New complex manganese vanadate materials were synthesized as high-quality single crystals in multi-millimeter lengths using a high-temperature, high-pressure hydrothermal method. One compound, ...Mn5(VO4)2(OH)4, was grown from Mn2O3 and V2O5 in 3 M CsOH at 580 °C and 1.5 kbar. Changing the mineralizer to 1 M CsOH/3MCsCl leads to the formation of another product, Mn6O(VO4)2(OH). Both compounds were structurally characterized by single-crystal X-ray diffraction (Mn5(VO4)2(OH)4: C2/m, Z = 2, a = 9.6568(9) Å, b = 9.5627(9) Å, c = 5.4139(6) Å, β = 98.529(8)°; Mn6O(VO4)2(OH): P21/m, Z = 2, a = 8.9363(12) Å, b = 6.4678(8) Å, c = 10.4478(13) Å, β = 99.798(3)°), revealing interesting low-dimensional transition-metal features. Mn5(VO4)2(OH)4 possesses complex honeycomb-type Mn–O layers, built from edge-sharing MnO6 octahedra in the bc plane, with bridging vanadate groups connecting these layers along the a-axis. Mn6O(VO4)2(OH) presents a more complicated structure with both octahedral MnO6 and trigonal bipyramidal MnO5 units. A different pattern of planar honeycomb sheets are formed by edge-shared MnO6 octahedra, and these sublattices are connected through edge-shared dimers of MnO5 trigonal bipyramids to form corrugated sheets. Vanadate groups again condense the sheets into a three-dimensional framework. Infrared and Raman spectroscopies indicated the presence of OH groups and displayed characteristic Raman scattering due to vanadate groups. Temperature-dependent magnetic studies indicated Curie–Weiss behavior above 100 K with significant anti-ferromagnetic coupling for both compounds, with further complex magnetic behavior at lower temperatures. The data indicate canted anti-ferromagnetic order below 57 K in Mn5(VO4)2(OH)4 and below 45 K in Mn6O(VO4)2(OH). Members of another class of compounds, K2M3(VO4)2(OH)2 (M = Mn, Co), also containing a honeycomb-type sublattice, were also synthesized to allow a comparison of the structural features across all three structure types and to demonstrate extension to other transition metals.
Two-dimensional triangular-lattice materials with spin-1/2 are perfect platforms for investigating quantum frustrated physics with spin fluctuations. Here we report the structure, magnetization, heat ...capacity, and inelastic neutron scattering (INS) results on cesium ytterbium diselenide, CsYbSe2 . There is no long-range magnetic order down to 0.4 K at zero field. The temperature-dependent magnetization, M(T), reveals an easy-plane magnetic anisotropy. A maximum is found in M(T) around T ≈ 1.5 K when magnetic field H is applied in the a b plane, indicating the short-range interaction. The low-temperature isothermal magnetization M(H) shows a one-third plateau of the estimated saturation moment, which is characteristic of a two-dimensional (2D) frustrated triangular lattice. Heat capacity shows field-induced long-range magnetic order for both H||c and H||ab directions. The broad peak in heat capacity and highly damped INS magnetic excitation at T = 2 K suggests strong spin fluctuations. The dispersive in-plane INS, centered at the (1/3 1/3 0) point, and the absence of dispersion along the c direction suggest 120 ∘ noncollinear 2D-like spin correlations. All these results indicate that the two-dimensional frustrated material CsYbSe2 can be in proximity to the triangular-lattice quantum spin liquid. We propose an experimental low-temperature H−T phase diagram for CsYbSe2.
The niobium hemicarbide (Nb
2
C) has at least three known polymorphs: α (Pnma or Pbcn),
β
(P
3
¯
1
m
)
, and
γ
(P6
3
/mmc) as a function of temperature. Identification of these phases has been ...notoriously difficult particularly for the lower-temperature variations (α and β) because of their long-range vacancy ordering. In the current study, an overall Nb
2
C composition has been processed by hot isostatically pressing NbC and Nb powders together which did not fully homogenize. Using neutron diffraction and selected area electron diffraction, the C6 (P
3
¯
m
1
) structure was identified in the Nb
2
C. The formation pathway for this phase is postulated from the high density of stacking faults observed in the NbC.
A hydrothermal route to single crystals of rare earth stannates RE2Sn2O7 (RE = La–Lu) in the cubic pyrochlore structure is reported. Growth reactions were performed in aqueous fluids at 700 °C and ...200 MPa with CsF mineralizers in concentrations ranging from 0 to 30 M, with 20 M CsF providing the most consistent results. Single crystals of the entire range of lanthanides were grown and characterized by single crystal X-ray diffraction and found to be isostructural in the Fd3̅m space group. The unit cell sizes range from 10.7106(16) Å for La2Sn2O7 to 10.3005(9) Å for Lu2Sn2O7. Both the unit cell size and RE–O distances are found to be essentially linear with respect to the ionic radius of the rare earth ion. The high quality diffraction data strongly suggests that there is very little site disorder or lattice defects in the sample. Of particular interest is the synthesis and single crystal growth of Ce2Sn2O7, which represents one of the few f1 pyrochlore samples. Specific heat measurements were obtained down to 50 mK on both Yb2Sn2O7 and Ce2Sn2O7, with the Yb material displaying a single sharp peak at 138 mK suggesting that the sample does not contain any site disorder or “lattice stuffing”. The Ce analog shows only a broad featureless transition below 100 mK, with this lack of long-range magnetic order consistent with quantum spin liquid behavior.
The magnetic properties are reported for three members of the glaserite series of compounds, Na2BaM(VO4)2, M = Mn, Mn0.6Co0.4, and Co. Large single crystals are grown using a high-temperature ...hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M2+O6 octahedra are interconnected with nonmagnetic (VO4)3– groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group P3̅m1); however, at lower temperatures both Na2BaMn(VO4)2 and Na2BaMn0.6Co0.4(VO4)2 undergo a structural transition to lower symmetry (monoclinic, C2/c). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na2BaMn0.6Co0.4(VO4)2 does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na2BaMn(VO4)2 and Na2BaCo(VO4)2 were determined using neutron powder diffraction. At zero magnetic field, Na2BaMn(VO4)2 possesses an antiferromagnetic structure with the moments ordered in a Néel-type arrangement and aligned along the C4 axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na2BaCo(VO4)2 represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the c-axis direction. The relationships between these structures and magnetic properties are discussed.
The triangular lattice compound TlYbS2 was prepared as large single crystals via a molten flux growth technique using sodium chloride. Anisotropic magnetic susceptibility measurements down to 0.4 K ...indicate a complete absence of long-range magnetic order. Despite this lack of long-range order, short-range antiferromagnetic interactions are evidenced through broad transitions, suggesting frustrated behavior. Variable magnetic field measurements reveal metamagnetic behavior at temperatures ≤2 K. Complex low temperature field-tunable magnetic behavior, in addition to no observable long-range order down to 0.4 K, suggest that TlYbS2 is a frustrated magnet and a possible quantum spin liquid candidate.
A new member of the descloizite family, a cobalt vanadate, SrCo(VO4)(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. ...SrCo(VO4)(OH) crystallizes in the orthorhombic crystal system in space group P212121 with the following unit cell parameters: a = 6.0157(2) Å, b = 7.645(2) Å, c = 9.291(3) Å, V = 427.29(2) Å3, and Z = 4. It contains one-dimensional Co–O–Co chains of edge-sharing CoO6 octahedra along the a-axis connected to each other via VO4 tetrahedra along the b-axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO4)(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co2+ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO6 octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a-axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO4)(OH).
The temperature–magnetic field phase diagram of the mixed honeycomb-triangular lattice systemK2Mn3(VO4)2CO3is investigated by means of magnetization, heat-capacity, and neutron-scattering ...measurements. The results indicate that triangular and honeycomb magnetic layers undergo sequential magnetic orderings and act as nearly independent magnetic sublattices. The honeycomb sublattice orders at about 85 K in a Neél-type antiferromagnetic structure, while the triangular sublattice displays two consecutive ordered states at much lower temperatures, 3 and 2.2 K. The ground state of the triangular sublattice consists of a planar “Y” magnetic structure that emerges from an intermediate collinear “up-up-down” state. Applied magnetic fields parallel or perpendicular to thecaxis induce exotic ordered phases characterized by various spin-stacking sequences of triangular layers that yield bilayer, three-layer, or four-layer magnetic superstructures. The observed superstructures cannot be explained in the framework of quasiclassical theory based only on nearest-neighbor interlayer coupling and point towards the presence of effective second-nearest-neighbor interactions mediated by fluctuations of the magnetic moments in the honeycomb sublattice.
The structural and magnetic properties of a glaserite-type Na2BaFe(VO4)2 compound, featuring a triangular magnetic lattice of Fe2+ (S = 2), are reported. Temperature dependent X-ray single crystal ...studies indicate that at room temperature the system adopts a trigonal P3̅m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (T s = 288 K). This structural transition involves a tilting of Fe–O–V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na2BaFe(VO4)2. The magnetic structure of the ordered state is modeled in the magnetic space group C2′/c′ that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Overall, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO6 octahedron.
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