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.     

Pressure dependence of the magnetic transition fields of the quasi two-dimensional antiferromagnet (C2H5NH3)2 CuCℓ4

Using magnetic susceptibility measurements, we have studied the pressure dependence of the magnetic transition fields of powder samples of antiferromagnetic (C₂H₅NH₃)₂CuC?₄, for hydrostatic pressures up to P ～ 7 kbar. The spin-flop field H₁ decreases linearly with pressure $\text{(}\text{dH}{}_1\text{dP}\text{= ?(20±4)}\text{G}\text{kbar}\text{)}$, while the field corresponding to the paramagnetic transition H₂ shows an increase, ～ 70% for P～7 kbar, that is roughly quadratic with P.     

Etude des proprietes thermodynamiques d'un groupement tetranucleaire: Application au compose Na3RuO4

The exact eigenvalues spectrum of the spin hamiltonian $\text{H}\text{= ?2}\text{∑}\text{(i, j)}\text{J}{}_{\mathrm{ij}}\text{S}\text{?}{}_{\mathrm{i}}\text{S}\text{?}{}_{\mathrm{j}}$ have been calculated for a tetranuclear cluster formed by four spins 3/2 at the vertices of a lozenge. Two isotrope exchange interactions J₁ and J₂ are able to explain the thermodynamic properties (magnetic susceptibility, entropy, specific heat). A ground state transition from singlet to triplet state occurs when the $\text{J}{}_2\text{J}{}_1$ ratio reaches the value $\text{4}\text{3}$. The magnetic susceptibility data of Na₃RuO₄ fit well with the theoretical values proposed for $\text{J}{}_1\text{K}\text{= (?19,5 K)}$ and J₂/k (?22,5 K).     

On the frequency dependence of the transition temperature in spin glasses

For the first time, the frequency dependence of T_f (temperature of the maximum of the a.c. susceptibility of spin-glasses) is shown to obey a Fulcher law $\text{τ = τ}{}_{\mathrm{o}}\text{exp}\text{[}\text{E}{}_{\mathrm{a}}\text{k(}\text{T}{}_{\mathrm{f}}\text{?}\text{T}{}_{\mathrm{f}}\text{)}\text{]}$. This is observed as well in the case of dilute alloys (or R.K.K.Y. spin-glasses : $\text{Cu}$Mn, $\text{Au}$Fe, …) as for frustrated systems (Eu_1?xGd_xS, Eu_xSr_1?xS …). For R.K.K.Y. spin-glasses, only in the case of a very small amplitude, V_o of the R.K.K.Y. interaction, this time dependence approaches an Arrhenius law. In the case of “frustrated” spin-glasses the concentration is the main parameter to determine the kind of frequency dependence of T_f. These properties are evidence for a glass-like phase transition in spin-glasses. The scaling of the frequency dependence of T_f with V_o is justified for R.K.K.Y. spin-glasses from present data.     

Temperature and pressure dependence of the elastic property of GeS2 glass

The sound velocities in GeS₂ glass have been measured by means of ultrasonic interferometry as a function of temperature or pressure up to 1.8 kbar. The bulk modulus K_s = 117.6 kbar and shear modulus G = 60.60 kbar were obtained for GeS₂ glass at 15°C and 1 atm. The temperature derivatives of both sound velocities and elastic moduli are negative :

$\text{(}\text{?ν}{}_1\text{?T}\text{)}$

_p =

$\text{?1.54 × 10}{}^{\mathrm{?4}}\text{km}\text{sec}$

°C,

$\text{(}\text{?ν}{}_1\text{?T}\text{)}$

_p =

$\text{?1.27× 10}{}^{\mathrm{?4}}\text{km}\text{sec}$

°C and

$\text{(}\text{?K}{}_{\mathrm{s}}\text{?T}\text{)}$

_p =

$\text{?1.27 × 10}{}^{\mathrm{?2}}\text{kbar}\text{°C}$

,

$\text{(}\text{?G}\text{?T}\text{)}$

_p = ?1.23 × 10^?2 kbar/°C,

$\text{(}\text{?Y}\text{?T}\text{)}$

_p = ?2.93 × 10^?2 their pressure derivatives are positive:

$\text{(}\text{?ν}{}_1\text{?P}\text{)}$

_T = 4.43× 10^?2km/kbar,

$\text{(}\text{?ν}{}_1\text{?P}\text{)}$

_T =

$\text{0.633 × 10}{}^{\mathrm{?2}}\text{km}\text{kbar}$

and (?K_s?P0)_T=6.81,

$\text{(}\text{?G}\text{?P)}{}_{\mathrm{T}}$

= 1.03, (?Y?T_T= 3.57. The Grüneisen parameter, γ_th= 0.298, and the second Grüneisen parameter, δ_s = 3.27, have also been calculated from these data. The elastic behavior of GeS₂ glass has proved to be normal despite the structural similarity among the tetrahedrally coordinated SiO₂, GeO₂ and GeS₂ glasses.     

The cooperative Jahn-Teller effect in uranium (4+) hydroxy-sulphate

The magnetic susceptibility measurements of orthorhombic U(OH)₂SO₄ within the temperature range 4.2–300 K have revealed a magnetic anomaly at T_D = 21 K associated with crystallographic transition induced by the cooperative Jahn-Teller effect. Above 21 K the magnetic susceptibility of the uranium (4+) ion corresponds to the electronic ground doublet ${}_{\mathrm{¡}}\text{M}{}_{\mathrm{J}}\text{= ± 2〉}$ confirming thus the antiprismatic symmetry (D_4d) of the crystal field at the uranium site. Below T_D the system of two singlets ($\text{1}\text{√2}$)|2〉 ± ($\text{1}\text{√2}$)|$\text{2}$〉 separated by δ(T) is the ground state of the uranium ion.     

Photoluminescence of GaSb under hydrostatic pressure

Photoluminescence measurements on GaSb samples were carried out at low temperatures (2 – 5 K) and high pressures (0 – 9 kbar). The energy shift of the direct gap was determined: $\text{d}\text{E}{}^{\mathrm{\Gamma }}\text{/dP= 14.5± 0.3 meV/kbar}$. At pressures above 8 kbar the spectra showed additional structure from the indirect L-conduction band minima. The energy shift of the L-conduction bands were determined: $\text{d}\text{E}\text{L/d}\text{P}\text{= 5.5± 1. meV/kbar.}$ From these data the critical pressure for the inversion of the two conduction bands was calculated: $\text{p}{}^{\mathrm{h}}{}_{\mathrm{c}}\text{= 10.5 ± 1. kbar}$.     

A model for phase transitions in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ)

The observed phase transitions in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ) are discussed using a simple model for the interchain coupling of charge density waves. Estimates based on Coulomb energies show that for 38 K < T < 49 K the components $\text{q}{}_{\mathrm{x}}\text{=}\text{π}\text{a}\text{+ q′}{}_{\mathrm{x}}$ and q_z of the wave vector associated with the charge density wave satisfy $\text{q}{}_{\mathrm{z}}\text{c}\text{q′}{}_{\mathrm{x}}\text{a?0.1}$, with $\text{q′}{}_{\mathrm{x}}\text{a～(T}{}_2\text{? T)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and T₂ = 49 K. A possible mechanism for the first order transition at 38 K is proposed. The results are compared with neutron and X-ray scattering and with isotope shifts of the transition temperatures.     

Raman spectroscopic study of tris-sarcosine calcium chloride in the antiferroelectric phase

The polarized Raman spectra of (CH₃NHCH₂COOH)₃CaCl₂ (TSCC) have been obtained applying hydrostatic pressure in the paraelectric, ferroelectric and in the pressure-induced antiferroelectric phase. The phase transition between the paraelectric or the ferroelectric and the antiferroelectric phase appears to be of first order. No cell doubling could be observed in the antiferroelectric phase. The space group $\text{P2}{}_1\text{a (C}{}^5{}_{\mathrm{2h}}\text{)}$ for TSCC in this phase is compatible with our experimental results. The pressure-dependence of the Raman-active soft mode is discussed qualitatively.     

The pressure dependence of elastic moduli of NiF2 to 10 kbar

At room temperature, the pressure dependence of the elastic stiffness moduli of NiF₂ have been measured by pulse superposition method to 10 kbar. The observed strong decreasing behavior in the modulus $\text{C}{}_{\mathrm{<mtext>s</mtext>}}\text{=}\text{1}\text{2}\text{(C}{}_{11}\text{? C}{}_{12}\text{)}$ is discussed.     

Antiferromagnetism in K2[FeCl5(H2O)] and its dependence upon hydrostatic pressure

We have measured the isothermal magnetization M of antiferromagnetic K₂[FeCl₅(H₂O)] as a function of applied magnetic field H, for fields up to 80 kOe and for several hydrostatic pressures up to P ～ 10 kbar. We obtained the P =1 atm exchange parameters of this material for an orthorhombic model. The spin-flop boundary of K₂[FeCl₅(H₂O)] was studied for several pressures. For the spin-flop fields H_SF, a relation of the form $\text{H}{}_{\mathrm{SF}}\text{(T) = H}{}_{\mathrm{SF}}\text{(0) (1 + BT}\text{3}\text{2}\text{+ CT}{}^2\text{+ DT}\text{5}\text{2}\text{)}$ adequately describes the experimental points up to relatively high temperatures, for all pressures. The isotherms H_SF vs. P show an abrupt change of slope at P ? 1 kbar, possibly due to a pressure-induced exchange of the intermediate and hard magnetic axes.     

Anisotropic magnetic susceptibility and phase transitions in intercalated graphite: KC24

We measured the magnetic susceptibility of KC₂₄ from 4.2 to 300 K and found no anomalies near the phase transitions at 95 and 123 K as observed in the resistivity. We conclude that the transitions must be due to order- disorder transitions of the K atoms and not charge density wave formation. The susceptibility is anisotropic; at room temperature $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{6}\text{c}\text{)= + 1.50 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 2\%}$ and $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{⊥}\text{c}\text{)= + 0.045 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 50\%}$. This anisotropy is not understood in terms of simple rigid band extensions of the band structure of graphite.     

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.     

Structure magnetique en champ applique nul de l'acetate de manganese tetrahydrate Mn(CH3COO)2, 4H2O

The magnetic structure of manganous acetate Mn(CH₃COO)₂, 4H₂O has been solved by neutron diffraction. Manganous acetate crystallizes in the space group $\text{P2}{}_1\text{c}$ with Z = 6. Manganese atoms (in position 2a and 4e) are located in (100) planes. Below T_N = 3.18 K this compound is antiferromagnetic in a zero applied field with the k vector [$\text{1}\text{2}$ 00]. The plane (100) is ferromagnetic. The magnetic group is $\text{P}{}_{\mathrm{2a}}\text{2}{}_1\text{c}$.     

Method for studying metal ion self-diffusion in some inorganic compounds

A method for studying metal ion self-diffusion in oxides (or other inorganic compounds) is described. The method involves oxidation of an appropriate metal to form a dense, single-layered scale of the lowest valent oxide (e.g. MnO on Mn). The specimen is then treated in high vacuum, and the evaporation of metal diffusing through the scale is measured. From the rates of metal diffusion/evaporation as a function of scale thickness information about the defect structure is obtained. The metal ion self-diffusion coefficient is determined from the rate of metal transport (evaporation) through a scale with known thickness. The requirements and limitations of the method are discussed. The use of the method is illustrated for Mn self-diffusion in MnO at 1100°C. The self-diffusion coefficient of Mn in MnO is proportional to the square root of the oxygen pressure, , in t MnO phase field near the MnO/Mn₃O₄ phase boundary. It is also tentatively concluded that the predominating defects near the Mn/MnO phase boundary are manganese interstitials.     

High-temperature incommensurate phase and superstructure of LiKSO4

The phase transitions of LiKSO₄ above RT have been investigated by means of X-ray powder diffraction, neutron single-crystal diffraction and thermal analysis. Below ca. 675°C the α phase (most probably isomorphous to α -K₂So₄ with a structure P6₃/mmc) is incommensurately modulated. From the large number of satellite diffractions, a hexagonal triple $\text{k}$ modulation was found with $\text{k}{}_1\text{=}\text{K}\text{a}{}^{\mathrm{☆}}{}_1$ and $\text{K}\text{?}\text{1}\text{2}$. The lock-in transition takes place at 470°C, as $\text{K}$ suddenly becomes $\text{1}\text{2}$. This commensurate phase has the P6₃ symmetry, and is a superstructure of the RT phase. At 439°C the superlattice disappears with the release of latent heat. The dynamical behavior of SO^-2₄ and Li⁺ ions in the higher-temperature phases is discussed. No incommensurate phase is found below RT.     

Two-electron,one-photon transitions in nickel

The two emission lines, Kα₁α₃^h and Kα₂α₃^h resulting from the two-electron transitions $\text{1s}{}^{\mathrm{?2}}\text{→ 2s}{}^{\mathrm{?1}}\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}$ and $\text{1s}{}^{\mathrm{?2}}\text{→ 2s}{}^{\mathrm{?1}}\text{2p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}$ were resolved for elemental nickel. Their measured energies agree well with calculations. Their relative intensity $\text{I(Kα}{}_1\text{α}{}_3{}^{\mathrm{h}}\text{)/I(Kα}{}_2\text{α}{}_3{}^{\mathrm{h}}\text{) ≈}\text{3}\text{4}$ and their intensity relative to that of the Kα diagram lines is about 10^?4. This is some 10⁴ times larger than both theoretical results and the results of ion-atom collision experiments.     

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.     

The Wilson-Andrei number for the SU(3) Kondo model

We give asymptotic forms for the high- and low-field magnetic susceptibility for the SU(3) linear dispersion Kondo model for T = 0. The ratio $\text{T}{}_{\mathrm{<mtext>K</mtext>}}\text{T}{}_{\mathrm{<mtext>H</mtext>}}$ is also calculated for the standard SU(2j + 1) Kondo model for general j. From these results the Wilson number W_j, defined by $\text{χ0 =}\text{Wj(gμ)}{}^2\text{j(j + 1)}\text{3kT}{}_{\mathrm{<mtext>K</mtext>}}$ where χ0 is the zero-temperature zero-field susceptibility, which has been calculated by Andrei and Lowenstein for $\text{j =}\text{1}\text{2}$, is deduced for the SU(3) model j = 1.     

X-ray compressibility measurements of the graphite intercalates KC8 and KC24

The isothermal compressibilities of pristine graphite and stages 1 and 2 potassium-graphite have been measured at room temperature. Diamond anvil X-ray diffraction techniques were employed to determine the c-axis lattice constant as a function of hydrostatic pressure up to 12 kbar. The compressibilities $\text{k}{}_{\mathrm{c}}\text{?}\text{1}\text{C}{}_{33}$ were found to be (2.73±0.09)×10^-12, (2.13±0.09)×10^-12, (5.3±0.8)×10^-12 and $\text{(1.6±0.2)×10}{}^{-12}\text{cm}{}^2\text{dyn}$ for graphite, KC₈, stage 2 KC₂₄ and stage 3 KC₂₄, respectively. The compressibility of KC₈ was comparable to that of RbC₈ deduced from neutron scattering experiments.