0428同步年報-2021-全

Neutron Science 077 perovskites; the key regime has been established that x = 3/8 in narrow-bandwidth Pr 1− x Ca x MnO 3 and La 1− x Ca x MnO 3 systems corresponds to a maximum MR effect. With HPHT-synthesized 134 quadruple perovskites AMn 3 Mn 4 O 12 (or AMn 7 O 12 ) solid solution, in which A = La 1− x Ca x and Na 1− x Ca x at intervals Δ x =0.1, the system can serve as a prototype to revisit the x =0–1/2 doped regime of the LCMO manganites in detail. 2 There are several advantages that make the current 134 system a sufficient model for the present study. The small variations in cell volume (< 2%) and octahedral tilt angles (138.7° ±1.2°) of the entire solid solution prevent the band-narrowing physics to the electronic doping accordingly. The octahedral tilt pattern of the series is locked as a + a + a + in Glazer’s notation with phase transitions so that further lattice distortion becomes indicative of long-range orbital ordering. The degree of crystallinity of these HPHT synthesized samples is much greater than simple perovskites of similar composition; the microstrain remains exceptionally low across phase transitions ( e 0 ≤ 0.015%), evident of the absence of significant intrinsic phase segregation. From detailed variable- temperature SXRD and NPD experiments, four distinct crystallographic phases and a comprehensive Fig. 3 : Structural phase transitions of HMO from high-temperature cubic to intermediate-temperature rhombohedral and to low-temperature orthorhombic phases. (a,b) Evolution of lattice parameters, volume and selected SXRD pattern across the phase transitions. (c,d) Detailed structure and symmetry-element analysis from SXRD and NPD joint Rietveld refinements of data at 300 and 100 K. [Reproduced from Ref. 4] Fig. 2 : (a) Magnetic phases observed from heat map of NPD patterns at base temperature. (b) Detailed temperature and compositional phase diagram of AMn 3 Mn 4 O 12 solid solution. (c) Four distinct structural phases with various orbital ordering at representative A-site compositions. [Reproduced from Ref. 2]