Magnetic interactions in the S = 1/2 square-lattice antiferromagnets Ba₂CuTeO₆ and Ba₂CuWO₆: parent phases of a possible spin liquid

Abstract

The isostructural double perovskites Ba2CuTeO6 and Ba2CuWO6 are shown by theory and experiment to be frustrated squarelattice antiferromagnets with opposing dominant magnetic interactions. This is driven by differences in orbital hybridisation of Te 6+ and W 6+ . A spin-liquid-like ground state is predicted for Ba2Cu(Te1-xWx)O6 solid solution similar to recent observations in Sr2Cu(Te1-xWx)O6.Magnetic frustration can stabilise novel quantum ground states such as quantum spin liquids or valence bond solids. 1 Frustration occurs when not all of the magnetic interactions in a material can be satisfied simultaneously as a result of lattice geometry or competing interactions. We have recently shown that a quantumspin-liquid-like state forms in the double perovskite solid solution Sr2Cu(Te1-xWx)O6 with a square lattice of Cu 2+ (3d 9 , S = 1/2) cations. 2,3 This was the first observation of a spin-liquid-like state in a square-lattice compound after 30 years of theoretical predictions. [4][5][6][7] The parent compounds Sr2CuTeO6 and Sr2CuWO6 are frustrated square-lattice (FSL) antiferromagnets. [8][9][10][11][12] The FSL model ( Fig. 1) has two interactions: nearest-neighbour J1 interaction (side) and next-nearest-neighbour J2 interaction (diagonal). Dominant antiferromagnetic J1 leads to Néel type antiferromagnetic order and dominant J2 leads to columnar magnetic order. Magnetic frustration arises from the competition of J1 and J2, and a quantum spin liquid state has been predicted for J2/J1 = 0.5 where frustration is maximised. [4][5][6][7] Sr2CuTeO6 and Sr2CuWO6 are the first known isostructural FSL systems with different dominant interactions and magnetic structures: dominant J1 and Néel order for Sr2CuTeO6 and dominant J2 and columnar order for Sr2CuWO6 respectively. 8,9 The two compounds have a tetragonal I4/m double perovskite structure with nearly identical bond distances and angles. 10,12 The magnetism becomes highly two-dimensional as a result of a Jahn-Teller distortion as the only unoccupied Cu orbital 3 2 − 2 is in the ab square plane. The major differences in dominant magnetic interactions are due to the diamagnetic Te 6+ d 10 and W 6+ d 0 cations located in the middle of the Cu 2+ square (Fig. 1c), which hybridise differently with O 2p allowing different superexchange paths between the Cu 2+ cations. 13,14 The spinliquid-like ground state forms when these two perovskites are mixed into a Sr2Cu(Te1-xWx)O6 solid solution. 2,3,15 Muon spin relaxation experiments revealed the absence of magnetic order or static magnetism in a wide composition range of x = 0.1-0.6. 2,3 The specific heat displays T-linear behaviour suggesting gapless excitations in a similar composition range. 2,3,15 The ground state has been proposed to be a random-singlet state with a disordered arrangement of non-magnetic valence bond singlets. 16 Motivated by these exciting findings in the Sr2Cu(Te1-xWx)O6 system, we have investigated the magnetic interactions of the isostructural barium analogues Ba2CuTeO6 and Ba2CuWO6. Ba2CuWO6 is know...

Site-Selective d10/d0 Substitution in an S = 1/2 Spin Ladder Ba2CuTe1–xWxO6 (0 ≤ x ≤ 0.3)

“…10 This limits Te 6+ participation in J2 exchange, where a d-orbital contribution from the B'' cation is required. We also highlighted that the d 10 /d 0 effect extends beyond simple cubic structures to a large range of 3d transition metal B' = Co 11,12 , Ni [13][14][15] and Cu [16][17][18][19] double perovskites, all of which follow the same principle based on the nonmagnetic B''-site: d 0strong J2 (type II) or d 10 -strong J1 (type I/Néel order).…”

“…17,20,21 The tetragonal unit cell has square lattice ab-planes of Cu 2+ cations in which superexchange is described using in-plane J1 (NN) and J2 (NNN) interactions, but with additional weak inter-plane interactions (J3 and J4) along c (Figure 1b). 17,18 Following the principles of the d 10 /d 0 effect, Sr2CuTeO6 is Néel ordered, while a strong J2 leads to columnar ordering for Sr2CuWO6. 16,19,[21][22][23][24] Upon introducing a mixture of W 6+ and Te 6+ , the material evades ordering over a wide portion (x = 0.05 -0.6) of the Sr2CuTe1-xWxO6 solution.…”

Diamagnetic d-Orbitals Drive Magnetic Structure Selection in the Double Perovskite Ba2MnTeO6

“…5 This d 10 /d 0 effect has been firmly established in B-site ordered Cu 2+ double perovskites Sr2Cu(Te,W)O6 and Ba2Cu(Te,W)O6, where Te 6+ is a 4d 10 cation and W 6+ is a 5d 0 cation on the B''-site linking the CuO6 octahedra. [6][7][8][9][10][11][12][13] Despite being isostructural, the interactions in the Te 6+ and the W 6+ compounds are completely different: both Sr2CuTeO6 and Ba2CuTeO6 have dominant nearest-neighbor J1 interactions whereas Sr2CuWO6 and Ba2CuWO6 have dominant next-nearestneighbor J2 interactions. 8,10 Furthermore, a novel spin-liquid-like state has been reported for the solid solution Sr2CuTe1-xWxO6.…”

Fractional corner magnetization of collinear antiferromagnets

“…It is interesting to note that recently the compound was successfully fabricated in the form of a single layer [24]. We note many other candidates for the S = 1/2 square antiferromagnets, e.g., Sr 2 CuTeO 6 [25], MoOPO 4 [26], and Ba 2 CuTeO 6 and Ba 2 CuWO 6 [27]. For three-dimensional systems, per-ovskite materials, which develop G-type antiferromagnetic orders, such as ATiO 3 with A = La, Ce, Pr, Nd, and Sm, would be good candidates [28,29].…”