Coexistence of two spin frustration pathways in the quantum spin liquid Ca_10Cr_7O_28

Abstract

Kagome antiferromagnetic lattices are of high interest because the geometric frustration is expected to give rise to highly degenerated ground states that may host exotic properties such as quantum spin liquid (QSL). Ca_10Cr_7O_28 has been reported to display all the features expected for a QSL. At present, most of the literature reports on samples synthesized with starting materials ratio CaO/Cr_2O_3 3:1, which leads to a material with small amounts of CaCrO_4 and CaO as secondary phases; this impurity excess affects not only the magnetic properties but also the structural ones. In this work, samples with starting material ratios CaO/Cr_2O_3 3:1, 2.9:1, 2.85:1, and 2.8:1 have been synthesized and studied by X-ray diffraction with Rietveld refinements, selected area electron diffraction measurements, high-resolution transmission electron microscopy (HRTEM), low-temperature magnetometry, and magnetic calorimetry. This result shows that a highly pure Ca_10Cr_7O_28 phase is obtained for a CaO/Cr_2O_3 ratio of 2.85:1 instead of the 3:1 usually reported; the incorrect stoichiometric ratio leads to a larger distortion of the corner-sharing triangular arrangement of magnetic ions Cr+5 with S = 1/2 in the Kagome lattice. In addition, our study reveals that there exists another frustration pathway which is an asymmetric zigzag spin ladder along the directions [211], [12-1], and [1- 1-1], in which the Cr-Cr distances are shorter than in the Kagome layers.