Lattice distortions measured in actinide ferromagnets PuP, NpFe2 and NpNi2

Abstract

— X-ray low-temperature measurements indicate that : (1) cubic PuP distorts to tetragonal below Tc = 125 K with accompanying line broadening, (2) NpFe2 exhibits a rhombohedral angle distortion from 60 to 60.53° below T c ~ 500 K, and (3) in NpNi2 the rhombohedral angle changes 0.19° ( ± 0.02°) below Tc = 32 K. Actinide U and N p ferromagnets have been shown [1] to exhibit a large distortion from cubic symmetry below T c , with all previous examples exhibiting a rhombohedral symmetry compatible with the ( 1 1 1 ) easy axes of magnetization found in these compounds . W e report here the results of X-ray experiments at low tempera ture to examine the symmetry of PuP ( T c = 125 K) , N p F e 2 ( T c ~ 500 K) , and NpNi 2 ( T c = 32 K) . In PuP a (100) easy axis was found with neutron measurements [2] and, as expected , we find a tetragonal distortion such that (c -a)/a = ( 3 1 ± 1 ) 1 0 4 at 5 K. The variation of the lattice parameters and the strain are shown as functions of temperature in figures 1 and 2, respectively. Below T c the diffraction peaks also broaden , presumably a consequence of strain induced by the magnetoelastic interactions (see Fig. 3). Fig. 1. — Variation of the lattice parameters as a function of temperature for PuP. (*) Work supported by the U.S. Department of Energy. Fig. 2. — Tetragonal distortion for PuP as a function of temperature. In NpFe 2 neut ron exper iments [3] determined a ( 1 1 1 ) easy axis and we find a rhombohedral distortion such that the rhombohedral angle changes from 60° to 60.53°. An alternative description of the rhombohedral distortion is to define a length c as a dis tance along the unique trigonal axis and a as a dis tance in the plane perpendicular to c such that c /a = 1.00 in the cubic phase . This definition is especially useful when comparing the magnitude of trigonal and tetragonal distort ions. In this case the strain in N p F e 2 is (120 ± 5) 10 , which is the largest found in any actinide compound. In NpNi 2 the quality of the powder pat terns is ra ther poor , but we est imate the change in the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979421 LATTICE DISTORTIONS MEASURED IN ACTINIDE FERROMAGNETS C4-69 rhombohedral angle to be 0.19°~0.020 from the broadening of the lines below T,. The absolute value of the strain is then (43 -t 5) Our results are compatible with the theory that all actinide ferromagnets exhibiting localized Sf moments reduce their symmetry below T, as a consequence of strong magnetoelastic interactions. PUP is the first system to be found with a tetragonal distortion, and NpFe, has the largest rhombohedral distortion found so far. The behaviour of the ferromagnetic compounds is in contrast to the actinide antiferromagnets, in which the distortions are either small or negligible [I, 41. This difference between the ferroand antiferromagnets is not understood. 0 . 2 5 1 1 1 1 1 , 1 1 1 1 1 1 20 40 60 80 100 120 140 160 180 200 220 Fig. 3. -Diffraction peak broadening observed in selected reTEMPERATURE ( K) flections from PUP as a function of temperature.