First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

In this work we explore the overall structural behaviour of the [(CH3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly proof the presence of structural phase transition at Tt ~187 K (temperature at which the dielectric transition occurs) that involves a symmetry change from R-3c to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order-disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 $\pm$ 0.1 K/kbar), so that it should be feasible to shift it to room temperature using adequate thermodynamic conditions, for instance by application of external pressure

Strongly Coupled Magnetic and Electronic Transitions in Multivalent Strontium Cobaltites

The topotactic phase transition in SrCoOx (x = 2.5-3.0) makes it possible to reversibly transit between the two distinct phases, i.e. the brownmillerite SrCoO2.5 that is a room-temperature antiferromagnetic insulator (AFM-I) and the perovskite SrCoO3 that is a ferromagnetic metal (FM-M), owing to their multiple valence states. For the intermediate x values, the two distinct phases are expected to strongly compete with each other. With oxidation of SrCoO2.5, however, it has been conjectured that the magnetic transition is decoupled to the electronic phase transition, i.e., the AFM-to-FM transition occurs before the insulator-to-metal transition (IMT), which is still controversial. Here, we bridge the gap between the two-phase transitions by density-functional theory calculations combined with optical spectroscopy. We confirm that the IMT actually occurs concomitantly with the FM transition near the oxygen content x = 2.75. Strong charge-spin coupling drives the concurrent IMT and AFM-to-FM transition, which fosters the near room-T magnetic transition characteristic. Ultimately, our study demonstrates that SrCoOx is an intriguingly rare candidate for inducing coupled magnetic and electronic transition via fast and reversible redox reactions

Rediscovery of Atelopus cruciger (Anura: Bufonidae): current status in the Cordillera de La Costa, Venezuela

The genus Atelopus (Anura: Bufonidae) has suffered one of the most drastic declines recorded in the Neotropics. Nine of 10 Venezuelan species are categorized as Critically Endangered on the IUCN Red List. None of these species had been observed since the mid 1980s until recently, when an adult female of Atelopus mucubajiensis and several adults of A. cruciger were rediscovered. To assess the current distribution and status of A. cruciger we explored 15 locations where it was formerly known. Two populations were detected. Historical and current distribution maps were constructed based on these field explorations and data from museum collections. Using PCR assays we detected infection with Batrachochytrium dendrobatidis in one morbid and three live specimens. Our surveys suggest A. cruciger populations from lower altitudes on the northern slope of the Cordillera de La Costa are recovering, although the presence of B. dendrobatidis could jeopardize their long-term survival unless active conservation strategies are adopted

Magnetic neutron diffraction study of the charge ordered chain compounds Rb11Mn8O16 and Cs3Mn2O4

The magnetic ordering patterns of Rb11Mn8O16 and Cs3Mn2O4 were determined by neutron powder diffraction with and without applied magnetic fields. The crystal structures of these compounds exhibit infinite chains of edge sharing MnO4 tetrahedra with periodically alternating Mn2 and Mn3 valence. Both Rb11Mn8O16 and Cs3Mn2O4 show collinear magnetic order with antiferromagnetic alignment of Mn moments along the chains below the N el temperatures TN 38 1 K and 13.5 5 K, respectively. In Cs3Mn2O4 the Mn2 and Mn3 moments could be separately refined. The full magnetic structure in zero magnetic field can be viewed as a set of ferrimagnetic chains whose net moments are coupled antiferromagnetically perpendicular to the chain direction. For this compound, we further observe a magnetic field induced transition into a high field phase with uniformly aligned ferrimagnetic moment