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Papers by shoji kashida

Research paper thumbnail of Ultrasonic Velocities of K 2 PbCu(NO 2 ) 6 near the Cooperative Jahn-Teller Phase Transitions

Journal of the Physical Society of Japan, 1978

Research paper thumbnail of Structural phase transition in KHCO3

Journal of Solid State Chemistry, 1990

ABSTRACT The structural phase transition of KHCO3 has been studied using three-dimensional data c... more ABSTRACT The structural phase transition of KHCO3 has been studied using three-dimensional data collected both below and above the transition point. Above 318 K, the crystal structure changes from the room temperature phase (monoclinic space group ) to a high temperature phase (monoclinic space group ), both containing four formula units. The cell dimensions are a = 15.195(1) Å, b = 5.640(1) Å, c = 3.734(1) Å, and β = 104.86(1)°, and v = 309.28 A3 (at 353 K). The structure is refined by the full-matrix least-squares method and final R-factor is 0.023. From the hydrogen atoms revealed in the difference Fourier map, it is concluded that the phase transition is ascribed to the order-disorder of the (HCO3)2 dimer.

Research paper thumbnail of X-Ray Study of the Room Temperature Structurein Monoclinic TlS

Journal of the Physical Society of Japan, 1993

Research paper thumbnail of Ultrasonic Velocities in BaTiO3

Journal of the Physical Society of Japan, 1973

ABSTRACT The sound velocities in BaTiO3 has been observed near its paraelectric to ferroelectric ... more ABSTRACT The sound velocities in BaTiO3 has been observed near its paraelectric to ferroelectric phase transition point. The anomalous part of the sound velocity of the longitudinal wave propagating along [100] direction depends on temperature as A[(T-T0)/T0]-\zeta with \zeta{=}0.41 and A{=}1.79× 104 cm\cdotsec-1, where T0 is the paraelectric temperature. The critical index \zeta is close to the theoretical value of 1/2 as predicted by Dvorák. On the other hand, the velocity of the transverse wave does not show any appreciable critical behavior. The value of A is also compared with the theory given by Dvorák. The observed curve for the longitudinal wave is in good agreement with the theoretical curve, while that for the transverse wave shows a notable discrepancy.

Research paper thumbnail of Successive Jahn-Teller Phase Transitions in K 2 PbCu(NO 2 ) 6

Journal of the Physical Society of Japan, 1978

Research paper thumbnail of X-Ray Investigation of the High Temperature Structure of (CH 3 ) 3 NHCdCl 3

Journal of the Physical Society of Japan, 1983

ABSTRACT The structural phase transitions in a linear chain compound (CH3)3NHCdCl3 were investiga... more ABSTRACT The structural phase transitions in a linear chain compound (CH3)3NHCdCl3 were investigated by an X-ray photographic method. It was found that the high temperature phase above 374 K belongs to the orthorhombic Pbnm (Z=8) rather than to the hexagonal P63/m (Z=6) which was proposed by Chapuis and Zuniga. The structure of this high temperature phase was argued to be characterized by an orientationally disordered state of the trimethyl ammonium ions. An analogy to the triangular Ising lattice is also refered.

Research paper thumbnail of The Crystal Structure of (CH 3 ) 3 NHCdBr 3 in the Partially Disordered(\(\sqrt{3}{\times}\sqrt{3}\))R30°-Phase

Journal of the Physical Society of Japan, 1986

Successive structural phase transitions in a linear chain compound (CH3)3NHCdBr3(TrMCB) have been... more Successive structural phase transitions in a linear chain compound (CH3)3NHCdBr3(TrMCB) have been studied by X-ray diffraction. The structure of the (\sqrt{3}×\sqrt{3})R30° phase, which exists above 379 K is refined by a block-diagonal least squares method; the final R-factor is 0.065. The structure belongs to the hexagonal space group P\bar{6}2m, where a partial disorder of CdBr3, chains occurs; the chains are rotated in antiphase in two of the three sublattice sites, while in the third site the chains are in disorder and can be represented by a superposition of the states in the former two sites. A close resemblance to the ``partially disordered state'' which was recently found in some triangular Ising antiferro-magnets is referred.

Research paper thumbnail of X-Ray Study of the Cation Distribution in the Ternary Oxide, 6 Bi2O3–V2O5

Journal of Solid State Chemistry, 1996

The structure of the ternary oxide 6 Bi2O3–V2O5has been investigated using single crystal X-ray d... more The structure of the ternary oxide 6 Bi2O3–V2O5has been investigated using single crystal X-ray diffraction data. It has a fluorite-type superstructure with a pseudo monoclinic cell,a= 20.023 Å,b= 11.668 Å,c= 20.472 Å, and β = 107.13°, (the relation to the fluorite subcell isa≃ 3/2[1, 1, 2]c,b≃ 3/2[1, 1, 0]c, andc≃ 1/2[5, 5, 2]c), but the real crystal symmetry isP1. The positions of metal atoms are refined toR= 0.12. The structure is characterized by six cation layers along thecaxis, each containing 18 metal atoms. The cation arrangement can be represented using the numbers of V atoms as 4–0–4–4–0–4. Along thebaxis, the crystal has square wave type displacements (≃0.6 Å). The origin of this large distortion is attributed to the Coulomb repulsion between V atoms in the nearest neighbor sites.

Research paper thumbnail of One-Dimensional Stacking Disorder in a Linear Chain Compound (CH 3 ) 3 NHCdCl 3

Journal of the Physical Society of Japan, 1985

ABSTRACT The structural phase transition of a linear chain compound (CH3)3NHCdCl3 (TrMCC) was stu... more ABSTRACT The structural phase transition of a linear chain compound (CH3)3NHCdCl3 (TrMCC) was studied by X-ray diffraction method. Accompanying the phase transition from the (1×\sqrt{3}) structure to the (2×\sqrt{3}) structure, strong diffuse streaks were observed along the \mbi{b}* direction. In order to explain the observed peak profiles, a simplified model was presented. In this model, the layers made of rather tightly coupled CdCl3 chains were assumed to stack with probabilities. The diffraction intensity was analyzed using the matrix method given by Kakinoki and Komura. The calculation which incorporates the effect of the phase shift due to the displacements normal to the stacking direction was found to reproduce the experimental results fairly well. The streaks observed in the (3× 2) phase were also discussed.

Research paper thumbnail of Superstructures in the vanadium-containing bismuth oxides

Acta Crystallographica Section A Foundations of Crystallography

Research paper thumbnail of Inelastic neutron scattering study of the localized dynamics of protons in KHCO 3 single crystals

In the crystal of potassium hydrogen carbonate (KHCO 3) all OH¯O hydrogen bonds are equivalent an... more In the crystal of potassium hydrogen carbonate (KHCO 3) all OH¯O hydrogen bonds are equivalent and virtually parallel to each other. The inelastic incoherent scattering function measured with oriented single crystals is presented as maps of intensity for energy transfer values corresponding to proton vibrations. All maps resemble those anticipated for harmonic oscillators with effective masses close to 1 amu. Contributions from the lattice dynamics, vibrational coupling with heavy atoms, anharmonicity, mixing of states, and Fermi resonance are analyzed. It is concluded that anharmonicity plays a major role and proton dynamics cannot be represented with normal coordinates within the harmonic force-field approximation.

Research paper thumbnail of X-ray study of the incommensurate phase of

Journal of Physics: Condensed Matter, 1999

ABSTRACT The successive structural phase transitions in the layer compound have been studied usin... more ABSTRACT The successive structural phase transitions in the layer compound have been studied using single-crystal x-ray diffraction. The intermediate phase between 194 K and 214 K is found to be incommensurate; the satellite maps show that the modulation mode which characterizes the intermediate phase has . It is shown that the previously reported satellite reflections at are ascribable to twins. A structure model is presented for the incommensurate phase. In the low-temperature phase below 197 K, the satellite reflections are shifted to a commensurate position, .

Research paper thumbnail of Indirect nuclear exchange coupling and electronic structure of the layered semiconductor TlS: NMR study and band structure calculation

Journal of Physics: Condensed Matter, 2004

ABSTRACT We have studied the electronic structure of the layered semiconductor TlS by means of 20... more ABSTRACT We have studied the electronic structure of the layered semiconductor TlS by means of 203Tl and 205Tl NMR and band structure calculation. The crystal structure of this compound is built from the metal–chalcogen layers formed of linked Tl3+S42− tetrahedra and Tl+ ions located between the layers. Our experiments show significant interlayer indirect exchange coupling of thallium nuclei. Since the distances between thallium ions essentially exceed the sum of their ionic radii, it is concluded that such coupling is realized due to the overlap of the Tl electron wavefunctions across the intervening S atom. The exchange interaction has been evaluated by means of the quantitative spectral analysis. The band structure calculation shows that the top of the valence band is composed of mixed Tl+ 6s6pz–S 3s3pz–Tl3+ 6s6pz states, yielding overlap of the Tl+–S–Tl3+ type. Such electronic structure gives rise to the effective interlayer coupling of Tl nuclear spins observed in the experiment. This effect is discussed along with the recently obtained NMR results in the chain thallium sulfide and selenide.

Research paper thumbnail of Photoemission study of chain- and layer-type TlS crystals

Journal of Physics: Condensed Matter, 1997

ABSTRACT Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl... more ABSTRACT Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl ions. The compound crystallizes in two different structures, the tetragonal TlSe type and the monoclinic type. Photoemission studies were carried out, using a synchrotron photon source, on TlS crystals with both the TlSe- and -type structures. Comparison of the spectra shows that the line shape of the Tl 5d core level is more asymmetric in the TlSe type than the type. The asymmetry reflects the density of states near the Fermi level of the crystals. It was also found that as the incident photon energy increases the intensities of the valence band peaks increase relative to those of the Tl 5d core levels. Comparing the intensity ratios with the calculated photoionization cross section ratios, the characters of the valence bands were discussed.

Research paper thumbnail of Successive structural phase transitions in monoclinic thallium monosulphide

Journal of Physics: Condensed Matter, 1993

The structural phase transitions in newly found monoclinic TlS have been studied using single-cry... more The structural phase transitions in newly found monoclinic TlS have been studied using single-crystal X-ray diffraction methods. The room temperature ferroelectric phase has a commensurate structure; the satellite reflections are observed at qc=(0, 0, 1/4) suggesting fourfold lattice modulations along the c axis. The intermediate phase between 318.6 K and 341.1 K is incommensurate; the satellites shift from qc to qi=(0.04, 0, 1/4). In the high-temperature paraelectric phase above 341.1 K, these satellite reflections disappear. A phenomenological treatment of the successive phase transitions in TlS is given. The temperature dependence of the dielectric constant is also discussed. The experimental results are compared with those for TlGaSe2.

Research paper thumbnail of Single-crystal NMR for the layered semiconductor TlGaSe 2

Journal of Physics: Condensed Matter, 2008

ABSTRACT We report on a 69Ga and 205Tl NMR study of single-crystal thallium gallium selenide. Our... more ABSTRACT We report on a 69Ga and 205Tl NMR study of single-crystal thallium gallium selenide. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from Tc = 107.5 to Ti = 118 K. On approaching phase transition at Ti from above, the crystal exhibits soft mode behavior, which is somewhat different for thallium and gallium substructures. Redistribution of 69Ga line intensities with temperature in the ferroelectric phase indicates a variation of the domain structure of this phase.

Research paper thumbnail of An X-ray study of the incommensurate structure in digenite (Cu 1.8 S)

Journal of Physics: Condensed Matter, 1991

ABSTRACT The structure of a non-stoichiometric copper compound Cu1.8S (digenite) has been studied... more ABSTRACT The structure of a non-stoichiometric copper compound Cu1.8S (digenite) has been studied by X-ray diffraction. Although digenite has a cubic anti-fluorite structure, satellite reflections were seen along the (111)c axis. Single-crystal X-ray diffraction data, including first- and second-order satellites, were collected using a four-circle diffractometer. A rhombohedral twinning model was used to determine the modulation densities of the copper ions. The final R-factor is about 10% including the main and satellite reflections. The present analysis shows that the crystal is composed of domain-like structures: in each domain, the dimensions of the regular lattice, formed from copper ions, are slightly larger than those of the basic sulphur lattice. The origin of this domain-like structure, based on the Frenkel-Kontorova model, is attributed to the non-stoichiometry and apparently smaller cation-anion interaction.

Research paper thumbnail of Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

Journal of Physics and Chemistry of Solids, 2003

ABSTRACT The electronic structures of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te ha... more ABSTRACT The electronic structures of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.

Research paper thumbnail of The crystal structure of (CH3)3NHCdCl3 in the high-temperature (3 × 3) phase

Journal of Solid State Chemistry - J SOLID STATE CHEM, 1987

The structure of a linear chain compound (CH3)3NHCdCl3 is studied at 375 K using three-dimensiona... more The structure of a linear chain compound (CH3)3NHCdCl3 is studied at 375 K using three-dimensional X-ray diffraction data. The structure is refined by a block-diagonal least-squares method with the space group P6. The obtained final R-factor is 0.065 for 2684 reflections. There are nine sublattice sites in the (3 × 3) unit cell; in eight of these sites, CdCl3 chains are rotated in antiphase, and the last site can be represented by a linear combination of states rotated in the opposite directions (a partially disordered phase). It is also argued that the ``ice rule'' will be fulfilled in the arrangement of the hydrogen bondings in this structure.

Research paper thumbnail of Dielectric and ac conductivity studies in as-grown Ga2Te3crystals with the defect zinc-blende structure

Physica Status Solidi A-applications and Materials Science - PHYS STATUS SOLIDI A-APPL MAT, 2010

Ga 2 Te 3 crystallizes in the zinc-blende structure where one-third of the cation sites are vacan... more Ga 2 Te 3 crystallizes in the zinc-blende structure where one-third of the cation sites are vacant. The dielectric properties of asgrown Ga 2 Te 3 crystals have been studied in the frequency range from 10 2 to 10 9 Hz. Below room temperature, the dielectric constant is around 20 and almost independent of temperature. The ac conductivity is found to obey a power law s ac $ v s , with s $ 1 indicating that the conduction is due to hopping of localized charge carriers. Above room temperature the dielectric constant shows an S-shaped step-like increase up to a very large value around 10 4. The relaxation is found to be of thermal activated Debye type, the relaxation time changes as t ¼ t 0 expðE t =kTÞ (E t % 0:27 eV). The activation energy of the relaxation nearly coincides with that deduced from conductivity. These results are analyzed using the Maxwell Wagner model and the space charge model.

Research paper thumbnail of Ultrasonic Velocities of K 2 PbCu(NO 2 ) 6 near the Cooperative Jahn-Teller Phase Transitions

Journal of the Physical Society of Japan, 1978

Research paper thumbnail of Structural phase transition in KHCO3

Journal of Solid State Chemistry, 1990

ABSTRACT The structural phase transition of KHCO3 has been studied using three-dimensional data c... more ABSTRACT The structural phase transition of KHCO3 has been studied using three-dimensional data collected both below and above the transition point. Above 318 K, the crystal structure changes from the room temperature phase (monoclinic space group ) to a high temperature phase (monoclinic space group ), both containing four formula units. The cell dimensions are a = 15.195(1) Å, b = 5.640(1) Å, c = 3.734(1) Å, and β = 104.86(1)°, and v = 309.28 A3 (at 353 K). The structure is refined by the full-matrix least-squares method and final R-factor is 0.023. From the hydrogen atoms revealed in the difference Fourier map, it is concluded that the phase transition is ascribed to the order-disorder of the (HCO3)2 dimer.

Research paper thumbnail of X-Ray Study of the Room Temperature Structurein Monoclinic TlS

Journal of the Physical Society of Japan, 1993

Research paper thumbnail of Ultrasonic Velocities in BaTiO3

Journal of the Physical Society of Japan, 1973

ABSTRACT The sound velocities in BaTiO3 has been observed near its paraelectric to ferroelectric ... more ABSTRACT The sound velocities in BaTiO3 has been observed near its paraelectric to ferroelectric phase transition point. The anomalous part of the sound velocity of the longitudinal wave propagating along [100] direction depends on temperature as A[(T-T0)/T0]-\zeta with \zeta{=}0.41 and A{=}1.79× 104 cm\cdotsec-1, where T0 is the paraelectric temperature. The critical index \zeta is close to the theoretical value of 1/2 as predicted by Dvorák. On the other hand, the velocity of the transverse wave does not show any appreciable critical behavior. The value of A is also compared with the theory given by Dvorák. The observed curve for the longitudinal wave is in good agreement with the theoretical curve, while that for the transverse wave shows a notable discrepancy.

Research paper thumbnail of Successive Jahn-Teller Phase Transitions in K 2 PbCu(NO 2 ) 6

Journal of the Physical Society of Japan, 1978

Research paper thumbnail of X-Ray Investigation of the High Temperature Structure of (CH 3 ) 3 NHCdCl 3

Journal of the Physical Society of Japan, 1983

ABSTRACT The structural phase transitions in a linear chain compound (CH3)3NHCdCl3 were investiga... more ABSTRACT The structural phase transitions in a linear chain compound (CH3)3NHCdCl3 were investigated by an X-ray photographic method. It was found that the high temperature phase above 374 K belongs to the orthorhombic Pbnm (Z=8) rather than to the hexagonal P63/m (Z=6) which was proposed by Chapuis and Zuniga. The structure of this high temperature phase was argued to be characterized by an orientationally disordered state of the trimethyl ammonium ions. An analogy to the triangular Ising lattice is also refered.

Research paper thumbnail of The Crystal Structure of (CH 3 ) 3 NHCdBr 3 in the Partially Disordered(\(\sqrt{3}{\times}\sqrt{3}\))R30°-Phase

Journal of the Physical Society of Japan, 1986

Successive structural phase transitions in a linear chain compound (CH3)3NHCdBr3(TrMCB) have been... more Successive structural phase transitions in a linear chain compound (CH3)3NHCdBr3(TrMCB) have been studied by X-ray diffraction. The structure of the (\sqrt{3}×\sqrt{3})R30° phase, which exists above 379 K is refined by a block-diagonal least squares method; the final R-factor is 0.065. The structure belongs to the hexagonal space group P\bar{6}2m, where a partial disorder of CdBr3, chains occurs; the chains are rotated in antiphase in two of the three sublattice sites, while in the third site the chains are in disorder and can be represented by a superposition of the states in the former two sites. A close resemblance to the ``partially disordered state'' which was recently found in some triangular Ising antiferro-magnets is referred.

Research paper thumbnail of X-Ray Study of the Cation Distribution in the Ternary Oxide, 6 Bi2O3–V2O5

Journal of Solid State Chemistry, 1996

The structure of the ternary oxide 6 Bi2O3–V2O5has been investigated using single crystal X-ray d... more The structure of the ternary oxide 6 Bi2O3–V2O5has been investigated using single crystal X-ray diffraction data. It has a fluorite-type superstructure with a pseudo monoclinic cell,a= 20.023 Å,b= 11.668 Å,c= 20.472 Å, and β = 107.13°, (the relation to the fluorite subcell isa≃ 3/2[1, 1, 2]c,b≃ 3/2[1, 1, 0]c, andc≃ 1/2[5, 5, 2]c), but the real crystal symmetry isP1. The positions of metal atoms are refined toR= 0.12. The structure is characterized by six cation layers along thecaxis, each containing 18 metal atoms. The cation arrangement can be represented using the numbers of V atoms as 4–0–4–4–0–4. Along thebaxis, the crystal has square wave type displacements (≃0.6 Å). The origin of this large distortion is attributed to the Coulomb repulsion between V atoms in the nearest neighbor sites.

Research paper thumbnail of One-Dimensional Stacking Disorder in a Linear Chain Compound (CH 3 ) 3 NHCdCl 3

Journal of the Physical Society of Japan, 1985

ABSTRACT The structural phase transition of a linear chain compound (CH3)3NHCdCl3 (TrMCC) was stu... more ABSTRACT The structural phase transition of a linear chain compound (CH3)3NHCdCl3 (TrMCC) was studied by X-ray diffraction method. Accompanying the phase transition from the (1×\sqrt{3}) structure to the (2×\sqrt{3}) structure, strong diffuse streaks were observed along the \mbi{b}* direction. In order to explain the observed peak profiles, a simplified model was presented. In this model, the layers made of rather tightly coupled CdCl3 chains were assumed to stack with probabilities. The diffraction intensity was analyzed using the matrix method given by Kakinoki and Komura. The calculation which incorporates the effect of the phase shift due to the displacements normal to the stacking direction was found to reproduce the experimental results fairly well. The streaks observed in the (3× 2) phase were also discussed.

Research paper thumbnail of Superstructures in the vanadium-containing bismuth oxides

Acta Crystallographica Section A Foundations of Crystallography

Research paper thumbnail of Inelastic neutron scattering study of the localized dynamics of protons in KHCO 3 single crystals

In the crystal of potassium hydrogen carbonate (KHCO 3) all OH¯O hydrogen bonds are equivalent an... more In the crystal of potassium hydrogen carbonate (KHCO 3) all OH¯O hydrogen bonds are equivalent and virtually parallel to each other. The inelastic incoherent scattering function measured with oriented single crystals is presented as maps of intensity for energy transfer values corresponding to proton vibrations. All maps resemble those anticipated for harmonic oscillators with effective masses close to 1 amu. Contributions from the lattice dynamics, vibrational coupling with heavy atoms, anharmonicity, mixing of states, and Fermi resonance are analyzed. It is concluded that anharmonicity plays a major role and proton dynamics cannot be represented with normal coordinates within the harmonic force-field approximation.

Research paper thumbnail of X-ray study of the incommensurate phase of

Journal of Physics: Condensed Matter, 1999

ABSTRACT The successive structural phase transitions in the layer compound have been studied usin... more ABSTRACT The successive structural phase transitions in the layer compound have been studied using single-crystal x-ray diffraction. The intermediate phase between 194 K and 214 K is found to be incommensurate; the satellite maps show that the modulation mode which characterizes the intermediate phase has . It is shown that the previously reported satellite reflections at are ascribable to twins. A structure model is presented for the incommensurate phase. In the low-temperature phase below 197 K, the satellite reflections are shifted to a commensurate position, .

Research paper thumbnail of Indirect nuclear exchange coupling and electronic structure of the layered semiconductor TlS: NMR study and band structure calculation

Journal of Physics: Condensed Matter, 2004

ABSTRACT We have studied the electronic structure of the layered semiconductor TlS by means of 20... more ABSTRACT We have studied the electronic structure of the layered semiconductor TlS by means of 203Tl and 205Tl NMR and band structure calculation. The crystal structure of this compound is built from the metal–chalcogen layers formed of linked Tl3+S42− tetrahedra and Tl+ ions located between the layers. Our experiments show significant interlayer indirect exchange coupling of thallium nuclei. Since the distances between thallium ions essentially exceed the sum of their ionic radii, it is concluded that such coupling is realized due to the overlap of the Tl electron wavefunctions across the intervening S atom. The exchange interaction has been evaluated by means of the quantitative spectral analysis. The band structure calculation shows that the top of the valence band is composed of mixed Tl+ 6s6pz–S 3s3pz–Tl3+ 6s6pz states, yielding overlap of the Tl+–S–Tl3+ type. Such electronic structure gives rise to the effective interlayer coupling of Tl nuclear spins observed in the experiment. This effect is discussed along with the recently obtained NMR results in the chain thallium sulfide and selenide.

Research paper thumbnail of Photoemission study of chain- and layer-type TlS crystals

Journal of Physics: Condensed Matter, 1997

ABSTRACT Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl... more ABSTRACT Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl ions. The compound crystallizes in two different structures, the tetragonal TlSe type and the monoclinic type. Photoemission studies were carried out, using a synchrotron photon source, on TlS crystals with both the TlSe- and -type structures. Comparison of the spectra shows that the line shape of the Tl 5d core level is more asymmetric in the TlSe type than the type. The asymmetry reflects the density of states near the Fermi level of the crystals. It was also found that as the incident photon energy increases the intensities of the valence band peaks increase relative to those of the Tl 5d core levels. Comparing the intensity ratios with the calculated photoionization cross section ratios, the characters of the valence bands were discussed.

Research paper thumbnail of Successive structural phase transitions in monoclinic thallium monosulphide

Journal of Physics: Condensed Matter, 1993

The structural phase transitions in newly found monoclinic TlS have been studied using single-cry... more The structural phase transitions in newly found monoclinic TlS have been studied using single-crystal X-ray diffraction methods. The room temperature ferroelectric phase has a commensurate structure; the satellite reflections are observed at qc=(0, 0, 1/4) suggesting fourfold lattice modulations along the c axis. The intermediate phase between 318.6 K and 341.1 K is incommensurate; the satellites shift from qc to qi=(0.04, 0, 1/4). In the high-temperature paraelectric phase above 341.1 K, these satellite reflections disappear. A phenomenological treatment of the successive phase transitions in TlS is given. The temperature dependence of the dielectric constant is also discussed. The experimental results are compared with those for TlGaSe2.

Research paper thumbnail of Single-crystal NMR for the layered semiconductor TlGaSe 2

Journal of Physics: Condensed Matter, 2008

ABSTRACT We report on a 69Ga and 205Tl NMR study of single-crystal thallium gallium selenide. Our... more ABSTRACT We report on a 69Ga and 205Tl NMR study of single-crystal thallium gallium selenide. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from Tc = 107.5 to Ti = 118 K. On approaching phase transition at Ti from above, the crystal exhibits soft mode behavior, which is somewhat different for thallium and gallium substructures. Redistribution of 69Ga line intensities with temperature in the ferroelectric phase indicates a variation of the domain structure of this phase.

Research paper thumbnail of An X-ray study of the incommensurate structure in digenite (Cu 1.8 S)

Journal of Physics: Condensed Matter, 1991

ABSTRACT The structure of a non-stoichiometric copper compound Cu1.8S (digenite) has been studied... more ABSTRACT The structure of a non-stoichiometric copper compound Cu1.8S (digenite) has been studied by X-ray diffraction. Although digenite has a cubic anti-fluorite structure, satellite reflections were seen along the (111)c axis. Single-crystal X-ray diffraction data, including first- and second-order satellites, were collected using a four-circle diffractometer. A rhombohedral twinning model was used to determine the modulation densities of the copper ions. The final R-factor is about 10% including the main and satellite reflections. The present analysis shows that the crystal is composed of domain-like structures: in each domain, the dimensions of the regular lattice, formed from copper ions, are slightly larger than those of the basic sulphur lattice. The origin of this domain-like structure, based on the Frenkel-Kontorova model, is attributed to the non-stoichiometry and apparently smaller cation-anion interaction.

Research paper thumbnail of Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

Journal of Physics and Chemistry of Solids, 2003

ABSTRACT The electronic structures of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te ha... more ABSTRACT The electronic structures of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.

Research paper thumbnail of The crystal structure of (CH3)3NHCdCl3 in the high-temperature (3 × 3) phase

Journal of Solid State Chemistry - J SOLID STATE CHEM, 1987

The structure of a linear chain compound (CH3)3NHCdCl3 is studied at 375 K using three-dimensiona... more The structure of a linear chain compound (CH3)3NHCdCl3 is studied at 375 K using three-dimensional X-ray diffraction data. The structure is refined by a block-diagonal least-squares method with the space group P6. The obtained final R-factor is 0.065 for 2684 reflections. There are nine sublattice sites in the (3 × 3) unit cell; in eight of these sites, CdCl3 chains are rotated in antiphase, and the last site can be represented by a linear combination of states rotated in the opposite directions (a partially disordered phase). It is also argued that the ``ice rule'' will be fulfilled in the arrangement of the hydrogen bondings in this structure.

Research paper thumbnail of Dielectric and ac conductivity studies in as-grown Ga2Te3crystals with the defect zinc-blende structure

Physica Status Solidi A-applications and Materials Science - PHYS STATUS SOLIDI A-APPL MAT, 2010

Ga 2 Te 3 crystallizes in the zinc-blende structure where one-third of the cation sites are vacan... more Ga 2 Te 3 crystallizes in the zinc-blende structure where one-third of the cation sites are vacant. The dielectric properties of asgrown Ga 2 Te 3 crystals have been studied in the frequency range from 10 2 to 10 9 Hz. Below room temperature, the dielectric constant is around 20 and almost independent of temperature. The ac conductivity is found to obey a power law s ac $ v s , with s $ 1 indicating that the conduction is due to hopping of localized charge carriers. Above room temperature the dielectric constant shows an S-shaped step-like increase up to a very large value around 10 4. The relaxation is found to be of thermal activated Debye type, the relaxation time changes as t ¼ t 0 expðE t =kTÞ (E t % 0:27 eV). The activation energy of the relaxation nearly coincides with that deduced from conductivity. These results are analyzed using the Maxwell Wagner model and the space charge model.