Pressure and isotope effects in the manganese-oxide perovskites

Book · Thu Dec 31 00:00:00 EST 1998

OSTI ID:323429

Goodenough, J B; Zhou, J S ^[1]

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1. Univ. of Texas, Austin, TX (United States). Center for Materials Science and Engineering

Measurements of the temperature dependence of the resistivity {rho}(T) and thermoelectric power {alpha}(T) under several hydrostatic pressures on {sup 18}O/{sup 16}O isotope-exchanged (La{sub 1{minus}x}Nd{sub x}){sub 0.7}Ca{sub 0.3}MnO{sub 3} polycrystalline samples spanning the structural O{prime}-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-O bond. They have also indicated a change at the O{prime}-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below {Tc}, and a phase segregation in the O phase above {Tc} that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through {Tc} in the O phase, and a single-crystal study of La{sub 1{minus}x}Sr{sub x}MnO{sub 3}, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below {Tc} within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above {Tc} and the existence of a ferromagnetic glass below {Tc} in the O phase. The intrinsic colossal magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above {Tc}, and the dramatic rise in {Tc} with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above {Tc}.

• Sponsoring Organization: National Science Foundation, Washington, DC (United States); Welch (Robert A.) Foundation, Houston, TX (United States)
• OSTI ID: 323429
• Report Number(s): CONF-971201--; ISBN 1-55899-399-1
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CALCIUM OXIDES
ELECTRIC CONDUCTIVITY
EXPERIMENTAL DATA
ISOTOPE EFFECTS
LANTHANUM OXIDES
MAGNETIC PROPERTIES
MANGANESE OXIDES
NEODYMIUM OXIDES
PRESSURE DEPENDENCE
STRONTIUM OXIDES
THERMOELECTRIC PROPERTIES