Magnetization, resistivity and heat capacity of the anisotropic RVSb3 crystals (R=La–Nd, Sm, Gd–Dy)

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 320 (2008) 120–141 www.elsevier.com/locate/jmmm

Magnetization, resistivity and heat capacity of the anisotropic RVSb3 crystals (R ¼ La–Nd, Sm, Gd–Dy) Athena S. Sefat, Sergey L. Bud’ko, Paul C. Canfield Ames Laboratory, Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Received 20 December 2006; received in revised form 9 May 2007 Available online 25 May 2007

Abstract Single crystals of rare-earth vanadium triantimonides RVSb3, with R ¼ La–Nd, Sm and Gd–Dy are grown out of solution. The substitution of heavier rare-earths makes the orthorhombic structure contract and accompanies changes in the magnetic properties throughout the series. Characterization of RVSb3 family was made with single-crystal X-ray diffraction, temperature and field-dependent magnetization M(T, H), heat capacity C(T) and resistivity r(T). All of the compounds are metallic, and all, with the exceptions of nonmagnetic LaVSb3 and ferromagnetic CeVSb3, show features typical of antiferromagnetic order. Thermodynamic and transport measurements indicate that these materials are highly anisotropic magnetically, due to the crystal electric field splitting of the Hund’s ground state, but only manifest a moderate electrical anisotropy. Given the relative scarcity of Ce-based ferromagnets, the temperaturedependent magnetization of CeVSb3 was measured under applied hydrostatic pressure up to 10 kbar, and Tc increases at the rate of 0.14(1) K kbar1. Published by Elsevier B.V. Keywords: Triantimonide; Flux growth; Single crystal x-ray; Magnetization; Heat capacity; Resistivity; Anisotropic behavior

1. Introduction Over the past decade, the rare-earth (R) families of binary and ternary antimonides have been of considerable interest due to a wide variety of electronic and magnetic phenomena associated with reduced dimensionality of the unit cell and point symmetry of the local moments. Studies of the anisotropic properties in single-crystal form include those of the R-diantimonide crystal series of RSb2 (R ¼ La–Nd, Sm) [1,2] on the ternary RAgSb2 (R ¼ Y, La–Nd, Sm, Gd–Tm) [3,4] and on the RTSb3 (R ¼ La, Sm, Gd) series with chromium [5–8] and vanadium [5–7] transition metals (T). RSb2 crystals were found to grow as malleable plates that could be easily peeled apart [2]. RSb2 has a C-centered orthorhombic crystal structure (Cmca, Z ¼ 8) that is composed of infinite sheets of Sb atoms and layers of Sb2 pairs separated by the R3+ ions [9] (Fig. 1a). The unique Corresponding author. Tel.: +1 515 294 3455.

E-mail address: [email protected] (A.S. Sefat). 0304-8853/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jmmm.2007.05.013

rare-earth site in RSb2 is positioned in a crystallographically unique (8f) m point-symmetry. Despite its simple structure, members of the RSb2 system have been reported to have diverse and complex properties, including anisotropic magnetoresistance and metamagnetism possessed by R ¼ Ce–Nd members [2]. In comparison to RSb2 diantimonides, the RAgSb2 crystals are grown out of solution as plates that are mechanically more robust [3]. RAgSb2 crystallizes in the primitive tetragonal structure (P4/nmm, Z ¼ 2) and consists of layers of Sb, RSb and Ag [10] (Fig. 1b). The unique rareearth site (2c) was found to be higher in symmetry than in RSb2 and tetragonal (4 mm). For some members of RAgSb2 series, the crystalline electric field anisotropies are strong enough to confine magnetic moments along c-unit cell axis in the case of R ¼ Er and Tm, or along ab-plane in R ¼ Dy [3]. Similar to RSb2, the unique rare-earth site in the triantimonide RTSb3 (T: Cr, V) structure is m (4d). RTSb3 is primitive orthorhombic (Pbcm, Z ¼ 4) and can be considered as being built by the insertion of a TSb layer into the structure of RSb2, perpendicular to the long

ARTICLE IN PRESS A.S. Sefat et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 120–141

(a)

121

(b)

Sb (2)

Ag Sb (1) R

R b

Sb (2)

Sb (1) c

(c) Sb (3) R Sb (1) T

TSb6

Sb (2) b

a

Inserted layer

Fig. 1. Crystal structures of (a) RSb2, (b) RAgSb2 and (c) RTSb3 structure depicting TSb6 coordination. The unit cells are outlined in all cases (color online).

translation direction a [11,12]. In RTSb3, the transition metal atoms have distorted octahedral antimony coordination and the strings of TSb6 are interconnected by edges to form two-dimensional finite sheets in the bc-plane (Fig. 1c). Measurements on LaTSb3 show that for T ¼ Cr, Cr3+ ions order ferromagnetically. A discrepancy for the Curie

transition has been found for LaCrSb3 samples as values in the range of 125–142 K [5,13–17] are reported, due to various synthetic procedures influencing the stoichiometry of the crystals. For example, in literature the growth of RCrSb3 crystals in particular includes slow-cooled solidstate reaction [14], arc-melt and annealing of the stoichio-

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metric mixture [8,19], and also flux [5,6]. The magnetic properties of RCrSb3 compounds are proven to be sensitive to the degree of Cr site-filling, and there has been evidence for Cr atomic occupancy to as low as 90% in the singlecrystal reports [11,15,18]. The RCrSb3 members with R ¼ Ce, Pr, Nd and Sm also show a net ferromagnetic moment from Cr spins below Tc105–147 K, with an additional transition temperatures between 10 and 35 K [6,13]. The lower temperature magnetic order has been associated with the presence of either an antiferromagnetic [13,16], canted antiferromagnetic [17] or ferromagnetic [12] spin structure of the R and Cr moments for R ¼ Ce, Pr and Nd. The late members of R ¼ Gd, Tb, Dy and Yb, however, do not display ferromagnetic ordering associated with just the Cr3+ and for them, the simultaneous ordering of the R–Cr predominates [8,12,16,19]. Compared to RCrSb3, RVSb3 members have only been sparsely studied. For LaVSb3, vanadium is not moment bearing [5,13]. Magnetic susceptibility and field-dependent magnetization measurements on the polycrystalline R ¼ La–Nd, Sm [13] and Dy [16] have revealed magnetic order in Ce, Pr, Nd and Dy systems below 10 K. Although the magnetic properties of RVSb3 compounds are grossly characterized, little is currently known of the anisotropic properties of single crystals. Anisotropic measurements of R ¼ La [5], Sm [6] and Gd [7] have only been recently reported, and are the sole reports on the physical properties of the crystals. As part of our effort to discover and understand new magnetic and electronic behavior in R–T–Sb system, the RVSb3 series has been chosen for study here. In this paper, we present a systematic study of the anisotropic properties of the RVSb3 series with R ¼ La–Nd, Sm and Gd–Dy, establishing the trends caused by the change of the rare-earth on the physical properties. Using single-crystal samples, we have performed detailed analyses of the field dependence and temperature dependence of magnetization, heat capacity and resistivity. Because partial occupancy has been implicated in the analyses of almost all of the reported RTSb3 single crystals, the refinement of the site occupancies becomes crucial and so we have also performed detailed structural studies. This paper will first present a brief review of experimental methods, followed by results of structure, thermodynamic and transport anisotropic properties for each compound. For the case of CeVSb3, the hydrostatic pressure dependence of the low-temperature ferromagnetic ground state is also examined. 2. Experimental details Single crystals of the RVSb3 samples were grown out of antimony flux, in a manner similar to other self-flux growths [5,20]. The procedure involved placing high-purity elements (43N) of R:V:Sb in the ratio of 8:8:84 into alumina crucibles, heating to 1180 1C under partial argon atmosphere and then cooling. For R ¼ La–Nd and Sm, the

cooling rate was 7 1C h1 from 1180 to 780 1C and was followed by decanting of the molten Sb-flux. For R ¼ Gd–Dy members, the cooling rate was 2 1C h1 from 1050 to 850 1C. Attempts at crystal growth for the RVSb3 series for R heavier than dysprosium failed, resulting in V15Sb18 and RSb crystalline, binary phases. The RVSb3 crystals had rectangular morphologies with smooth surfaces and dimensions of 0.5:3:6 mm or smaller in a,b,c directions, respectively (Fig. 2a). These crystals have been reported to form with thin face parallel to a-axis and longest side parallel to c-axis [5]. To confirm the orientation, the b- and c-crystallographic directions were checked for each of PrVSb3, TbVSb3 and SmVSb3 crystals by back-scattering Laue diffraction (Mo source) and, in addition, by using a four-circle diffractometer. Two procedures verified that the RVSb3 crystals formed with thin face parallel to a-axis and longest side along c-axis (Fig. 2a). First, the lattice parameters agreed very well with Ref. [9] for all three crystallographic directions. Second, special reflection conditions for the space group Pbcm were checked; for example, the [9 1 0] reflection could be found in contrast to pure background signal at the position of the [9 0 1] reflection. The initial phase purity and structural identification were made using Rigaku Miniflex X-ray diffractometer (Cu Ka radiation) by indexing powder diffraction data on a small ground piece of crystal. Subsequent single-crystal analyses were made using a Bruker APEX CCD or STOE (S2) image plate diffractometer. The crystal structures were solved with the SHELXTL program suite and direct methods. Details of data collection and refinement conditions are given in Table 1. To check the deviation from the ideal compositions in all of RVSb3 samples, occupancy parameters were varied together with thermal parameters in series of least square cycles. Temperature-dependent electrical resistance measurements were performed on a Quantum Design PPMS unit with a frequency of 16 Hz and an excitation current of 3–5 mA, in the range of 1.8–300 K. The electrical contacts were placed on samples in standard four-probe geometry, using Pt wires and silver epoxy (EPO-TEK H20E). The largest source of error in determining sample resistivity (r) is the estimation of sample geometry and the positions of electrical contacts. Based on this, absolute values of r are accurate to 710%. The notation JJc or rc denotes current running or resistivity measured along c-crystallographic axis. Residual-resistivity ratios are defined as RRR ¼ r(300 K)/r(1.8 K) and no attempts to extrapolate the data below 1.8 K were made. The resistive transition temperatures were estimated from peaks in dr/dT [21] and, as will be shown, corroborate the transition temperatures inferred from magnetization and heat capacity measurements. DC magnetization was measured as a function of temperature and magnetic field using a Quantum Design MPMS unit. For a typical temperature sweep experiment, the sample was cooled to 1.8 K in zero-field (zfc), and then

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123

(a)

(b)

(c) Sb (2)

b

c

R

a Sb (3)

Fig. 2. Single crystal of NdVSb3 on a millimeter-scale paper, depicting typical dimensions of the RVSb3 crystals and the crystallographic directions. (b) The change of unit cell lattice parameters in RVSb3 compounds, refined from single-crystal X-ray diffraction data and (c) coordination geometry of R3+ ion. Table 1 Crystallographic data for RVSb3 at 293 K LaVSb3 Wavelength Space group Formula weight (g mol1) Cell volume (A˚3) rcalcd (mg m3) 2y range (1) Reflections collected Data/parameters Extinction coefficient Drmax/Drmin (e A˚3) Goodness-of-fit on F2 Conventional residual, R1, Weighted residual, wR2 [I42s(I)]

CeVSb3

PrVSb3

NdVSb3

SmVSb3

555.10 505.18(18) 7.298 6.10–58.32 5415 730/32 0.031(7) 7.101/6.299 1.260 0.0646

556.31 495.99(17) 7.450 6.18–58.34 3413 730/32 0.034(1) 2.405/1.926 1.215 0.0281

557.10 490.45(15) 7.545 3.10–56.60 3683 649/32 0.036(1) 1.654/2.181 1.273 0.0221

0.71073 A˚ Pbcm (No. 57) 560.43 566.54 486.86(17) 478.68(10) 7.646 7.861 6.26–58.28 3.16–56.46 5407 3577 721/32 630/32 0.029(3) 0.0138(4) 4.651/3.988 1.492/1.725 1.760 1.361 0.0370 0.0209

0.1535

0.0786

0.0521

0.1035

data were collected by warming from 1.8 K in an applied field. In the case of CeVSb3, the sample was subsequently cooled in the applied field (fc), and the measurement repeated from 1.8 K. The notation HJc or Mc denotes

0.0488

GdVSb3

TbVSb3

DyVSb3

573.44 474.60(16) 8.025 3.18–56.52 3646 622/32 0.0135(4) 1.356/1.634 1.289 0.0211

575.11 469.78(14) 8.131 3.20–56.34 2706 609 /32 0.0051(7) 6.342/6.458 0.903 0.0501

578.69 463.6(2) 8.291 3.22–56.56 2239 585/32 0.0035(4) 2.784/2.634 1.269 0.0378

0.0476

0.1436

0.0927

measurement made with the applied field along the c-axis, i.e. [0 0 1]. For low temperatures, polycrystalline averages of susceptibilities of the orthorhombic crystals were estimated by M(T) ¼ w(T) ¼ (wa+wb+wc)/3 and will be

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represented in the form of d(wT)/dT [22], to infer transition temperatures. At higher temperatures (T440 K), the magnetic susceptibility of ground single crystals was measured and fitted to a modified Curie–Weiss law by inclusion of a temperature-independent term w0: w ¼ C/ (Typ)+w0, where C is the Curie constant and yp is the Weiss temperature. If the rare-earth is the only momentbearing ion, the term C is related to the effective moment in paramagnetic state and meff ¼ gJ (J(J+1))1/2. The results of these analyses are summarized in Table 4. The temperature-independent contributions which arise from Pauli and Van Vleck paramagnetism, and also core and Landau diamagnetism, are found consistently o103 emu mol1 across the RVSb3 series. The Curie temperature of CeVSb3 was investigated under pressure up to 10 kbar using a pressure cell designed to fit MPMS instrument. The cell is a version of the one described in Ref. [23] made of Ni–Cr–Al alloy analog of 40KhNYu alloy [24], R&D Support Corporation, Tokyo, Japan. An approximately 1:1 mixture of light mineral oil and n-pentane was used as a pressure transmitting media. The shift in the superconducting transition temperature of Pb was used to determine the pressure at low temperatures [25]. Here, the magnetization was measured in a zfc and warming in an applied field of 50 Oe roughly parallel to c-axis, as this longest axis took the shape of a capsule which confined the sample. Heat capacity C(T) of RVSb3, as well as the LaSb2 and LaAgSb2 crystals [2,3], was measured using a Quantum Design PPMS via the relaxation method. Data for ternary antimonides with R ¼ La, Ce and Pr were collected down to 0.4 K using a 3He insert, and on those of R ¼ Nd, Sm, Gd, Tb and Dy were measured down to 1.8 K. The upper temperature limit for C(T) measurements is 15 K for all RVSb3, with the exception of LaVSb3 and GdVSb3 which were both measured up to 150 K. For LaVSb3, at low temperatures, a plot of C/T against T2 at low temperatures gave a straight line with a slope b and intercept g, associated with the lattice and electronic contributions, respectively. The value of the Debye temperature (yD) was subsequently calculated in the low-temperature limit (b ¼ 12p4R/5y3). For the rest of RVSb3 members, the direct measurement of g and b from heat capacity data was hampered by the lowtemperature magnetic orderings. In order to obtain the magnetic heat capacity, non-magnetic contributions were approximated by the non-magnetic LaVSb3 data, and subtracted from the measured heat capacity of RVSb3 (R ¼ Ce, Pr, Nd, Sm, Tb, Dy). The magnetic entropy, S, was estimated by integration of C/T versus T after an extrapolation of the data down to the origin (i.e. for T ¼ 0 K, S ¼ 0) via polynomial fit. The fact that R ln 8 was recovered for GdVsb3 sample is taken as confirmation of our assumption that LaVSb3 is an adequate approximation of the non-magnetic excitations.

Table 2 Positional and equivalent isotropic displacement parameters for RVSb3 Occupancy

x

y

z

Ueq (A˚2)

LaVSb3 La V Sb1 Sb2 Sb3

0.99(1) 0.97(2) 1 1 1

0.3097(1) 0.9096(2) 0.0664(1) 0.2157(1) 0.5009(1)

0.0017(1) 1/4 0.1079(2) 0.4981(1) 1/4

1/4 0 1/4 1/4 0

0.008(1) 0.009(1) 0.010(1) 0.010(1) 0.010(1)

CeVSb3 Ce V Sb1 Sb2 Sb3

1.00(1) 0.99(1) 1 1 1

0.3102(1) 0.9079(1) 0.0668(1) 0.2190(1) 0.5010(1)

0.0021(1) 1/4 0.1082(1) 0.4970(1) 1/4

1/4 0 1/4 1/4 0

0.010(1) 0.012(1) 0.012(1) 0.012(1) 0.012(1)

PrVSb3 Pr V Sb1 Sb2 Sb3

0.992(3) 0.96(1) 1 1 1

0.3101(1) 0.9067(1) 0.0671(1) 0.2205(1) 0.5013(1)

0.0024(1) 1/4 0.1087(1) 0.4964(1) 1/4

1/4 0 1/4 1/4 0

0.006(1) 0.006(1) 0.008(1) 0.007(1) 0.008(1)

NdVSb3 Nd V Sb1 Sb2 Sb3

0.99(1) 0.96(1) 1 1 1

0.3108(1) 0.9058(1) 0.0672(1) 0.2218(1) 0.5014(1)

0.026(1) 1/4 0.1087(1) 0.4960(1) 1/4

1/4 0 1/4 1/4 0

0.008(1) 0.008(1) 0.010(1) 0.010(1) 0.010(1)

SmVSb3 Sm V Sb1 Sb2 Sb3

1.00(1) 0.98(1) 1 1 1

0.3113(1) 0.9041(1) 0.0676(1) 0.2243(1) 0.5016(1)

0.0031(1) 1/4 0.1091(1) 0.4948(1) 1/4

1/4 0 1/4 1/4 0

0.003(1) 0.004(1) 0.005(1) 0.004(1) 0.004(1)

GdVSb3 Gd V Sb1 Sb2 Sb3

1.000(3) 0.97(1) 1 1 1

0.3115(1) 0.9026(1) 0.0672(1) 0.2259(1) 0.5018(1)

0.0037(1) 1/4 0.1085(1) 0.4935(1) 1/4

1/4 0 1/4 1/4 0

0.007(1) 0.008(1) 0.009(1) 0.008(1) 0.008(1)

TbVSb3 Tb V Sb1 Sb2 Sb3

0.99(1) 0.97(1) 1 1 1

0.3119(1) 0.9015(2) 0.0674(1) 0.2272(1) 0.5020(1)

0.0040(1) 1/4 0.1087(2) 0.4930(2) 1/4

1/4 0 1/4 1/4 0

0.004(1) 0.004(1) 0.005(1) 0.005(1) 0.005(1)

DyVSb3 Dy V Sb1 Sb2 Sb3

1.007(5) 1.00(1) 1 1 1

0.3121(1) 0.9004(2) 0.0676(1) 0.2284(1) 0.5021(1)

0.0044(1) 1/4 0.1086(1) 0.4924(1) 1/4

1/4 0 1/4 1/4 0

0.005(1) 0.007(1) 0.006(1) 0.006(1) 0.006(1)

3. Results 3.1. Structure of RVSb3 Given that variations in stoichiometry have been identified in the RTSb3 series [11,18,19] and may be a cause for discrepancies in magnetic order [5,13–17], in this

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study we start with a detailed structural evaluation of the RVSb3 (R ¼ La–Nd, Sm, Gd–Dy) flux-grown single crystals. The starting structural parameters were taken from the reported CeTSb3 (T ¼ Cr, V) structure, Ref. [9]. For CeVSb3 compound, the lattice parameters obtained here are a ¼ 13.172(3) A˚, b ¼ 6.2419(12) A˚ and c ¼ 6.0327 (12) A˚. These values are in close agreement with those reported on a polycrystalline sample: a ¼ 13.190(2) A˚, b ¼ 6.2392(8) A˚ and c ¼ 6.0303(8) A˚ [11]. The contraction of the RVSb3 structure with substitution of a smaller R is clearly seen from the unit cell volumes, see Fig. 2b and Table 1. The relative decrease in parameter a, on going from LaVSb3 to DyVSb3, is 5.4%. This change is much more significant than the decrease in parameters b and c, being 0.9% and 2.2%, respectively. For each RVSb3 member, room-temperature structure refinements consisted of atomic site and anisotropic displacement parameters (Table 2). To check for deviation from the 1:1:3 composition in each compound, the site occupancies of the three Sb positions were initially refined together. For each RVSb3, the antimonide stoichiometry was determined to be ideal. For example, in the case of LaVSb3, the occupancy factors were 1.0016(12) for Sb(3) position (4c), 1.005(11) for Sb(2) position (4d) and 1.02(1) for Sb(1) position (4d). These small deviations for antimony atoms were consistent throughout RVSb3 series and were considered to be insignificant. As a result, during the final refinement cycles, only the vanadium (4c) and rare-earth R (4d) positions were together allowed to deviate from the full occupancies. These refined occupancies are listed in Table 2. Similar to the reported CeTSb3 (T ¼ Cr, V) analog [11,18], the V sites are found to be slightly deficient, noteworthy for R ¼ Pr, Nd, Gd and Tb samples (Table 2). However, analogous to literature [11], Sb(1) sites are fully occupied in our Sb-flux-grown crystals. From the selected interatomic distances listed in Table 3, it is evident that the substitution of La for Dy results in

125

small changes in the V–Sb and Sb–Sb distances: The V–Sb distances in LaVSb3 (2.7207(9)–2.751(2) A˚) and DyVSb3 (2.7014(10)–2.724(2) A˚) are similar as are the Sb(3)–Sb(3) distances in square sheets of LaVSb3 (3.0268(6)–3. 1269(6) A˚) and DyVSb3 (2.9615(9)–3.0998(9) A˚). This manifests itself as the small change in parameters b and c, as noted above (Fig. 2b). However, a more significant difference is evident in the R–Sb distances which decrease from LaVSb3 (3.2764(9)–3.3898(12) A˚) to DyVSb3 (3.1446 (9)–3.3454(14) A˚). For heavier rare-earths, the monocapped square antiprisms become flatter along a (Fig. 2c), and although, if approximately the same Sb(3)–Sb(3) distances are kept, the structure contracts along a. Since the R atoms are also bonded to Sb(1) and Sb(2), the collapse of the V octahedral layer is a consequence of the substitution of heavier rare-earths (Fig. 1c). We have made several attempts to grow crystals of the late rare-earth vanadium triantimonides for R heavier than Dy, but have not been able to produce them. The only report of rare-earth substitution beyond Dy in RTSb3 is that reported for YbCrSb3. In YbCrSb3, however, the ionic radius of Yb (1.33 A˚), was found to be similar to Sm3+ (1.27 A˚ for a coordination number of 9), which hints at divalent Yb2+ [19]. The reported unit cell volume of YbCrSb3 is 477.6(2) A˚3 and near to that of SmCrSb3 and 478.68(10) A˚3 [19]. 3.2. Physical properties of RVSb3 3.2.1. LaVSb3 For LaVSb3 the magnetic susceptibility is positive, weakly temperature dependent and slightly anisotropic with Mc4Mab (Fig. 3). Unlike the previously reported result on LaVSb3 [5], there is no Curie tail at low temperatures, which indicates negligible amounts of magnetic impurities in our sample. The temperature-

Table 3 Selected interatomic distances (A˚) and angle (1) for chosen RVSb3 members R

La

Nd

Tb

Dy

R–Sb(2) 2  R–Sb(2) R–Sb(1) R–Sb(3) 2  R–Sb(3) 2  R–Sb(2) R–V V–Sb(1) 2  V–Sb(1) 2  V–Sb(2) 2  V–V 2  Sb(1)–Sb(2) Sb(3)–Sb(3) 2  Sb(3)–Sb(3) 2  Sb(3)–R–Sb(3) angles

3.2764(9) 3.3482(12) 3.3136(16) 3.3398(12) 3.3493(12) 3.3898(12) 3.651(3) 2.7207(9) 2.731(2) 2.751(2) 3.0268(6) 3.1500(15) 3.0268(6) 3.1269(6) 53.73(2), 55.74(2), 81.70(3)

3.2132(7) 3.2860(10) 3.2428(11) 3.2734(8) 3.2852(8) 3.3641(10) 3.5631(15) 2.7141(7) 2.7291(14) 2.7423(12) 2.9966(6) 3.1441(10) 2.9966(6) 3.1165(6) 54.27(2), 56.74(2), 82.48(2)

3.1620(7) 3.2230(13) 3.1762(15) 3.2101(10) 3.2235(10) 3.3524(13) 3.474(2) 2.7076(10) 2.726(2) 2.7273(18) 2.9729(5) 3.1354(16) 2.9729(5) 3.1067(6) 55.17(2), 57.75(2), 83.88(2)

3.1446(9) 3.2041(13) 3.1541(15) 3.1909(10) 3.2046(10) 3.3454(14) 3.446(2) 2.7014(10) 2.724(2) 2.7177(17) 2.9615(9) 3.1274(15) 2.9615(9) 3.0998(9) 55.30(2), 57.98(2), 84.19(3)

The number multiplying the distance indicates the multiplicity, i.e. the number of particular Sb atoms at this distance.

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⊥

ρ μΩ

ρ μΩ

Fig. 3. Temperature dependence of magnetization at 15 kOe for LaVSb3.

dependent resistivity of LaVSb3 (Fig. 4) shows metallic behavior with RRRs of 4.3 and 8.8 for currents along crystallographic c and b directions, respectively. Although our RRR values are slightly larger than literature values [5], we appear to have a larger temperature-independent r0 term for JJc. For LaVSb3 and at room temperature, rcE145 mO cm and rbE140 mO cm. The occurrence of a AT2 term in the resistivity, as depicted in Fig. 4b, is suggestive of a low-temperature Fermi-liquid state. For LaVSb3 and fits below 13 K (Fig. 5b) give A coefficients of 0.0025(2) mO cm K2 for rc and 0.0041(3) mO cm K2 for rb. Our result is comparable to the LaCrSb3 system [14]. For this ferromagnetic system, however, there are Critinerant moments, and in another report [5] T3/2 behavior in resistivity was found below Tc due to a reduced scattering length. Fig. 5a shows the heat capacity of LaVSb3, LaAgSb2 [3] and LaSb2 crystals. C(T) increases monotonically with

ρ μΩ

126

Fig. 4. (a) Temperature dependence of electrical resistivity for LaVSb3 and (b) a linear fit in r versus T2 between 1.8 and 13 K.

Fig. 5. Temperature dependence of heat capacity for LaVSb3, LaSb2 and LaAgSb2 (a) in the form of C(T), also (b) C/T versus T2. The linear fit of data in (b) is for the range of 0.6–5 K for LaSb2, 0.9–5 K for LaVSb3 and 2–5 K for LaAgSb2.

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electronic contribution of g ¼ 17(1) mJ mol1 K2 are found. It is worth noting that g for LaVSb3 is substantially higher than the other diantimonides systems, LaSb2 and

increasing temperature. By extrapolating a linear region found in C/T versus T2 between o1 and 5 K (Fig. 5b), for LaVSb3, a Debye temperature of yDE155 K and a large Table 4 Some of magnetic and transport properties of the RVSb3 series Compound

LaVSb3 CeVSb3 PrVSb3 NdVSb3 SmVSb3 GdVSb3 TbVSb3 DyVSb3

TM (K)

– 4.6(1) 2.0(1), 7.1(1) 4.9(1) 3.6(1) 4.5(1), 6.3(1) 3.9(1), 7.1(1) 5.7(1)

Type

– F AF AF AF AF AF AF

meff (mB)

yp (K)

m (mB), 2 K

C–W w fit

C–W w fit

Jc

Ja

–

– 3 (3) 2 (3) 1 (2) – 19(1) 10(1) 3(1)

1.46 1.17 0.85 – – 3.15 4.33

0.33 2.89 2.90 0.47 2.30 8.89 9.00

2.45 3.52 3.54 – 7.74 9.6 10.6

μΒ

μΒ

The types of order are antiferromagnetic (AF) and ferromagnetic (F) below magnetic ordering temperatures (TM).

Fig. 6. (a) Temperature dependence of magnetization in zero-field and field-cooled forms for CeVSb3 measured with applied fields of 25 Oe along c, 500 Oe along a and 5 kOe along b. (b) For CeVSb3, the third power of magnetization versus internal magnetic field at temperatures close to Curie temperature. (c) Magnetization versus applied field of CeVSb3 along different crystallographic axes, at 2 K; the inset demonstrates the M(H) hysteresis, at low-field region, along c.

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LaAgSb2. The derived electronic contributions are 2.0(5) and 3.4(5) mJ mol1 K2, respectively, for LaSb2 and LaAgSb2. This is probably due to a combination of different band structures and partially filled 3d bands of vanadium. Also note that, although g is higher, there is no indication of Stoner enhancement of w(T). For LaSb2 and LaAgSb2, the derived Debye temperatures are comparable and both are E150 K. In the case of LaVSb3, the origin of the low-temperature deviation from linearity (Fig. 5b), for T2o0.90 K2, is not known at this time.

Fig. 7. Temperature dependence of electrical resistivity for CeVSb3; the inset is the enlarged low-temperature data.

3.2.2. CeVSb3 The only report of CeVSb3 [13] gives a brief description of field dependence and temperature dependence of magnetization, on a polycrystalline sample which was found to be a ferromagnet with Tc ¼ 4.5(1) K. For CeVSb3, the polycrystalline magnetic susceptibility data above 40 K was found to follow the modified Curie–Weiss behavior and gives meff ¼ 2.45 mB, close to 2.54 mB for a

Fig. 8. For CeVSb3: (a) temperature dependence of heat capacity along with the derivative of resistivity (dr/dT); (b) temperature dependence of magnetic heat capacity in the form of C/T and also the entropy change (S); (c) heat capacity in the form of C/T versus T2 and linear fit between 0.4–2 K and 13–15 K (inset).

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free Ce3+, and yp ¼ 3(3) K (see Table 4). This result disagrees with the reported magnetic moment of 1.80(3) mB f.u.1 and the notion of a mixed or intermediate Ce3+,4+ valence [13]. The temperature dependence of magnetization on a CeVSb3 single crystal (Fig. 6a) is found to be highly anisotropic, with largest magnetization found along the c-crystallographic axis. The zfc and fc measurements clearly diverge below the ferromagnetic transition temperature for the applied field along c and a. In order to determine the Curie point for CeVSb3, magnetization isotherms at temperatures around Tc have been measured and the data have been used for the construction of an Arrott plot [26] (as illustrated in Fig. 6b) which implies a Curie temperature between 4.6 and 4.7 K.

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Field-dependent magnetization at 2 K (Fig. 6c) rises very rapidly up to 300 Oe for HJc and varies weakly thereafter, reaching only 1.46 mB f.u.1 at 54 kOe. The magnetization for applied field along a- and b-axes rises more slowly, reaching values of 0.33 and 0.64 mB f.u.1 at 54 kOe, respectively. The report on a polycrystalline CeVSb3 found saturated moment of 1.26(3) mB f.u.1 at 2 K and 50 kOe [13], compared to theoretical 2.14 mB for Ce3+. The magnetization of a polycrystal should be theoretically equal to the average of magnetization along various directions of a single crystal. However, in practice, a polycrystalline sample may suffer from preferred orientation and plausible reorientation of grains in an applied magnetic field, thus less reliable. For the single-crystal result (Fig. 6c), the induced moment at 2 K is highly

Fig. 9. (a) The isobaric magnetization curves for CeVSb3 under hydrostatic pressure. The pressure cell load was Pb (pressure gauge) and CeVSb3. Pressure values for different curves are 0, 0.36, 1.9, 3.7, 5.9, 7.9, 9.5 and 9.7 kbar (arrow indicates the direction of pressure increase). Inset (bottom) is a subset of curves representing CeVSb3 transition with pressure. Inset (top) is the enlarged part of a representative set of curves illustrating the shift of Pb superconducting transition with pressure. (b) Pressure dependence of Curie temperature for CeVSb3.

Fig. 10. The temperature-dependent magnetization of PrVSb3 measured with an applied field of (a) 0.1 kOe along the three crystallographic axes, and along a with (b) 0.1, 2 and 5 kOe.

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anisotropic and the magnetization isotherms suggest that at low fields CeVSb3 has an ordered moment primarily along the c-axis. For HJc, a small coercive field of 16(2) Oe and remanent magnetization of 0.39(2) mB f.u.1 (Fig. 6c inset) are found. The values Bc ¼ 30(10) Oe and Mr ¼ 0.15(3) mB f.u.1 were reported for a polycrystalline sample at 2 K [13]. Fig. 7 shows the anisotropic temperature dependence of the electrical resistivity for CeVSb3. For currents along c and b, the RRRs are 3.4 and 5.9, respectively; however, these are clearly lower limits since r(T) is still dropping at 1.8 K. For CeVSb3, at room temperature rcE140 mO cm and rbE115 mO cm. The resistivity shows a sharp drop below 4.8 K (Fig. 7 inset) mainly due to the loss of spindisorder scattering below Tc. Low-temperature heat capacity result as well as dr/dT data for CeVSb3 are shown in Fig. 8a. Both plots manifest anomalies which peak at approximately 4.5 K for C(T) and 4.6 K for dr/dT, marking the transition to an ordered ferromagnetic state below. In addition, the relation of M3 versus H data (Fig. 6b) indicates Tc ¼ 4.65(5) K for CeVSb3. Based on all of these data, we conclude that Tc ¼ 4.6(1) K. The magnetic entropy associated with this transition was estimated by the integration of C/T versus T and is shown in Fig. 8b. The magnetic entropy released up to 4.7 K is approximately 0.51 J mol1 K1 which is close to R ln 2, suggesting a doubly degenerate ground state for CeVSb3. The magnetic entropy does not reach its maximum value of R ln 6 by 15 K. The 4f levels of Ce3+ most likely split into three Kramers doublets by crystal electric field (CEF) effect and the remaining portion of the theoretical entropy would be removed through the population of another set of CEF levels, above the ground state. By extrapolating linear regions found in C/T versus T2 for CeVSb3 (Fig. 8c), an electronic contribution of

E162 mJ mol1 K2 is found between 0.4 and 2 K, whereas the linear contribution is E400 mJ mol1 K2 for higher temperatures (inset). Given the intervening ordering, this discrepancy is not unusual, and it is clear that the hightemperature fit less likely represents an electronic heat capacity. Well above Tc (Fig. 8b inset), the extracted Debye temperature is E180 K, similar to that of LaVSb3 (E155 K). Given the relative rarity of Ce-based ferromagnets and their potential proximity to exotic superconducting states [27], an investigation of the effect of hydrostatic pressure on CeVSb3 was performed. As noted above, CeVSb3 has Tc ¼ 4.65(5) K at atmospheric pressure, a temperature that

Fig. 12. Temperature dependence of electrical resistivity for PrVSb3 along two crystallographic axes. The inset is the enlarged low-temperature data with shifts of resistivity scale.

Fig. 11. For PrVSb3, (a) magnetization versus applied field along different crystallographic axes at 1.8 K, (b) the M(H) isotherms along a at 1.8, 5 and 10 K; inset is the low-field region at 1.8 K.

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is well separated from the superconducting transition temperature of the chosen Pb manometer. Also for CeVSb3, the rare-earth moments start to saturate at an applied field of 300 Oe along c crystallographic direction. The temperature dependences of magnetization under various pressures, for CeVSb3 along with Pb pressure gauge, are shown in Fig. 9a. The sample was roughly fixed with HJc-axis. Shifts in the transition temperatures were measured via an onset criterion (see illustrations in Fig. 9a insets). There is a clear increase in Curie temperature with increasing pressure, one that is substantially larger than the decrease in the Tc of the Pb manometer. The pressure dependence of the ferromagnetic ordering temperature for CeVSb3 is presented in Fig. 10b and over 10 kbar range; Tc rises with 0.14(1) K kbar1.

Fig. 13. Temperature-dependent heat capacity of PrVSb3. Short arrows indicate inferred magnetic ordering temperatures. Low-temperature data are a fit to nuclear Schottky anomaly (CN), shown as solid line and described in text.

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3.2.3. PrVSb3 The previous magnetization measurement on a PrVSb3 polycrystalline sample found an antiferromagnet ground state with TNE7 K [13]. The results of PrVSb3 singlecrystal magnetization data at 0.1 kOe, indicate strong anisotropy (Fig. 10a) with Ma4Mc4Mb. The magnetization for HJa reveals magnetic transitions at low temperature and low applied fields (Fig. 10b). At H ¼ 0.1 kOe, there are three noticeable transitions at 2, 2.5 and 7.3 K, but at higher applied field, these features become less prominent. Field-dependent magnetization isotherms of PrVSb3 along the three crystallographic axes are shown in Fig. 11. Whereas M(H) at 2 K increases almost linearly for HJc and b, the magnetization along a features metamagnetic transitions. The temperature-dependent evolution of the M(H) for HJa is shown in Fig. 11b. The change of magnetization associated with the lowest field transition for HJa and at 1.8 K below Ho4.5 kOe is gradual (Fig. 11binset), but is followed by two sharp transitions at H ¼ 4.5 and 9.5 kOe. For H49.5 kOe, the magnetic moment starts to saturate and is 2.89 mB f.u.1 at 65 kOe. This value is close to the theoretical Pr3+ free ion value of 3.2 mB. The anisotropic magnetization is consistent with the reported 1.15 mB f.u.1 polycrystalline value at 50 kOe [13]. For M(H) at 5 K (Fig. 11b), a saturated paramagnetic state is probably obtained with comparable moment and a similar saturation value, approached in the 10 K data as well. The fit of the polycrystalline magnetic susceptibility data above 40 K to a modified Curie–Weiss law gives meff ¼ 3.52 mB, close to the free ion 3.58 mB value for Pr3+ and yp ¼ 2(3) K (see Table 4). The temperature-dependent electrical resistivity for PrVSb3 is shown in Fig. 12. For resistivity along c and b, the RRRs are 4.3 and 6.3, respectively, and rc4rb. For PrVSb3, at room temperature rcE90 and rbE70 mO cm. The resistivity in both crystallographic directions decreases with decreasing temperature down to 7.5 K followed by an increase, then a drop below 2 K (Fig. 12 inset).

Fig. 14. (a) Temperature dependence of heat capacity for PrVSb3 shown along with the average susceptibility form of d(wT)/dT (H ¼ 2 kOe) and also the derivative in resistivity, dr/dT. (b) Temperature dependence of magnetic heat capacity and entropy change (S) for PrVSb3.

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Fig. 15. For NdVSb3 and along crystallographic axes, (a) temperature dependence of magnetization at 5 kOe and (b) magnetization versus applied field at 2 K.

Fig. 16. Temperature dependence of electrical resistivity for NdVSb3; inset is the enlarged low-temperature data with shifts of resistivity scale.

Heat capacity measurements manifest a sharp rise with decreasing temperature below 10 K, which peaks at 7.3 K followed by an anomaly at 2.0 K (Fig. 13). The low temperature upturn below T2 K is likely associated with a nuclear Schottky-type anomaly, similar to the observation in many other Pr-based intermetallics [28]. The observed Schottky contribution is likely associated with nuclear spin I ¼ 5/2 state of 141Pr isotope, results from the intrasite, hyperfine coupling between the nuclei and the 4f electrons. At these low temperatures, the total heat capacity is approximately the sum of electronic, lattice, magnetic, but predominantly nuclear contributions. The heat capacity can be estimated with the following expression: C ¼ gT þ bT 3 þ C mag þ C N ,

where CN ( ¼ aT2) represents the Schottky term. The fit in Fig. 13 represents the T2 for 0.4oT (K)o1.8 and is shown to deviate above this temperature range due to the anomaly at 2 K. The Schottky subtracted heat capacity of PrVSb3 is shown in Fig. 14a along with plots of low-field d(wT)/dT and also dr/dT. The heat capacity features a magnetic transition at 7 K and an anomaly centered at 2 K. Features of d(wT)/dT depicts two peaks at 2.5 and 7.3 K, and a drop below 2 K; dr/dT has two peaks at 2 and 7 K. The data of PrVSb3 crystal here present at least one more transition in addition to the one noted between 6 and 7 K in literature [13]. Given the large field dependence, the heat capacity and resistivity zero-field data would most accurately represent the Ne´el temperature. Therefore, for PrVSb3, TN ¼ 2.0(1) and 7.1(1) K. Subsequent to Schottky subtraction, the electronic and lattice contributions can be estimated from the nonmagnetic LaVSb3 data and also deducted. In the case of Pr3+, which is non-Kramer’s ion, magnetic entropy for PrVSb3 is derived and shown in Fig. 14b. At 7.3 K, the magnetic ground state is may be a pseudo-doublet, with the derived entropy of R ln 2. The magnetic entropy does not seem to reach its maximum value of R ln 9 (ln 9 ¼ 2.20) by 15 K and signifies a contribution from crystal-field effects in this sample. 3.2.4. NdVSb3 For NdVSb3 single crystal, magnetization is found to be highly anisotropic with Ma4Mb,c (Fig. 15a). The temperature dependence of susceptibility above 40 K for the polycrystalline sample follows the Curie–Weiss law (Fig. 15a, inset) giving yp ¼ 1(2) K and meff ¼ 3.54 mB (see Table 4), close to the 3.62 mB Nd3+ free ion value, and comparable to the reported 3.27 mB f.u.1 [13]. The isothermal magnetization data at 2 K (Fig. 15b) show metamagnetic behavior for HJa, with an approximately

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Fig. 17. For NdVSb3, temperature dependence of (a) heat capacity shown along with the average susceptibility d(wT)/dT form and resistivity drb/dT. (b) Magnetic heat capacity in the form of C/T and entropy change (S) as a function of temperature.

Fig. 18. (a) Temperature dependence of magnetization for SmVSb3 measured with applied field of 0.5 kOeJa, b and c. (b) Magnetization versus applied field for SmVSb3 along the three crystallographic axes at 2 K.

Fig. 19. Temperature dependence of electrical resistivity for SmVSb3; the inset is the enlarged low-temperature data with shifts of resistivity scale.

linear M(H) for H?a. The saturated 2.90 mB f.u.1 moment here at 65 kOe is close to the calculated full gJ ¼ 3.27 mB, and substantially higher than the reported 1.54 mB found in polycrystalline sample [13], a value closer to the average of three crystallographic orientations of M(H) at 50 kOe. The anisotropic temperature dependence of resistivity for NdVSb3 single crystal is shown in Fig. 16. For currents along c and b, the RRRs are 3.6 and 5.2. At room temperature, rcE95 and rbE55 mO cm. The low-temperature expanded scale (Fig. 16 inset), a change of slope is evident at 5 K. The temperature dependence of heat capacity, d(wT)/dT and dr/dT indicate an antiferromagnetic transition at 4.9(1) K (Fig. 17a). The change in magnetic entropy associated with this transition is close to R ln 2 at TN (Fig. 17b). However, it should be noted that for NdVSb3, the estimated magnetic entropy suffers from lack of points below 1.8 K and has been estimated by a polynomial fit of C(T) to origin, as described in Section 2. The increase in entropy beyond Ne´el temperature signifies contribution from crystal-field effect in the sample. The

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Fig. 20. (a) Temperature dependence of heat capacity for SmVSb3 shown along with the average d(wT)/dT susceptibility form and resistivity (dr/dT). (b) Temperature dependence of magnetic heat capacity in the form of C/T and entropy change (S) of SmVSb3.

Fig. 21. (a) For GdVSb3, temperature dependence of magnetization measured with applied field of 5 kOeJa, b and c. (b) Magnetization versus applied field of GdVSb3 at 2 K; the inset is the enlarged higher field region along a.

S at 15 K is 1.5R and is less than the theoretical maximum of 2.3R (J ¼ 9/2).

Fig. 22. Temperature dependence of electrical resistivity for GdVSb3; the inset is the enlarged low-temperature data with shifts of resistivity scale.

3.2.5. SmVSb3 Antiferromagnetic order in SmVSb3 was observed in a single crystal at TN ¼ 4 K and b direction were found to be the easy magnetization axis [6]. The anisotropic temperature-dependent magnetization for SmVSb3 crystal (Fig. 18) shows clear but comparatively minor anisotropy with the magnetic transition clearly seen along each of the three crystallographic axes. The magnetization for ToTN implies that the ordered moment is probably close to the b-axis. For SmVSb3, the 2 K isothermal magnetization up to 60 kOe (Fig. 18b) is anisotropic and essentially linear. For SmVSb3, at room temperature rcE80 mO cm and rbE55 mO cm (Fig. 19), much smaller than reported in Ref. [6]. Below the transition, the in-plane resistivities rc and rb, increase and decrease, respectively, as the temperature is lowered (Fig. 19 inset). Our RRR values are larger than literature values [6] and are 3.1 along c and 5.7 along b.

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Fig. 20a shows C(T) of SmVSb3 along with d(wT)/dT polycrystalline averaged data and dr/dT. The sharp l anomaly in heat capacity as well as the sharp feature in d(wT)/dT place the magnetic ordering transition at 3.6 K. The derivatives of both rb and rc also show clear feature at this temperature, although drb/dT fits to the approximation of magnetic heat capacity. Based on these data, we conclude that SmVSb3 orders below TN ¼ 3.6(1) K. The magnetic heat capacity along side entropy is shown in Fig. 20b. The accumulated magnetic entropy at TN is very close to R ln 2 and is essentially leveled beyond this temperature region, indicating that the doublet ground state is well separated from excited levels.

Fig. 23. For GdVSb3, temperature dependence of heat capacity shown up to 10 K, along with the average susceptibility d(wT)/dT form and derivative of resistivity dr/dT.

3.2.6. GdVSb3 The temperature-dependent magnetization along the three-crystallographic directions of GdVSb3 is shown in Fig. 21a and is essentially isotropic in the paramagnetic

Fig. 24. (a) For GdVSb3, temperature dependence of heat capacity up to 150 K is shown along with LaVSb3 non-magnetic analog. Inset is the enlarged high-temperature data. (b) Temperature dependence of magnetic heat capacity estimate in the form of CM/T, and entropy change (S) of GdVSb3.

Fig. 25. (a) Temperature dependence of magnetization for TbVSb3 measured with H ¼ 2 kOe along a, b and c, shown along with the polycrystalline average. The inset is the enlarged M(T) for applied field along c and b. (b) The magnetization versus applied field at 2 K.

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state. The paramagnetic moment and Weiss temperature, from the inverse susceptibility above 40 K, are 7.74 mB and 19(1) K, respectively. The antiferromagnetic order in a single crystalline piece of GdVSb3 has been reported at 5 K with a paramagnetic effective moment of 8.3 mB [7], slightly higher than 7.94 mB, which is expected for Gd3+ free ion value. The field-dependent magnetization for GdVSb3 at 2 K (Fig. 21b) is anisotropic with a metamagnetic transition near 45 kOe seen for HJa and M(H) for HJc becoming super-linear, at a comparable field. GdVSb3 has RRRs of 2.2 and 2.7 along c- and b-axes, respectively (Fig. 22), and an anomaly below 6.5 K (depicted in inset). For GdVSb3, at room temperature rcE115 and rbE100 mO cm. The magnetic feature that is observed here for rb(T) is similar to that reported in Ref. [7] for rc(T), however, in literature, no feature was highlighted in the low temperature rb(T).

The low temperature-dependent heat capacity, in Fig. 23, manifests two sharp l-like transitions at 4.5(1) and 6.3(1) K. These magnetic ordering temperatures are in agreement with the polycrystalline average of d(wT)/dT data and consistent with features in r(T). The pair of magnetic ordering temperatures for GdVSb3 is observed for the first time. The measured range of heat capacity data is shown in Fig. 24a, along with the non-magnetic analog LaVSb3. For GdVSb3 at high temperatures, heat capacity falls slightly below that of LaVSb3 for T460 K (Fig. 24a inset). However, in estimating the magnetic component of heat capacity for GdVSb3, for To35 K no scaling of LaVSb3 analog needs to be applied. The magnetic heat capacity and entropy are depicted in Fig. 24b. In GdVSb3, where no crystal-field effects should be present due to Gd3+ S-state, entropy is the theoretical maximum (J ¼ 7/2, R ln 8E2.08R) by 25 K.

Fig. 26. Temperature dependence of electrical resistivity for TbVSb3; the inset is the enlarged low-temperature data with shifts of resistivity scale.

3.2.7. TbVSb3 As part of this systematic study of the RVSb3 series, we grew, apparently for the first time, single crystals of TbVSb3. The temperature-dependent magnetization of TbVSb3 at 2 kOe is strongly anisotropic with Ma4McMb, see Fig. 25a. The inverse of the polycrystalline temperature-dependent susceptibility gives an effective moment of 9.6 mB, comparable to the expected meff ¼ 9.72 mB for the free Tb3+ and Weiss temperature of 10(1) K, consistent with antiferromagnetic exchange interactions. Metamagnetism and anisotropy are evident in the 2 K field-dependent magnetization (Fig. 25b), especially along a. At the highest measured field of 65 kOe, the magnetization reaches 8.9 mB f.u.1, consistent with saturated moment of 9 mB for free Tb3+ ion. For TbVSb3, the electrical resistivity decreases with lowering temperature and features a minimum at 10 K followed by an increase in r and two features at 7 K and 4 K (Fig. 26); TbVSb3 gives a RRRs of 2.0 and 3.1 along

Fig. 27. For TbVSb3: (a) temperature dependence of heat capacity shown along with the d(wT)/dT average susceptibility form and dr/dT resistivity; (b) magnetic heat capacity in the form of C/T and also entropy change (S).

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c- and b-axes, respectively. For TbVSb3, at room temperature rcErb105 mO cm. The heat capacity for TbVSb3 shows a shoulder at 4 K followed at higher temperature by a sharp l transition at 7 K (Fig. 27a). Based on these magnetic contributions, also manifested in the behavior of drc/dT and averaged polycrystalline d(wT)/dT, for TbVSb3 TN ¼ 3.9(1) and 7.1(1) K. The magnetic contribution to heat capacity for TbVSb3 was roughly estimated by subtraction of non-magnetic LaVSb3 data, and is shown in Fig. 27b alongside entropy. For TbVSb3, the magnetic entropy at 20 K is found to be 0.82R, which is much less than the expected value of 2.56R (J ¼ 6); this signifies contribution from crystal-field effects in the sample. The total entropy below TN is found to be 52% R ln 2. Tb, a non-Kramer ion, may have a singlet ground state, with the first excited level within TN so as to allow for magnetic order in the compound. 3.2.8. DyVSb3 The magnetic susceptibility of single crystalline DyVSb3 is anisotropic with Ma 4Mc4Mb (Fig. 28a) and a magnetic feature just below 6 K. The w(T) of the polycrystalline sample fit of Curie–Weiss above 40 K yields meff ¼ 10.6 mB f.u.1, same as Dy3+ free ion value and a Weiss constant of 3(1) K. Field-dependent magnetization isotherms, shown in Fig. 28b, are clearly anisotropic; for HJa, different metamagnetic states occur as a function of applied magnetic field. For DyVSb3, rcE120 mO cm and rbE80 mO cm at room temperature. For DyVSb3, the electrical resistivity reveals a clear break in at 6 K (Fig. 29), due to the loss of spindisorder scattering in the antiferromagnetic ordered state. The RRRs are 2.0 and 3.3 along c- and b-crystallographic axes, respectively. The sharp anomaly in heat capacity at 5.7(1) K (Fig. 30a), TN, coincide with the

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position of d(wT)/dT peak and dr/dT feature. This result casts doubt on the literature value of TN ¼ 12 K for DyVSb3 polycrystalline sample [16]. For DyVSb3, magnetic entropy S rapidly increases up to the magnetic transition temperature (Fig. 30b). The magnetic entropy change up to 5.7 K is 5.75 Jmol1 K1 (R ln 2) indicating a doublet ground state for this compound. The entropy at 15 K is 1.35R, quite small compared to the expected R ln 16 value ( ¼ 2.77R) for Dy3+, indicating the presence of strong crystalline electric field splitting.

4. Discussion and conclusions Motivated by studies of rare-earth antimonides [1–8], we have synthesized and performed thermodynamic and

Fig. 29. Temperature dependence of electrical resistivity for DyVSb3; the inset is the enlarged low-temperature data with shifts of resistivity scale.

Fig. 28. For DyVSb3, (a) the temperature dependence of magnetization measured with applied field of 5 kOe along a, b and c and (b) magnetization versus applied field at 2 K.

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Fig. 30. (a) Temperature dependence of heat capacity for DyVSb3 shown along with the average d(wT)/dT form and derivative in r, dr/dT. (b) Temperature dependence of magnetic heat capacity in the form of C/T and entropy change (S) of DyVSb3.

transport properties on RVSb3 series, which crystallize in a quasi-two-dimensional crystal structure (Fig. 1c). The unit cell volume of RVSb3 crystals reveals the expected decrease from the lanthanide contraction on going from R ¼ La to Dy. Analogous to CeTSb3 (T ¼ Cr, V) single-crystal diffractions [11,18], all RVSb3 members, grown out of self-Sb-flux, show full antimony site occupancies (two 4d, and 4c). However, similar to the literature, vanadium sites are found to be slightly deficient, specifically for the R ¼ Pr, Nd and Gd, RVSb3 members (Table 2). These crystallographic vacancies, as suggested by single-crystal X-ray diffraction, are small and do not seem to correlate with trends in the bulk physical property results. In RVSb3, vanadium is not moment bearing: LaVSb3 manifests a weakly temperature-dependent susceptibility (Fig. 3). The replacement of various rare-earths in RVSb3 changes the ground state of these materials (Table 4) from paramagnetic (R ¼ La) to ferromagnetic (R ¼ Ce) to antiferromagnetic (R ¼ Pr, Nd, Sm, Gd-Dy). For RVSb3 compounds, the low ordering temperatures (o10 K) are attributed to R3+ local moments, and for some, multiple magnetic transitions are revealed. Similar to RSb2, for RVSb3 the unique rare-earth site has low point symmetry (m) and anisotropic properties are observed. CeVSb3 is the only ferromagnet in RVSb3 series, having a Tc ¼ 4.6(1) K. The rest of RVSb3 members are lowtemperature antiferromagnets and demonstrate the equivalence of C(T) and the Fisher d(wT)/dT for determining the transition temperatures, and as well, manifest similar anomalies in dr/dT. For PrVSb3, magnetic transitions are found at 2.0(1) and 7.1(1) K. RVSb3 samples with R ¼ Nd and Sm have TN ¼ 4.9(1) and 3.6(1) K, respectively. For GdVSb3, there are two antiferromagnetic transitions at 4.5(1) and 6.3(1) K. In addition, for TbVSb3, TN are seen at 3.9(1) and 7.1(1) K. Finally, DyVSb3 orders antiferromagnetically below 5.7(1) K (Table 4). The high-temperature magnetic behavior of the Ce, Pr, Nd, Gd, Tb and Dy members of RVSb3 series is local moment-like and the values of the effective magnetic

Fig. 31. For RVSb3, paramagnetic Curie–Weiss temperatures, yP, and magnetic ordering temperatures, TM, as a function of de Gennes factor dG ¼ (gJ1)2J(J+1). Open symbols represent light rare-earths.

moment in the paramagnetic state are close to the theoretical values; meff values are summarized in Table 4, along and with Weiss temperatures yp. Similarly, in RSb2 with R ¼ Ce–Nd, magnetic susceptibility showed local moment behavior at high temperatures [2]. One effect that is the characteristic of RKKY interaction is the scaling of quantities that are dependent on value of exchange interaction (yp and TM) and deGennes factor, dG ¼ (gJ1)2J(J+1). Here gJ is the Lande´ g factor and J is the total spin angular momentum [31]. The trend of

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RVSb3 paramagnetic Weiss temperature versus dG is reasonable (Fig. 31, top). However, similar to many rareearth series [2,3,32], there is a poor scaling of magnetic ordering temperatures for the rare-earth members (Fig. 31, bottom). In addition, it is worth noting that the local maximum in TN (for R ¼ Tb) is also non-deGennes like and is probably associated with extreme CEF induced anisotropy in Tb and DyVSb3 [33]. For estimating the magnetic ground state for RVSb3 (R6¼La) with ordering temperatures below 7 K (Table 4), the heat capacity data up to 15 K were sufficient. The validity of this was ascertained by measuring heat capacity for non-magnetic LaVSb3, along with GdVSb3, up to 150 K (Fig. 24a). In finding magnetic component of heat capacity for GdVSb3, no scaling of LaVSb3 analog needed to be applied. Among the magnetic RVSb3, GdVSb3 (Sstate) has 6% smaller unit cell volume, compared with LaVSb3. Also, given only a slightly larger (8%) decrease in the cell volume across the series (La to Dy), a scaling of LaVSb3 did not seem crucial with respect to the later Tb or Dy. As a result, conclusions regarding the magnetic ground state are based on the magnetic entropy achieved up to 15 K. The entropy analysis of the estimated magnetic heat capacity indicates that member of RVSb3 with R ¼ Ce, Pr, Nd, Sm and Dy have doublet or pseudo-doublet ground states at magnetic ordering temperatures, TM. For TbVSb3, however, the magnetic ground state at TN is probably singlet. The continuous increase of S above TN in Ce, Pr, Nd, Tb and Dy indicates that the electronic levels of R ions in these compounds are CEF split with closely spaced sublevels. On the other hand, for SmVSb3, well-defined plateau of S above TN indicates that the next higher CEF level is at a considerably higher energy. The entropy at 15 K in all magnetic RVSb3 cases, except for the 8-fold GdVSb3, is considerably less than the respective calculated value of S, which suggests strong CEF effects. All RVSb3 compounds described here are metallic and manifest long-range order of their local moments (R6¼La) in the loss of spin-disorder scattering in r(T). The RRR across the series ranges from 2 to 9 (Fig. 32, top). As a relation between RRR and the crystallographic quality of the sample is generally drawn, our values would suggest a fair crystal quality. Although some of RVSb3 are found to be slightly vanadium deficient, no correlation is seen between the values of RRR based on bulk resistivity measurements and site vacancies in crystals of R ¼ Pr, Nd, Gd and Tb (Table 2 and Fig. 32) found in single-crystal Xray diffraction results. Based on this, it appears that the crystallographic off-stoichiometries noted, are not important. On one hand, the moderate RRR values may imply small plane defects such as stacking faults or stoichiometric line defects, i.e. edge dislocations, which would increase the value of residual resistivity, lowering RRR. On the other hand, some of the scatterings in RVSb3 compounds have magnetic origin at low temperatures, as RRR of rb in the non-magnetic LaVSb3 gives the highest value of 8.8. Our RRR are comparable to the literature values for RVSb3

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Fig. 32. For RVSb3, a summary of residual-resistivity ratios (RRR) and resistivities at room temperature (r300 K), with excitation current running along the two crystallographic directions of b (rb) and c (rc).

system [6,9,10] and better than those reported for RCrSb3 crystal system [2,4,11]. Across the RVSb3 series, RRR along b- is consistently larger than that along c-crystallographic axis (Fig. 32, top). For all of RVSb3 with R ¼ La–Nd, Sm, Gd–Dy, we report the anisotropic resistivities along two crystallographic axes. Across the series, the resistivity at room temperature (r300 K) shows that rc4rb (Fig. 32, bottom), with the exception of R ¼ La and Tb, where rcErb. In literature, the data of r300 K for RVSb3 with R ¼ La, Sm and Gd contradictory show that rb4rc [5–7]. Our RRR values for LaVSb3 (Fig. 32, top) are slightly larger than literature values of 4.4 along c-axis and 4.7 along b-axis [5]. For LaVSb3, our magnetic susceptibilities are consistent with those reported and largest for wc. In addition, for SmVSb3 sample and below the transition (Fig. 19 inset), the in-plane resistivities, rc and rb, increase and decrease, respectively, as the temperature is lowered. The previous study on the anisotropic properties of SmVSb3 did not find this behavior, reported much higher r300 K [6]. In our study, the crystallographic orientations were confirmed (see Section 2) using sample pieces that were prepared for resistance measurements. SmVSb3 crystal quality here is slightly better (Fig. 32, top), compared to the reported RRR of 3.7 and 1.6, along b and c, respectively [6]. Concerning M(H), SmVSb3 is consistent with literature, as TN is most clearly observed for HJb. Finally, for GdVSb3, our result in Fig. 22

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contradicts and challenges literature [7] for two reasons: one is the issue of sample orientation with respect to resistance measurements, the other is the puzzling nonfeature in the literature low temperature rb(T), that is clearly observed here. There are no detailed magnetization data reported for GdVSb3. Heavy-fermion (HF) behavior may be seen in strongly correlated electron systems, which in lanthanides, is associated with a specific character of 4f electron states. A large value of the linear coefficient g of electron heat capacity, is a feature of HF behavior and may hint at large effective electron mass m*. For the non-magnetic member LaVSb3, an electronic contribution of g ¼ 17(1) mJ mol1 K2 is found below 5 K (Fig. 5b). This value is relatively large, compared to LaSb2 and LaAgSb2, and likely to persist throughout the entire RVSb3 series. In fact, for the diantimonide systems LaSb2 and LaAgSb2, the derived electronic contributions are about an order of magnitude smaller and g ¼ 2.0(5) and 3.4(5) mJ mol1 K2, respectively. For CeVSb3, more enhanced g values of 162(3) mJ mol1 K2 below 2 K is obtained. Such a value gives insight into possible moderate HF characteristic of this compound. The Tc of CeVSb3 is found to increase linearly under pressure with a rate of 0.14(1) K kbar1 (Fig. 9b). This behavior is consistent with a Doniach one-dimensional Kondo necklace model [29]. At relatively low applied pressures, the RKKY interactions [30] dominate and the Kondo necklace model gives TcaJ2N(Ef), where J is the magnetic exchange constant between the conduction electrons and the 4f local moments, and N(Ef) is the density of conduction electrons at the Fermi level. At higher pressures than measured here, it is expected for Tc of CeVSb3 to pass through a maximum when Kondo and the RKKY energies are close, and then drop to zero with increasing pressure to a possible quantum-critical point. Recently, there has been growing interest in the study of materials located close to a ferromagnetic quantum-critical point. For example, for two similar systems, the magnetic phase of CeAgSb2 with Tc ¼ 9.6 K [34] and CeNiSb3 with Tc ¼ 6 K [35] have been investigated as a function of pressure. In both samples, the application of pressure increases the hybridization strength between the localized cerium f states and conduction electrons: the magnetic ordering temperature first increases, then passes through a maximum. For CeAgSb2 [34], the ferromagnetic phase is suppressed at 35 kbar, and another magnetic phase, a possible antiferromagnet, found above 27 kbar. The TN of this phase is reported to reach a maximum value of 6 K at 44 kbar and is completely suppressed by 50 kbar. Here, no superconductivity was observed up to 46 kbar down to 0.2 K. For CeNiSb3, however [35], the ferromagnetism is only found to be slightly suppressed for pressures higher than 35 kbar, with coexistence of two magnetic phases above in the range 35 kbarpPp55 kbar. The critical pressure for suppression of the low-temperature phase is 55 kbar, while a quantum-critical point is associated with

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