Crystal and Magnetic Structures and Properties of BiMnO_3+δ

Abstract

Crystal and magnetic structures of BiMnO(3+delta) (delta = 0.03, 0.08, and 0.14) have been determined by the Rietveld method from neutron diffraction data at 8-10 and 290 K. BiMnO(3.03) (= Bi(0.99)Mn(0.99)O(3)) crystallizes in a monoclinic system (the refinement was performed in space group C2/c; Z = 8; a = 9.5313(3) A, b = 5.57791(17) A, c = 9.7375(4) A, beta = 108.951(2) degrees at 290 K). BiMnO(3.08) (= Bi(0.974)Mn(0.974)O(3)) crystallizes in space group P2(1)/c (Z = 8; a = 9.5565(4) A, b = 5.51823(16) A, c = 9.7051(4) A, beta = 109.442(3) degrees at 290 K). It was found that Mn vacancies are localized mainly in one Mn site (among three sites) in Bi(0.974)Mn(0.974)O(3). Vacancy-ordering and charge-ordering scenarios are suggested as possible reasons for the crystal symmetry change compared with Bi(0.99)Mn(0.99)O(3). BiMnO(3.03) and BiMnO(3.08) are ferromagnetic below T(C) = 82 and 68 K, respectively, with magnetic moments along the monoclinic b axes. Refined magnetic moments at 10 K are 2.88(2)micro(B) in BiMnO(3.03) and 2.33(2)micro(B) in BiMnO(3.08). BiMnO(3.14) (= Bi(0.955)Mn(0.955)O(3)) crystallizes in an orthorhombic system (space group Pnma; Z = 4; a = 5.5136(4) A, b = 7.8069(8) A, and c = 5.5454(5) A at 290 K), and its structure is similar to that of LaMnO(3.11)-LaMnO(3.15). No magnetic reflections were found in BiMnO(3.14) down to 8 K, in agreement with its spin-glass magnetic state. Magnetic and chemical properties of BiMnO(3+delta) (0.02 < or = delta < or = 0.14) have also been investigated and compared with those of LaMnO(3+delta). Systematic changes of magnetic parameters in BiMnO(3+delta) were found to depend on delta.

Evidence of the Griffiths phase in multiferroic BiMnO3 and BiFe0.5Mn0.5O3 films

“…Figure 1 shows (a) the high-magnification cross-sectional HREM image and (b) the typical selected-area electron diffraction (SAED) pattern along the [010] zone axes for the BMO film. Because the alternative SAED patterns were not observed, one can conclude that the major phase of the BMO film has the orthorhombic crystal structure with lattice parameters a b 0.56 nm, c 0.7838 nm and = α = = β γ 90° which are very close to the published results [20,21]. However, the small regions of long-periodic stripdomain phase, represented by the inset of Fig.…”

Perovskite solid solutions with multiferroic morphotropic phase boundaries and property enhancement

“…Details of the space group determination can be found elsewhere. 38 This is the same crystal structure of the high temperature phase of BiMnO 3 , which transforms into the monoclinic one at 770 K. 41,42 The monoclinic to orthorhombic transformation can be also induced by high pressures, 43 and by oxygen hyperstoichiometry, 44 as well as by the simultaneous substitution of Pb 2þ for Bi 3þ and Ti 4þ for Mn 3þ as shown here. This might be an example of chemical pressure effects, like those described in the Pb(Zr,Ti)O 3 system that have been proposed to be responsible of the appearance of the MPB.…”

“…52 BiMnO 3 would be then isostructural to BiScO 3 , and a comparative Rietveld study of neutron diffraction data seemed to suggest this to be the case. 53 Subsequent additional neutron diffraction experiments confirmed this space group determination, 44 but an independent report using SAED indicated the presence of long-ranged monoclinic C2 order, in coexistence with short ranged P2 or P2 1 distortions, and thus, of a polar state. 54 The length of the short range order seems to increase with the oxygen hyperstoichiometry, 39 but a P2 1 /c space group has been determined for BiMnO 3:08 .…”

“…54 The length of the short range order seems to increase with the oxygen hyperstoichiometry, 39 but a P2 1 /c space group has been determined for BiMnO 3:08 . 44 Further insight into this issue can only be gained by a sound electrical characterization, which requires the processing of high quality ceramics. This is challenging for BiMnO 3 that thermally decomposes into a mixture of Bi 2 Mn 4 O 10 , Bi 12 MnO 20 and MnO 2 above 673 K. Note that Bi 2 Mn 4 O 10 is ferromagnetic with a Tc of 42 K, and most probably the magnetic impurity in Fig.…”

“…39 Oxygen hyperstoichiometry was controlled by XPS and magnetic measurements, by comparing the N eel temperature of 80 K to previous reports on the evolution of the magnetism with stoichiometry. 44 A composition of BiMnO 3:04 was estimated, so monoclinic C2/c phase can be assumed. 39 A full electrical characterization was carried out.…”