Spin-phonon coupling probed by infrared transmission spectroscopy in the double perovskite Ba$_2$YMoO$_6$

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by  Zhe Qu

Spin-phonon coupling probed by infrared transmission spectroscopy in the
double perovskite Ba YMoO
2 6
Zhe Qu,1,a) Youming Zou,1 Shile Zhang,1 Langsheng Ling,1 Lei Zhang,1 and Yuheng Zhang1,2
1)High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui, 230031,
China
2)High Magnetic Field Laboratory, University of Science and Technology of China, Hefei 230026,
China
Inthiswork,wereportastudyontheinfraredtransmissionspectroscopyofthedoubleperovskiteBa YMoO .
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At 300K,three bands are observedat∼ 255.1cm−1, ∼ 343.4cm−1, and∼ 561.5cm−1, which arerelatedto
the motion between the cation Ba2+ and the anion YMO−2, the Y-O stretching motion and the stretching
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3
vibration of the MoO octahedron, respectively. These modes continue to harden upon cooling owing to the
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1
shrink of the lattice constant. When the temperature decreases to T ≤ 130 K around which the spin singlet
0 dimer begins to form, an additionalphonon mode appears at ∼ 611cm−1, suggesting the occurrence of local
2
distortion of MoO6 octahedra. With further decrease of the temperature, its intensity enhances and its peak
n
positionkeepsunchanged. Theseresultsindicatethattheformationofthespinsingletdimersisaccompanied
a
with the occurrence of the local structure distortion of MoO octahedra, providing evidence for the strong
J 6
spin-phonon coupling in the double perovskite Ba YMoO .
3 2 6

] PACS numbers: 78.30.-j,75.10.Jm,63.20.-e
l
e
-
r Geometry frustrated magnetic materials have at- to d1 electronic configuration of Mo5+ ions, Ba YMoO
t 2 6
s tracted widespread attention in recent decades because maintains cubic symmetry down to 2 K and no static
.
t of the discovery of exotic ground states such as spin structure distortion was observed by neutron diffraction
a
m glass, spin ice, and spin liquid.1–4 Lattices based on tri- down to 2 K.14,16
angular or tetrahedral architectures usually show geom- It is known that the coupling between spin and lat-
-
d etry frustration. This includes the B-site ordered dou- tice degrees of freedom usually plays an important role
′
n ble perovskites with general formula A2BBO6. In these in geometry frustrated materials. Infrared spectroscopy
o materials, the magnetic ions occupy the B′ site while is very sensitive tool to probe the spin-phonon coupling.
[c alkaline-earthsandlanthanidesresideontheBsite,form- Strongspin-phononinteractionsareexpectedtomanifest
ingtwointerpenetratinglatticeswithface-centered-cubic themselvesaseitheranomalousfrequencyshiftsoroccur-

1 (fcc) symmetry. Since the magnetic moments form an rence of new phonon peaks with the magnetic transition
v edge-sharing tetrahedra network, the magnetism should in infrared spectra.17–21
3
be geometric frustrated. Considerate efforts have been
8 In this work, we measured the infrared transmission
devoted to experimentally investigate the magnetism of
3 spectra of Ba YMoO to explore the spin-phonon cou-
these double perovskites and a wealth of ground states 2 6
0 pling in this compound. At 300 K, we observed three
. have been revealed.5–12
1 bands corresponding to the motion between the cation
0 Here we focus on the double perovskite Ba2YMoO6, Ba2+ and the anionYMO−2, the Y-O stretching motion
which is an example of the extreme s = 1/2 case and 6
3 and the stretching vibration of the MoO octahedron,
has an ideal cubic double perovskite structure at room 6
1 respectively. Thesemodescontinuetohardenuponcool-
: temperature. While the Curie-Weiss temperature deter-
v ingowingtotheshrinkofthelatticeconstant. Whenthe
mined from bulk susceptibility measurements suggests
i temperature decreases to T ≤ 130 K around which the
X strong antiferromagnetic (AFM) exchange interaction,
spin singlet dimer begins to form, an additional phonon
r no sign of static long-range magnetic order is observed mode appears at ∼ 611cm−1, suggesting the occurrence
a above2K.13,14 Itisfoundthatwithdecreasingtempera-
of local lattice distortion of MoO octahedra. With fur-
turethespinsofadjacents=1/2Mo5+ graduallyfreezes 6
ther decrease of the temperature, the intensity of this
into a disordered pattern of spin singlets without signa-
phonon mode enhances and its peak position keeps al-
tures of a phase transition, resulting in an exotic ground
most unchanged. These results indicate that the forma-
state with coexisting paramagnetism and collective spin
tion of the spin singlet dimers is accompanied with the
singlets.13–15 Above ∼ 125 K, a paramagnetic state is
occurrence of the local structure distortion of MoO oc-
recoveredin the system, without obvious signatures of a 6
tahedra, providing evidence for the strong spin-phonon
phasetransition.13,15Previousresultsshowthatalthough
coupling in the double perovskite Ba YMoO .
Jahn-Teller distortion could be expected to occur owing 2 6
Polycrystalline Ba YMoO was prepared using the
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conventional solid-state reaction method.13,14 Stoichio-
metric proportions of high purity Y O , MoO and
2 3 3
a)Authortowhomcorrespondenceshouldbeaddressed. Electronic BaCO powders were mixed and heated at 900 oC for
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mail: zhequ@hmfl.ac.cn 12 hours. They are then pelletized, and then sintered at
2

using a Bruker v80v FTIR spectrometer equipped with
a Janis continuous flow cryostat (model ST-100-FTIR).
I Figure 1 displays the powder XRD pattern
obs of Ba YMoO at room temperature. Rietveld
units) bIccalkcgr rthefienseamm2epnlte2i2s,2s3i6ngolfepthhaeseXwRitDhapcautbtiecrnstruccotnufirrem(sFmth3amt
b. diff space group). The lattice parameter is determined to be
r
(a a = 8.3853 ˚A, which agrees well with previous reports
sity within the experimental error.13,14
en Figure2showsthetemperaturedependenceofthesus-
nt ceptibility χ(T) and its reciprocal. It can be seen that
I
there are two regions that follow the Curie-Weiss law.
The Curie-Weiss fitting for the susceptibility above 150
K yields a Curie-Weiss temperature of -172 K, indicat-
20 40 60 80 ing the strong AFM exchange. The effective moment
2q (degree) is determined to be 1.52 µB, which is smaller than the
spin only value for S = 1/2 due to the strong quan-
tum fluctuations10,13. Below ∼ 25 K, there is another
FIG.1. (Color online) Powder XRDpatternsof Ba2YMoO6. linear regime in χ−1. The Curie-Weiss fitting in this
ThesolidcurveisthebestfitfromtheRietveldrefinementus-
region shows a Curie-Weiss temperature of -2.2 K sug-
ing GSAS, with Rp = 5.97% and Rwp = 8.73%. The vertical
gesting the weak AFM interaction. The effective mo-
marks indicate the position of Bragg peaks and the bottom
curves show the difference between the observed and calcu- ment is found to be 0.57 µB, which corresponds to ∼
lated intensities.
1.0
o
1200-1250 C underflowing5%H2/Argas. Thestruc- (a) 10K
ture and the phase purity of the samples were checked 0.8 30K
by powder X-ray diffraction (XRD) at room tempera- 50K
taucroe.mMmaergcnieatlizsuatpieorncomnedauscutrienmgeqnutasnwtuemre ipnetrefrofremreendcewditeh- bance0.6 7900KK
r 110K
vice (SQUID) magnetometer (Quantum Design MPMS so0.4 130K
7T-XL).Theinfraredtransmissionspectrawerecollected Ab 150K
200K
0.2 250K

300K
0.0
200 300 400 500 600
10 c(T) under 1 T Wave number (cm-1)
Curie-Weiss fitting 1.5
10K (b) (c)
Oe) 8 Oe) 30K 1.0
-3c (10 emu/mole 246 -3c (10 emu/mole Oe)0000....02460 50 100T1 (5 K0)200250300 01..50-13c (10 emu/mole Absorbance 211175905310000000KKKKKKK 00..05II(T)/(10K)
0 580 wa6v0e0 numbe6r 2(0cm-1) 6400 100T (K2)00 300
0.0
0 50 100 150 200 250 300
T (K)
FIG. 3. (Color online) (a) The infrared transmission spectra
measured at various temperatures for Ba YMoO plotted as
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FIG.2. (Color online)Thesusceptibilityχanditsreciprocal theabsorbanceversusthewavenumber. Thedashedlinesare
χ−1 measuredunder1Tasfunctionofthetemperature. Red guide for the eyes. The arrow marks the additional phonon
lines represent the Curie-Weiss fitting for the susceptibility mode which appears below ∼ 130 K. Shown in (b) is the
in the two linear regions in χ−1. Shown in the inset is the enlarged view of the new phonon mode. The data havebeen
susceptibility after the substraction of the low temperature shifted for clarification. The temperature dependence of the
Curie tail. normalized intensity of this phonon mode is shown in (c).
3
10% fraction of all the S = 1/2 moments. The inset is strong spin-phonon coupling in the double perovskite
to Fig. 2 exhibits the temperature dependence of the Ba YMoO .
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susceptibility after the substraction of the low tempera- In summary, we have conducted an infrared trans-
tureCurietail. Itdisplaysagap-likefeaturewithapeak mission spectroscopy study in the double perovskite
at ∼ 130 K, which is related to the spin-gap opening Ba YMoO to investigate its spin-phonon coupling.
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at this temperature.14,15 All these results are consistent Three phonon bands have been identified at 300 K. We
with previousreports,13,14,16 confirmingthat our sample find that an additional phonon mode appears at ∼ 611
is of high quality. cm−1 when the temperature decreases to T ≤ 130 K
aroundwhichthe spin singletdimer begins to form,sug-
Wefurthermeasuredtheinfraredtransmissionspectra gesting the occurrence of local distortion of MoO octa-
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ofBa2YMoO6atvarioustemperaturetoprobethelattice hedra. With further decrease of the temperature, its in-
distortion in this compound. The results are shown in tensity enhances and its peak position keeps unchanged.
Fig. 3(a). Therearethreewelldefinedabsorptionbands Ourresultsindicatethattheformationofthespinsinglet
associated with the lattice vibrational modes (phonons) dimers is accompanied with the occurrence of the local
having the typical profile expected for an insulating ma- structure distortion of MoO octahedra, providing evi-
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terial. At300K,thesestrongphononmodesareobserved dence for the strong spin-phonon coupling in the double
near ∼ 255.1 cm−1, ∼ 343.4 cm−1, and ∼ 561.5 cm−1, perovskite Ba YMoO .
2 6
respectively. The phonon band centered at about 255.1
cm−1 shouldberelatedtothemotionbetweenthecation
Ba2+ andthe anionYMO−2 similarto otherdouble per- ACKNOWLEDGMENTS
6
ovskite compounds.24 The phonon band around ∼ 343.4
cm−1 canprobablyrelatedtotheY-Ostretchingmotion,
We thank Drs. Zhiquan Jiang, Ranran Zhang, Jun
asthatobservedinBa YNbO .25Thestronghigh-energy
2 6 Fang, and Wei Tong for helpful discussions. This work
phononbandlocatednear561.5cm−1 shouldbeassigned
is supported by National Natural Science Foundation of
tothestretchingvibrationmodeoftheMoO octahedron
6 China under contracts Nos. 11004198 and 11174291.
due to the higher charge of this cation.25
Z. Q. gratefully acknowledges supports from the Youth
Innovation Promotion Association, Chinese Academy of
With decreasing temperature, one can see that all
Sciences.
these three bands aredisplaced towardhigher energydi-
rection, implying an enhancement of the bond strength. 1A.P.Ramirez,Ann.Rev.Mater.Sci.24,453(1994).
This is in agreementwith the slight shrinkage of the lat- 2J.E.Greedan, J.Mater.Chem.11,37(2001).
tice constant upon cooling in Ba YMoO .14,16 More im- 3Introduction to Frustrated Magnetism, edited by C. Lacroix, P.
2 6 Mendels,andF.Mila(Springer,Heidelberg,2011).
portantly, as shown in Fig. 3 (b), an additional phonon 4L.Balents,Nature464,199(2010).
mode occurs at ∼ 611 cm−1 when the temperature de- 5C. R. Wiebe, J. E. Greedan, G. M. Luke, and J. S. Gardner,
creases to T ≤ 130 K. This fact suggests that a new Phys.Rev.B65,144413(2002).
stretching vibration mode of the MoO octahedron ap- 6C.R.Wiebe,et al.Phys.Rev.B68,134410(2003).
pears below ∼ 130 K, hinting that a lo6cal distortion of 7T.Aharen,et al.Phys.Rev.B81,064436(2010).
8T.Aharen,et al.Phys.Rev.B80,134423(2009).
MoO6 octahedron should occur. Since the occurrence 9S. J. Makowski, J. A. Rodgers, P. F. Henry, J. P. Attfield and
temperature of this mode corresponds to the peak tem- J.-W.G.Bos,Chem.Mater.21,264(2009).
perature in χ(T) curve around which the spin-singlet 10A.S.Erickson,et al.Phys.Rev.Lett. 99,016404 (2007).
pairs begin to form and the spin gap opens,14,15 the 11E. J. Cussen, D. R. Lynham, and J. Rogers, Chem. Mater. 18,
2855(2006).
occurrence of such phonon peak suggests that there is
12A.C.Mclaughlin,Phys.Rev.B78,132404(2008).
strong spin-phonon coupling in Ba2YMoO6. This ar- 13M.A.deVries,A.C.Mclaughlin,andJ.-W.G.Bos,Phys.Rev.
gument is further supported by the temperature depen- Lett.104,177202 (2010).
denceofthisadditionalphononpeak. Itcanbeseenthat 14T.Aharen,et al.Phys.Rev.B81,224409(2010).
its peak position keeps almost unchanged with further 15J.P.Carlo,et al.Phys.Rev.B84,100404(2011).
16A.C.Mclaughlin,M.A.deVries,andJ.-W.G.Bos,Phys.Rev.
decreaseofthe temperature,hinting thatthe strengthof B82,094424(2010).
thelocallatticedistortiondoesnotchange. Ontheother 17A.B.Kuz’menko,et al.Phys.Rev.B63,094303(2001).
hand, the intensity of this phonon mode enhances upon 18A.B.Sushkov,O.Tchernyshyov,W.Ratcliff,S.W.Cheong,and
coolingandapproachessaturationbelow∼30K(seeFig. H.D.Drew,Phys.Rev.Lett.94,137202(2005).
19G.Caimi,L.Degiorgi,H.Berger,andL.Forro´,Europhys.Lett.
3 (c)), which means that more and more MoO octahe-
6 75,496(2006)
drabecomedistorted. Thisresultscanbeunderstoodby 20J.Kim,et al.Phys.Rev.B74,052406(2006).
consideringthemagneticpropertiesofBa2YMoO6; since 21C.C.Homes,et al.Phys.Rev.B79,125101 (2009).
increasingnumbersofspinsingletsareformeduponcool- 22A.C.LarsonandR.B.VonDreele,General Structure Analysis
ing, the local lattice distortion is expected to occur in System(GSAS)(LosAlamosNationalLaboratoryReportLAUR
86-748,2000)
increasing number of MoO octahedra. All these results
6 23B.H.Toby,J.Appl.Cryst.34,2001(210).
indicate that the formation of the spin singlet dimers 24F.P.delaCruz,N.E.Massa,J.A.Alonso,M.J.Mart´ınez-Lope,
is accompanied with the occurrence of the local struc- andM.T.Casais,SolidStateCommun.127,703(2003).
turedistortionofMoO octahedra,suggestingthatthere 25A.E.Lavata, andE.J.Baran,Vib.Spec.32,167(2003)
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