TY - JOUR
T1 - Systematic study of compositional and synthetic control of vacancy and magnetic ordering in oxygen-deficient perovskites Ca 2Fe 2-xMn xO 5+yand CaSrFe 2-xMn xO 5+y (x = 1/2, 2/3, and 1; Y = 0-1/2)
AU - Ramezanipour, Farshid
AU - Greedan, John E.
AU - Cranswick, Lachlan M.D.
AU - Garlea, V. Ovidiu
AU - Donaberger, Ronald L.
AU - Siewenie, Joan
PY - 2012/2/15
Y1 - 2012/2/15
N2 - Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca 2Fe 2-xMn xO 5 and CaSrFe 2-xMn xO 5+y, where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T d chain (vacancy) disorder. In the special case of CaSrFeMnO 5 the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ∼160 Å. This reveals a systematic progression from Ca 2FeMnO 5 (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO 5 (Icmm, disordered tetrahedral chains, overall short-range order) to Sr 2FeMnO 5 (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T c, decreases for the same x when Sr substitutes for one Ca. A review of the changes in T c for the series Ca 2Fe 2-xM xO 5, taking into account the tetrahedral/octahedral site preferences for the various M 3+ ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca 2Fe 2-xMn xO 5 and the CaSrFe 2-xMn xO 5 series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca 2 phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ∼50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) Å.
AB - Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca 2Fe 2-xMn xO 5 and CaSrFe 2-xMn xO 5+y, where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T d chain (vacancy) disorder. In the special case of CaSrFeMnO 5 the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ∼160 Å. This reveals a systematic progression from Ca 2FeMnO 5 (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO 5 (Icmm, disordered tetrahedral chains, overall short-range order) to Sr 2FeMnO 5 (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T c, decreases for the same x when Sr substitutes for one Ca. A review of the changes in T c for the series Ca 2Fe 2-xM xO 5, taking into account the tetrahedral/octahedral site preferences for the various M 3+ ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca 2Fe 2-xMn xO 5 and the CaSrFe 2-xMn xO 5 series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca 2 phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ∼50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) Å.
UR - http://www.scopus.com/inward/record.url?scp=84856838655&partnerID=8YFLogxK
U2 - 10.1021/ja210985t
DO - 10.1021/ja210985t
M3 - Article
AN - SCOPUS:84856838655
SN - 0002-7863
VL - 134
SP - 3215
EP - 3227
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 6
ER -