Thompson C M, Carlo J P, Flacau R, Aharen T, Leahy I A, Pollichemi J R, Munsie T J S, Medina T, Luke G M, Munevar J, Cheung S, Goko T, Uemura Y J, Greedan J E
Department of Chemistry and Chemical Biology, McMaster University, Hamilton, ON L8S 4M1, Canada. Brockhouse Institute for Materials Research, McMaster University, Hamilton, ON L8S 4M1, Canada.
J Phys Condens Matter. 2014 Jul 30;26(30):306003. doi: 10.1088/0953-8984/26/30/306003. Epub 2014 Jul 8.
The B-site ordered double perovskite Ba2CaOsO6 was studied by dc magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) Å at 280 K and cubic symmetry [Formula: see text] is retained to 3.5 K with a = 8.3462(7) Å. Curie-Weiss susceptibility behaviour is observed for T > 100 K and the derived constants are C = 0.3361(3) emu K mol(-1) and ΘCW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu K mol(-1) for a 5d(2) Os(6+) configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50 K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (µSR) data show clear long-lived oscillations indicative of a continuous transition to long-range magnetic order below TC = 50 K. An estimate of the ordered moment on Os(6+) is ∼ 0.2 μB, based upon a comparison with µSR data for Ba2YRuO6 with a known ordered moment of 2.2 μB. These results are compared with those for isostructural Ba2YReO6 which contains Re(5+), also 5d(2), and has a nearly identical unit cell constant, a = 8.36278(2) Å-a structural doppelgänger. In contrast, Ba2YReO6 shows ΘCW = - 616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. The results on these 5d(2) systems are compared to recent theory, which predicts a variety of ferromagnetic and antiferromagnetic ground states. In the case of Ba2CaOsO6, our data indicate that a complex four-sublattice magnetic structure is likely. This is in contrast to the spin-disordered ground state in Ba2YReO6, despite a lack of evidence for structural disorder, for which theory currently provides no clear explanation.
通过直流磁化率、粉末中子衍射和μ子自旋弛豫方法对B位有序双钙钛矿Ba2CaOsO6进行了研究。晶格参数在280 K时为a = 8.3619(6) Å，具有立方对称性[公式：见正文]，在3.5 K时保持不变，a = 8.3462(7) Å。在T > 100 K时观察到居里-外斯磁化率行为，导出的常数为C = 0.3361(3) emu K mol(-1)和ΘCW = -156.2(3) K，与文献值非常吻合。这个居里常数比5d(2) Os(6+)构型的仅自旋值1.00 emu K mol(-1)小得多，表明自旋-轨道耦合有重大影响。先前的研究在50 K附近检测到了磁化率和热容异常，但关于基态的性质没有得出明确结论。虽然粉末中子衍射未检测到有序的Os磁矩，但μ子自旋弛豫(µSR)数据显示出明显的长寿命振荡，表明在TC = 50 K以下向长程磁有序的连续转变。基于与已知有序磁矩为2.2 μB的Ba2YRuO6的µSR数据比较，对Os(6+)上的有序磁矩估计约为0.2 μB。将这些结果与含有Re(5+)（同样为5d(2)）且晶胞常数几乎相同（a = 8.36278(2) Å）的同结构Ba2YReO6的结果进行了比较，Ba2YReO6是一种结构类似物。相比之下，Ba2YReO6的ΘCW = - 616 K，在50 K以下具有复杂的自旋无序且最终自旋冻结的基态，表明其几何失配程度比Ba2CaOsO6高得多。将这些5d(2)体系的结果与最近的理论进行了比较，该理论预测了多种铁磁和反铁磁基态。对于Ba2CaOsO6，我们的数据表明可能存在复杂的四亚晶格磁结构。这与Ba2YReO6中的自旋无序基态形成对比，尽管缺乏结构无序的证据，目前理论对此没有明确解释。
