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The lattice parameters of BiFeO3 were determined with the Straumanis method. At 25–13±0.02 °C, the hexagonal parameters are ah = 5.5799±0.0003, and ch = 13.8670±0.0005 Å. The temperature dependence of the lattice parameters in the range 20–325 °C is given by the equations: ah = 5.5764 Å + 6.06 × 10−5t, and ch = 13.8620 Å + 2.10 × 10−4t. In the range of 344–838 °C, the lattice parameters obey the following equations: ah = 5.5946 + 6.83 × 10−5 t, and ch = 13.7251 + 9.05 × 10−4 t−12.503 × 10−7 t2 + 9.40 × 10−10 t3− 3.57 × 10−13t4. By extrapolation of the angular separation of the 11.0 and the 10.4 reflections, the electrical Curie temperature was determined to be 845±5 °C.

was first synthesized1 and shown S to possess the perovskite structure,2 numerous studies of its nuclear, electrical, and magnetic characteristics have been reported. Despite all this work, the exact atomic structure explaining X-ray, e l e~t r o n ,~ and neutron' powder diffraction data has not been ascertained. Because the conductivity of BiFeO3 is too high for the necessary electrical data to be taken, it is not certain whether BiFeOt is ferroelectric or antiferroelectric.In the absence of single crystals, single-phase polycrystalline material must be provided to allow determination of the correct nuclear structure and to permit the reproduction of meaningful physical and chemical properties. The preparation of BiFeOa described by all investigators is by solid state reaction of BilOaFez03 mixturcs from 700' to 800°C for 0.5 to 2.0 hr. Some investigators report that this technique yields pure BiFeOa; more frequently the product is described as almost single phase or as containing small quantities of additional phases.All attempts by our group to prepare BiFeOa by this standard technique havc produced multiphase samples. Variation of sintering temperature, time, and atmosphere; repetitive grinding and sintering; and the addition of small (< 0.5% mole) quantities of third components failed to produce BiFeOs which analyzed as single phase by X-ray diffraction. Reaction below 700°C is incomplete, and BiFeOt becomes unstable above 750"C.p In the 700" to 750°C range Bi2Fe40B6 was energetically competitive with BiFeOa, forming in quantities up to several mole percent while leaving a corresponding amount of residual BieO3. Based on these experimental results, it must be concluded that many of the investigations appearing in the literature were conducted on impure samples containing low concentrations ( 5 5 mole % ) of decomposition products or BiZFa09 and BirOa.The following technique was developed t o prepare pure BiFeOa. The reactants are weighed in a 2Bi&.FaOa ratio and ground thoroughly. The 100% excess Biz01 in the reaction mixture prevents formation of BilFQOg during sintering. The sample, after a 3 hr sinter in a closed air atmosphere a t 750°C followed by air quenching, contains only BiFeOs and unreacted Bi203. The residual Bi?Oa is removed by a multiple-step concentrated HNOa leach. The acid leach is repeated twice to prevent the hydrolysis of BiON03 in the washing process. The leached residue is washed a minimum of three times with large volumes of distilled water t o neutralize the acidity. After being dried in a furnace, the sample is ground to a very fine powder.The purple-black polycrystalline BiFeOa prepared by this technique analyzes as single phase by X-ray powder diffraction. The following simple cell rhombohedra1 parameters were evaluated: a = 3.958 A a = 89"30' Figure 1 compares the low-angle X-ray diffraction patterns of two BiFeOa samples sintered 3 hr at i5O"C. The lower sample is a sintcred Biz08-Fe*03 mixture; its pattern shows definite evidence of thc two major peaks of both Biz03 and BiZFerOB...

Solid solutions of BiFeO3 with PbTiO3, PbTi0.5Zr0.5O3, and PbZrO3 were prepared. The crystallographic data on these solutions, which are basically perovskitic, are given. The dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C. Dielectric Curie points were found in solutions containing up to 90 mole % BiFeO3. These results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric. The extrapolated Curie point for BiFeO3 is above 850°C. BiFeO3 appears more likely to be ferroelectric than antiferroelectric, but the distinction between the two classifications may not be sharp.

The perovskite La1/3Sr2/3FeO3−δ was investigated by neutron diffraction, magnetic and Mössbauer spectroscopy measurements. La1/3Sr2/3FeO3−δ undergoes magnetic ordering at T = 190–200 K accompanied by charge disproportionation. Magnetic peaks due to charge ordering are observed below 200 K. The charge ordering is gradually developed below 200 K along with a charge disproportionation, 2Fe4+ ⇒ Fe3+ + Fe5+. La1/3Sr2/3FeO3−δ shows an antiferromagnetic structure at low temperature. Magnetic moments of about 3 and 1.3 μB were obtained from the neutron diffraction data refinement for Fe3+ and Fe5+ at 15 K, respectively, which suggest that both Fe ions are in a low spin state. These values are significantly lower than those reported by Battle et al for La1/3Sr2/3FeO2.98. Mössbauer spectra indicate that full charge ordering might be reached below 20 K with no Fe4+.

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4mm above 225K. At 16K, the BD3–ND3 unit stacks along the c axis with a tilt angle of about 16° between the N–B bond and the c axis. As the temperature is increased, the BD3–ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition.