Etude comparative des structures type K4NiF4 et pérovskite par la méthode des invariants

The study of K₂NiF₄ and perovskite structure type by the “method of invariants” leads to the relationship: $\text{(A-X)}{}_9\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? (A-X)}{}_{12}\text{=}\text{constant}$, where (A-X)₉ and (A-X)₁₂ are the invariant values associated with cation A in coordination number 9 and 12. In the case where A = K⁺ and X = F^?, we propose the relationship:

$\text{(K}{}^{\mathrm{+}}\text{?F)}{}_{\mathrm{R}}\text{= 2.832 R}{}^{\mathrm{<mtext>1</mtext><mtext>11.4</mtext>}}$

where R is the coordination number.     

NMR study of hydrogen molybdenum bronzes: H1.71MoO3 and H0.36MoO3

Proton NMR relaxation times (T₂T₁, and T_1?) and absorption spectra are reported for the compounds H_1.71MoO₃ (red monoclinic) and H_0.36MoO₃ (blue orthorhombic) in the temperature range 77 K < T < 450 K. Rigid lattice dipolar spectra show that both compounds contain proton pairs, as OH₂ groups coordinated to Mo atoms in H_1.71MoO₃ and as pairs of OH groups in H_0.36MoO₃. The room temperature lineshape for H_1.71MoO₃ shows that the average chemical shielding tensor has a total anisotropy of 20.1 ppm. The relaxation measurements confirm that hydrogen diffusion occurs and give E_A = 22 kJ mole^?1 and $\text{τ}{}^0{}_{\mathrm{<mtext>C</mtext>}}\text{? 10}{}^{\mathrm{?13}}\text{sec}$ for H_1.71MoO₃ and E_A = 11 kJ mole^?1 and $\text{τ}{}^0{}_{\mathrm{<mtext>C</mtext>}}\text{? 3 × 10}{}^{\mathrm{?8}}\text{sec}$ for H_0.36MoO₃.     

EPR and magnetization of GdGa2

Paramagnetic resonance and magnetic measurements were performed on powdered samples of GdGa₂. The magnetic data indicated ferrimagnetic behavior with $\text{T}{}_{\mathrm{c}}\text{? 181°}\text{K}$. Above 250° K the susceptibility obeys the Curie-Weiss law $\text{χ}{}_{\mathrm{g}}\text{=}\text{2.66}{}_2\text{× 10}{}^{\mathrm{?2}}\text{(T ? 27.6)}\text{emu/g-Oe}$ which corresponds to an effective moment of 7.95 Bohr magnetons. Over the range from 190 to 300°K the data obey a Néel type law, $\text{χ}{}_{\mathrm{g}}{}^{\mathrm{?1}}\text{= 35.95 (T ? 12.5) ?}\text{2.20 × 10}{}^4\text{(T ? 177)}$, which is indicative of ferrimagnetic order. The resonance measurements were performed at 9.013 gHz at 247, 296, and 349°K. The spectra were analyzed with a computerized curve-fitting technique that involves a linear combination of Lorentzian absorption and dispersion susceptibility components. Following demagnetization corrections, the g-factor was found to be 1.983₂ while the half-power, half-linewidth was 592.7 Oersteds.     

Diffusion of point defects participating in solid-phase chemical reactions (trap diffusion): Demonstration for the Ti3+ → Ti4+ transition in corundum

A problem of trap diffusion, that is diffusion of point defects in crystals participating in a solid-phase chemical reaction with motionless impurity ions, is solved. Time dependences of the reaction-front displacement, X_f, and its steepness, $\text{(}\text{?C}\text{?X}\text{)}{}_{\mathrm{f}}$ are determined analytically for N₀ ? C₀ and numerically for all relations of N₀ and C₀$\text{x}{}_{\mathrm{f}}{}^2\text{=2}\text{N}{}_0\text{C}{}_0\text{Dt; (}\text{ac}\text{ax}\text{)}{}_{\mathrm{f}}\text{=0.3C}{}_0{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{(}\text{g}\text{D}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2></mtext>}}$where C₀ and N₀ are the initial concentration of impurity and the eqilibrium defect concentration, respectively, D is a diffusion coefficient, and g is a chemical reaction constant. Dependence of X_f vs C₀ and t is confirmed for oxygen annealing of corundum crystals doped with titanium which, reacting with the point defects, changes its valency. The data are obtained for dependence of displacement X_f upon partial oxygen pressure and thermotreatment temperature as well as upon the sign of the constant electric field applied to the sample. From these data we conclude that the reaction of titanium impurity, changing from the three-valent to the tetravalent state at the activation energy of 80 ± 8.5 kcal/mole is due to anisotropic diffusion of charged aluminum vacancy and holes in the valence band. The diffusion coefficient for that process at 1500°C is estimated to be larger than 10^?5 cm²/sec. Using the trap-diffusion features, the concentration of optical centers of the 0.315-μm absorption band in ruby is also estimated.     

Low-temperature luminescence of Eu3+ in K2EuCl5

The luminescence associated with the Eu³⁺ ion in K₂EuCl₅ has been studied at cryogenic temperatures under conditions of high resolution. Emission was observed to originate from both the ⁵D₀ and ⁵D₁ excited states, and transitions to the ${}^7\text{F}{}_0\text{,}{}^7\text{F}{}_1\text{,}{}^7\text{F}{}_2\text{,}{}^7\text{F}{}_3$, and ⁷F₄ ground levels were observed. The fine structure observed within these emission bands was found to be consistent with the existence of an effective C₄ site symmetry for the emitting Eu(III) species, even though the crystal structure does not indicate the presence of a true or pseudo C₄ axis.     

Heat capacity and thermodynamic functions of CsCrCl3 and RbCrCl3. Structural phase transitions

We present the heat capacities measured by adiabatic calorimetry from 6 to 350 K, and by differential scanning calorimetry from 300 to 500 K, of CsCrCl₃ and RbCrCl₃. A first-order transition at T_c = (171.1±0.1) K was detected for CsCrCl₃. The RbCrCl₃ showed at T_c = (193.3±0.1) K a transition with thermal hysteresis at temperatures just below the maximum. At T₁ = (440±10) K a continuous transition was also detected. Furthermore, at T_N ≈ 16 K, and for both compounds, a small bump due to magnetic long-range ordering was observed. The thermodynamic functions at 298.15 K are

     

7.

Nd³⁺, Eu³⁺, and Gd³⁺ ions as local probes in the Na_xSr_3−2xLn_x(PO₄)₂ and KCaLn(PO₄)₂ rare earth phosphates     

C. Parent  P. Bochu  A. Daoudi  G. Le Flem 《Journal of solid state chemistry》1982,43(2):190-195

Use of Nd³⁺, Eu³⁺, and Gd³⁺ as local structural probes allows the determination of the rare earth positions in the Na_xSr_3?2xLn_x(PO₄)₂ (Ln = La to Tb) and KCaLn(PO₄)₂ phases (Ln = rare earth). Moreover, a common feature of both series is a particularly high splitting of the excitation ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ and ${}^6\text{P}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ levels of the Gd³⁺ ions.     

8.

Thermodynamic properties of two metal-rich tantalum sulfides: Ta₂S and Ta₆S     

B.U. Harbrecht  S.R. Schmidt  H.F. Franzen 《Journal of solid state chemistry》1984,53(1):113-119

The incongruent vaporization reactions of Ta₂S and Ta₆S have been investigated by mass-loss effusion in the temperature range 1576 to 1902 K. By extrapolation of P_S(obs) to equilibrium the enthalpies of the reactions $\text{3}\text{2}\text{Ta}{}_2\text{S(s)}\text{=}\text{1}\text{2}\text{Ta}{}_6\text{S(s) + S(g)}$ and Ta₆S = 6 Ta(s) + S(g) were found to be $\text{ΔH}{}^0{}_{298}\text{R}\text{= 53.0(0.3) · 10}{}^3\text{K}$ and $\text{ΔH}{}^0{}_{298}\text{R}\text{= 58.1(0.4) · 10}{}^3\text{K}$, respectively. Comparison between the above values, determined by a 2nd law treatment, and 3rd law values was used to derive fef (“free energy function”) values for Ta and S in the compounds. These postulated fef's, which apply only to the elements as present in the compounds measured, are compared to tabulated quantities for the pure solid elements to provide a criterion for 2nd and 3rd law evaluation.     

9.

Cryogenic luminescence studies of Eu³⁺ in LiEuCl₄     

Harry G. Brittain  Gerd Meyer 《Journal of solid state chemistry》1985,59(2):183-189

The luminescence associated with the Eu³⁺ ion in LiEuCl₄ has been studied at cryogenic temperatures under conditions of high resolution. Emission was observed to originate from both the ⁵D₀ and ⁵D₁ excited states, and transitions to the ${}^7\text{F}{}_0\text{,}{}^7\text{F}{}_1\text{,}{}^7\text{F}{}_2\text{,}{}^7\text{F}{}_3$, and ⁷F₄ ground levels were observed. The fine structure observed within these emission bands was found to be consistent with the existence of an effective D₄ site symmetry for the emitting Eu³⁺ species, even though the europium polyhedron was found to be that of a bisdisphenoid.     

10.

Electrical conductivity of Th₃P₄-type EnLn₂S₄     

Mineo Sato  Gin-Ya Adachi  Jiro Shiokawa 《Journal of solid state chemistry》1980,31(3):337-341

Electrical conductivity measurements of Th₃P₄-type EuLn₂S₄ (Ln = LaGd) compounds have been made as functions of temperature and sulfur vapor pressure. These compounds are all p-type semiconductors, and their conductivities at room temperature have almost the same values for the specimens from EuLa₂S₄ to EuNd₂S₄ but increase on going from EnNd₂S₄ to EuGd₂S₄. In addition, the conductivity of EuGd₂S₄ is sensitive to sulfur vapor pressure and obeys the relationship . The mechanism of electrical transport in these compounds is discussed.     

11.

Etude cristallochimique des phases de type perovskite du systeme Th_0.25 NbO₃NaNbO₃     

M. Labeau  J.C. Joubert 《Journal of solid state chemistry》1978,25(4):347-353

The compound Th_0.25 NbO₃ melts congruently at 1390°C. Single crystals obtained by slow cooling from the melt are transparent and show uniaxial optical properties. A single-crystal X-ray analysis confirms the tetragonal cell found by Kovba and Trunov from a powder data and gives a = 3.90 Å and c = 7.85 Å. No systematic absence of the hkl reflections is observed on precession films. The relative intensities of the main reflections are characteristic of a perovskite-like arrangement ABO₃ whose large dodecahedral A sites are only partly occupied. Several domains have been found in the perovskite-type solid solution (1 ? x) Th_0.25NbO₃-x NaNbO₃. For 0 ? x ? 0.5 the phases have a tetragonal cell with $\text{a ? a}{}_0$ and $\text{c ? 2a}{}_0$ as in pure Th_0.25 NbO₃. When 0.6 ? x ? 0.8 the corresponding phases crystallize with a small cubic cell ($\text{a}{}_0\text{? 3.9}$Å), while phases with 0.9 ? x ? 1 have an orthorhombic cell ($\text{a ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, b ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, c ? a}{}_0$).     

12.

Proprietes dielectriques d'une serie de polyoxyethylene (POE) presentant des masses moleculaires lentement variables ($\text{200 ≤}\text{M}{}_{\mathrm{w}}\text{≤ 4 × 10}{}^6$)     

Françoise Decossas  André Moliton  Eve Marchal 《European Polymer Journal》1982,18(12):1073-1084

In this paper, we present dielectric results for samples of POE (polyethyleneoxyde) divided into three classes. Compounds of low molecular weight ($\text{M}{}_{\mathrm{w}}\text{< 1000}$) show very weak absorption at lower temperatures. At higher temperatures (below the melting point), there appears a very important peak (stronger for lower $\text{M}{}_{\mathrm{w}}$) which corresponds to the supercooled phase. Compounds of intermediate molecular weight ($\text{1000 ≤}\text{M}{}_{\mathrm{w}}\text{≤ 10}{}^5$) show a β relaxation near 250 K (bis$\text{M}{}_{\mathrm{w}}\text{? 3000}$) which is due to the movement of chain-ends in the amorphous phase. This process increases with the importance of that part of the structure formed of shorter lamellae, thus explaining the marked diminution of this absorption for higher molecular weight compounds (the shorter lamellae cannot be obtained for $\text{M}{}_{\mathrm{w}}\text{> 4000}$). The α relaxation (obtained for T > 300 K) is perturbed by an important conduction; however, we have found a peak and a shoulder at lower frequencies, due to shorter and longer lamellae respectively. The shoulder is also present for high molecular weight compounds ($\text{M}{}_{\mathrm{w}}\text{≥ 0.1–10}{}^6$). At lower temperatures, they display a very broad γ absorption, superposed at higher frequencies with a β relaxation. The nature of this β relaxation is different from that observed for the intermediate molecular weight compounds.     

13.

Thermodynamics of hydrogen trapping in intermetallic compounds: Application to $\text{LaNi}{}_5\text{H}$     

Ted B. Flanagan  C.A. Wulff  B.S. Bowerman 《Journal of solid state chemistry》1980,34(2):215-221

It has been observed for several intermetallic compound/H systems that deviations from Sieverts' law of ideal solubility occur which are in the opposite direction from the deviations from ideality which occur in pure metal/H systems. The principal source of nonideality on both types of interstitial solutions arises from the changes of the relative partial molar enthalpy of H₂ solution, ΔH_H, with H content. The variations of ΔH_H with H content are, however, in the opposite directions for intermetallic/H and pure metal/H interstitial solutions. Relative partial molar entropies for solution of H₂ in the α-phase of the $\text{LaNi}{}_5\text{H}$ system are given at 298.2 K. These have been determined from calorimetric determinations of ΔH_H and measurements of . The partial entropies show unusual extrema as a function of H content. A model based on trapping of H atoms is proposed which explains the variations of ΔH_H and ΔS_H. Since the deviations from ideality arising from an increase of ΔH_H with H content seem to be limited to solids with at least two different ordered metal components, e.g., intermetallic compounds, it seems reasonable to assign the trapping sites to interstices which result from an interchange of metal atoms. This interchange may create interstices rich in those nearest-neighbor metal atoms which have the greatest affinity for H.     

14.

Nonstoichiometry and defect structure of the perovskite-type oxides La_1−xSr_xFeO_3−°     

Junichiro Mizusaki  Masafumi Yoshihiro  Shigeru Yamauchi  Kazuo Fueki 《Journal of solid state chemistry》1985,58(2):257-266

In order to elucidate the defect structure of the perovskite-type oxide solid solution La_1?xSr_xFeO_3?δ (x = 0.0, 0.1, 0.25, 0.4, and 0.6), the nonstoichiometry, δ, was measured as a function of oxygen partial pressure, P_O2, at temperatures up to 1200°C by means of the thermogravimetric method. Below 200°C and in an atmosphere of P_O2 ≥ 0.13 atm, δ in La_1?xSr_xFeO_3?δ was found to be close to 0. With decreasing log P_O2, δ increased and asymptotically reached $\text{x}\text{2}$. The value corresponding to $\text{δ =}\text{x}\text{2}$ was about ?10 at 1000°C. With further decrease in log P_O2, δ slightly increased. For LaFeO_3?δ, the observed δ values were as small as <0.015. It was found that the relation between δ and log P_O2 is interpreted on the basis of the defect equilibrium among Sr′_La (or V?_La for the case of LaFeO_3?δ), V^··_O, Fe′_Fe, and Fe^·_Fe. Calculations were made for the equilibrium constants K_ox of the reaction

$\text{1}\text{2}\text{O}{}_2\text{(g) + V}{}^{\mathrm{··}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= O}{}^{\mathrm{x}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe}}$

and K_i for the reaction

$\text{2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= Fe}{}^{\mathrm{\prime }}{}_{\mathrm{Fe}}\text{+ Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe·}}$

Using these constants, the defect concentrations were calculated as functions of P_O2, temperature, and composition x. The present results are discussed with respect to previously reported results of conductivity measurements.     

15.

Structure,spectral properties and interconversions of bis-niobocenes and their dianions. The crystal and molecular structure of [(η⁵-C₅H₄SiMe₂OSiMe₂-η⁵ : η¹-C⁵H³)²Nb²H²] [Na(OEt₂)₂]₂     

D.A. Lemenovskii  V.P. Fedin  A.V. Aleksandrov  Yu.L. Slovohotov  Yu.T. Struchkov 《Journal of organometallic chemistry》1980,201(1):257-268

Electron-acceptor properties of bis-niobocenes (η⁵-C₅H₄Y)(H)$\text{Nb(η}{}^5\text{: η}{}^1\text{-C}{}_5\text{H}{}_3\text{X)}{}_2\text{N}$b(H)(η⁵-C₅H₄Y) with X  Y  H and XY  SiMe₂OSiMe₂have been investigated. Bis-niobocenes are shown to readily add two electrons forming stable salts of the corresponding dianions [(η⁵-C₅H₄Y)(H)Nb(η⁵ : η¹- C₅H₃X)₂Nb(H)(η⁵-C₅H₄Y)]^2-. The surplus electrons can be removed to give quantitative regeneration of initial neutral bis-niobocenes. The crystal and molecular structure of the title compound has been determined; R  0.044, interatomic distance are Nb…Nb 3.93, NbH 1.62, average NbC(π) 2.36, NbC(σ) 2.31 Å, other distances correspond to the usually observed values. Unlike the neutral bis-niobocenes, there is no direct metalmetal bond in the dianionic structures. This conclusion is supported by electronic spectra of neutral and dianionic species.     

16.

A study of the spinel materials LiTi₂O₄ and $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4$ by photoelectron spectroscopy     

P.P. Edwards  R.G. Egdell  I. Fragala  J.B. Goodenough  M.R. Harrison  A.F. Orchard  E.G. Scott 《Journal of solid state chemistry》1984,54(2):127-135

HeI-excited valence-band ultraviolet photoelectron spectra and MgKα-excited Ti-2p X-ray photoelectron spectra are reported for the spinel materials LiTi₂O₄ and $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4$. The presence of a Fermi edge in the ultraviolet photoelectron spectrum of LiTi₂O₄ confirms the metallic nature of this material, although the measured density of states at the Fermi energy is much lower than that expected from an independent-electron interpretation of the magnetic susceptibility. This difference is attributed to a strong interaction of the conduction electrons with the lattice vibrations. The localization of conduction electrons that occurs in the final state in the Ti-2p X-ray photoelectron spectrum of LiTi₂O₄ is attributed to a Coulomb interaction with a core hole.     

17.

Crystallographic and magnetic properties of the Cd(OH)₂ layer structure compound TiS₂ containing extra iron     

T. Takahashi  O. Yamada 《Journal of solid state chemistry》1973,7(1):25-30

     

18.

Studies of spinels. VII. Order-disorder transition in the inverse germanate spinels $\text{Zn}{}_{\mathrm{2-x}}\text{(}\text{Co,Ni}\text{)}{}_{\mathrm{x}}\text{GeO}{}_4\text{(x ⋍ 1)}$     

J. Preudhomme  P. Tarte 《Journal of solid state chemistry》1980,35(2):272-277

A series of germanate spinels Zn_2?x(Ni, Co)_xGeO₄ has been synthesized and investigated by X-ray diffractometry and infrared spectroscopy. Synthesis at 1200°C leads to cubic phases characterized by an inverse, disordered distribution of Ge and bivalent cations (essentially Ni or Co) over the octahedral sites; however, the presence of some short-range order is suggested by the infrared spectrum. Tempering at an appropriate temperature (between 800 and 900°C, depending on the $\text{Co}\text{Ni}$ ratio) leads to tetragonal spinel phases, corresponding to the 1:1 order between Ge and bivalent cation on octahedral sites. The transformation is sluggish and proceeds at a significant rate only in a narrow temperature range, just below the transition temperature. A comparison with the behavior of known inverse, ordered titanate spinels shows that, within the family of inverse II–IV spinels M₂^IIM^IVO₄, the ratio of the octahedral cationic radii is one of the factors determining the order-disorder transition temperature, and the importance of the tetragonal distortion.     

19.

Standard enthalpies of formation of uranium compounds I. β-UO₃ and γ-UO₃     

E.H.P. Cordfunke  W. Ouweltjes  G. Prins 《The Journal of chemical thermodynamics》1975,7(12):1137-1142

The standard enthalpy of formation of γ-UO₃ has been critically assessed; the value ?(292.5 ± 0.2) kcal_th mol^?1 is suggested.The enthalpies of solution of β-UO₃ and γ-UO₃ in 3 M H₂SO₄ have been measured and used to derive:

$\text{ΔH}{}_{\mathrm{f}}{}^{\mathrm{°}}\text{(β?}\text{UO}{}_3\text{, 298.15}\text{K}\text{) = ?(291.6 ± 0.2)}\text{kcal}{}_{\mathrm{th}}\text{mol}{}^{\mathrm{?}}$

     

20.

Interactions magnétiques dans des groupements binucléaires [Fe₂O₇]⁸⁻ au sein de Na₈Fe₂O₇     

Henri Kessler  Son Ly 《Journal of solid state chemistry》1981,39(1):22-28

The magnetic interaction in the structural units [Fe₂O₇]^8?, built of two corner-sharing FeO₄ tetrahedra, in Na₈Fe₂O₇ ($\text{Na}{}_2\text{O}\text{Fe}{}_2\text{O}{}_3\text{=}\text{4}\text{1}$) has been studied by magnetic susceptibility measurements (4.2–500 K). An exchange integral $\text{J}\text{K}{}_{\mathrm{<mtext>B</mtext>}}$ of ?37 K is obtained by comparison of the experimental values and the calculated ones using a Heisenberg-Dirac-Van Vleck-type Hamiltonian ? = $\text{?2J}\text{S}\text{?}{}_1\text{S}\text{?}{}_2$. The hypothesis of magnetically isolated [Fe₂O₇]^8? groups is corroborated by Mössbauer spectroscopy between 1.5 and 77 K. The susceptibility measurements of the solid solutions Na₈Fe_2?xM_xO₇ (M = Al, Ga; 0 ≤ x ≤ 0.2 for Al; 0 ≤ x ≤ 2 for Ga) leads to the same conclusion of the existence of isolated Fe³⁺Fe³⁺ pairs in Na₈Fe₂O₇. The type of substitution of Fe by Al or Ga is determined; homonuclear Fe³⁺Fe³⁺ and M³⁺M³⁺ pairs and heteronuclear Fe³⁺M³⁺ pairs are formed.     

	$\text{C}{}_{\mathrm{<mtext>p,</mtext><mtext>m</mtext>}}\text{R}$	$\text{S}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{R}$	$\text{{H}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(T)?H}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(0)}}\text{R}\text{K}$	$\text{?{G}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(T)?H}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(0}}\text{RT}$
CsCrCl3	15.38	26.49	3503.2	14.735
RbCrCl3	15.76	25.99	3556.8	14.384

Nd3+, Eu3+, and Gd3+ ions as local probes in the NaxSr3−2xLnx(PO4)2 and KCaLn(PO4)2 rare earth phosphates

Use of Nd³⁺, Eu³⁺, and Gd³⁺ as local structural probes allows the determination of the rare earth positions in the Na_xSr_3?2xLn_x(PO₄)₂ (Ln = La to Tb) and KCaLn(PO₄)₂ phases (Ln = rare earth). Moreover, a common feature of both series is a particularly high splitting of the excitation ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ and ${}^6\text{P}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ levels of the Gd³⁺ ions.     

Thermodynamic properties of two metal-rich tantalum sulfides: Ta2S and Ta6S

The incongruent vaporization reactions of Ta₂S and Ta₆S have been investigated by mass-loss effusion in the temperature range 1576 to 1902 K. By extrapolation of P_S(obs) to equilibrium the enthalpies of the reactions $\text{3}\text{2}\text{Ta}{}_2\text{S(s)}\text{=}\text{1}\text{2}\text{Ta}{}_6\text{S(s) + S(g)}$ and Ta₆S = 6 Ta(s) + S(g) were found to be $\text{ΔH}{}^0{}_{298}\text{R}\text{= 53.0(0.3) · 10}{}^3\text{K}$ and $\text{ΔH}{}^0{}_{298}\text{R}\text{= 58.1(0.4) · 10}{}^3\text{K}$, respectively. Comparison between the above values, determined by a 2nd law treatment, and 3rd law values was used to derive fef (“free energy function”) values for Ta and S in the compounds. These postulated fef's, which apply only to the elements as present in the compounds measured, are compared to tabulated quantities for the pure solid elements to provide a criterion for 2nd and 3rd law evaluation.     

Cryogenic luminescence studies of Eu3+ in LiEuCl4

The luminescence associated with the Eu³⁺ ion in LiEuCl₄ has been studied at cryogenic temperatures under conditions of high resolution. Emission was observed to originate from both the ⁵D₀ and ⁵D₁ excited states, and transitions to the ${}^7\text{F}{}_0\text{,}{}^7\text{F}{}_1\text{,}{}^7\text{F}{}_2\text{,}{}^7\text{F}{}_3$, and ⁷F₄ ground levels were observed. The fine structure observed within these emission bands was found to be consistent with the existence of an effective D₄ site symmetry for the emitting Eu³⁺ species, even though the europium polyhedron was found to be that of a bisdisphenoid.     

Electrical conductivity of Th3P4-type EnLn2S4

Electrical conductivity measurements of Th₃P₄-type EuLn₂S₄ (Ln = LaGd) compounds have been made as functions of temperature and sulfur vapor pressure. These compounds are all p-type semiconductors, and their conductivities at room temperature have almost the same values for the specimens from EuLa₂S₄ to EuNd₂S₄ but increase on going from EnNd₂S₄ to EuGd₂S₄. In addition, the conductivity of EuGd₂S₄ is sensitive to sulfur vapor pressure and obeys the relationship . The mechanism of electrical transport in these compounds is discussed.     

Etude cristallochimique des phases de type perovskite du systeme Th0.25 NbO3NaNbO3

The compound Th_0.25 NbO₃ melts congruently at 1390°C. Single crystals obtained by slow cooling from the melt are transparent and show uniaxial optical properties. A single-crystal X-ray analysis confirms the tetragonal cell found by Kovba and Trunov from a powder data and gives a = 3.90 Å and c = 7.85 Å. No systematic absence of the hkl reflections is observed on precession films. The relative intensities of the main reflections are characteristic of a perovskite-like arrangement ABO₃ whose large dodecahedral A sites are only partly occupied. Several domains have been found in the perovskite-type solid solution (1 ? x) Th_0.25NbO₃-x NaNbO₃. For 0 ? x ? 0.5 the phases have a tetragonal cell with $\text{a ? a}{}_0$ and $\text{c ? 2a}{}_0$ as in pure Th_0.25 NbO₃. When 0.6 ? x ? 0.8 the corresponding phases crystallize with a small cubic cell ($\text{a}{}_0\text{? 3.9}$Å), while phases with 0.9 ? x ? 1 have an orthorhombic cell ($\text{a ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, b ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, c ? a}{}_0$).     

Proprietes dielectriques d'une serie de polyoxyethylene (POE) presentant des masses moleculaires lentement variables (200 ≤ Mw ≤ 4 × 106)

In this paper, we present dielectric results for samples of POE (polyethyleneoxyde) divided into three classes. Compounds of low molecular weight ($\text{M}{}_{\mathrm{w}}\text{< 1000}$) show very weak absorption at lower temperatures. At higher temperatures (below the melting point), there appears a very important peak (stronger for lower $\text{M}{}_{\mathrm{w}}$) which corresponds to the supercooled phase. Compounds of intermediate molecular weight ($\text{1000 ≤}\text{M}{}_{\mathrm{w}}\text{≤ 10}{}^5$) show a β relaxation near 250 K (bis$\text{M}{}_{\mathrm{w}}\text{? 3000}$) which is due to the movement of chain-ends in the amorphous phase. This process increases with the importance of that part of the structure formed of shorter lamellae, thus explaining the marked diminution of this absorption for higher molecular weight compounds (the shorter lamellae cannot be obtained for $\text{M}{}_{\mathrm{w}}\text{> 4000}$). The α relaxation (obtained for T > 300 K) is perturbed by an important conduction; however, we have found a peak and a shoulder at lower frequencies, due to shorter and longer lamellae respectively. The shoulder is also present for high molecular weight compounds ($\text{M}{}_{\mathrm{w}}\text{≥ 0.1–10}{}^6$). At lower temperatures, they display a very broad γ absorption, superposed at higher frequencies with a β relaxation. The nature of this β relaxation is different from that observed for the intermediate molecular weight compounds.     

Thermodynamics of hydrogen trapping in intermetallic compounds: Application to LaNi5H

It has been observed for several intermetallic compound/H systems that deviations from Sieverts' law of ideal solubility occur which are in the opposite direction from the deviations from ideality which occur in pure metal/H systems. The principal source of nonideality on both types of interstitial solutions arises from the changes of the relative partial molar enthalpy of H₂ solution, ΔH_H, with H content. The variations of ΔH_H with H content are, however, in the opposite directions for intermetallic/H and pure metal/H interstitial solutions. Relative partial molar entropies for solution of H₂ in the α-phase of the $\text{LaNi}{}_5\text{H}$ system are given at 298.2 K. These have been determined from calorimetric determinations of ΔH_H and measurements of . The partial entropies show unusual extrema as a function of H content. A model based on trapping of H atoms is proposed which explains the variations of ΔH_H and ΔS_H. Since the deviations from ideality arising from an increase of ΔH_H with H content seem to be limited to solids with at least two different ordered metal components, e.g., intermetallic compounds, it seems reasonable to assign the trapping sites to interstices which result from an interchange of metal atoms. This interchange may create interstices rich in those nearest-neighbor metal atoms which have the greatest affinity for H.     

Nonstoichiometry and defect structure of the perovskite-type oxides La1−xSrxFeO3−°

In order to elucidate the defect structure of the perovskite-type oxide solid solution La_1?xSr_xFeO_3?δ (x = 0.0, 0.1, 0.25, 0.4, and 0.6), the nonstoichiometry, δ, was measured as a function of oxygen partial pressure, P_O2, at temperatures up to 1200°C by means of the thermogravimetric method. Below 200°C and in an atmosphere of P_O2 ≥ 0.13 atm, δ in La_1?xSr_xFeO_3?δ was found to be close to 0. With decreasing log P_O2, δ increased and asymptotically reached $\text{x}\text{2}$. The value corresponding to $\text{δ =}\text{x}\text{2}$ was about ?10 at 1000°C. With further decrease in log P_O2, δ slightly increased. For LaFeO_3?δ, the observed δ values were as small as <0.015. It was found that the relation between δ and log P_O2 is interpreted on the basis of the defect equilibrium among Sr′_La (or V?_La for the case of LaFeO_3?δ), V^··_O, Fe′_Fe, and Fe^·_Fe. Calculations were made for the equilibrium constants K_ox of the reaction

$\text{1}\text{2}\text{O}{}_2\text{(g) + V}{}^{\mathrm{··}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= O}{}^{\mathrm{x}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe}}$

and K_i for the reaction

$\text{2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= Fe}{}^{\mathrm{\prime }}{}_{\mathrm{Fe}}\text{+ Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe·}}$

Using these constants, the defect concentrations were calculated as functions of P_O2, temperature, and composition x. The present results are discussed with respect to previously reported results of conductivity measurements.     

Structure,spectral properties and interconversions of bis-niobocenes and their dianions. The crystal and molecular structure of [(η5-C5H4SiMe2OSiMe2-η5 : η1-C5H3)2Nb2H2] [Na(OEt2)2]2

Electron-acceptor properties of bis-niobocenes (η⁵-C₅H₄Y)(H)$\text{Nb(η}{}^5\text{: η}{}^1\text{-C}{}_5\text{H}{}_3\text{X)}{}_2\text{N}$b(H)(η⁵-C₅H₄Y) with X  Y  H and XY  SiMe₂OSiMe₂have been investigated. Bis-niobocenes are shown to readily add two electrons forming stable salts of the corresponding dianions [(η⁵-C₅H₄Y)(H)Nb(η⁵ : η¹- C₅H₃X)₂Nb(H)(η⁵-C₅H₄Y)]^2-. The surplus electrons can be removed to give quantitative regeneration of initial neutral bis-niobocenes. The crystal and molecular structure of the title compound has been determined; R  0.044, interatomic distance are Nb…Nb 3.93, NbH 1.62, average NbC(π) 2.36, NbC(σ) 2.31 Å, other distances correspond to the usually observed values. Unlike the neutral bis-niobocenes, there is no direct metalmetal bond in the dianionic structures. This conclusion is supported by electronic spectra of neutral and dianionic species.     

A study of the spinel materials LiTi2O4 and Li43Ti53O4 by photoelectron spectroscopy

HeI-excited valence-band ultraviolet photoelectron spectra and MgKα-excited Ti-2p X-ray photoelectron spectra are reported for the spinel materials LiTi₂O₄ and $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4$. The presence of a Fermi edge in the ultraviolet photoelectron spectrum of LiTi₂O₄ confirms the metallic nature of this material, although the measured density of states at the Fermi energy is much lower than that expected from an independent-electron interpretation of the magnetic susceptibility. This difference is attributed to a strong interaction of the conduction electrons with the lattice vibrations. The localization of conduction electrons that occurs in the final state in the Ti-2p X-ray photoelectron spectrum of LiTi₂O₄ is attributed to a Coulomb interaction with a core hole.     

Studies of spinels. VII. Order-disorder transition in the inverse germanate spinels Zn2−x(Co,Ni)xGeO4(x ⋍ 1)

A series of germanate spinels Zn_2?x(Ni, Co)_xGeO₄ has been synthesized and investigated by X-ray diffractometry and infrared spectroscopy. Synthesis at 1200°C leads to cubic phases characterized by an inverse, disordered distribution of Ge and bivalent cations (essentially Ni or Co) over the octahedral sites; however, the presence of some short-range order is suggested by the infrared spectrum. Tempering at an appropriate temperature (between 800 and 900°C, depending on the $\text{Co}\text{Ni}$ ratio) leads to tetragonal spinel phases, corresponding to the 1:1 order between Ge and bivalent cation on octahedral sites. The transformation is sluggish and proceeds at a significant rate only in a narrow temperature range, just below the transition temperature. A comparison with the behavior of known inverse, ordered titanate spinels shows that, within the family of inverse II–IV spinels M₂^IIM^IVO₄, the ratio of the octahedral cationic radii is one of the factors determining the order-disorder transition temperature, and the importance of the tetragonal distortion.     

Standard enthalpies of formation of uranium compounds I. β-UO3 and γ-UO3

The standard enthalpy of formation of γ-UO₃ has been critically assessed; the value ?(292.5 ± 0.2) kcal_th mol^?1 is suggested.The enthalpies of solution of β-UO₃ and γ-UO₃ in 3 M H₂SO₄ have been measured and used to derive:

$\text{ΔH}{}_{\mathrm{f}}{}^{\mathrm{°}}\text{(β?}\text{UO}{}_3\text{, 298.15}\text{K}\text{) = ?(291.6 ± 0.2)}\text{kcal}{}_{\mathrm{th}}\text{mol}{}^{\mathrm{?}}$

     

Interactions magnétiques dans des groupements binucléaires [Fe2O7]8− au sein de Na8Fe2O7

The magnetic interaction in the structural units [Fe₂O₇]^8?, built of two corner-sharing FeO₄ tetrahedra, in Na₈Fe₂O₇ ($\text{Na}{}_2\text{O}\text{Fe}{}_2\text{O}{}_3\text{=}\text{4}\text{1}$) has been studied by magnetic susceptibility measurements (4.2–500 K). An exchange integral $\text{J}\text{K}{}_{\mathrm{<mtext>B</mtext>}}$ of ?37 K is obtained by comparison of the experimental values and the calculated ones using a Heisenberg-Dirac-Van Vleck-type Hamiltonian ? = $\text{?2J}\text{S}\text{?}{}_1\text{S}\text{?}{}_2$. The hypothesis of magnetically isolated [Fe₂O₇]^8? groups is corroborated by Mössbauer spectroscopy between 1.5 and 77 K. The susceptibility measurements of the solid solutions Na₈Fe_2?xM_xO₇ (M = Al, Ga; 0 ≤ x ≤ 0.2 for Al; 0 ≤ x ≤ 2 for Ga) leads to the same conclusion of the existence of isolated Fe³⁺Fe³⁺ pairs in Na₈Fe₂O₇. The type of substitution of Fe by Al or Ga is determined; homonuclear Fe³⁺Fe³⁺ and M³⁺M³⁺ pairs and heteronuclear Fe³⁺M³⁺ pairs are formed.