Neutron diffraction studies and the magnetism of an ordered perovskite: Ba2CoTeO6

The impact of A-site cations on the crystal structure and magnetism of the new double perovskites ALaCoTeO6 (A = Na and K)

In this work, we report the structural and magnetic characterization of two new B-site rock-salt ordered double perovskites ALaCoTeO6 (A = K+ and Na+) with mixed A-site cations. KLaCoTeO6 crystallizes in the space group P4/nmm with a long-range ordering degree of 84.8% for the A-site K+/La3+ cations, whereas NaLaCoTeO6 adopts an unexpected triclinically distorted I1̄-structure with Na/La3+ disordering, validated by combined Rietveld refinements against high-resolution neutron diffraction data and Cu Kα1 X-ray powder diffraction data. Magnetic susceptibility at low temperatures shows clear antiferromagnetic (AFM) transitions for both compounds. KLaCoTeO6 exhibits the highest AFM transition temperature of 20 K amongst all the Co/Te-ordered 3C-type A2CoTeO6 (A = Pb2+, Sr2+, and Ca2+) and ALaCoTeO6 double perovskites due to its larger Co2+-O-Te6+ bond angle and A-site cationic ordering-induced larger distortion of the Co2+-based face-centered cubic sublattice. Moreover, we found that the average radius of the A-site cations plays a decisive role in the AFM transition temperatures of all these ordered double perovskites, that is, a larger A-site cation always results in a higher AFM transition temperature. This provides a strategy to subtly manipulate the magnetic properties of ordered double perovskites.

Ba2-xBixCoRuO6 (0.0 ≤ x ≤ 0.6) Hexagonal Double-Perovskite-Type Oxides as Promising p-Type Thermoelectric Materials

A new series of Ba_2-xBi_xCoRuO₆ (0.0 ≤ x ≤ 0.6) hexagonal double perovskite oxides have been synthesized by a solid-state reaction method by substituting Ba with Bi. The polycrystalline materials are structurally characterized by the laboratory X-ray diffraction, synchrotron X-ray, and neutron powder diffraction. The lattice parameters are found to increase with increasing Bi doping despite the smaller ionic radius of Bi³⁺ compared to Ba²⁺. The expansion is attributed to the reduction of Co/Ru-site cations. Scanning electron microscopy further shows that the grain size increases with the Bi content. All Ba_2-xBi_xCoRuO₆ (0.0 ≤ x ≤ 0.6) samples exhibit p-type behavior, and the electrical resistivity (ρ) is consistent with a small polaron hopping model. The Seebeck coefficient (S) and thermal conductivity (κ) are improved significantly with Bi doping. High values of the power factor (PF ∼ 6.64 × 10^-4 W/m·K²) and figure of merit (zT ∼ 0.23) are obtained at 618 K for the x = 0.6 sample. These results show that Bi doping is an effective approach for enhancing the thermoelectric properties of hexagonal Ba_2-xBi_xCoRuO₆ perovskite oxides.

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba[Formula: see text]MnTeO[Formula: see text]

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation ([Formula: see text]SR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba[Formula: see text]MnTeO[Formula: see text], where Mn[Formula: see text] ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn[Formula: see text] spins and a phase transition at [Formula: see text] = 20 K. Below [Formula: see text], the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. [Formula: see text]SR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above [Formula: see text]. It further unveils critical slowing-down of spin dynamics at [Formula: see text] and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

Structural Chemistry and Magnetic Properties of the Hexagonal Double Perovskite Ba2CoOsO6

The double perovskite Ba₂CoOsO₆, synthesized using solid-state methods at ambient pressure, is shown as a rare example of an oxide adopting the 6L-trigonal (S.G.: P3̅m1) perovskite structure. The structure, refined using a combination of X-ray and neutron diffraction data, showed the Co and Os were ordered over the two dimer sites with additional ordering over the corner-sharing sites. Bond valence calculations show the presence of the Co(II) and Os(VI) valence states, and the latter was confirmed using X-ray absorption spectroscopy. Bulk magnetic susceptibility measurements show Ba₂CoOsO₆ to undergo antiferromagnetic ordering near 100 K, and neutron diffraction showed an ordered moment on the Co3, Co4, and Os2 sites; whereas the Os1/Co1 remained disordered.

Synthesis and characterisation of the vibrational and electrical properties of antiferromagnetic 6L-Ba2CoTeO6 ceramics

Optimal processing conditions for fabrication of dense single-phase 6L-Ba₂CoTeO₆ ceramics via the solid-state reaction method were determined. These ceramics possess a room-temperature crystal structure described by the centrosymmetric P3[combining macron]m1 space group. Polarized Raman spectroscopy enabled the observation of all the 25 predicted Raman modes and assignment of their symmetries. On cooling, BCTO ceramics exhibit two antiferromagnetic transitions at 3 K and 12.5 K, in broad agreement with a recent single-crystal study [P. Chanlert, N. Kurita, H. Tanaka, D. Goto, A. Matsuo and K. Kindo, Phys. Rev. B: Condens. Matter Mater. Phys., 2016, 93, 094420]. Low temperature Fourier-transform infrared reflectivity analyses suggest the antiferromagnetic phase transitions to be driven by small distortions of the CoO₆ octahedra, lowering locally their C_3v symmetry. This causes splitting of the associated vibrational modes, but without a long-range structural change. AC impedance spectroscopy revealed BCTO ceramics to be leaky insulators with an activation energy for conduction of ∼0.15-0.25 eV, which suggests electron hopping between mixed oxidation states of Co.

Ba3(Cr0.97(1)Te0.03(1))2TeO9: in Search of Jahn-Teller Distorted Cr(II) Oxide

A novel 6H-type hexagonal perovskite Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ was prepared at high pressure (6 GPa) and temperature (1773 K). Both transmission electron microscopy and synchrotron powder X-ray diffraction data demonstrate that Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ crystallizes in P6₃/mmc with face-shared (Cr_0.97(1)Te_0.03(1))O₆ octahedral pairs interconnected with TeO₆ octahedra via corner-sharing. Structure analysis shows a mixed Cr²⁺/Cr³⁺ valence state with ∼10% Cr²⁺. The existence of Cr²⁺ in Ba₃(Cr²⁺_0.10(1)Cr³⁺_0.87(1)Te⁶⁺_0.03)₂TeO₉ is further evidenced by X-ray absorption near-edge spectroscopy. Magnetic properties measurements show a paramagnetic response down to 4 K and a small glassy-state curvature at low temperature. In this work, the octahedral Cr²⁺O₆ component is stabilized in an oxide material for the first time; the expected Jahn-Teller distortion of high-spin (d⁴) Cr²⁺ is not found, which is attributed to the small proportion of Cr²⁺ (∼10%) and the face-sharing arrangement of CrO₆ octahedral pairs, which structurally disfavor axial distortion.

Structural phase transitions and magnetic and spectroscopic properties of the double perovskites Sr2Co1−xMgxTeO6 (x = 0.1, 0.2 and 0.5)

Structural and magnetic properties of a series of double perovskites are investigated by X-ray and Neutron diffraction, EPR and magnetic susceptibility. The structural and phase-transitions analysis are done using symmetry mode-analysis approach.