57Fe Mössbauer study of Prm(Fe, Mo)n compounds with m:n=2:17 and 1:12

Journal of Alloys and Compounds 285 (1999) 37–47

57

L

¨ Fe Mossbauer study of Pr m (Fe, Mo) n compounds with m:n52:17 and 1:12

a a a a, a a b M. Morariu , D.P. Lazar , A. Galatanu , N. Plugaru *, V. Kuncser , G. Filoti , G. Hilscher , b A. Kottar a

National Institute of Materials Physics, P.O. Box MG-07, Bucharest-Magurele, Romania b Technical University of Vienna, Wiedner Hauptstraße 8 -10, A-1040 Vienna, Austria Received 9 December 1998

Abstract The effects of Mo substitution in Pr 2 Fe 172x Mo x and PrFe 122x Mo x compounds were investigated by X-ray diffraction, magnetic ¨ measurements and Mossbauer spectroscopy. Different fitting procedures, using magnetic sextets at low temperature, magnetic sextets and quadrupole doublets at room temperature and paramagnetic doublets well above the Curie points, had to be considered in order to analyze ¨ ¨ the Mossbauer spectra. The Mossbauer data obtained on the Mo-containing compounds stand for distributions of the hyperfine fields due to the random distributions of Mo over the 6c sites in the 2:17 and 8i sites in the 1:12 compounds. The variations in the isomer shifts and hyperfine fields with composition are discussed in relationship to the local environment details at the different iron sites. Site-specific information on the electronic effects due to Fe(3d)–Mo(4d) states hybridization are derived.  1999 Elsevier Science S.A. All rights reserved. Keywords: Rare earth intermetallic compounds;

57

¨ Fe Mossbauer spectroscopy; Local environment analysis

1. Introduction In recent years, strongly motivated by the demand for high-performance permanent magnetic materials, the research in this field has been concentrated on the study of a wide range of iron-rich rare earth intermetallics. The improvement of the magnetic properties induced by the interstitial modification of alloys [1], has led to a resurgence of the interest in the investigation of R 2 Fe 17 compounds and their substitutional R 2 Fe 172x M x solid solutions [2–20], as well as the metastable RFe 122x M x compounds [21–27], with emphasis on their microscopic properties. The partial replacement of iron, which is essential for the structural stability of the RFe 122x M x phases, plays also a key role in asserting the magnetic properties by the electronic charge redistribution effects associated with the presence of the substituent. Amongst various types of iron substitutions, that of molybdenum has attracted much interest, mainly in the RFe 122x Mo x series, as a consequence of the relative easiness in compound formation and the rather extended molybdenum content range compared to R 2 Fe 172x Mo x . *Corresponding author.

Thus, it was found that the molybdenum solubility range is between 0.5#x#4.0 in YFe 122x Mo x [28–30] and 1.0#x# 3.0 in NdFe 122x Mo x [31]. Recently, Zinkevitch et al. [32] have shown that compounds with the ThMn 12 structure form in the Fe–Gd–Mo system in the composition domain defined by the formula Gd 11n Fe 122x 2y Mox h y , where 0# n#0.23, 1.16#x#3.56 and 0.60#y#0.90, with h representing iron vacancies. In the case of the R 2 Fe 172x Mo x compounds the molybdenum content in single-phase samples was determined in a range of x#1.0 [33] for R5Y and x#0.7 for R5Nd [34]. In the R 2 Fe 172x M x compounds with the Th 2 Zn 17 structure, the iron and the substitutional atoms occupy four crystallographically inequivalent lattice sites, labelled as 6c, 9d, 18f and 18h, whereas three different sites, assigned as 8i, 8j and 8f are occupied in the RFe 122x Mx with the ThMn 12 structure. Analyses of structural data obtained by X-ray [35] or neutron [36–38] diffraction assessed the strong molybdenum preferential occupancy of the iron 8i site in the ThMn 12 structure, independently of the rare earth partner. The magnetocrystalline anisotropy and spin reorientation transitions were studied by magnetic measurements, [31,39–46] as well as neutron scattering [36– 38,47–49]. Recently, magnetic data on RFe 122x Mo x single

0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )01055-X

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M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

crystals, with R5Er [50], R5Y [51], and R5Dy [52] have also been reported. These investigations show that the molybdenum substitution for iron strongly reduces the Curie temperature and the iron magnetic moment, weakens the magnetic anisotropy of the iron sublattice and favours a large variety of magnetic phase transitions. ¨ Several studies performed by 57 Fe Mossbauer spectroscopy on RFe 122x M x compounds were concerned with obtaining information on the local spin- and charge-densities at the magnetically inequivalent iron sites, [53–60]. The characteristic substitutionally-induced disorder in this class of compounds leads to a distribution of local environments at a specific crystallographic iron site, and consequently, a distribution of the hyperfine parameters on each site has to be taken into account. An approach considering that the iron atoms on a specific crystallographic site are represented by several magnetic sextets, each sextet having a relative area determined by the binomial probability of the local environ¨ ment at that site, was used to analyze the Mossbauer spectra measured at temperatures between 4.2 and 293 K of some RFe 122x M x compounds [53–55]. Up to four sextets had to be considered for an iron site, taking into account the sextets with relative intensities higher than 3%. ¨ The Mossbauer spectra of RFe 10 Mo 2 , with R5Y or Lu, acquired at temperatures between 5–300 K were studied by Christides et al. [56]. These authors suggested that two magnetic phases have to be considered to coexist even below the Curie temperatures and, consequently, simulated ¨ the Mossbauer spectra with a bimodal distribution of hyperfine fields, including one ferromagnetic and one paramagnetic component. The local distributions of the magnetic and paramagnetic component were taken into account in the work carried out by Ayres de Campos et al. [57], on the series RFe 9.5 Mo 2.5 . The authors used in their fits three magnetic components, corresponding to each of the crystallographic iron sites, at low temperature, one single quadrupole doublet assigned to each iron site, above T C , and two different components (magnetic and nonmagnetic) for each site, in the intermediate temperature range. The existence of the nonmagnetic components was observed in the high temperature ¨ Mossbauer spectra and accounted for by pure quadrupole interaction doublets. The onset of the magnetic transitions versus temperature variation was unambiguously marked both by the presence of regions with anomalous behaviour in the AC susceptibility curves and in the same cases by muon spin relaxation results [57]. Much less data on the molybdenum substituted R 2 Fe 17 compounds are reported. A study undertaken by X-ray diffraction and magnetic measurements of the Y 2 Fe 172x M x (N y ), with M5Mo, V and Ti was reported in Ref. [33]. It was found that the substitution of molybdenum for iron slightly expands the unit cell, increases the Curie temperature by about 20 K per Mo atom, significantly less than V (98 K per V atom) or Ti (80

K per Ti atom), and strongly decreases the mean iron magnetic moment, by about 24% per Mo atom. Recently, the structural parameters obtained by Rietveld refinement of the neutron diffraction patterns of several Nd 2 Fe 172x Mx compounds [34], have indicated that molybdenum atoms preferentially substitute for the iron on the 6c site, which has also been supported by calculations of the enthalpy of mixing, used as a measure of the bond strength for Nd–X and Fe–X, in Miedema’s model. A survey of the published work reveals a lack of data on the Pr 2 Fe 172x Mox compounds and only a few results concerning the PrFe 122x Mox system [45,46,61]. In this ¨ article we present structural, magnetic and 57 Fe Mossbauer spectral results obtained on Pr 2 Fe 172x Mox and PrFe 122x Mo x compounds and we show evidence of the electronic charge redistribution effects on the hyperfine parameter variations and related site-specific quantities with molybdenum content.

2. Experimental Samples of Pr 2 Fe 172x Mo x , with x50 and 0.5 and PrFe 122x Mox , with x51.5 and 2.2 were prepared using a conventional arc-melting furnace, in argon atmosphere, starting with high purity elements. Firstly, a (Fe–36 wt.% Mo) pre-alloy was prepared and then remelted together with the necessary quantities of iron and praseodymium to match the desired stoichiometries. An excess of praseodymium, of about 20% in each case, was used to compensate the losses during melting. The as-cast samples were wrapped in tantalum foil, sealed in a quartz tube under high vacuum and annealed at 9508C for 1 week. Subsequently, the samples were quenched in water. The crystal structure of the alloys was investigated by X-ray diffraction on powders with grain size of about 20 mm, using a Siemens D500 diffractometer with Co K a ˚ selected by a graphite monoradiation (1.789007 A) chromator in the secondary beam, at room temperature (RT). The X-ray spectra, plotted in Fig. 1, show the formation of compounds with the Th 2 Zn 17 structure for x50 and 0.5, and the ThMn 12 structure for x51.5 and 2.2 compositions. The presence of a-Fe secondary phase in amounts of about 6% and 4% was detected in the samples with x50 and 0.5, respectively. The Curie temperatures, T C , were derived from the thermal variation of magnetization curves, presented in Fig. 2, measured in the temperature range 293–900 K, in a magnetic field of 200 Oe. Magnetization isotherms were recorded at 2 K using a SQUID magnetometer type S603 from Cryogenics, in applied fields up to 6 T. The saturation magnetizations, Ms , were derived by fitting the experimental data at fields larger than 2.5 T to the approach to saturation law M5Ms (12b /H 2 ) 1 x0 H, where the b parameter is related to the magnetocrystalline anisotropy constants and x0 is a Pauli-type susceptibility.

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

39

fine powders mixed with polystyrene dissolved in toluene and fixed on iron-free mica foil. The spectra were analyzed using the MOSSFIT software package, in the approximation that treats the quadrupole interaction as a perturbation to the magnetic hyperfine interaction. In our fitting procedure a sextet was represented by the hyperfine field, Hhf , the isomer shift, d, the quadrupole shift, ´, and a single linewidth, G, as fit parameters. We used Lorentzian lines and constrained their relative intensities to the ratio 3:2x:x 2 :x 2 :2x:3, with x in the range 1.1–1.3, to allow for a slight texture of the samples. The estimated errors are 63 kOe for Hhf , 60.015 mm / s for d and 60.03 mm / s for ´. The d values are given relative to a-Fe foil at RT.

3. Structural and magnetic results

Fig. 1. X-ray diffraction patterns of the Pr 2 Fe 172x Mo x and PrFe 122x Mo x at 293 K.

¨ The Mossbauer spectra were collected at 18 K, 293 K (room temperature, RT) and above the Curie temperatures using two spectrometers in the constant-acceleration mode: a Promeda-type for the RT and high temperature measurements and a home-made spectrometer equipped with a closed-cycle two-stage helium refrigerator for the low temperature measurements. For all the measurements, sources of 57 Co in rhodium were used and the velocity scales were calibrated using an a-Fe absorber at RT. The ¨ samples, consisting in homogeneous Mossbauer absorbers of about 35 mg / cm 2 effective thickness, were prepared of

The values of the lattice constants, unit cell volumes, Curie temperatures and saturation magnetizations are given in Table 1. The lattice constants of Pr 2 Fe 17 obtained in this work agree fairly well with those reported in Ref. [62] and are somewhat smaller than those in Ref. [63]. One may observe that the substitution of the larger molybdenum atom for the smaller iron one in Pr 2 Fe 16.5 Mo 0.5 (Mo and ˚ Fe atomic radii for a coordination number of 12 are 1.39 A ˚ respectively) leads to a slight contraction in and 1.26 A, the basal plane, Da /a 0 520.16% and a greater expansion of the unit cell along the c-axis, Dc /c 0 50.49%. Such anisotropic changes in the lattice constants were systematically determined by neutron diffraction in Nd 2 Fe 172x M x , with M5Mo, W [34], M5Cr [64] or M5V, Cr [65]. The correlation of these results with the substituent preferential occupancy of the iron 6c sites found in all cases, suggest a molybdenum preferential occupancy of the iron 6c sites in Pr 2 Fe 16.5 Mo 0.5 . This may be accounted for by the special arrangement of the iron 6c (dumbell) sites as c-axis-oriented pairs of transition metal atoms exhibiting the shortest Fe–Fe distance in the Th 2 Zn 17 structure. It is worth mentioning that neutron diffraction results indicate that for a certain substituent type, its site-affinity is independent of the rare earth partner in R 2 Fe 172x M x [12–19,66]. The Curie temperatures increase by about 32 K / Mo atom as the nonmagnetic molybdenum substitutes for iron, following the general trend observed in R 2 Fe 172x M x for various iron substitutions [67]. This behaviour may be assigned to the competition between the increase in T C due Table 1 Lattice constants, unit cell volumes, Curie temperatures and saturation magnetizations at 2 K for Pr 2 (Fe,Mo) 17 and Pr(Fe,Mo) 12

Fig. 2. TMA curves of Pr 2 Fe 172x Mo x and PrFe 122x Mo x .

Compound

˚ a (A)

˚ c (A)

˚ 3) V (A

T C (K)

Ms ( mB / f.u.)

Pr 2 Fe 17 Pr 2 Fe 16.5 Mo 0.5 PrFe 10.5 Mo 1.5 PrFe 9.8 Mo 2.2

8.591 8.575 8.597 8.632

12.465 12.526 4.803 4.814

796.7 797.6 355.0 358.7

298 314 430 385

36.3 32.6 21.1 17.2

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M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

to the diminution of the strong negative Fe(6c)–Fe(6c) exchange interactions and the decrease in T C due to the decrease in the mean iron magnetic moment that weakens the overall Fe–Fe exchange coupling. It was already shown that in Pr 2 Fe 17 iron and rare earth sublattices magnetizations couple ferromagnetically and favour an easy magnetization direction along the b-axis in the basal plane [68]. The data listed in Table 1 indicate a decrease in Ms by 3.7 mB / f.u. in Pr 2 Fe 16.5 Mo 0.5 relative to Pr 2 Fe 17 . Under the assumption that the Pr magnetic moment is not affected by substitution and consequently, the decrease in Ms is due only to the decrease in the mean iron magnetic moment, Dm Fe , one can estimate this variation by using the 9 53.7 mB , where m 9Fe 5m Fe 2Dm Fe . relation 17m Fe 216.5m Fe Taking for the mean iron moment the value m Fe 52.17 mB , obtained by neutron diffraction on Pr 2 Fe 17 at 2 K [62], then one derives for Dm Fe the value of 0.16 mB as x increases from 0 to 0.5. This is in reasonable agreement with the value of 0.39 mB obtained for the decrease of m Fe in Y 2 Fe 172x Mo x , with x increasing from 0 to 1.0 [33]. In the PrFe 122x Mo x compounds, both lattice constants increase with Mo content and their values are in good agreement with those reported in Ref. [46]. The crystal lattice distorts stronger in the basal plane, which is evidenced by the relative variation Da /a 0 50.41% comparing to Dc /c 0 50.23%. The effect of increasing the Mo content is to strongly decrease the Curie temperature, as previously observed for the YFe 122x Mo x series [67], and

the saturation magnetization. The Curie temperatures and the saturation magnetizations given in Table 1 compare well with those obtained for the similar PrFe 122x Mo x compounds in Ref. [46], excepting the T C of the x52.2 compound, for which these authors report a lower value (323 K). The PrFe 122x Mo x compounds order ferromagnetically and exhibit basal plane magnetic anisotropy up to T C [46]. A comparison of the saturation magnetizations for the x51.5 and 2.2 compounds given in Table 1, with the saturation magnetizations measured for several corresponding YFe 122x Mox compounds yields for the Pr magnetic moment values which are either lower [30,41], or higher [43,69], than the free-ion value (3.2 mB ).

¨ 4. Mossbauer spectral results and analysis

4.1. Pr2 Fe172 x Mox compounds ¨ The experimental Mossbauer spectra of the 2:17 compounds, measured at 18 and 293 K, as well as their fit results are shown in Fig. 3. Comparing the 18 K spectrum of Pr 2 Fe 17 to the 15 K one reported in Ref. [70] or the 85 K one reported in Ref. [63], one may observe a high similarity in the experimental data, with a small increase in the magnetic hyperfine interaction at low temperatures compared to 85 K. In the fit of the 18 K spectrum of Pr 2 Fe 17 we have used seven

¨ Fig. 3. Measured and calculated Mossbauer spectra of Pr 2 Fe 172x Mo x at 18 K (top) and 293 K (bottom).

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

sextets, designated by 6c, 9d6, 9d3, 18f12, 18f6, 18h12 and 18h6 and relative areas in the ratios 2:2:1:4:2:4:2, as previously ascertained for the rhombohedral R 2 Fe 17 exhibiting basal plane magnetic anisotropy [63]. The sextets correspond to the seven magnetically different iron sites, having in view the different angles between the principal axis of the electric field gradient tensor and the hyperfine field at the four inequivalent iron sites. The linewidth in our fit is between 0.32–0.34 mm / s. The substitution of as small a content as 0.5 Mo atoms per formula unit (f.u.) for ¨ Fe leads to significant changes in the Mossbauer spectrum, consisting in a broadening of the absorption lines and a decrease of the overall magnetic hyperfine interaction. In the analysis of the 18 K spectrum of Pr 2 Fe 16.5 Mo 0.5 we have considered several magnetic sextets for a specific iron site, in order to account for the distribution of the hyperfine fields produced by the variation of local environments [53]. Assuming a random distribution of the Mo atoms over the 6c sites, we have calculated the binomial probabilities of the different local Fe configurations and subsequently, the relative areas of the sextets. A satisfactory fit was obtained by decomposing the spectrum into 11 sextets, with relative intensities higher than 3%, with one sextet for each of the 6c, 9d3 and 18h6 magnetic sites and two sextets for each of the 9d6, 18f12, 18f6, 18h12 sites. The linewidth increased to 0.35–0.37 mm / s. The sextets were assigned to the inequivalent iron sites on the basis of their relative areas and the correlation between the hyperfine field and the number of iron nearest neighbours (NN), i.e. the larger the number of iron NN, the larger the Hhf value. In the Th 2 Zn 17 structure the local coordinations of the 6c, 9d, 18f and 18h iron sites are (1,3,6,3,1), (2,0,4,4,2), (2,2,2,4,2) and (1,2,4,2,3), respectively, where the numbers in parentheses stand for the (6c,9d,18f,18h,R(6c)) nearest neighbors. Therefore, the

41

decreasing order of the hyperfine fields is Hhf (6c). Hhf (9d)$Hhf (18f).Hhf (18h), reflecting the trend in the Fe NN, i.e. 13, 10, 10, 9. The refined hyperfine interaction parameters are listed in Table 2. Inspection of the 293 K experimental spectra of Pr 2 Fe 172x Mo x compounds, see Fig. 3, obviously shows reduced magnetic hyperfine interactions due to the proximity of the measurement point to the Curie temperatures. The shapes of the spectra give evidence for increased areas at low velocities, which should be expected to arise from collapsed sextets contributed by those Fe sites with low Hhf values. Therefore, the spectra were fitted with seven magnetic sextets to account for the Fe sites complying to the condition that the quadrupole interaction is a perturbation to the magnetic one and four symmetric quadrupole doublets in the ratio 2:3:6:6, in order to account for the distribution of collapsed sextets, with low Hhf values, corresponding to the four different iron sites. We also imposed the constraint that the summed areas of the sextets and quadrupole doublet at a specific iron site be equal to the crystallographic weight of the site. Reasonable fits were obtained with a linewidth in the range 0.35–0.38 mm / s and a total area of the collapsed sextets fraction of 20.6% for x50, and 17.7% for x50.5. One may note that these fractions decrease as the T C of the compounds increase due to the Mo addition. The magnitude of the pure quadrupole interaction was ¨ determined from the Mossbauer spectra measured at 373 K, in the paramagnetic temperature range. These spectra were well fitted with four doublets with the relative areas corresponding to the occupation number. The hyperfine parameters obtained at RT and 373 K are also displayed in Table 2 and the composition dependencies of the site averaged hyperfine fields and isomer shifts are shown in Fig. 4. Substitution of molybdenum for iron decreases both

Table 2 ¨ Mossbauer hyperfine parameters for Pr 2 Fe 172x Mo x x

0

T (K)

Hhf (kOe)

d (mm / s)

´ (mm / s) QS (mm / s) 0.5

Hhf (kOe)

d (mm / s)

´ (mm / s) QS (mm / s)

Site

Wt.

6c

9d6

9d3

18f12

18f6

18h12

18h6

avg.

18 293 18 293 373 18 293 373

356 263 0.259 0.045 20.027 20.01 20.04 20.10

293 202 0.031 20.156 20.170 0.11 0.19 20.65

316 221 0.031 20.156 – 20.37 20.34 –

310 190 0.094 20.076 20.138 0.05 0.17 0.54

268 163 0.094 20.076 – 20.24 20.06 –

273 159 0.053 20.083 20.142 0.13 0.09 20.59

274 162 0.053 20.083 – 20.35 20.10 –

295 190 0.088 20.078 20.132

18 293 18 293 373 18 293 373

343 212 0.072 20.016 20.090 0.03 0.02 0.18

267 151 20.015 20.176 20.200 0.21 0.14 20.60

300 166 20.015 20.176 – 20.19 20.12 –

289 136 0.018 20.156 20.193 20.06 20.01 0.52

255 105 0.018 20.156 – 0.18 0.13 –

245 114 0.019 20.136 20.198 0.10 0.15 20.41

247 115 0.019 20.136 – 20.35 20.12 –

273 139 0.019 20.155 20.187

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M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

the hyperfine fields and the isomer shifts at the four different iron sites. Also, as predicted from the Debye approximation, the weighted average isomer shifts decrease linearly with increasing temperature, by 26.2310 24 mm / s K for x50 and 25.9310 24 mm / s K, for x50.5.

4.2. PrFe122 x Mox compounds

Fig. 4. Composition dependencies of the site averaged hyperfine fields (left) and isomer shifts (right) of Pr 2 Fe 172x Mo x .

¨ The experimental Mossbauer spectra of the PrFe 122x Mo x compounds, shown in Fig. 5, are characteristic for this class of intermetallics, obviously standing for distributions of hyperfine fields as the random Mo(8i) site occupancy leads to statistical distributions of local environments at the three different Fe sites [53–55]. The 18 K spectra were fitted with 11 sextets per spectrum, with relative areas determined by the binomial probability of local environments, following the model analysis described in Refs. [53,54]. The resolution of the spectra at high velocities enabled us to take into account sextets with relative areas as low as 2.4%. Satisfactory fits were obtained with linewidths in the range 0.35–0.37 mm / s. The local environments of the 8i, 8j and 8f iron sites in the ThMn 12 structure are (5,4,4,1), (4,2,4,2) and (4,4,2,2), respectively, with the numbers in parentheses referring to the (8i,8j,8f,1a) nearest neighbours. The largest hyperfine field was unambiguously assigned to the 8i site, with most Fe NN, (1325x / 4). In the assignment of the two lower

¨ Fig. 5. Measured and calculated Mossbauer spectra of PrFe 122x Mo x at 18 K (top) and 293 K (bottom).

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

hyperfine fields corresponding to the 8j and 8f sites, both having (102x) Fe NN as well as identical decompositions, we based on recent neutron diffraction results obtained on the RFe 9.5 Mo 2.5 compounds [49], that indicate for the iron magnetic moment the order m Fe (8j).m Fe (8f). Therefore, in view of the correlation between the hyperfine fields and the magnetic moments, we assigned the higher Hhf to the Fe(8j) site and the lower one to the Fe(8f) site, also in agreement with predictions of band structure calculations [71,72]. The RT spectra of the PrFe 122x Mo x compounds, although measured below the Curie temperatures (see Table 1), show contributions from both a ferromagnetic phase and a paramagnetic phase. Recently, experimental evidence has been obtained that the two magnetic phases may coexist in a rather broad temperature range below T C in this class of compounds [49,57]. Therefore, the spectra were analyzed using a magnetic component and a paramagnetic one, represented by a doublet, for each of the three inequivalent iron sites. The paramagnetic fractions were 12.1% for x51.5 and 18.5% in the case x52.2. The linewidths in these fits were within the range 0.36–0.38 mm / s. Each of the high temperature spectra (573 K and 423 K) was well fitted with three paramagnetic doublets in the ratio (42x):4:4. Their assignment was based on their relative areas, for the 8i site and on the correlation between the isomer shift and the Wigner–Seitz (WS) cell volume, in the case of the 8j and 8f sites, as discussed in Section 5. The hyperfine parameters obtained for the PrFe 122x Mo x compounds are given in Table 3 and the composition dependencies of the site averaged hyperfine field and isomer shift are plotted in Fig. 6. Whereas at 18 K and 293 K the Hhf values decrease similarly at the inequivalent iron sites as the Mo content increases, the composition dependencies of the site averaged d values show different slopes at the three measuring temperatures, suggesting Fe sitedependent second-order Doppler shift contributions, which are related to different Debye temperatures.

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Fig. 6. Composition dependencies of the site averaged hyperfine fields (left) and isomer shifts (right) of PrFe 122x Mo x .

5. Discussion In the substitutional systems under current investigation the variations in the hyperfine parameters as molybdenum replaces iron reflect the influences due to (i) a redistribution of the electronic charge density as a result of the decrease in the valence electron concentration, (ii) an overall increase in the electronic charge associated with the extra filled states of Mo, and (iii) an increase in the structural disorder. Because of the small compositional modifications, the structural parameters show only slight

Table 3 ¨ Mossbauer hyperfine parameters for PrFe 122x Mo x x

1.5

T (K)

Hhf (kOe)

d (mm / s)

QS (mm / s) 2.2

Hhf (kOe)

d (mm / s)

QS (mm / s)

Site

Wt.

8i

8j

8f

avg.

18 293 18 293 573 573

323 251 0.094 20.053 20.100 0.48

251 174 20.013 20.213 20.340 0.30

208 159 20.073 20.279 20.350 0.65

252 191 20.011 20.189 20.287

18 293 18 293 423 423

309 242 0.065 20.014 20.040 0.53

211 151 20.073 20.221 20.280 0.34

187 139 20.071 20.254 20.300 0.63

220 168 20.046 20.190 20.244

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

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Table 4 Specific iron site parameters for Pr 2 (Fe,Mo) 17 compounds

VWS

˚ 3) (A

˚ d¯ Fe – Fe (A) Fe NN Mo NN 2dd / dT (310 24 mm s 21 K 21 ) 2DHhf /Dy (kOe)a MFe (m B / atom) a

Pr 2 Fe 172x Mo x

Iron site

x

6c

9d

18f

18h

0.0 0.5 0.0 0.5 0.0 0.5 0.5 0.0 0.5

12.302 12.454 2.67 2.69 13.00 12.75 0.25 7.9 4.2 52 2.46 2.36

11.237 11.176 2.49 2.50 10.00 9.50 0.50 6.0 5.4 46 2.07 1.91

11.780 11.942 2.57 2.60 10.00 9.50 0.50 6.4 6.0 38 2.04 1.91

12.152 12.143 2.56 2.55 9.00 8.75 0.25 5.4 6.0 108 1.89 1.70

0.0 0.5

Dy denotes the variation of the average Mo NN number.

variations and therefore, one should expect the electronic effect to outweigh the structural effect.

5.1. Isomer shifts We analyze the isomer shift variations in relationship to the changes in the local environment at the inequivalent Fe sites and the variations of the atomic volumes represented by the Wigner–Seitz cell volumes, VWS . These were calculated using the BLOKJE software [73], the lattice constants obtained in this work (Table 1) and the positional parameters given in Ref. [62] for Pr 2 Fe 17 at 300 K and those reported in Ref. [36] for ErFe 10.5 Mo 1.5 at 290 K. The WS volumes, the type and number of nearest neighbours, the mean Fe–Fe interatomic distances, as well as quantities derived from the spectral hyperfine parameters are given in Tables 4 and 5. A survey of the isomer shift data plotted in Figs. 4 and 6 and the high temperature data listed in Tables 2 and 3 shows that the isomer shifts scale with the Wigner–Seitz cell volumes, i.e. the trends in the sequences of increasing isomer shifts 9d,18f#18h,6c and 8f,8j,

8i for the 2:17 and 1:12 compounds, respectively, follow the sequences of increasing VWS in the compounds. The decrease in the d values with increasing the molybdenum content is in agreement with previous results on Fe–Mo alloys with Mo as a substitutional impurity [74]. In a compound, the site averaged isomer shifts have different temperature variations, Dd /DT, indicating different effective recoil masses at the inequivalent Fe sites. The substitution of Mo for Fe leads to a decrease in absolute value of the thermal shifts in both systems, suggesting an increase in the effective recoil masses and hence, stronger Fe bonding to its neighbors, with the exception of the 18h and 8f sites in the 2:17 and 1:12, respectively. For the last two mentioned sites increased Dd /DT values were observed and that could arise from the fact that both patterns have the lowest Hhf and are subject to highest errors in the fitting. The 18 K isomer shifts as a function of the WS volumes, modified by Mo substitution, are plotted in Fig. 7. The isomer shift variations, Dd, may be expressed by the relation Dd 5Ddv 1Dde , where Ddv accounts for the volume

Table 5 Specific iron site parameters for Pr(Fe,Mo) 12 compounds

VWS

˚ 3) (A

˚ d¯ Fe – Fe (A) Fe NN Mo NN 2dd / dT (310 24 mm s 21 K 21 ) DHhf /Dy (kOe)a MFe ( mB / atom) a

PrFe 122x Mo x

Iron site

x

8i

8j

8f

1.5 2.2 1.5 2.2 1.5 2.2 1.5 2.2 1.5 2.2

13.517 13.889 2.73 2.74 11.12 10.25 1.87 2.75 3.5 2.6 17 2.23 2.13

11.806 11.847 2.60 2.61 8.50 7.80 1.50 2.20 5.9 5.2 56 1.73 1.46

11.372 11.397 2.52 2.53 8.50 7.80 1.50 2.20 5.0 5.8 29 1.45 1.29

1.5 2.2

Dy denotes the variation of the Mo NN average number.

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

45

Fig. 7. Wigner–Seitz cell volume dependencies of the site averaged isomer shifts of Pr 2 Fe 172x Mo x (left) and PrFe 122x Mo x (right).

scaling and Dde represents an electronic contribution due to screening and hybridization effects. Whereas the Ddv term has a positive contribution as VWS expands and a negative one for VWS contracting, the Dde term may be associated with a negative contribution arising from an increase in the s-like character of the Fe 3d electrons as a result of 3d–4d states hybridization and the larger electronegativity of Mo as compared to Fe (Pauling’s electronegativities of Mo and Fe are 2.16 and 1.83, respectively). Consequently, in the 2:17, the d (9d) and d (18h) decrease as Mo substitutes for Fe and this correlates with the VWS contraction estimated for the 9d and 18h sites. One may deduce a small Dde contribution at these sites. Although the WS volumes at the 6c and 18f sites expand, relatively large decreases of d (6c) and d (18f) may be observed. Fe(6c) site has one 6c (Fe,Mo) NN at a very ˚ in x50 and 2.30 A ˚ in short distance, of about 2.39 A x50.5 compound, and accordingly, a large Dde term should be responsible for the large decrease in the isomer shift evidenced for this site. As concerns the 18f site, with two 6c (Fe,Mo) NN, it is likely that the Dde term outweighs the Ddv one, resulting in the observed decrease in the isomer shift. Similar considerations may be used also in the case of the 1:12 compounds. Thus, the WS volume dependences of the d (8i) and d (8j) sites reveal strong electronic contributions, dominating the effect of volume scaling on d. Following these considerations, a surprisingly small variation Dd may be observed at the 8f site, with the smallest WS volume and four 8i (Fe,Mo) NN, for which a larger Dde contribution should be expected.

5.2. Hyperfine fields Using the 18 K Hhf values listed in Tables 2 and 3 and the average numbers of Mo nearest neighbors at a specific

iron site, y, we derived the local hyperfine field variations, DHhf /Dy, which are given in Tables 4 and 5. A comparison of the effects produced on the hyperfine fields by the substitution of one Mo atom for Fe at the crystallographically inequivalent iron sites in the 2:17 compounds, shows similar reductions in the Hhf at the 9d and 18f sites, both having the same numbers of Mo NN, y50.5, Fe NN (9.5) and Pr NN (2). Significant differences may be observed between the Hhf variations at the 6c and 18h sites, both with y50.25 Mo NN, but largely different Fe and Pr coordinations, i.e. 12.5 Fe NN and 1 Pr NN at the 6c site and 9.5 Fe NN and 3 Pr NN at the 18h site. Similarly, in the 1:12 compounds, the less Fe NN and the more Pr NN, the larger decreases in the Hhf are evidenced, as observed in the case of the 8j and 8f sites comparing to the 8i sites. The local Fe magnetic moments calculated from the site-averaged hyperfine fields using a proportionality constant A hf 5145 kOe /mB are also given in Tables 4 and 5. Although this proportionality is strictly valid only for the Fermi contact hyperfine field, as it neglects the orbital and dipolar contributions, is generally used to estimate the ¨ local iron magnetic moments from the Mossbauer hyperfine fields in large classes of rare earth-iron intermetallics [55,71]. Inspection of the VWS and m Fe values in Tables 4 and 5 shows that the correlation found in a large variety of Fe-based rare earth compounds and stated as the larger the VWS the larger the m Fe [75], is fully respected in the 1:12 compounds, following the trend 8i.8j.8f, and only partially in the 2:17 compounds. In the latter case, the WS volumes order in the sequence 6c.18h.18f.9d, whereas the local iron moments order as 6c.9d$18f. 18h. The iron moment at the 18h site, with the least (9) Fe atoms as nearest neighbors and most (3) Pr atoms as nearest neighbours is greatly reduced by the Fe(3d)– Pr(5d) states hybridization. Using the weighted averages of the hyperfine fields one

46

M. Morariu et al. / Journal of Alloys and Compounds 285 (1999) 37 – 47

obtains for the mean iron moments in Pr 2 Fe 172x Mo x the values m Fe 52.03 mB for x50 and 1.88 mB for x50.5, which gives a variation Dm Fe 50.15 mB , in fair agreement with the value of 0.16 mB estimated from magnetization measurements. Thus, the observed decrease in the saturation magnetization, DMs 53.7 mB , may primarily be ascribed to the decrease in the iron sublattice magnetization, DMFe 53.5 mB . In the PrFe 122x Mo x compounds, the mean iron moments are 1.74 mB for x51.5 and 1.52 mB for x52.2. In this case the decrease in the iron sublattice magnetization, DMFe 53.4 mB , is sensibly smaller than the decrease observed in the saturation magnetizations, DMs 5 3.9 mB and points to a trend in the Pr magnetic moment to be reduced by crystal field effects as the Mo content increases.

6. Concluding remarks Small amounts of Mo substituted for Fe strongly decrease the saturation magnetizations in Pr 2 Fe 172x Mo x and PrFe 122x Mo x compounds, cause a steep decrease in the Curie temperatures in the 1:12 and lead to a slight increase of T C in the 2:17 compounds, much less than expected, which suggests a large electronic effect associated with the Mo atoms in these ferromagnetic compounds. Analysis of the WS volume dependencies of the isomer shifts shows that the electronic term accounting for screening and hybridization effects largely outweighs the volume ¨ scaling term. The evolution of the Mossbauer parameters as a function of Mo content and versus temperature points to increasing Debye temperatures with Mo content as an effect of electron states hybridization and strong modification of the exchange integrals for the sites having larger numbers of surrounding Mo atoms. The hyperfine fields decrease significantly with increasing Mo content, as also expected from the Ms decrease. However, the site-specific Hhf values do not show a decrease that is correlated with the number of Mo atoms nearest neighbors, i.e. one may not assume a realistic decremental field DHhf per substituted Mo atom. A survey of the quadrupole interaction data shows that no significant changes in this quantity are induced by the substitution of Mo for iron in the studied composition ranges.

Acknowledgements The authors wish to thank Dr. W. Kappel for his strong support in performing the magnetic measurements. The helpful discussions with Dr. S. Constantinescu and Dr. G. ¨ Wiesinger on theoretical aspects of Mossbauer effect data analysis are gratefully acknowledged.

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