P.E. Bierstedt

Ferroelectric Rare‐Earth Molybdates

The molybdates of Sm, Eu, Tb, and Dy are isotypic with ferroelectric Gd2(MoO4)3. Other compositions of this structure occur in the pseudoternary system Gd–La–Y‐molybdate and in the pseudobinary system Gd2(MoO4)3–Gd2(WO4)3. A representative number of these compositions were grown as single crystals and tested for ferroelectricity. Each was found to be ferroelectric as evidenced by a ferroelectric hysteresis loop and by visual observation of domains. The Curie temperatures (TC), spontaneous polarizations (Ps), and coercive fields (Ec) were determined, with values ranging as follows: TC, 148°–190°C; Ps, 0.14–0.23 μC/cm2 at 25°C; Ec, 5.0–14.5 kV/cm at 25°C.

New transition metal dichalcogenides formed at high pressure

Abstract Copper dichalcogenides CuX2 with the pyrite-type structure have been synthesized at high pressure. These materials are superconductors. New pyrite-type dichalcogenides of Ni, Co and Fe have also been prepared by the same technique.

New Modified Mn2Sb Compositions Showing Exchange Inversion

Transformations from ferrimagnetic to antiferromagnetic ordering with decreasing temperature have now been observed in manganese antimonides modified with V, Co, Cu, Zn, Ge, and As as well as with Cr. The magnetic structure change is accompanied by a first‐order change in the volume of the unit cell as well as a change in electrical resistivity. Crystallographic data for these materials in the exchange inversion region and at room temperature are presented as a function of the concentration of modifying element.

SUPERCONDUCTIVITY OF SOME NEW HEXAGONAL TUNGSTEN BRONZES

Abstract The nonstoichiometric tungsten bronzes MxWO3 with hexagonal structure (maximum x = 0.33) have been described by Magneli1 for the alkali metals potassium, rubidium, and cesium. These materials exhibit metallic-type conduction, and, as recently reported by Sweedler et al .2 bronzes having hexagonal or tetragonal I symmetry show superconductivity of a series of new hexagonal tungsten bronzes containing the alkaline earths, calcium, strontium, or barium. In addition, the hexagonal thallium and indium bronzes prepared in this laboratory3 are shown to be superconducting.

Designing zeolite catalysts for shape-selective reactions: Chemical modification of surfaces for improved selectivity to dimethylamine in synthesis from methanol and ammonia

The relative contributions of external and intracrystalline acidic sites of small pore H-RHO zeolite for the selective synthesis of methylamines from methanol and ammonia have been studied. Nonselective surface reactions which produce predominantly trimethylamine can be eliminated by “capping” the external acidic sites with trimethylphosphite (TMP) and other reagents, thus improving the selectivity toward the formation of dimethylamine. For small pore zeolites, neither the zeolite pore size nor the internal acidic sites is significantly affected by this treatment. In situ infrared and MAS-NMR studies show that TMP reacts irreversibly with the zeolite acidic sites via a modified Arbusov rearrangement to form surface-bound dimethylmethylphosphonate.

Topochemical reduction of the high temperature superconductor YBa2Cu3O7: The structure of YBa2Cu3O6 by neutron diffraction

Abstract Superconducting orthorhombic YBa 2 Cu 3 O 6.8 has been reduced to tetragonal YBa 2 Cu 3 O 6 without structural collapse by heating under a nitrogen atmosphere at 900°C. Further reduction above ∼950°C results in destruction of the perovskite-related structure. Neutron powder diffraction data were fitted by the Rietveld profile refinement method to obtain the structure of YBa 2 Cu 3 O 6 . The reduced material is viewed as containing isolated linear Cu 1+ O 2 units and square pyramidal Cu 2+ O 5 units that share oxygen atoms to form a two-dimensional copper-oxygen network. Semiconducting behavior is observed for YBa 2 Cu 3 O 6 from 4.2 K to 300 K consistent with such a localized-electron model. The structural refinement of YBa 2 Cu 3 O 6 shows that the linear (Cu − 0) n chains present in YBa 2 Cu 3 O 7 are now completely absent.

Stability of Te–Cu amorphous alloy thin films for optical recording

We have studied the structure and optical stability of Te–Cu thin film alloy candidates for write‐once optical recording. Films prepared by rf diode sputtering with 20–50 at. % Cu are amorphous, as‐sputtered. One of these, Te65 Cu35 , has a relatively high crystallization temperature (150 °C), as determined by x‐ray diffraction. Near the eutectic composition (∼29 at .% Cu), alloy films have stable optical properties after accelerated aging at 60 °C and 85% relative humidity. The mechanism for film stability near the eutectic was studied by x‐ray photoelectron spectroscopy and depth profiling using ion scattering spectroscopy. We found that a Cu‐enriched surface oxide, formed at ambient conditions, passivates the film and is responsible for its subsequent stability after accelerated aging. We also demonstrated that a 14 in. diam, multilayer optical disk with a Te65 Cu35 recording medium exhibits excellent linearity for 3 and 8 MHz pulses, good written pulse length stability, and high signal‐to‐noise ratio....

Mixed-cation transition metal pyrite dichalcogenides—High pressure synthesis and properties

Abstract Ternary, pyrite-type dichalcogenides Zn y Mn 1− y X 2 , Cd y Mn 1− y X 2 , Zn y Cd 1− y X 2 , Zn y Cu 1− y X 2 , Cd y Cu 1− y X 2 , Mn y Cu 1− y X 2 , Cu ∼0.75 Fe ∼0.25 S 2 , and Ni 0.4–0.6 Fe 0.6-0.4 X 2 (0 y 2 was elevated upon substitution with less than 20 a/o of Zn or Cd. A noncolinear spin structure is proposed to explain the ferromagnetism observed in the Mn y Cu 1− y X 2 compounds.

Superconducting oxides of rhenium and molybdenum with tungsten bronze type structures

Abstract Superconducting transitions have been found for hexagonal KξReO3 ( a = 7.335 A , c = 7.480 A , x ∼ 0.3) and tetragonal KξMoO3 ( a = 12.36 A , c = 3.86 A , x ∼ 0.5) at 3.6° and 4.2°K, respectively. However, no superconducting transitions were found for the cubic bronzes NaξMoO3, KξMoO3, NaξReO3, or ReO3 down to 1.3°K.

Magnetic Anisotropy and Electrical Resistivity in Exchange Inversion Compounds

Magnetic anisotropy and electrical resistivity have been determined for several materials of composition Mn2−xCrxSb, which exhibit a ferrimagnetic to antiferromagnetic phase transformation with decreasing temperature. Magnetocrystalline anisotropy is similar in its temperature behavior to that of Mn2Sb with slight differences attributed to changes in magnetic moment of the Mn sublattices. Resistivity is attributed to electron-magnon scattering. The magnetocrystalline anisotropy at the two Mn sites of nearly equal magnitude but opposite sign is consistent with the small anisotropy of the magnetoresistance.