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Physica B 378–380 (2006) 332–333

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Elastic softening mode change in Y1�xCaxTiO3 by Ca-doping

Takashi Suzukia,�, Shinya Moritaa, Haruhiro Higakia, Isao Ishiia, Masaki Takemuraa,Fumitoshi Igaa, Toshiro Takabatakea, Masami Tsubotab

aDepartment of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, JapanbJAERI/SPring-8, Hyogo 679-5198, Japan

Abstract

We measured various elastic moduli of single-crystalline Y1�xCaxTiO3 (x ¼ 0, and 0.2) as a function of temperature by means of an

ultrasonic technique. Remarkable elastic softening has been found in transverse C44 for the sample with x ¼ 0 around 60K, indicating

structural symmetry lowering corresponding to the eyz strain. In contrast, the sample with x ¼ 0:2 shows large softening in C55 in

response to ezx around 130K. These results reveal that Y1�xCaxTiO3 (x ¼ 0, and 0.2) undergo a structural transition and the crystal

symmetry of the low temperature phase is changed by Ca-doping.

r 2006 Elsevier B.V. All rights reserved.

PACS: 62.65.+k; 71.30.+h; 71.27.+a; 61.50.Ks

Keywords: Elastic softening; Structural transition; Ultrasonic measurement; Mott insulator; Orbital ordering

Among various Mott insulators, YTiO3 is a rareexample, which undergoes the ferromagnetic transi-tion at TC ¼ 30K [1]. YTiO3 and the Ca doped alloysY1�xCaxTiO3 have received much attention because ofhaving Jahn–Teller-type distortion in connection with anantiferro-orbital order. Recently, Iga et al. determined theorbital polarization in YTiO3 at room temperature usingsoft X-ray linear dichroism [2]. Since the TiO6 octahedronsare elongated along the z-direction (with a small deforma-tion in the xy plane) due to the Jahn–Teller distortion, thetriply degenerate t2g states split into an excited jxyi stateand nearly degenerate ground states consisting of jyzi andjzxi orbits. One can expect a structural change in such acompound with the orbital degeneracy because a strain cancouple to the orbits mediated by the Jahn–Teller effect orcoupling between multipoles of orbitals. In fact, thestructural transition from the GdFeO3-type orthorhombicphase (space group: Pbnm) to a monoclinic phase (spacegroup: P21=n) was found in Y1�xCaxTiO3 with x ’ 0:40 at230K in the vicinity of an insulator-metal transition [3].For YTiO3, we recently reported preliminary data of elastic

front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2006.01.121

ng author. Tel.: +8182 424 7040; fax: +81 82 424 7044.

ss: tsuzuki@hiroshima-u.ac.jp (T. Suzuki).

stiffness C44 indicating a lattice anomaly in a temperaturerange higher than TC [4].In order to investigate the structural change correlated to

the orbital ordering in single-crystalline Y1�xCaxTiO3

(x ¼ 0 and 0.2), we measured various elastic moduli as afunction of temperature T since the elastic moduli aresensitive quantity to the structural transition and theorbital ordering [5].The single-crystalline samples were grown by a floating-

zone method using an image furnace with four halogenlamps under a reductive atmosphere of Ar and H2 mixture[6]. Powder X-ray diffraction analysis confirmed that thesamples are crystallized into a single phase with theGdFeO3-type orthorhombic symmetry.Temperature dependence of sound velocity vðTÞ was

measured by an ultrasonic phase-comparison type pulse-echo technique. The ultrasound frequency was about10MHz. The sizes of samples were 2:697� 3:433�3:024mm3 for the sample with x ¼ 0 and 2:685� 2:606�3:136mm3 for x ¼ 0:2. The magnitude of elastic moduli Cii

was calculated using the relation C ¼ rv2, where r(¼ 5:341 g=cm3 for x ¼ 0 and 5:035 g=cm3 for x ¼ 0:2 atroom temperature) is the mass density. We measured all ofsix independent stiffness moduli in the orthorhombic

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52.0

51.5

51.0

50.5

C44

/ G

Pa

300250200150100500T / K

YTiO3

Fig. 1. Temperature dependence of C44 for YTiO3.

70

68

66

64

62

C55

/ G

Pa

300250200150100500T / K

Y0.8Ca0.2TiO3

Fig. 2. Temperature dependence of C55 for Y0:8Ca0:2TiO3.

T. Suzuki et al. / Physica B 378–380 (2006) 332–333 333

symmetry: longitudinal C11, C22 and C33, and transverseC44, C55 and C66.

Figs. 1 and 2 represent temperature dependencies of C44

for the sample with x ¼ 0 and of C55 for x ¼ 0:2,respectively. Each modulus shows substantial softening

around 60K for x ¼ 0 or around 130K for x ¼ 0:2,indicating the structural symmetry lowering from orthor-hombic below these temperatures. Only C44 shows theelastic anomaly in YTiO3. However, weak anomalies weredetected around 130K in other moduli apart from C55 forY0:8Ca0:2TiO3 probably due to the secondary effect of thetransition. The symmetry of the low temperature phase ispossibly monoclinic due to emergence of the spontaneousstrain eyz or ezx. The moduli C44 and C55 are the linearresponse to the eyz and ezx strains, respectively. If thesetransitions are closely related to the orbital ordering, wecan estimate that jyzi would be a main component of theground state wave function for YTiO3 below 60K, and jzxi

would be the main component for Y0:8Ca0:2TiO3 below130K.It is very interesting to note that the spontaneous strain

is switched from eyz to ezx by Ca-doping. This mayoriginate from the change in the interaction strength and/or direction between the orbitals of neighboring Ti ions[7,8] since a Ti-O-Ti bond angle increases with increasingCa concentration [9]. However, we may have to take intoconsideration of at least the influence of randomnessintroduced by the Ca-doping. A precise structural analysisof the low temperature phase is demanded to elucidate thetransition mechanism. In order to clarify the switchingprocess, elasticity measurements for the sample with x ¼

0:1 are under way.

Summary

We have found the elastic anomalies in Y1�xCaxTiO3

(x ¼ 0 and 0.2) for the first time, revealing that bothcompounds undergo the structural transition. The crystalsymmetry of the low temperature phase is changed by theCa-doping.

Acknowledgment

This work was partly supported by the Grant-in-Aids(13CE2002 and 17340113) from MEXT, Japan and an aidfund from Energia.

References

[1] J.E. Greedan, J. Less-Common Met. 111 (1985) 335.

[2] F. Iga, et al., Phys. Rev. Lett. 93 (2004) 257207.

[3] K. Kato, et al., J. Phys. Soc. Japan 71 (2002) 2082.

[4] T. Suzuki, et al., Physica B 329–333 (2003) 868.

[5] T. Suzuki, et al., Phys. Rev. B 62 (2000) 49.

[6] M. Tsubota, et al., Physica B 281–282 (2000) 622.

[7] M. Mochizuki, M. Imada, J. Phys. Soc. Japan 70 (2001) 1777.

[8] H. Nakao, et al., J. Phys. Soc. Japan 73 (2004) 2620.

[9] K. Uchihira, et al. (unpublished).