Dielectric observation of a probably antiferroelectric high pressure phase in the ferroelectric Tris-Sarcosine Calcium Chloride (TSCC)

Dielectric susceptibility and ferroelectric hysteresis measurements on TSCC show that the 2nd-order paraelectric-ferroelectric (PE-FE) phase boundary beginning at (130.8 K, 1 bar) runs with pronounced curvature to (176.8 K, 5.03 kbar), where a new phase of low dielectric susceptibility appears. This phase is hypothesized to be antiferroelectric (AFE) with 3 possible structures: orthorhombic (P2₁2₁2₁, piezoelectric) or monoclinic ($\text{P2}{}_1\text{a}$ or $\text{P2}{}_1\text{n}$, centrosymmetric). The FE-AFE phase boundary was determined down to (81.0 K, 2.60 kbar) and the PE-AFE boundary up to (225.5 K, 6.82 kbar). Both the FE-AFE and PE-AFE transitions are of 1st order over the range studied.     

Anomalous dielectric behaviour of La(III) substituted lead titanate ceramics

In La(III) substituted PbTiO₃ ceramics the behaviour of the reciprocal dielectric constant in the paraelectric phase can be described by the relation $\text{1}\text{?～}\text{(T ? T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{\gamma }}$ with the exponent 1 ?γ<2 depending on the composition. This γ-value is an intermediate of values for Curie-Weiss behaviour (γ = 1) and of values connected with diffuse phase transitions (γ = 2) and increases with increasing La(III) concentration.An excellent agreement between experimental and calculated permittivity curves in the paraelectric phase can be achieved using an alternative equation consisting of a linear and a quadratic term with coefficients varying systematically with the composition. The last equation can be derived and interpreted by means of a physical model assuming the existence of heterophase fluctuations.     

Dielectric susceptibility and correlation length of one-dimensional CDW with impurties. II. weak disorder

Dependence of static dielectric susceptibility and correlation length of charge density waves (CDW) with weak defects on parameter of incommensurability with lattice is investigated. In almost commensurate phase (h?h_ch_c), $\text{χ ～ (h?h}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{In}{}^{\mathrm{<mtext>-4</mtext><mtext>3</mtext>}}\text{h}{}_{\mathrm{c}}\text{/h?h}{}_{\mathrm{c}}$ and R_c ～ (h ? h_c)^{$\text{2}\text{3}$}. In${}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{h}{}_{\mathrm{c}}\text{/h ? h}{}_{\mathrm{c}}$. Far from commensurability $\text{(h?h}{}_{\mathrm{c}}\text{) χ～ (a+h}{}^2{}_{\mathrm{c}}\text{/h}{}^2\text{)}{}^{\mathrm{<mtext>-2</mtext><mtext>3</mtext>}}$, $\text{R}{}_{\mathrm{c}}\text{～ (a + h}{}^2{}_{\mathrm{c}}\text{/h}{}^2\text{)}{}^{\mathrm{<mtext>-2</mtext><mtext>3</mtext>}}$, where a is the dimensionless ratio of random potential intensities, corresponding to backward and forward scattering impurities.     

Pressure-induced low-temperature transitions in TMMC

Measurements of the pressure dependence of the low-frequency dielectric constant ? and high pressure, low temperature X-ray diffraction photographs on (CH₃)₄ (TMMC) have shown that the low temperature $\text{P2}{}_1\text{aa}$ phase of TMMC transforms to the low temperature $\text{P2}{}_1\text{m}$-like phase of (CH₃)₄ NCdCl₃ (TMCC) at high pressure. The pressure-temperature phase diagram has been deliniated from the triple point at 2.0 kbar and 137° K to lower temperatures, and, from changes in the dielectric constant anomaly, it appears that another triple point may occur at ～ 1.0 kbar and 95° K. The pressure and temperature dependences of ? together with the observed doubling of the unit cell suggest that the $\text{P2}{}_1\text{a}$ low temperature phase of TMMC may be antiferroelectric.     

Structure of positive parity states in 29P and two-step processes in (p,t) reactions

From a study of (p,t) reactions on ³¹P and ³⁰Si it is suggested that in ²⁹P the states with $\text{J}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}_1{}^{\mathrm{+}}$ and $\text{1}\text{2}{}_2{}^{\mathrm{+}}$, the pair $\text{3}\text{2}{}_2{}^{\mathrm{+}}$, $\text{5}\text{2}{}_1{}^{\mathrm{+}}$, and the pair $\text{7}\text{2}{}_3{}^{\mathrm{+}}$, $\text{9}\text{2}{}_1{}^{\mathrm{+}}$ are related by weak coupling of a s${}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ proton with the states 0₁⁺, 0₂⁺, 2₁⁺ and 4₁⁺ respectively of ²⁸Si. Completely atypical L = 2 angular distributions have been obtained for the $\text{3}\text{2}{}_1{}^{\mathrm{+}}$ and $\text{5}\text{2}{}_2{}^{\mathrm{+}}$ states in ²⁹P and it is suggested that this is due to contribution by two-step processes.     

Coriolis intensity perturbations in the SO2 molecule: Experimental determination of the relative signs of (∂μ∂Qi)

The Coriolis interactions between ν₁ and ν₃, and between ν₂ and ν₃ in SO₂ have been analyzed to obtain the signs of the products $\text{ζ}{}_{3.1}{}^{\mathrm{c}}\text{(}\text{?μ}{}^{\mathrm{a}}\text{?Q}{}_3\text{)(}\text{?μ}{}^{\mathrm{b}}\text{?Q}{}_1\text{)}$ and $\text{ζ}{}_{3.2}{}^{\mathrm{c}}\text{(}\text{?μ}{}^{\mathrm{a}}\text{?Q}{}_3\text{)(}\text{?μ}{}^{\mathrm{b}}\text{?Q}{}_2\text{)}$. It has been found that both of the signs of these products are positive. Then, relative signs of ($\text{?μ}\text{?Q}{}_1$) have been determined using the calculated values of the Coriolis zeta constants for the present definition of the normal coordinates. The obtained sign combination of ($\text{?μ}\text{?Q}{}_{\mathrm{i}}$) is ±(+?+), which agrees with the one predicted by the molecular orbital calculations. Using the sign combination (+?+), the polar tensors of S and O atoms were also calculated.     

Spectral and temperature characteristics of Cs2Na(Er,Yb, Y)Cl6 upconverters

Spectral luminescence characteristics of the Cs₂Na(Er, Yb, Y)Cl₆ type upconverter have been measured in the range 300–650 K. A nearly sixfold decrease of the emission intensity of an upconverter was observed in the green area. This emission corresponds to two luminescence transition of Er³⁺ ions: ${}^2\text{H}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$ and ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$. A mechanism of the luminescence intensity decrease has been proposed, in which deexcitation of the ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level to the ${}^4\text{I}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ level is caused by a neighbouring Er³⁺ ion being excited to the ${}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ level.     

The spectrum and structure of the He2 molecule: Multichannel quantum defect analyses of the triplet levels associated with (1σg)2(1σu)npλ

Emission spectra for the electronic transitions $\text{6–17pπ}{}^3\text{Π}{}_{\mathrm{g}}\text{-2s a}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ and $\text{7–16pσ}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{-2s a}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ of He₂ are reported and the electronic structures of $\text{npπ}{}^3\text{Π}{}_{\mathrm{g}}{}^{\mathrm{?}}$ and $\text{np(}{}^2\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{,}{}^3\text{Π}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{)}$ characterized. The energy levels associated with $\text{(1σ}{}_{\mathrm{g}}\text{)}{}^2\text{(1σ}{}_{\mathrm{u}}\text{)np(}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{,}{}^3\text{Π}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{)}$ exhibit extensive channel mixing, which leads to a breakdown of conventional band models for the higher n¹-members of these Rydberg “series.” However, a model based on multichannel quantum defect theory quantitatively correlates the observed level structures. The higher-energy ($\text{n}{}^1\text{> ～5}$) portions of the $\text{np(}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{,}{}^3\text{Π}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{)}$ channels can be represented by two eigen-quantum defects μ₁ = 0.225 and μ₂ = 0.930 and the close- to loose-coupling matrix elements $\text{U}{}_{11}\text{= U}{}_{22}\text{= [N(2N + 1)}{}^{\mathrm{?1}}\text{]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{U}{}_{12}\text{= ?U}{}_{21}\text{= [(N + 1)(2N + 1)}{}^{\mathrm{?1}}\text{]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. The inclusion of energy dependence in the μ_α's leads to quantitative correlations for all n¹-values.     

Laser excitation spectra of He2

Metastable $\text{a(2sσ)}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ He₂ molecules are produced by a dc discharge in a flowing He stream. Laser excitation downstream of the discharge produces excitation spectra for a number of He₂ states. LIF spectra are observed for the $\text{(npπ)}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ series for n = 4–9, excepting 5 and the $\text{(npπ)}{}^3\text{Π}{}_{\mathrm{g}}$ series for n = 5–15.     

Chemiluminescence of IF in the gas phase reaction of I2 with F2

Emission from both the state and the previously unreported $\text{A}{}^3\text{Π}{}_1$ state of IF has been observed in the gas phase reaction of I₂ with F₂ at low pressures. For the state the transition moment and vibrational populations were extracted from the spectra by a least-squares method whereby theoretical band shapes were fit to the experimental data. The effect of flow rates of reactants and Ar on the relative emission from the two electronic states, the effect of pressure on the state, and extinction of emission near 470 nm all favor the population of excited electronic states through a four-center reaction complex, rather than association of F and I atoms.It is argued that there is an avoided curve crossing between the lowest two ³Π_0⁺ states of IF, and that the ground state dissociation energy is 23 229 ± 100 cm^?1. The radiative lifetime of the state is estimated to be 10^?3 sec and to be much shorter than that of the $\text{A}{}^3\text{Π}{}_1$ state.     

The ã3A2 ← X̃1A1 absorption spectrum of thioformaldehyde: A vibrational and rotational analysis

The results of a vibrational and rotational analysis of the banded $\text{a}\text{?}{}^3\text{A}{}_2\text{←}\text{X}\text{?}{}^1\text{A}{}_1$ transition in $\text{CH}{}_2\text{S}\text{CD}{}_2\text{S}$ are presented. Only three of the six vibrational modes are active in the spectrum with $\text{ν′}{}_2\text{=}\text{1320}\text{1012}$, $\text{ν′}{}_3\text{=}\text{859}\text{798}$, and $\text{2ν′}{}_4\text{=}\text{711}\text{516}\text{cm}{}^{\mathrm{?1}}$. The spin forbidden transition gains intensity primarily by a mixing of the ${}^1\text{A}{}_1\text{(π}{}^1\text{,π)}$ and ${}^3\text{A}{}_2\text{(π}{}^1\text{,n)}$ states. This is confirmed by a rotational analysis of the 0₀⁰ band of both isotopes. The rotational analysis shows that the coupling in the $\text{a}\text{?}{}^3\text{A}{}_2$ state is near Hund's case b and that the spin constants are nearly 10 times greater than those observed for CH₂O. A $\text{CNDO}\text{2}$ calculation shows that this difference is due to the greater spin orbit coupling of S in CH₂S and to the smaller energy differences between the $\text{B}\text{?}{}^1\text{A}{}_1\text{(π}{}^1\text{,π)}$, $\text{b}\text{?}{}^3\text{A}{}_1\text{(π}{}^1\text{,π)}$, $\text{X}\text{?}{}^1\text{A}{}_1$, and the $\text{a}\text{?}{}^3\text{A}{}_2\text{(π}{}^1\text{,n)}$ states. The r₀ structure calculated from the rotational constants is $\text{r}{}_{\mathrm{<mtext>CS</mtext>}}\text{= 1.683}\text{A}\text{?}$, $\text{r}{}_{\mathrm{<mtext>CH</mtext>}}\text{= 1.082}\text{A}\text{?}$, β_HCH = 119.6°, and α (out of plane) = 16.0°. A simultaneous fit of the vibrational levels in ν′₄ of CH₂S and CD₂S to a double minimum potential function yielded a barrier to molecular inversion of 13 cm^?1 and an equilibrium out-of-plane angle of 15°.     

Ni2+ emission in MgO,KMgF3, KZnF3 and MgF2

The emission of Ni²⁺ ions in MgO, KMgF₃, KZnF₃ and MgF₂ crystals has been investigated. The fine structure on the bands at about 20 000 cm^-1 and 13 000 cm^-1 has been studied in detail and from this and the excitation spectra these bands are assigned to ${}^1\text{T}{}_{\mathrm{2g}}\text{→}{}^3\text{A}{}_{\mathrm{2g}}$ and ${}^1\text{T}{}_{\mathrm{2g}}\text{→}{}^3\text{T}{}_{\mathrm{2g}}$ transitions respectively.     

A new ferroelectric compound: Methylammonium trichloromercurate (CH3NH3HgCl3)

Measurements of the dielectric constants revealed a ferro- to paraelectric transition in methylammonium trichloromercurate (CH₃NH₃HgCl₃). Thermal analysis by differential scanning calorimetry indicates a transition temperature of Tc ≈ 60°C. The structures of both phases were determined from single crystal X-ray measurements. The ferroelectric phase crystallizes at room temperature in the polar trigonal space group $\text{P}\text{3}{}_2\text{(a = b =}\text{7.8117(3)}\text{, c =}\text{9.826(3)}\text{A}\text{?}\text{, Z =}\text{3}\text{)}$. The refinement of the ferroelectric structure included the fractional contribution of the two domains present. The paraelectric phase has monoclinic symmetry (space group C2 with $\text{a =}\text{13.816(2)}\text{, b =}\text{7.880(1)}\text{, c =}\text{9.734(3)}\text{A}\text{?}\text{, β =}\text{90.49(5)°}\text{, Z =}\text{6}\text{)}$ and contains an almost completely disordered methylammonium group while order with pronounced thermal motion is observed in the ferroelectric phase.     

N2+ bound quartet and sextet state potential energy curves

Potential energy curves for the ${}^4\text{Σ}{}^{\mathrm{+}}{}_{\mathrm{u}}$, ${}^4\text{Π}{}_{\mathrm{g}}$ and ${}^6\text{Σ}{}^{\mathrm{+}}{}_{\mathrm{g}}$ states of N⁺₂ that dissociate to N (${}^4\text{S}{}^0$) and $\text{N}{}^{\mathrm{+}}\text{(}{}^3\text{P)}$, have been determined from a complete active space self-consistent field calculation. The ${}^6\text{Σ}{}^{\mathrm{+}}{}_{\mathrm{g}}$ state is found to be significantly bound (D_e = 2.68 eV) with a minimum at 1.72 Å.     

The ultraviolet absorption spectrum of thioformaldehyde

The spectra of H₂CS and D₂CS were surveyed over the wavelength region from 230 to 180 nm and four distinct absorptions were identified. These are assigned to transitions from the $\text{X}\text{?}{}^1\text{A}{}_1$ ground state to the $\text{B}\text{?}{}^1\text{A}{}_1\text{(π, π}{}^1\text{)}$, $\text{C}\text{?}{}^1\text{B}{}_2\text{(n, 3s)}$, $\text{D}\text{?}{}^1\text{A}{}_1\text{(n, 3p}{}_{\mathrm{y}}\text{)}$, and $\text{E}\text{?}{}^1\text{B}{}_2\text{(n, 3p}{}_{\mathrm{z}}\text{)}$ electronic states. A vibrational and rotational analysis of the second system was undertaken. The results indicate that the molecule is planar in the $\text{C}\text{?}{}^1\text{B}{}_2\text{(n, 3s)}$ state and that while the CH and CS bond lengths remain near their ground-state values, the HCH angle increases substantially.     

Magnetic Phase transitions and hysteresis in FeCl2 · 2H2O

The magnetic phase diagram of FeCl₂ · 2H₂O has been determined by means of single crystal neutron diffraction experiments. Isothermal and isobaric measurements reveal the existence of first order and second order phase transitions separated by tricritical points at $\text{t}{}_1{}^1\text{= 0.5, h}{}_1{}^1\text{= 0.91}\text{and}\text{t}{}_2{}^1\text{= 0.39, h}{}_2{}^1\text{= 0.99}$. Considerable hysteresis effects were observed at the antiferro—ferrimagnetic phase boundary at temperatures t < 0.33.     

Activation energy of field emission flicker noise power due to adsorbed potassium on tungsten

The temperature dependence of the field emission flicker noise spectral density functions has been investigated for potassium adsorbed on tungsten (112) planes by a probe hole technique. By integration of the spectral density functions $\text{W(?) = B}{}_{\mathrm{i}}\text{?}{}^{\mathrm{?ge}}\text{i}$ the noise power $\text{(δn}{}^2{}_{\mathrm{\Delta ?}}$ for different frequency intervals Δ? is obtained. From the exponential temperature dependence of $\text{(δn}{}^2{}_{\mathrm{\Delta ?}}$ noise power “activation energies” q_Δ? are determined. Plots of these energies versus coverage show a similar “oscillating” behaviour as recently found for $\text{W(?}{}_{\mathrm{j}}\text{)}$ or which indicates phase transitions of the adsorbed potassium submonolayers. The noise activation energies are discussed in terms of existing models and a comparison is made between the experimental q values and surface diffusion energies E_d as determined by conventional methods.     

Energies and orbital sizes for some Rydberg and valence states of the nitrogen molecule

Accurate SCF computations are reported on the Rydberg states of N₂ of electron configurations $\text{---1π}{}_{\mathrm{u}}{}^3\text{2π}{}_{\mathrm{u}}$, $\text{---1π}{}_{\mathrm{u}}{}^3\text{3σ}{}_{\mathrm{u}}$, and ---3σ_g2π_g, also on the valence states of the configuration $\text{---1π}{}_{\mathrm{u}}{}^3\text{1π}{}_{\mathrm{g}}$. The Rydberg state calculations supplement those of Lefebvre-Brion and Moser. A comparison is made between the $\text{---1π}{}_{\mathrm{u}}{}^3\text{2π}{}_{\mathrm{u}}$ states and the parallel set of states of the $\text{1π}{}_{\mathrm{u}}{}^3\text{1π}{}_{\mathrm{g}}$ configuration. This comparison shows a sharp difference in the ¹Σ⁺ states of the two configurations, the ¹Σ⁺ state being very high in the latter but relatively low in the former configuration. Recknagel coefficients are given for the several states of the two configurations; as expected, these are much smaller for the $\text{1π}{}_{\mathrm{u}}{}^3\text{2π}{}_{\mathrm{u}}$ configuration. Also, the ¹Δ state is relatively lower for the latter configuration.     

Magnetic circular dichroism of transition metal ions in solids—II K2NaGaF6: Cr3+

The absorption and MCD spectra of the ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{2g}}$, ${}^4\text{A}{}_{\mathrm{2g}}$, ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{1g}}{}^{\mathrm{a}}$ and ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{1g}}{}^{\mathrm{b}}$ spin-allowed transitions of Cr³⁺ in K₂NaGaF₆ are reported. It is shown that transitions to the ${}^4\text{T}{}_{\mathrm{1g}}$. states are induced by T_1u vibratio the other spin-allowed transition, ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{2g}}$, there are three competing intensity mechanisms: electric dipole induced by T_1u vibrations, electric dipole induced by T_2u vibrations and magnetic dipole, and an estimate is made of the relative importance of these. The magnetic dipole ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^2\text{E}{}_{\mathrm{g}}$ zero-phonon line is observed to be accompanied by a vibrational sideband for which the coupling is predominantly with T_2u vibrations. Other weak transitions are observed in MCD spectra and their origin briefly discussed.     

Infrared diode laser spectroscopy of the NS radical

The v = 1 ← 0 vibration-rotation bands of the NS radical in the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electronic states were observed by using a tunable diode laser. From the least-squares analysis the band origins were determined to be 1204.2755(12) and 1204.0892(19) cm^?1, respectively, for $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. The rotational and centrifugal distortion constants and the internuclear distance in the X²Π electronic state were obtained as follows: B_e = 0.775549(10) cm^?1, D_e = 0.00000129(33) cm^?1, and $\text{r}{}_{\mathrm{e}}\text{= 1.49403(4)}\text{A}\text{?}$, with three standard deviations indicated in parentheses.