Cl35 NQR study of the structural phase transition in (CH3NH3)HgCl3

The Cl³⁵ nuclear quadrupole resonance spectra of (CH₃NH₃)HgCl₃ have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around T_c? 60°C. The transition may be related to a disordering of the CH₃NH₃ groups which are reorienting both above and below T_c. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH₃NH₃ motion.     

EPR study of Mn2+ around the ferroelastic transition point of Pb3(PO4)2

The spin Hamiltonian parameters of Mn²⁺ have been measured above and below the transition point (180°C) of the lead phosphate. They show that Mn²⁺ substitutes a Pb_I ion. Between 175 and 180°C the principal axis OX of the fine tensor is parallel to the wave vector of the soft mode which condensates at the transition point. An exaltation of the linewidth is observed. The linewidth remains constant within 5°C of T_c; in this temperature range, the “static regime” is achieved, and the correlation time of the fluctuations is less than 10^?8 sec.     

Temperature dependent core photoemission in Ce24Co11

We present Ce 3d photoemission results (XPS with Al Kα) in the temperature range 100–660°K. The mixed valence behaviour of Ce is very clear with an increase of the valence at lower temperature. A model analysis (of the Gunnarsson and Schönhammer type) gives the weight of the ?₀ configuration equal to 0.19 at 300°K and equal to 0.23 at 100°K. This soft temperature dependence is discussed in connection with the temperature dependence of magnetic properties and with the chemistry of Ce intermetallics.     

The electric field gradient at Ta in HfB2

Using the time-differential perturbed-angular correlation technique, we have observed the nuclear electric quadrupole interaction at ¹⁸¹Ta (482 keV) as an impurity in the refractory compound HfB₂. The measured interaction frequency is ν_Q=730±5 MHz which corresponds to an electric field gradient of |eq|=(1·19±0·05)×10¹⁸ V/cm² at room temperature. By considering a second measurement at 4·2°K, which yields the same results as above, and by comparison with available results for Hf in HfB₂, we conclude that the d-electron density of states at the Fermi level is quite small in agreement with trends observed by others.     

Calculation of absorption coefficient for the ν2 and ν3 bands of sulfur dioxide

Absorption coefficients S/d of sulfur dioxide were calculated in the ν₂ and ν₃ bands for a temperature range from 300 to 1200°K. Calculations were carried out for such vibrational transitions that their integrated absorption coefficients α at T°K are greater than 0.1% of the total integrated absorption coefficients α^obs observed at 300°K. Rotational lines with intensities greater than 10^-6 cm^-2 atm^-1 STP were included in the calculation. The line intensities were averaged over small wavenumber intervals of Δω = 10 cm^-1 throughout the bands to obtain the absorption coefficients S/d. They were compared with the experimental values of other investigators and good agreement was obtained.     

Magnetic properties of V5S8 single crystals

The magnetic properties of V₅S₈ single crystals have been investigated by susceptibility and torque measurements. The susceptibilities parallel and perpendicular to the magnetic easy axis show a remarkable anisotropy below the Neel temperature of 32 K and are nearly Isotropie above that temperature. The easy axis is found to be nearly along the c-axis of the monoclinic lattice, being inclined at an angle of 9.6 ± 1.0° from the c-axis toward the a-axis. The torque curves, measured up to 24.4 kOe at liquid helium temperature, deviate from the usual form of a sine function with increasing magnetic field. The analysis of these torque curves suggests that spin flopping may occur at 43 kOe, a comparatively low critical field. Using these experimental results, a localized d-electron model for a particular site of the vanadium-ion, proposed by previous investigators, is examined.     

Thermal expansion of ZrFe2 and some rare-earth iron compounds

The thermal expansion coefficient of the compounds ZrFe₂, TiFe₂, YFe₂, ErFe₂, ErFe₃, Er₆Fe₂₃ and Er₂Fe₁₇ has been determined from room temperature up to 700°C. The anomalous thermal expansion of some of these compounds observed at temperatures below their Curie temperature (T_c) is shown to be related to a strong pressure dependence of T_c. (dT_c/dP is large and negative).     

Polymorphism in Bi2Sn2O7

Single crystals of Bi₂Sn₂O₇ were grown in a Bi₂O₃ flux. Phase transitions were identified at about 90 and 680° using X-ray, SHG, DSC, dielectric, and optical data. γ-Bi₂Sn₂O₇, which exists above 680°C is centric and cubic with a = 10.73 Å at 700°, and it probably has the ideal pyrochlore structure. β-Bi₂Sn₂O₇, which exists between 680° and about 90°C, is acentric but remains cubic with a = 21.40 Å. α-Bi₂Sn₂O₇, which exists from about 90°C to below room temperature, is acentric and noncubic, probably tetragonal with a = 21.328 and c = 21.545 Å. The α-β transition is first order, and the β-γ transition appears to be second order. Substitutions of Pb²⁺ or Cd²⁺ for Bi³⁺ and of Ga³⁺, Rh³⁺ Sc³⁺, In³⁺, Sb⁵⁺ Nb⁵⁺ or Ta⁵⁺ for Sn⁴⁺ lower the α-β transition temperature.     

Magnetic ordering of a d1 compound: VF4

Magnetic susceptibility and nuclear magnetic resonance above and below the ordering temperature show that VF₄ is an $\text{S =}\text{1}\text{2}$, 3d¹ canted antiferromagnet with a transition temperature near 28°K and a canting angle of ～ .02 rad. The compound is not an example of a proposed nonmagnetic singlet ground state of resonating covalent bonds.     

Temperature dependence of the electric quadrupole interaction in PbHfO3

The temperature dependence of the Electric Field Gradient (EFG) in PbHfO₃ was studied in the temperature range 25–225°C by the Differential Perturbed Angular Correlation method. In the two anti-ferroelectric phases below 215°C, not too close to either transition temperature, the EFG decreases as the temperature increases toward T_c. Just above T_c an abrupt rise of EFG was observed indicating a critically behaving contribution to EFG. The results are interpreted in terms of a model based on the local static as well as time-dependent changes of the electric environment, at a lattice site. In accord with the predictions of this model the results exhibit qualitatively the P²_s temperature dependence of EFG far from T_c, where P_s is the sub-lattice polarization, while in close proximity to T_c the dominant contribution to EFG is due to the susceptibility X_q connected with the soft-mode fluctuations. The derived critical exponents are in agreement with previous experimental results on related compounds and with theoretical predictions. A hitherto unobserved additional component of V_zz was established, behaving critically at the antiferro-paraelectric transition at 215°C. This new component is interpreted to originate in local fluctuations connected with the central mode.     

Superstructure and phase transitions in CsPbCl3

Superstructure of CsPbCl₃ is investigated by X-ray diffraction experiments. In the room temperature phase, three types of superlattice reflections are observed which correspond to the X, M and R points of the cubic Brillouin zone. On heating from room temperature, an integrated intensity of the X point reflection exhibits a discontinuous decrease at 37°C, while the R and M point reflections disappear at 42 and 47°C respectively. Explanations of these phase transitions in terms of condensation of soft-zone boundary phonon modes are presented.     

Covalent distribution of spin in V2O3:O17 nuclear resonance

O¹⁷ nuclear magnetic resonance has been observed in metallic V₂O₃ with frequency shifts from (?0.10 ± 0.02)-(?0.05 ± 0.02) per cent between 170 and 460°K respectively, a linewidth of 37 ± 5 oe and spin-lattice relaxation rate 1/T₁ ≈ 60 sec^?1 at 296°K. From these quantities, covalency parameters f_s/2S = ? 0.35 × 10^?3 and ?_π/2S ≈ ? 0.07 are calculated. One of the two vanadium 3d electrons in the antiferromagnetic state below the 170°K metal-insulator transition is inferred to lie in a non-magnetic state, while covalent charge transfer augments the spin moment of the other 3d electron to the observed 1.2 μ_B.     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

Ferroelectric and optical properties of Ba6Ti2Nb8O30 single crystals

The dielectric, optical and non-linear optical properties of Ba₆Ti₂Nb₈O₃₀ single crystals were examined from room temperature up to the Curie temperature of 245°C. The spontaneous polarization at room temperature was estimated as 0·22±0·01 C/m². The linear electrooptic constants were measured as r₃₃^T=(1·17±0·02)×10^?10 and r₁₃^T=(0·42±0·01)×10^?10 m/V. The non-linear optical coefficients were d₃₃=(15·1±2·0)×10^?12 and d₃₁=(11·0±2·0)×10^?12 m/V, which are comparable to those of Ba₄Na₂Nb₁₀O₃₀. Temperature dependences of δ₃₃ and δ₃₁ (Miller's δ) were found to be proportional to that of P_s.     

Ferroelasticity in BiVO4

A transition in BiVO₄ crystals has been observed at 255°C. Optical and X-ray studies indicate that this is a ferroelastic transition of the type 4/m F2/m.     

The mixed conductor properties of AgCrS2

A reinvestigation of the mixed conductivity of AgCrS₂ shows that the ionic conductivity increases from 3 × 10^-5 (Ω-cm)^-1 at 25°C to 0.3 (Ω-cm)^-1 just above the order-disorder transition temperature of 400°C, whereas the electronic part is much smaller. Structural evidence is presented, that a fraction of the silver ions occupy off-site octahedral positions in the disordered phase.     

High temperature phase transition in NH4H2PO4

NH₄H₂PO₄ (ADP) has been investigated by infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and d.c. conductivity in the temperature range of 25–180° C. Sharp reversible changes were observed in the region from 400 to 500 cm^?1 of the infrared spectra in the temperature range of 138–174° C. Similar and supportive data were obtained with DSC, TGA and DC conductivity measurements. The results clearly suggest a high temperature phase transition for ADP before its melting point.     

Raman scattering from three phases in LiIO3

We studied the temperature dependence of the Raman spectrum of LiIO₃, from room temperature up to t ～ 350°C. Two discontinuous changes in the spectrum are observed as temperature increases. The first one is reversible and occurs in a temperature range between 215°C and 260°C, depending upon sample origin (single crystal or powder) and thermal history. The second occurs at t ? 290°C and becomes irreversible once the samples are heated above 340°C. Each phase has a characteristic spectrum, distinct from that of the other two. Although the occurence of these phase changes are in complete agreement with studies made with X-ray diffraction and differential thermal analysis (DTA), it is at variance with previous Raman and infrared work which report no qualitative change in spectrum at the α ? γ phase transition. We believe this disagreement comes about because our measurements are the first ones so far to have actually passed through the transition.     

Infrared study of cis-trans isomerization of (CH)x films

Cis-trans isomerization of polyacetylene has been investigated by infrared spectroscopy over the temperature range 115–180°C. It is shown that isomerization is slow at 115°C and becomes rather fast above 140°C. The chemical processes during the isomerization appear to be complex, and a beginning of decomposition occurs above 140°C.     

High efficiency second-harmonic generation in KNbO3 crystals

Second-harmonic generation in KNbO₃ crystals has been studied with CW and pulsed Al_xGa_1?xAs and dye lasers. High conversion efficiencies were obtained by noncritical Type I phase-matching at room temperature for the wavelength λ_PM = (860±2) nm using the nonlinear optical coefficient d₃₂ = 21 pm/V. The measured peak power was P_2ω=13.6 μW for a fundamental power P_ω=21 mW with a crystal of l = 8.97 mm in length. The phase-matching wavelength in the orthorhombic phase of KNbO₃ can be temperature tuned between 838 nm (for T = -36°C) and 950 nm (for T = +180°C).