CaCrO3: An Anomalous Antiferromagnetic Metallic Oxide

Abstract

Combining infrared reflectivity, transport, susceptibility and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transitionmetal oxide with a three-dimensional electronic structure. LSDA calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizeable pd hybridization remarkably strong next-nearest neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magneto-elastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.Strongly correlated electron systems including the wide class of transition-metal oxides exhibit a quite general relation between magnetic order and electrical conductivity [1]: ferromagnetism typically coexists with metallic conductivity, whereas insulators usually exhibit antiferromagnetism. It is always a challenge to understand exceptions from this rule. The rare observations of ferromagnetism in insulating transition-metal oxides most often are due to a particular type of orbital ordering [2]. The few examples of antiferromagnetic (AFM) metals, e.g., (La/Sr) 3 Mn 2 O 7 [3] or Ca 3 Ru 2 O 7 [4], are characterized by reduced electronic and structural dimensionality, and the antiferromagnetic order corresponds to a stacking of ferromagnetic (FM) layers. Here we report the discovery of a three-dimensional transition-metal oxide with metallic conductivity, antiferromagnetic exchange interactions, and C-type antiferromagnetic order: the perovskite CaCrO 3 .Perovskites containing Cr 4+ (CaCrO 3 , SrCrO 3 , and PbCrO 3 ) were already studied previously [5,6,7,8,9,10], but neither the details of the crystal structure nor the nature of the magnetic ordering are known. Only very recently evidence for C-type AFM order was reported in multi-phase samples of SrCrO 3 [10]. Regarding the conductivity, the existing data are controversial. In Refs. [7,9] CaCrO 3 was claimed to be metallic, but more recently insulating behavior has been reported [5]. A similar controversy persists also for SrCrO 3 , which should definitely be more metallic than CaCrO 3 due to the less distorted crystal structure, but metallic behavior was observed in Ref.[5] only under pressure. These controversies most likely are connected with the difficulty to prepare high-quality stoichiometric materials and with the lack of large single crystals.CaCrO 3 exhibits an orthorhombic GdFeO 3 -type perovskite structure and early magnetization measurements indicate a magnetic transition at 90 K [8], which is confirmed in our samples. Two electrons occupy the Cr 3d shell (S=1), rendering the material electronically similar to insulating RVO 3 [11] (also 3d 2 ) and to metallic (Ca/Sr)RuO 3 (4d 2 ) [12]. CaCrO 3 shows an unusually high transition-metal valence, Cr 4+ , which may lead to a small or even negative charge-transfer gap [13,14], i.e., holes in the O band. In CrO 2 with ruti...

Orbital-ordering driven structural distortion in metallicSrCrO3

“…They suggested CaCrO 3 is itinerant, but close to being localized, implying importance of correlation effects. [9,10] Additionally, no evidence of orbital ordering within the t 2g shell was observed. [9] Attfield and coworkers have concentrated on SrCrO 3 , using neutron diffraction and synchrotron powder x-ray diffraction studies.…”

“…[9,10] Additionally, no evidence of orbital ordering within the t 2g shell was observed. [9] Attfield and coworkers have concentrated on SrCrO 3 , using neutron diffraction and synchrotron powder x-ray diffraction studies. [7,8] Below T N ≈40 K, most of their sample underwent a structure transition from a NM cubic phase to a AFM tetragonal phase, but the two phases coexist even at low T .…”

“…[4] Weiher, Chamberland, and Gillson obtained a metallic single crystal sample. [5] The susceptibility measurements showed two anomalies, a kink at 325 K and an upturn at 90 K. At the latter, which is recently identified as the Neel temperature T N , [6,9] a kink in the resistivity data and decrease in volume by 2 % from the powder diffraction studies were observed. The Curie-Weiss moment is high spin 3.7 µ B , recently confirmed by Zhou et al [6] The reason of large difference from spin-only value of 2.8 µ B for S=1 system is unresolved, however it was found that the susceptibility in SrCrO 3 did not follow a CurieWeiss behavior so no local moment value could be identified.…”

“…[6,7,8,9] Whether these systems are metallic, strongly correlated, and spin ordered is still controversial. [6,7,8,9,10] Roth and DeVries reported an ordered moment of 1.9 µ B and Curie-Weiss moment of 2.83 µ B in the isovalent compound PbCrO 3 , consistent with S = 1 Cr 4+ (d 2 ). [1] Chamberland and Moeller synthesized a single crystal, which was semiconducting with 0.27 eV activation energy.…”

High-pressure and high-temperature synthesis and physical properties of Ca2CrO4 solid

“…[26][27][28][29] The infrared reflectivity measurement suggests the compounds of CaCrO 3 and SrCrO 3 are metallic, 30,31 while for the resistivity measurement they are reported to be insulators and insulator-metal transition happens under high pressure, 26,32,33 which is not observed in Ca 2 CrO 4 . The orthorhombic distorted CaCrO 3 is more insulating than cubic SrCrO 3 with a narrower band width, 26 axis, 30,35 while it is elongated in the layered Ca 2 CrO 4 and Sr 2 CrO 4 , which leads to quite different orbital and spin distribution. The insulating behavior of the layered perovskites possibly due to the suppression of orbital degree of freedom and the two dimensional crystal structure.…”