Takehiko Nakagawa

Dielectric and Magnetic Properties of Pb(Fe1/2Ta1/2)O3

Single crystals of Pb(Fe1/2Ta1/2)O3 were grown from ternary melts with excess PbO. The crystals, having cubic perovskite structure at room temperature, undergo a ferroelectric phase transition at about 243°K, changing to rhombohedral structure. The dielectric constant passes through a maximum of nearly 9500 at 1 KHz. The spontaneous polarization along [100] direction is ca 28 µC/cm2 at liquid nitrogen temperature. A magnetic anomaly at about 180°K is related to the onset of antiferromgnetic ordering.

Magnetic and Electrical Properties of Ordered Perovskite Sr2(FeMo)O6 and Its Related Compounds

A series of mixed metal oxides with perovskite-type structure, A 2 (FeMo)O 6-λ (where A=Ba, Sr and Ca), were synthesized. The ferrimagnetic Neel temperatures of Ba 2 (FeMo)O 6 , Sr 2 (FeMo)O 6 and Ca 2 (FeMo)O 6 , were respectively found to be 351°K, 450°K and 398°K, with the saturation moments of 3.6∼3.2 µ B . The Mossbauer absorption spectra of Fe 57 in Sr 2 (FeMo)O 6 and Ca 2 (FeMo)O 6 were measured at room temperature. They have the isomer shift of 0.27 mm/sec and 0.35 mm/sec relative to that of Pd(Co 57 ) and the values of internal magnetic field at 0°K were deduced to be -570 kOe and -490 kOe, respectively. From the measurements of electrical conductivity and infra-red transmission spectra, all of these compounds A 2 (FeMo)O 6 were found to be highly conductive (ρ≃10 -1 ∼10 -3 ohm·cm) and their temperature dependence is characteristic of metallic conduction.

Electrical Properties and Mössbauer Effect in the System Sr2(FeMoxW1-x)O6

A series of solid solutions between Sr 2 (FeMo)O 6 and Sr 2 (FeW)O 6 were synthesized and their electrical properties were studied by the measurements of both the temperature dependence of the electrical conductivity and the Seebeck coefficient. The valence states of iron ions were studied by the Mossbauer effect measurements. The compounds in the range (0.5≤ x ≤1) have high conductivity (ρ≃10 -3 ohm·cm) with the metallic type. The valence distribution between the B sublattice ions is considered to be Sr 2 (Fe 3+ Mo x 5+ W 1- x 5+ )O 6 . The compounds in the range (0.3≤ x ≤0.4) is considered to be a semiconductor with a very small energy gap or a degenerate semiconductor. And the compound in the range (0≤ x ≤0.2) may be called semiconducting. A mixed valence compound Sr 2 (Fe 1- x 2+ Fe x 3+ Mo x 5+ W 1- x 6+ )O 6 will be formed in the range (0≤ x ≤0.1). The rapid change in valence in the range (0.1≤ x ≤0.4) is evidenced by a rapid replacement of the Fe 3+ ions for the Fe 2+ ions.

Magnetic and Mössbauer Studies of the Ordered Perovskites Sr2Fe1+xRe1-xO6

Structural, magnetic, electrical properties and Mossbauer effect are investigated on Sr 2 FeReO 6 , SrFe 2/3 Re 1/3 O 3 and their solid solutions. Sr 2 FeReO 6 and SrFe 2/3 Re 1/3 O 3 (Sr 2 Fe 4/3 Re 2/3 O 6 ) are ordered perovskites of types Sr 2 [Fe][Re]O 6 and Sr[Fe][Fe 1/3 Re 2/3 ]O 6 respectively, both of which are tetragonal at room temperature. Sr 2 FeReO 6 is metallic (ρ∼10 -3 \(\varOmega\)-cm) and ferrimagnetic below 419 K, while SrFe 2/3 Re 1/3 O 3 is semiconductive (ρ r · t · ∼10 -1 \(\varOmega\)-cm, E ρ ∼0.09 eV) and ferrimagnetic below 475 K. The SrFe 2/3 Re 1/3 O 3 compound is diffcult to saturate in the magnetization. Mossbauer spectra showed that irons in these compounds are essentially trivalent. The spectra of SrFe 2/3 Re 1/3 O 3 are composed of two lines which can be attributed to the two kinds of Fe 3+ ions located on the two sublattices. Electrical and magnetic properties of Sr 2 FeReO 6 and SrFe 2/3 Re 1/3 O 3 are discussed, taking the crystallographic structured and ionic states of ...

High pressure synthesis of Pb(B, B′)O3 type perovskite

A number of newly synthesized compounds of the type Pb3B2WO9 and Pb2BWO6 with the perovskite structure are reported where B is Cr, Co, Sc, In, Ni, Mn, Fe or Zn. Synthesis of these compounds was carried out at a pressure between 20 and 40 kb using a piston-cylinder apparatus. It is concluded that lead containing perovskite with general formula Pb(B, B′)O3 are stabilized well under high pressure.

Structural and Magnetic Transitions in Sr2 (NiMo) O6

Structural, electrical and magnetic properties of Sr 2 (NiMo)O 6 were studied. The compound undergoes a phase transition at 230°C, where the structure which is of the ordered perovskite-type changes from tetragonal to cubic. Dielectric anomaly is not observed at this transition point. The crystal becomes antiferromagnetic below 71.5°K. The magnetic structure seems analogous to that of Sr 2 (NiW)O 6 after Blasse.

Mössbauer Studies on the Phase Transition in the Sr2(FeW)O6-Ba2(FeW)O6 System

Tetragonal-cubic phase transitions in the Sr 2 (FeW)O 6 -Ba 2 (FeW)O 6 system were investigated by means of X-ray diffraction and Mossbauer effect in the temperature range from room temperature to about 400°C. Sr 2 (FeW)O 6 undergoes a phase transition at about 350°C. The transition temperature decreases almost linearly by replacing Sr 2+ ions by Ba 2+ ions. Room temperature Mossbauer spectra of the system, with the exception of Ba 2 (FeW)O 6 , are characterized by the appearance of two well separated absorption peaks due to the quadrupole interaction of Fe 2+ ions. Temperature dependence of the quadrupole splitting of Sr 2 (FeW)O 6 is compared with that of the tetragonality. Cubic Ba 2 (FeW)O 6 has a single absorption peak (δ=1.39 mm/sec) in the Mossbarer spectra. Due to the disordered arrangement of Sr 2+ and Ba 2+ ions, quadrupole splittings are observed even in the cubic phase of the system.

Magnetic Properties and Optical and Paramagnetic Spectra of Divalent Nickel in Sr2(NiW)O6

Small single crystals of ordered perovskite Sr 2 (NiW)O 6 were synthesized by the flux method using SrCl 2 6H 2 O. The crystal becomes antiferromagnetic below 59°K and undergoes a tetragonal-cubic transition at 523°K. Optical and paramagnetic spectra of divalent nickel in Sr 2 (NiW)O 6 were investigated. The optical absorption lines are interpreted as transitions between the singlet ground state and possible Stark levels, following the scheme given by Low for MgO:Ni. Parameters of the energy levels were determined as follows: 10 D q =8575 cm -1 , E p =13430 cm -1 , E D =10860 cm -1 and E G =17120 cm -1 . Paramagnetic resonance occurred at the field strength of 7562 Oe and the frequency of 23.73 GC /sec. Values of g and spin-orbit coupling constant λ were evaluated at 2.242 and -256 cm -1 respectively.

Crystal Distortion in Sr(Fe1/2Ta1/2)O3 and Some of Its Solid Solutions

The tetragonal-cubic transition was studied in Sr(Fe1/2Ta1/2)O3, together with its solid solutions with SrTiO3, Ba(Fe1/2Ta1/2)O3 and BaTiO3. At the transition temperature, the magnetic susceptibility and d.c. electrical resistivity showed small anomalies, whereas the dielectric constant did not. The tetragonal distortion in Sr(Fe1/2Ta1/2)O3 was found to be not ferroelectric but of the same type as in SrTiO3 below 110°K. The effective magnetic moment of the Fe3+ ion was found to be 5.5 µB.

X-Ray and Mössbauer Studies in the Sr(FeTa)1/2O3 and Sr(CrTa)1/2O3 System

Sr(FeTa) 1/2 O 3 is a disordered perovskite, tetragonal at room temperature, while Sr(CrTa) 1/2 O 3 is an ordered perovskite, cubic at room temperature. These compounds make solid solutions in the whole range of composition. Tetragonal-cubic transition occurs at about 90 mol% of Sr(FeTa) 1/2 O 3 . Intensity of superstructure line of X-ray diffraction which shows the ordered arrangement of the B ions decreases gradually with increase of Sr(FeTa) 1/2 O 3 . Isomer shift is nearly constant, 0.64 mm/sec, for all compounds, the value of which is reasonable for Fe 3+ . The curve of the quadrupole splitting vs composition is rather complicated. We can give it a qualitative interpratation in terms of local distortion arising from the disordered distribution of the Ta, x Fe and (1- x )Cr ions.