Solid state studies on substituted CuCr2O4 spinel

Changes in crystallographic, electrical, and thermal properties of CuCr₂O₄ spinel were investigated by replacing Cu with Mg, i.e., Cu_1?xMg_xCr₂O₄, and Cr with Al, i.e., CuCr_2?xAl_xO₄. The tetragonal distortion in CuCr₂O₄ disappeared with 60% replacement of Cu by Mg (x = 0.6) or 50% replacement of Cr by Al (x = 1.0). The temperature variation of electrical resistivity for all the tetragonal samples was similar to that of CuCr₂O₄. The first order, diffusionless phase transition was manifest in the hysteresis loops of log ? vs $\text{1}\text{T}$ plots. The resistivity and activation energy for conduction changed sharply near the phase transition composition. With the replacement of Cr by Al, the conduction in CuCr₂O₄ was found to change from p type to n type. The low thermal stability of the spinel was found to be due to a high concentration of tetrahedral Cu²⁺ ions (>80%) and compressed tetragonal distortion which strains the spinel lattice. This strain is removed by replacing either Cu with Mg or Cr with Al, whereby the spinel becomes stable.     

Proprietes magnétiques et étude par spectroscopie Mössbauer des tellurites de fer Fe2Te3O9 et Fe2Te4O11: Structure magnétique de Fe2Te4O11 à 4,2 K

Magnetic study and Mössbauer resonance measurements of the tellurites Fe₂Te₃O₉ and Fe₂Te₄O₁₁ characterize antiferromagnetic ordering. The transition temperatures determined by Mössbauer resonance, are 34 and 27 K, respectively. At 295 K the values of chemical shifts, 0.35 and 0.39 mm/sec, are typical of high-spin Fe(III) in octahedral coordination. Neutron powder diffraction was used to determine the magnetic structure of Fe₂Te₄O₁₁ at 4.2 K. It shows antiferromagnetic interactions between Fe³⁺ ions belonging to [Fe₂O₁₀] groups. The magnetic space group is $\text{P}{}_{\mathrm{2a}}\text{2}{}_1\text{c}$.     

Magnetic susceptibility of the one-dimensional conductor (H3O)1.6[Pt(C2O4)2]nH2O

The magnetic susceptibility of the dioxalatoplatinate acid (H₃O)_1.6[Pt(C₂O₄)₂]nH₂O, a mixed valency planar (MVP) compound with columnar structure and one-dimensional metallic conduction properties, has been measured in the temperature range 1.5–300°K. The observed paramagnetism, like in other MVP compounds, may be described by a linear $\text{1}\text{T}$ dependence with a break in the slope at a characteristic temperature. The Curie constants as well as the point of discontinuity have been found to depend on the crystal water content. The results are discussed in terms of different theoretical concepts.     

Phase equilibria in the FeMnTiO system for the compositions Fe0.297Mn0.097Ti0.606O

This paper reports equilibrium-phase data for the FeMnTiO system at oxygen fugacities between 10^?18.2 and 10^?10.0 atm and at temperatures between 1249 and 1514K. An equilibration-and-quench method was used, and the oxygen fugacities were controlled with mixtures of hydrogen and carbon dioxide. The phase relations are presented on a log f_O2 vs $\text{1}\text{T}$ diagram and are discussed interms of the phase relations in the two systems FeTiO and MnTiO.     

Properties of compounds of type NaxSi46 and NaxSi136

A qualitative account is given of the electrical and magnetic properties of compounds of the type Na_xSi₁₃₆. It is suggested that a metal-insulator transition takes place with decreasing x. It is conjectured that in sintered powders the resistance is due to amorphous contacts between grains, leading to a resistivity proportional to $\text{A}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$.     

Heat capacity and thermodynamic properties of ramsdellite,Li2Ti3O7, and spinel,Li43Ti53O4

The heat capacity and heat content Li_{$\text{4}\text{3}$}Ti_{$\text{5}\text{3}$}O₄ and Li₂Ti₃O₇ have been measured in the temperature range 198—960 K. The lattice and dilation contributions to the heat capacity have been estimated. The standard thermodynamic functions and the high temperature enthalpy and entropy have been derived. The lattice heat capacity of Li_{$\text{4}\text{3}$}Ti_{$\text{5}\text{3}$}O₄ spinel appears to be consistent with the phonon model put forward by Grimes.     

Phase diagram and some physical properties of V2O3+x1 (0 ? x ? 0.080)

The magnetic and electric properties of V₂O_3+x were investigated by measurements of magnetic susceptibility, electrical resistivity, magnetotorque, Mössbauer of doped ⁵⁷Fe, and NMR of ⁵¹V, and the results were compared with those of the (V_1?xTi_x)₂O₃ system or highly pressured V₂O₃. The results obtained are as follows: (1) The metallic state shows an antiferromagnetic ordering at T_N (x). The value of T_N for metallic V₂O₃, obtained by interpolation to x = 0, shows the coincidence between V₂O_3+x and the (V_1?xTi_x)₂O₃ system. (2) Magnetic susceptibility of V₂O_3+x is expressed as χ_M(V₂O_3+x) = (1?x)χ_M(V³⁺) + xχ_M(V⁴⁺). χ_M(V⁴⁺) obeys the Curie-Weiss law $\text{(χ}{}_{\mathrm{<mtext>M</mtext>}}\text{(}\text{V}{}^{\mathrm{4+}}\text{) =}\text{0.77}\text{T}\text{+ 17)}$. (3) In the insulating phase, the electrical resistivity ? is expressed as a common equation: $\text{? = 10}{}^{\mathrm{?1.8}}\text{exp}\text{(}\text{E}\text{kT}\text{)}$. This implies that the substitution of Ti or nonstoichiometry (V⁺⁴ + metal vacancies) has little influence on the carrier mobility (or bandwidth). (4) There is a critical length in the c-axis (? 14.01 Å) where the metal-insulator transition takes place. This suggests that the length of the c-axis plays an important role in the metal-insulator transition of V₂O₃-related compounds.     

Crystallography and phase relations of MeOM2O3TiO2 systems (Me = Fe,Mg, Ni; M = Al,Cr, Fe)

Subsolidus phase relations of ternary oxide systems containing divalent Fe, Mg, or Ni, trivalent Al, Cr, or Fe, and tetravalent Ti are characterized by solid solutions at metal/oxygen ratios $\text{3}\text{4}$, $\text{2}\text{3}$, and $\text{3}\text{5}$. At low temperatures only compounds with cubic or hexagonal close-packed oxygen and uniform oxygen coordination remain stable in the crystal structures NaCl, spinel, ilmenite-α-Al₂O₃, TiO₂. The pseudobrookite phases FeTi₂O₅, MgTi₂O₅, Al₂TiO₅, Fe₂TiO₅, the V₃O₅ structure phase Cr₂TiO₅, and the Andersson phases Cr₂Ti_n?2O_2n?1 (n = 4,6,7,8,9) decompose. Additional phases with close-packed oxygen as predicted by a simple structure model for metal/oxygen ratios $\text{7}\text{12}$, $\text{5}\text{6}$, and $\text{11}\text{12}$ do not form but presumably are important for nonstoichiometric solid solutions. Most differences between systems containing transition metals and the MgOAl₂O₃TiO₂ system can be attributed to crystal field effects.     

Studies on Compensated Cu-Cr-Al Spinel Oxide Semiconductors

Electrical resistivity (ρ) and thermoelectric power (α) measurements were carried out in air on CuCr_2-xAl_xO₄(0.06 ≤ x ≤ 0.10) spinel oxides in the temperature range 300-675 K. Carrier concentration (n), optical phonon frequency (γ_o), and mobility (μ) values have been calculated from resistivity and thermoelectric power data. The results show that CuCr_2-xAl_xO₄ spinel oxides are compensated semiconductors and the total compensation of the hole carriers by electrons occur when 0.09 chromium ions are replaced by aluminium ions.     

Neutron powder diffraction on RbCrl3 and magnetic measurements on RbCrl3 and CsCrl3

β-RbCrI₃ (a = 13.772(3), b = 8.000(2), c = 7.069(2) Å β = 95.85(1)°, $\text{Z = 4,}\text{C2}\text{m}$ at 293 K) and γ-RbCrI₃ (a = 13.586(2), b = 7.923(2), c = 14.094(3) Å, β = 96.88(1)°, Z = 8, C2 at 1.2 K) are isostructural to β-RbCrCl₃ and γ-RbCrCl₃ and are both Jahn-Teller distorted BaNiO₃ structures. In both compounds elongated octahedra occur. γ-RbCrI₃ most probably has a magnetic spiral structure at 4.2 and 1.2 K. Theoretically, a spiral propagating along the b axis is expected. A model with k = 9/19b1 yielded the best result. However, no good fit was obtained possibly because of a misfit in k and canting of the magnetic moments due to anisotropy. χ vs T single-crystal measurements on β-CsCrI₃ are in accordance with its magnetic structure. The three-dimensional magnetic ordering temperature T_c is estimated as 27(1) K. From the χ vs T curves of γ-RbCrI₃, T_c could not be determined. From fits to χ vs T powder data $\text{J}\text{k}$ of CsCrI₃ and RbCrI₃ are estimated to be ?14(2) and ?11(1) K, respectively.     

Low-temperature phases of the solid electrolyte Ag26I18W4O16

Single crystal X-ray diffraction photographs taken with a Buerger precession camera, at temperatures 250, 214, and 122 K, corroborate the existence of three low-temperature phases of Ag₂₆I₁₈W₄O₁₆. These phases are labeled α′, β, and γ in order of decreasing temperature. The α′ phase is monoclinic, space group P2₁, Z = 2; the β phase is triclinic, space group $\text{P}\text{1}$ or P1, Z = 2; and the γ phase is triclinic, space group P1, Z = 1. Lattice constants at the aforementioned temperatures are given. Twins in the β and γ phases are related by the albite and pericline laws, as are twins in the feldspars. The highest symmetry known to be attained by the (W₄O₁₆)^8? entity is 2(C₂), which, strictly, it must lose at the transition to the α′ phase.     

A deuterium NMR study of D(UO2XO4), 4D2O; X = P,As

Deuterium NMR spectra have been obtained by the solid echo technique for polycrystalline samples of DUO₂XO₄, 4D₂O, with X = P (DUP) and As (DUAs). Transitions (II → I) were found, by DSC measurements, to occur at 260 and 290 K for DUP and DUAs, respectively. In the high temperature tetragonal phase I, the NMR lineshapes are consistent with motionally averaged axially symmetric electric field gradient tensors, with values for $\text{〈}\text{e}{}^2\text{qQ}\text{h}\text{〉}{}_{\mathrm{<mtext>av</mtext>}}$ of 39.2 ± 0.2 and 38.2 ± 0.2 kHz for DUP and DUAs, respectively. This requires fast chemical exchange of the deuterium atoms between all of the hydrogen-bond sites in a water layer, and participation of all of the deuterium atoms in the diffusion process. There are no discernible discontinuities in the spectra at the transition temperature suggesting that there is little change in the local structure of the water layers at the transition. The spectra disappeared at about 10 K below T_c and for DUP a distorted broader spectrum became discernible below about 200 K.     

Structural aspects of the metal-insulator transition in V4O7

V₄O₇ has a transition with decreasing temperature at 250 K and the structure has been refined at 298 and 200 K. The triclinic structure (A$\text{1}$) consists of rutile-like layers of VO₆ octahedra extending indefinitely in the a-b plane and four octahedra thick along the c-axis. The average VO distances for the four independent V atoms are 1.967, 1.980, 1.969, and 1.984 Å at 298K and 1.948, 1.992. 1.961, and 2.009 Å at 200K. At 200K there is a clear separation into strings of V³⁺ or V⁴⁺ ions running parallel to the pseudorutile c-axis. In addition, all of the 3+ and half of the 4+ sites are paired to form short VV bonds. The remaining V⁴⁺ atom is displaced toward one oxygen so as to balance its electrostatic charge. The distortion at the metal-insulator transitions in V₄O₇, Ti₄O₇, VO₂ + Cr, and NbO₂ are compared.     

An EPR study of solid solutions ZnAl2O4ZnCr2O4

We studied the EPR spectra of ZnAl₂O₄-ZnCr₂O₄ solid solutions. The changes observed in the spectrum with increasing chromium concentration are attributed to the gradual development of magnetic interactions between paramagnetic ions in the solid solution. From the concentration dependence of the intensity of the isolated ions spectrum an approximate value of the range of the exchange interactions is deduced. The spectrum observed at high chromium concentrations is attributed to clusters of chromium ions coupled by exchange; the temperature dependence of its intensity indicates an antiferromagnetic character of the concentrated solid solutions.     

A novel case of magnetic relaxation in the 99Ru Mössbauer spectrum of Na3RuO4

The magnetic properties of Na₃RuO₄ have been studied using ⁹⁹Ru Mössbauer spectroscopy. Magnetic hyperfine splitting is seen at temperatures below 30°K. The value of the flux density at 4.2°K (58.58 T) is compatible with a saturation field at an $\text{S =}\text{3}\text{2}\text{Ru}{}^{\mathrm{V}}$ ion, and at intermediate temperatures approximately follows an $\text{S =}\text{3}\text{2}$ Brillouin function. The published interpretation of magnetic susceptibility data in terms of tetranuclear intracluster antiferromagnetism is shown to be incorrect, and it is concluded that Na₃RuO₄ shows antiferromagnetic three-dimensional long-range order with a Néel temperature of T_N = 30 ± 1°K. Between 25 and 30°K motional narrowing of the spectrum is seen, which is probably due to a slow spin-spin relaxation within the crystal-field levels of the ⁴A_2g ground state of the Ru^V ion. This is the first observation of relaxation effects in a ⁹⁹Ru spectrum.     

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.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

Structure de SnPb2O4 à quatre températures: relation entre dilatation et agitation thermiques

The structural study of SnPb₂O₄ oxide, an isomorphic compound belonging to the general family “MeX₂O₄” like Pb₃O₄, is made from accurate X-ray and neutron diffraction techniques on powdered samples. The structural evolution of SnPb₂O₄ is analyzed from 300 to 5 K: no phase transition is observed, contrary to Pb₃O₄, which exhibits a tetragonal → orthorhombic transition at 170 K. The thermal expansion tensor is practically isotropic in this temperature range: the α_a, α_c and α_V coefficients are neighboring those observed in the Pb₃O₄ tetragonal phase at the same temperature. On the other hand, the thermal vibrations are strongly anisotropic, with large amplitudes in the (a, b) plane. In this study the thermal vibrations are connected to the thermal expansion. $\text{B}\text{ab}$ and $\text{B}\text{c}$ temperature factors are considered as functions of the a and c cell parameters. The relation established by Grüneisen between the mean-square amplitudes of vibrations and the thermal volume expansion is discussed. The interatomic distances found show that the bindings are similar to that of Pb₃O₄: only the [Sn⁴⁺O₆] octahedrons are smaller than [Pb⁴⁺O₆] octahedrons.     

The crystal structure of Na7Mg4.5(P2O7)4

The crystal structure of Na₇Mg_4.5(P₂O₇)₄ has been solved by direct methods from the three-dimensional X-ray data. The space group is $\text{P}\text{1}$. The crystal structure consists of Mg²⁺, Na⁺, and P₂O^4?₇ ions. One magnesium atom at symmetry center (0,0,0) and two sodium atoms at ±(?0.0421, ?0.0596, 0.2230) display occupation factors 0.5 each. A short interatomic distance between these Na⁺ and Mg²⁺ ions (1.80 ± 0.01 Å) excludes the occupation of both sites in the same unit cell. The crystal structure of Na₇Mg_4.5(P₂O₇)₄ consists of unit cells containing Na₈Mg₄(P₂O₇)₄ or Na₆Mg₅(P₂O₇)₄ with a statistical occurrence 1:1.Each Mg²⁺ ion is octahedrally coordinated by six O^2? ions at distances 1.979 – 2.270 Å. The coordination polyhedra around the Na⁺ ions are ill-defined. The bond angles POP in the P₂O^4?₇ groups are 126.6 and 133.6° (±0.3°). The final reliability factor R is 7.1%.