Evidence of a lattice distortion in NpO2 below 25 K from neutron magnetic inelastic scattering

Solid State Communications, Vol. 79, No. 2, pp. 197-200 Printed in Great Britain.

0038-1098/91 $3.00 + .00 Pergamon Press plc

EVIDENCE OF A LATTICE DISTORTION IN NpO2 BELOW 25 K FROM NEUTRON MAGNETIC INELASTIC SCATTERING R. CaciuffoLS, G. Amoretti2, J.M. Fournier 3"4, A. Blaise4, R. Osborn 5, A.D. Taylor5, J. Larroque 6, M.T. Hutchings7 ~Dipartimento di Scienze dei Materiali e della Terra, Universit/t di Ancona, Via Brecce Bianche, 1-60131 Ancona, Italy, and Istituto di Struttura della Materia del Consiglio Nazionale delle Ricerche, 1-00044 Frascati, Italy 2Dipartimento di Fisica, Universit~i di Parma, Viale delle Scienze, 1-43100 Parma, Italy 3Universit~ Joseph Fourier, F-38000 Grenoble, France 4D+partement de Recherche Fondamentale, Centre d'Etudes Nucl6aires de Grenoble, 85 X, F-38041 Grenoble, France 5ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OXI 1 0QX, United Kingdom 6Centre d'Etudes Nucl6aires de Cadarache, BP 01, F-13108 Saint-Paul-lez-Durance, France 7N.D.T. Department, A.E.A. In. Tec., Harwell Laboratory, Didcot, Oxon OX11 0RA, United Kingdom

(Received 24 February 1991 by R. Fieschi) Modifications of the crystal field ground state of NpO 2 below the phase transition at 25 K have been investigated by magnetic neutron spectroscopy. The results show a splitting in the low temperature phase of the cubic F~2) quartet ground state into two doublets separated by 7.2 meV. This supports the hypothesis that the phase transition involves the quadrupolar ordering of the Np 4+ ions by a collective Jahn-Teller distortion of the oxygen sublattice. The observed amplitude of the splitting is consistent with an oxygen displacement of the order of 0.02 A, which is below the present limits of resolution of neutron diffraction experiments. A CURIOUS phenomenon is shown by NpO2, an apparently simple tetravalent ionic oxide with fluoritetype crystal structure. A large anomaly is observed in the specific heat [1] and magnetic susceptibility [2] at T,-- 25K but neither M6ssbauer [3] nor neutron diffraction [4-6] experiments have found evidence for either magnetic ordering or lattice distortion. Similar anomalies observed in the isostructural compound UO2 are due to a first-order phase transition from a paramagnetic to a type-I antiferromagnetic state [7, 8] associated with a triple-k Jahn-Teller distortion of the oxygen sublattice [9, 10]. The oxygen displacement is A3k = 0.008/~, corresponding to A = 0.014/~ if a monoclinic distortion is assumed [7]. Neutron diffraction experiment on NpO2 are, however, more difficult because the available single crystals are quite small, the volume of the largest being of the order of 0.5mm 3. As a consequence, only distortions larger than 0.02-0.03/~ could be resolved in the diffraction experiment so far performed. The upper limit on the ordered magnetic moment set by Mrssbauer spectroscopy is /~0 < 0.01 #B, to be compared with the effective Curie-Weiss paramagnetic moment of

3 #B. If the magnetic moment is not ordered in the low-temperature phase, the magnetic susceptibility should diverge as the temperature decreases towards zero. However, the measured susceptibility reaches a constant value of 8.4 x 10-3emumole 1 at 5K [2], which implies that the magnitude of the moment is almost zero. This is surprising since Np 4+ is a Kramers ion (5f 3 electronic configuration) which cannot have a non-magnetic singlet as the crystal field (CF) ground state. There are several theoretical models designed to explain these observations. Friedt et al. [3] have proposed a dynamic internal distortion of the oxygen sublattice below Tc = 25 K. The resulting reduction in the local symmetry induces a splitting of the cubic CF multiplets such as to reduce the magnetic susceptibility ~((T) as the temperature falls below To. This hypothesis is consistent with the Mrssbauer results but a search for anharmonic effects in NpOz at low temperature by neutron diffraction [4] shows no observable deformation of either the neptunium or oxygen restoring potential below T,, which effectively excludes a dynamical distortion model. Zolnierek et

197

198

EVIDENCE O F A L A T T I C E D I S T O R T I O N IN Np02 BELOW 25 K

al. [11] have advanced the hypothesis that in NpO2 the cation is trivalent. The fourth electron in the 5f 4 shell is provided by the oxygen 2p bands which, consequently, contain one localized hole per unit cell. The phase transition is then described either as an antiferromagnetic ordering of the holes sublattice, if these are localized in the empty anion cubes, or as an order-disorder transition if the holes are attached to the oxygen ions. In both cases the Np 3÷ ions would be in a non-magnetic singlet state. This model is, however, in contradiction with both the isomer shift in M6ssbauer spectra and the magnetic form factor measured by neutron diffraction [12] which imply a tetravalent 5f 3 electronic configuration. This has been recently confirmed by neutron spectroscopy measurements of the CF excitations [13, 14] which are consistent with a 1"8(2) quartet as ground state of t h e 419/2 multiplet, followed by a F~~) quartet at ~ 50 meV and a I"6 doublet at ,-~220 meV. The CF potential derived in [13] is just that required to explain the zero magnetic moment according to the model of Solt and Erd6s [15] which, in analogy to the UO2 case, assumes a collective Jahn-Teller monoclinic distortion of the oxygen sublattice driven by the quadrupolar interaction. As a consequence of this distortion, the Np 4+ quadrupoles order in an antiferroquadrupolar configuration and the F~2) quartet is split into two Kramers doublets, the lowest one being nearly non-magnetic provided that the relative strength of the 4th and 6th order cubic CF assumes a particular value and that the anisotropy confines the magnetic moment to the [0 0 1] direction. The previous neutron diffraction experiments have looked for this static distortion without success. We have therefore done a neutron spectroscopy experiment with the aim of detecting the splitting of the ground quartet below T,. Measurements were performed using the directgeometry chopper spectrometer H E T [16] at the ISIS spallation neutron source at the Rutherford Appleton Laboratory. Scattered neutrons are detected by three arrays of 3He detectors covering a scattering angle range A~b between 3° and 136 °. Normalization of the spectra recorded at different angles is performed by measurements on a vanadium standard. Incident energies of 25 and 60 meV were used to study the scattering at two sample temperatures, 5 and 37 K. That sample consisted of 32 g of NpO2 powder doubly encapsulated in a special AI holder. It was prepared at the Centre d'Etudes Nucl6aires de Cadarache, France, by firing Np metal in an oxygen atmosphere. A sample of the isostructural nonmagnetic compound ThO2, encapsulated in an identical container, was also measured in order to determine the phonon contribution to the neutron spectra.

Vol. 79, No. 2

The inelastic magnetic neutron scattering cross section is proportional to the imaginary part of the dynamic susceptibility Z"(Q, to):

S(Q, to) ,,~ f2(Q)

Z"(Q,/to) 1 -

exp

(1)

kBTJ

where Q is the scattering vector, hto is the energy transfer and f(Q) is the magnetic form factor. The poles of z"(Q, to) corresponding to CF excitations are observed in the neutron spectra as peaks with intensities proportional to the magnetic dipole transition probability between two CF states. The dependence of the intensity on the scattering vector Q is entirely given by the f2(Q) factor. The magnetic neutron scattering S(~b, to) from NpO2 at temperatures above and below the phase transition, with an incident energy of 60meV and scattering angle q~ = 5°, is shown in Fig. 1. The phonon scattering has been subtracted by scaling similar measurements on ThOz. At T = 37 K the signal consists of a broad component having the shape of a Lorentzian multiplied by the Bose thermal factor. Below the phase transition temperature, at T = 5 K, this broad signal is superimposed to a Gaussianshaped inelastic peak with the centre at 7.2 meV and a full width at half maximum of 2.4 meV, comparable to the instrument resolution at that energy transfer. The intensity of the broad component is almost independent both on scattering angle and temperature. A neutron polarization analysis experiment with a tripleaxis spectrometer is planned to check if this signal has a physical origin or if it is completely, or in part, due to an artefact introduced by the procedure adopted to subtract the phonon scattering. On the other hand, the difference between the spectra obtained at 5 and 37 K cannot be due to an artefact and reflects a change in the physical properties of the sample. This difference can hardly be attributed to a variation of the phonon density of states (PDS) induced by a structural phase transition. In fact, no temperature effects are visible in the spectra recorded at a scattering angle ~b = 136°, where only vibrational terms contribute to the scattering function (Q ~ 10A i,f2(Q) ~ 10 3). Moreover, the expected lattice distortion should be characterized by atomic displacements of the order of a few hundreds o f / ~ [7], as in the case of UO2 for which no appreciable changes of the PDS have been observed through the phase transition temperature. At larger scattering angle the intensity of the inelastic peak rapidly decreases, confirming its magnetic origin. This is shown in Fig. 2 where the integrated intensity of the inelastic peak estimated at different values of the scattering vector Q is compared with the square of the

Vol. 79, No. 2 I

I

199

EVIDENCE OF A L A T T I C E D I S T O R T I O N IN NpO2 BELOW 25 K I

(a)

I

I

I

I

I

I

I

I

I

I

I

T= ,5K

,

1'2

8

OE N

0"4

"~

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I lllJ I "-- I

"~

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I

I

~k.

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I TIIIIIttlTIIIITI ! I lqTl

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'

'

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(b)

I

~

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;

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' I

8

10

12

14

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T: 57 K

Fig. 2. Integrated intensity of the inelastic peak estimated at different values of the scattering vector Q (dots) compared with the square of the Np 4+ magnetic form factor f(Q) (solid line). The inset shows the magnetic signal obtained at Q values of 0.56/~-' (dots) and 1.97~ -~.

4 2

'

6

O (~-1)

E

O0

4

10

20

30

'h co ( m e V )

F i g . 1. I n e l a s t i c m a g n e t i c n e u t r o n

s c a t t e r i n g cross-

section obtained for NpO2 at (a) T = 5 K and (b) T = 37 K with an incident energy of 60meV and a full scattering angle of ff = 5 °. The phonon contribution has been removed by scaling similar measurements on ThO2. The solid line is a fit to the data using a lorentzian function multiplied by the Bose thermal factor at T = 37 K and the sum of a lorentzian and a gaussian function at T -- 5 K. The contribution to the intensity due to the inelastic peak appearing below the phase transition temperature is shown by the broken line. Np 4+ magnetic form factorf(Q), measured by neutron diffraction [12]. The inset of Fig. 2 shows, as an example, the magnetic signal obtained for Q values of 0.56 and 1.97/~ x. The agreement between the experimental data and the f2(Q) curve is only qualitative because of the difficulty in separating the contribution to the intensity coming from the Lorentzian and Gaussian line-shapes, so that we do not believe the observed differences to be significative. Therefore, the results support the assignment of the peak at 7.2 meV to a CF transition. The appearance of a magnetic excitation below T, = 25 K suggests a splitting of the 1"8(2)quartet due to an internal distortion of the oxygen-ligand cage surrounding the Np 4+ ions. If, as suggested in [15], we assume a monoclinic distortion corresponding to the condensation of the M5 optic phonon mode, the point symmetry at the Np site is lowered to C2h. In a refer-

ence frame with the z-axis parallel to the [001] crystallographic direction and the x-axis along the [1 1 0] direction, pointing toward the pair of oxygen ions which become closer under distortion (as it was assumed in the UO2 case [10]), the CF Hamiltonian can be written in the form HCF = //cubic Odist

//cubic + Hdist, =

~

84( 1~0 -

5044)

(2) +

B6(0°6 "Jff 2104),

2 ~2 + B~024 + B~O2 + B~6066, B202

(3) (4)

where 0 mare the Stevens operator equivalents and the parameters B~, are the coefficients of the CF potential. The omission of the other symmetry-allowed terms in equation (4) causes an error in the energy levels' position which is negligible for the small distortions considered here. Using the estimates of [13] for the cubic part of the Hamiltonian (2), namely W = - 1.74 meV and x = - 0 . 7 5 , we obtain B4 = 2.175 x 10-2meV and B6 = - 1 . 7 2 6 x l0 4meV. By calculating the distortion term with the nearest-neighbour point charge model in the Russell-Saunders coupling approximation, one obtains a splitting of the ground state quartet into two Kramers doublets separated by ~ 7 . 5 m e V for an oxygen displacement of A -0.022~. The transition probability between the two doublets is comparable in magnitude with that observed in [13] for the transition between the F~2) and the F8(~) quartets. Solt and Erd6s calculated in [15] that for the assumed distortion, the magnetic moment of the lowest doublet vanishes for a ratio between the B6 and B4 parameters very close to the one given by the values quoted above, provided that an anisotropy mechanism exists which forces the magnetic moment

200

EVIDENCE OF A LATTICE DISTORTION IN NpO2 BELOW 25 K

along the [0 0 1] axis. It has also been shown that more symmetrical distortions (tetragonal, trigonal or orthorhombic) cannot lead to a non-magnetic ground doublet [11]. In conclusion, our neutron spectroscopy results strongly support the hypothesis proposed by Solt and Erd6s [15] that the phase transition at 25K in NpO2 consists of a quadrupolar ordering driven by a collective Jahn-Teller distortion of the oxygen sublattice. As a result of the distortion, the CF ground state is split into two doublets the lower one being nearly non-magnetic because of the particular value of the CF potential. The amplitude of this splitting, is consistent with an oxygen displacement of the order of 0.02 A, which is below the present resolution limits of neutron diffraction experiments.

Acknowledgements - We are grateful to Dr G.H. Lander for many helpful discussions and for his continuous interest in this work. We should also like to acknowledge the assistance of Dr R.O.A. Hall in handling the sample. This work was supported in part by INFM and GNSM, Italy, and by the Underlying Research Programme of the United Kingdom Atomic Energy Authority (UKAEA). REFERENCES 1. 2. 3. 4.

D.W. Osborne & E.F. Westrum, Jr., J. Chem. Phys. 21, 1884 (1953). P. Erd6s, G. Solt, Z. Zolnierek, A. Blaise & J.M. Fournier, Physica 102B, 164 (1980). J.M. Friedt, F.J. Litterst & J. Rebizant, Phys. Rev. B32, 257 (1985). R. Caciuffo, G.H. Lander, J.C. Spirlet, J.M.

5. 6. 7. 8. 9.

10. I I. 12. 13.

14. 15. 16.

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Fournier & W.F. Kuhs, Solid State Commun. 64, 149 (1987) and references cited therein. A. Boeuf, R. Caciuffo, J.M. Fornier, L. Manes, J. Rebizant, J.C. Spirlet & A. Wright, Phys. Status Solidi 79, K1 (1983). A. Boeuf, J.M. Fournier, G. Heger, L. Manes, J. Rebizant, F. Rustichelli & J.C. Spirlet, J. Phys. (Paris) Lett. 42, L-401 (1981). J. Faber, Jr. & G.H. Lander, Phys. Rev. BI4, 1151 (1976). G.H. Lander, J. Faber, Jr., A.J. Freeman & J.P. Desclaux, Phys. R'e¢. B13, 1177 (1976). R. Osborn, A.D. Taylor, Z.A. Bowden, M.A. Hackett, W. Hayes, M.T. Hutchings, G. Amoretti, R. Caciuffo, A. Blaise & J.M. Fournier J. Phys. C." Solid State Phys. 21, L931 (1988). G. Amoretti, A. Blaise, R. Caciuffo, J.M. Fournier, M.T. Hutchings, R. Osborn & A.D. Taylor, Phys. Rev. !!40, 1856 (1989). Z. Zolnierek, G. Solt & P. Erdrs, J. Phys. Chem. Solids 42, 773 (1981). A. Delapalme, M. Forte, J.M. Fournier, J. Rebizant & J.C. Spirlet, Physica 102B, 171 (1980). J.M. Fournier, A. Blaise, G. Amoretti, R. Caciuffo, J. Larroque, M.T. Hutchings, R. Osborn & A.D. Taylor, Rutherford Appleton Laboratory Report No. RAL-89-135 (1989); Phys. Rev. !]43, 1142 (1991). S. Kern, J. Morris, C.-K. Loong, G. Goodman, G.H. Lander & B. Cort, J. Appl. Phys. 63, 3598 (1988). G. Solt & P. Erd6s, J. Magn. Magn. Mater. 15-18, 57 (1980). A.D. Taylor, B.C. Boland, Z.A. Bowden & T.J.L. Jones, Rutherford Appleton Laboratory Report No. RAL-87-012 (1987).