Metal-Insulator Transition and Orbital Ordering in Y1-xCaxTiO3
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Many of transition-metal oxides exhibit metal-insulator (MI) transitions induced bystrong interactions between d-electrons. In the vicinity of the MI transition, variousinteresting phenomena, such as high-TC superconductivity in cuprates and colossal magnetoresistancein manganites, have been discovered. To understand these phenomena, itis recognized that the orbital degree of freedom, in addition to the charge and spin degreesof freedom, should be taken into account. Recently, the role of orbital ordering in thedramatic change in transport and magnetic properties of manganites has been revealedby resonant x-ray scattering (RXS) techniques.
Among a large variety of transition-metal oxides, the perovskite-type titanium oxide,YTiO3 and the Ca substituted alloys Y1¡xCaxTiO3 have received much attention becauseof having smaller Jahn-Teller distortion relative to manganites. The parent materialYTiO3 is a Mott-Hubbard insulator in which one 3d electron of Ti3+ occupies the t2gorbital keeping the eg orbitals unoccupied. It is a rare example of having ferromagnetismbelow the Curie temperature TC = 30K. The origin of this ferromagnetic order has beenconsidered to be a ferromagnetic superexchange interaction accompanied by antiferroorbital ordering of the t2g orbitals. By the substitution of Ca2+ for Y3+ in Y1¡xCaxTiO3,the ferromagnetic order disappears at xFP » 0:2, while the system remains insulating upto xMI » 0:4. For x > xMI, a metallic state becomes stable at room temperature. At thecritical concentration x = 0:39, a first-order transition occurs from the insulating stateto the metallic state upon decreasing temperature below about 150 K. The issues are notonly the origin of the MI transition but also the reason why the value of xMI is far abovethat of xFP. In analogy with the manganites, the orbital degree of freedom would play animportant role in both the magnetic and metal-insulator transitions
To clarify these issues on Y1¡xCaxTiO3 , three approaches have been taken in thepresent study: (1) a comparison between the Ca substitution effect and pressure effecton the MI transition, (2) Rietveld analysis of the crystal structure by taking account ofthe GdFeO3-type distortion (difference in bond angles of Ti-O-Ti) and the Jahn-Tellerdistortion (difference in Ti-O bond lengths), (3) study of orbital ordering by the techniquesof RXS and polarized neutron diffraction (PND).
In Chapter 1, transport and magnetic properties of the 3d transition-metal oxides andMott-Hubbard model for the MI transition are briefly described. The previous studies ofYTiO3 and Y1¡xCaxTiO3 are also referred. Then, the purpose of the present study ispresented.
Chapter 2 presents the methods of single-crystal growth of Y1¡xCaxTiO3 and characterizationsby x-ray diffraction analysis and electron-probe microanalysis.
Chapter 3 gives descriptions of experimental methods and techniques including thetransport, magnetic and structural measurements. The PND, which provides the wavefunctions from the analysis of magnetic form factors, has been used to study orbitalordering in the ferromagnetic state for x < 0:2. The RXS technique, which is able toobserve the orbital ordering from the analysis of the anisotropy of the atomic scatteringfactor, has been applied to study the substitution effect on the orbital ordering in thepresent system for x · 0:75.
In Chapters 4.1-4.3, the results of transport, magnetic, thermoelectric and structuralmeasurements for Y1¡xCaxTiO3 are presented and discussed, which are summarized as below.
(1) In Chapters 4.1 and 4.2, we made comparison between the Ca substitutioneffect and pressure effect on the transport and the structural properties. Forseveral samples with x ' xMI, we measured the electrical resistivity ½(T)and powder x-ray diffraction (XRD) at ambient pressure. For x = 0:37 and0:39, the MI transitions occur at TMI = 100 K and 150 K on cooling, respectively.Powder x-ray diffraction analysis showed that the monoclinic phasedecomposes into a monoclinic phase and a low-temperature orthorhombic(LTO) phase on cooling below TMI. This LTO phase is found to be a metallicphase from the fact that the residual resistivity decreases when the volumefraction of LTO increases. For x = 0:37, we measured ½(T) and XRD underpressure. The value of TMI (100 K at P = 0 GPa) increases with increasingpressure, and eventually the metallic phase is stabilized even at roomtemperature under P = 1:5 GPa. The phase separation temperature agreeswell with the TMI, and both temperatures increase linearly with increasing xor pressure. These results indicate that the MI transition in Y1¡xCaxTiO3is not a simple Mott-Hubbard type but is caused by the percolation of themetallic LTO domains.(2) In Chapter 4.3, Rietveld analysis of room-temperature powder x-ray diffractiondata is presented for the samples in the whole range 0 · x · 1. It isfound that the tilting angle of the TiO6 octahedron (the Ti-O-Ti bond anglewhich is related to the magnitude of superexchange interaction) increasesmonotonously with increasing x from 0 to 1. On the other hand, the anglebetween the local coordination axes in the TiO6 octahedron decreases from93.5± for x = 0 to 92± for x = 0:2. Above x = 0:2, two Ti-O bond lengthsin the ab plane become almost equal, i.e., Jahn-Teller distortion is released.This structural change should be responsible for the disappearance of theferromagnetism at x = 0:2.
In Chapters 4.4 and 4.5, the results of PND and RXS are presented and discussed, as summarized below.(3) In the ferromagnetic region x · 0:2, we measured the magnetic form factorsof Ti ions by means of PND in external fields parallel to the c¡axis. ThePND intensities have been observed at “forbidden" reflections in the conditionsof h + k = 2n + 1, where h and k are the Miller indexes and n is aninteger. By comparing the data with the model of orbital ordering configuration,we have determined the wave functions assuming c1jzxi + c2jxyi(c21+ c22= 1) at site 1, for example. The coefficient c1 is determined to be0.77 for x = 0 and x = 0:05. Above x = 0:1, however, c1 could not be determined uniquely. It is suggested that the ferromagnetic order becomesunstable when the order of orbitals is weakened.The RXS experiments were performed for 0 < x · 0:75 at room temperature.The main-edge RXS intensity at the 1s ! 4p transition energy ofE = 4:982 keV decreases linearly with increasing x up to 0.2, and graduallydecreases up to x = 0:75. On the other hand, the pre-edge intensityat the 1s ! 3d transition energy of E = 4:972 keV decreases rapidly withincreasing x up to xFP and vanishes at xMI. The x dependence of the RXSintensity at the main-edge has no dramatic change at both xFP and xMI andis unlike the x dependence of any local lattice distortion. The x dependenceof RXS intensity at the pre-edge, on the other hand, is similar with that ofJahn-Teller (JT) distortion of the TiO6 octahedron. This means that theRXS intensity at the pre-edge reflects the orbitally ordered state. Thus, wehave found that the ordering is weakened above x = xFP but remains in thewhole insulating phase for x < xMI.
By combining the above results (1), (2), and (3), it is found that the temperatureinducedMI transition in Y1¡xCaxTiO3 at x » 0:39 is not a simple Mott-Hubbard typebut is a result of percolation of domains of the metallic low-temperature orthorhombicphase. For the insulating phase for 0 < x < 0:2, the strong evidence of orbital ordering isobtained by both polarized neutron diffraction and resonant x-ray scattering experiments.It is concluded that the magnetic order becomes unstable when the order of orbitals isweakened, and the metallic phase appears when the order of orbitals melts for x > 0:4 inY1¡xCaxTiO3.
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