Very recently, a 3 d based honeycomb cobaltate Na 2 Co 2 TeO 6 has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its 4 d /5 d counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on Na 2 Co 2 − x Zn x TeO 6 by structural, magnetic, and thermodynamic measurements, and track the doping evolution of its magnetic ground states. Due to the extremely close radii of Zn 2+ and high-spin Co 2+ ions, the substitution can be easily achieved. X-ray diﬀractions reveal no structural transition but only minor changes on the lattice parameter c over a wide substitution range 0 ≤ x ≤ 1 . 5. Magnetic susceptibility and speciﬁc heat measurements both suggest an antiferromagnetic ground state which is gradually suppressed with doping. It can survive with x up to ∼ 1 . 0. Then it evolves into a spin-glass phase with short-range order that is rapidly supplanted by a magnetically disordered state when x ≥ 1 . 3. By summarizing all these data, we construct a magnetic phase diagram of Na 2 Co 2 − x Zn x TeO 6 . Our results demonstrate that the Zn doping can eﬀectively suppress the magnetic order and induce a possibe quantum paramagnetic state. These may serve as a platform to investigate the Kitaev physics in this system.

Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material Na$_2$Co$_2$TeO$_6$

Synthesis and investigation on the morphological, structural, and electrical proprieties of a cesium magnesium phosphate prepared using solid-state reaction

Very recently, a $3d$ based honeycomb cobaltate ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its $4d/5d$ counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{TeO}}_{6}$ by structural, magnetic, and thermodynamic measurements, and track the doping evolution of its magnetic ground states. Due to the extremely close radii of ${\mathrm{Zn}}^{2+}$ and high-spin ${\mathrm{Co}}^{2+}$ ions, the substitution can be easily achieved. X-ray diffractions reveal no structural transition but only minor changes on the lattice parameter $c$ over a wide substitution range $0\ensuremath{\le}x\ensuremath{\le}1.5$. Magnetic susceptibility and specific heat measurements both suggest an antiferromagnetic ground state which is gradually suppressed with doping. It can survive with $x$ up to $\ensuremath{\sim}1.0$. Then it evolves into a spin-glass phase with short-range order that is rapidly supplanted by a magnetically disordered state when $x\ensuremath{\ge}1.3$. By summarizing all these data, we construct a magnetic phase diagram of ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{TeO}}_{6}$. Our results demonstrate that the Zn doping can effectively suppress the magnetic order and induce a possible quantum paramagnetic state. These may serve as a platform to investigate the Kitaev physics in this system. 

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Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material 
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Structure and optical properties have been successfully determined for a series of niobium- and tantalum-containing layered alkaline-earth silicate compounds, Ba3(Nb6−xTax)Si4O26 (x = 0.6, 1.8, 3.0, 4.2, 5.4). The structure of this solid solution was found to be hexagonal P-62m (No. 189), with Z = 1. With x increases from 0.6 to 5.4, the lattice parameter a increases from 8.98804(8) to 9.00565(9) A and c decreases from 7.83721(10) to 7.75212(12) A. As a result, the volume decreases from 548.304(11) to 544.479(14) A3. The (Nb/Ta)O6 distorted octahedra form continuous chains along the c-axis. These (Nb/Ta)O6 chains are in turn linked with the Si2O7 groups to form distorted pentagonal channels in which Ba ions were found. These Ba2+ ions have full occupancy and a 13-fold coordination environment with neighboring oxygen sites. Another salient feature of the structure is the linear Si–O–Si chains. When x in Ba3(Nb6−xTax)Si4O26 increases, the bond valence sum (BVS) values of the Ba sites increase slightly (2.09–2.20), indicating the size of the cage becoming progressively smaller (over-bonding). While SiO cages are also slightly smaller than ideal (BVS range from 4.16 to 4.19), the (Nb/Ta)O6 octahedral cages are slightly larger than ideal (BVS range from 4.87 to 4.90), giving rise to an under-bonding situation. The bandgaps of the solid solution members were measured between 3.39 and 3.59 eV, and the x = 3.0 member was modeled by density functional theory techniques to be 3.07 eV. The bandgaps of these materials indicate that they are potential candidates for ultraviolet photocatalyst.

Structural and optical properties of Ba3(Nb6−xTax)Si4O26 (x = 0.6, 1.8, 3.0, 4.2, 5.4)

The honeycomb-lattice cobaltate Na$_3$Co$_2$SbO$_6$ has recently been proposed to be a proximate Kitaev quantum spin liquid~(QSL) candidate. However, non-Kitaev terms in the Hamiltonian lead to a zigzag-type antiferromagnetic~(AFM) order at low temperatures. Here, we partially substitute magnetic Co$^{2+}$ with nonmagnetic Zn$^{2+}$ and investigate the chemical doping effect in tuning the magnetic ground states of Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$. X-ray diffraction characterizations reveal no structural transition but quite tiny changes on the lattice parameters over our substitution range $0\leq x\leq0.4$. Magnetic susceptibility and specific heat results both show that AFM transition temperature is continuously suppressed with increasing Zn content $x$ and neither long-range magnetic order nor spin freezing is observed when $x\geq0.2$. More importantly, a linear term of the specific heat representing fermionic excitations is captured below 5~K in the magnetically disordered regime, as opposed to the $C_{\rm m}\propto T^3$ behavior expected for bosonic excitations in the AFM state. Based on the data above, we establish a magnetic phase diagram of Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$. Our results indicate the presence of gapless fractional excitations in the samples with no magnetic order, evidencing a potential QSL state induced by doping in a Kitaev system.

Signatures of a gapless quantum spin liquid in the Kitaev material Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$

Structural characterization and X-ray reference powder pattern determination have been conducted for the Co- and Zn-containing tridymite derivatives Ba(Co1−xZnx)SiO4 (x = 0.2, 0.4, 0.6, 0.8). The bright blue series of Ba(Co1−xZnx)SiO4 crystallized in the hexagonal P63 space group (No. 173), with Z = 6. While the lattice parameter “a” decreases from 9.126 (2) A to 9.10374(6) A from x = 0.2 to 0.8, the lattice parameter “c” increases from 8.69477(12) A to 8.72200(10) A, respectively. Apparently, despite the similarity of ionic sizes of Zn2+ and Co2+, these opposing trends are due to the framework tetrahedral tilting of (ZnCo)O4. The lattice volume, V, remains comparable between 626.27 A3 and 626.017 (7) A3 from x = 0 to x = 0.8. UV-visible absorption spectrum measurements indicate the band gap of these two materials to be ≈3.3 and ≈3.5 eV, respectively, therefore potential UV photocatalytic materials. Reference powder X-ray diffraction patterns of these compounds have been submitted to be included in the Powder Diffraction File (PDF).

N. King

Papers

Structural and optical properties of Ba(Co1-xZnx)SiO4 (x = 0.2, 0.4, 0.6, 0.8)