., ARNOTT, R. J. and WOLD, A,, J. Solid State Chem. 7 (1973) 262.

[9] GERARD, A., GRANDJEAN, F. and WAUTELET, M., IV Inter- nattonal Conference on Solid Compounds of Transition Elements, Geneva (1973).

[lo] CARLSSON, B., GOLIN, M. and RUNDQVIST, S., J. Solid State Chem. 8 (1973) 57.

[ l l ] GBRARD, A., BuEI. SOC. Belge Phys. 1 (1966) 43.

[12] BAILEY, R. E. and DUNCAN, J. F., Znorg. Chem. 6 (1967) 1444.

[13] SATO, K., ADACHI, K. and ANDO, E., J. Phys. Soc. Japan 26 (1969) 855.

[14] WAPPLING, R., HAGGSTROM, L., RUNDQVIST, S. and KARLS- SON, E., J. Solid State Chem. 3 (1971) 276.

[15] GOODENOUGH, J. B., J. Solidstate Chem. 7 (1973) 428.

[16] BARRETT, P. H. and MICKLITZ, H., Perspectives in Moss- bauer Spectroscopy (Plenum Press, New York-London), p. 117, 1973.

[17] BLOMQUIST, J., ROOS, B. and SUNDBOM, M., J. Chem. Phys.

55 (1971) 141.

[18] SHIRLEY, D. A., Rev. Mod. Phys. 36 (1964) 339.

[19] BEAN, C. P. and RODBELL, D. S., Phys. Rev. 126 (1962) 104.