Phase competition inL0.5A0.5MnO3perovskites

Abstract

Single crystals of the systems Pr 0.5 (Ca 1-x Sr x ) 0.5 MnO 3 , (Pr 1-y Y y ) 0.5 (Ca 1-x Sr x ) 0.5 MnO 3 , and Sm 0.5 Sr 0.5 MnO 3 were grown to provide a series of samples with fixed ratio Mn(III)/Mn(IV)=1 having geometric tolerance factors that span the transition from localized to itinerant electronic behavior of the MnO 3 array. A unique ferromagnetic phase appears at the critical tolerance factor t c = 0.975 that separates charge ordering and localized-electron behavior for t<t c from itinerant or molecular-orbital behavior for t>t c . This ferromagnetic phase, which has to be distinguished from the ferromagnetic metallic phase stabilized at tolerance factors t>t c , separates two distinguishable Type-CE antiferromagnetic phases that are metamagnetic. Measurements of the transport properties under hydrostatic pressure were carried out on a compositions t a little below t c in order to compare the effects of chemical vs. hydrostatic pressure on the phases that compete with one another near t=t c .

V–V bond length fluctuations in VO x

“…Suppression of κ ph (T) proves to be an excellent indicator of the presence of bondlength fluctuations whether these fluctuations derive from disordered Jahn-Teller deformations 15 , spin-lattice coupling in charge-transfer-gap paramagnets 16 , or two-phase fluctuations 17 . The electronic component κ e (T) of the measured κ(T) = κ ph (T) + κ e (T) was subtracted out by calculation from the Wiedermann-Franz law and the measured electronic conductivity.…”

Competing crystal structures inLa0.5Ca0.5MnO3: Conventional charge order versus Zener polarons

“…There is conflicting experimental evidence from resonant XRD experiments [15] for a CB CO pattern in Pr 0.60 Ca 0.40 MnO 3 and a recent refinement of the structure of Pr 0.5 Ca 0.5 MnO 3 using high resolution x-ray and neutron powder data favored a CB structure [16]. Both CB and ZP phases have been proposed to exist in a phase diagram for half-doped manganites in which the tolerance ratio for the rare earth and alkaline earth ions is varied [17]. Powder neutron and x-ray diffraction experiments [2] and x-ray absorption near edge structure (XANES) [18,19] and resonant x-ray diffraction (RXD) [20] support a CB CO picture for La 0.5 Ca 0.5 MnO 3 .…”

“…These two states are therefore similar in energy and the actual ground state may depend on specific A and B ions [17], or even sample preparation, history and ambient conditions. The predominant difference in charge populations of the (Table I) (Table I) are indicated by small and large dashed circles.…”

Chemical, structural, and transport properties ofNa1−xCoO2

“…1 shows the in-plane resistivity ρ ab (T 3/2 ) for Na 0.57 CoO 2 . In view of a transition from polaronic to itinerant in-plane conduction as x increases beyond x≈0.25 in Na 1-x CoO 2 12 , we have previously 13 attributed the T 3/2 behavior of ρ ab (T) below 175 K, where the compound exhibits 3D metallic behavior, to the presence of strong-correlation fluctuations persisting into the compositions with x > 0.25. This departure from conventional Fermi-liquid behavior was rationalized within the context of a 3D electron gas, a condition that breaks down progressively above 170 K where the c-axis conductivity becomes thermally activated.…”