吾思汝什么意思The nature of the first order isostructural transition in
GdRhSn
Sachin Gupta a ,K.G.Suresh a ,⇑,A.K.Nigam b ,Y.Mudryk c ,D.Paudyal c ,V.K.Pecharsky c ,d ,K.A.Gschneidner Jr.c ,d
a
Department of Physics,Indian Institute of Technology Bombay,Mumbai 400076,India b
Tata Institute of Fundamental Rearch,Homi Bhabha Road,Mumbai 400005,India c
Ames Laboratory,Iowa State University,Ames,IA 50011-3020,USA d
Department of Materials Science and Engineering,Iowa State University,Ames,IA 50011-2300,USA
a r t i c l e i n f o Article history:
Received 15May 2014
Received in revid form 3June 2014Accepted 4June 2014
Available online 16June 2014Keywords:
Rare earth intermetallic Iso-structural transition Magnetocaloric effect
a b s t r a c t
We prent structural,magnetic,thermal,magnetocaloric,and electrical transport properties of polycry
s-talline GdRhSn.Magnetization data show that it orders antiferromagnetically at T N =16.2K.The com-pound has the ZrNiAl type hexagonal crystal structure at room temperature and undergoes a first order iso-structural transition in the paramagnetic state at 245K.The unit cell volume change at the tran-sition is small (À0.07%)but discontinuous,in agreement with the first-order nature of the transition obrved by magnetic,transport,and heat capacity measurements.The anisotropic changes of the lattice parameters are D a /a =0.28%and D c /c =À0.64%on cooling.A substantial change in the 4f and conduction electron hybridization,giving ri to an incread integrated DOS,occurs when the high temperature pha transforms to the low temperature pha.A moderate magnetocaloric effect at T N (D S M =À6.5J/kg K and D T ad =4.5K for D H =50kOe)has been measured using both magnetization and heat capacity data.
Ó2014Elvier B.V.All rights rerved.
1.Introduction
For many decades,rare earth intermetallics have been attract-ing a lot of attention of rearchers due to their interesting physical properties associated with the localized 4f electrons of the rare earths.The partially filled 4f shell of the rare earth ions in the compounds has made them promisin
g materials for theoretical and experimental investigations [1].Many of the compounds show unusual structural,magnetic,thermal,and electrical properties.
面相疤痕图解
Ternary equiatomic rare earth stannides RTSn (R =rare earth element and T =transition metal)have been extensively studied due to their versatile low temperature magnetic and electrical properties [2–6].The compounds crystallize in veral crystal structures depending on the transition metal,and the most frequently occurring structures are the orthorhombic TiNiSi,hexagonal AlB 2,and hexagonal ZrNiAl-type structures [5–8].
Among the RTSn family,RRhSn shows interesting magnetic and related properties,which change with varying the rare earth ele-ments.Magnetic and other physical properties of compounds across this ries change considerably,even though all of the RRhSn compounds (R =La–Nd,Sm,Gd–Lu)are iso-structural with
the ZrNiAl type hexagonal crystal structure (space group P 62
m ).S
´lebarski [9]reported that at low temperature (T (T Kondo )CeRhSn shows non-Fermi liquid behavior,
while Kondo/intermediate valence behavior is obrved at high temperature.Recently,Zocco et al.[10]carried out high-pressure studies on CeRhSn and obrved the interplay of spin glass-like and non-Fermi liquid behavior.PrRhSn orders ferromagnetically below 3K with a strong uniaxial anisotropy (an anisotropy field $65T)[11,12].Mihalik et al.[13]obrved that NdRhSn has a ferromagnetic ground state below 7.6K,which is preceded by an incommensurate antiferro-magnetic state with a propagation vector (0,0,1/11)between 9.8and 7.6K.It was obrved that in both magnetic phas of NdRhSn the magnetic moments are locked along the c axis (the easy-mag-netization direction)[13].A transition from the paramagnetic to a ferromagnetic state was obrved at 14.9K in SmRhSn [14].In addition to the ferromagnetic–paramagnetic transition,another transition at 8.2K in a field of 100Oe was also obrved for SmRhSn,which was described as a spin reorientation transition [14].YbRhSn shows no magnetic ordering down to 2K in magnetic susceptibility data,however a decrea in resistivity data has been obrved below 1.8K which indicates the magnetic ordering below 1.8K [15].The resistivity of YbRhSn decreas on the application of magnetic field,which suggests that the low temperature behavior
dx.doi/10.1016/j.jallcom.2014.06.027
0925-8388/Ó2014Elvier B.V.All rights rerved.
⇑Corresponding author.
E-mail address:suresh@phy.iitb.ac.in (K.G.Suresh).
of the resistivity may be associated with the ont of
effect or/and with the manifestation of an interplay of
and Kondo like interactions[16].
Basic structural and magnetic properties of GdRhSn
reported earlier[3,17].Ła˛tka et al.[3]found the
temperature(h p)from DC magnetization data,and
magnetic susceptibility data they reported that there is
ence of the drivingfield frequency(f)orfield amplitude(
the real component of ac susceptibility,while imaginary
nent of ac susceptibility shows frequency and H ac Furthermore,they obrved non-vanishing anomalies in
third andfifth harmonics of magnetic susceptibility[3] obrvations suggest that the antiferromagnetic
GdRhSn in not simple collinear,as corroborated by119
155Gd Mössbauer data[3].
Structural,magnetic,electronic and Mössbauer
studies of RRhSn ries have been done by many authors[18–28].
Recently we have studied magnetic and related properties of RRhSn (R=Tb–Tm)ries[7].The ternary compounds show interesting magnetic,electrical,and magnetocaloric properties.All of the com-pounds in RRhSn(R=Tb–Tm)ries[7],except HoRhSn,are antifer-romagnetic.HoRhSn shows ferromagnetic ordering below6.2K. TbRhSn has the highest magnetic ordering temperature at T N=18.3K,while ErRhSn does not exhibit any magnetic ordering down to1.8K.Bad on a thorough literature review,it appears that none of the compounds in the RRhSn ries shows any sign of first-order structural transformation from room temperature down to very low temperatures($2K).However,
we found that GdRhSn has some differences compared to the other members of the RRhSn ries.
In this paper,we report the results of our studies on this com-pound in detail by means of temperature-dependent X-ray powder diffraction,magnetization,heat capacity,magnetocaloric,and elec-trical transport measurements.Thefirst-principles calculations have been performed to explain the nature of the obrved phenomena.
2.Experimental details
Polycrystalline sample of GdRhSn was synthesized by arc melting stoichiome-tric amounts of the constituent elements(with purity99.9%for Gd and99.99% for Rh and Sn)on a water cooled copper hearth with a titanium
atmosphere.The formed ingot wasflipped over and re-melted
ensure homogeneity.An as-cast sample was aled in a quartz tube
of10À6torr and annealed for a week at800°C to reach equilibrium The X-ray powder diffraction(XRPD)data for pha analysis were Panalytical X’PERT PRO powder diffractometer using Cu K a1
zation measurements,M(T)and M(H),were performed using a
Magnetometer(VSM)attached to a Physical Property Measurement
tum Design,PPMS-6500).Heat capacity measurements with and
also performed on the PPMS using the thermal relaxation technique.
resistivity was measured using a four probe technique on the
excitation current of150mA.
All temperature-dependent X-ray measurements were performed
Rigaku TTRAX diffractometer(Bragg–Brentano geometry,Mo K
equipped with a continuousflow helium cryostat,and a split-coil
magnet[29].The2h range of the measured Bragg angles for the
terns was from9°to49°.Temperature range of data collection was
The collected X-ray powder diffraction patterns were analyzed
refinement program LHPM Rietica[30].Thefinal profile residuals(
than11%,and the derived Bragg residuals(R B)were less than6%.
3.Experimental results
The XRPD pattern of GdRhSn measured at room
and analyzed using the Rietveld refinement is shown
has been confirmed that the compound prepared as
above is a single pha material,and it crystallizes in the
ZrNiAl type structure with the space group P 62m. parameters obtained from the refinement are a=c=3.8916(6)Å.The parameters are in good agreement with ear-lier reported values[3].
Fig.2shows the temperature dependence of dc magnetic sus-ceptibility obtained in an appliedfield of500Oe and over the tem-perature range2–300K in zerofield cooled(ZFC)andfield cooled (FC)mode
s.The magnetization shows a cusp at T N=16.2K indicat-ing antiferromagnetic ordering.Below T N there is a small upturn in thefield cooled susceptibility data,which may ari due to the reorientation of moments at low temperatures.The Neel tempera-ture obtained by us is clo to the value reported byŁa˛tka et al.[3] T N=16.0(1)K.
It is worth noting from Fig.2,that in addition to an upturn in the susceptibility data below T N when measured in FC mode,a clear step-like feature in the inver susceptibility is obrved near 245K.The magnetic susceptibility data between40and240K werefitted using modified Curie–Weiss law,ðvÀv0ÞÀ1¼ðTÀh pÞ=C m,where v0=0.00418emu/mol Oe is temperature inde-
pendent factor that aris due to diamagnetic and Van-Vleck con-tributions.The effective magnetic moment[p eff=(8C m)1/2] calculated from thefit is7.91l B/Gd3+,which is clo to the theoret-
ically calculated,p
th
¼g
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
JðJþ1Þ
p
;value for the Gd3+free ion (p th=7.94l B/Gd).The Weiss temperature(h p)calculated from the fit is13.9K.It is noted that the h p value is lower than T N.A similar behavior has also been obrved in other intermetallic compounds Room temperature X-ray powder diffraction pattern of GdRhSn.The line at the bottom shows difference between the experimental data points
calculated using crystallographic parameters obtained during the Rietveld refinement.The tick marks indicate the calculated positions of the Bragg reflections GdRhSn and the Miller indices are shown for the high intensity peaks only. interpretation of the references to color in thisfigure legend,the reader is referred web version of this article.)
Temperature dependence of dc magnetic susceptibility at500Oe(left-hand Temperature dependence of inver susceptibility along with the Curie–Weiss (right-hand axis).The int shows susceptibility plot at low temperatures.
S.Gupta et al./Journal
slightly lower value for p eff =7.56l B /Gd 3+and a larger value of h p =39.9K.This indicates that in the abnce of the structural transformation (e below)the T N of GdRhSn would have been higher than that of TbRhSn (T N =18.3K),in formal agreement with the de Gennes factor.It is noted that a small change in p eff value is likely statistically insignificant becau of much smaller number of data points in the high temperature region.While there is a nota-ble difference in the value of h p above and below the structural transition,the sign of h p is positive in both regions.
An interesting feature en in the inver susceptibility plot is the anomaly at about 245K.Similar anomaly was obrved in Er 5Si 4,where it is coupled with the structural transformation from one paramagnetic pha into another [33].Moreover,structural anomalies (step-like changes of the c /a ratio)are common for other intermetallics with the ZrNiAl type of crystal structure [8].There-fore,it was reasonable to assume that a previously unknown struc-tural transformation also occurs at 245K in GdRhSn.In order to investigate this anomaly,detailed temperature dependent XRPD measurements were performed on GdRhSn powder (particle size <25l m,mixed with GE varnish)at temperatures ranging from 7to 290K.A clor look at the Rietveld refinement of the room temperature XRPD data has confirmed that the sample con-tained at least 96%of the main pha.Weak peaks of the impurity phas,Gd 2O 3and an unidentified pha,possibly of the Cu 2Al-type structure,were obrved,indic
ating that a minor decomposition of the GdRhSn powder may have possibly occurred during grinding.
木马查杀In the XRPD pattern collected at 250K,the Bragg peaks of the low-temperature pha became visible,while the intensity of the Bragg reflections of the high-temperature pha decread.At 245,240K,and 235K pha coexistence was obrved and Bragg peaks of both high-temperature and low-temperature phas are simultaneously prent in the XRPD patterns (Fig.3).Interestingly,both the locations and intensities of (121)and (131)Bragg reflections are invariant to the transition as their positions and intensities do not change.Due to peak overlap at high Bragg angles,it was not possible to e if this behavior is universal for all (1k 1)reflections.Below 230K,only the peaks of the low-temperature pha can be clearly identified.The crystal structure of the low temperature pha remains practically the same as the structure of the high-temperature pha,and only a substantial change of the c /a ratio occurs at the first-order transition without a change of symmetry.Thus,the low-temperature pha also belongs to the ZrNiAl type.The rever transformation was obrved when
the sample was heated up to room temperature with $5K thermal hysteresis.The unit cell volume change at the transition is small (À0.07%)but discontinuous,supporting the first-order type of the tran
sition.The anisotropic changes of the lattice parameters are much larger:D a /a =0.28%and D c /c =À0.64%(e Fig.4).When the sample was cooled down to 7K,no additional anomalies were obrved.A minor upturn in the value of the c parameter that reflects spontaneous magnetostriction occurs below the magnetic ordering temperature,but the overall variation of the unit cell vol-ume with temperature follows a conventional thermal contraction.Fig.5(a)displays the field dependence of magnetization at different temperatures for fields up to 50kOe.Below T N ,the mag-netization increas nonlinearly with increasing temperature up to a field of 10kOe and after that the M vs.H dependence becomes linear due to a change in the alignment of spins in high magnetic fields.As the temperature ris above the transition temperature,all of the magnetic isotherms become progressively more linear as expected for the paramagnetic region.It is also worth noting from Fig.5(a)that there is no tendency toward saturation of mag-netization even at the highest measured field.All of the Arrott plots between 3and 21K show a positive slope (e Fig.5(b)),thereby implying the cond order nature of the magnetic pha transition [34].
In order to obtain a further insight into the magnetic nature of GdRhSn,heat capacity measurements were carried out at zero field and at 50kOe from 2to 300K,Fig.6(a).As en in Fig.6(a)the zero field heat capacity data show three anomalies:the first anomaly is en near 15K,which is due to antiferro
magnetic pha transition;the cond anomaly is in the form of a broad hump at low temper-atures (e the int of Fig.6(a))and is attributed to the spin reori-entation,which is also en in dc magnetic susceptibility data.The third anomaly is the peak at 244K,which is due to the first order iso-structural transition.With the application of magnetic field,the peak associated with the magnetic order–disorder transition is suppresd and shifts towards lower temperatures,confirming the antiferromagnetic nature of this compound.The k -shaped peak at the ont of magnetic ordering in zero field heat capacity data confirms the cond order magnetic pha transition in GdRhSn.The value of the total heat capacity near room temperature reaches the Dulong–Petit ,C =3nR =74.8J/mol K for n =3atoms/f.u.,where R is the gas constant.蓝色星球第一季
We also note that the temperature of the heat capacity anomaly related to the structural transition at 244K is clearly suppresd by the magnetic field despite the material being purely paramagnetic.
Bragg angle, 2θ(deg)
Bragg angle, 2θ(deg)
(a)T e m p e r a t u r e , T (K )
(b)
Temperature evolution of the XRPD pattern in GdRhSn across the first-order transition on cooling and (b)temperature-dependent contour map of the 282S.Gupta et al./Journal of Alloys and Compounds 613(2014)280–287
A similar effect has been obrved in Er5Si4[35]which has been explained by the extreme pressure dependence of the structural-only transition[36,37].Even in the paramagnetic state,strong magneticfield may induce weak but measurable magnetostriction, and therefore,relatedfield-induced strain has a measurable effect on the structural-only transformation in the paramagnetic state. The mechanism of thefield-induced magnetostriction in GdRhSn is different from Er5Si4simply becau of the spherical symmetry of the4f charge density of Gd3+ion compared to the aspherical one for Er3+.Pressure dependent studies to further understand this unusual behavior are underway and will be reported parately.
It is well known that the total heat capacity in a metallic mag-netic material can be expresd as the sum of independent elec-tronic,lattice(phonon),and magnetic contributions.CðTÞ¼C elðTÞþC phðTÞþC magðTÞð1Þwhere C el is the electronic part,C ph is phonon contribution,and C mag is the magnetic contribution to the total heat capacity of a compound.
C elðTÞþC phðTÞ¼c Tþ9NRðT=h DÞ3
Z h D=T
x4e x
ðe xÀ1Þ
dxð2Þ红曲米怎么吃
Here,c is electronic coefficient of the heat capacity,N=3is number of atoms per formula unit,h D is Debye temperature,and R=8.31J/mol-K is the molar gas constant.The Debye model was ud tofit the non-magnetic ,lattice+electronic)of total heat capacity data as shown by solid line in Fig.6(b).The value of c and h D is estimated by thefit of Eq.(2)to the heat capacity data and are
Magnetization isotherms at lected temperatures forfields up to50kOe and(b)Arrott plots at different temperatures
S.Gupta et al./Journal of Alloys and Compounds613(2014)280–287283
c =5.85mJ/mol-K 2,h D =230K.The values of c an
d h D ar
e in the
same range as obrved for other compounds of RRhSn ries [7].The magnetic heat capacity (magnetic plus elastic at high temperature transition,e below)has been estimated by subtract-ing non-magnetic contributions from total heat capacity data and is shown in Fig.6(c).Since neither LaRhSn data nor the C el+ph (T )curve have a first-order peak at 245K,the peak obrved at the high tem-perature transition in magnetic heat capacity is purely structural.The magnetic entropy was calculated by the equation S m ¼R T
0C m dT from the magnetic heat capacity data (e Fig.6(c))and utilizing the linear extrapolation between the lowest temper-ature of the measurement (2K)and 0K.The magnetic entropy estimated from magnetic heat capacity data below iso-structural transition is 17.9J/mol-K and is clo to the expected value R ln(2J +1)for Gd 3+.The excess in the magnetic entropy around 245K is due to the iso-structural transition.
The magnetocaloric effect (MCE)was calculated in terms of both isothermal magnetic entropy change (D S M )and adiabatic temperature change (D T ad )using the C –H –T data.The D S M and D T ad for GdRhSn compound have been calculated from the heat capacity data,in the usual manner,[38]utilizing the following thermodynamic relations:
D S M ðT ;H Þ¼
Z
T
C ðT 0;H ÞÀC ðT 0;0ÞT 0红酒小知识
dT
ð3ÞD T ad ðT ÞD H ffi½T ðS Þf ÀT ðS ÞH i S
ð4Þ
The D S M estimated from M –H –T data using Maxwell’s relation:
D S M ¼
Z
H
户外活动教案大班@M散文阅读
@T H
dH ð5Þ
The temperature dependence of D S M and D T ad for field change of 50kOe are plotted in Fig.7(a)and (b),respectively.The maxi-mum values of D S M and D T ad are 6.5J/kg K and 4.5K for the field change of 50kOe,respectively.The MCE calculated from the
M –H –T data is shown in Fig.7(c).The MCE calculated from both C –H –T and M –H –T data show positive entropy change (negative MCE)near 10K,which is due to spin reorientation as en in mag-netization data.
The electrical resistivity measurement of GdRhSn in the tem-perature range 2–300K in zero field is shown in Fig.8.The resistiv-ity shows metallic behavior over the entire temperature range.The sharp drop below 16K occurs due to the magnetic pha transition and the conquent reduction in spin disorder scattering.The resistivity also falls sharply near 250K,which coincides with the structural transition at the same temperature.
4.Theoretical investigations
In order to understand the underlying first order pha transfor-mation in the nonmagnetic regime of GdRhSn,we have carried out local density approximation (including the Hubbard U ),LDA +U [39],calculations in conjunction with the tight-binding linear muffin tin orbital and full potential linear augmented plane wave (FP-LAPW)band structure methods [40,41].The conventional von Barth and Hedin exchange correlation potentials have been ud.The k -space integration has been performed with 32Â32Â32Brillouin zone mesh.
Total energy calculations as a function of ratio of lattice con-stants show a global and a local energy minimum indicating a pos-sibility of first order pha transformation (Fig.9(a)).The global total energy minimum corresponds to c /a =0.501(experimentally the ground state structure has c /a =$0.507),and the local mini-mum falls between c /a =0.506and c /a =0.508(experimentally,the high temperature structure has c /a =0.512,e Fig.4(d)).Even though the absolute values of the computed c /a ratios correspond-ing to the two minima are slightly lower than the experimentally obrved values,the predicted trend matches the experiment.The 4f states shift rigidly towards higher energy upon the transition from the high temperature (HT)pha to the
low
dependence of heat capacity at zero and 50kOe magnetic field.The int shows low temperature heat capacity data LaRhSn,the solid line shows the fit of Eq.(2).(c)Temperature dependence of the magnetic plus elastic heat capacity
