Mössbauer study of hyperfine interactions in NpAs at high pressure

148

Journal of Magnetmm and Magnetic Matermls 63 & 64 (1987) 148-150 North-Holland, Amsterdam

MOSSBAUER STUDY OF HYPERFINE INTERACTIONS IN NpAs AT HIGH PRESSURE** Ulnke POTZEL, J MOSER, W POTZEL, F J M BOGIe*, J C H A P P E R T * and J C S P I R L E T I

LITTERST, G M

KALVIUS,

Phystk-Department E15, Techmsche Unwers~taI Munchen, D-8046 Garchmg, Fed Rep Germany * CEN, F-38041 Grenoble, France t Europ lnsI for Transuramum Elements, D- 7500 Karlsruhe, Fed Rep Germany

Using the 60 keV Mossbauer transmon m 2~7Np we have investigated NpAs at pressures up to 85 kbar m the temperature range between 4 2 and 200 K At high pressures two magnetically spht subspectra are observed which suggest a longitudinal-wave modulation of the two ordered moments

1. Introduction In recent years the 5f electron structure of actmides has been a major goal of research, both for experimentalists and theorists [1] This is mainly due to the observation, that the 5f electrons can exhibit 3d-hke (bandllke) as well as 4f-hke (localized) character The change from locahzed to bandhke f electron structure IS particularly well reflected in the magnetic properties of actmide metals and mtermetalhc compounds It was first pointed out by Hdl [2] that one crucial parameter determining the magnetic properties of actlmde compounds ~s the actinlde-actlnide separation If It decreases below a critical value, the "Hill limit", the overlap and delocahzatlon of 5f electrons Is sharply increased and magnetic order can no longer be supported For neptunium the Hdl hmit is ~ 3 25 A d~rect way to change the actinide separation is the application of external pressure In recent years we have applied the high pressure techtuque in Mossbauer experiments to the cubic Laves phase compounds NpAI2 and NpOs2, with N p - N p separations fairly close to the Hill limit Rather drastic changes of the ordering temperature and the hyperfine parameters [3,4] have been found which supports the notion of rapid 5f delocahzatlon with reduced volume ** Work supported by the Bundesmmlsterlum fur Forschung und q echnologle

In this communication we report on high pressure Mossbauer experiments on NpAs It has the NaCl-structure with a lattice parameter of a ~ 5 84/~ at room temperature Its N p - N p spacing Is ~ 4 1 ~ and hence this c o m p o u n d lies far above the Hill hmlt For this reason NpAs ~s considered a c o m p o u n d with localized 5f electron structure A completely different response to external pressure is expected when compared to the Laves phases Already at ambient pressure the phase diagram of NpAs is fairly complex [5] At 4 2 K it is a type I antlferromagnet At 1~8 K a transition to a tetragonal phase occurs and a commensurate (4+, 4 - ) magnetic phase is formed At 168 K - but still m the tetragonal p h a s e - a n incommensurate magnetic phase is observed A b o v e 173 K the NaCl-structure ts found again w~th NpAs in the paramagnetlc state

2. Experimental details The high pressure Mossbauer spectrometer for the 6 0 k e V resonance In 237Np has been described in some detail in ref [3] However, some important modifications have been made The high pressure clamp is mounted m a separate stainless steel contamer which Is filled with 4He exchange gas at reduced pressure Thus the clamp can be heated by a heating cod to elevated temperatures whereas the source can always be kept at -~ 5 K The source is 24~Am metal evaporated onto a

0304-8853/87/$03 50 O Elsevier Science Publishers B V tNorth-Holland Physics Publishing Division)

149

U Potzel et al / Mossbauer studzes of hyperIfne mteracnons m NpAs

tantalum disk A source actlvlty of ~ 2 5 0 m C I resulted in countrates of ~75000/s in the 60 keV window after transmission through the absorber and the B4C anvils 3. Results and discussion Fig 1 shows Mossbauer absorption spectra of NpAs at 4 2 K and various pressures At ambient pressure (fig 1, top) a magnetic hyperfine spectrum is observed with a hyperfine field of Bhf 5 4 2 T This IS expected for a simple type I antlferromagnet The relatively high hyperfine field Bhf, the induced quadrupole interaction e 2 q Q - - - 2 7 0 mm/s as well as the isomer shift

100

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NpAs 998

996

100

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Bhf[~4 o ,[...

0

~ 998 E

t

520 5O0 480

'

~ 997 I.--

100

998

S = 15 4 m m / s (rel to NpAI2) clearly indicate a 5f 4 (Np ~+) configuration In fact, the numbers for Bh~ and for e 2 q Q are very close to the corresponding Np ~+ free ion values At high pressures rather drastic changes are observed Most Important Is that above 54 kbar and within the temperature range between 4 2 and ~ 1 0 0 K two subspectra with saturation hyperfine fields of t, hf°tl) ~ 503 T and o<2),.,hf~ 458 T are found The Mossbauer spectrum at 81 kbar and 4 2 K is also depicted in fig 1 If the pressure is reduced to 11 kbar and 4 2 K is also depicted in fig 1 If the pressure is reduced to I 1 kbar (fig l, bottom) only one magnetic hyperfine spectrum is found However, both the hyperfine splitting and the isomer shift are notlcably different when compared to the spectrum at ambient pressure The slightly curved baseline which appears in all spectra of fig 1 is caused by geometrical effects due to the rather large Doppler velocity required It has been taken into account in the fitting procedure and has no further consequences for the evaluatton of the data The regime between 11 and 54 kbar is under investigation at present The pressure dependences of the magnetic hyperfine field Bhf and of the isomer shift S are summarized in fig 2 The

'P~i~ 11kber

1

460 440 i

S 16 •i mm

996

;

I.. 8 6

Velocity [mm/sec] Fig 1 Mossbauerabsorption spectra of NpAs at 4 2 K and at various pressures The velocity scale refers to the source of 24~Am-metal

d

' 10' 8' Pressure [kbor]

' 8b

Fig 2 Pressure dependences of magnetic hyperfine field Bhf and of isomer shift S

150

U Potzel et al / Mossbauer studtes o/hyperfine mteractmns m N p A s

changes of S and Bhf are surpnsmgly large In addition, Bhf decreases when pressure ts applied and the same holds for the ordering temperature TN In table 1 the pressure coefficients of Bht, TN and S are c o m p a r e d wtth those of NpOs2 and NpAl2 which are representattves of more itinerant 5f electron behaviour, and with NpCo2Sl2 and DyAl2, which are considered typical for localized f-electron configurations T h e numbers given m table l are based on a hnear mterpolatlon with pressure From the data of table 1 we are forced to conclude that N p A s is situated m between the itinerant and the Iocahzed matertals It is most astontshmg that desptte tts locatton on the Hill plot and tts large hyperfine field N p A s does not belong to the localized group T h e appearance of two magnetic subspectra in the high pressure regime ts also puzzhng Although at present the evaluatton of our data ts only prehmlnary we tend to believe that above 54 kbar N p A s shows a stmllar behaviour as NpP m its tetragonal phase at ambtent pressure [6] At low temperature m the tetragonal phase NpP exhlbtts two magnetic sites with relative lntenSttles of 2 1 From Mossbauer and neutron dtffractton studtes it was concluded that the ordered magnettc m o m e n t s at the Np sttes form an almost squared-up longltudmal-wave moduTable I Comparison of the pressure coeffioents of the ordering temperature T,,ra, the magnetic hyperfine field Bhr, and the Isomer shift S m some f-electron c o m p o u n d s T h e change of S with pressure is n o r m a h z e d to the difference of the isomer shift between trwalent and tetravalent c o m p o u n d s Compound

NpOs2 NpA12 NpAs NpCo2SI 2 DyAlz

din T,,rd dp (Mbar t)

din Bhf dp (Mbar i)

1 AS IS TM - S~+[ Ap (Mbar t)

- 9 -11 - 17 + 22 +4 4

- 53 -30 - 07 + 0 005 +0 2

0 l 2 0 0

6 8 2 6 7

latton [6] T h e Mossbauer spectra at -~4 K of N p A s at high pressures and of NpP at ambient pressures look very slmxlar indeed This would mdtcate that N p A s at high pressures shows a crystallographtc phase transition from NaCI to a tetragonal structure In the tetragonal phase the (c/a)-ratio is probably very close to unity and hardly measurable even by htgh prectsion X-ray expertments T h e Mossbauer effect, however, can clearly detect thts transmon via the strongly d~fferent magnettc properties

4. Summary The high pressure data show that the 5f electrons of NpAs are not fully locahzed At low pressures (below 11 kbar) N p A s remains a type I anttferromagnet At high pressures (above 5 4 k b a r ) the expertmental results can be explamed by a crystallographtc phase transttton from NaCI to a tetragonal structure T h e ordered magnettc m o m e n t s at the Np sites suggest a longitudinal-wave modulation of a similar type as m the tetragonal phase of NpP at ambient pressure

References [1] H a n d b o o k of the Physics and Chemistry of the Actmldes, vol 2, eds A J F r e e m a n and G H Lander (Elsevier Science Publishers, A m s t e r d a m , 1985) [2] H H Hdl, m P l u t o n m m 1970 and other Actmldes, part I, ed W M Miner, Nuclear Metallurgy, vol 17 (Metallurgical Society A I M E , New York, 1970) p 2 [3] J Moser, J Gal, W Potzel, G W o r t m a n n , G M Kalvms, B D Dunlap, D J L a m and J C Splrlet, Phys~ca 102B (1980) 199 [4] J Moser, W Potzel, B D Dunlap G M Kalvlus J Gal, G W o r t m a n n , D J Lain, J C Splrlet and I Nowlk m Physics of Sohds under High Pressure, eds J S Schdhng and R N Shelton (North-Holland, A m s t e r d a m , 1981) p 271 [_5] P Burlet, S Quezel, M Kusmetz, D Bonmsseau, 1 Rossat-Mlgnod, J C Splrlet, J Reblzant and O Vogt, Proc of ' A c t m l d e s 85", A~x-en-Provence, France (1985) [6] A T Aldred, B D Dunlap, A R Harvey D J Ldm, G H Lander and M H Mueller, Phys Rev B9 (1974) 3766