The rich physics of A-site-ordered quadruple perovskite manganites AMn₇O₁₂

Perovskite-structure AMnO3 manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron–phonon coupling, and Jahn–Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn7O12 were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn7O12 compounds host rich physics beyond the canonical AMnO3 materials

Competing electronic instabilities in the quadruple perovskite manganite PbMn₇O₁₂

Structural behavior of PbMn_{7}O_{12} has been studied by high resolution synchrotron x-ray powder diffraction. This material belongs to a family of quadruple perovskite manganites that exhibit an incommensurate structural modulation associated with an orbital density wave. It has been found that the structural modulation in PbMn_{7}O_{12} onsets at 294 K with the incommensurate propagation vector ks=(0,0,∼2.08). At 110 K another structural transition takes place where the propagation vector suddenly drops down to a quasicommensurate value ks=(0,0,2.0060(6)). The quasicommensurate phase is stable in the temperature range of 40-110 K, and below 40 K the propagation vector jumps back to the incommensurate value ks=(0,0,∼2.06). Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the quasicommensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behavior recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the quasicommensurate phase is likely to be associated with a competition between the Pb^{2+} lone electron pair and Mn^{3+} Jahn-Teller instabilities

Pressure-induced Jahn-Teller switch in the homoleptic hybrid perovskite [(CH3)(2)NH2]Cu(HCOO)(3): orbital reordering by unconventional degrees of freedom

Through in situ, high-pressure X-ray diffraction experiments we have shown that the homoleptic perovskite-like coordination polymer [(CH3)2NH2]Cu(HCOO)3 undergoes a pressure-induced orbital reordering phase transition above 5.20 GPa. This transition is distinct from previously reported Jahn–Teller switching in coordination polymers, which required at least two different ligands that crystallize in a reverse spectrochemical series. We show that the orbital reordering phase transition in [(CH3)2NH2]Cu(HCOO)3 is instead primarily driven by unconventional octahedral tilts and shifts in the framework, and/or a reconfiguration of A-site cation ordering. These structural instabilities are unique to the coordination polymer perovskites, and may form the basis for undiscovered orbital reorientation phenomena in this broad family of materials

BiMn7 O12: Polar antiferromagnetism by inverse exchange striction

Despite extensive research on magnetically induced ferroelectricity there exist relatively few studies on how a preexisting electric polarization affects magnetic order. Given that well-established magnetoelectric coupling schemes can in principle work in reverse, one might anticipate that primary, polar magnetic structures could be uniquely stabilized in ferroelectric crystals, however, this scenario is apparently rare. Here, we show that in ferroelectric BiMn7O12, a pure, polar E-type antiferromagnetic order emerges below T1=59 K, and we present a phenomenological model of trilinear magnetoelectric coupling consistent with Bi3+ lone-pair driven polar distortions uniquely stabilizing the polar antiferromagnetism via modulation of Heisenberg exchange pathways, i.e., inverse exchange striction. In addition, below T2=55 K there occurs large commensurate canting of the E-type structure due to the onset of ferrimagnetic order on a separate crystallographic sublattice that may be exploited for additional magnetoelectric functionality

A plethora of structural transitions, distortions and modulations in Cu-doped BiMn7O12 quadruple perovskites

The presence of strongly competing electronic instabilities in a crystalline material can produce fascinating structural phenomena. For example, the A-site-ordered quadruple perovskite BiMn7O12 hosts both active polar instabilities of the Bi3+ lone pair electrons and Jahn–Teller instabilities of Mn3+ cations that drive the following sequence of phase transformations on cooling, Im-3 > I2/m > Im > P1, corresponding to orbital ordering and polar distortions. Carrier doping by Cu2+ tunes the two instabilities in BiCuxMn7−xO12 solid solutions and significantly complicates the system behavior. The x = 0.05 and 0.1 members show the following sequence of phase transformations on cooling, Im-3 > I2/m > R-1(αβγ)0 > R3(00γ)t, and are examples of materials with the electric dipole helicoidal texture in the ground state and a dipole density wave structure in the intermediate R-1(αβγ)0 phase (Science 2020, 369, 680–684). Here, the detailed behavior of the BiCuxMn7−xO12 solid solutions with x = 0.2–0.8 was investigated by laboratory X-ray, synchrotron X-ray, and neutron powder diffraction between 5 K and 620 K, and differential scanning calorimetry measurements. Nearly every composition (with a step Δx = 0.1) has a unique behavior when considering both the sequence of phase transitions and the presence of incommensurate superstructure reflections. The sequence Im-3 > HT-Immm(t)* > Immm* > LT-Immm(t)* is realized for x = 0.2 and 0.3 (where t denotes pseudo-tetragonal), Im-3 > I2/m* > Immm(t)* – for x = 0.4, Im-3 > I2/m* > I2/m* – for x = 0.5, Im-3 > I2/m* > Im-3 – for x = 0.6 and 0.7, and Im-3 > R-3 > I2/m > Im-3 – for x = 0.8, where asterisks denote the presence of additional incommensurate reflections. Re-entrance of the high-temperature cubic phase was observed at low temperatures for x = 0.6–0.8 suggesting strong competition between the different electronic instabilities. The re-entrant cubic phases have nearly zero thermal expansion

Correlated physical and mental health summary scores for the SF-36 and SF-12 Health Survey, V.1

<p>Abstract</p> <p>Background</p> <p>The SF-36 and SF-12 summary scores were derived using an uncorrelated (orthogonal) factor solution. We estimate SF-36 and SF-12 summary scores using a correlated (oblique) physical and mental health factor model.</p> <p>Methods</p> <p>We administered the SF-36 to 7,093 patients who received medical care from an independent association of 48 physician groups in the western United States. Correlated physical health (PCS_c) and mental health (MCS_c) scores were constructed by multiplying each SF-36 scale z-score by its respective scoring coefficient from the obliquely rotated two factor solution. PCS_c-12 and MCS_c-12 scores were estimated using an approach similar to the one used to derive the original SF-12 summary scores.</p> <p>Results</p> <p>The estimated correlation between SF-36 PCS_cand MCS_cscores was 0.62. There were far fewer negative factor scoring coefficients for the oblique factor solution compared to the factor scoring coefficients produced by the standard orthogonal factor solution. Similar results were found for PCS_c-12, and MCS_c-12 summary scores.</p> <p>Conclusion</p> <p>Correlated physical and mental health summary scores for the SF-36 and SF-12 derived from an obliquely rotated factor solution should be used along with the uncorrelated summary scores. The new scoring algorithm can reduce inconsistent results between the SF-36 scale scores and physical and mental health summary scores reported in some prior studies.</p> <p>(Subscripts C = correlated and UC = uncorrelated)</p

The inevitable QSAR renaissance

QSAR approaches, including recent advances in 3D-QSAR, are advantageous during the lead optimization phase of drug discovery and complementary with bioinformatics and growing data accessibility. Hints for future QSAR practitioners are also offered