A structural and ¹²¹Sb Mössbauer-spectroscopic study of PrPdSb and NdPdSb

1 Introduction

The equiatomic RETX (RE=rare earth element; T= electron-rich transition metal; X=element of the 3^rd, 4^th or 5^th main group) compounds are one of the dominating families of intermetallic phases [1]. Given the large number of RE, T and X elements, these phases show an extremely large variability concerning the valence electron count and their broadly varying physical properties [2]. The several thousand RETX phases crystallize with more than 30 different structure types [3], [4], [5]. An important issue concerns the ordering of the T and X atoms. Its precise determination is definitely a prerequisite for a deeper understanding of the structure-property relations. Although these RETX phases have been the subject of detailed investigations for more than 40 years in both solid state chemistry and solid state physics, so far, many phases are structurally still not fully characterized.

The antimonides PrPdSb and NdPdSb [6], [7], [8], [9], [10], [11], [12], [13], [14] are examples for such insufficiently characterized compounds. Both phases were first reported by Marazza et al. [6] and these authors assigned the CaIn₂ type on the basis of X-ray powder diffraction data with a statistical distribution of the palladium and antimony atoms on the indium network. Magnetic data were reported shortly thereafter. NdPdSb shows a modulated magnetic structure below the Néel temperature of 10 K while PrPdSb shows no long-range magnetic order at 1.5 K [11]. So far, all structural studies on PrPdSb and NdPdSb have relied on powder data. Mehta et al. [9] presented an orthorhombic model for NdPdSb with two crystallographically independent neodymium sites. The weakness of this model is the low resolution and the exact √3 ratio between the a and c axis, giving no hint for an orthorhombic distortion. Subsequent neutron powder diffraction data [11] on both PrPdSb and NdPdSb confirmed the hexagonal symmetry and refinements with an ordered model (LiGaGe vs. CaIn₂ type) resulted in better residuals; however, again with moderate resolution.

Herein we report on the growth of small single crystals of PrPdSb and NdPdSb and a precise structural study on the basis of single crystal X-ray diffractometer data along with an investigation of the samples by ¹²¹Sb Mössbauer spectroscopy.

2 Experimental

2.2 X-ray diffraction

Guinier patterns (Enraf-Nonius FR552 camera, imaging plate detector, Fujifilm BAS-1800) of PrPdSb and NbPdSb were recorded using CuKα₁ radiation and α-quartz (a=491.30, c=540.46 pm) as an internal standard. The hexagonal lattice parameters were deduced from least-squares refinements and the accuracy of the indexing was ensured through intensity calculations [17]. The present data is in good agreement with those published previously (Table 1). The experimental and simulated powder diagram of the PrPdSb sample is presented exemplarily in Fig. 1.

Table 1:

Lattice parameters for PrPdSb and NdPdSb with standard deviations given in parentheses.

Compound	Type	Space group	a (pm)	b (pm)	c (pm)	V (nm3)	Reference
PrPdSb	CaIn2	P63/mmc	459.3	a	780.5	0.1426	[6]
PrPdSb	CaIn2	P63/mmc	458.5	a	780.2	0.1420	[7]
PrPdSb	CaIn2	P63/mmc	458.8(8)	a	783.6(9)	0.1428	[10], [11]
PrPdSb	NdPtSb	P63mc	458.6(2)	a	780.2(4)	0.1421	This work
NdPdSb	CaIn2	P63/mmc	458.0	a	771.6	0.1402	[6]
NdPdSb	CaIn2	P63/mmc	458.0	a	771.2	0.1401	[7]
NdPdSb	CaIn2	P63/mmc	457.7(8)	a	767.6(9)	0.1393	[10]
NdPdSb	NdPdSb	Pmmb	458.33(6)	771.89(3)	793.7(1)	0.2808	[9]
NdPdSb	CaIn2	P63/mmc	457.92(7)	a	772.60(9)	0.1403	[11]
NdPdSb	NdPtSb	P63mc	458.0(1)	a	771.5(1)	0.1402	This work

Fig. 1:

Experimental and simulated X-ray powder pattern (CuKα₁ radiation) of PrPdSb.

Single-crystal fragments were isolated from pieces of the crushed ingots and glued to quartz fibers using beeswax. Their quality was checked on a Buerger camera (white Mo radiation). Intensity data sets were collected with an IPDS-II diffractometer from Stoe (graphite monochromatized MoKα radiation; oscillation mode). Numerical absorption corrections were applied to the obtained data. Details about the data collections and the crystallographic parameters are summarized in Table 2.

Table 2:

Crystal data and structure refinement for PrPdSb und NdPdSb, NdPtSb type, space group P6₃mc, Z=2.

Empirical formula	PrPdSb	NdPdSb
Formula weight, g mol−1	369.1	372.4
Unit cell dimension (Guinier powder data)
a, pm	458.70(5)	458.18(4)
c, pm	780.55(6)	771.25(6)
Cell volume V, nm3	0.1422	0.1402
Calculated density, g cm−3	8.62	8.82
Crystal size, μm3	20×40×40	30×40×60
Transm. ratio (min/max)	0.420/0.584	0.477/0.598
Absorption coefficient, mm−1	32.2	33.8
Detector distance, mm	60	70
Exposure time, s	600	480
ω range/increment, deg	0–180/1	0–180/1
Integr. Param. A/B/EMS	14.0/−4.0/0.03	13.0/2.7/0.014
F(000), e	312	314
θ range, deg	5.1–33.3	5.1–33.3
Range in hkl	±7, ±7, ±12	±7, ±7, ±11
Total no. of reflections	3553	3453
Independent reflections/Rint	244/0.0752	229/0.0334
Reflections with I>2 σ(I)/Rσ	208/0.0152	208/0.0091
Data/parameters	244/11	229/11
Goodness-of-fit on F2	0.82	1.17
R1/wR2 for I>2 σ(I)	0.0119/0.0258	0.0129/0.0309
R1/wR2 for all data	0.0196/0.0272	0.0182/0.0317
Twin ratio	0.48(7)	0.46(6)
Extinction coefficient	68(5)	367(15)
Largest diff. peak/hole, e Å−3	0.71/−0.72	1.20/−1.88

3 Crystal chemistry

The new single crystal X-ray diffractometer data clearly confirm hexagonal space group symmetry and palladium-antimony ordering for the antimonides PrPdSb and NdPdSb. Before starting the crystal chemical discussion we need to comment on the slightly enhanced U₃₃ displacement parameters of the palladium atoms, which might be an indication of a required symmetry reduction. Since no additional reflections were evident from the diffraction patterns, all klassengleiche symmetry reductions can be ruled out. The only translationsgleiche subgroup allowing a splitting of the palladium position is P3m1. Refinement is this trigonal setting did not result in an improved structure solution. All single crystal structure refinements of such hexagonal phases have shown this slight anisotropy. Representative examples are NdPtSb [25], CeAuGe [26], ErAgSn [27], CeCuSn [28] or samples from the solid solution Ca_1−xRE_xAg_1−ySb (RE=La–Nd, Sm) [29].

Exemplarily we discuss the crystal structure of PrPdSb. The palladium and antimony atoms form slightly puckered Pd₃Sb₃ hexagons (Fig. 2) with Pd–Sb distances of 269 pm which perfectly match with the sum of the covalent radii [30] for Pd+Sb of 269 pm. These layers are rotated by 60° in every other layer, leading to an AB stacking sequence. The praseodymium atoms have a quasi-sandwich-like coordination by two of these Pd₃Sb₃ hexagons. Due to the puckering and the rotation of the hexagons, three palladium atoms at 312 pm Pr–Pd are the closest neighbors to praseodymium.

Fig. 2:

The crystal structure of PrPdSb. Praseodymium, palladium and antimony atoms are draws as gray, blue and red ellipsoids (99% probability), respectively. The puckered [Pd₃Sb₃] networks are emphasized.

The structure shown in Fig. 2 is clearly reminiscent of the AlB₂ structure. Indeed PrPdSb and NdPdSb are superstructure variants of this aristotype. The corresponding group-subgroup relation has been discussed in a review article [31]. An important question concerns the assignment of the correct structure type for PrPdSb and NdPdSb. In the older literature [6], [7], [8], [9], [10], [11], [12], [13], [14], the structure is always ascribed to the LiGaGe type [32]. This is not correct since the dimensionalities of the [GaGe]⁻ and the [PdSb]³⁻ polyanions are distinctly different. The gallium atoms in LiGaGe have an almost ideal tetrahedral germanium coordination with 3×254 and 1×258 pm Ga–Ge, and the [GaGe]⁻ substructure corresponds to the wurtzite type. In PrPdSb we observe a pronounced two-dimensional character for the [PdSb]³⁻ polyanion. The fourth antimony neighbor is at the much longer distance of 346 pm which is definitely not a bonding contact. Thus, in view of the distinctly different bonding situation LiGaGe and PrPdSb are rather isopointal [33], [34] than isotypic and belong to two different structural branches.

All the phases that adopt the space group type P6₃mc with a Wyckoff sequence b²a have the unit cell ratio c/a as well as the atomic z parameters as degrees of freedom. This allows structures with almost flat hexagonal networks (2D) up to structures with strong puckering and almost tetrahedral arrangements (3D). This is basically a function of the radii of the atoms forming such an equiatomic compound. An impressive example is the series of REAuGe germanides [35], [36] where the dimensionality of the [AuGe] network changes as a function of the rare earth element. The large cerium atoms separate the [Au₃Ge₃] hexagons in CeAuGe (3×260 and 1×364 pm Au–Ge) [26], i.e. we observe no inter-layer Au–Ge bonding. In going to the smaller rare earth atoms (i.e. playing with the lanthanide contraction), one observes increasing inter-layer bonding with the shortest values in ScAuGe (3×258 and 1×275 pm Au–Ge) [36]. All these examples nicely underline that a precise structure determination even of such simple compounds is a prerequisite for understanding the bonding situation.

4 ¹²¹Sb Mössbauer spectroscopy

The ¹²¹Sb Mössbauer spectra of PrPdSb and NdPdSb (Fig. 3) could be well reproduced with single signals at isomer shifts of −7.55 mm·s⁻¹ for PrPdSb and −7.47 mm·s⁻¹ for NdPdSb, in good agreement with the single crystallographic antimony sites determined from the diffraction experiments. The isomer shifts are in the typical range for intermetallic antimonides [37], [38], [39]. Both compounds show a small quadrupole splitting of around 2 mm·s⁻¹, resulting from the asymmetric 3+1 coordination of the antimony atoms within the [PdSb] networks. The line width parameters were kept fixed at 3.0 mm s⁻¹ (the typical value observed for related antimonides) during the fitting procedure in order to avoid correlation with the quadrupole splitting parameters.

Fig. 3:

Experimental and simulated ¹²¹Sb Mössbauer spectra of PrPdSb and NdPdSb at T=5 K.