O Y Zelinska, A V Tkachuk, A P Grosvenor - Structure and physical properties of ybzn2sb2 and ybcd2sb2 - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 1 (2008) 204-209 Ivan Franko National University of Lviv www. chemetal-j ournal. org

Structure and physical properties of YbZn2Sb2 and YbCd2Sb2

Oksana Ya. ZELINSKA1,2, Andriy V. TKACHUK1, Andrew P. GROSVENOR1, Arthur MAR1*

1 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2

2 Department of Inorganic Chemistry, Ivan Franko National University of Lviv, 79005 Lviv, Ukraine Corresponding author. Fax: +1 780 492 8231; e-mail: arthur.mar@ualberta.ca

Received March 26, 2007; accepted June 16, 2008; available on-line September 10, 2008

The ternary ytterbium transition-metal antimonides YbM2Sb2 (M = Zn, Cd) were confirmed by single-crystal X-ray diffraction studies to adopt the CaAl2Si2-type structure (P 3 ml; a = 4.4424(6) A, c = 7.4184(9) A for YbZn2Sb2; a = 4.6512(7) A, c = 7.5661(11) A for YbCd2Sb2). Electrical resistivity measurements on single crystals revealed weakly metallic behaviour, with a resistivity minimum near 80 K in the case of YbCd2Sb2. The Yb atoms are predominantly divalent, as indicated by band structure calculations and X-ray photoelectron spectroscopy on YbZn2Sb2, but nonzero magnetic moments derived from magnetic susceptibility measurements and additional features in the experimental valence band spectrum provide evidence for the presence of some trivalent Yb species.

Antimonide / Ytterbium / Electronic structure / Magnetic properties

Introduction

Among ternary rare-earth transition-metal antimonides R-M-Sb where M is Zn or Cd, the most widespread phases are RM1-xSb2 (HfCuSi2-type) and R6ZnSb15 (La6MnSb15-type) found for the earlier rare earths, and

RM2Sb2 (CaAl2Si2-type) and R14ZnSb11 (Ca14AlSb11-

type) found for the divalent rare earths R = Eu and Yb [1]. Some of the ytterbium-containing compounds, such as Yb14ZnSb11, have been of particular interest because of the anomalous physical properties that might arise from the occurrence of intermediate or mixed valence of Yb [2,3]. Substitution of Ca for Yb has led to solid solutions CaxYb1-xZn2Sb2 that may be promising thermoelectric materials [4]. Other new phases recently discovered in the Yb systems include Yb9Zn4+xSb9 [5,6] and Yb2CdSb2 [7].

Investigation of the YbM2Sb2 (M = Zn, Cd) phases remains incomplete, and some inconsistent results are noted in the literature. For YbZn2Sb2, only cell parameters are known [8,9], the electrical resistivity curve at low temperature (2 to 300 K) [10] is several orders of magnitude greater than that at high temperature (300 to 800 K) [4], and the magnetic properties have been described as either diamagnetic [10] or following Curie-Weiss behaviour [9]. For YbCd2Sb2, a single-crystal structure determination (R(F) = 0.038) has been reported [11], but no electrical or magnetic properties have been measured so far. To resolve some of these ambiguities, we report  here   a  systematic   study  of these two compounds, including crystal structure determination, electrical and magnetic property measurements, electronic band structure calculations, and X-ray photoelectron spectroscopy.

Experimental

Starting materials were powders of Yb (99.9%, Cerac), Zn (99.9%, Cerac), Cd (99.999%, Cerac), and Sb (99.999%, Alfa-Aesar). Crystals of YbZ^Sb2 and YbCd2Sb2 were originally identified as byproducts in attempts to prepare "YbM07Sb2" (in analogy to other RM1-xSb2 phases) [12,13] by reaction of the elements in evacuated fused-silica tubes heated at 1050 °C for 3 d, followed by slow cooling to 600 °C over 8 d, annealing at 600 °C for 8 d, and quenching in water. The compounds were subsequently prepared in quantitative yield by reaction of the elements, in the correct stoichiometry, placed in alumina crucibles jacketed by outer fused-silica tubes, heated at 1000­1050 °C for 1 d followed by slow cooling to 800­850 °C over 4 d and then to 600 °C over 2 d. Products

were evaluated by powder X-ray diffraction patterns collected on an Inel powder diffractometer equipped with a CPS 120 detector, and by energy-dispersive X-ray (EDX) analysis on a Hitachi S-2700 scanning electron microscope.

Single-crystal intensity data for YbZn2Sb2 and YbCd2Sb2 were collected on a Bruker Platform / SMART 1000 CCD diffractometer at 22 °C using

Table 1 Crystallographic data for YbM2Sb2 (M = Zn, Cd).

Formula

Formula mass (amu)

Space group

a (A)

c (A)

V (A3)

Z

pcalcd (g cm-3)

Crystal dimensions (mm)

Radiation

m(Mo Ka) (mm-1)

Transmission factors

29 limits

Data collected

No. of data collected

No. of unique data

No. of variables

R(F) (F2 > 2o(F2)) a

Rw(F2) b

Goodness of fit

(Ap)max, (Ap)min (e A-3)

a R(F )= E| |Fo| - |Fc(/£| Fo|

YbZn2Sb2 54_7.28

P 3 m1 (No. 164)

4.4424(6) 7.4184(9)

126.79(3)

1

7.168

0.24 x 0.13 x 0.09

Graphite monochromated Mo Ka,

37.91

0.011-0.097

5.50° £ 29(Mo Ka) £ 66.02° -6 £ h, k £ 6; -11 £ l £ 11 1671 (Rint = 0.046) 219 (214 with F2 > 2o(F2))

10

0.023 0.056 1.168

3.06, -1.06

YbCd2Sb2 64J.34

P 3 m1 (No. 164)

4.6512(7) 7.5661(11) 141.75(4) 1

7.513

0.21 x 0.12 x 0.05

l = 0.71073 A

32.94

0.035-0.216

5.38° £ 29(Mo Ka) £ 60.78° -6 £ h, k £ 6; -10 £ l £ 10

1663 (Rint = 0.042)

200 (198 with F2 > 2o(F2))

10

0.026 0.061 1.302

1.67, -3.00

Rw (Fo2) = Z ^(Fo2 - Fc2 )2 jzwFo4     , w- = [a2 (Fo2)+ (Ap)2 + Bp] where p = [max(Fo2,0)+ 2Fc2 ^3 Table 2 Positional and equivalent isotropic displacement parameters (A2) a for YbM2Sb2 (M = Zn, Cd).

 

YbZn2Sb2

YbCd2Sb2

Yb in 1a (0, 0, 0)

 

 

Ueq

0.0144(2)

0.0174(2)

M in 2d (!/, %, z)

 

 

z

0.63267(15)

0.63064(11)

Ueq

0.0176(3)

0.0195(3)

Sb in 2d (!/, %, z)

 

 

z

0.25640(6)

0.23749(9)

Ueq

0.0121(2)

0.0145(3)

Ueq is defined as one-third of the trace of the orthogonalized tensor.

W scans. Calculations were carried out with use of the SHELXTL (version 6.12) package [14]. Face-indexed numerical absorption corrections were applied. The centrosymmetric space group P 3 m1 was chosen and initial atomic positions were located by direct methods, confirming the expected CaAl2Si2-type structure [15]. The crystal structures were refined by full-matrix least-squares methods on F2 with anisotropic displacement parameters. Table 1 summarizes crystallographic data, Table 2 lists the final values of the atomic positions and displacement parameters, and Table 3 lists selected interatomic distances. Further data, in CIF format, have been sent to Fachinformationszentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany, as supplementary material No. CSD-419689 and 419690 and can be obtained by contacting FIZ (quoting the article details and the corresponding CSD numbers).

Silver plate-shaped crystals of YbZn2Sb2 and YbCd2Sb2 were selected for electrical resistivity measurements after their identities were confirmed by EDX analysis. The electrical resistivity within the ab plane was measured by standard four-probe techniques on a Quantum Design Physical Property Measurement System (PPMS) equipped with an ac transport controller (Model 7100). The current was 100 |jA and the frequency was 16 Hz. Measurements of dc magnetic susceptibility were made on ground samples (20-30 mg) between 2 and 300 K on a Quantum Design 9T-PPMS dc magnetometer / ac susceptometer. Susceptibility values were corrected for contributions from the holder and the underlying sample diamagnetism.

Tight-binding linear muffin tin orbital (TB-LMTO) band structure calculations were performed on YbZn2Sb2 within the local density and atomic

b

a

Table 3 Selected interatomic distances (A) and angles (°) in YbM2Sb2 (M = Zn, Cd).

 

YbZn2Sb2

YbCd2Sb2

Yb-Sb (x6)

3.1931(4)

3.2311(5)

Yb-M (x6)

3.7422(9)

3.8757(7)

M-M (x3)

3.2231(14)

3.3346(11)

M-Sb (x3)

2.6936(5)

2.8647(5)

M-Sb

2.7914(13)

2.9747(11)

Sb-M-Sb (x3)

111.10(2)

110.38(2)

Sb-M-Sb (x3)

107.79(2)

108.55(2)

spheres approximations using the Stuttgart TB-LMTO program [16]. The basis sets consisted of Yb 6s/6p/5d/4f, Zn 4s/4p/3d, and Sb 5s/5p/5d/4f orbitals, with the Yb 6p and Sb 5d/4f orbitals being downfolded. Integrations in reciprocal space were carried out with an improved tetrahedron method over 131 independent k points.

The valence band spectrum of YbZn2Sb2 was collected on a Kratos AXIS Ultra spectrometer equipped with a monochromatic Al Ka X-ray source. The pressures throughout the analysis chamber were 10-6-10-7 Pa. The resolution function for this instrument has been determined to be 0.4 eV by analysis of the cobalt Fermi edge. Powders of YbZn2Sb2 were pressed onto C tape in air and transferred into the vacuum chamber of the instrument where they were sputter-cleaned with an Ar+ ion beam (4 kV, 10 mA) to remove any surface oxide or contaminates formed. The valence band spectrum was collected using a pass energy of 20 eV and a step size of 0.05 eV. Sample charging was corrected for by calibrating the adventitious C 1s peak to the accepted value of 284.8 eV. The spectrum was analysed with use of the CasaXPS software package [17].

Results and discussion

YbM2Sb2 (M = Zn, Cd) adopts the trigonal CaAl2Si2-type structure [15], with cell parameters (Table 1) in good agreement with those previously reported (a = 4.444(1) A, c = 7.424(1) A for YbZn2Sb2 [8]; a = 4.650(1) A, c = 7.565(2) A for YbCd2Sb2 [11]). The structure is built up by stacking [M2Sb2] double layers of edge-sharing MSb4 tetrahedra along the c axis, separated by hexagonal nets of Yb atoms that are octahedrally coordinated by Sb atoms (Fig. 1). Consistent with the larger size of Cd relative to Zn, there is an expansion of the structure on going from YbZn2Sb2 to YbCd2Sb2, as manifested by the ~0.2 A increase in the M-Sb distances within the MSb4 tetrahedra. The ~3.2 A Yb-Sb distances in YbM2Sb2 are within the range found in YbSb2 (3.19(1)-3.57(1) A) [18] which contains divalent Yb, but longer than that found in YbSb (3.041 A) [19] which contains trivalent Yb. If divalent Yb is assumed, the electron-precise formulation Yb2+(M2+)2(Sb3-)2 indicates that the Zintl concept is satisfied.

Fig. 1 Structure of YbM2Sb2 (M = Zn, Cd). The large lightly shaded circles are Yb atoms, the small solid circles are M atoms, and the medium open circles are Sb atoms.

The electrical resistivity profile from 2 to 300 K for YbZn2Sb2 suggests a small band gap semiconductor or a semimetal (Fig. 2a). The absolute resistivity at room temperature is high (p300 = 0.53 mQ-cm), similar to that found in a previous measurement from 300 to 800 K (p300 = 0.32 mQ-cm) [4] but in considerable disagreement with another measurement from 5 to 300 K (p300 = 0.25 Q-cm) [10] (note the difference in units). Moreover, the temperature coefficient found here is essentially identical to that in the previous high-temperature measurement (dp/dT = 0.0012 mQ-cm/K) [4]. In contrast, the resistivity for YbCd2Sb2 is substantially higher, with p300 = 50 mQ-cm, and exhibits a characteristic minimum near 80 K suggestive of Kondo lattice behaviour in which dilute magnetic impurities cause scattering of conducting electrons at low temperatures (Fig. 2b). Because the resistivity measurements were performed on discrete single crystals, this observation suggests the presence of some trivalent Yb that is intrinsic to YbCd2Sb2.

The magnetic susceptibility reveals nearly temperature-independent paramagnetism throughout most of the temperature range superimposed with a Curie tail at low temperatures (Fig. 3). For YbZn2Sb2, these results agree qualitatively with the paramagnetism (|meff = 2.38 |mB, 9p = -108 K) reported

Е о

d

cp

>

a:

0.40

0.30

0.20

0.10

0.00 +

0       50      100     150     200     250 300

Temperature, T (K)

(a)

60

E о

d

E

Cp

> (Л

40

30

20

10

0       50      100     150     200     250 300

Temperature, T (K)

(b)

Fig. 2 Electrical resistivity of single crystals of

(a) YbZn2Sb2 and (b) YbCd2Sb2 measured

within the ab plane.

0.06

0.05

о E "3 E

IP

cp

о

(Л (Л

!S I 1 о 0.01-к

5 \ї

0.04

0.03

0.02

0.004

E .

0     10    20    30    40 50

T (K)

0     50    100   150   200   250 300 Temperature, T (K)

Fig. 3 Magnetic susceptibility of YbZn2Sb2 (open squares) and YbCd2Sb2 (solid circles). The inset shows fits of the inverse susceptibility to the Curie-Weiss law at low temperature.

by Zwiener et al. from 80 to 290 K [9], but differ from the large diamagnetic response seen at all temperatures by Pfleiderer et al. [10], who also reported the inconsistently high resistivity described above. Fitting of the low-temperature portion of the inverse susceptibility to the Curie-Weiss law gives Heff = 2.1 Цв for YbZn2Sb2 and 0.9 |їв for YbCd2Sb2. Given that the theoretical magnetic moments are 0 | B for Yb2+ (4f14) and 4.5 |їв for Yb3+ (4f13), the observed nonzero moments suggest the presence of some trivalent Yb. For comparison, YbZn2As2 also exhibits a nonzero magnetic moment of 2.3 | B/Yb [20]. It is customary to dismiss a small nonzero moment in divalent Yb compounds to minor amounts of impurities such as Yb2O3. However, the resistivity minimum seen earlier provides evidence that a small amount of trivalent Yb may be intrinsic, on the order of ~20% in YbZn2Sb2 and ~4% in YbCd2Sb2, if the nonzero moment is attributed entirely to these compounds and not to another unidentified impurity. Overall, our observations tend to support the earlier proposal that YbZn2Sb2 exhibits an intermediate Yb valence [9].

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O Y Zelinska, A V Tkachuk, A P Grosvenor - Structure and physical properties of ybzn2sb2 and ybcd2sb2