Toshikazu Hirose

Structural Phase Transitions and Semiconductor-Metal Transition in WO3

Semiconducting WO 3 with the donor electrons trapped in oxygen vacancies shows so-called `semiconductor-metal transition at the triclinic-monoclinic (I) phase transition temperature at about 17°C. Resistivity discontinuities as large as a factor 10 2 have been observed at the monoclinic: (II)-triclinic phase transition at about -40°C. The semiconductor-metal transition is interpreted by the consideration that the energy gap between the t 2 g orbitals of 5 d 1 conduction electrons and the donor level are enlarged by the crystalline distortions due to the condensations of the soft phonon modes. The resistivity discontinuities are interpreted as due to the different energy gaps in the monoclinic (II) and triclinic phases. The essential difference between WO 3 and ReO 3 having the same crystal structure is discussed.

Condensed Phonon Modes in Successive Phases of WO3

The irreducible representations of normal modes of WO 3 are presented at the points (varGamma), X , M and R in or on the Brillouin zone of the hypothetic cubic phase. The atomic displacements at each phase are decomposed into the normal modes referring to the diffraction data. The normal modes concerned with W atoms condense at all the transition points except for the orthorhombic-monoclinic transition, where a linear combination of three rotational modes of WO 6 octahedra, R 25 x + R 25 z + M 3 y , condenses. Antiferrodistortive structures in the phases above room temperature are due to the condensations of the M , X and R modes. Ferridistortive structure in the monoclinic (II) phase is due to the condensation of both (varGamma_{15}^{z}) and M 3 z modes. WO 3 is shown to have ferridistortive structures with first-order successive phase transitions.

EPR Study of Strong Axial Cr3+-Vo Centers in WO3

The EPR spectra of Cr 3+ ion accompanied with a nearest neighbour oxygen vacancy (Cr 3+ -V o center) were observed over a temperature range from -150 to 200°C and showed `strong axial field lines. The crystalline fields have been obtained as follows; | D |=84 GHz, | E |=2.6 GHz at 25°C, and | D |=120 GHz, | E |=23 GHz at -100°C. These fields are very large compared with those observed in many other crystals. The angular dependence of the EPR spectra has been interpreted by a rotation of oxygen octahedra about a quasi-cubic diagonal axis and the displacements of tungsten atoms. From the temperature dependence of the EPR spectra the softening of the rotational condensed phonon modes of the oxygen octahedra was observed. The relations between the EPR spectra and the structural phase transitions in WO 3 are discussed.

Condensed Modes of WO_3 Represented by the Normal Modes in the Space Group D^7_

The prototypic phase of WO 3 is considered to be the tetragonal phase with the space group D 4 h 7 , that is the highest temperature phase, to explain systematically the displasive successive phase transitions of WO 3 . The normal modes at the points ( varGamma ), Z , M and A in the tetragonal Brillouin zone are studied with the help of the group theory and are compared with the normal modes in the cubic WO 3 that is the hypothetic prototype with the space group O h 1 . The condensed modes in each phase of WO 3 are described by the irreducible representations of the normal modes in the tetragonal WO 3 , referring to the results of X-ray and neutron diffraction studies.

EPR Study of V4+ Ions in Tungsten Trioxide WO3

The EPR spectra from V 4+ ions in WO 3 single crystal were studied in the temperature range from 300 K to 77 K by using a K-band spectrometer. At 77 K, the g values and hf parameters A of V 4+ ions were strong c -axis anistropy, so that g X =1.928, g Y =1.945, g Z =1.815, (Eulerian angles; θ g =3.9°, φ g =45.8°, ψ g =-0.2°), and A X =-60.5 G, A Y =-55.0 G, A Z =-190.4 G, (θ A =4.4°, φ A =43.6°, ψ A =1.5°). The ground state of 3 d 1 electron of V 4+ ion was determined to be | x y > state by hf structure analysis. WO 3 is determined to be a ferroelectric material with the spontaneous polarization along the c -axis connected to the condensed ferroelectric (varGamma_{15}^{z}) phonon modes of W 6+ ions in the monoclinic II phase at 77 K. At the relations of g av vs A iso for V 4+ and VO 2+ ions in some host crystals, WO 3 is shown to be a remarkable material.

Condensed Modes in BaMF4 Type Crystals

The prototype structure of BaMF 4 type crystal is considered to be a structure composed of slabs of the perovskite, with the space group D 17 2 h . Normal modes at the (varGamma) point in the orthorhombic Brillouin zone of the hypothetical D 17 2 h phase and the C 12 2 v phase below the melting point were analyzed with a help of the group theory. The condensed modes to give the crystal structure at room temperature were assigned and discussed in comparison with the Sr 2 Ta 2 O 7 structure.

Acoustic Instability near the Monoclinic-Orthorhombic Phase Transition in WO3

The X-ray scattering profiles from WO 3 single crystal are carefully examined to investigate the acoustic instability near the monoclinic-orthorhombic phase transition temperature T c =310°C. The spontaneous shear strain e zx is 0.016 at 25°C, decreases with increasing temperature and becomes zero at T c . The temperature dependence of X-ray diffuse scattering intensity indicates that the elastic stiffness consant C 55 becomes vanishingly small just above T c .