Semi-Heusler-type intermetallics MPtSn (M=Ti, Zr, Hf, Th): a magnetic susceptibility and NMR study

Intermetallics 13 (2005) 756–763 www.elsevier.com/locate/intermet

Semi-Heusler-type intermetallics MPtSn (MZTi, Zr, Hf, Th): a magnetic susceptibility and NMR study A. Grykałowska, K. Wochowski, B. Nowak* W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław 2, Poland Received 19 June 2004; received in revised form 11 November 2004; accepted 24 November 2004 Available online 1 February 2005

Abstract A careful analysis of magnetization isotherms recorded in the temperature range 1.9–300 K, and in magnetic field strengths up to 5 T shows that the TiPtSn, ZrPtSn, HfPtSn and ThPtSn all are diamagnetic with intrinsic susceptibilities of K2.76, K7.54, K8.67 and K10.18 m3/mol, respectively. It is shown that an application of nuclear magnetic resonance (NMR) of 47Ti, 49Ti and 91Zr quadrupolar nuclei is extremely useful in controlling the microscopic structure of semi-Heusler-type intermetallics, e.g. structural order/disorder at local sites. No valuable information on structural order/disorder in MPtSn compounds can, however, be directly obtained from static NMR spectra of spin 1/2 117Sn, 119 Sn and 195Pt nuclei since the observed spectra are broadened far beyond the rigid-lattice dipolar width due to indirect spin couplings of these heavy nuclei through the chemical bond. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Ternary alloy systems; B. Magnetic properties; D. Site occupancy

1. Introduction The semi-Heusler-type compounds MPtSn (MZTi, Zr, Hf, Th) crystallize in the cubic MgAgAs-type of structure [1]. It can be represented as consisting of four interpenetrating cubic fcc sublattices A(000), B(1/4 1/4 1/4), C(1/2 1/2 1/2) and D(3/4 3/4 3/4) one of which, e.g. B is empty. Alternatively, this structure may be considered as zincblende crystal structure of tin and platinum atoms in which the M atoms occupy the octahedral intersitial sites (Fig. 1). Thus each Sn and M atom is tetrahedrally coordinated by Pt atoms, whereas each Pt atom is doubly tetrahedrally coordinated by Sn and M atoms. Early study by Palstra et al. [2] classify the TiPtSn, HfPtSn and ThPtSn as Pauli paramagnets while the specific heat measurements indicate that TiPtSn and HfPtSn have a zero value for electronic term g (fN(EF)) in the formula C/TZgTCaT3. For ThPtSn gz2 mJ/mol K2 and no data * Corresponding author. Tel.: C48 71 343 50 21–29; fax: C48 71 344 10 29. E-mail address: [email protected] (B. Nowak). 0966-9795/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.11.003

are reported so far for ZrPtSn. To clarify the situation we have performed accurate magnetization measurements for TiPtSn, ZrPtSn, HfPtSn and ThPtSn. All compounds of family MPtSn exhibit anomalous large electrical resistivities which unusual temperature dependencies and gap formation are strongly dependent on annealing conditions and quality of the samples investigated [2–4]. Under proper annealing conditions the formation of a gap in electron energy spectrum near the Fermi level is also observed in isostructural and isoelectronic nickel based semi-Heusler compounds MNiSn (MZTi, Zr, Hf) [3,5–8]. From computer simulation of the X-ray diffraction patterns of ZrNiSn it was concluded that Ni atoms and empty B sublattice (Vac) are located on their regular positions, but Zr and Sn atoms substitute each other because of closeness of Zr and Sn atomic radii [3,6]. In this situation the chemical formula of ZrNiSn may be written as Zr1KxSnxNi(Vac)Sn1KxZrx. The more ordered samples (x!0.2) exhibit semiconductor-like temperature dependence of resistivity R for all temperatures studied (0.1!T!1000 K) while the less ordered (xO0.2) show

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Table 1 NMR parameters of relevant nuclides of the MPtSn intermetallics [11] Nucleus

Natural abundance (%)

I

Magnetogyric ratio (g/2p) (MHz/T)

Nuclear quadrupole moment (Q) (10K28 m2)

47

7.44 5.41 11.2 7.68 8.59 18.60 13.62 33.832

5/2 7/2 5/2 1/2 1/2 7/2 9/2 1/2

2.4040 2.4048 3.9748 15.2610 15.9659 1.7284 1.0856 9.2923

0.302 0.247 0.176 – – 3.365 3.793 –

Ti Ti 91 Zr 117 Sn 119 Sn 177 Hf 179 Hf 195 Pt 49

Fig. 1. Crystal structure of MPtSn intermetallics (MgAgAs-type).

a metallic R(T) behavior for T!100 K. No irregularities in the position of Ti and Sn atoms have been found in TiNiSn samples, most probably due to essential difference of the Ti and Sn atoms’ radii [6]. Also theoretical total-energy and band-structure calculations for Zr1KxSnxNi(Vac)Sn1KxZrx [9] show that at around xZ0.15 the gap closes to form semimetal. Therefore, to achieve experimental parameters comparable to those calculated for the ideal crystals, special attention to sample quality is needed. Several methods, like e.g. X-ray, metallographic and microprobe analyses, are used to determine quality of the samples. The main purpose of present work is to find and test another reliable method for characterization of sample quality for this type of materials. In this paper we present the results of nuclear magnetic resonance (NMR) obtained for a family of Pt based semi-Heusler compounds MPtSn (MZTi, Zr, Hf, Th). Most valuable information can be obtained when all components of a compound are accessible to the NMR technique. Such an opportunity appears in the case of intermetallic compounds TiPtSn and ZrPtSn where NMR experiments can be performed either for Ti, Zr, Sn and Pt nuclei. Table 1 gives the relevant NMR parameters for all nuclides in compounds studied. NMR differs from X-ray diffraction in being sensitive to local symmetry, as opposed to long range symmetry. The NMR of quadrupolar nuclei (47Ti, 49Ti and 91Zr) might provide new insights into, e.g. structural disorder of local sites, since NMR is sensitive to local-site environments rather than the bulk environment. Since the electric field gradient (EFG) tensor is zero by symmetry at the position of these nuclei in ordered compound and nonzero in disordered compound, the above problem could be definitely solved by quadrupole perturbed Ti or Zr NMR. If for example, ZrPtSn have some Zr and Sn atoms at wrongly occupied sites, the low local symmetry about zirconium atom should

considerably broaden the 91Zr NMR signals in powder samples through the nuclear electric quadrupole interaction despite the long-range cubic symmetry of the crystal lattice. The same is expected for the case of 47Ti and 49Ti resonance in TiPtSn. Sensitivity of NMR spectra of spin IZ1/2 of 119 Sn and 195Pt nuclei on structural disorder in MPtSn compounds is also discussed.

2. Experimental details 2.1. Samples preparation The samples were prepared by arc melting the constituent elements of at least 99.9% purity under purified argon gas. The samples were repeatedly flipped and remelted to enhance the homogeneity. The weight losses after melting were negligible. Both the as-cast and annealed at 800 8C by 1 week samples of TiPtSn were examined. The samples A and B of ZrPtSn were annealed at 850 8C by 2 weeks and at 800 8C by 1 week, respectively. HfPtSn and ThPtSn were subjected to annealing conditions of 1 week at 800 8C and 2 weeks at 850 8C, respectively. 2.2. X-ray The X-ray study of our samples was carried out with Siemens D5000 powder diffractometer using either Cu Ka or Co Ka radiation in order to ascertain the crystallographic and phase purity of prepared specimens. In our magnetic and NMR investigations only the samples with diffraction patterns not revealing the presence of other phases were used. Our room-temperature X-ray diagrams reveal narrow Bragg peaks indexed successfully in a cubic MgAgAs-type crystal structure. The lattice parameters calculated from these spectra are collected in Table 2. 2.3. Magnetic measurements The magnetic susceptibility of polycrystalline samples of TiPtSn, ZrPtSn, HfPtSn and ThPtSn was determined with

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Table 2 Summary of experimental results obtained for MPtSn intermetallics TiPtSn

ZrPtSn

6.339 Lattice parameter 6.170 ˚) a0 (A K2.76 K7.54 Magnetic susceptibility c (10K10 m3/mol) Chemical shift d (ppm) 47,49 Ti 2215 91 Zr 2249 119 Sn 1623 1206 195 Pt 3625 3400 Line width FWHM (kHz) 47 Ti (49Ti) 0.138 (0.138) 91 Zr 0.317 119 Sn 2.8 2.2 195 Pt 1.21 1.16

HfPtSn

ThPtSn

6.315

6.754

K8.67

K10.18

found that the use of magic angle spinning (MAS) technique has practically no effect on the 119Sn and 195Pt NMR line shapes in HfPtSn, so the reported 119Sn and 195Pt chemical shifts are only approximate for this compound. Determinations of nuclear spin-lattice relaxation times (T1) were made using conventional inversion recovery (IR) or saturation recovery (SR) pulse sequences.

3. Results and discussion 1009 3682

1691 4140

3.05 2.95

1.7 1.24

a SQUID magnetometer (Quantum Design, Inc.) between 1.9 and 300 K with the magnetic field strength, H, up to 5 T. 2.4. NMR measurements NMR measurements were performed using a Bruker Avance DSX-300 spectrometer operating at a field of 7.05 T. The spectra were obtained by Fourier transform of the free-induction decay (FID) or quadrupolar-echo pulse sequence (90+x K tK 90+y ) with extended phase cycling [10]. The spectra of 47Ti and 49Ti nuclei were obtained at nominal frequency of 16.9 MHz by ordinary single pulse sequence. The optimum pulse width used on solid sample of TiPtSn was 7.5 ms, the same as the 908 pulse width in the liquid reference sample of TiCl4. Up to 3000 scans were required to achieve an adequate signal-to-noise ratio. No exponential line broadening was applied before Fourier transformation of the FID signal. The 91Zr NMR spectra obtained at 28 MHz are the Fourier transformations of typically 1024 the FID signals after a single pulse. The 908 pulse (9.5 ms) was adjusted on the sample of ZrPtSn. It was found that 119Sn and 195Pt NMR line shapes recorded at about 112 and 64.4 MHz, respectively, were independent of the thermal treatment of the samples. Evaluated values of chemical shifts presented in Table 2 are reported in parts per million (ppm). More positive values correspond to high-frequency, low-field, paramagnetic, deshielded values (IUPAC d scale [11]). The 47Ti and 49Ti chemical shifts were determined relative to 47Ti and 49Ti NMR signals in TiCl4. The 91Zr, 119Sn and 195Pt chemical shift are given with reference to X 91ZrZ0.9296298, X 119 SnZ0.37290632 and X 195PtZ0.21400000, respectively [11]. Here X is defined as the ratio of the isotope-specific frequency to that of 1H in tetramethylsilane (TMS) in the same magnetic field [11]. In all cases the reported 119Sn and 195 Pt chemical shifts were measured at the peak position, even in HfPtSn, exhibiting an asymmetric line shape. It was

3.1. Magnetic measurements Measured magnetization isotherms (M vs. H plots) exhibit curvilinear dependence, especially at lowest temperatures (see Figs. 2–4). Observed maximum in M vs. H plots and negative value of M at high magnetic fields indicates that measured magnetization M is a sum of dia intrinsic diamagnetic contribution Mint and contributions para ferro Mimp and Mimp from paramagnetic and ferromagnetic-like para dia impurities, respectively. We assume that Mint and Mimp are ferro linear in H while the Mimp would saturate in sufficiently strong magnetic fields. para ferro MðHÞ Z cdia int H C cimp H C Mimp ðHÞ

(1)

para where cdia int and cimp represent the intrinsic diamagnetic and impurity paramagnetic susceptibilities, respectively. Thus, for any given temperature, the slope vM/vH in strong fields para gives cZ cdia int C cimp , and subtraction of cH gives the ferromagnetic-like impurity magnetization with no further ferro assumption about the form of Mimp ðH; TÞ. Quite frequently, when the volume fraction of the ferromagnetic impurities is low, the finely divided ferromagnetic particles behave

Fig. 2. Magnetization isotherms recorded at TZ1.9 K. Solid lines are guides to eyes.

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Fig. 5. Temperature dependence of the magnetic susceptibility c of HfPtSn. Solid line is least-squares fit of the data to Eq. (2). Fig. 3. Magnetization vs. applied magnetic field for HfPtSn at TZ1.9 K. The total magnetization Mexp is decomposed into a ferromagnetic-like ferro and magnetization linear in H, where c is the impurity magnetization Mimp sum of intrinsic diamagnetic susceptibility cdia int and impurity paramagnetic susceptibility cpara imp . Solid lines are guides to eyes.

superparamagnetically. Alternatively, if the impurity magnetization is attributed to isolated paramagnetic impurities, it can also be assumed to saturate in sufficiently strong magnetic fields. We do not make an attempt to analyze these effects in details. The problem was to determine correctly the intrinsic susceptibility of MPtSn compounds as this is the only contribution in which we are interested in the present work. Fig. 3 shows the measured magnetization

ferro curve M and deconvoluted contributions cH and Mimp vs. H para at 1.9 K. The contribution cimp should exhibit a Curie– Weiss temperature behavior according to the relation cpara imp Z C=ðT K qÞ, what is observed, indeed (see Fig. 5). By fitting the magnetic susceptibility data to the formula

cðTÞ Z cdia int C C=ðT K qÞ

(2)

we have evaluated intrinsic magnetic susceptibilities cdia int . The data, supported by temperature independent chemical shifts of titanium, zirconium, tin and platinum NMR, show unambiguously that the intrinsic magnetic susceptibilities of all semi-Heusler phases MPtSn (MZTi, Zr, Hf, Th) are temperature independent and diamagnetic. Numerical values are presented in Table 2. 3.2. Crystal structure and NMR

Fig. 4. Magnetization M vs. applied magnetic field for HfPtSn at various temperatures. Solid lines are guides to eyes.

In the series MPtSn the lattice constants, a0, and structural disorder of local sites (if present) are associated with the size of constituent atoms. Mutual exchange Ti4Pt is probable in the TiPtSn due ˚ ) and Pt to similarity of covalent radii of Ti (1.32 A ˚ ). An exchange Zr (Hf)4Sn can take place in (1.30 A ˚ ), Hf ZrPtSn (HfPtSn) due to closeness of Zr (1.45 A ˚ ) and Sn (1.41 A ˚ ) covalent radii. From simple (1.44 A geometrical considerations no disorder is expected in ˚. ThPtSn, where covalent radius of Th is equal to 1.65 A All numerical values of respective covalent radii are taken from [12]. In the above considerations, the use of covalent radii is justified by the fact that the distances M–Pt, equal (a0O3)/4, are very close to the sum of the covalent radii rMCrPt. For Ti, Zr and Hf but not Th based compounds, the sum of covalent radii, rSnCrPt, also matches closely the distance (a0O3)/4. These observations strongly suggest

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an essential covalent bonding Pt–M and Pt–Sn in the series MPtSn.

Table 3 Total calculated and respective contributions to the second moment, M2, calculated and experimental line width (FWHM) of 47Ti (49Ti) NMR in TiPtSn

3.2.1. TiPtSn The 47,49Ti NMR spectra of TiPtSn at 294 K are shown in Fig. 6. The two sharp lines attributed to the 47Ti and 49Ti isotopes are separated by 4.5 kHz due to the different values of the magnetic moments of the two isotopes. The line widths of both isotopes, defined as full width at half maximum (FWHM), of the as-cast sample (A) are twice as large as those of sample (B) annealed at 800 8C by 1 week. To estimate how large can be an influence of structural disorder on NMR spectra, we have calculated the second moments and derived from them the line widths of 47Ti and 49 Ti absorption resonance lines. In these calculations the pure dipolar interactions were assumed, and derived line widths were calculated assuming a Gaussian line shape of the resonance lines. The Van Vleck’s expression [13] for the nuclear dipole second moment, M2, is given in units of (Hz2) as:

47

M2I Z ð3=5ÞðgI =2pÞ4 h2 IðI C 1ÞSj NI rK6 oj C ð4=15Þ (3) !ðgI =2pÞ2 h2 Sf ðgf =2pÞ2 If ðIf C 1ÞNf rK6 of where h is the Planck’s constant, (gI/2p), I and NI are the magnetogyric ratio, nuclear spin and abundance of nuclei which resonance is under observation while (gf/2p), If and Nf represent other nuclei. The roj (rof) is the distance between the atom under consideration and j-th (f-th) atom,

49

Ti

Ti

Interaction 47

47

Ti– Ti Ti–49Ti 47 Ti–195Pt 47 Ti–119Sn 47 Ti–117Sn Total calculated FWHM calculated (Hz) FWHM experimental (Hz) 47

a b

2

M2 (Hz )

Interaction

M2 (Hz2)

120.03 85.64 1700.00 839.72 686.07 3431.46 138

49

157.28 53.35 1700.91 840.18 686.44 3438.16 138

138a, 240b

49

Ti– Ti Ti–47Ti 49 Ti–195Pt 49 Ti–119Sn 49 Ti–117Sn Total calculated FWHM calculated (Hz) FWHM experimental (Hz) 49

Annealed at 800 8C by 1 week. As-cast.

and summation over j includes identical nuclei and over f includes all different nuclei. The data are summarized in Table 3. As expected, the dominant contributions to the dipolar width of both isotopes come from heteronuclear interactions Ti–Pt and Ti–Sn. An excellent agreement is observed between calculated and experimental line widths of 47Ti and 49Ti resonances of annealed sample, thus confirming its perfect ordering. Any structural disorder broadens the resonance of titanium nuclei, mainly by the quadrupolar interaction. The resonance of 47Ti nuclei is more sensitive in this respect since the 47 Ti nuclei have a larger quadrupole moment than the 49Ti. The spin-lattice relaxation rate of titanium nuclei is expected to be the sum of three independent contributions: ð1=T1 Þ hR1 ¼ ðR1 Þdip þ ðR1 Þimp þ ðR1 Þq

Fig. 6. 47Ti and 49Ti NMR line shapes in TiPtSn at TZ294 K. The scale zero has arbitrarily been set at the frequency of the 47Ti resonance. (A) ascast sample, (B) sample annealed at 800 8C by 1 week.

138a, 240b

(4)

where (R1)dip results from nuclear dipolar coupling, (R1)imp results from the dipolar coupling to the electronic magnetic dipole moment of the magnetic impurity ions, and (R1)q results from interaction of quadrupole moment of 47Ti or 49 Ti nucleus with the crystalline electric field gradient at its site. With perfect structural order and perfect tetrahedral symmetry about titanium sites there would be zero static electric field gradient at these sites. However, even in crystals with cubic symmetry, quadrupole relaxation process may be effective by way of the Waller mechanism of lattice vibrations which might cause the electric field at the position of the nucleus to fluctuate from cubical symmetry leading to effective electric quadrupole interaction [14]. The first two terms in Eq. (4) represent magnetic interactions. Therefore, each such a term is proportional to square of the nuclear magnetogyric ratio of the respective nucleus [15]. Since 47gy49g (see Table 1), a measurable difference between R1(47Ti) and R1(49Ti) could be attributed to the difference in their quadrupole contributions, D(R1)q.

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Our measurements at 294 K give T1(49Ti)Z9.6 s and T1(47Ti)Z2.7 s. Moreover, it is known [16] that ðR1 Þq f fð2I C 3Þ=½I 2 ð2I K 1Þgðe2 qQ=hÞ2

(5)

Then the ratio T1(49Ti)/T1(47Ti) will be proportional to the ratio of the squares of the quadrupole moments multiplied by the appropriate ratio of spin factors. The calculated value for this ratio is 3.52, which is in good agreement with the experimental value of 3.55. It is then evident that although the static electric field gradient at titanium nuclei is close to zero, its fluctuations entirely determine nuclear spin-lattice relaxation rates of 47 Ti and 49Ti nuclei in TiPtSn. Relatively long relaxation times are in line with conclusion derived from the analysis of the NMR spectra that quadrupole coupling constants, e2qQ/h, must be weak. The 119Sn and 195Pt NMR spectra in TiPtSn are shown in Figs. 9 and 10. The line widths of both resonances largely exceed the dipolar values. Origin of this broadening is discussed in the next section concerning the 119Sn and 195Pt NMR spectra in ZrPtSn. 3.2.2. ZrPtSn The 91Zr NMR spectra of ZrPtSn at 294 K are shown in Fig. 7. Two independently prepared samples, one annealed at 800 8C by 1 week, another annealed at 850 8C by 2 weeks, both reveal narrow resonance lines, very similar in shape and line width. The sharp line of sample (B) is accompanied by a broad, low intensity background component. We believe that the broad component is due to unresolved Zr

761

NMR satellites: nmK1 4 nm ¼ nL K nq ðm K 1=2Þð3 cos2 Q K 1Þ=2

(6)

i.e. due to the (mK1)4m transitions K5/24K3/2, K3/ 24K1/2, 1/243/2, 3/245/2 of 91Zr which appear in addition to the central K1/241/2 line if quadrupole coupling is nonzero. Here nL is the nuclear Larmor frequency, nqZ3e2qQ/[h2I(2IK1)] is the quadrupole frequency, and Q is the angle between the direction of the external magnetic field and the largest principal axis of the EFG tensor. The calculated contributions to the dipolar second moment of the 91Zr NMR line are given in Table 4. As can be seen, in both cases the experimental width of the central line exceeds the calculated one thus suggesting some structural disorder. Even in samples (A), not revealing a broad component, the charge redistribution due to structural disorder produces small electric field gradient at the position of zirconium nuclei and corresponding line broadening via quadrupole interaction. Measurements of 91Zr spin-lattice relaxation rate have been performed in the temperature range 150–294 K for a sample annealed at 850 8C. A characteristic feature of R1(91Zr) is its linear dependence on the square of the temperature (see Fig. 8). It thus represents the phononinduced quadrupolar relaxation. It is caused by an ordinary two-phonon (Raman) process creating a fluctuating EFG tensor at the site of the nuclear probes. For TR0.5QDebye the relaxation can be expressed as (R1)qZaT2 [17]. The QDebye for ZrPtSn is unknown, but for isostructural and isoelectronic ZrNiSn the QDebyeZ(310G10) K [6]. From Fig. 8 a(91Zr)Z3.16!10K6 sK1 KK2. The 119Sn and 195Pt NMR spectra of ZrPtSn shown in Figs. 9 and 10 exhibit a rather broad absorption lines, which line widths are insensitive to the thermal treating of the sample. To estimate how large can be an influence of structural disorder on NMR spectra of a spin IZ1/2 of 119Sn and 195Pt nuclei, we have calculated dipolar second moments of these nuclei in perfectly ordered ZrPtSn compound as well as in disordered compound. Three models of disorder were considered. In model (A) the Zr Table 4 Total calculated and respective contributions to the second moment, M2, calculated and experimental line width (FWHM) of 91Zr NMR in ZrPtSn Interaction

M2 (Hz2)

91

1129 3918 1935 1581 8563 218 317a, 317b

Zr– Zr Zr–195Pt 91 Zr–119Sn 91 Zr–117Sn Total calculated FWHM calculated (Hz) FWHM experimental (Hz) 91

Fig. 7. 91Zr NMR spectra in ZrPtSn at TZ294 K. (A) sample annealed at 850 8C by 2 weeks, spectrum of sample (B) annealed by 1 week at 800 8C reveals weak unresolved satellite background on expanded scale (inset).

91

a b

Annealed at 850 8C by 2 weeks. Central line of sample annealed at 800 8C by 1 week.

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Fig. 8. 91Zr nuclear spin-lattice relaxation rate in ZrPtSn plotted against the square of the absolute temperature.

Fig. 10. 195Pt NMR spectra of MPtSn compounds at TZ294 K.

and Sn atoms substitute each other, Pt atoms being at their regular positions. This leads to the formula (Zr1KxSnx)Pt(Sn1KxZrx), where x represents a fraction of substituted atoms. In model (B) the Zr atoms are at their regular crystallographic positions while the Sn and Pt atoms substitute each other. In this case a chemical formula can be written as Zr(Pt1KxSnx)(Sn1KxPtx). Model (C) describes the situation in which the Sn atoms are at their correct positions while Zr and Pt atoms substitute each other leading to the formula (Zr1KxPtx)(Pt1KxZr)Sn.

Intentionally, the calculations have been performed for a less favourable case, i.e. when xZ0.5 (maximum of disorder). The data obtained for the 119Sn resonance and summarized in Table 5 contain a rather remarkable result. Even in the less favourable case the calculated value of the dipolar line width can be at most 17% larger than its value in perfectly ordered compound while the experimentally observed width is about 2.5 times larger. Similar calculations (not shown) of second moments and line widths of the 195Pt NMR give similar results. It is then evident that for the 117,119Sn and 195Pt NMR spectra other broadening mechanisms, e.g. the indirect spin couplings through the chemical bond [18], should be taken into consideration, in addition to the dipolar interaction.

Fig. 9. 119Sn NMR spectra of MPtSn compounds at TZ294 K.

3.2.3. HfPtSn and ThPtSn Very large quadrupole moments of the 177Hf and 179Hf nuclides make them extremely sensitive detectors of any structural disorder in materials studied here. On the other hand, very low magnetogyric ratios and low isotopic abundances render these nuclides somewhat unfavourable from an NMR standpoint. Unfortunately, we were unable to observe the resonances of hanium nuclei since they occur below the frequency range of our spectrometer. Thorium has no NMR active isotopes at all. So, only 119Sn and 195Pt NMR were studied in HfPtSn and ThPtSn. The 119Sn and 195 Pt NMR spectra of HfPtSn and ThPtSn at 294 K are shown in Figs. 9 and 10. Again, the calculations similar to those presented in Table 5 show that experimental values of the line width are several times larger than calculated ones. We postulate that pseudodipolar and/or scalar interactions 119 Sn–195Pt are responsible for this effect.

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Table 5 Total calculated and respective contributions to the second moment, M2, calculated and experimental line width (FWHM) of 119Sn NMR in ZrPtSn Interactions

119

Sn–119Sn Sn–117Sn 119 Sn–195Pt \119 Sn–91Zr Total calculated FWHM calculated (Hz) FWHM experimetnal (Hz) 119

M2 (Hz2) Perfect order ZrPtSn

Exchange Sn4Zr Zr1KxSnxPtSn1KxZrx

Exchange Sn4Pt Zr(Pt1KxSnx)(Sn1KxPtx)

Exchange Zr4Pt (Zr1KxPtx)(Pt1KxZrx)Sn

19,166 6960 62,968 29,440 118,534 811 2200

44,574 16,186 62,968 18,751 142,479 889

64,337 23,362 36,994 37,754 162,447 949

19,166 6960 51,604 37,754 115,484 800

4. Conclusions Determination of magnetic susceptibility for the semiHeusler-type intermetallics MPtSn (MZTi, Zr, Hf, Th) is complicated by the nonlinear magnetic field H dependence of the magnetization resulting from the presence of small concentrations of magnetic impurities. Such impurities are common in transition-metal alloys where trace impurity concentrations are often significant. The nonlinearity is particularly strong at low temperatures. A careful analysis of magnetization isotherms performed between 1.9 and 300 K with the magnetic field strength H up to 5 T shows that intrinsic susceptibility is diamagnetic for all samples investigated. This work shows that NMR of quadrupolar nuclei is very useful in controlling the microscopic structure of semiHeusler intermetallics, e.g. structural disorder at local sites. Our results show that in ZrPtSn, even in carefully annealed samples, some structural disorder exists in Zr and Sn atom sublattices, most probably due to closeness of Zr and Sn atoms’ radii. This agrees with the interpretation of the diffuse X-ray scattering in isostructural ZrNiSn [3,6]. On the other hand, perfectly ordered TiPtSn can be obtained by appropriate annealing of the samples. In the course of present study we have realized that no valuable information on structural order/disorder in MPtSn compounds can be directly obtained from static NMR spectra of 117,119Sn and 195Pt nuclei. Although all these nuclei have spin IZ1/2 and the second moments are not affected by quadrupole effects, the observed spectra are broadened far beyond the rigid-lattice dipolar width due to indirect spin couplings of these heavy nuclei through the chemical bond. High-resolution solid-state tin and platinum

NMR study and characterization of these electron mediated interactions in MPtSn compounds will be reported in a separate paper.

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