High resolution X-ray study reveals the wave vector change in the modulated structure of the quasi-one dimensional compound beta'-Cu vanadium bronze. Structural modulation of the reduced wave vector q0 = (0, 0.305, 0) emerges below 220 K in beta'-Cu0.29V2O5 . For beta'-Cu0.39V2O5, not the single q modulation but two kinds of modulations were observed. A three-fold superlattice structure with q1 = (0, 0.333, 0) appears below 210 K. An incommensurate modulated structure with q2 = (0, 0.26 ? 0.29, 0) coexists below 175 K, whose satellite intensity and b* component Qb have temperature and passing-time dependencies between 140 K and 175 K. The competition between q1 and q2 modulations was also observed. It seems that the q2 is deeply related to the physical property change between 140 K and 180 K confirmed by the decrease in the magnetic susceptibility and the increase in the resistivity.</p

Nagao, Nobuaki

Nogami, Yoshio

Oshima, Kokichi

Ueda, Yutaka

Yamada, Hiroyuki

English

Okayama University Scientific Achievement Repository

PhysicsPhysics fieldsOkayama University Year 2003X-ray study of modulated structures ofbeta-CuxV2O5Nobuaki Nagao∗ Yoshio Nogami† Kokichi Oshima‡Hiroyuki Yamada∗∗ Yutaka Ueda††∗Okayama University, nagao@science.okayama-u.ac.jp†Okayama University‡Okayama University∗∗University of Tokyo††University of TokyoThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/physics general/2X—ray study ofmodulated structures ofβ′—CuxV2O5(x = 0.29, 0.39)Nobuaki Nagao a, Yoshio Nogami a,1, Kokichi Oshima a Hiroyuki Yamada b, Yutaka Ueda b,aGNST, Okayama University, 3—1—1 Tsushima, Okayama 700—8530, JapanbISSP, University of Tokyo, 5—1—5 Kashiwanoha, Kashiwa, Chiba 277—8581, JapanAbstractHigh resolution X-ray study reveals the wave vector change in the modulated structure of the quasi-one dimensionalcompound β′—Cu vanadium bronze. Structural modulation of the reduced wave vector q0= (0, 0.305, 0) emergesbelow 220 K in β′—Cu0.29V2O5. For β′—Cu0.39V2O5, not the single q modulation but two kinds of modulations wereobserved. A three—fold superlattice structure with q1= (0, 0.333, 0) appears below 210 K. An incommensuratemodulated structure with q2= (0, 0.26 ∼ 0.29, 0) coexists below 175 K, whose satellite intensity and b* componentQb have temperature and passing-time dependencies between 140 K and 175 K. The competition between q1 andq2modulations was also observed. It seems that the q2is deeply related to the physical property change between140 K and 180 K confirmed by the decrease in the magnetic susceptibility and the increase in the resistivity.Key words: X-ray, modulated structure, low dimensional conductor, vanadium bronze, β′—CuxV2O5,1. Introductionβ and β′ type vanadium bronzes have a quasi-onedimensional crystal structure with the space groupC2/m. There are three sites for vanadium(V1,V2,V3);V1 forms a zig-zag chain by sharing the edges of VO6octahedron. V2 forms a two leg ladder by sharing thecorners of VO6 octahedron. V3 forms another zig-zag chain sharing the corners of VO5 pyramid. Thesechains and ladder run along the one-dimensional b—axis. Metal ion is inserted in the tunnel formed byV2O5 framework[1]. β type vanadium bronze has beeninvestigated systematically and its electronic propertyhas been understood in terms of a bipolaron formationin the V1 chain[2]. Knowledge of physical propertiesof β′ type bronze was limited, modulated structuresobserved in β′—CuxV2O5 have been also explained in1 Corresponding author: Graduate School of Natural Sci-ence and Technology, Okayama University, 3—1—1 Tsushima,Okayama 700-8530, Japan. E-mail: nogami@science.okayama-u.ac.jp, fax: +81—86—251—7830terms of the bipolaron ordering[3]. However, recentmagnetic measurement of vanadium bronze showednot the bipolaron spin-singlet state but another mag-netic ground state[4]. Furthermore Yamada et. alpresented electrical, magnetic and structural proper-ties of β′—CuxV2O5 systematically[5]. β′—CuxV2O5 isa quasi one-dimensional conductor for x > 0.60 buta semiconductor for low Cu concentration. Aroundx = 0.40, anomaly in the activation energy has beenobserved. Modulated structure formation and anoma-lies in the resistivity and the magnetic susceptibilityhave been observed around 210 K for x = 0.29. Forx = 0.39, both an increase in the resistivity and adecrease in the magnetic susceptibility which dependon the cooling rate, have been observed between 140K and 180 K. Furthermore a change in the periodic-ity of the modulated structure has been observed. Tounderstand these relationship between physical prop-erties and structure, we performed high resolutionX-ray measurement of x = 0.29 and x = 0.39 usingHUBER 5042 four-circle diffractometer and singlecrystals grown by the improved method.Preprint submitted to LT23 Proceedings 18 September 20022. Results and DiscussionFigure 1 shows temperature dependencies of the in-tegrated intensities of the first-, second- and third-order satellite reflections observed in β′—Cu0.29V2O5.Fig. 1. Temperature dependencies of the integrated intensitiesof the first-, second- and third-order satellite reflections ofβ′—Cu0.29V2O5.These reflections are located at Q±n = G ± nq0,where G is a reciprocal vector of the lattice andq0= (0, 0.305, 0). Within the experimental accuracy,we observed no temperature dependence of q0. Differ-ence in the width between of satellite reflections andof fundamental ones indicates that the structural cor-relation length is more than 1000 A˚. Since intensitiesof the second- and third-order satellite reflections arerelatively strong, these reflections are not due to thediffraction harmonics(Bessel function contribution)but the inharmonicity of the modulation. In otherwords, the modulation is not sinusoidal.Fig. 2. Temperature dependencies of the integrated intensitiesof satellite reflections of β′—Cu0.39V2O5. The successive mea-surements were performed on heating process after slow coolingof 0.5 K/min. The temperature of the crystal was hold dur-ing five hours for one measurement, and heating rate betweenmeasurements was 0.1 K/min.Figure 2 shows temperature dependencies of the in-tegrated intensities of two kinds of satellite reflectionsobserved in β′—Cu0.39V2O5. Satellite reflection withthe commensurate wave vector of q1= (0, 0.333, 0) ispresent below 210 K. Satellite reflection with the in-commensurate q2= (0, 0.26 ∼ 0.29, 0) coexists below175 K. Both the satellite intensity and the b* compo-nent Qb of q2 have temperature and passing-time de-pendencies between 140 K and 175 K. Both of themincrease with the passage of time at 160 K. Below 175K the q1satellite intensity decreases with the evolu-tion of the q2modulation. On the other hand, below140 K the q1modulation become dominant with thedecrement of the q2modulation. This indicates thesemodulations are not independent.Then we will survey the structural modulations everobserved in β′—CuxV2O5 of the lower doping level x.For x = 0.29, the inharmonic modulation of q0=(0, 0.305, 0) is observed. Similarly For x = 0.33, theinharmonic modulation of q = (0, 0.325, 0) has beenobserved[3]. This similarity suggests that these modu-lations have the common origin. With the increase ofCu doping level x, the Qb increases, and the three-foldcommensurate superstructure with q1= (0, 0.333, 0)appears for x = 0.39(this work) and x = 0.40[3]. Notethat positive relation between the Qb and the dopinglevel x. This modulation may be originated from theCu configurational ordering. This ordering of the metalhas been proposed for the superstructures observed inseveral β and β′ type vanadium bronzes[5]. Indeed,this proposal is plausible for several vanadium bronzes.However, this seems not the case in this β′—Cu vana-dium bronze. This is because the Qb is not equal to1.5x. From the chemical formula and the crystal struc-ture, the wave vector of metal site ordering should beequal to 1.5 times x. In all the β′—Cu vanadium bronzeabove, the Qb is too small compared with 1.5x. In fact,there is no apparent indication of the Cu ordering inthe three-fold superstructure for x = 0.39[6]. Alterna-tively, from the gradual increase in Qb with the valueof the doping level, this modulated structure may berelated to carrier number in the V2O5 framework.Finally, the incommensuratemodulated structure q2with smaller Qb of 0.26 ∼ 0.29 is very particular andobserved only in the x = 0.39 vanadium bronze. Al-though the origin of the modulation remains unclear,this modulated structure will be related to the decre-ment in the magnetic susceptibility and the incrementin the resistivity[5] observed in the same temperaturerange of the q2modulation prominence.References[1] A. -M. Savariault et. al, Z. fur Kristal. 209 (1994) 405.[2] H. Nagasawa et. al, J. Phys. Coll. C3. 44 (1983) 1737.[3] Y. Kanai et. al, Synth. Met. 9 (1984) 369.[4] H. Yamada et. al, J. Phys. Soc. Jpn. 68 (1999) 2735.[5] H. Yamada et. al, J. Phys. Soc. Jpn. 69 (2000) 1437.[6] N. Nagao et. al, in preparation.2

X-ray study of modulated structures of beta-CuxV2O5

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X-ray study of modulated structures of beta-Cu<sub>x</sub>V<sub>2</sub>O<sub>5</sub>

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