Physica B 156 & 157 (1989) 829-831 North-Holland, Amsterdam

ANTIFERROMAGNETISM OF UN&P, AND UNiAs, BY NEUTRON DIFFRACTION

P. FISCHER’, A. MURASIK’,* D. KACZOROWSKI* and R. TROC* ‘Labor fir Neutronenstreuung ETHZ, CH-5303 Wiirenlingen, Switzerland ‘Institute for Low Temperature and Structure Research, Polish Academy of Science, 50-950 Wroclaw, Poland

Neutron diffraction studies on polycrystalline UNi,P, and UNiAs, indicate antiferromagnetic ordering of AFI type, below T, = 101 and 195 K, respectively. Our results show that only the uranium ions carry localized magnetic moments. At 7.5 K they amount to (1.59 5 0.07)~~ for UN&P, and (1.85 + 0.06)~~ for UNiAs,. In both compounds the moments are aligned along the c axis. In the course of this study structural parameters have been also refined.

1. Introduction

The magnetic properties of U-pnictogen ter- naries containing transition metal Cu, or Ni have recently attracted considerable interest [ 11. While the compounds based on Cu, like UCu,P, and UCuP,, are ferromagnets, the corresponding phases based on nickel order antiferromagneti- tally with the susceptibility maxima occurring at 111 and 228 K for UNi,P, and UNiAs,, respec- tively. In ref. 1 considerable departures of the observed effective magnetic moments ~l.,rr from the free ion values of either U4+ or U3+, are reported, indicating a large crystal-field splitting of the ground multiplet. In both compounds the relatively large U-U separation (3.8-4.0 A) re- nders direct exchange interactions unlikely and favours indirect ones, especially superexchange via P3- and As3- anions.

2. Experimental details

The polycrystalline samples of UNi,P, and UNiAs, were obtained by synthesis of the mix- ture of powdered nickel metal with UP, or UAs,, placed in evacuated quartz ampoules and annealed at 900°C for two weeks, as described previously [l]. The neutron diffraction meas- urements were carried out at the reactor Saphir in Wiirenlingen, using the multi-detector powder

* Permanent address: Institute of Atomic Energy Swierk, 05-400 Otwock, Poland.

diffractometer DMC and a two-axis spectrome- ter. For the structure analysis, neutron diffrac- tion patterns were measured at 7.5, 148 and 293 K with neutrons of wavelength A = 1.708 A, using a close-cycle helium refrigerator. The in- tensities of the largest magnetic reflections were carefully measured as a function of temperature on the two-axis diffractormeter with a neutron wavelength A = 2.34 A. The analysis of ex- perimental data was made using the profile- fitting procedure developed by Rietveld [2]. The neutron scattring lengths: b, = 8.42 fm, bNi =

10.3 fm, b, = 5.13, and b,, = 6.48 fm were used [3]. The magnetic form factors for U and Ni ions were taken in dipole approximations from refs. 4 and 5, respectively.

3. Crystal structure refinement

Polycrystalline UNi,P, was first obtained by Hoffman and Jeitschko [6], and its crystal struc- ture was reported to be of the ThCr,Si, type. Assuming the ideal ThCr,Si,-type unit cell, a comparison of observed and calculated nuclear neutron intensities revealed a rather poor agree- ment (R,, = 8.5%, R,, = 16%) as well as un- realistic temperature factors for the U and P atoms, both being negative even at 293 K. This led us to conclude that the ideal ThCr,Si,-type structure may not be the case for the UNi,P, phosphide. Because of the large difference in the scattering amplitudes of Ni and P,.@e effect of a

0921-4526/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

830 P. Fischer et al. I Antiferromagnetism of lJNi,P2 and UNiAs,

partial disorder on the Ni and P sites may be readily detected.

Hence in the next step, the atomic occupancies for the Ni and P sites are refined too. The best fit was achieved for a 25% mixing of Ni and P, yielding R,, = 6% and R,,, = 13%. This resulted in increasing the Debye-Waller factor for P from -0.4 to 1.6 A’, leaving the temperature factor for U almost unchanged (negative). We con- cluded, therefore, that the partial disorder can- not be considered as a main cause of the poor fit.

On the other hand, in seveal ternary nickel pnictides, it was observed that the ThCr,Si,-type compounds are formed with a slightly lower nickel content [7,8]. Therefore, to test the de- fect formation for the nickel sites, in subsequent least-squares refinements only the occupancy pa- rameter of Ni was varied, yielding the composi- tion UNi ,,56(4jP2. The corresponding final con- ventional residuals were R,, = 4.0%, R,, = 12%. The temperature factors have now reason- able values for all atoms (see table II below). The departure from a 1: 2 : 2 composition can be qualitatively explained from bonding considera- tions [7].

(4 (b) (Cl

Fig. 1. The tetragonal cells of (a) UNi?P, (the ThCr,Si,-type

structure), (b) the CaBe?Ge,-type structure, and (c) UNiAs,.

UNiAs, represents a new structural type for a compound of the composition 1: 1: 2 [9]. In P4/nmm space group the atoms occupy the fol-

lowing positions: U and As, in 2(c) ? (l/4,1/4, z), Ni in 2(b)? (3/4, l/4, l/2) and As, in 2(a) ? (3/4,1/4,0). It shows a close simi- larity with ThCr,Si, or CaBe,Ge,-type struc- tures, which can be visualized as XBBX or alter- nating XBBX and BXXB layer sequences along c of the transition metal (B) and metalloid (X), respectively [lo]. With regard to the structure of UNiAs,, it appears that entire planes of nickel atoms are missing from the BXXB layer (fig. 1). The values of the structure parameters obtained for both compounds in the course of the least- squares profile fitting are summarized in tables I and II.

Table I

Lattice constants of UNi,Pz and UNiAs,. R,, and R,, are the reliability

factors for integrated and weighted profile intensities, respectively. R,, is the

reliability factor for integrated magnetic intensities. Estimated standard devia-

tions correspond to the last digit and are given in parentheses

Compound T(K) a(A) c(A) R,,(s) R,J%) R,,(s)

UN&P, 293 3.828( 1) 9.431(2) 4.0 11.7

UNiAs, 293 3.951(l) 9.138(2) 5.4 11.9

UN&P, 7.5 3820(l) 9.412(2) 4.3 12.0 7.6

UNiAs, 7.5 3.937(l) 9.136(2) 5.5 12.3 12.6

Table II Atomic coordinates and isotropic temperature factors of UNi,P, and UNiAs,

Compound T zu

(IQ ZP 2.4, BU B,, B, B As

CA’)

UN&P,

293 0.3758(4) 0.28(5) 0.48(5) 0.80(8) 7.5 0.3765(4) 0.03(7) 0.09(6) 0.47(9)

293 0.2460(4) 0.6528(4) 0.04(5) 0.56(5) 0.27(5)

UNiAs, 7.5 0.2456(4) 0.6531(4) -0.13(5) ~0.08(5)

P. Fischer et al. I Antiferromagnetism of UNizPz and UNiAs, 831

a

1

0 100 200 300 T(K)

Fig. 2. Temperature dependence of peak intensity of mag- netic reflection (100) for (a) UNiAs, and (b) UN&P,. As insets the corresponding structures of these compounds are shown.

4. The magnetic ordering in UNi,P, and

W*s*

Neutron diffraction patterns of UN&P, and UNiAs,, taken at 7.5 and 148 K, reveal magnetic Bragg peaks, indexable on the basis of chemical unit-cell and obeying the rule h + k + I = 2n + 1. Such a rule is characteristic for collinear antifer- romagnetic ordering of the type AFI. The mag- netic moments are coupled ferromagnetically within (001) sheets, but adjacent layers are ar- ranged antiferromagnetically, i.e. their sequence in the c direction is + - + - -. . In both com- pounds the magnetic moments are aligned paral- lel to the c axis. At 7.5 K the ordered magnetic moments of U amount to (1.59 + 0.07)~~ and (1.85 + 0.06)1*, for UNi,P, and UNiAs,, respec-

tively. The temperature dependence of the (100) magnetic peak intensity yields the NCel tempera- tures TN = 101 and 195 K for UNi,P, and UNiAs,, respectively (fig. 2). These values are considerably lower than those found from the magnetic susceptibility measurements PI- Hence, one cannot rule out the possibility of another magnetic phase before reaching TN. The intensities of magnetic peaks as a function of the magnitude of the momentum transfer Q decrease in ap roximate accordance with a squared U4’ or U p+ form factor [4]; the scattering therefore arises from antiferromagnetically ordered mag- netic moments due to 5f electrons.

Acknowledgements

One of us (A.M.) is greatly indebted to E.T.H. Zurich and the Swiss National Science Foundation for financial support.

References

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