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X-ray investigation of the lattice parameters of NpP at low temperature
Volume
44A, number
4
PHYSICS
X-RAY
INVESTIGATION
LETTERS
18 June
OF THE LATTICE
1973
PARAMETERS
OF NpP AT LOW TEMPERATURE* M.H. MUELLER, G.H. LANDER, H.W. KNOTT and J.F. REDDY Argonne National Laboratory, Argonne, Ill. 60439, Received
29 March
USA
1973
A low-temperature X-ray investigation of the anti-ferromagnetic compound NpP shows that the cubic unit cell becomes tetragonal when the magnetic structure becomes commensurate with the chemical lattice at 74°K. No anomaly in the lattice parameter is observed at the Nell temperature of 125°K.
The low-temperature measurement of lattice parameters of rare-earth monopnictides [l] has proved useful in understanding the magnetic interactions of these compounds. For example, both CeSb [2] and NdCl[3] are NaCl type and order antiferromagnetically near 20°K with ferromagnetic (001) sheets stackedalong the c-axis. The magnetic symmetry in the ordered state is tetragonal. At the ordering temperature both compounds exhibits a tetragonal distortion of the unit cell [ 11, such that c/a = 0.997. These distortions reflect the interaction between the exchange and crystal-field potentials. The distortion of the crystal lattice removes the degeneracy of the crystal-field eigenstates, and lowers the energy of the system. The Jahn-Teller distortion [e.g. 41 is the classic example of this process. A comparison between the light rare-earth monopnictides and the corresponding actinide compoimds suggests that strong similarities should exist. The magnetic structures of all the uranium monopnictides UN, UP, UAs, USb and URi [5] are similar to those described above for NdSb and CeSb. However, no tetragonal distortion is observed in either UN or UP at low temperatures [6]. In both cases a small volume expansion occurs below the ordering temperature, but this presumably arises from a negative magnetic Gruneisen parameter [7]. Recent X-ray experiments by the present authors indicate that in UAs, as in UN and UP, no tetragoanl distortion occurs below the ordering temperature of 127OK. The magnetic properties of the neptunium monopnictides, also with the NaCl crystal structure, have * Work performed under the auspices of the U.S. Atomic Energy
Commission.
recently been investigated in this laboratory [8,9] . NpP becomes antiferromagnetic at - 125°K with an incommensurate magnetic structure. The magnetic ordering is, however, still characterized by (001) ferromagnetic sheets, with the propagation direction parallel to [OOl] . At 74°K the magnetic structure becomes commensurate with the lattice with a stacking of three chemical unit cells along the c-axis. At low temperature the (001) ferromagnetic sheets are arranged in a + + + ~ - - sequence. The low-temperature X-ray experiments were performed on a portion of the NpP polycrystalline sample used in the neutron experiments [9]. The powder was die-pressed (7500 lbs/sq. in.) into a flat disc and placed in a milled recess in a copper sample holder. The disc was covered with a Mylar window (0.002” thick), sealed with a small copper frame and epoxy resin. The sample holder was attached with vacuum grease to a large copper block in the 5.621
,
,
,
,
/
,
,
,
,
,
561[iy.--560 t
I
TEMPERATURE.‘K
Fig. 1. Variation of the lattice parameter with temperature for NpP. The variation of c/a in the tetragoal region is also shown.
249
cryostat.
Heaters
and calibrated
platinum
and get-m:l-
nium resistors were embedded in the large block. Tempcraturc control to i0.2 K” was attained with an exchange
gas system.
on a cl)nvcntional ('(I K
cy
diffractometer
were made
table with I’ilteted
01. the lattice
with rempet-ature
is shown
ofNpP
parameters
in fig. I The I-OOIII telnpcn-
is 5.6 I48 k 7 A. Bc’twecn 2 150 alld
valut2
3NJ”E: the thermal and is independent
expansion
is 9.6 X 10~ 5 “K
’
is most pur.zling.
In
NpP the tetragonal
distortion
occurs
but rather
at 3 Io\ver tcmpcraturc
symmetry
not at the xekl temperature
( 125°K).
\vhcn the magnetic
of the actinidc’ and lanthanide
monopnictide:
suggests that direct ~omparisonx bctwern the propciticb 01‘ the two series may 1101 necessarily lead to ;I Jeepel u:~dcrst;mding
of temperature. This value ih with that obtained for the l-al-e-earth
comparable
pnictides
structure becomes COliillieiislIt-att with the lattice. The lack ~rt‘similarity between the l~~u’-lc‘l~i~~er:~tiIrelattiit‘
radiation.
The variation ture
hleasuremcnts
compounds are the rule gather than the exception. and their abscncc in the antit‘et-roniagnetic. uranium mtmc~-
of the actinide
compounds.
111. and the value of S. I X IO h “K ’
monvpnictides
for UP 161 in the same temperature 90 and 150°K the thermal
range. Bctwcen
expansion
is I-educed.
as in UP. no anomaly is observed at the Necl temperature (2 115°K). However. at 74.5 t 03°K we obscrvc a large tetragonal t10n experiments the magnctic
distortion in NpP. Ncutroll-dil‘fra~[(I] indicate that at this temperatui-c
structure
ofNpP
hecon~s
References
bum. [ I I I,. L.&y, Ph) s. liondens Rlater. IO ( 1969) X5. [ 71 II. 1Lcbech. P. I:ischer and B.1). Ksini‘ord, in Rare Earth\ and actmides, ( The Inrtitutc of Physics, London 19? 1) p. 204.
COiiilllelistll-;1ti’
Lvith the lattice. The c/a ratio (see insert of’ fig. I ) changes abruptly to O.W75 on cooling 5” below the transition.
and reaches
The volume of the unit cell (= a-)~,) show\
tcmpet-ature. a discontinuity perature. periments
a value of 0.995X ? 2 at helium
at 73°K and expands
below
this teni-
Indications from this and other I”] cxare that the 74°K tralktion is a f‘irst c)rde~
olle.
The present larticc distortion compound.
experiment in
Lattice
311
is the first IW~CW t ot‘ :I
alltiferromagnetic
distortions
actinide
in analogous
I-arc-cartli
[S 1 I:or a review SW; Xl. Kutnict\. in Rare earths and .Ictinides (The Institute of l’hyks. London 1971) p. 162. 161 J.A.C. 5larples, 1. l’hy\. (‘hem. Solid< 31 (1970) 2431. 17 I (;.I<. b\%ite, l’roc. Phps. Sot. (London) 86 ( 1965) 159 1x1 D.J. Lam. l3.D. Dulllap. A.T. Aldrcd and 1. Nowik. American institute 01‘ I’hy‘;ic\. Conf. I’roc. IO ( 1973 I lo lx published.
44A, number
4
PHYSICS
X-RAY
INVESTIGATION
LETTERS
18 June
OF THE LATTICE
1973
PARAMETERS
OF NpP AT LOW TEMPERATURE* M.H. MUELLER, G.H. LANDER, H.W. KNOTT and J.F. REDDY Argonne National Laboratory, Argonne, Ill. 60439, Received
29 March
USA
1973
A low-temperature X-ray investigation of the anti-ferromagnetic compound NpP shows that the cubic unit cell becomes tetragonal when the magnetic structure becomes commensurate with the chemical lattice at 74°K. No anomaly in the lattice parameter is observed at the Nell temperature of 125°K.
The low-temperature measurement of lattice parameters of rare-earth monopnictides [l] has proved useful in understanding the magnetic interactions of these compounds. For example, both CeSb [2] and NdCl[3] are NaCl type and order antiferromagnetically near 20°K with ferromagnetic (001) sheets stackedalong the c-axis. The magnetic symmetry in the ordered state is tetragonal. At the ordering temperature both compounds exhibits a tetragonal distortion of the unit cell [ 11, such that c/a = 0.997. These distortions reflect the interaction between the exchange and crystal-field potentials. The distortion of the crystal lattice removes the degeneracy of the crystal-field eigenstates, and lowers the energy of the system. The Jahn-Teller distortion [e.g. 41 is the classic example of this process. A comparison between the light rare-earth monopnictides and the corresponding actinide compoimds suggests that strong similarities should exist. The magnetic structures of all the uranium monopnictides UN, UP, UAs, USb and URi [5] are similar to those described above for NdSb and CeSb. However, no tetragonal distortion is observed in either UN or UP at low temperatures [6]. In both cases a small volume expansion occurs below the ordering temperature, but this presumably arises from a negative magnetic Gruneisen parameter [7]. Recent X-ray experiments by the present authors indicate that in UAs, as in UN and UP, no tetragoanl distortion occurs below the ordering temperature of 127OK. The magnetic properties of the neptunium monopnictides, also with the NaCl crystal structure, have * Work performed under the auspices of the U.S. Atomic Energy
Commission.
recently been investigated in this laboratory [8,9] . NpP becomes antiferromagnetic at - 125°K with an incommensurate magnetic structure. The magnetic ordering is, however, still characterized by (001) ferromagnetic sheets, with the propagation direction parallel to [OOl] . At 74°K the magnetic structure becomes commensurate with the lattice with a stacking of three chemical unit cells along the c-axis. At low temperature the (001) ferromagnetic sheets are arranged in a + + + ~ - - sequence. The low-temperature X-ray experiments were performed on a portion of the NpP polycrystalline sample used in the neutron experiments [9]. The powder was die-pressed (7500 lbs/sq. in.) into a flat disc and placed in a milled recess in a copper sample holder. The disc was covered with a Mylar window (0.002” thick), sealed with a small copper frame and epoxy resin. The sample holder was attached with vacuum grease to a large copper block in the 5.621
,
,
,
,
/
,
,
,
,
,
561[iy.--560 t
I
TEMPERATURE.‘K
Fig. 1. Variation of the lattice parameter with temperature for NpP. The variation of c/a in the tetragoal region is also shown.
249
cryostat.
Heaters
and calibrated
platinum
and get-m:l-
nium resistors were embedded in the large block. Tempcraturc control to i0.2 K” was attained with an exchange
gas system.
on a cl)nvcntional ('(I K
cy
diffractometer
were made
table with I’ilteted
01. the lattice
with rempet-ature
is shown
ofNpP
parameters
in fig. I The I-OOIII telnpcn-
is 5.6 I48 k 7 A. Bc’twecn 2 150 alld
valut2
3NJ”E: the thermal and is independent
expansion
is 9.6 X 10~ 5 “K
’
is most pur.zling.
In
NpP the tetragonal
distortion
occurs
but rather
at 3 Io\ver tcmpcraturc
symmetry
not at the xekl temperature
( 125°K).
\vhcn the magnetic
of the actinidc’ and lanthanide
monopnictide:
suggests that direct ~omparisonx bctwern the propciticb 01‘ the two series may 1101 necessarily lead to ;I Jeepel u:~dcrst;mding
of temperature. This value ih with that obtained for the l-al-e-earth
comparable
pnictides
structure becomes COliillieiislIt-att with the lattice. The lack ~rt‘similarity between the l~~u’-lc‘l~i~~er:~tiIrelattiit‘
radiation.
The variation ture
hleasuremcnts
compounds are the rule gather than the exception. and their abscncc in the antit‘et-roniagnetic. uranium mtmc~-
of the actinide
compounds.
111. and the value of S. I X IO h “K ’
monvpnictides
for UP 161 in the same temperature 90 and 150°K the thermal
range. Bctwcen
expansion
is I-educed.
as in UP. no anomaly is observed at the Necl temperature (2 115°K). However. at 74.5 t 03°K we obscrvc a large tetragonal t10n experiments the magnctic
distortion in NpP. Ncutroll-dil‘fra~[(I] indicate that at this temperatui-c
structure
ofNpP
hecon~s
References
bum. [ I I I,. L.&y, Ph) s. liondens Rlater. IO ( 1969) X5. [ 71 II. 1Lcbech. P. I:ischer and B.1). Ksini‘ord, in Rare Earth\ and actmides, ( The Inrtitutc of Physics, London 19? 1) p. 204.
COiiilllelistll-;1ti’
Lvith the lattice. The c/a ratio (see insert of’ fig. I ) changes abruptly to O.W75 on cooling 5” below the transition.
and reaches
The volume of the unit cell (= a-)~,) show\
tcmpet-ature. a discontinuity perature. periments
a value of 0.995X ? 2 at helium
at 73°K and expands
below
this teni-
Indications from this and other I”] cxare that the 74°K tralktion is a f‘irst c)rde~
olle.
The present larticc distortion compound.
experiment in
Lattice
311
is the first IW~CW t ot‘ :I
alltiferromagnetic
distortions
actinide
in analogous
I-arc-cartli
[S 1 I:or a review SW; Xl. Kutnict\. in Rare earths and .Ictinides (The Institute of l’hyks. London 1971) p. 162. 161 J.A.C. 5larples, 1. l’hy\. (‘hem. Solid< 31 (1970) 2431. 17 I (;.I<. b\%ite, l’roc. Phps. Sot. (London) 86 ( 1965) 159 1x1 D.J. Lam. l3.D. Dulllap. A.T. Aldrcd and 1. Nowik. American institute 01‘ I’hy‘;ic\. Conf. I’roc. IO ( 1973 I lo lx published.