Exciton spectra, valence band splitting, and energy band structure of CuGaXIn1−XS2 and CuGaXIn1−XSe2 crystals

Exciton spectra are studied in CuGa_XIn_1−XS₂ solid solutions by means of photoreflectivity and wavelength modulation spectroscopy at liquid nitrogen temperature. The exciton parameters, dielectric constants, and free carrier effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The crystal field and spin orbit valence band splitting is calculated as a function of X taking into account the energy position of excitonic lines. The energy band structure of CuGa_XIn_1−XS₂ and CuGa_XIn_1−XSe₂ compounds is derived from optical spectra at photon energies higher than the fundamental band gap. The energies of optical transitions are tabulated for X values from 0 to 1.     

Ab initio calculation of the structure and optical properties of lead oxyhalides Pb<Subscript>3</Subscript>O<Subscript>2</Subscript><Emphasis Type="Italic">X</Emphasis><Subscript>2</Subscript> (<Emphasis Type="Italic">X</Emphasis> = Cl,Br, I)

Using ab initio methods, we obtained information on the crystal structure, calculated the phonon spectra, and determined the optical properties of oxyhalides Pb₃O₂Br₂ and Pb₃O₂I₂. A compound of Pb₃O₂Br₂ is synthesized, and its phonon spectra are recorded. The experimentally observed vibrational bands are assigned. A comparative analysis of the crystal lattice parameters, the phonon spectra, and the anisotropy parameters of lead oxyhalides Pb₃O₂ X ₂ (X = Cl, Br, I) is performed.     

Polarized Raman scattering from oriented single crystals of A2BX4 halides (A = Cs,B = Zn,X = Cl,Br, I)

Raman spectra of isostructural tetrahalozincates viz. Cs₂ZnI₄, Cs₂ZnBr₄ and Cs₂ZnCl₄ single crystals have been measured in various scattering orientations using polarized laser excitation. The lattice dynamics of the crystals is probed down to 70 K in order to cover the successive phase sequences. Except Cs₂ZnI₄, none of the other salts shows the characteristic commensurate-incommensurate features of lattice instabilities. The unit cell dynamics is understood in terms of 84 phonon branches originating from various internal modes of tetrahedral ZnX²⁻₄ group and external lattice modes. The enormous splittings, responsible for a complete loss of degeneracy in the internal modes for all the three salts, suggest a considerable distortion particularly at low temperatures in the tetrahedral geometry of the ZnX²⁻₄ group.     

Short-Range Characterization of the MeAr (Me=Zn, Cd) Ground-State Potentials from Fluorescence Spectra

First-time observed D1(¹Π)_v′=10→X0⁺(¹Σ) fluorescence in ZnAr, and A0⁺(³Π)_v′=4→X0⁺ and D1(¹Π)_v′=7,8→X0⁺ fluorescence in CdAr van der Waals (vdW) molecules were produced in a continuous supersonic molecular beam crossed with a pulsed dye-laser beam, following excitation of single vibronic levels. The dispersed fluorescence spectra displayed characteristic Condon internal diffraction (CID) patterns consisting of bound-free, reflection type, continuous spectra, and, in certain cases, bound-bound discrete features. An analysis of the A0⁺→X0⁺ and D1→X0⁺ bound-bound spectra indicates that Morse functions are adequate representations of the X0⁺ potential energy (PE) curves below their dissociation limits. In simulation of the A0⁺→X0⁺ and D1→X0⁺ bound-free spectra, the Morse, Lennard-Jones L-J(n−6), and Maitland-Smith M-S(n₀,n₁) functions were tested, and the respective M-S(11.3, 9.0) and M-S(10.6, 7.0) potentials were found to be good representations for the repulsive walls of the X0⁺ PE curves of ZnAr and CdAr, respectively, over the short range, R=2.45-4.38 Å (ZnAr) and R=2.85-4.31 Å (CdAr), of internuclear separations.     

Solid solution formation in the systems CuMIIIX2-AgMIIIX2 where MIII=Al,Ga, In and X2=S,Se

A study of solid solution formation in the systems CuAlX₂-AgAlX₂, CuGaX₂-AgGaX₂ and CuInX₂-AgInX₂, where X=S, Se, has shown that when M^III=Al and Ga, CuM^IIIX₂ and AgM^IIIX₂ were not completely miscible. The CuInX₂-AgInX₂ system, however, showed complete solid solubility. The limits of solid solubility are explained in terms of the c/a ratio, and the internal atomic coordinate u. The lattice constant a is a linear function of composition, while c bows upwards. This behavior is also discussed in terms of trends in 2-c/a.     

Synthesis and properties of Mn1−XFeXS single-crystals at room temperature

Results of the first synthesis of Mn_1−XFe_XS single-crystals and its structural, electrical and magnetic properties at room temperature are presented. The Mn²⁺→Fe²⁺ substitution in Mn_1−XFe_XS solid solutions is accompanied by a compression of the NaCl lattice and a small deformation of the octahedral environments, and the concentration transition from dielectric to semimetal. Single-line Mossbauer spectra indicate the paramagnetic state Mn_1−XFe_XS sulfides at room temperature.     

Electronic excitation of HgX (X = Cl,Br, I) due to electron and methyl mercury halide collisions

Electronic excitation of HgX (X = Cl, Br, I) radicals in the B-state has been observed as the result of collisions with low energy electrons and methyl mercury halide (CH₃HgX) molecules. The emission intensity has been observed to be much weaker than that observed for electron-HgX₂ collisions under similar experimental conditions. Using the strongest band head of the B-X band system, an attempt has been made to calculate the emission cross section due to electron CH₃HgX collisions at 10 eV electron kinetic energy. For HgCl, HgBr, and HgI radicals, these cross sections are 1 × 10^-18, 7 × 10^-17, and 2 × 10^-17 cm², respectively, with an estimated uncertainty of ±30%. Our measured threshold electron energy for excitation of CH₃HgX molecules and observation of the B-X emission band system and emission cross sections measured at 10 eV are greatly different from those measured by Allision and Zare [Chem. Phys. 35, 263 (1978)].     

XPS and UPS studies of pyrite compounds (CoX2 → CuX2, X: S; Se): Investigation of the insulator-metal transition in the NiS2−xSexsystem

We present in this paper a photoemission study (XPS and UPS) of several transition metal disulphides and diselenides (CoX₂ → CuX₂) with the pyrite structure. Our results show that all these compounds have narrow conduction bands and that the spectra for the disulphides and the diselenides are very similar. Nevertheless some changes occur in the electronic structure when going in the same series from CoX₂ → CuX₂, which are qualitatively well understood from simple theoretical considerations. Our main purpose is to discuss the influence of correlations between electrons in these narrow band systems and particularly in the case of the half-filled band (Mott-Hubbard insulator NiS₂). Considering the energy shift of the core levels deduced by XPS experiments we show that the effects of these correlations become smaller when going from CoX₂ to CuX₂, whereas they seem to remain unchanged in NiS_2?xSe_x, where an insulatormetal transition takes place. We present an attempt to quantitatively analyse our UPS results on the conduction bands. According to our results the particular behaviour corresponding to the Mott-Hubbard insulating state of NiS₂ and NiS_2?xSe_x (x < 0.5) mixed systems can be revealed by photoemission studies.     

New compounds in the Mn-X-Bi system where X = Ni, Cu, RhorPd

A new class of magnetic compounds has been discovered in the temary system Mn-X-Bi where X is Ni, Cu, Rh or Pd. The approximate compositions of these compounds are Mn₅Ni₂Bi₄, Mn₃Cu₄Bi₄, Mn₅Rh₂Bi₄, and Mn₅Pd₂Bi₄. The Bravais lattice is face-centered cubic, and the lattice constants are 12.16 Å (X = Ni), 12.18 Å (X = Cu), 12.31 Å (X = Rh) and 12.44 Å (X = Pd). These compounds are probably ferromagnetic and have Curie temperatures in the range -7°C to 183°C. A crystal structure is proposed for these compounds which contains 88 atoms/unit cell.     

The bΣ(b0) → XΣ(X10,X21) and aΔ(a2) → X21 transitions of AsBr

Emission spectra of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺,X₂1) and a¹Δ(a2) → X₂1 transitions of AsBr have been measured in the near-infrared spectral region with a Fourier-transform spectrometer. The arsenic bromide radicals were generated in fast-flow systems by reaction of arsenic vapor (As_x) with bromine and were excited by microwave-discharged oxygen. The most prominent features in the spectrum are the Δv = +1,0,−1, and −2 band sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺) transition in the range 11 700-12 700 cm⁻¹. With lower intensities, the Δv = 0 and −1 sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₂1) sub-system show up in the same range. Further to the red, between 6000 and 6700 cm⁻¹, the Δv = 0, +1, and −1 sequences of the hitherto unknown a¹Δ(a2) → X₂1 transition are observed. Analyses of medium- and high-resolution spectra have yielded improved molecular constants for the X₁0⁺, X₂1, and b0⁺ states and first values of the electronic energy and the vibrational constants of the a2 state.     

Nonreciprocal light birefringence in the R 3B7O13 X Boracites (R=Co, Cu, Ni; X=I, Br)

The field and angular dependences of nonreciprocal birefringence (NB), which is linear in magnetic field B and is due to magnetic-field-induced spatial dispersion, have been studied in the cubic (symmetry class T _d) paraelectric phase of the R ₃B₇O₁₃ X boracites (R=Co, Cu, Ni; X=I, Br) at a wavelength λ=633 nm. It is shown that the NB in crystals with different 3d and halogen ions exhibits the same anisotropy. The relation between the A and g parameters, A=2g, which determine the NB anisotropy, suggests that the microscopic mechanism of the NB is the manifestation of second-order magnetoelectric susceptibility at optical frequencies.     

The vapor phase Raman spectra,Raman band contour analyses,Coriolis constants,force constants,and values for thermodynamic functions of the trihalides of group V

The vapor phase Raman spectra have been recorded of the molecules PX₃ (X = F, Cl, or Br), AsX₃ (X = F, Cl, Br, or I), and SbX₃ (X = Cl, Br, or I) in sealed tubes at 22–365°C as appropriate. Attempts to record the vapor phase Raman spectra of the molecules PI₃, SbF₃, and BiX₃ (X = F, Cl, Br, or I) were unsuccessful. The infrared and Raman rotational branch separations of the a₁ species fundamentals for these molecules are in agreement with the theoretically calculated values. The Raman band contours of the e-species fundamentals have been analyzed to yield, in favorable cases, values for the corresponding Coriolis constants which have been compared with those obtained from infrared band contour analyses, and which have been employed to constrain the e-species force constants. It is concluded that the Raman contour method for defining Coriolis and thus force constants should be regarded as further, but not necessarily the best, experimental method with which to help to constrain the field. Values for various thermodynamic functions of the molecules have been calculated from the new values for the fundamental frequencies and from the most recent structural data.     

Solid-State NMR Studies on Ionic <Emphasis Type="Italic">closo</Emphasis>-Dodecaborates

Alkali metal dodecahydro-closo-dodecaborates M₂[B₁₂H₁₂] (M = K, Rb, Cs, NH₄, N(CH₃)₄) and the perhalogenated cesium salts Cs₂[B₁₂X₁₂] (X = Cl, Br, I) are studied by solid-state ¹¹B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B₁₂X₁₂]²⁻ icosahedra which is examined by variable-temperature ¹¹B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M⁺. All alkali metal dodecahydro-closo-dodecaborates M₂ [B₁₂H₁₂] exhibit at elevated temperatures ¹¹B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic ¹¹B NMR spectra with a dominant homonuclear ¹¹B–¹¹B dipolar splitting. The per-halogenated cesium salts Cs₂[B₁₂X₁₂] behave differently. Hence, from the experimental ¹¹B NMR spectra at room temperature a substantial mobility is only seen for the [B₁₂Cl₁₂]²⁻ anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B₁₂X₁₂]²⁻ clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental ¹¹B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M₂ [B₁₂H₁₂] is achieved by line shape simulations, considering [B₁₂H₁₂]²⁻ ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M⁺, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction. Authors' address: Klaus Müller, Institut für Physikalische Chemie, Universit?t Stuttgart, Pfaffen-waldring 55, Stuttgart 70569, Germany     

Magnetic interactions in the layer compounds MPX3 (M = Mn,Fe, Ni; X = S,Se)

The mpx₃ phases (M = Mn, Fe, Ni; X = S, Se) with sheet structures are insulators with localized moments. They show antiferromagnetic ordering at low temperatures. From the magnetic structures determined by neutron diffraction the magnetic interactions are considered and shown to be characteristic of 2D behavior.     

Spiral spin structures of RMn6Ge6 (R=Gd, Tb, Dy)

The field dependences of the magnetic moment of several Gd_XY_1−XMn₆Ge₆ compounds have been analysed by a mean-field theory of several spin structures. The assumption of spiral spin structures at low temperatures in these compounds has been confirmed. In addition to this, the magnetic phase diagrams of TbMn₆Ge₆ and DyMn₆Ge₆ have been deduced from magnetisation measurements of powder samples. In contrast to GdMn₆Ge₆, the crystal fields of these compounds produce a variety of additional spin phases.     

Penetration depths of superconducting U6X (X = Fe,Co, Mn)

The penetration depths, λ(T), of the heavy-fermion uranium-based superconductors U₆X (X = Fe, Co, Mn) have been measured as a function of temperature at ∼ 35 MHz. It was found that the temperature dependence of λ(T) for these compounds agrees well with the BCS theory. However, the values of λ(0) are large. By comparing the Slater-Pauling curve of the X elements with λ(0)^-1 for the U₆X superconductors, we have found that λ(0)^-1 is proportional to the saturation spin moments of elemental X, suggesting that there is a correlation between the superconductivity of these U₆X superconductors and the magnetic nature of the X elements.     

Vibrational spectra of Matrix isolated binary and mixed cadmium dihalides

The Raman spectra CdX₂ and CdXY (X, Y = Cl, Br, I) molecules isolated in solid krypton at 20°K are reported together with the matrix isolation infrared spectrum of CdX₂ and CdXY (X, Y = Br, I) molecules. Isotopic effects and force constants are discussed and several irregularities in them are interpreted as possible indications of slightly bent structures. The bending vibration frequency of CdClI as well as several bands assigned to dimers are also reported.     

Accurate wavenumbers of the AΣ → XΠ (0, 0) and (1, 0) bands of OH and OD

The important A²Σ → X²Π (0, 0) absorption band of OH has been remeasured from spectra obtained by flash photolysis of H₂O₂ vapor. Accurate wavenumber measurements using thorium standards show that the previous rotational line measurements are about 0.1 cm^-1 too large. Measurements are also given for the (1, 0) band of OH and the (0, 0) and (1, 0) bands of OD.     

Preparation and properties of disordered NaBi(XO4)2, X=W or Mo, crystals doped with rare earths

NaBi_1−xRE_x(XO₄)₂, X=W or Mo and RE=Pr, Nd, Ho, Er and Yb single crystals have been grown by the Czochralski technique. Rare earth concentrations about 3.5×10²⁰ cm⁻³ have been achieved in crystals with good optical quality. Melt stability is obtained by synthesising NaBi(XO₄)₂ from the precursor Na₂X₄O₁₃ phase and minimising Mo volatility. The strength of W and Mo compounds to chemical attack and thermal annealing in several atmospheres is reported. Mo compound is etched by inorganic acids and becomes coloured after vacuum annealing. The optical absorption, photoluminescence and refractive indices of the hosts are characterised and show a dichroic character. The lattice disorder induces broadening of the 10 K optical absorption of the rare earth impurities.     

Chemical shifts of copper and cobalt K absorption discontinuities in the spinels CuCr2X4 (X =O,S, Se,Te), CoCr2X4 (X =O,S) and Cu0.5Co0.5Cr2−xRhxS4 (x = 0, 1, 2)

The present paper reports the chemical shifts of the copper and cobalt K absorption discontinuities in the spinels CuCr₂X₄ (X = O, S, Se, Te), CoCr₂X₄ (X = O, S) and Cu_0.5Co_0.5Cr_2?xRh_xS₄ (x = 0, 1, 2). The magnitudes of the chemical shifts show that copper is monovalent (except in CuCr₂O₄) and cobalt is bivalent in all these spinels. The valence structure for the spinels containing copper shows that the conductivity of such spinels (except CuCr₂O₄) is due to the holes in the anion p-orbitals. Levine's theory of ionicity has been extended to spinels which has made it possible to calculate the various bond parameters for the spinels CuCr₂X₄ and CoCr₂X₄. It has been found that a linear relation exists between the X-ray chemical shift ΔE and C, E_g, and f_i, the ionic energy, total energy and ionici parameters respectively. This fact has been used to determine the various bond parameters in the spinels Cu_0.5Co_0.5Cr_2?xRh_xS₄. It is found that in these spinels a natural balance of ionicity at the A site is maintained, when chromium is gradually replaced by rhodium.